Photoactive donor-acceptor conjugated macrocycles: New opportunities for supramolecular chemistry

Photoactive donor-acceptor conjugated macrocycles: New opportunities for supramolecular chemistry

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Shanyuan Zhong^a, Lingyun Zhu^a, Shuai Wu^a, Yuanming Li^a, Meijin Lin^*^,^a^,^b

Chinese Chemical Letters | 2023, 34(8) : 108124

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Chinese Chemical Letters | 2023, 34(8): 108124

• Review •

Photoactive donor-acceptor conjugated macrocycles: New opportunities for supramolecular chemistry

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Shanyuan Zhong^a, Lingyun Zhu^a, Shuai Wu^a, Yuanming Li^a, Meijin Lin^*^,^a^,^b

Affiliations

^aKey Laboratory of Molecule Synthesis and Function Discovery, and Fujian Provincial Key Laboratory of Electrochemical Energy Storage Materials, College of Chemistry, Fuzhou University, Fuzhou 350116, China

^bCollege of Materials Science and Engineering, Fuzhou University, Fuzhou 350116, China

Published: 2023-08-15 doi: 10.1016/j.cclet.2022.108124

Outline

Abstract

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The design and synthesis of photoactive macrocyclic molecules continue to attract attention because such species play important roles in supramolecular chemistry as well as photoelectronic applications. Donor-acceptor (D-A) conjugated macrocycles are an emerging class of photoactive molecules due to their D-A conjugated structural characteristics and tunable optical properties. In addition, the well-defined cavities in such D-A macrocycles endow them with versatile host-guest properties. In this review, we provide a comprehensive summary of D-A conjugated macrocycle chemistry, detailing recent progress in the area of synthetic methods, optical properties, host-guest chemistry and applications of the underlying chemistry to chemical sensors, bioimaging and photoelectronic devices. Our objective is to provide not only a review of the fundamental findings, but also to outline future research directions where D-A conjugated macrocycles and their constructs may have a role to play.

Key words

Conjugated macrocycles / Donor-acceptor / Photoactive / Supramolecular chemistry / Photoelectronic materials

Cite this Article

Shanyuan Zhong, Lingyun Zhu, Shuai Wu, Yuanming Li, Meijin Lin. Photoactive donor-acceptor conjugated macrocycles: New opportunities for supramolecular chemistry[J]. Chinese Chemical Letters, 2023 , 34 (8) : 108124 - . DOI: 10.1016/j.cclet.2022.108124

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1 Introduction

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Design and synthesis of new macrocycles with intriguing structures and peculiar functions (e.g., optical, electronic and magnetic properties) are of great significance since their important roles in supramolecular chemistry and materials science [1-6]. The traditional macrocycles such as crown ether [7-10], cyclodextrins [11-14], calixarenes [15-18], cucurbiturils [19-21], pillararenes [22-25] and their structurally similar scaffolds usually possess tunable cavities for binding guests but no other photoelectronic functions for further applications. Moreover, their skeletons are incapable of being changed and functional substituents can be introduced only on their portals. Therefore, it is challenging to create a class of customizable functional macrocycles with a wide range of cavity sizes and diverse functions.

Donor-acceptor (D-A) conjugated molecules containing alternate donor (D) and acceptor (A) motifs are an emerging class of functional molecules with tunable optical properties, electronic structures as well as redox properties [26-30], which have potential applications in various fields such as organic optoelectronic devices [31-36], sensors and bioimaging [37-42]. In these molecules, the electronic-rich building blocks (called D motifs) generally govern the highest occupied molecular orbitals (HOMOs), while the electron-deficient building blocks (called A motifs) determine the lowest unoccupied molecular orbitals (LUMOs). More importantly, due to the tailorable HOMO-LUMO energy gaps and facilitated electron transports in the π-conjugations, these D-A conjugated molecules are always photoactive and could be excited by the UV-vis lights. Recently, chemists have introduced this design strategy into the synthetic macrocycles, and rolled such linear D-A conjugated molecules into different macrocycles to develop a new class of functional macrocycles named "D-A conjugated macrocycles (DACMs)", which is anticipated to combine the bilateral advantages of host-guest properties from synthetic macrocycles and photoactive properties from the D-A conjugated molecules.

Indeed, a myriad of D-A π-conjugated macrocycles with different applications has been reported in the past decades. Compared with the traditional macrocycles, the D-A π-conjugated macrocycles possess the following unique characteristics: (1) Their cyclic structures are often shape-persistent but controllable, which are beneficial towards supramolecular self-assembly and accommodation of guest molecules with electronic activity [43-45]. (2) Their electronic structures as well as photoactive properties could be modulated by their π-conjugations and introduction of the appropriate donor and acceptor units [46,47]. (3) Compared with linear conjugated molecules, macrocycles can not create charge defects thanks to lacking end-groups [48-51]. Accordingly, such photoactive DACMs hold a broad application prospect in supramolecular chemistry and functional materials.

In this review, we provide a comprehensive summary of DACMs chemistry. The synthesis of DACMs is presented by the different linkages between their D and A motifs, such as vinylene, ethynylene, arylene, B/N-containing linkers in their cyclic backbones. Unique optical properties with controllable electronic structures as well as rich host-guest chemistry are also illustrated to shed light on the relationships between their structures and properties. Moreover, their potential applications in chemical sensors, bioimaging, and photoelectronic devices especially in organic photovoltaics and organic light-emitting diodes are also discussed. Our objective is to provide not only a review of the fundamental findings, but also to outline future research directions where DACMs and their constructs may have a role to play.

2 Synthesis of DACMs

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In the past decades, a variety of D and A units have been introduced into the macrocycles to gain tunable optical and electronic structures. In general, D and A moieties are connected via π-systems, such as vinylene, ethynylene, and arylene [52,53]. Also, different D and A units can be connected directly to form DACMs in alternating fashions, as illustrated in Fig. 1, in which the blue color represents the D while the flesh color represents the A [54].

2.1 Vinylene-linked macrocycles

As we know, the vinylene-linked π-conjugations could be commonly created by the Wittig reactions [55], McMurry cross-coupling reactions [56], and glyoxylic Perkin condensation reactions [57,58]. In 2017, a D-A structural carboxyfunctionalized 1,6-pyrenophane-tetraene macrocycle was firstly reported by Robert and co-workers (Fig. 2) [59] via glyoxylic Perkin condensation reactions. 1,6-Pyrenylenediglyoxylic acid 1 was reduced to 1,6-diacetic acid 2 under H₃PO₂/NaI. Glyoxylic Perkin condensation was taken place between 1 and 2 under high dilution conditions to afford macrocycle 3 in ca. 25% isolated yield. By changing the linkers, when a large excess of diglyoxylic acid 5 was reacted with dibromophenylene diacetic acid 4 in an attempt to generate linear three-block biamaleate 6 (Fig. 3) [60]. Unexpectedly, a D-A conjugated macrocycle 7 was synthesized in ca. 25% yield, which can reach up to 69% under high dilution conditions.

Inspired by these unexpected results, our group developed a facile and efficient approach for the synthesis of imides embedded DACMs 10 via glyoxylic Perkin condensation and imidization reaction under mild conditions recently [61]. As shown in Fig. 4, the conformationally adaptive macrocycle 10 was prepared by glyoxylic Pekin condensation of precursors pyrenylene-2,7-digloxylic acid 8 and 1,4-diphenylenediacetic acid 9 using Et₃N as the catalyst, and followed by an imidization with 2,6-diisopropylaniline. Our group also swapped the pyrenylene-2,7-digloxylic acid 8 with pyrenylene-1,6-digloxylic acid to generate different macrocycles via a similar synthetic procedure, and the related optical property will be demonstrated in Section 3 [52]. Notably, the present macrocyclization yield (58%) has a good lead over other π-conjugated macrocycles prepared by metal catalysis (ca. 5%-52%), which might be related to the formation of dynamic covalent bonds during the Perkin condensations under thermodynamic control [62,63]. Due to the dynamic and reversible bonding characteristics, it is helpful to generate the low-energy macrocyclic intermediates. More importantly, these dynamic and reversible bonds could be fixed by the following imidization reactions, which is further in favour of the formation of the stable 10 in high yields. Thus, dynamic covalent methods are indeed mighty strategies to carry out the macrocyclization process.

2.2 Ethynylene-linked macrocycles

Sonogashira cross-coupling [64,65], Glaser homo-coupling and Eglington coupling reactions are the general methods to synthesize D-A structural ethynylene macrocycles [66]. In 2004, a dehydroannulene-type cyclophane 15 furnished with fluorescence ion sensory properties was synthesized by the Baxter group via Eglinton/Galbaraith coupling, in which the ring skeleton was composed of thiophene as donor units and pyridine as acceptor units with ethynylene linkage (Fig. 5) [67]. Silylated-intermediate 13 was obtained by coupling 12 and 11 under Sonogashira coupling reactions. 14 was desilylated with TBAF in aqueous THF and followed by a terminal alkyne coupling under high dilution afforded the target D-A conjugated macrocycle 15.

In 2005, Traber et al. applied palladium-copper-catalyzed Sonogashira coupling to construct arylene ethynylene macrocycles (Fig. 6) [68]. Bimolecular macrocyclization between precursors 16 and 17 in one-pot under the catalysis of Pd(PPh₃)₄/CuI was performed to afford product 18. However, the purification was problematic due to the poor solubility of 18. In this case, two hexyl substituents were introduced in amine group to change this situation. The desired macrocycle 21 was attained by reacting 19 and 20 under Sonogashira coupling conditions in a yield of 20% after purification.

Leu et al. have developed m-phenylene-bridged cross-conjugated macrocycle 30 and 2,5-thiophene-bridged macrocycles 31 via Sonogashira cross-coupling reaction (Fig. 7) [53]. Sonogashira coupling reaction was taken place between TIPS-protected m-phenylene ethynylene monomer and o-diiodoarene, treating with TBAF to deprotect leading to 26 and 28 with moderate yields after purification. Notaly, macrocyclization was accomplished by using Pd(P^tBu₃)₂ as catalyst under copper-free conditions which plays a critical role in improving the yield of resulting macrocycles. Similar synthetic procedures were capitalized to generate 2,5-thiophene-containing macrocycle 31. It was found that the cross-conjugated bridges are conducive to charge transfer.

Phulwale et al. utilized the Sonogashira coupling reaction for the construction of the triangular shape-persistent fully conjugated macrocyclic structures 37 and 39 based on the building block phenanthrylene (Fig. 8) [69]. The reaction of the terminal alkyne 32 and aryl halide 33 catalyzed by Pd(PPh₃)₄ and CuI system was employed to afford the open-chain precursor and followed by the Sonogashira cross-coupling reaction between the resultant and acetylene gas under a dilute concentration leading to macrocyclic molecule 34. The D-A conjugated macrocycle 35 was obtained after deprotection by TFA. Diamine 36 and 38 were applied to convert o-quinone into dibenzophenazine and dibenzoquinoxaline respectively, which perform as acceptor units in macrocycles 37 and 39.

The naphthalenediimide-based enantiomeric pair 42 and 43 with D-A system reported by Takeuchi were synthesized by Sonogashira coupling reaction (Fig. 9) [70]. 2,6-Dibromonaphthalenediimide 40 and 1,8-diethynylanthracene 41 were subjected to intermolecular macrocyclization under the catalysis of Pd(PPh₃)₄ and CuI in refluxing THF, leading to the final products in 15% isolated yield. Interestingly, the enantiomeric pair exhibited absorption and fluorescence response in varied temperatures, which holds great prospective in the area of molecular thermometer.

In 2021, dos Santos and co-workers demonstrated the construction of a vinylene-ethynylene-fused conjugated macrocycle 45 for the application of organic electronics based on thiophene moiety via a McMurry coupling process (Fig. 10) [71]. The Sonogashira coupling was carried out under Pd(PPh₃)₄ and CuI to give the precursor 44, and then, intermolecular macrocyclization between two 44 fragments was performed via McMurry coupling to afford the final cyclic structure 45 in 17% yield.

2.3 Arylene-linked macrocycles

The construction of arylene and/or heteroarylene DACMs in alternating patterns mainly depends on multinuclear macrocyclic Pt complexes and follow by a reductive elimination process. In 2015, the building blocks of thiophene and perylene diimide (PDI) derivative were utilized by Ball and co-workers to synthesize chiral conjugated macrocycles in an alternating D-A-D-A pattern via cyclic platinum complex [72]. The A fragment, 1,7-diphenyl-PDI possesses high stability of n-type charge carrier and excellent electronic characteristics, which is an ideal building block to construct optoelectronic materials. As shown in Fig. 11, stannylated 46a was treated with PtCl₂(COD) to afford 46b. The conversion of 46b into four-cornered tetraplatinum macrocycle-intermediate was implemented by reacting with 5,5′-bis(trimethylstannyl)-2,2′-bithiophene for 40 h. The target macrocycle was produced through a reductive elimination by treating with PPh₃ in a yield of 8%. The linkage at 1,7-position of diphenyl-PDI results in three stereoisomers, enantiomers (S,S) and (R,R) and meso isomers (R,S). X-ray analysis of single crystals indicated that the meso isomer possesses a larger ring strain owing to the bowl-shaped linker. Based on this result, Liu and co-workers replaced the units bithiophene with trithiophene to design new macrocycles via theoretical analysis [73]. Trithiophene possesses a red-shift absorption with respect to bithiophene, which is expected to increase the ability of harvesting light in the newly designed macrocycles. Furthermore, NH₂ and NO₂ groups were incorporated into the trithiophene moiety to explore the influence of absorption spectra by employing theoretical calculation.

In 2018, Zhang's group further built up a trimer macrocycle 48 with stannylated 1,7-dithienyl-PDI subunit through a platinum complex (Fig. 12) [74]. Subsequently, to endow the macrocycle with the ability of self-assembly through halogen bonding interactions, bromine atoms were introduced into the thiophene rings to afford the functionalized 48-Br₁₂. As expected, the brominated macrocycle can assemble into a capsular structure, which can be applied as active layer in field transistor devices. At the same time, they also integrated helical perylene diimide ribbons into macrocyclic backbone 50 and 52 through the same synthetic pathway as that of 47 to explore the relationship between molecular conformation and electronic property (Fig. 13) [75].

Interestingly, Würthner reported an ingenious D-A conjugated macrocycle 57 in 2021 that the PDI units located inside the macrocycle, in which the ring backbone was constructed based on oligothiophene with phenylene as linkages (Fig. 14) [76]. Imidization was performed between 53 and amines 54 under microwave irradiation and high temperature. Pt-intermediate 55c was obtained after stannylation and Sn-Pt exchange with Pt(COD)Cl₂. Intermolecular cross-coupling reactions between Pt-mediated 55c and stannylated 56 were fulfilled to give target products 57. It is worth noting that elevated temperature plays a decisive role in ensuring a syn-conformation of thiophene units. In 2022, Würthner further utilized the same building blocks PDI and thiophene to construct a series of D-A conjugated half-cycles 63-66 via the same synthetic pathway [77]. As shown in Fig. 15, ring closure was carried out by the combination of PDI-thiophene precursor 58 and stannylated α-oligothiophene structures bearing two (59) to five (62) thiophene subunits under high-dilution conditions in order to suppress the intermolecular polymerization. They also attached PDI units extraannularly the thiophene macrocycle via an oxidative coupling pathway to investigate the influence on supramolecular assembly with the introduction of PDI segments [78].

Similar to the synthesis of 48, Li created a diketopyrrolopyrrole-based D-A conjugated macrocycle 69 which can be utilized as non-fullerene acceptor to construct organic solar cells (Fig. 16) [54]. The macrocyclic platinum complex 68a was prepared by reacting 67 with [PtCl₂(COD)]. Treated with dppf and underwent a reductive elimination process with PPh₃ yielded the required macrocycle 69. The yield after purification was up to 16%.

The synthesis of twisted macrocycle 72 with significant fluorescence solvatochromism was carried out by Itami through Suzuki–Miyaura coupling [79]. As shown in Fig. 17, the building block with axially chirality, 4,5-diphrnylphenanthrene with diboronic acids, coupled with 3,6-dibromonaphthalimide through quadruple Pd-catalyzed Suzuki–Miyaura cross-coupling, leading to the nonplanar aromatic macrocycle 72 with 8% yield.

Very recently, Wang et al. have made an available approach for the construction of DACMs via post-functionalization (Fig. 18) [80]. On the basis of their previous results, the D-A system was incorporated into the synthesized conjugated macrocycles 73 and 74 by oxidizing methoxy groups and then condensation with aniline to generate 75 and 76. The resulted dibenzo[a,c]-phenazine moieties acted as acceptor units, which endow the newly designed macrocycles with a narrowing energy gap.

The Shikita group reported the formation of an alternating macrocycle 79 with cyclo-meta-phenylene motif comprising of three 5-(N-carbazolyl-phenylen-1,3-diyl) and three 6-phenyl-1,3,5-triazin-2,4-diyl via Suzuki–Miyaura cross coupling reaction (Fig. 19) [81]. Macrocyclization was carried out between trimeric dichloride 77 and diboronate precursor 78 under dilution conditions and the target product 79 was generated in an 18% isolated yield.

Coupling reactions play a significant role in synthesizing DACMs, but there still exist disadvantages. It is inevitable to form undesired oligomers and polymers during the process of macrocyclization. In this regard, a highly dilute concentration of reactants is needed to inhibit the formation of linear conjugated molecules.

2.4 Cycloparaphenylene (CPP)-based macrocycles

Cycloparaphenylene (CPP) as emerging aromatic macrocycles comprised of arylene and heteroarylene based on coupling reactions [82]. The curved macrocycles 84 and 85 designed by Itami were synthesized via stepwise coupling reactions (Fig. 20) [83]. The Suzuki−Miyaura cross-coupling reaction between 80 and 81 were taken place to achieve C-shaped precursor 82 in a yield of 76%. The final structure 84 was obtained by means of Ni-mediated cyclization and followed by an aromatization process. The conversion of anthraquinone (AQ) into tetracyanoanthraquinodimehane (TCAQ) endows acceptor moiety of 85 with a strengthened electron-withdrawing ability.

Jasti achieved a series of aza[8]cycloparaphenylenes via replacing the acceptors by the alkylated pyridine moieties in 2015 [84]. Macrocyclization was carried out between 86 and 87 via a palladium-catalyzed Suzuki cross-coupling process (Fig. 21). The resulting cyclic compound 88 was then submitted to a reductive aromatization, leading to two nitrogen-doped nanohoop structures 89 and 90. Alkylation of 89 and 90 was conducted respectively to obtain the D-A hoops 91 and 92. The key steps are the construction of U-shaped precursors 86 and 87 on account of ring strain. In those structures, the alkylated pyridine ring acts as an acceptor while the oligophenylene chain acts as a donor unit.

Then, Jasti further reported a nitrogen-doped [6]CPP nanohoop 96 (Fig. 22) with higher ring strain owing to the smaller diameter by contrast with 91 and 92 in 2015 [85]. The precursor 93 was obtained through lithium-halogen exchange and Miyaura borylation. Macrocyclization was carried out by means of sequential Miyaura borylation and treating with an oxidative homocoupling under mild conditions to give 95 with a yield of 51%. The key to constructing the highly strained aza[6]CPP 95 lies in the strategy of boronate homocoupling reaction.

Wang and co-workers reported the construction of 100,101 and 102 bearing one, two, or three 2,7-bis(2-thienyl)-9H-fluoren-9-one (TFOT) units (Fig. 23) [86]. The precursors 97 was submitted to Ni(COD)₂-mediated intramolecular homocoupling and followed by an oxidative aromatization with DDQ afforded the target product 100 bearing one TFOT unit in a yield of 86%. Similarly, macrocycles 101 and 102 were generated via Ni(COD)₂-mediated coupling reaction of 97a and 97b and followed by an oxidative aromatization process with yields of 7% and 14%, respectively.

Suzuki macrocyclization was applied for the construction of CPP-based D-A macrocycle 106 with bright orange-emitting (Fig. 24) [87]. The acceptor unit, benzothiadiazole (BT) fragment was introduced by the reaction of 103 and 4,7-dibromobenzo[c]-1,2,5-thiadiazole 104 under Suzuki coupling conditions. The resulting product 105a was subjected to triethylsilyl deprotection and followed by a reductive aromatization process under H₂SnCl₄ to generate the final macrocycle 106 in 22% isolated yield.

The Tan group recently reported the synthesis of D-A hoop 108 (Fig. 25) containing tetra-benzothiadiazole (TB) acceptors units via platinum-complex intermediate, in which the macrocycle 108 possessed bright emission and supramolecular assembly behaviour [88]. Macrocyclization was accomplished by using platinum-mediated assembly approach with para-borylated 4,7-diphenyl-benzothiadiazole 107. Then a reductive elimination was taken place under the condition of PPh₃ in refluxing toluene for 48 h to afford target molecule 108 in a yield of 10%. In 2021, they further utilized the same building block 107 to fabricate macrocycle 109 with bright red emission through a gold-mediated trimerization method [89]. Macrocyclization procedure was carried out by reacting 107 with [Au₂Cl₂(dcpm)] to afford a triangular cyclic intermediate. Subsequent oxidative chlorination gave the target molecule 109 with 24% isolated yield.

2.5 B/N-containing macrocycles

The tricoordinate organoboranes attract extensive attention owing to their strong electron-accepting ability. The embedding of organoboranes as A units into macrocycles would provide a mighty strategy to modulate electronic properties [90]. In 2012, the π-conjugated B-π-N macrocyclic compound 111 synthesized by Jäkle via organometallic condensation was firstly reported, in which the ring backbone was consisted of amine donor units and borane acceptor units linked by para-phenylene (Fig. 26) [91]. Reaction of linear silylated oligomer 110 with 2 equiv. of BBr₃ afforded a borylated product, then followed by treating with 4-(^tBu)-N,N-bis(4-(Me₃Sn)C₆H₄) aniline under pseudo high-dilution conditions gave rise to the B-Br functionalized macrocyclic compound. The final B-π-N macrocycle 111 was acquired after dealing with triisopropylphene copper (TiPCu) in refluxing toluene in an overall yield of 38% after purification, in which the boron can be protected in the shield of bulky TiP groups.

In 2015, the Chen group utilized this method to further fabricate two novel unstrained B-containing macrocycles 113 and 115 employing the building blocks of 3,6-disubstituted carbazole and 2,7-disubstituted fluorene (Fig. 27) [92]. For the sake of maximizing close to the degree of endocyclic C-B-C at 120°, the DFT calculation was utilized and revealed that the skeleton of 113 comprised of two carbazole moieties and two fluorene moieties linked by four boranes and that of 115 comprised of two carbazole moieties and four fluorene moieties linked by two boranes exhibited less ring strain. The synthetic procedure of 113 was identical to that of 111 by starting with precursor 112 via an organometallic condensation reaction. However, macrocyclization of 115 was carried out via Pd-catalyzed Stille coupling under high dilution. More B-containing macrocycles with different kinds of building blocks are expected to construct via this modular synthetic approach.

A new D-π-A macrocycle whose skeleton is composed of fluorene and arylamine as well as organoboranes was reported by the Baser-Kirazli group [93]. The combination of organoborane and π-conjugated system endows the structure with excellent electronic characteristics owing to the electron affinity of organoboranes. As shown in Fig. 28, the precursors 116 and 117 were acquired through lithium-halogen exchange and silicon-boron exchange respectively. Subsequently, organometallic condensation was performed between 116 and 117 at high-dilution conditions to access the final macrocycle 118.

Russell and co-workers exploited Eglinton–Glaser coupling reactions in the synthesis of two macrocycles (Fig. 29) with well-defined cavities that can absorb electron-deficient C₇₀ [94]. The terminal alkynes of 119 were bonded under the conditions of palladium-copper-catalyzed Eglinton–Glaser coupling, resulting in asymmetrical ring configuration 120 with a yield of 21.4%. Another macrocycle 122 with a larger diameter, was obtained through an intermolecular macrocyclization that two fragments 121 were coupled to form 122 in a yield of 8.7% with a symmetric structure. In those cyclic structures, the electron-rich triphenylamine (TPA) serve as donor units, while the electron-deficient 4,7-diphenyl-2,1,3-benzothiadiazole (BTTh₂) acts as acceptor units.

Pd-catalyzed Buchwald–Hartwig amination reaction was employed by Izumi and co-workers to build up an alternating D-A conjugated macrocycle 127 (Fig. 30) [95]. The double amination between dibromodibenzophenazine 124 and donor unit 123 was performed under the catalyst Pd₂(dba)₃ and ligand Qphos to generate 125 in a yield of 94%. After deprotected the N-Boc of 125 via TFA, macrocyclization was fulfilled by Pd-catalyzed Buchwald–Hartwig double amination of corresponding intermediate 126, resulting in the target product 127 being obtained in with a relatively high yield of about 45%. The U-shaped dibenzo[a,j]phenazine (DBPHZ) is chosen to act as an acceptor to adjust D-A dihedral angles, while the donor part is N,N-diphenyl-p-phenyelendiamine, which is conducive to relieve strain and add flexibility.

In summary, three common synthetic approaches have been employed in the past years. Transition-metal-mediated coupling reactions, such as palladium-catalyzed Suzuki coupling [96], McMurry coupling between carbonyls catalyzed by TiCl₃/LiAlH₄ or Zn/TiCl₄ [97], palladium-copper-catalyzed Eglinton–Glaser coupling between terminal alkynes [66], are the common strategies used in the synthesis of DACMs. Another strategy to generate macrocyclic structures is to form a cyclic platinum complex and then experience a reductive elimination process to afford the cyclic compounds [98,99]. However, despite high efficiency, the noble metals used are the main disadvantages in these synthetic methods. The glyoxylic Pekin condensation is a potential approach to achieve DACMs with high yield owing to the dynamic covalent process.

3 Photophysical properties of DACMs

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As stated above, the D-A π-conjugated macrocycles have received a great deal of attention as a result of their unique characteristics. Understanding the relationship between structure and property is critically important for the further development of functional materials. Generally, the HOMO in conjugated molecules is determined by electron donors, while the LUMO depends on electron acceptors [100]. As a result, the energy gap of D-A π-conjugated macrocycles can be tailored by selecting appropriate building blocks with the different ability of electron affinity, thus the corresponding optical and electronic properties will be adjusted. In this regard, such a strategy holds great promise in organic optoelectronic devices. Furthermore, it is typical for D-A structures to exhibit solvatochromic luminescent properties that the polarity of solvents plays a decisive role in the emission wavelength and intensity because of charge separation in the excited state [101]. At the same time, the intramolecular charge transfer (ICT) between donor units and acceptor units can give rise to the absorption band red-shifted.

It's typical for D-A molecules to exhibit the phenomenon of solvatochromic behaviour that the emission maximum red-shifted larger with the polarity of solvents increasing, which is attributed to the separation of HOMO and LUMO in the excited states [102]. The DACMs, with spatial localization of the HOMO and LUMO, undergoes ICT upon excitation, resulting in solvatofluorochromism in non-polar solvents. When the polarity of solvents increases, non-radiative relation is more favoured in the excited stated. The incorporation of acceptor BT unit into 128 ([10]CPP) skeleton to form the D-A system remarkedly altered its optical properties. As shown in Fig. 31b, the absorption maximum of 106 was 334 nm, which was almost identical to that of parent 128 [87]. However, the emission maximum of 106 was 571 nm, which exhibited a significant Stokes shift in 237 nm and a pronounced bathochromically shifted (105 nm) compared with the parent 128 (466 nm). Interestingly, different from those acceptor-containing CPP-derivatives such as 85 with a low quantum yield (5%) account for ICT [83], 106 possessed a quantum yield up to 59% owing to the absence of ICT, which was confirmed by theoretical calculation [87]. Similarly, 108, with four BT units incorporated into the cyclic structure, also exhibited a large red-shift emission by 119 nm in chloroform, by comparison with that of parent 128 ([12]CPP) [88]. The Stokes shift of 108 was 142 nm, which was attributed to the broken of symmetry in an excited state.

As shown in Fig. 32, the AQ-incorporated CPP molecule 84 exhibited more obvious red-shift of emission maximum when the polarity of solvents increases (∆λFL max = 95 nm from carbon tetrachloride to chlorobenzene) [83]. The DFT and time-dependent DFT calculations of 84 showed that the HOMO locates at the curved oligoparaphenylene moiety while the LUMO is localized on the AQ moiety, which resulted in the solvatochromic behaviour of 84. The HOMO-LUMO gap of acceptor-introduced 84 and 85 reduced to 2.68 eV and 1.93 eV respectively, compared with that of the parent 128 in 3.61 eV.

Similarly, the DACMs 30 and 31 also exhibited solvatochromic behaviour [53]. As shown in Fig. 33, the fluorescence spectra showed more pronounced bathochromic shifts in more polar solvents, which means charge-transfer experienced. The theoretical calculations indicated that the HOMOs are localized on veratrole moieties while the LUMOs are localized on phthalimide moieties, which accounted for the fluorescence solvatochromism. Owing to the spatial localization of the HOMO on two dialkoxyphenanthrenes moieties and 1,2-ethynylidene, and LUMO on A dibenzophenazine, a positive solvatochromism was observed in emission spectra of 37a (Fig. 34) [69].

It is interesting that when biocompatible encapsulation agent, DSPE-mPEG₅₀₀₀ was applied to produce 109 nanodots, the emission maximum of 109 nanodots redshifted to 650 nm (Fig. 35) [89]. The 109 nanodots exhibited a bright red 3PF upon excitation at 440 nm through a femtosecond laser at 1320 nm, which was confirmed to be a nonlinear optical process of 3PF via a power dependence relationship experiment.

Nuckolls designed and synthesized a series of cyclic and acyclic π-conjugated molecules based on diphenyl-perylenediimide (P) and bithiophene (B) to investigate the influences of macrocyclization and the D-A system toward the optical and electronic properties and performance being applied in n-type electronic materials (Fig. 36) [103]. As evidenced by absorption spectra and estimated band gap, the DACMs 47 showed a broader absorption range and narrower energy gap by contrast with that of either linear acyclic fragments 131 and 132 or cyclic compound 133 with only PDI derivatives fragments. Furthermore, the macrocyclic structures are more easily to be reduced than acyclic molecules according to cyclic voltammetry.

Based on above findings, Liu and co-workers made subtle structural modifications on conjugated macrocycle 47 with intention to understand its photophysical properties and improve its performance as organic photovoltaics by theoretical calculations [73]. The time-dependent DFT functional theory was employed and revealed that when bithiophene was replaced with trithiophene or its derivatives, not only the energy gap decreased effectively but also the oscillator strengths and electron reorganization energy changed a lot, which offers a feasible way to improve the performance of DACMs to be employed as organic photovoltaics. Moreover, such subtle structural modifications on macrocycles are expected to gain higher first hyperpolarizability, which holds great prospective in second-order nonlinear optical (NLO) materials.

The water-soluble macrocycle 134 (WCMT-1) that comprised of two 1,6-pyrene and two phenyl units linked by four maleimides constructed by our group exhibited aggregation-induce emission (AIE) effect in low polarity solutions and the fluorescence can be quenched in aqueous solution owing to the twist intramolecular charge transfer (TICT) (Fig. 37) [104]. Interestingly, the incorporation of surfactant, sodium lauryl sulfonate (SLS), into 134 aqueous solution can keep a lid on the TICT process thus "turn on" far red/near-infrared (FR/NIR) fluorescence 134 aqueous solution.

Not only band gap can be adjusted in cyclic skeleton by incorporating variable building blocks, but the different side chains can also tune the energy gap effectively. Zahid et al. also designed a series of D-π-A macrocycles (135-140) (Fig. 38), in which the donor macrocycle that consisted of arylborane and 9-methylcarbazole bridged by electron-withdrawing 9,9′-dimethylfluorene performed as donors [105]. Linked by a thiophene spacer, several acceptors (A1-A5) with different electron affinity were attached to the cyclic structure and theoretical calculation was applied to explore the impacts of end-capped moieties on photoelectronic properties. The frontier molecular orbitals of these macrocycles are show in Fig. 38c, for 135, 136, 139 and 140, HOMO is primarily distributed on the bridge and acceptor units and LUMO is composed of bridge and acceptor units. As for 137 and 138, HOMO is inhibited by bridge and donor units while LUMO mainly populated by acceptor and bridge units. All these macrocycles possess narrower energy gap in relation to that of 135. The theoretical calculation results substantiated that the newly designed compounds all possessed a narrower energy gap, lower binding energy, higher dipole moment and higher charge transport abilities compared with those of the reference 135 that comprised of donor cycle with four mesityl linked at B atom. There is no doubt that such modifications have made available protocols to design functional molecules with high performance to be exploited in organic photoelectronic devices.

4 Host-guest systems of DACMs and their photophysical properties

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Another peculiar property of DACMs is their well-defined cavities, which are of great importance in host-guest chemistry. These cavities can accommodate molecules with electronic activity such as electron-deficient C₆₀ and C₇₀. In addition, the shape and size of guest molecules need to match up with the cavities of hosts to fix into the inner cavities. More importantly, the complexation will make a significant impact not only on the behaviour of self-assembly and the structure of host molecules to form stable host-guest systems, but also on electronic properties. For example, in 2015, Ball and co-workers developed fully conjugated chiral macrocycles by using 2,2-bithiophene (D) and (R/S-1,7-diphenyl-PDI) (A) as building blocks (Fig. 39) [72]. The novel DACMs possess well-defined elliptical cavities with a distance between two bithiophenes and two PDI is 1.0 nm and 1.6 nm, respectively. Thus, small molecules or ions could be accommodated in to the cavity, which holds a promising chance for host-guest chemistry. In 2020, they further investigated the host-guest interaction between (R,R)-47 and fullerene C₇₀ and its derivative PC₇₁BM [106]. Characterization and quantification of the binding guest molecules fullerenes uncovered strong host-guest interaction with association constant (K_a) up to 9278 L/mol. A pronounced absorption and emission spectra perturbation were observed, especially the quench of (R,R)-47 fluorescence during the process of complexation. Remarkably, the host-guest complex (R,R)-47/PC₇₁BM outperformed macrocycle alone in electron mobility, which improved the OFET device performance more than five-fold.

In 2017, Russell group synthesized two DACMs, pentagonal 120 and hexagonal 122, possessing cavities with diameter from 1.1 nm to 1.8 nm and 1.8 nm to 2.6 nm respectively, in which the ring skeletons are comprised of TPA and BTTh₂ linked by acetylene (Fig. 40) [94]. In the cyclic skeleton, the TPA is functioned as electron-donor, while BTTh₂ serves as electron-acceptor. Given the suitable electronic structure and large cavity of the macrocycles 120 and 122 respectively, the formation of host-guest systems with electronically active molecules are expected. Electron-deficient C₆₀ and C₇₀ or its derivative PC₇₁BM can be incorporated into the cavity of 120 with the ratio of 1:1 to form host-guest complex, while one 122 molecular can accommodate two C₇₀ in its inner cavity. Interestingly, the C₇₀ was existed in the side of the cavity near BTTh₂ instead of the central location, which is ascribed to the S‧‧‧π interaction that can stabilize the 120⊃C₇₀ complex. Fluorescence titration of 120 and 122 with C₇₀ exhibited an obvious quench of fluorescence intensity and the association constants were calculated to be 1.95 × 10⁴ L/mol for 120⊃2C₆₀ and 1.33 × 10⁴ L/mol for 122⊃2C₆₀, which was in accordance with the results that the complexation between 120 or122 and C₆₀ exists. The blended film made by complex of 120⊃PC₇₁BM for organic solar cell afforded a significant power conversion efficiency, which can be considered as functional materials for photovoltaic application.

In 2018, based on the above discovery, Cheng and co-workers further researched what differences will make if the guest molecular, coronene (COR), was added into the host-guest systems formed by complexes 120⊃C₇₀ and 122⊃C₇₀ (Fig. 41) [107]. When processed at the 1-phenyloctane/HOPG interface, both of macrocycles 120 and 122 exhibit different self-assembled patterns at different concentrations according to the scanning tunneling microscopy. At low concentration, the monomeric 120⊃C₇₀ complexes direct the same position. However, as the concentration increases slowly, the 120⊃C₇₀ complexes tend to exhibit dimerized and turn to zigzag patterns. Once COR solution is added into the 120⊃C₇₀ system, the arrangement patterns mentioned before are gone, replaced by a loose packing model. As shown in picture 41, not only inside the cavity is a guest COR molecular, but near the cycle skeleton and adjacent macrocycles still exist COR molecules on account of adsorption and π conjugation. As a result, the host-guest system of 120⊃COR emerged with a ratio of 1:3 (120: COR). Similarly, there are two guest COR incorporated into the cavity of 122. Interestingly, the hole formed by long alkyl chains of adjacent 122 can accommodate guest molecules as well, thereby resulted in a host-guest network in a ratio of 1:5 (122: COR). Obviously, macrocycles/COR complexes are more stable than macrocycles/C₇₀. The DFT calculation indicates that the macrocycles/COR benefit more in thermodynamics. At the meantime, the planarity and π-conjugated structure of COR play a crucial role in stabilizing macrocycles/COR complexes, which can form strong π-π interactions with the HOPG interface.

The macrocycle 7 with intrinsic inner cavity also possesses great potential in binging small guest molecules [60]. As evidenced by sing-crystal X-ray diffraction (Fig. 42), one p-xylene molecule trapped into the cavity of macrocycle 7 when methanol was slowly diffused into a p-xylene solution of 7 to get its single crystal. Therefore, more stable host-guest system is anticipated to attain if suitable guest molecules are found.

Owing to the large skeleton of 106, C₆₀ can be absorbed into its inner cavity [87]. As shown in Fig. 43b, the host-guest interaction between 106 and C₆₀ led to a completely quench of fluorescence the fluorescence was quenched. A high binding constant, ca. 2.06 × 10⁴ L/mol was obtained.

Benefiting from the large size of 108, the supramolecular assembly of 108 with fullerene was also investigated [88]. Besides, the incorporation of the guest molecules also exerted a great influence on the conformation of host compound. As evidenced by single crystal X-ray diffraction, the average diameter of the inner cavity of 108 reached up to 16.5 Å, which was enough to accommodate large host molecules. Interestingly, when assembled with anthracene-C₆₀, 108 underwent a deformation into elliptical conformation with long and short axis of 17.8 and 14.7 Å, respectively, as shown in Fig. 44a. The self-assembly of anthracene-C₆₀@108 was stabilized by π‧‧‧π interactions of fullerene and CH⋯π interactions of anthracene and the binding constant was determined to be ca. 1.87 × 10⁴ L/mol calculated by fluorescence titration. Ternary cocrystallization of 108, C₆₀/C₇₀ and trithiasumanene were also performed. The hoop of 108 transformed into an oval shaped structure in the supramolecular assembly of (trithiasumanene⊃C₇₀)@108 due to the larger size of C₇₀. As a result, 108 can serve as an adaptive host that the structure of macrocycle 108 can adjust its conformation according to different guest molecules.

5 Applications of DACMs

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5.1 Chemical sensors

The incorporation of heteroatom into macrocycles is a useful tool to furnish macrocycles with different functions. In 2004, the dehydroannulene-type cyclophane 15 developed by Baxter possessed a special ability of coordination toward certain metal ions, in which the cyclic structure encompassed donor thiophene and acceptor pyridine units linked by ethynylene [67]. It was found that a yellow precipitate emerged when 15 and AgCl were mixed in 10% MeOH/CHCl₃ in five minutes. However, the precipitate disappeared completely when a saturated 10% H₂O/MeOH solution of KCN was added, which confirmed the coordination is a reversible process. Moreover, a bright orange luminescence was observed in the precipitate of 15/AgⅠ. By contrast, a visually pronounced fluorescence quenching occurred in the solution of 15/NiⅡ. The special phenomenon of chromogenic fluorescence and precipitate of 15 toward AgⅠ provide a platform for 15 to be utilized as AgⅠ sensors. Additionally, the D-A conjugated macrocycle 15 holds great promise in applying for NiⅡ sensors in biological media owing to the fluorescence quenching/precipitate caused by NiⅡ.

Recently, our group prepared a DACMs 141 comprising of 1,6-pyrenylene and 1,4-phenylene linked by maleimides, which exhibited vapochromic behavior [52]. Two crystals with different colours, the yellow crystallite 141α and the red crystallite 141β (Fig. 45c), were obtained via different crystal growth methods. The frontier orbitals were separated that HOMO is occupied by pyrene moiety and LUMO is mainly distributed on the maleimide and benzene units, which indicated the D-A character of 141. Interestingly, when exposed to saturated vapour of volatile solvents such as dichloromethane, tetrahydrofuran and toluene overnight, the yellow crystallite 141α transferred into the red crystallite 141β along with the changes of conformation (Fig. 45d), which have been sustained by powder X-ray diffraction (PXRD). This conversion was attributed to the flip of pyrene induced by the vapor of solvent and was an irreversible process, which creates the possibility of being applied for environmental management and industrial monitoring.

5.2 Bioimaging

Organic materials with near-infrared (NIR) fluorescence hold great potential in the field of bioimaging [108,109]. In order to extend their emissions to NIR region, the common strategies are the introduction of strong electron-donating and electron-accepting units or scale-up of π-conjugation [110]. In this regard, DACMs are anticipated to enter this field. In 2021, our group constructed a water-soluble DAMC, namely 134, in which the main backbone comprised of 1,6-position pyrenes and 1,4-position benzenes linked by four maleimides (Fig. 46) [104]. Interestingly, the incorporation of surfactant, sodium lauryl sulfonate (SLS), into 134 can keep a lid on TICT process thus "turn on" far-red/near-infrared (FR/NIR) fluorescence in 134 aqueous solution. Further analysis of controlled experiments shed light on both electrostatic and hydrophobic interactions are critically important in creating supra-amphiphile for this switch. Hela and HepG2 cancer cells were chosen to carry out bioimaging experiments. It is found out that the two types of cells were capable of fluorescent response when deal with SLS and 134. Meanwhile, the existence of 134&SLS has no impact on cell viability.

The above results provide us with reference and guidance for subsequent molecular design. To address the issue of lacking specificity, triphenylphosphonium cation (TPP) was integrated into four maleimides and the skeleton of macrocyclic structure remains the same (Fig. 47) [111]. The novel designed compound, 142, can achieve a 650+ nm emission and large Stokes shift of about 200+ nm, which is expected to offer new opportunities in the field of FR/NIR bioimaging. The AIE effect is discovered when the ratio of water in the mixed solvent is more than 80%, for which the solubility of 142 is worse in an aqueous solution than that in organic solvents. Meanwhile, the addition of SLS to 142 aqueous solutions can further enhance fluorescence owing to the electrostatic interaction. More importantly, the distributed synchronicity of fluorescence intensity between Mito-Tracker Green FM and 142 as well as high biocompatibility of Hela or HepG2 and make TPP-CMT a promising candidate in the field of mitochondrial-targeting bioimaging.

The macrocycle 109, with red emission and unique three-photon fluorescence (3PF) characteristic, exhibited promising prospective in fluorescence cerebrovascular imaging [112]. As shown in Fig. 48, the imaging of mouse cerebrovascular exhibited a clear contrast owing to its bright emission. A higher signal-to-background ratio (SBR), 14.7 in 2.6 µm blood vessel at imaging depth of 100 µm was observed in comparison to that of (1.92) in 2.7  µm vessel at imaging depth of 800 µm. Also, the penetration depth can even reach up to 900 µm.

5.3 Photoelectronic devices

5.3.1 Organic photovoltaics (OPVs)

The energy gap of DACMs can be tailored effectively by introducing appropriate D and A units into macrocyclic backbones, resulting in modular optical properties and electronic structures. Moreover, DACMs with controllable structures can align into ordered 2D and 3D self-assembly that are conducive to charge transfer. Additionally, introducing different building blocks into cyclic skeletons equip macrocycles with desired functions that broaden their applications as functional materials. For example, thiophene is a common building block to construct functional materials in organic electronic devices as a result of its high stability and polarizability as well as outstanding charge transport ability [113,114]. Cooke reported a thiophene-based macrocycle to be applied in organic field-effect transistors (OFETs) [71].

In 2016, Ball and co-workers compared the performance of the macrocycles 47 and their corresponding acyclic analogues 131 and 132 whose basic structural units were bithiophene and diphenyl-perylenediimide in organic optovoltaics for the first time [103]. The X-ray crystallographic and spectra analyses, as well as DFT calculations of these macrocyclic structures and corresponding acyclic fragments revealed the importance of molecular structure and supramolecular self-assembly in enhancing the performance of optovoltaic and optoelectronic materials. The UV-vis spectra exhibited larger absorption range of cyclic structures with respect to acyclic counterparts. The electron mobility was estimated by fabricating thin-film semiconductors and the results indicated that cyclic molecules 47 and 133 exhibited a pronounced advantage than acyclic analogues 132 and 132. Further analysis of atomic force microscopy displayed cyclic molecules showed greater potential in bulk heterojunction (BHJ) solar cells. The overall excellent properties endow the cyclic structures with higher power conversion efficiency (PCE) than that of acyclic analogues. This investigation sheds light on the importance of geometry toward the performance of optovoltaic devices and make it a promising modular method for electron transport materials.

The DACMs 120 and 122 consisting of TPA and BTTh₂ were synthesized by Zhang and co-workers [94]. Taking advantage of their large cavities that absorb guest molecules, host-guest complexes with C₇₀ or PC₇₁BM were successfully constructed. The 120, serving as electron donor material, was blended with PC₇₁BM to fabricate BHJ organic solar cells (OSCs). Different ratios of D/A weight (form 1:1 to 1:4) were investigated and the results exhibited the highest PCE in 2.66% with the ratio of 1:3 while 122 in the 1:4 blend with PC₇₁BM obtained highest PCE of 1.28%. Adding PC₇₁BM to pristine film of 120 resulted in entirely quench of fluorescence, which indicates the electron transfer from D unit 120 to A unit PC₇₁BM with high efficiency (Fig. 49). To this end, such host-guest systems provide excellent opportunities for fabricating OSCs. Remarkably, the ratio of conjugated macrocycle donor and fullerene acceptor has great impacts on the PCE of the device. This work makes D-A macrocycles available for fabricating OSCs with fullerene acceptor and offers a promising direction for optimizing the performance of OSC.

In 2018, Li et al. reported on the first synthesis of a diketopyrrolopyrrole (DPP) based D-A macrocycle, in which the cyclic backbone linked electron-deficient DPP and electron-rich bithiophene in an alternating D-A-D-A pattern [54]. The broad emission ranging from 700 nm to 1000 nm enables 69 to be utilized in near-infrared OLEDs. Interestingly, 69 exhibited amorphous morphology in thin film but can still possess good ability of charge transporting. When 69 was applied as non-fullerene acceptors in OSCs, a PCE of 0.49% was obtained, which was in direct contrast to that of linear analogue in 0.03% PCE. dos Santos and co-workers achieved two macrocyclic structures based on thiophene moiety 143 and 45, which can perform as donors to be applied in OSCs [71]. Four benzothiadiazole (BT) units was introduced into the skeleton of 143 to form the macrocycle 45 with a D-A system. As shown in Fig. 50c, the HOMO and LUMO electron density distribution of 143 are almost overlapped in the whole skeleton. However, 45 exhibited D-A character that LUMO is primarily populated by the BT units. The maximum absorption of 45 red-shifted in comparison with that of 143, which stemmed from the ICT effect of the D-A system. The cyclic voltammetry (CV) revealed that the two molecules can be served as donor materials, which was confirmed by DFT calculation. As a result, PC₇₁BM was selected as acceptor to fabricated OSCs with the two macrocycles. The devices obtained PCEs of 1.1% for 143 and 0.63% for 45 (Fig. 50). Such poor PCEs were attributed to narrow absorption. However, it provides access to be applied in photodetectors with the external quantum efficiency (EQE) of 40% for 143 and 25% for 45.

5.3.2 Organic light-emitting diodes (OLEDs)

Since the special phenomena, thermally activated delayed fluorescence (TADF) was first exploited in OLEDs by Endo et al., it has attracted extensive attention [115]. Compared to traditionally luminous and phosphorescent OLED materials, TADF materials are a class of highly efficient OLED emitters. Triplet excitons can transfer to singlet excitons through thermally conversion by a fast reverse intersystem crossing (RISC) process. Thus, the energy gap of highly efficient TADF emitters between singlet and triplet must be as small as possible [116,117]. The strategy of incorporating D units and A units can exactly meet this requirement. Conjugated macrocycles with D-A system can separate HOMO and LUMO in space, which is conducive to get a small bandgap.

In 2020, Izumi et al. developed a D-A-D-A conjugated macrocycle by employing two U-shaped N,N-diphenyl-p-phenyelendiamine as donor units and dibenzo[a,j]phenazine (DBPHZ) as acceptor units [95]. Given the fact that DBPHZ plays a significant role in narrowing energy gap and its U-shaped structure, it is a suitable building block to construct a macrocyclic structure for TADF materials. In order to gain insights into the impact of cyclization on photophysical and redox properties, the author synthesized a linear molecule 144 and cyclic structure 127 for comparison. The emission of linear analogue exhibited a larger red-shifted than that of 127, which is ascribed to the free rotation of the D-A system. As shown in Fig. 51b, the HOMO of 127 is localized on the whole ring while the LUMO is mainly inhibited by A units DBPHZ. As for 144, the electron density in the HOMO and LUMO was primarily occupied by D and A, respectively. Theoretical analysis of time-resolved luminescence spectroscopy indicated that the macrocycle 127 possesses a greater advantage in both RISC process and delayed fluorescence contribution than linear analogue counterpart. As evidenced by the fabrication and characteristics of OLED emitters, the cyclization afforded the conjugated macrocycle 127 for higher efficient TADF property, whose external quantum efficiency (EQE) reached up to 11.6% (Fig. 51) when served as an OLED emitter while that of linear oligomer 144 was 6.9%. It is a prominent progress that the EQE of the twisted D-A conjugated macrocycle is far beyond conventional fluorescent materials.

Shikita et al. also demonstrated that cyclic configuration possessed a superior performance when served as an OLED emitter than the acyclic counterpart [81]. As shown in Fig. 52b, the HOMO of DACMs 79 is populated by three carbazole units and the LUMO is inhibited by entire macrocyclic skeleton, which is different from that of 145 with LUMO distributed on the central triazine and adjacent phenylene rings. The macrocyclic compound 79, with a low S₁-T₁ energy gap (0.14 eV) determined by experiment, possessed the TADF ability while the linear molecule 145 did not. When fabricated as OLED emitters, the 79-based device exhibited a 4-fold increase in electroluminescence efficiency than that of acyclic analogue 145.

5.3.3 Organic fields effect transistor (OFET)

The electron mobility is an important parameter in OFET devices for which low carrier mobility is unfavourable for carrier extraction. Two different D-A conjugated macrocyclic conformation, trans-52 and cis-50, composing of chiral, helical perylene diimide ribbons and bithiophene were achieved by Ball and co-workers (Fig. 53) [75]. The LUMO determined by cyclic voltammetry was estimated to be –3.80 eV for trans-52 and –3.82 eV for cis-50, which are similar to the parent helical perylene diimide and anticipated to perform as n-type semiconductors. Pronounced discrepancy of the two molecular conformations was observed. The cis-50 presented a "tent" shape with less strain and more flexibility, whereas the trans-52 showed an "upright" with more rigidity, leading to a 4-fold increase in electron mobility of cis-50 by contrast with that of trans-52.

The strong interaction between (R,R)-47 and PC₆₁BM exert a pronounced improvement on the performance of OFET device [106]. When (R,R)-47⊃PC₆₁BM was fabricated into thin films in the active layer (Fig. 54), the electron mobility increased more than five-fold compared with that of (R,R)-47. The DACMs 69, with amorphous morphology in film, also exhibited good charge transport ability by OFET measurements [54]. These investigations show that DACMs possess great potential application in OFET.

Functionalization also offers a feasible strategy to broaden the applications of D-A macrocycles. As shown in Fig. 55a, functionalization with bromine atoms in thiophene rings facilitate 48-Br₁₂ self-assembly, thus a three-dimensional capsular structure was obtained in the solid state and in films [74]. The blended film made by 48-Br₁₂ exhibited electron mobility more than 20-folds by contrast with that of 48. The cellular films in OFET active layer possesses interior spaces showed response to different hydrocarbons, which is expected to be applied in sensing and nanoreactors.

6 Conclusions and outlook

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In this review, we have presented important works of photoactive donor-acceptor conjugated macrocycles during the past decades, including the synthesis of DACMs with different D, A units and linkage moieties, excellent photophysical properties, intriguing host-guest chemistry, as well as various potential applications especially in chemical sensors, bioimaging, photoelectronic devices. This new emerging class of photoactive DACMs have offered a prospective strategy to combine the bilateral advantages of synthetic macrocycles and D-A molecules, which pave an avenue for the development of supramolecular chemistry and functional materials. Despite the great breakthroughs have achieved in this promising field, there are still issues needed to be solved in the near future: (1) Coupling-based pathways with tedious synthetic steps and low overall yield impede their further applications. One promising strategy is the dynamic covalent methods that the macrocyclization are carried out under thermodynamic control, such as Perkin condensation and alkyne metathesis [66,118]. (2) Diverse building blocks are needed to construct macrocyclic architecture with novel properties and unique functions. (3) Though altered optical and electronic properties of photoactive DACMs provide many possibilities for various applications, more fine modification on structure and property should be taken into consideration in order to enhance their performances as functional materials. The construction of DACMs with suitable inner cavity are anticipated to form host-guest systems, which provides an excellent way to tune self-assembly and are promising to be applied in OSCs. Also, tunable energy gap of DACMs that reflect in photo and redox properties offer the possibility for further applications in organic photoelectronic materials. Moreover, DACMs with high first hyperpolarizability are expected to obtain, which holds great potential in the fields of second-order NLO materials.

The past decades have witnessed the remarkable progress of photoactive DACMs in the synthetic chemistry and supramolecular chemistry. Undoubtedly, the development of D-A π-conjugated macrocycles is still going to flourish and opportunities and challenges coexist in this promising domain. The great potential of photoactive DACMs inspires us spare no efforts to explore and investigate their further application as photofunctional materials.

Declaration of competing interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

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This work is supported by the National Natural Science Foundation of China (Nos. 21971041 and 22001039) and Natural Science Foundation of Fujian Province (No. 2020J01447).

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Year 2023 volume 34 Issue 8

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doi: 10.1016/j.cclet.2022.108124

Receive Date：2022-10-31
Online Date：2025-11-21
Published：2023-08-15

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Received：2022-10-31
Revised：2022-12-07
Accepted：2022-12-27

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^bCollege of Materials Science and Engineering, Fuzhou University, Fuzhou 350116, China

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Fig. 1. Schematic of three connection methods of D-A conjugated macrocycle mentioned in this review.

Fig. 2. Synthesis of macrocycle 3. Conditions: (a) H₃PO₂, NaI, AcOH, reflux, 16 h, 92%; (b) Ac₂O, NEt₃, THF, reflux, 72 h, then add EtOH, EtBr, DBU, reflux, 24 h, 25%. DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene.

Fig. 3. Synthesis of linear bismaleate 6 and conjugated macrocycle 7. Conditions: Ac₂O, NEt₃, THF, reflux,16 h, then EtBr, EtOH, DBU, reflux, 24 h. Reaction conditions: (A) 8 equiv. of 5, concentrated, 6: 0%, 7: 25%; (B) Stoichiometric, high dilution, 6: 0%, 7: 69%.

Fig. 4. Synthesis of DACMs 10. Conditions: Ac₂O, Et₃N, THF, reflux, then 2,6-diisopropylaniline, 58%.

Fig. 5. Synthesis of macrocycle 15. Conditions: (a) PdCl₂(PPh₃)₂, CuI, toluene, 20 ℃, 5 d (54%); (b) 1 mol/L (ⁿBu)₄NF, THF, H₂O, 20 ℃, 20 h (88%); (c) [Cu₂(OAc)₄]2H₂O, pyridine, 20 ℃, 7 d (46%).

Fig. 6. Synthesis of macrocycles 18 and 21. Conditions: (a) Pd(PPh₃)₄, CuI, ⁱPr₂NH, THF; (b) Pd(PPh₃)₄, CuI, C₆H₆, NEt₃.

Fig. 7. Synthesis of macrocycles 30 and 31. Conditions: (a) Pd(OAc)₂, PPh₃, CuI, HNⁱPr₂; (b) TBAF, THF; (c) Pd(P^tBu₃)₂, NEt₃, toluene; (d) Pd(P^tBu₃)₂, DABCO, toluene. TBAF = tetrabutylammonium fluoride, DABCO = N,N-dimethylethanolamine.

Fig. 8. Synthesis of D-A macrocycles 37 and 39. Conditions: (a) Pd(PPh₃)₄, CuI, Et₃N, THF, 70 ℃, 3 h, 58%; (b) acetylene, Pd(PPh₃)₄, CuI, Et₃N, THF, 70 ℃, 3 h, 48%; (c) Na₂S·H₂O, toluene, MGE, 150 ℃, 12 h; (d) TFA, DCM, H₂O, rt, 12 h, 83%; (e) 36, PTSA, AcOH, EtOH, DCM, 110 ℃, 12 h; (f) 38, PTSA, AcOH, EtOH, DCM, 100 ℃, 12 h. PTSA = p-toluenesulfonamide, TFA = trifluoroacetic acid.

Fig. 9. Synthesis of enantiomeric pairs 43 and 44. Conditions: Pd(PPh₃)₄, CuI, THF, reflux, 24 h.

Fig. 10. Synthesis of macrocycle 45. Conditions: TiCl₄, Zn, pyridine, THF, reflux, 22%.

Fig. 11. Synthesis of macrocycle 47. Conditions: (a) Pt(COD)Cl₂, toluene, 100 ℃, 24.5 h, 45%; (b) 5,5′-bis(trimethylstannyl)-2,2′-bithiophene, THF, 50 ℃, 40 h; (c) PPh₃, toluene, 100 ℃, 24 h, 8% (2 steps). COD = 1,5-cyclooctadiene.

Fig. 12. Synthesis of macrocycle 48-Br₁₂. Conditions: Br₂, I₂, CH₂Cl₂, r.t.

Fig. 13. Synthesis of macrocycles 50 and 52. Conditions: (a) 1,4-Bis-(tributylstannyl)benzene, P-(2-furyl)₃, Pd₂dba₃, THF, 55 ℃, 12 h; (b) Pt(COD)Cl₂, toluene, 100 ℃, 12 h; (c) 5,5′-Bis(tributylstannyl)-2,2′-bithiophene, THF, 55 ℃, 40 h. (d) PPh₃, toluene, 100 ℃, 12 h.

Fig. 14. Synthesis of macrocycles 57. Conditions: (a) Zn(OAc)₂, imidazole, microwave irradiation; (b) Sn(C₄H₉)Cl, n-BuLi, THF, r.t., overnight; (c) Pt(COD)Cl₂, toluene, 95 ℃; (d) toluene, 75 ℃, overnight, then dppf, CH₂Cl₂, r.t., 6 h, then m-xylene, 120 ℃, overnight. dppf = 1,1'-bis(diphenylphosphino)ferrocene.

Fig. 15. Synthesis of macrocyclic D-A dyads 63, 64, 65 and 66. Conditions: (a) toluene, 75 ℃, overnight, then dppf, CH₂Cl₂, r.t., 6 h, then m-xylene, 120 ℃, overnight.

Fig. 16. Synthesis of D-A conjugated macrocycle 69. Conditions: (a) [Pt(COD)Cl₂], THF, 70 ℃, 24 h; (b) (dppf), CH₂Cl₂, r.t., 12 h; (c) PPh₃, toluene, 110 ℃, 12 h, yield 16% after three steps.

Fig. 17. Synthesis of macrocycle 72. Conditions: Pd(OAc)₂, XPhos, NaOH, 1,4-Dioxane, 80 ℃, 16 h, 8%.

Fig. 18. Synthesis of macrocycles 75 and 76. Conditions: (a) Ce(NH₄)₂(NO₃)₆, THF/ACN, r.t., 30 min; (b) 1,2-Diaminobenzene, glacial acetic acid, chloroform, 70 ℃, 16 h.

Fig. 19. Synthesis of macrocycle 79. Conditions: Pd(PPh₃)₄, Cs₂CO₃, THF, reflux, 18%.

Fig. 20. Synthesis of 84 and 85. Conditions: (a) Pd(PPh₃)₄, Na₂CO₃, ⁿBu₄NBr, THF, reflux, 41 h, 76%; (b) Ni(COD)₂, 2,2′-bipyridyl, THF, reflux, 25 h, 56%; (c) NaHSO₄·H₂O, o-chloranil, m-xylene, water, 150 ℃, 75 h, 27%; (d) Malononitrile, TiCl₄, pyridine, CH₂Cl₂, 0 ℃ to r.t., 26 h, 81%.

Fig. 21. Synthesis of D-A aza[8]CPPs 91 and 92. Conditions: (a) K₃PO₄, SPhos Pd G2, 80 ℃; (b) sodium napthalenide, THF, 78 ℃; (c) MeOTf, SPhos Pd G2 = chloro(2-dicyclohexylphosphino-2′,6′-dimethoxy-1,1′-biphenyl)(2′-amino-1,1′-biphenyl-2-yl)palladium(Ⅱ).

Fig. 22. Synthesis of 96. Conditions: (a) Pd(PPh₃)₂Cl₂, KF, B(OH)₃, O₂, THF/H₂O, 51%; (b) i. sodium naphthalenide, THF, -94 ℃; ii. I₂, THF, 45%; (c) Methyl iodide, CH₂Cl₂, 100 ℃, µW, 83%.

Fig. 23. Synthesis of TFOT-containing macrocycles 100, 101 and 102. Conditions: (a) 98, Pd(PPh₃)₄, K₂CO₃, toluene/ethanol/water, 80 ℃, 10 h; (b) MeI, 120 ℃, 10 h; (c) Ni(COD)₂, bpy, THF, reflux, 24 h; (d) DDQ, 100 ℃, 3 h; (e) Ni(COD)₂, bpy, THF, reflux, 24 h; (f) DDQ, 75 ℃, 2 h. TBS = tert-butyldimethylsilyl, bpy = bipyridine, DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.

Fig. 24. Synthesis of 106. Conditions: (a) SPhos Pd Gen Ⅲ, 2 mol/L K₃PO₄, 1,4-dioxane, 80 ℃, 38%; (b) TBAF, THF; (c) H₂SnCl₄, THF, 22% (2 steps).

Fig. 25. Synthesis of 108 and 109. Conditions: (a) Pt(COD)Cl₂, CsF, THF, reflux, 24 h; (b) PPh₃, toluene, 110 ℃, 48 h; (c) [Au₂Cl₂(dcpm)], Cs₂CO₃, toluene/EtOH/H₂O; (d) PhICl₂, DMF. Dcpm = dicyclopropylmethyl.

Fig. 26. Synthesis of D-π-A macrocycle 111. Reagents: (a) BBr₃; (b) 4-(tert-butyl)-N,N-bis(4-(trimethylstannyl)phaenyl)aniline; (c) triisopropylphenyl copper (TipCu).

Fig. 27. Synthesis of macrocycles 113 and 115. Conditions: (a) BBr₃; (b) 2,7-bis(trimethylstannyl)-9,9′-dimethylfluorene; (c) TipCu; (d) Pd₂(dba)₃, ^tBu₃P.

Fig. 28. Synthesis of 118. Conditions: ^FMesLi, toluene, r.t.

Fig. 29. Synthesis of 120 and 122. Conditions: (a) Pd(PPh₃)Cl₂, CuI, ⁱPr₂-NH, r.t., 5 d.

Fig. 30. Synthesis of macrocycle 127. Conditions: (a) Pd₂(dba)₃, QPhos, NaOt-Bu, toluene, 60 ℃, 12 h; (b) TFA, CH₂Cl₂, r.t., 40 min; (c) 124, (1.0 equiv.), Pd₂(dba)₃, QPhos, K₂CO₃ (2.2 equiv.), 1,4-dioxane, 100 ℃, 24 h. dba = dibenzylideneacetone, Qphos = 1,2,3,4,5-pentaphenyl-1′-(di-tert-butylphosphino)ferrocene.

Fig. 31. (a) Structure of parent compound 128 ([10]CPP) and control compound 129 and 130; (b) Absorption and emission spectra of 106 compared to 128 and 130 in dichloromethane; (c) The calculated frontier molecular orbitals of 128, 106, 85. Reproduced with permission [87]. Copyright 2020, Wiley-VCH; (d) Absorption and emission spectra of 108 in chloroform solution (red), powder (blue), and polymethylmethacrylate film (black). Reproduced with permission [88]. Copyright 2020, Wiley-VCH.

Fig. 33. UV-vis and fluorescence spectra of DACMs 30 (a) and 31 (b); (c) HOMO and LUMO of 30 and 31 calculated at the TD/PCM (cyclohexane)/CAM-B3LYP/6-311+G(2d, 2p) level. Reproduced with permission [53]. Copyright 2012, American Chemical Society.

Fig. 35. (a) Absorption (solid lines) and emission spectra (dashed lines) (excitation wavelength: 434 nm) of 109 in chloroform solution (blue) and its nanodots in aqueous dispersion (red); (b) 3PF spectrum of 109 nanodots in aqueous dispersion measured under 1320 nm fs laser excitation. Reproduced with permission [89]. Copyright 2021, Wiley-VCH.

Fig. 36. (a) Schematic of diphenyl-perylenediimide denoted P and bithiophene denoted B; Acyclic structures of (b) 131 (aBPB) and (c) 132 (aBPBP); Cyclic structure of (d) 47 and (e) 133 (cP₄).

Fig. 37. (a) Structure of 134 (WCMT-1); (b) Fluorescence spectra of 134 in water with different proportion of dioxane; (c) Plot of fluorescence intensity at 650 nm versus Dioxane% (C₁₂₈ = 2.5 × 10 ^-5 mol/L; (d) Direct fluorescence titration of 134 in water with SLS (λ_ex = 440 nm). Reproduced with permission [104]. Copyright 2021, Elsevier.

Fig. 38. (a) Sketch map of 135-140; (b) Calculated HOMO and LUMO energy levels and band gaps of 135-140; (c) HOMO and LUMO of 135-140. Reproduced with permission [105]. Copyright 2021, American Chemical Society.

Fig. 39. DFT-minimized model (side-on and face-on views) of stereoisomer (a) (S,S)-47; (b) (R,S)-47. Reproduced with permission [72]. Copyright 2015, American Chemical Society; (c) (R,R)-47⊃PC₇₁BM complex. Reproduced with permission [106]. Copyright 2020, Wiley-VCH.

Fig. 40. Energy-minimized geometry for complexes of (a, b) 120⊃C₇₀; (c, d) 122⊃C70 (gray: C; blue: sulfur; yellow: nitrogen); Fluorescence titration of (e) 120 and (f) 122 with C₇₀. Reproduced with permission [94]. Copyright 2017, American Chemical Society.

Fig. 42. Crystal structure of macrocycle 7, inside the cavity is a p-xylene molecule (gray: C; green: Br; red: O).

Fig. 44. Structure of the supramolecular assemblies of 108. (a) Anthracene-C₆₀@108; (b) trithiasumanene⊃C₆₀@104; (c) (trithiasumanene)⊃C₇₀@108 (gray: C; blue: sulfur; yellow: nitrogen).

Fig. 45. (a) Structure of 141; (b) The frontier orbitals of 141α and 141β; (c) The PXRD and the photographs of 141α before and after adsorption different solvents; (d) The model and main view in the space-filling model of 141α and 141β; (e) The interconversion process between 141α and 141β and related thermal free energies. Reproduced with permission [52]. Copyright 2021, Elsevier.

Fig. 46. (a) Confocal fluorescence images of HeLa cells and HepG2 cells pretreated with or without SLS (30 µmol/L) and then incubated with 134 (10 µmol/L); (b) Cell viability of HeLa cells and HepG2 cells treated with different concentrations of 134 & SLS (1:3) supra-amphiphile. Reproduced with permission [104]. Copyright 2021, ELSEVIER.

Fig. 47. (a) Structure of 142; (b) Fluorescence spectra of 142 in acetonitrile/water mixed solvent; (c) Subcellular localization experiments in living HeLa and HepG2 cells. Reproduced with permission [111]. Copyright 2021, WILEY-VCH.

Fig. 48. (a) 3D reconstruction of the vasculature (0–900 µm); (b) 3PF cerebrovascular imaging at various vertical depths, scale bar = 100 µm; (c) SBR ratio analysis of blood vessels at 100 (left) and 800 µm (right), excitation: 1320 nm fs laser (1 MHz). Reproduce with permission [89]. Copyright 2021, Wiley-VCH.

Fig. 49. (a) Suggested molecular models of the assembly structures for 120 before and after adding PC₇₁BM; (b) Photoluminescence spectra. Reproduced with permission [94]. Copyright 2017, American Chemical Society.

Fig. 50. (a) Structure of 143; (b) Spectral responsivity curves of the corresponding photovoltaic devices estimated from their EQE spectra; (c) HOMO and LUMO of 143 and 45; J-V characteristics of the (d) 143: PC₇₁BM and (e) 45: PC₇₁BM blends; External quantum efficiency (EQE) spectra of the (f) 143: PC₇₁BM and (g) 45: PC₇₁BM blends. Reproduced with permission [71]. Copyright 2021, The Royal Society of Chemistry.

Fig. 51. (a) Structure of linear analogue 144; (b) Energy-level and frontier molecular orbitals of 127 and 144; (c-f) OLED device characteristics fabricated with macrocycle 127 (DEV₁) and corresponding linear oligomer 144 (DEV₂). Reproduced with permission [95]. Copyright 2020, American Chemical Society.

Fig. 52. (a) Structure of acyclic compound 145; (b) Frontier molecular orbitals of 79 and 145; (c) External electroluminescence efficiency of OLED devices based on 79 (device A) and 145 (device B). Reproduced with permission [81]. Copyright 2021, WILEY-VCH.

Fig. 53. DFT calculated lowest energy geometry for cis-50 (a) and trans-52 (b); (c) Schematic of the OFET device; (d) Transfer characteristics for cis-50 and trans-52. Reproduced with permission [75]. Copyright 2018, American Chemical Society.

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