Robust free-base and metalated corrole radicals with reduction-induced emission

Robust free-base and metalated corrole radicals with reduction-induced emission

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Pengfei Li^a, Chulin Qu^a, Fan Wu^a^,^b^,*, Hu Gao^a^,^c, Chengyan Zhao^a, Yue Zhao^a, Zhen Shen^a^,*

Chinese Chemical Letters | 2025, 36(2) : 110292

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Chinese Chemical Letters | 2025, 36(2): 110292

• Communication •

Robust free-base and metalated corrole radicals with reduction-induced emission

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Pengfei Li^a, Chulin Qu^a, Fan Wu^a^,^b^,*, Hu Gao^a^,^c, Chengyan Zhao^a, Yue Zhao^a, Zhen Shen^a^,*

Affiliations

^a State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China

^b School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China

^c College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China

Published: 2025-02-15 doi: 10.1016/j.cclet.2024.110292

Outline

Abstract

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Preparing free-base porphyrinoid radicals that can function as coordination ligands is a challenging task. Here we report the synthesis of a stable, free-base benzocorrole (BC) radical containing only two inner NH protons via a retro-Diels-Alder conversion. The radical character of BC was fully supported by crystallographic analysis, spectroscopic evidence, and theoretical calculations. This neutral radical ligand allowed easy insertion of Zn(Ⅱ), Ga(Ⅲ), and Pd(Ⅱ) ions to produce radical complexes. All these radicals exhibited luminescence-on responses under weak reducing atmosphere, corresponding to the conversion to their aromatic anions. The red fluorescence was observed for BC and its Zn(Ⅱ) and Ga(Ⅲ) complexes, and the near-infrared phosphorescence (> 900 nm) was detected for Pd(Ⅱ) complex at room temperature. Furthermore, Ga(Ⅲ) corrole exhibited a variation in fluorescence in response to axial coordination. Our findings provide a promising radical platform for coordination and developing novel functional materials with switchable spin and emission.

Key words

Radicals / Porphyrinoids / Macrocyclic ligands / Luminescence / Redox chemistry

Cite this Article

Pengfei Li, Chulin Qu, Fan Wu, Hu Gao, Chengyan Zhao, Yue Zhao, Zhen Shen. Robust free-base and metalated corrole radicals with reduction-induced emission[J]. Chinese Chemical Letters, 2025 , 36 (2) : 110292 - . DOI: 10.1016/j.cclet.2024.110292

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Organic radicals have attracted considerable interest in various application fields because of their distinct redox and magnetic properties offered by the unpaired electrons [1-3]. However, the active nature of radicals makes their synthesis and manipulation very challenging. Porphyrinoids are typically aromatic macrocycles with [4n + 2] π-conjugation systems (e.g., 18 π tetrapyrrolic porphyrin, corrole, and phthalocyanine) (Fig. 1a). The redox activity of porphyrinoids enables the generation of [4n + 1]/[4n + 3] π radical ions via one-electron redox reactions. The porphyrinoid radicals could be stabilized via spin delocalization on their large π-conjugated systems [4], but most of them only exist in their coordinated forms with the insertion of boron [5,6], phosphorus [7], and various metal ions [8-10]. The absence of free amines in coordinated porphyrins allows for easier radical stabilization. The direct redox reactions on free-base porphyrins usually involve complicated deprotonation-protonation processes and other side reactions [11-13], making the identification and isolation of desired radicals extremely difficult. Free-base porphyrinoid radicals are assumed to be flexible, sensitive to outer stimulus, and easy to be functionalized via coordination, but reports of such molecules are still rare [14-18].

Corroles are one-carbon-short porphyrins that typically have three inner NHs in their free-base forms. The electron-rich nature and squeezed inner cavity of corrole provide non-innocent coordination properties for sustaining radicals [19]. When coordinated with some metal ions such as Cu(Ⅱ), Ni(Ⅱ), and Pt(Ⅳ) [20-23], corroles could undergo ring-oxidation to function as dianion radical ligands. The unique coordination performances of corroles lead to a wide range of diversified applications including biomedical theranostics [24-26], catalysis [27-33], and spintronics [34-37]. Benzo-fusion is a highly effective method for raising the energy of the π HOMO of corrole and regulating the d-π interaction between the corrole ligand and the central ion. Paolesse and co-workers reported the first free-base tetrabenzocorrole via a cross-coupling method, which is a 3NH closed-shell molecule bearing eight peripheral -CO₂CH₃ groups. In addition, they have effectively synthesized a variety of benzocorrole complexes with the insertion of Cu, Co, Sn and Al ions [38,39]. We also successfully synthesized Cu(Ⅱ) and Ni(Ⅱ) benzocorrole complexes that were metalated using one-pot procedures. These complexes demonstrated intriguing capabilities in sustaining stable π-radicals and have been employed in the study of spin modulation [34,36], aromatic-antiaromatic conversion [20], and photothermal therapy [25]. Although ligand-noninnocence was commonly found in metallocorroles, the synthesis of metal-free corrole radicals remained difficult, and Bröring et al. reported the first and only such free monoradical that was stabilized by β-chlorination [15]. It is still desirable to develop stable free-base corrole radicals for controllable coordination to create a variety of radical complexes with unique features.

In this work, we report a free-base benzocorrole (BC) that is highly stable in its neutral radical state at ambient conditions (Fig. 1b). BC consists of elements C, N, and H only, and contains two inner NHs. As a radical ligand, BC demonstrates easy coordination with both divalent and trivalent metal ions (Zn(Ⅱ), Pd(Ⅱ), and Ga(Ⅲ)), and prefers to stabilize the complexes in their radical states. Red fluorescence or near-infrared phosphorescence could be activated on BC and its complexes via ligand reduction to aromatic anions under various treatments.

The benzo-fused BC was prepared using the retro-Diels-Alder approach [40]. The bicyclo[2.2.2]octadiene-fused 3H-corrole 1 was newly synthesized as a diastereomeric mixture via the direct acid-catalyzed condensation between 4, 7-dihydro-4, 7-ethano-2H-isoindole and 3, 5-di-tert-butylbenzaldehyde in a 3:1 (v: v) methanol-water mixture solvent, followed by the oxidative cyclization by p-chloranil in chloroform. Heating solid 1 at 250 ℃ in vacuo eliminated its ethylene bridges and afforded radical BC spontaneously (Fig. 2a). The high-resolution ESI mass spectrometry for BC provided the exact mass at m/z = 1061.6427 consistent with a 2H formula C₇₇H₈₁N₄ (m/z 1061.6456) (Fig. S2 in Supporting information). Compound 1 was ¹H NMR active, indicative of its closed-shell character. In contrast, BC was nearly ¹H NMR silent, and only broad signals of tert-butyl protons of BC could be found (Fig. S15 in Supporting information). The electron spin resonance (ESR) measurement of BC provided an intense and sharp peak at g = 2.0029, clearly indicating its free organic radical character (Fig. 2b). The toluene solution of BC stored at ambient conditions exhibited unchanged absorption during 30-day monitoring, indicative of its intriguing stability (Fig. S25 in Supporting information).

Single crystals of BC were obtained from the slow diffusion of methanol into its chloroform solution and analyzed by X-ray diffraction (Figs. 2c-e). The structure of BC contains only two NH moieties with trans arrangement. Compared to 3H-corroles [41-43], the benzocorrole plane of BC demonstrates higher planarity with a mean plane deviation of only 0.056 Å due to smaller steric repulsion within the inner cavity. The amino-type pyrroles of BC have wider C_a-N-C_a angles (111.74° and 112.45°) than the imino-type pyrroles (109.22° and 109.74°), but imino-type pyrroles have more significant long-and-short alternation in C_α-N bond lengths (Table S3 in Supporting information). The three meso-aryl groups are nearly perpendicular to the corrole plane. The directly linked C_α-C_α bond length is 1.395(6) Å, which is significantly shorter than those of the reported free-base corroles ranging between 1.41 Å and 1.44 Å [15,41-43]. There is no close intermolecular stacking observed owing to the bulky 3, 5-tert-butyl-phenyl groups.

Cyclic voltammetry and differential pulse voltammetry of BC reveal its reversible redox properties with oxidation peaks at E_ox1 = −0.12 V, E_ox2 = 0.71 V, and a reduction peak at E_red1 = −0.65 V (vs. Fc/Fc⁺) (Fig. 3a). There is no protonation-deprotonation observed during the redox processes. The gap between E_ox1 and E_red1 is only 0.53 V, which is typical for a free π-radical and significantly lower than the approximately 2 V gaps found in 18 π porphyrins and corroles [44,45].

The absorption spectrum of corrole 1 displays a sharp Soret band at 415 nm in CH₂Cl₂, with its emission spectrum showing a fluorescence band peaking at 654 nm (Fig. 3b). Comparatively, BC is fluorescence-silent in CH₂Cl₂, and exhibits an intense red-shifted absorption band at 454 nm and several weak broad bands in the range of 500–750 nm. BC was loaded into water-soluble nanoparticles (BC-NPs) by mixing with amphiphilic polymer DSPE-PEG, which exhibited near identical absorption in water as in CH₂Cl₂ and toluene. The solution color of BC in CH₂Cl₂ was light green and rapidly changed to pink upon HCl addition (Fig. 3c), and by adding TEA, the original color was reversibly recovered. Stepwise titration of BC by adding HCl incrementally revealed spectral changes with unchanged isobestic points, thus a one-step diprotonation process forming [H]²⁺ is purposed (Fig. S16 in Supporting information). The optimized structure of [H]²⁺ is highly saddle-distorted owing to the steric effect of the four inner pyrrole N-H protons. Such distortion is commonly observed in other tetrapyrrolic macrocycles that are fully protonated [46]. The absorbance maxima of [H]²⁺ shifts to 519 nm (Fig. 3d). The notable redshift is verified by TD-DFT calculations. The detailed compositions of the electronic transitions of [H]²⁺ are presented in Table S8 (Supporting information). Similar protonation-induced bathochromic shifts of absorption bands have also been observed in aromatic porphyrins and phthalocyanines [47].

The redox behaviors of BC associated with absorption changes were further investigated using thin-layer spectroelectrochemistry (Fig. 3e). BC was gradually oxidized to its cation BC⁺ and reduced to anion BC⁻ by shifting the applying potential positively and negatively, respectively. Isobestic points were observed on the spectral change, indicating no side reactions occurred during the redox processes. The band at 454 nm gradually decreases from BC to BC⁺, corresponding to the conjugation change from [4n + 1] to [4n] π. In contrast, aromatic characteristic Soret bands at 456 and 465 nm, as well as the Q band at 641 nm, intensify from BC to BC⁻, indicating the formation of a [4n + 2] π system after reduction.

After adding trace 1, 8-diazabicyclo[5.4.0]undec-7-ene (DBU) into the DMF solution of BC, the same spectral change as the reduction in spectroelectrochemistry was observed, indicating the formation of anion BC⁻ (Fig. 3f). The Soret band splitting of BC⁻ reflects the x and y polarizations (Table S7 in Supporting information) [48]. The chemical reduction also occurred by using THF, CH₃CN and DMSO as solvents or hydrazine, NaBH₄ and SnCl₂ as reductant (Figs. S18–S24 in Supporting information). The DMF solution of BC⁻ exhibits red fluorescence, with a maximum at 648 nm and a shoulder at 707 nm in its emission spectrum. DBU has been used as a reductant in a few reactions, including the reduction of a nickel corrole radical [21,49-52]. We assume that there is a direct electron transfer from DBU to the corrole radical. forming a DBU radical that is easily quenched (Fig. S13 in Supporting information). The Hückel aromaticity of BC⁻ was identified by magnetic circular dichroism (MCD) spectroscopy [53]. The coupled intense MCD Faraday B terms with a + / - sign at 645 and 628 nm in ascending energy are assigned to the Q bands of 18 π aromatic porphyrinoids, corresponding to an angular orbital momentum change of ±9. The weak MCD signals with a – / + sign at 466 and 455 nm are ascribed to the angular orbital momentum change of ±1.

Density functional theory (DFT) calculations (B3LYP) were performed on BC and its redox ions (tert-butyl groups omitted). Two common 18 π aromatic free-base porphyrinoids, 5, 10, 15-triphenylcorrole (TPC) and 5, 10, 15, 20-tetraphenylporphyrin (TPP) were also calculated for comparison. The bond lengths of DFT-optimized BC show good consistency with those obtained from the single-crystal structure (Fig. S28 in Supporting information). Free-base porphyrinoids exhibit tautomerism with core-hydrogen alteration [54,55], and the trans-NH state of BC is preferred with lower energy than the possible cis-NH tautomers (Fig. S44 in Supporting information). The spin density of BC exhibits very small distribution on peripheral-benzo- and meso-carbons, but large coefficients on the pyrrolic carbons (Fig. 4a). The molecular orbital (MO) energy level diagram indicates that BC exhibits better one-electron donating and accepting capabilities in comparison to TPC and TPP (Fig. 4a). Both the a₂-type lowest singly unoccupied MO (SUMO) and highest singly occupied MO (SOMO) of BC closely resemble the distribution of its spin density (Fig. S36 in Supporting information) [56]. Notably, the SUMO of BC (−3.28 eV) is more than 1 eV lower than the lowest (doubly) unoccupied MO (LUMO) of TPC (−2.12 eV) and TPP (−2.21 eV), indicating the strong electron-accepting ability of BC that facilitates electron pairing. Thus, the electron-transfer process could easily occur on BC upon adding the strong Lewis base DBU. The SOMO-SUMO gap of BC (1.08 eV), which is much smaller than the highest (doubly) occupied MO (HOMO)-LUMO gaps of TPC and TPP. Upon a one-electron reduction, the HOMO-LUMO gap of BC⁻ is determined to be 2.44 eV, a value that closely resembles that of TPC (2.47 eV) (Fig. 4b). The solvent effect on the stabilization of the anion of BC was further evaluated by employing SMD solvent models in the calculations. The HOMO and LUMO energy levels, as well as the total energy of BC in DMF, are significantly lower than those in toluene and gas phase, whereas their HOMO-LOMO gaps remain consistent (Fig. S43 in Supporting information). The time-dependent DFT (TD-DFT) calculations (B3LYP) nicely reproduce the absorption spectra of BC, BC⁺, and BC⁻, as well as the protonated [H₂BC]²⁺ (Figs. S49-S51, S56 in Supporting information). The calculated Q and Soret bands of BC⁻ completely originate from its frontier four orbitals with M_L = ±4 and ±5 (Table S7). The S₁ state of BC⁻ was optimized, predicting a fluorescence band at 647.5 nm corresponding to the S₁ → S₀ transition, which exactly matched the experimental finding at 648 nm (Fig. 4c). The electron and hole distributions on the S₁ state of BC⁻ were analyzed by Multiwfn, revealing no spatial separation of the π-excitation (Fig. 4d) [47,57]. The Sr index, which quantifies the degree of electron-hole overlap, exhibits a value of 0.94 within the range of 0 to 1, suggesting the S₁ state of BC⁻ is a locally-excited (LE) state.

DFT calculations were also performed on a variety of inner-3NH corroles with the same phenyl substituents at meso positions, including TPC, dibenzo-fused DBC-3H, tetrabenzo-fused TBC-3H, and octacarboxymethyl-substituted tetrabenzo-fused OCTBC-3H, to address the reason for the energy level and vertical ionization potential due to the π-extension (Table S5 in Supporting information). This indicates that it is the most unstable among the four and may tends to lose one electron and one proton under ambient conditions [13], resulting in the generation of a neutral radical.

Electrostatic potential (ESP) and Iso-chemical shielding surface (ICSS) calculations [58,59] were conducted on BC, TPC, and TPP, using their optimized structures (Fig. 4e). The ESP maps show that the central coordination atmosphere of BC is closer to that of TPP rather than TPC. BC has a high concentration of negative charges (blue) surrounding its two imine N atoms, indicating its Lewis base nature, which allows for further protonation or coordination. In contrast, TPC has significant positive charge (red) on the amine N—H, which is located opposite the negatively charged imine N atom, indicating that TPC is more acidic than TPP and BC [60]. The ICSS maps clearly show that both TPC and TPP are typically aromatic with the shielded region depicted in red and yellow. The nonaromatic character of BC is revealed by the green-cyan region with close-to-zero nucleus-independent chemical shifts (NICS) values inside the corrole ring [61]. The one-electron reduced anion BC changes to aromatic, exhibiting an inner shielding area similar to neutral TPC and TPP. The NICS(1) values of BC⁻ and TPP at the molecular center are −11.6, −11.3, and −13.3 ppm, respectively. The induced ring currents calculated by anisotropy of the induced current density are clockwise for BC⁺, and anticlockwise for BC⁻, but interrupted on the meso carbons of BC (Fig. S57 in Supporting information) [62].

The zinc, gallium, and palladium complexes of BC were synthesized (Fig. 5a), and all were identified as π-radicals by their sharp single-line ESR signals and spin density maps with near-zero density on central metals (Figs. S28 and S44 in Supporting information). The direct insertion of Zn(Ⅱ) into 3H-corrole was difficult, which easily led to ring degradation [21,63]. Treating BC with zinc acetate in pyridine under reflux afforded stable Zn(Ⅱ) complex Zn-BC in high yield. Ga(Ⅲ) corroles were typically fluorescent molecules with axial pyridine coordination and have shown good tumor imaging and therapeutic performance [64-66]. Intriguingly, treating BC with GaCl₃ resulted in the formation of non-emitting, chloride-ligated radical Ga-BC, as evidenced by the single-crystal X-ray diffraction. Such radical could only be obtained by treating common aromatic gallium corroles with the additional chemical oxidant N(4-BrC₆H₄)₃SbCl₆ [67]. The five-coordinate Ga(Ⅲ) center in Ga-BC has a distorted square pyramidal geometry. The Ga(Ⅲ) ion is 0.487 Å above the 4 N plane, and the Ga-Cl length is 2.2208(17)Å. By adding pyridine incrementally into the toluene solution of Ga-BC, the fluorescence appears and gradually enhances at 637 and 700 nm (Fig. 5b), corresponding to its ligand reduction to aromatic anion. However, the reduction was not observed for BC, Zn-BC, and Pd-BC in pure pyridine. The mass spectrum shows that the axial chloride of Ga-BC could be substituted by treating it with excess pyridine (Fig. S12 in Supporting information), which may facilitate ligand reduction. The palladium insertion of 3H-corrole could afford a closed-shell anionic complex accompanied by a counterion [68]. A neutral palladium radical complex Pd-BC was synthesized for the first time via the reflux of BC and Pd(OAc)₂ in pyridine. Unlike Ga-BC, the single-crystal structure of Pd-BC reveals no axial ligand and the Pd(Ⅱ) ion perfectly lies on the highly planar corrole plane. After adding DBU into the DMF solution of Pd-BC, the appearance of sharp B and Q bands indicates the formation of anion Pd-BC⁻ (Fig. 5c). Interestingly, Pd-BC⁻ displays near-infrared phosphorescence in the anoxic solution at room temperature owing to the promoted intersystem crossing (ISC) to triplet by palladium. The long-lived emission shows a peak at 930 nm with a microsecond-level lifetime (9.82 µs) and extends beyond 1100 nm, exhibiting a "mega" shift of 306 nm relative to the lowest energy Q band. The quantum yields and lifetimes of all reduced compounds are listed in Table S4 (Supporting information). The electrochemical measurements were performed on the complexes in CH₂Cl₂ (Fig. S31 in Supporting information). The difference in potential between E_ox1 and E_red1 is 0.49 V for Zn-BC, 0.59 V for Ga-BC, and 0.46 V for Pd-BC. These small gaps indicate the radical nature of the complexes.

To get a better understanding of the room-temperature phosphorescence caused by Pd coordination, the excited-state energy gaps, and spin-orbit coupling matrix elements (SOCME) between singlet state S₁ and triplet states T_n were further calculated for BC and Pd-BC⁻ (B3LYP) (Fig. 6) [69,70]. BC⁻ exhibits four triplet states, and Pd-BC exhibits three triplet states that are located below their respective S₁ state. The T₄ state of Pd-BC⁻ has nearly the same energy as its S₁ state. BC and Pd-BC⁻ have large S₁-T₁ gaps (ΔE_S1-T1) of 0.58 eV and 0.60 eV, respectively, while their T₃ and T₄ states have small S₁-T_n gaps (< 0.3 eV) that facilitate the ISC processes. All the S₁-T_n (n = 1–4) SOCMEs of BC are smaller than 0.65 cm⁻¹. For Pd-BC⁻, although the S₁-T₁ and S₁-T₄ SOCMEs are minor (0.17 and 0.13 cm⁻¹, respectively), the S₁-T₂ and S₁-T₃ SOCMEs exhibit significant values of 24.44 and13.83 cm⁻¹, respectively. The ISC process is supposed to efficiently take place in the S₁→T₃ channel of Pd-BC due to the narrow energy gap (ΔE_S1-T3 = 0.17 eV) and large SOCME. The higher excited triplet states T_n (n > 1) can undergo fast internal conversion (IC) to T₁, subsequently deactivate to the ground state S₀ and emit phosphorescence [71].

In summary, we have prepared a free-base benzocorrole radical BC and its various radical complexes, which were fully studied by crystallographic analysis, spectroscopic measurements, and theoretical calculations. The protonation at the inner imines of BC largely red-shifted its main absorption band. The fluorescence turn-on response of BC was discovered under the mild reducing atmosphere, corresponding to a reductive conversion to its aromatic anion. The Zn(Ⅱ), Pd(Ⅱ), and Ga(Ⅲ) complexes of BC are all stable radicals and readily reduced to their emissive anions in a manner similar to BC. Ga-BC was ligated by an axial chloride, which could be replaced by pyridine accompanied by enhanced fluorescence. The anion of Pd-BC exhibits NIR phosphorescence beyond 900 nm at room temperature. The reduction-driven luminescence of radicals offers their potential applications in responsive imaging. This radical ligand will be used to produce new complexes with tunable redox, magnetic and emissive properties, thus broadening the application of functional radical 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.

CRediT authorship contribution statement

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Pengfei Li: Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Chulin Qu: Writing – review & editing, Investigation, Formal analysis, Data curation. Fan Wu: Writing – review & editing, Writing – original draft, Methodology, Funding acquisition, Conceptualization. Hu Gao: Methodology, Investigation. Chengyan Zhao: Investigation, Formal analysis, Data curation. Yue Zhao: Visualization, Software, Resources, Methodology, Data curation. Zhen Shen: Writing – review & editing, Resources, Project administration, Funding acquisition, Conceptualization.

Acknowledgments

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This work was supported by the National Natural Science Foundation of China (Nos. 22271140, 22071103, and 22371118). The theoretical calculations are performed using the supercomputing resources at the High-Performance Computing Center of Nanjing University.

Supplementary materials

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110292.

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Year 2025 volume 36 Issue 2

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doi: 10.1016/j.cclet.2024.110292

Receive Date：2024-05-23
Online Date：2025-11-12
Published：2025-02-15

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Received：2024-05-23
Revised：2024-07-14
Accepted：2024-07-23

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^a State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China

^b School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China

^c College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, 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 (a) Structures of free-base aromatic porphyrin, corrole, and phthalocyanine. (b) Structure of the free-base benzocorrole radical and schematic of its coordination performance and reduction-induced luminescence.

Fig. 2 (a) Synthesis of BC: (i) CH₃OH, H₂O, HCl, r.t.; (Ⅱ) CHCl₃, p-chloranil, reflux; (Ⅲ) 250 ℃, in vacuo. (b) ESR spectrum of BC in CH₂Cl₂. (c) Top and (d) side view of the single-crystal structure of BC with thermal ellipsoids at 30% probability. (e) Packing view of BC with space-fill representation.

Fig. 3 (a) Electrochemical measurements of BC in CH₂Cl₂. (b) Absorption and fluorescence spectra of 1 in CH₂Cl₂ (λ_ex = 415 nm), and absorption spectra of BC in various solvents. (c) Images of BC solutions displaying the color change upon protonation and the fluorescence after reduction. (d) Absorption spectral change of BC upon adding HCl in the CH₂Cl₂ solution. (e) Absorption spectral changes of BC during oxidation and reduction recorded by spectroelectrochemistry. (f) Absorption, fluorescence (λ_ex = 458 nm), and MCD spectra of BC⁻ in DMF (10 µmol/L, 3 mL) upon adding DBU (0.5 µL).

Fig. 4 (a, b) Frontier molecular orbital diagrams of BC, TPC, TPP, and BC⁻, with the spin density map of BC. (c) TD-DFT simulated emission spectra of BC⁻. (d) The electron (blue) and hole (yellow) distribution of the S₁ excited state on BC⁻. (e) Molecular electrostatic potential and ICSS maps (1 Å above the XY plane) of BC, TPC, and TPP.

Fig. 5 (a) Synthesis and chemical structures of the radical complexes. (i) Zn-BC: Zn(OAc)₂·2H₂O, pyridine, reflux; (Ⅱ) Ga-BC: GaCl₃, pyridine, reflux; (Ⅲ) Pd-BC: Pd(OAc)₂, pyridine, reflux. Single-crystal structures of Ga-BC and Pd-BC are presented (thermal ellipsoids at 30% probability) with central coordination structures highlighted. (b) Fluorescence spectra of Ga-BC in toluene as a function of the volume proportion of the added pyridine (λ_ex = 467 nm). (c) Absorption and emission spectra, and time-resolved photoluminescence (PL) decay curve of Pd-BC⁻ in deaerated DMF upon adding DBU (λ_ex = 624 nm).

Fig. 6 Schematic diagram of the calculated energy gaps, spin-orbital coupling constants between S₁ and T_n (n = 1–4), and proposed energy transfer processes for fluorescence (Fluo.) and phosphorescence (Phos.).

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