Due to the pyridine moieties were sensitive to acid, the acid-induced fluorescence regulation behavior of compounds
1–
6 was further investigated. As shown in
Fig. 2, upon adding trifluoroacetic acid (TFA) into
1 in CH
2Cl
2, a noticeable decrease in the absorption at 335 nm accompanied by an increase in the absorption at 380 nm were observed. The existence of isoabsorptive point at 352 nm also indicated the formation of a new species. In addition, addition of TFA into
1 induced a significant fluorescence quenching at 406 nm, accompanied by the appearance of a new peak at 549 nm (
Fig. 2b), corresponding to the protonation form of
1 (
1 + H
+). The red-shift of absorption and emission wavelengths and decrease of fluorescence intensity might be due to the stronger electron-withdrawing ability of protonated pyridine than that of free pyridine, indicating intramolecular charge transfer (ICT) was mainly involved into the fluorescence mechanism [
53]. Interestingly, when 7.0 equiv. TFA was added to
1 in CH
2Cl
2, the solution showed an obvious white light emission (CIE: 0.31, 0.33) resulting from the complex of blue fluorescence of
1 and yellow fluorescence of
1 + H
+ (
Figs. 2c and d). It should be noted that the common white light emission materials need the combination of multiple chromophores in one system with accurate ratios [
54-
56], which was high-cost and unaccessible. Thus, the acid-induced white light emission of
1 afforded a simple way to construct white light emission materials by making use of the single fluorophore. Additionally,
1H NMR spectroscopy was performed to investigate the protonation process and the signals H
a-d corresponding to the protons of pyridine rings displayed an apparent down-field shift upon the addition of TFA, resulting from the decreased electron densities upon protonation (Fig. S15 in Supporting information). Furthermore, upon addition of trimethylamine (TEA) into the solution of
1 + H
+, the fluorescence spectrum could be almost completely recovered to the initial state, revealing the protonation process was reversible (Fig. S16 in Supporting information). Compound
4 showed a similar absorption and fluorescence response to TFA with
1 (Fig. S19 in Supporting information). However, the acid-induced absorption and fluorescence change of
3 was different to
1 or
4 (Fig. S18 in Supporting information). For example, the absorption spectrum of
3 was slightly blue-shifted and the fluorescence spectrum showed a 49 nm blue shift accompanied by an increase in fluorescence emission upon addition of TFA, which revealed that the PET process was inhibited upon protonation [
51]. Moreover, the absorption spectrum of
2, 5 and
6 was also red-shifted and the fluorescence was quenched without wavelength changes upon acidification due to the enhanced ICT character upon addition of TFA (Figs. S17, S20 and S21 in Supporting information). Therefore, the substituent effect had a great effect on the acid-induced fluorescence regulation of TPA derivatives and tunable multicolor fluorescence emission including white light emission was achieved through simple acidification.