A quantum-chemical insight into the tunable fluorescence color and distinct photoisomerization mechanisms between a novel ESIPT fluorophore and its protonated form

Enol-keto proton tautomerization and cis-trans isomerization reactions of a novel excited-state intramolecular proton transfer (ESIPT) fluorophore of BTImP and its protonated form (BTImP⁺) were explored using density functional theory/time-dependent density functional theory (DFT/TD-DFT) computational methods with a B3LYP hybrid functional and the 6-31+G(d,p) basis set. In addition, the absorption and fluorescence spectra were calculated at the TD-B3LYP/6-31+G(d,p) level of theory. Our results reveal that both BTImP and BTImP⁺ can undergo an ultrafast ESIPT reaction, giving rise to the single fluorescence emission with different fluorescence colors, which are nicely consistent with the experimental findings. Calculations also show that following the ultrafast ESIPT, BTImP and BTImP⁺ can experience the distinctly different cis-trans isomerization processes. The intersystem crossing between the first excited singlet S₁ state and triplet T₁ state is found to play an important role in the photoisomerization process of BTImP⁺. In addition, the energy barrier of the trans-keto→cis-keto isomerization in the ground state of BTImP⁺ is calculated to be 10.49kcalmol^-1, which implies that there may exist a long-lived trans-keto species in the ground state for BTImP⁺.