Substituent Position Effect on Excited-State Intramolecular Proton Transfer and Twisting Reaction of Salicylideneaniline Derivatives

The excited-state intramolecular proton transfer (ESIPT) mechanism has important applications in fluorescent probes, molecular photoswitches, and luminescent materials. To reveal substituent position effects and fluorescence mechanism, we comprehensively investigated excited-state behavior and photophysical properties of three salicylideneaniline derivatives with ortho-, meta-, and para-substitutions by time-dependent density functional theory, focusing on ESIPT and five twisting reactions. Potential energy curves show that the proton-transfer barrier is lower for para-substitution, while twisting reactions are easier for ortho- and meta-substitutions. Importantly, the mutual influence between stilbene isomerization and ESIPT is first revealed. Both C═N and stilbene C═C twists are competitive with ESIPT for meta-substitution, whereas the C═C twisting channel can be negligible for para-substitution. Interestingly, the calculated emissions of enol and keto forms are not separated well. The experimental long-wavelength fluorescence could be ascribed to the trans keto form generated by phenolic twist after ESIPT. Para substitution can effectively promote intramolecular charge transfer, which is favorable for developing two-photon fluorescent probes. A delicate uncoupled electronic structure of meta-substituted compound is discerned by electron-hole analysis, which inspires us to reveal the reason for nonseparated enol and keto emissions. This research can provide guidance for choosing suitable substituent positions for specific applications.