Theoretical study on the effects of N-confused and substituted groups on ESIPT mechanism and photophysical properties of NCTPP derivatives

Recently, the solvation preferences of N-confused tetraphenylporphyrin (NCTPP) within dichloromethane mixed solvent systems have been investigated. Findings indicate that the significance of interactions between solvent molecules is comparable to that between the solute and solvent molecules. However, there are limited reports on the photoinduced tautomerism mechanism of NCTPP. In this paper, the effects of N-confusion and substituents on the excited state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of NCTPP derivatives are studied by density functional theory and time-dependent density functional theory. Initially, the molecular geometric structure is fully optimized. Electronic structure analysis indicates that the molecule has the local excitation characteristic of ππ* transition. Subsequently, the observed large Stokes shift in emission spectra is attributed to electron perturbations in the π-conjugated ring, induced by N-confusion. Aromaticity analysis confirms that this shift is due to the enhanced localization of electron rearrangement. Finally, potential energy curves are constructed, and the ESIPT reaction rate is computed, along with excited-state non-adiabatic dynamics simulations, demonstrating the dynamic equilibrium of ESIPT. Theoretical studies reveal that N-confusion outside the ring, coupled with benzene substitution, induces electron rearrangement, resulting in enhanced electron localization and a pronounced Stokes shift. This alteration in photophysical properties opens new avenues for applications in photochemistry and materials science.