Restriction of photoisomerization from the perspective of deviation from head-on conical deactivation: Insights from DFT-based computations with a demonstrative example

Understanding the intricate molecular photophysics of newly synthesized Schiff base probes with structural variability and molecular flexibility often requires a comprehensive correlation between their inherent photophysical properties and spectral behavior, from both experimental and theoretical standpoints.In this study, we present DFT-based quantum chemical calculations, including potential energy curves and surfaces, to investigate the feasibility of cistrans isomerization in the ground and excited states using the conical deactivation model.For the purpose we employed the ESIPT-prone Schiff base, 2-((E)-(naphthalene-2-ylimino)-methyl)phenol (NMP), as a representative probe featuring a flexible double bond.Despite its apparent susceptibility to cistrans isomerization, our detailed photophysical analysis, both experimental and theoretical, reveals that NMP's behavior is predominantly governed by prototropic processes.A subtle deviation from the ideal head-on conical deactivation, combined with a thorough analysis of potential energy curves and surfaces for the prospective cistrans isomerization process in the S 0 and S 1 states, reveals and justifies the non-occurrence of photoisomerization.This study highlights the effectiveness of our straightforward DFT/TDDFT based calculations, demonstrating that these methods are often sufficient to capture the trajectory of conical deactivations, without the need for more complex multiconfigurational approaches like CASPT2/CASSCF.The results underscore the critical influence of deviations from a head-on conical deactivation on the occurrence or suppression of cis-trans isomerization, a principle that extends to a wide variety of molecular systems with flexible double bonds.