Theoretical Study on the Nitrile Substitution Effect in a Schiff Base Derivative: Suppression of Excited-State Intramolecular Proton Transfer and Dominance of Torsional Decay

Schiff bases represent a fundamental class of organic compounds that exhibit widespread applications in fluorescence probes, biological imaging, and optoelectronic materials due to their efficient excited-state intramolecular proton-transfer (ESIPT) processes. However, the targeted modulation of their photophysical properties through specific chemical substitution remains a significant challenge. In particular, the role of nitrile functional groups in modifying ESIPT dynamics and enabling alternative nonradiative decay pathways has not been thoroughly elucidated. In this study, we investigate the excited-state dynamics of the Schiff base derivative 2-(2-hydroxyphenyl)-2-(methylamino)acetonitrile (HMAN), focusing on the effect of nitrile substitution using high-level electronic structure calculations (CASSCF//CASPT2) and on-the-fly surface-hopping nonadiabatic dynamics simulations. The results reveal that upon excitation to the S₁ state, the dominant decay mechanism involves torsion around the C═N bond (dihedral ≈ ± 90°), rather than the conventional ESIPT process, leading to ultrafast S₁ → S₀ decay within ∼600 fs through two twisted S₁/S₀-CN conical intersections. The ESIPT process is entirely suppressed in S₁ due to the significantly reduced proton-acceptor ability of the imine nitrogen, which is caused by electron density depletion induced by the strongly electron-withdrawing nitrile group. This electronic effect fundamentally disrupts the hydrogen-bond preorganization that is essential for proton transfer. These findings elucidate how targeted nitrile substitution fundamentally reshapes relaxation pathways in Schiff base systems by suppressing proton transfer and promoting torsional decay. This work provides a mechanistic framework for the rational design of Schiff base derivatives with tailored excited-state dynamics via strategic functionalization.