Substituent Effects on Excited-State Relaxation of N , N ′-Bis(salicylidene)- p -phenylenediamine: A Comparative Study of BSP and S-BSP

N, N ′-Bis(salicylidene)- p -phenylenediamine (BSP) is a prototypical Schiff-base ligand that has been widely used as a model system for investigating intramolecular proton transfer and electronic structures. In this work, the excited-state intramolecular proton transfer (ESIPT) mechanism and excited-state relaxation dynamics of BSP and its asymmetrically substituted derivative S-BSP are systematically investigated using electronic structural calculations combined with OM2/MRCI nonadiabatic molecular dynamics simulations. The results show that, upon photoexcitation to the bright S 1 state, both systems exhibit two competing nonradiative deactivation pathways. One channel involves an ultrafast ESIPT process occurring on a subpicosecond time scale, followed by relaxation to the ground state via an S 1 /S 0 conical intersection (CI). The other pathway proceeds without proton transfer and is dominated by molecular conformational twisting, leading to nonradiative decay through an alternative S 1 /S 0 CI. It is worth noting that the ESIPT pathway involves two proton-transfer directions (denoted as ESIPT-1 and ESIPT-2). For the fully symmetric BSP system, ESIPT-1 and ESIPT-2 display essentially equivalent dynamical behavior. In contrast, for S-BSP, ESIPT-1 initiated on the NO 2 -substituted side accounts for 28.2%, whereas only 3.9% proceed via the ESIPT-2 process. Overall, these results demonstrate that the substituent effects can significantly modulate the competing excited-state deactivation pathways. This study provides detailed mechanistic insights into multichannel excited-state relaxation in Schiff-base systems and clarifies the role of substituent-induced asymmetry in ESIPT photochemistry