Revisiting the intersystem crossing mechanisms in chromophore dimers through the lens of excitonic coupling: a case study of naphthalimide.

Intersystem crossing (ISC) efficiency is primarily governed by the magnitude of spin-orbit coupling, which is typically enhanced by heavy-atom effects and molecular symmetry effects. Recent studies have suggested that charge transfer and/or excitonic coupling between neighboring chromophore units, either covalently linked or spatially associated, can also play a key role in this process. Here, we demonstrate a molecular design strategy to enhance triplet generation based on excitonic coupling in covalently linked homodimers. A series of naphthalimide dimers bridged by chalcogen linkers (O, S, Se) was synthesized to systematically modulate interchromophoric electronic communication, probed by spectroscopic and theoretical investigation. Our results show that the oxygen-linked dimer (Napht2[O]) exhibits minimal changes in absorption and emission spectra relative to the parent monomer. In contrast, Napht2[S] and Napht2[Se] dimers display pronounced excitonic signatures, accompanied by luminescence quenching and enhanced singlet oxygen generation, attributed to ISC enhancement. Time-dependent density functional theory calculations, combined with excitonic analysis based on transition density matrix, revealed how dimerization-induced modifications in excited-state character are crucial for maximizing ISC rates. Our findings demonstrate that increasing excitonic coupling through heavier chalcogen bridges can dramatically accelerate ISC in homodimers by inducing changes in excited-state character between singlet and triplet states, without invoking a full charge-separation step. Beyond these specific results, this work highlights excitonic coupling as a versatile handle for engineering ISC in molecular assemblies, opening new perspectives for the design of photoactive materials for optoelectronics, photocatalysis, and photodynamic applications.