Geometry-Enabled π-Conjugation, Rather Than Substituent Electron-Donating or Withdrawing Ability, Governs Photoinduced Electron Transfer in 1,8-Naphthalimide Fluorophores

Photoinduced electron transfer (PeT) represents a fundamental mechanism in the design of fluorescent probes and molecular switches, typically modulated by tuning the electronic donating or withdrawing capacity of substituents. This widely adopted strategy implicitly assumes that substituent electronic effects directly translate into effective electronic coupling. Through density functional theory (DFT) and time-dependent DFT (TD-DFT) investigations, we demonstrate that this assumption is insufficient for 1,8-naphthalimide (NI) derivatives. Our results reveal that the occurrence of PeT is not governed by intrinsic electron-donating or -withdrawing ability, but rather by a simple geometric criterion: the coplanarity between the substituent and the NI core. Coplanarity activates π-conjugation between the substituent and the fluorophore, which redistributes π-electron density and attenuates the electron-accepting capacity of the NI unit, thereby suppressing PeT. In contrast, non-coplanar substituents block π-conjugation and facilitate PeT regardless of their intrinsic electronic properties. Notably, we clarify that the experimentally observed pH-dependent fluorescence enhancement originates from deprotonated anionic species rather than neutral forms. Overall, the research calls for a reassessment of conventional PeT-based molecular design strategies and provides a simple, predictive framework for the rational development of PeT-controlled luminescent materials and fluorescent probes.