Twisting, Hydrogen Bonding, Exciplex Formation, and Charge Transfer. What Makes a Bright Fluorophore Not So Bright?

Excited-state charge transfer (CT) and the formation of twisted conformers govern the optical properties of a wide range of dyes. Aminonaphthalimide (ANI) derivatives, popular photosensitizers for blue light, are no exception. The focus herein is on an ANI derivative with an aliphatic amine, which ensures its solubility in a broad variety of solvents including water. Increasing solvent polarity quenches ANI fluorescence because of the expected formation of a dark twisted intramolecular charge-transfer (TICT) state. Contrary to the expectations, however, viscous polar alcohols, which inhibit conformational changes leading to the TICT state, only marginally recover ANI emission. Our analysis reveals that the "solubilizing" aliphatic amine forms an intramolecular exciplex, providing alternative nonradiative deactivation pathways without significant conformational changes and eliminating the viscosity dependence of ANI fluorescence. The findings also show a substantial dependence of ANI photophysics on hydrogen bonding with the solvating media, considerably enhancing the polarity effects. As a result, the fluorescence quantum yield of ANI (exceeding 0.4 in moderately polar solvents) drops below 0.001 when transferred to aqueous media. This feature allows one to showcase the utility of such dyes for imaging bacterial cells, complementing their growing popularity for CT, spintronics, materials, and biomedical applications.