Solvatofluorochromic Behavior of Naphthalene-Based π-Conjugated Compounds Governed by Intramolecular Charge Transfer

Solvatofluorochromic compounds are of great interest because their fluorescence properties sensitively respond to solvent polarity, enabling applications in molecular sensing and optoelectronic devices. However, understanding the structure-fluorescence property relationships governing their solvatofluorochromic behavior remains limited. In this study, we conducted a detailed analysis of the solvatofluorochromism of seven novel naphthalene-based π-conjugated compounds in six solvents of different polarities. The ultraviolet-visible absorption and fluorescence spectra of each compound were measured, and changes in the fluorescence wavelengths were compared. Absolute fluorescence quantum yields and lifetimes were also determined. Five methyl ester compounds exhibited solvatofluorochromic behavior, and the relationship between their Stokes shift and solvent polarity was successfully established. Furthermore, theoretical calculations were performed using density functional theory and time-dependent density functional theory. An analysis based on the Lippert-Mataga equation quantitatively evaluated the difference in dipole moments between the ground state (S₀) and excited state (S₁), indicating that an intramolecular charge transfer mechanism was involved in the fluorescence behavior.