Single-Benzene Fluorophore with Enhanced Emission: Insights into Photophysical Properties through Proton Transfer Mapping.

A comprehensive study of a single-benzene fluorophore, characterized by a low molecular weight and a significant Stokes' shift, is reported. The fluorophore exhibits crystallization-induced emission enhancement and pH-dependent luminescence, attributable to a conformational lock in the crystalline state and suppression of nonemissive forms upon deprotonation. Extensive photophysical analysis reveals a quantum yield increase from 1% in solution to 22% in the solid state, along with a consistent emission wavelength irrespective of the state. Theoretical calculations show that the Excited-State Intramolecular Proton Transfer (ESIPT) mechanism is facilitated by the unique orientation of acetyl substituents and a strong intramolecular hydrogen bond in the solid state and rationalized the large Stokes' shift observed. We explore the role of crystallization and intramolecular interactions in modifying the photophysical behavior of the fluorophore, emphasizing the absence of solvatochromism and the distinctive absence of strong intermolecular interactions in the solid state. Further theoretical modeling underscores the interplay between geometric conformation and proton transfer in determining emission properties, which is dramatically altered upon deprotonation. Therefore, detailed theoretical approaches to comprehensively map the complete photophysical landscape offer critical insights for the design and synthesis of small molecules that outperform larger fluorophores while retaining stimuli-responsive properties.