Monomeric Fluorescence of H‐Aggregates in a Series of 2‐(Hydroxyphenyl)benzoxazoles Derivatives

The exploitation of molecular self-assembly, which leads to the formation of aggregates, represents one of the most crucial approaches in the fabrication of advanced functional materials. A fundamental aspect in this area is the development of novel strategies to construct supramolecular architectures with new (unusual) properties, in particular, strongly fluorescent H-aggregates. Using 2-(2-carboalkoxy-3,4,5-trichloro-6-hydroxyphenyl)benzoxazoles (HBOs) as building blocks, H-aggregates were successfully obtained in both aqueous binary mixtures and the crystalline state. These aggregates exhibit intense fluorescence despite the prohibition imposed by Kasha's rule. This violation seems to result from the deactivation mechanism of the excited excitonic state in HBO H-aggregates, which involves, along with internal conversion, a photochemical channel induced by excited-state intramolecular proton transfer (ESIPT) rather than a radiative one. The consequence of ESIPT is monomeric fluorescence from the keto forms of HBOs. The fluorescence quantum yields of HBOs in the crystalline state are significantly higher than those in solution, ranging from 0.81 to 0.91. This enhancement is due to intramolecular hydrogen bonds as well as the dense molecular packing of HBOs, which suppresses conformational transformations.