The effect of the inclusion of guest molecules on the solid-state fluorescence of naphthooxazole-type fluorophores

Although naphthooxazole-type fluorophores did not exhibit fluorescence in solution, they exhibited strong fluorescence in the solid state. Interestingly, a large red-shift in colour and drastic decrease in fluorescence intensity were observed upon inclusion of guest molecules in the crystalline state. To elucidate the enclathrated guest effects on the solid-state photophysical properties, the X-ray crystal structures of the guest-free and several guest-inclusion crystals have been determined. Semi-empirical molecular orbital calculations (PM3 and INDO/S) confirmed that the reasons for the red-shift of the absorption and fluorescence wavelength maxima and drastic fluorescence quenching behaviour upon formation of host–guest inclusion compounds are attributable to strong intermolecular π–π interactions and to the formation of continuous molecular linking by intermolecular hydrogen bonds.