From 2-phenylbenzoxazole to diphenyl-bibenzoxazole derivatives: Comparative advantages of mono- and bis-chromophores for solution and solid-state fluorescence
Résumé
The optical properties of three bis-benzoxazole derivatives, namely 2,2’-diphenyl-6,6’-bibenzoxazole and its derivatives bearing methyl and tert-butyl groups at the para position, were investigated and compared to those of the corresponding subunits. As expected from the increase of the π-electron conjugated system, the absorption and fluorescence spectra were red-shifted when passing from mono-to bis-chromophores. The three bis-benzoxazoles showed superior fluorescence quantum yields in solution, but they were only weakly emissive in the solid state, the unsubstituted derivative even being virtually not photoluminescent. The solid-state behaviour was explained by the fact that bis-chromophoric molecules were locked in a non-planar conformation, as discussed on the basis of DFT calculations and X-ray data.