Strong intramolecular charge transfer emission in benzobisoxazole cruciforms: solvatochromic dyes as polarity indicators.

A photophysical study was carried out on a series of nine X-shaped cross-conjugated compounds based on the benzobisoxazole nucleus. These "molecular cruciforms" were terminally substituted with electron-rich 4-(N,N-dimethylamino)phenyl, electron-neutral phenyl or electron-poor 4-pyridyl groups. Intramolecular charge transfer (ICT) transitions in molecular cruciforms were shown to be activated to a different extent by the combinations of these substituents located in different positions along vertical and/or horizontal axes of the molecules, showing strong emission throughout the visible spectrum. The high fluorescence efficiency and bathochromic shifts associated with the increasing polarity of the solvent make these cruciforms interesting candidates for use as fluorescence polarity indicators operating in a broad solvatochromic range.