Excited state properties and acid–base equilibria of trans-2-styrylbenzoxazoles

The photophysics of some styryl dyes namely; 2-(4′-R-styryl)benzoxazole, (R-SBO where R = NMe2, 2′,4′-OCH3, OCH3, CH3, H, Cl and CN) were investigated in several organic solvents by using steady-state absorption and emission as well as frequency modulation spectroscopic techniques. The absorption and fluorescence characteristics (such as fluorescence maximum, quantum yield and lifetime) are tuned by substitution, solvent and protonation. Depending on the substituent, the benzoxazole ring can be an electron donor (as in the case of CN-SBO) or an electron acceptor (as in the other derivatives). All of the solid derivatives exhibit intense emission with colours changing from blue to green and yellow, depending on the substituent. The protonation constants of the benzoxazole nitrogen atom, in both the ground and excited state, were also determined. The obtained values are strongly substitutent dependent, and indicate that the basicity of all derivatives (except for CN-SBO) increases upon excitation. In addition, the trans → cis photoisomerization quantum yields were determined in some solvents. The values depend on the nature of the substituent and the solvent characteristics like the polarity and viscosity. The effects of solvent polarity and viscosity as well as the substituent on both the fluorescence and isomerization quantum yields, suggested that the two-deactivation pathways are competitive. The role of the intramolecular charge transfer interaction in controlling the properties of the ground and excited state of the present styryl dyes is also discussed.