DRASTIC CHANGES IN THE FLUORESCENCE PROPERTIES OF NBD PROBES WITH THE POLARITY OF THE MEDIUM - INVOLVEMENT OF A TICT STATE

The spectroscopic and photophysical properties of two 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) derivatives with different substituents on the nitrogen group are reported in 18 solvents. The solvatochromic shifts were analysed by correlating with polarity scales. The results, together with the help of modified neglect of diatomic overlap (MNDO) calculations, enable the polarity of the ground and first singlet excited states to be determined. Experiments based on variations in temperature and viscosity establish that the two probes undergo different de-excitation pathways. The possibilities of internal rotation leading to a twisted intramolecular charge transfer (TICT) state in the case of diethylamino-NBD are discussed. A study in binary solvent mixtures outlines specific solvent-solute interactions. Appropriate restrictions are emphasized on the utilization of NBD probes in biological fields.