EFFECTS OF SMALL CHEMICAL VARIATION, FUNCTIONALITY, AND SOLVENT ON THE SPECTROSCOPIC AND PHOTOPHYSICAL PROPERTIES OF THE 3H-INDOLE PROBE FLUOROPHORE

The spectroscopic and photophysical properties were obtained for three substituted derivatives of 2-[p-(dimethylamino)-phenyl]-3,3-dimethyl-3H-indole (4) in a series of five characteristic apolar, polar aprotic, and polar protic solvents. Probe 3 is equivalent to 4 but with an ester group in position 5 of the indole ring. Probe 2 is equivalent to 3 but with a methylamino group on the Ph(c) ring, and probe 1 is also equivalent to 3 but with an amino group on the Ph(c) ring. The fluorescence lifetime (tau(F)) was found to vary from 50 ps to 2 ns for these four probes in the various media studied here. In nonpolar media, tau(F) was found to vary with solvent viscosity (eta) according to the Forster-Hoffmann type equation (tau(F) = Keta2/3) for three out of the four probes studied. The K values are shown to be dependent, apart from the molecular volume of the librating moiety, to the extent of charge delocalization throughout the entire molecule. The excited-state dipole moments have been measured to be 14.2 (1), 14.9 (2), 15.9 (3), and 13.9 (4) and were practically the same as those calculated by INDO/S semiempirical method. Due to higher charge delocalization experienced by the excited state of molecule 3, the librational movement of the dimethylanilino moiety is prohibited, giving rise to an increase in the K values and to a marked decrease in the radiationless internal conversion process. Molecules 1 and 4 are suggested as excellent candidates for probing the microviscosity of hydrophobic environments. On the other hand, all these molecules can be used as polarity probes of microheterogeneous oil/water interfaces.