PHOTOPHYSICS OF SOME SUBSTITUTED 3H-INDOLE PROBE MOLECULES AND THEIR CHARGED SPECIES

The spectroscopic and photophysical parameters of neutral and cationic species of the following molecules have been discus sed: 2-phenyl-3,3 -dimethyl-3H-indole (1), 2- [(p-amino)phenyl]-3,3-dimethyl-3H-indole ( 2), 2- [(p -dimethylamino)phenyl] 3,3-dimethyl-3H-indole (3), amino)ph enyl]-3,3-dimethyl-5-carboethoxy-3H-indole (4), 2- [(p-methylamino)phe nl]-3,3-dimethyl-5 -carboethoxy-3H-indole (5), 2 [(p-dimethylamino)phe nyl] -3,3-dimethyl- 5-carboethoxy-3H-indole (6). Solvatochromic shifts have been interpreted in terms of the nature of the substituent group s and the state of solute-solvent interactions and complexation. The t heoretical radiative decay rate constant (k(F)(t)) along with the band width of the absorption profile of the different species involved have been used to discuss the geometrical changes from one species to the other in the ground state. The mirror-image relationship between absor ption and fluorescence spectra has proven to be a good tool to discuss any geometrical changes occurring in the excited state. A radiationle ss torsional mechanism takes place in the excited state relaxation of the various species. The protonation of the ring nitrogen atom generat es a highly planar cationic species which retains its conformation in the relaxed excited state. The very effective quenching of the monocat ion fluorescence is interpreted by the formation of a non-emissive TIC T state.