PROTON-TRANSFER EQUILIBRIUM REACTIONS IN SOME SUBSTITUTED 3H-INDOLE PROBE MOLECULES - ROLE PLAYED BY THE TICT STATE

Five substituted derivatives of the main chromophore, 2-phenyl-3,3-dim ethyl-3H-indole (1), have been studied over a wide range of acidity by UV absorption and steady state fluorescence spectroscopy. These are a s follows: 2-[(p-amino)phenyl]-3,3-dimethyl-3H-indole (2), 2-[(p-dimet hylamino)phenyl]-3,3-dimethyl-3H-indole (3), mino)-phenyl]-3,3-dimethy l-5-carboethoxy-3H-indole (4), amino)phenyl]-3,3-dimethyl-5-carboethox y-3H-indole (5), and amino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indol e (6). The dependence of absorption and fluorescence spectral characte ristics upon acidity has allowed us to characterize four species, name ly, the anion (A), the neutral molecule (N), the cation (C), and the d ication (DC), for molecules 2 and 3. Molecules 4, 5, and 6 give rise t o an additional anion due to the hydrolysis of the ester group. The re lative order of protonations to available sites is discussed. The ring -protonated species have shown the Ph(c) and indole rings to be coplan ar in both the ground and first excited singlet states. A low-energy n on-emissive twisted intramolecular charge transfer (TICT) state taking its origin in the dimethylamino moiety is suggested from INDO-S calcu lations to explain the strong fluorescence quenching of these ring-pro tonated species. Acidity constants for the various prototropic equilib ria have been determined experimentally both in the ground and in the excited singlet states. Forster cycle calculations have been used to s upplement the various equilibrium constants. Possible applications in organized assemblies are discussed.