Proton transfer equilibrium reactions in some substituted 3H-indole probe molecules: role played by the TICT state

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Abstract
Five substituted derivatives of the main chromophore, 2-phenyl-3,3-dimethyl-3H-indole (1), have been studied over a wide range of acidity by UV absorption and steady state fluorescence spectroscopy. These are as follows: 2-[(p-amino)phenyl]-3,3-dimethyl-3H-indole (2), 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-3H-indole (3), 2-[(p-amino)-phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (4), 2-[(p-methylamino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (5), and 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (6). The dependence of absorption and fluorescence spectral characteristics upon acidity has allowed us to characterize four species, namely, the anion (A), the neutral molecule (N), the cation (C), and the dication (DC), for molecules 2 and 3. Molecules 4, 5, and 6 give rise to an additional anion due to the hydrolysis of the ester group. The relative order of protonations to available sites is discussed. The ring-protonated species have shown the Phc and indole rings to be coplanar in both the ground and first excited singlet states. A low-energy non-emissive twisted intramolecular charge transfer (TICT) state taking its origin in the dimethylamino moiety is suggested from INDO-S calculations to explain the strong fluorescence quenching of these ring-protonated species. Acidity constants for the various prototropic equilibria have been determined experimentally both in the ground and in the excited singlet states. Förster cycle calculations have been used to supplement the various equilibrium constants. Possible applications in organized assemblies are discussed.

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