PROTON-TRANSFER EQUILIBRIUM REACTIONS IN DONOR-ACCEPTOR 3H-INDOLE DERIVATIVES - ELECTRONIC SPECTROSCOPY AND PHOTOPHYSICS OF THE NEUTRAL MOLECULES AND THEIR CHARGED SPECIES IN WATER

Substituted derivatives of the main chromophore 2-phenyl-3,3-dimethyI- 3H-indole [I] have been studied over a wide range of acidity by UV abs orption and fluorescence spectroscopy. The spectroscopic and photophys ical parameters of the various species have been discussed. The molecu les are: 2-[(p-amino) phenyl]-3,3-dimethyl-3H-indole (II), 2-[(p-dimet hylamino)phenyl]-3,3-dimethyl-3 (III), 2-[(p-amino)phenyl]-3,3-dimethy l-5-cyano-3H-indole (IV), and methylamino)phenyl]-3,3-dimethyl-5-cyano -3H-indole (V). Spectral characteristics allow us to characterize thre e species namely the neutral molecule (N), the monocation (C) and the dication (DC) for molecules IV and V. Protonation of the ring nitrogen atom generates a highly planar ground-state cationic species which re tains its conformation in the relaxed excited state. Acidity constants have been determined experimentally. A radiationless torsional mechan ism is involved in the excited state relaxation of the various species . A low energy non-emissive twisted intramolecular charge transfer (TI CT) excited state taking its origin in the amino or anilino groups moi ety is suggested from INDO/S calculations to explain the strong fluore scence quenching of the neutral and monocation species in water. It is concluded that molecule V which is the most planar exhibits the stron gest TICT excited state formation.