Dual fluorescence of 2-(4′-N,N-dimethylaminophenyl)pyrido [3,4-d]imidazole in a TritonX-100/n-hexanol/water reverse micelle in cyclohexane:: Effect of pH and trifluoroacetic acid

The spectral characteristics of 2-(4'-N,N-dimethylaminophenyl)pyrido[3,4-d]imidazole (DMAPPI) have been studied in a TritonX-100 (TX-100)/n-hexanol/water reverse micelle in cyclohexane as a function of water (w(0)), surfactant, cosurfactant, pH, and trifluoroacetic acid. Under the neutral conditions, dual fluorescence (normal and twiste intramolecular charge transfer) is observed, even at w(0) = 0, suggesting that the TICT state is stabilized by the hydrogen bonding from n-hexanol. These studies indicate that DMAPPI molecules are present near the interface of the water pool and the micellar phase toward the micellar side, and the changes observed in the spectral characteristics with change of w(0) are due to the formation of the reverse micelles and the alignment of cosurfactant around DMAPPI. Variation of pH in the range 3-10 has no effect on the spectral characteristics of DMAPPI, suggesting that the protons do not penetrate the reverse micelles, whereas the trifluoroacetic acid protonates DMAPPI to form monocations (MCs). At w(0) = 0, MC2 and MC3 (see Scheme 1) are the MCs present both in the S-0 and S-1 states, whereas with an increase in w(0), the MC2 shifts toward MC1. Biprotonic phototautomerism is observed in MC1, which leads to the formation of MC2 in the S1 state. (C) 2000 Academic Press.