Photoinduced intermolecular electron transfer interaction between coumarin dyes and aromatic amines has been investigated in sodium dodecyl sulphate micellar solutions using steady-state and time-resolved fluorescence quenching measurements. Steady-state fluorescence quenching of the coumarin dyes by the amine quenchers always shows a positive deviation from linear Stern-Volmer relationship, which arises due to the localized high quencher concentrations at the micellar Stern layer. In time-resolved fluorescence measurements, the analysis of the fluorescence decays following a micellar quenching kinetics model assuming a unified quenching constant (k q Ј) per quencher occupancy does not give satisfactory results, especially for the higher quencher concentrations used. The observed fluorescence decays are, however, seen to fit reasonably well following a bi-exponential analysis for all the quencher concentrations used. The average fluorescence lifetimes of the coumarin dyes in the micellar solution as estimated from the bi-exponential decay analysis are seen to undergo a systematic reduction with the effective mean quencher concentrations. The bimolecular quenching constants (k q ) thus estimated are seen to be much smaller than those reported in the homogeneous solutions ͑e.g., in acetonitrile͒, indicating that the electron transfer in the micellar media is inherently inefficient. Correlation of the observed k q values in the micellar solutions with the free-energy changes (⌬G 0 ) for electron transfer reactions show an inversion in the observed rates as predicted by Marcus' outer sphere electron transfer theory at exergonicities more that ϳ0.65 eV. To the best of our knowledge this is the first report on the Marcus inverted region observed for the electron transfer reactions in micellar solution.

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Abstract— The effect of solvent polarity on the electronic absorption and fluorescence properties of neutral red (NR), a phenazine‐based dye of biological importance has been investigated in several neat and mixed solvents. An unusual dual solvatochromic behavior has been observed that reveals the existence of two closely spaced electronic excited states in NR. In low‐polarity solvents the fluorescence of the NR is mainly emitted from the localized excited state, whereas in high‐polarity solvents the emission from the charge transfer state dominates. The dipole moments of the localized and charge transfer states of NR have been estimated from the solvatochromic shifts. The dipole moment of the localized excited state (4.8 D) is only slightly higher than that of the ground state (2.0 D), while that of the charge transfer state is drastically higher (17.5 D). Fluorescence quantum yields and the life‐times of NR have been determined in different solvents and correlated with the solvatochromic shifts.

Photophysical properties and the rotational relaxation dynamics of a phenazine based dye, neutral red, have been investigated in aqueous solutions in the presence of β-cyclodextrin (β-CD) using ground-state absorption and steady-state and time-resolved fluorescence measurements. It has been observed that while the neutral form (NR) of the dye forms an inclusion complex with β-CD, its protonated form (NRH + ) does not. In the presence of β-CD (10 mM), the pK a value of the dye is estimated to be about 6.06 ( 0.05, much lower than the pK a value of 6.81 ( 0.05 measured in aqueous solution. Both photophysical properties and the rotational relaxation dynamics of NR undergo substantial changes in the presence of β-CD. Steady-state and timeresolved fluorescence studies suggest simultaneous formation of 1:1 and 1:2 complexes of NR/β-CD in aqueous solutions. Time-resolved fluorescence anisotropy measurements confirm that only NR forms an inclusion complex with β-CD and the NR/β-CD complex rotates as a whole with a relaxation time much longer than that observed for free NR in aqueous solution.