DUAL SOLVATOCHROMISM OF NEUTRAL RED

The effect of solvent polarity on the electronic absorption and fluore scence properties of neutral red (NR), a phenazine-based dye of biolog ical importance has been investigated in several neat and mixed solven ts. An unusual dual solvatochromic behavior has been observed that rev eals the existence of two closely spaced electronic excited states in NR. In low-polarity solvents the fluorescence of the NR is mainly emit ted from the localized excited state, whereas in high-polarity solvent s the emission from the charge transfer state dominates. The dipole mo ments of the localized and charge transfer states of NR have been esti mated from the solvatochromic shifts. The dipole moment of the localiz ed excited state (4.8 D) is only slightly higher than that of the grou nd state (2.0 D), while that of the charge transfer state is drastical ly 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.