Photophysical properties and thermochromic shifts of electronic spectra of Nile Red in selected solvents. Excited states dipole moments

Photophysical parameters, the rate constants for radiative (k(F)) and nonradiative (k(IC)) for Nile Red (NR) in aprotic polar solvents incapable of hydrogen bonding with NR have been determined. No regular polarity dependence on k(IC) were observed in contrast to other authors. The effect of temperature on absorption and fluorescence spectra of NR in two selected solvents: ethyl acetate and 1,2-dichloroethane was studied and the electric dipole moment in the excited state S-1 was determined using the Bilot and Kawski theory [L Bilot, A. Kawski, Z. Naturforsch. 17a (1962) 621, L. Bilot, A. Kawski, Z. Naturforsch. 18a (1963) 10, 256]. For the previously obtained dipole moment in the ground state mu(g) = 8.2 D [A. Kawski, P. Bojarski, B. Kuklinski, Chem. Phys. Lett. 463 (2008) 410] the change from thermochromic measurements upon excitation of NR is equal to Delta mu = mu(e) - mu(g) = 1.75 +/- 0.5 D and is in good agreement with the value Delta mu = 1.8 +/- 1D from solvatochromic method. A comparison of the obtained Delta mu value with those of other authors is given and discussed. It has been concluded that the difference between the excited mu(e) and ground mu(g) state dipole moments for NR is too small to create a TICT state. (C) 2009 Elsevier B.V. All rights reserved.