Dipole moment of benzonitrile in its excited S1 state from thermochromic shifts of fluorescence spectra

The effect of temperature T ranging from 293 to 393 K on absorption and fluorescence spectra of benzonitrile (BN) in ethyl acetate is studied. The absorption spectra of BN remain unchanged with increasing T. The analysis of fluorescence band shift by the Bilot and Kawski theory [L. Bilot, A. Kawski, Z. Naturforsch. 17a (1962) 621], for the known dipole moment value in the ground state μg = 4.18 D and α/a3 = 0.5 (α is the polarizability and a is the Onsager interaction radius of the solute), yield the average value of excited state dipole moment μe = 4.42 D. This value is in satisfactory agreement with 4.45, 4.51 and 4.57 D obtained previously from the Stark shift of rotational lines.