Electronic and Vibrational States of Azulene

Abstract

The absorption and emission spectra of azulene have been studied in a substitutional solid solution in napthalene at 20°K. The spectra are polarized and are very sharp, and from the polarization properties of the absorption transitions it has been determined that the first excited state at 14 652 cm^—1 is L_b, ¹B₁ [we will designate an electronic state by giving first the Platt symbol (J. R. Platt, J. Chem. Phys. 17, 484 (1949) and then the C_2v group symbol, Herzberg, Infrared and Raman Spectra of Polyatomic Molecules (D. Van Nostrand Company, Inc., New York, 1945)] (the transition with the ground state is transversally polarized), whereas the second excited state at 28 050 cm^—1 is L_a, ¹A₁ (the transition with the ground state is long‐axis polarized), in agreement with the theoretical predictions of Platt and Moffitt. Vibrational analyses have been performed for the two absorption transitions, and the second transition appears to be perturbed by vibrational‐electronic interaction. A sensitized emission spectrum has also been found, in which excitation of the napthalene leads to emission from the second excited state of azulene to the ground state. A vibrational analysis of the emission transition yields ground state vibrational frequencies which are in excellent agreement with the vibrations deduced from the infrared absorption spectrum. Environmental effects are discussed, and an attempt is made to correlate the vibrations in the different electronic states.