Spectrum of azulene: Part II. The 7000-A and 3500-A absorption systems

Abstract The first two electronic transitions of azulene have been photographed in the vapor, and the first transition in the pure crystal as well (4°K). The pure crystal spectrum is very diffuse; this is attributed to the disordered structure of the crystal. For the first vapor transition, which is also somewhat diffuse, the positions of 76 bands are recorded; most of them can be analyzed in terms of seven upper state vibrational frequencies and one difference interval. The sharper and richer second transition is more complicated; nine upper state frequencies and four difference intervals form the basis of a tentative analysis. It is provisionally concluded that azulene retains C 2 v symmetry in the two excited states. There are substantial geometrical changes, especially in the 7000-A transition. Upper state vibrational frequencies are difficult to correlate with those of the ground state. Attention is drawn to a large decrease in a very low frequency (probably the “butterfly” vibration), an effect observed in other molecules as well. The vapor spectrum and the mixed-crystal spectrum of Sidman and McClure are compared. They are difficult to correlate when, in the dilute mixed crystal, the absorption of the guest molecule (azulene) approaches the onset of absorption by the host (naphthalene). A partial explanation is suggested. There are also anomalies in respect of the role of nontotally symmetric ( b 1 ) vibrations in the two spectra. These, while apparent in the mixed crystal transition at 3500 A, do not appear to be present in the vapor. On the other hand, it is concluded (from consideration of the Franck-Condon principle and rough numerical comparisons of the effectiveness of intensity stealing in other molecules) that the greater part of the intensity of the 3500-A system derives from intensity stealing by totally symmetric ( a 1 ) vibrations. The unusual emission and internal converion properties of azulene are considered to be explicable in terms of the geometrical changes accompanying excitation. Internal conversion, by tunnelling, is then offered as the reason for the diffuseness of the weak long-wavelength system. Diffuseness in other polyatomic spectra is also discussed.