Spectral Effects of Strong Exciton Coupling in the Lowest Electronic Transition of Perylene

Measurements have been made of the absolute intensity of the lowest-energy electronic state of perylene. The intensities of the Au and Bu factor group states have been found to deviate from an oriented gas model to such an extent that the model proves valueless in this case. The perturbation treatment of crystal-induced intensity stealing has been very briefly reviewed to examine the effect of the orientation of molecules on the relative importance of second-order terms. It has been shown that these terms can be responsible for most of the intensity when the active molecular axis is very nearly 90° to the crystal symmetry axis. Numerical values have been obtained for the influence of five higher-energy states on the intensity of the lowest state. The oriented gas polarization ratio is Ia/Ib=344: 1; the observed value is 1:1.76; the calculated value is 1: 2.05. The calculated and experimental values of the absolute a and b polarized intensity are in fair agreement. The observed splitting (5250 cm—1) between the origin of the b spectrum and the peak of the a spectrum has been considered as a strong coupling Davydov splitting which is calculated to be 5013 cm—1 using a dipole approximation. Vibrational frequencies of 350 and 1270 cm—1 have been obtained from absorption spectra data with perylene at 4.2°K. The possible role of vibronic coupling and charge resonance on the spectrum, and the nature of the diffuse fluorescence are also discussed.