Theory of Vibronic Spectra in Molecular Crystals with Four Molecules in the Unit Cell: Perylene

A theory of vibronic coupling in molecular crystals which – like perylene – contain four molecules in the unit cell is presented. The vibronic Hamiltonian, selection rules, and expressions for the integral properties of an absorption spectrum are derived and discussed. The vibronic equation contains parameters which as shown can be obtained from the widths and location of the gravity centres of the vibronic absorption bands. The absorption spectra computed from the vibronic equation derived agree well in both frequency and intensity distribution with the observed crystal absorption bands of different polarization. All main features of the absorption bands in the perylene crystal result from vibronic coupling only and inclusion of charge transfer states is not necessary.