Charge-transfer bands in crystal spectra of molecular complexes

Abstract

The polarized absorption spectra in the 240–1300 mμ region were measured by means of a microspectrophotometer on crystals of six molecular complexes which involve pyrene and perylene as the electron donor and tetracyanoethylene, p-fluoranil, and 1,3,5-trinitrobenzene as the electron acceptor. The charge-transfer band in the crystalline state is observed at a wave number approximately equal to that of the corresponding band of the 1:1 complex formed in solution. The first charge-transfer band is exclusively polarized in the direction of the crystal axis along which donor and acceptor molecules are alternately stacked on each other if it is well separated from any local-excitation band. The splitting between the first and second charge-transfer bands is 13.6 ∼ 14.5 kK for the perylene complexes and 7.0 ∼ 7.3 kK for the pyrene complexes, which roughly corresponds to the energy difference between the highest and next occupied SCF molecular orbitals of the donor. It is also confirmed that a vibrational structure does often appear on the charge-transfer band in the crystalline state, while it has never been observed on the charge-transfer band of the 1:1 complex formed in solution.