Exciton transitions in quasi-one-dimensional crystals. 9,10-dichloroanthracene

Abstract

Polarized reflection spectra of the first singlet transition of the α-crystalline form of 9,10-dichloroanthracene are reported. Crystal faces (001), (011) and (010) were examined in spectral range 450 to 350 nm at two temperatures, 5 K and 300 K. Two systems of transitions were observed. The first system is assigned to neutral excitons. Spectral similarities with unsubstituted anthracene and arguments based on the one-dimensional stacking of molecules are used to construct a model of the exciten band structures. The M-polarized ππ* molecular transition gives rise to a four branch band with two allowed transitions. The 0-0b (Ag → Au) transition lies 50-100 cm-1 above the bottom of the exciton band and the 0-0c′ (Ag → Bu) transition lies at the top of the band. In the reflection spectrum the Davydov splitting c′b for transverse excitons is 210 cm-1. The exciton band of the 00 molecular transition is not isolated but overlaps the two-particle manifold of the 0-1 vibronic transition. As a result of the 0-1c′ transition is unexpectedly strong in the spectra of the (010) face. The second system is polarized along the stack-axis a and starts 2500 cm-1 above the first system. It is tentatively assigned as |a(Ag → Bu) charge transfer exciton transition in agreement with earlier observations. © 1977.