Absorption and Emission Spectra of Anthracene Dimers

Abstract

The preparation of two types of anthracene dimer is described and their absorption and fluorescence spectra are reported. The basic spectral features of each dimer, together with a consideration of the method of formation, enable us to assign a geometry to each pair. One dimer, prepared by photolytic dissociation of dianthracene, has a symmetrical sandwich configuration. The geometry of the other dimer, prepared in solution at low temperatures, is deduced from an analysis of the vibronic structure of its first absorption band (¹A_1g→¹B_2u transition of a single molecule) using the theory of vibronic coupling in dimers. The two molecules have a configuration related to that of the two molecules in the unit cell of the crystal; one in which the short in‐plane axes make an angle of 55 deg with each other while the long axes are parallel. The resonance splittings of the ¹B_2u and ¹B_3u states in each dimer are deduced from the spectra and the results are discussed in relation to the theories of the energy levels of dimers. The fluorescence of the sandwich dimer is of the excimer type while the fluorescence from the other dimer is excited dimer emission with a vibrational intensity distribution different from that in the corresponding monomer spectrum.