Fluorescence and Raman spectra of pure and doped anthracene crystals at 4 K

Abstract

The fluorescence of pure crystalline anthracene at 4 K is distinguished from that of dopants. Two persistent impurities are identified as 2-methylanthracene and (tentatively) 2-hydroxyanthracene. Molecular vibrations active in host fluorescence are, with one exception, totally symmetric; assignments are based on both fluorescence polarization and newly measured Raman depolarization ratios for molten anthracene. Phonon bands in the host spectrum result from intrinsic perturbations.The effect of doping is described; three categories of dopant are distinguished. Some are inert, some change the shape of host emission lines and some give guest fluorescence. Guest emission is compared with the Raman spectrum of the guest; anthrone is shown to enter the lattice as 9-hydroxyanthracene.The structure of phonon bands in host and guest fluorescence is related to distortions of the lattice consequent on electronic excitation. Excitons trapped at guests are weakly delocalized over neighbouring host molecules whereas excitons responsible for host emission are partially localized, extending over a region which includes about a hundred molecules.