Host and guest emission in pentacene-anthracene mixed crystals

Abstract

Below the concentration level for substitutional solution of pentacene in host anthracene crystal, pentacene absorbs and emits in transitions to and from the first singlet excited state with mirror symmetry in the absorption and fluorescence spectra. Emission from the host anthracene is almost unchanged from the pure crystal. At progressively higher concentrations, the guest and host spectra are changed in connected ways, both being displaced to the red to the extent of up to 40 cm^–1(guest) and ∼600 cm^–1(host), reflecting the progressive stabilisation of the energy levels of host and guest molecules in the surface and boundary aggregates of the crystal. Approximate calculations of the energy stabilisation by dispersion forces through the different polarizabilities in ground and excited states have been made taking as models anthracene, tetracene and pentacene. It is shown that the observed spectral shifts can be interpreted broadly in terms of changes in dispersion energy for different situations in the lattice as a consequence of different host-host and host-guest interactions when interblock solution takes the place of substitutional solution of the dopant in the host lattice.