Triplet-triplet spectra of aromatic molecules in mixed crystals

Polarized triplet-triplet absorption spectra are reported for naphthalene, 1-chloronaphthalene, and phenanthrene in various host crystals. The transitions in all three cases are long-axis, confirming the results of photoselection experiments. The coupling of guest T—T transitions to host S—S transitions is discussed. Where the upper triplet state falls within the exciton band of a host singlet excited level, the guest in its upper triplet state can act as a shallow exciton trap, and in hosts where π-overlap with the guest is possible a broadening of the spectral transitions is then expected. This effect is tentatively identified in biphenyl and fluorene hosts, in contrast to hexamethylbenzene and durene.