High resolution excitation spectroscopy on triplet excitons in organic molecular crystals comparison of naphthalene and anthracene

Abstract Triplet exciton absorption spectra of naphthalene and anthracene have been measured indirectly applying phosphorescence and delayed fluorescence excitation spectroscopy in the temperature range between 2 K and 300 K. The temperature dependence of the exciton line shape and line width is very similar in both crystals indicating comparable exciton phonon coupling. The line positions however, are shifted in different directions between helium and room temperature, to the red for naphthalene and to the blue for anthracene. This is explained with the thermal expansion of the crystal affecting the solvent shift with opposite sign in these crystals. In the naphthalene spectra below 30 K a substructure in the upper 0.0 Davydov component is observed, which can not be explained so far.