Some effects of molecular orientation on fluorescence emission and energy transfer in crystalline aromatic hydrocarbons

An examination of their fluorescence spectra in the dissolved and crystalline forms reveals a close similarity both in position and structure for long, thin, aromatic molecules where Davydov splitting of the first absorption band in the crystal is small. The crystal fluorescence spectra of disk-shaped aromatic hydrocarbons on the other hand undergo a pronounced red-shift and loss of vibrational structure in comparison with the corresponding molecular fluorescence observed in solution; this is attributed to a much closer spacing and larger overlap of molecular planes in a different type of crystal lattice, which promotes very strong molecular interaction, and it is suggested that the spectrum of crystal emission is diagnostic of the type of molecular lattice adopted. The transfer of energy from solvent to solute in solid solutions of aromatic hydrocarbons appears to depend on the type of molecular lattice exhibited by both solute and solvent and may provide an additional criterion of crystal structure.