Spectra and Structure of Mixed Organic Crystals

The multiplet structure in the electronic spectra of phenanthrene in biphenyl is interpreted in terms of there being inequivalent sites available for the guest in the host lattice. The interconversion of these inequivalent sites has been observed at temperatures above 10°K, and such interconversion has been shown to occur for ground states and triplet excited states only. The dispersion shifts for phenanthrene in biphenyl are +4 cm−1 (ground state), 14 cm−1 (lowest-excited singlet state), and −13 cm 1 (lowest triplet state). All of these results are based on interpretations of (i) spectroscopic splittings and (ii) variations of the line intensities with temperature in the range 1.8°–30°K. The results of calculations of one-dimensional potential-energy curves for the motion of guest molecules in host lattices suggest that it is not uncommon for a guest molecule to have a number of energetically similar lattice dispositions. The results of the calculations are supported by the appearance of multiplet structure in a number of other mixed-crystal systems, namely: pyrene in biphenyl, anthracene in biphenyl, phenanthrene in naphthalene. The calculations also suggest that the total repulsive energy for a guest gives an indication of its solubility in a given host. The calculations also suggest that one should not in general expect exact alignment of the principal axes of a guest and host molecule although this seems most likely to occur when the guest molecule is larger than the host molecule.