ELECTRONIC SPECTRA OF ORGANIC SOLID SOLUTIONS: EFFECTS OF LATTICE DISORDER ON SPECTRAL LINE WIDTHS AND MULTIPLICITIES

Line widths and multiplicities observed in solute electronic spectra of dilute organic solid solutions depend to a great extent on lattice disorder and solute site inhomogeneities. A definition of the structure of different types of organic solids is followed by a discussion on defects in these solids and of some of their photophysical consequences. The formation of organic solid solutions and the effect of various thermal treatments are considered in the light of the previous discussion. The influence of lattice phonon structure on spectral line widths is discussed for near-perfect and increasingly disordered solid solutions, in terms of the optical analogue of the Mössbauer effect. At low temperature effect is less important than static and dynamic solute site inhomogeneities in determining observed spectral line widths. The latter should be narrowest for substitutional solutions. The formation of such solutions depends on a number of parameters amongst which are the solute and solvent molecular geometries and volumes. The effect of these two parameters on solute spectral line widths has been studied. Multiplet structure in the electronic spectra of crystalline organic solutions is discussed. The origin of solute site plurality is considered in terms of the mechanism of crystal growth. I