REFLECTION AND PIEZOREFLECTION SPECTROSCOPY OF SQUARYLIUM DYE POL YMORPHS

An investigation of the electronic excitations of collective inter-actions in strongly coupled dye crystals has been carried out. This has relevance to the collective interactions observed when aggregation occurs in concentrated solutions of dyes. The study of aggregation in solution is hampered by a lack of structural knowledge of the aggre-gates. A model system has been found in the polymorphic forms of the squarylium dye, 2,4-bis-(4-diethylamino-2-hydroxyphenyl) cyclobute-nediylium-1,3-diolate (HEAPS). The packings of the dye polymorphs are similar to those proposed for the aggregates in solution and afford an opportunity to make the kind of structure-spectra correlations necessary to understand aggregation phenomena. A polarized single crystal specular reflection study was undertaken of the monoclinic and triclinic polymorphs of HEAPS with particular emphasis on determining the dipole-dipole coupling and the effect of internal fields on reflectivity. Application of a four oscillator molecular exciton-polariton theory was made and required an extension of the theory to include two singlets as well as a frequency dependent lattice damping. The first observation of a stopping band, characterized by high reflectivity over a broad range of energy, in both principal extinction directions, is made on the monoclinic (20(\u27)11) face. An intermolecular charge transfer band was also assigned in the triclinic polymorph spectra. In addition, a uniaxial piezoreflection study was carried out on the HEAPS polymorphs, to more clearly define the broad structure observed and test current piezomodulation theories. Complementary microscopic and phenomenological theories were applied to the piezoreflection spectra with good success. Polymorphism has been exploited to provide a structural pertur-bation enabling a detailed analysis of crystal spectra and interactions. The disparate optical responses of the polymorphs of the squarylium dye HEAPS graphically demonstrate the effects of solid state interactions on spectra