Some studies on mixed molecular compounds

Mixed donor π-π* molecular complexes (donor\(_1\) )\(_x\)(donor\(_2\) )\(_{i-x}\)/TCNQ with donors anthracene, chrysene, dibenzofuran, dibenzotniophen, përylene, phenanthrene, phenazine and pyrene, have been prepared, and analysed by spectroscopic and chromatographic methods. Formation of a continuous series of isostructural crystalline mixed complexes with 0<x<l requires similar parent complex solubilities, formation constants and crystal structure, and similar donor shapes and sizes. Where these requirements are not met, mixed complexes of limited composition range may be formed having either a completely new lattice structure or that of one of the parents. The crystal structure of dibenzothiophen/TCNQ has been determined. The donor acceptor relative orientation and separation are consistent with weak charge transfer interactions. Interstack donor-donor and acceptor- acceptor overlap is poor. Dibenzofuran/TCNQ has a similar structure, with possible donor disorder. Diffuse reflectance spectral data on relative intensities of charge transfer transitions in different complexes show the same trends as predictions from SCF-MO-CIcalculations. These trends have been qualitatively interpreted in terms of donor and acceptor orbital symmetries and relative orientations. Comparison with relative intensities of such transitions in mixed complexes has been used to deduce whether or not donor-acceptor orientations are similar in mixed and parent complexes. Regular variations in single crystal semi conductivity as a function of mixed complex composition are obscured by the influence of physical defects. Such defects also alter the magnitude of photoconduction, by acting as recombination centres (e.g. in anthracene/pyromellitic dianhydride) or sites for dissociation of charge transfer excited state to charge carriers (e.g. in dibenzothiophen/dibenzofuran/TCNQ). Photoconduction studies on mixed complexes have shown that the action spectrum of a mixed complex will only correspond to the appropriately weighted sum of the action spectra of the parent complexes if the density and distribution of defects, degree of interstack donor-donor and acceptor orbital overlap and relative orientations of donors and acceptor are similar in parent and mixed complexes.