RECENT ADVANCES IN NONLINEAR SPECTROSCOPY AND NONLINEAR OPTICAL-MATERIALS

High-symmetry, pi-electron, ladder oligomers and polymers, theoretically capable of supporting polarons and bipolarons, have been synthesized and examined by degenerate four wave mixing experiments employing picosecond and femtosecond pulses. The results of these experiments have been analyzed employing density matrix theory with explicit consideration of radiation field-matter interactions and of molecular relaxation processes consisting of (1) electron-hole recombination/exciton decay, (2) structural relaxation to gap states, and (3) decay of the populations of gap states. On the basis of correlation of observed effects with optical changes induced by chemical/electrochemical doping, we tentatively assign these gap states to bipolaron states and rationalize the ultrafast generation of these structurally-relaxed, bond-alternation defect states along the lines suggested by Su and Schrieffer (Proc. Natl. Acad. Sci. U.S.A. 1980, 77, 5626). An alternate model, which also describes the temporal response, is structural relaxation of free excitons into self-trapped excitons followed by relaxation of the self-trapped excitons to the ground state as suggested by Kobayashi (Synth. Met. 1992, 49-50, 565). The femtosecond and picosecond structural relaxation times observed for these and many other pi-electron materials are important both in a fundamental sense of understanding how ultrafast charge separation occurs and in terms of developing materials for nonlinear optical applications, e.g., development of materials for sensor protection and exploiting excited-state optical nonlinearities (Nature 1992, 359, 269).