PROBING OF PI CONJUGATION IN TRANSPOLYACETYLENE USING NEAR-INFRARED PHOTOLUMINESCENCE SPECTROSCOPY

Near-infrared photoluminescence from undoped trans-polyacetylene was attributed to emission from "cis defects," structures composed of isolated cis-(-CH = CH-) bonds embedded within conjugated trans-(CH)x segments. Spectra were found to be independent of residual transition-metal impurities in the polymer, but quantum efficiencies decreased with increasing concentrations of neutral defects and free carriers. The percolation limit for free carriers was achieved at about 30% overall trans-(CH)x. The photoluminescence energy varied inversely with the length of the conjugated cis defects, with a different dependence than for polyenes in solution. Interchain interactions were found to be significant in the solid state, distorting bonding geometries and perturbing the electronic structure of the chains; we postulate they are essential for bulk conduction. Huckel calculations yielded beta-0 = 1.08 eV for the pi-interaction energy, predicting an energy of 0.39-0.43 eV for the pi-pi* transition of a typical bound neutral soliton. The band gap for trans-polyacetylene of infinite conjugation length was estimated to be 1.20 eV by extrapolation of the solid-state luminescence data. We conclude that thermally isomerized trans-polyacetylene is best described as being a three-dimensional ensemble of short, conjugated segments that are bounded and interconnected by defects and crosslinks. Mean segment lengths are probably less than 13 (-CH = CH-) units.