Structure−Property Relationships for Photoconduction in Substituted Polyacetylenes

New photoconductive materials are explored from three groups of polyacetylenes: poly(phenylacetylenes) [HCC(C6H5-p-R)]n, poly(3-thienylacetylenes) [HCC(3-C4H2S-β-R‘)]n, and poly(1-alkynes) {HCC[(CH2)mR‘ ‘]}n, where R = CH3 (2), CO2(CH2)6OCO-Biph-OC7H15 (Biph = 4,4‘-biphenylyl; 3); R‘ = Si(CH3)3 (4), Br (5); and R‘ ‘ = CO2(CH2)6OCO-Biph-OC9H19 (m = 2; 6), 9-carbazolyl (m = 3; 7) and OCO-Biph-OC7H15 (m = 9; 8). Photoconduction in the polyacetylenes under illumination of visible light is investigated using photoinduced xerographic discharge technique. In the pure (undoped) state, all the polyacetylenes except 5 show higher photosensitivity than do poly(phenylacetylene) (R = H; 1), a well-studied photoconducting polyacetylene, and poly(9-vinylcarbazole), the best-known photoconducting vinyl polymer. Among the polyacetylenes, photoconduction performance of the polymers with electron-donating and/or hole-transporting moieties is superior to those with electron-accepting ones. The liquid crystalline polymer 6 exhibits very high photosensitivity, probably due to the formation of crystalline aggregates of its mesogenic pendants induced by the thermal treatment in the photoreceptor preparation process. C60 acts as a photoconductivity enhancer when doped to amorphous 3, but functions as a crystallinity-breaking plasticizer when doped to liquid crystalline 6, leading to a large decrease in photoconductivity. While 3 shows a low photosensitivity (2.8 × 10-3 lx-1 s-1) to a 573 nm light in the undoped state, doping with I2 and sensitization with crystal violet (CV) dramatically increase its photosensitivity (up to 41.2 × 10-3 lx-1 s-1). The CV-sensitized 4 exhibits high photoconductivity in the near-infrared spectral region, which may find technological applications in the digital photoimaging systems.