Photoexcitations in polycarbazolyldiacetylenes

Abstract

We present the results of a photoinduced-absorption study carried out on an unsymmetrical polydiacetylene having a carbazolyl group directly bound to the acetylene moiety, i.e., poly-1(-N- carbazolyl)penta-1,3-diyn-5-acetoxy (polyCPDA). The absorption spectrum of microcrystalline samples in a KBr pellet shows a broad maximum at 2.2 eV. Photoexcitation of these samples at 2.54 eV gives rise to a luminescence emission from 1.4 to 2 eV with a maximum at 1.8 eV and two well-resolved photoinduced-absorption (PA) peaks at 0.45 and 1.1 eV. From the temperature, laser intensity, and chopper frequency dependence of the PA signals, we demonstrate that the dominant long-lived photocarriers are charged bipolarons formed by interchain charge separation. The lifetimes of the bipolarons in polyCPDA have been found of the order of seconds at 80 K. The intermolecular nature of the observed PA bands appears to be confirmed by the absence of any photoinduced signal in a solution of oligomers of the same polydiacetylene. © 1992 The American Physical Society.