Optical properties and long-lived charged photoexcitations in polydiacetylenes

Abstract

We have studied the vibrational and electronic structure and the elementary excitations of poly [1,6-di(N-carbazolyl)-2,4-hexadiyne] (polyDCHD), poly[1-(N-carbazolyl)penta-1,3-diyn-5-ol] (polyCPDO), and poly[1-(N-carbazolyl)penta-1,3-diyn-5-acetoxy] (polyCPDA) by electronic absorption, Raman scattering, and photoinduced absorption in the infrared region. The long-lived photoexcitations are charged bipolarons with associated infrared-active vibrational (IRAV) modes. The results are also discussed in relation to the degree of chain order of the different polydiacetylenes as evidenced by the electronic spectra. Only poly(DCHD) shows an absorption spectrum dominated by a single exciton, suggesting a one-dimensional ordered structure. Consequently, for this polymer only, triplet excitons with a lifetime of the order of ms are photogenerated in addition to bipolarons. Among polydiacetylenes, only poly(CPDA) shows photoinduced IRAV modes not affected by Fano coupling between electronic and vibrational states.