Photoinduced absorption in disubstituted polyacetylenes: Comparison of theory with experiments

Priya Sony and Alok Shukla

Phys. Rev. B 71, 165204 – Published 11 April 2005

Abstract

In a recently performed experiment, [Korovyanko et al.Phys. Rev. B 67, 035114 (2003)] have measured the photoinduced absorption (PA) spectrum of phenyl-disubstituted polyacetylenes (PDPA’s) from the $1 B_{u}$ and $2 A_{g}$ excited states. In the $1 B_{u}$ PA spectrum they identified two main features, namely, PA1 and PA2, while in the $2 A_{g}$ spectrum they identified only one feature called $P A_{g}$ . In this paper we present a theoretical study of the $1 B_{u}$ and $2 A_{g}$ PA spectra of oligo-PDPA’s using the correlated-electron Pariser-Parr-Pople model and various configuration interaction methodologies. We compare the calculated spectra with the experiments, as well as with the calculated spectra of polyenes of the same conjugation lengths. The calculated spectra are in good agreement with the experiments. Based upon our calculations, we identify PA1 as the $m A_{g}$ state and $P A_{g}$ as the $n B_{u}$ state of the polymer. Regarding the PA2 feature, we present our speculations. Additionally, it is argued that the nature of the excited states contributing to the $2 A_{g}$ -PA spectra of oligo-PDPA’s is qualitatively different from those contributing to the spectra of polyenes.