Photoinduced absorption in disubstituted polyacetylenes: Comparison of theory with experiments

TL;DR

This study theoretically analyzes the photo-induced absorption spectra of phenyl-disubstituted polyacetylenes, comparing calculations with experiments to identify excited states and differences from polyenes.

Contribution

It applies correlated-electron models and CI methods to interpret experimental PA spectra and distinguishes the excited state contributions in PDPA from polyenes.

Findings

Calculated spectra agree well with experimental data.

PA1 is identified as the mA_g state.

PA_g corresponds to the nB_u state.

Abstract

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