Apparent phonon side band modes in pi-conjugated systems: polymers, oligomers and crystals

TL;DR

This paper presents a time-dependent model explaining the appearance of apparent phonon side bands in the emission spectra of pi-conjugated systems, attributing them to vibrational damping effects that reshape vibronic structures.

Contribution

The study introduces a damping-based vibrational reshaping model that accurately reproduces vibronic features in emission spectra of pi-conjugated polymers, oligomers, and crystals.

Findings

Damping of transition dipole correlations causes vibrational modes to appear as side bands.

The model quantitatively matches experimental vibronic spectra across various systems.

Raman spectra measurements can predict emission spectral features.

Abstract

The emission spectra of many pi-conjugated polymers and oligomers contain side-band replicas with apparent frequencies that do not match the Raman active mode frequencies. Using a time dependent model we show that in such many mode systems, the increased damping of the time dependent transition dipole moment correlation function results in an effective elimination of the vibrational modes from the emission spectrum; subsequently causing the appearance of a regularly spaced progression at a new apparent frequency. We use this damping dependent vibrational reshaping to quantitatively account for the vibronic structure in the emission spectra of pi-conjugated systems in the form of films, dilute solutions and single crystals. In particular, we show that by using the experimentally measured Raman spectrum we can account in detail for the apparent progression frequencies and their relative intensities in the emission spectrum.