Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s

TL;DR

This study investigates the breaking of mirror symmetry in the optical absorption and emission spectra of oligo(para-phenylene)s, linking it to anharmonic librational modes through ab-initio calculations.

Contribution

It provides a detailed quantum chemical analysis explaining the spectral asymmetry in para-linked phenylene systems, supported by experimental agreement.

Findings

Spectral asymmetry correlates with anharmonic librational modes.

Good agreement between calculations and experimental spectra.

Temperature effects influence the degree of spectral asymmetry.

Abstract

The absorption and emission spectra of most luminescent, pi-conjugated, organic molecules are the mirror image of each other. In some cases, however, this symmetry is severely broken. In the present work, the asymmetry between the absorption and fluorescence spectra in molecular systems consisting of para-linked phenyl rings is studied. The vibronic structure of the emission and absorption bands is calculated from ab-initio quantum chemical methods and a subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is achieved. A clear relation can be established between the strongly anharmonic double-well potential for the phenylene ring librations around the long molecular axis and the observed deviation from the mirror image symmetry. Consequences for related compounds and temperature dependent optical measurements are also discussed.