Chromophoric disorder in conjugated polymers: the cursious case of P3HT

The origin of the broad absorption spectrum of conjugated polymers is discussed. Motivated by the open questions posed in the recent experimental literature, we investigate theoretically the chromophoric disorder in single molecule poly(3­hexyl)thiophene (P3HT) at the atomic level using quantum­classical simulations. We reproduce the absorption spectrum and confirm qualitatively the prediction of simplified models ­ the localization length of the first excited state decreases with increasing temperature. Counter to expectation, the same trend is observed for the the average energy of the chromophore: in spite of a shorter localization length, the spectrum of the hot chromophore is red­shifted with respect to its cold counterpart. We trace this peculiarity to the anharmonicity of the underlying torsional potential which allows preferential access to more planar inter­ring conformations at high temperature. The contributions of bending to the transitional energies, the origi n of inhomogeneous broadening and the possibility of classification of the chromophore as planar/twisted/bent atlow and high temperatures are discussed.