Signatures of intramolecular vibrational and vibronic Q_{mathrm{x}}–Q_{mathrm{y}} coupling effects in absorption and CD spectra of chlorophyll dimers

Abstract

An electron-vibrational coupling model that includes the vibronic (non-adiabatic) coupling between the Q_{mathrm{y}} and Q_{mathrm{x}} transitions of chlorophyll (Chl), created by Reimers and coworkers (Scientific Rep. 3, 2761, 2013) is extended here to chlorophyll dimers with interchlorophyll excitonic coupling. The model is applied to a Chl a dimer of the water-soluble chlorophyll binding protein (WSCP). As for isolated chlorophyll, the vibronic coupling is found to have a strong influence on the high-frequency vibrational sideband in the absorption spectrum, giving rise to a band splitting. In contrast, in the CD spectrum the interplay of vibronic coupling and static disorder leads to a strong suppression of the vibrational sideband in excellent agreement with the experimental data. The conservative nature of the CD spectrum in the low-energy region is found to be caused by a delicate balance of the intermonomer excitonic coupling between the purely electronic Q_{mathrm{y}} transition and the Q_{mathrm{y}} transition involving intramolecular vibrational excitations on one hand and the coupling to higher-energy electronic transitions on the other hand.