Signatures of intramolecular vibrational and vibronic Q(x)-Q(y) coupling effects in absorption and CD spectra of chlorophyll dimers

An electron-vibrational coupling model that includes the vibronic (non-adiabatic) coupling between the Q(y) and Q(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 highfrequency 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(y) transition and the Q(y)transition involving intramolecular vibrational excitations on one hand and the coupling to higher-energy electronic transitions on the other hand.