Quantum chemical evidence for an intramolecular charge-transfer state in the carotenoid peridinin of peridinin-chlorophyll-protein

We present theoretical confirmation of an intramolecular charge-transfer (CT) state in peridinin in agreement with experimental results of Frank and co-workers (J. Phys. Chem. B 1999, 103, 8751 and J. Phys. Chem. B 2000, 104, 4569). Quantum chemical calculations using time-dependent density functional theory under the Tamm-Dancoff approximation were made on two structures: fully optimized peridinin and a molecule from the crystal structure of peridinin-chlorophyll-protein. The CT state appears as the third and second excited singlet state, respectively, for the two structures. A dipole-in-a-sphere model is used to estimate the solvation stabilization energies of each state in a variety of solvents. The energy of the CT state is shown to decrease dramatically in solvents of increasing polarity while the energy of the dark S-1 (Ag--like) state remains comparatively constant.