DFT Calculations on Charge-Transfer States of a Carotenoid-Porphyrin-C₆₀ Molecular Triad

We present a first-principles study on the ground and excited electronic states of a carotenoid-porphyrin-C60 molecular triad. In addition, we illustrate a method for using DFT-based wave functions and densities to simulate complicated charge-transfer dynamics. Since fast and efficient calculations of charge-transfer excitations are required to understand these systems, we introduce a simple DFT-based method for calculating total energy differences between ground and excited states. To justify the procedure, we argue that some charge-transfer excitations are asympototically ground-state properties of the separated systems. Further justification is provided from numerical experiments on separated alkali atoms. The donor-chromophore-acceptor system studied here can absorb and store light energy for several hundreds of nanoseconds. Our density-functional calculations show that the triad can possess a dipole moment of 171 D in a charge-separated state. The charge-transfer energy technique is used to obtain the energies of the excited states. The charge separated excited states with a large dipole moment will create large polarization of the solvent. We use a model to estimate the stabilization of the excited-state energies in the presence of polarization. The calculated excited-state energies are further used to calculate the Einstein's A and B coefficients for this molecular system. We use these transition rates in a kinetic Monte-Carlo simulation to examine the electronic excitations and possible charging of the molecule. Our calculations show that the solvent polarization plays a crucial role in reordering the excited-state energies and thereby in the charge-separation process.