Ab Initio Calculations of Intramolecular Exciton Transfer with Reduced Modes in Donor-Bridge-Acceptor Species
We present a new, fully ab initio approach for computing intramolecular charge and energy transfer rates. Using a time-convolutionless master-equation approach, parameterized with couplings obtained from an accurate quantum-chemical approach, we benchmark the approach against experimental results and predictions from Marcus theory for triplet energy transfer for a series of donor-bridge-acceptor systems. An important component of our analysis is the use of a projection operator scheme that parses out specific internal nuclear motions that accompany the electronic transition. Using an iterative Lanczos method, we concentrate the coupling between the electronic and nuclear degrees of freedom into a small number of reduced harmonic modes. We find that using only a single reduced mode--termed the "primary mode" or "Lanczos modes", one obtains an accurate evaluation of the golden-rule rate constant and insight into the nuclear motions responsible for coupling the initial and final electronic states.
In particular, the irreducible representation of the primary mode reveals hidden details of the dynamics. For the cases considered here, the primary modes belong to totally symmetric irreducible representations of the donor and acceptor moieties. Upon investigating the molecular geometry changes following the transition, we propose that the electronic transition process can be broken into two steps, in the agreement of Born-Oppenheimer approximation: a fast excitation transfer occurs, facilitated by the "primary Lanczos mode" (PLM), followed by slow nuclear relaxation on the final electronic diabatic surface.
We apply the method to a larger, "star" molecule, that has been experimentally shown that its exciton transfer pathway can be radically modified by mode-specific infrared excitation of its vibrational mode. The primary mode and rate constants we obtain generally agree with the experiments.