Photochemistry and Electron Transfer Kinetics in a Photocatalyst Model Assessed by Marcus Theory and Quantum Dynamics

The present computational study aims at unraveling the competitive photoinduced electron transfer (ET) kinetics in a supramolecular photocatalyst model. Detailed understanding of the fundamental processes is essential for the design of novel photocatalysts in the scope of solar energy conversion that allows unidirectional ET from a light-harvesting photosensitizer to the catalytically active site. Thus, the photophysics and the photochemistry of the bimetallic complex RuCo, [(bpy)(2)Ru-II(tpphz)-Co-III(bpy)(2)](5+), where excitation of the ruthenium(II) moiety leads to an ET to the cobalt(III), were investigated by quantum chemical and quantum dynamical methods. Time-dependent density functional theory (TDDFT) allowed us to determine the bright singlet excitations as well as to identify the triplet states involved in the photoexcited relaxation cascades associated with charge-separation (CS) and charge-recombination (CR) processes. Diabatic potential energy surfaces were constructed for selected pairs of donor-acceptor states leading to CS and CR along linear interpolated Cartesian coordinates to study the intramolecular ET via Marcus theory, a semiempirical expression neglecting an explicit description of the potential couplings and quantum dynamics (QD). Both Marcus theory and QD predict very similar rate constants of 1.55 x 10(12) - 2.24 x 10(13) s(-1) and 1.21 x 10(13)-7.59 x 10(13) s(-1) for CS processes, respectively. ET rates obtained by the semiempirical expression are underestimated by several orders of magnitude; thus, an explicit consideration of electronic coupling is essential to describe intramolecular ET processes in RuCo.