Nature and location of excited charge-transfer states in porphyrin dimers: Development of preliminary design characteristics for biomimetic solar energy conversion systems

Abstract

The ground and excited electronic states of a magnesium porphyrin–porphyrin (MgP–P) dimer in a variety of geometries have been investigated by ab initio configuration interaction calculations. The lowest four excited singlet states were found to be locally excited monomer-like (π, π*) states in each geometry, except for dimers in which the macrocycles were constrained to lie within the van der Waals separation. The lowest MgP+–P− radical pair state CT1 was found much higher in energy than the lowest (π, π*) state; CT1←S0 transition energies varied approximately linearly with macrocyclic center-to-center distance, but were only weakly affected by relative macrocyclic orientation. Coordination of the Mg atom by chloride ion was shown to differentially stabilize CT1 relative to (π, π*) states, to the extent that CT1 is positioned near or below the lowest (π, π*) state. Moreover, estimates of the differential stabilization of CT1 by a polarizable medium suggest that combined point-charge and medium effects are sufficient to stabilize CT1 below the lowest (π, π*) state in a variety of geometries of the complex.