Ground and excited states of linked and fused zinc porphyrin dimers: Symmetry adapted cluster (SAC)-configuration interaction (CI) study

The symmetry adapted cluster (SAC)/SAC-configuration interaction method was applied to calculate the ground and excited states of zinc porphyrin monomers (without and with phenyl groups, ZnP and ZnPPh, respectively) and meso-meso linked (Zn2PMM) and doubly fused (Zn2PDF) zinc porphyrin dimers. Various features of the absorption spectra are studied, clarified, and assigned theoretically. The calculated electronic spectrum of ZnPPh, in comparison with that of ZnP, showed that the phenyl groups affect the spectrum in both the peak positions and intensities. In the dimers, Zn2PMM and Zn2PDF, the interactions of the monomer's four-orbitals result in an eight-orbital model of the dimers, which plays an important role in the interpretation of the excited states observed in the spectra. In Zn2PMM, the interaction is smaller and each peak in the split Soret (BI and BII) bands consists of two peaks, in contrast to the prediction based on Kasha's exciton rule. In Zn2PDF, the interaction between the two monomer units is so strong that the small highest occupied molecular orbital-lowest unoccupied molecular orbital gap causes the red-shifted Q (1.16 eV) and BI (2.13 eV) bands. In addition, the excitations out of the eight orbitals appear in the low-energy region of the spectrum, which is very different from the case of the monomer. A comprehensive and pictorial analysis is given for the excited states of the dimers in comparison with those of the monomer. (C) 2002 American Institute of Physics.