Electronic States and Optical Properties of Porphyrins in van der Waals Contact: Th^IV Sandwich Complexes

Ground-state and time-resolved excited-state absorption spectra and fluorescence and phosphorescence spectra of three Th^IV sandwich complexes, Th^IV(TPP)₂, Th^IV(OEP)₂, and Th^IV(OEP)(TPP), are reported (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinate, TPP = 5,10,15,20-tetraphenylporphyrinate). These complexes, in which the nitrogen planes of the two porphyrin macrocycles are ~2.9 Å apart, exhibit a number of prominent optical characteristics: (i) monoporphyrin-like Q and B absorption bands, (ii) a new absorption between the Q and B bands, (iii) a weak, low-energy absorption that is substantially red-shifted relative to the Q bands of analogous monoporphyrin complexes, (iv) fluorescence and phosphorescence emission bands that are even further red-shifted relative to typical emission bands from porphyrin monomers, and (v) a moderately intense near-infrared ³(π, π^*) excited-state absorption not observed in monomeric porphyrins. These characteristic optical properties of the sandwich complexes are all accounted for by a relatively simple molecular orbital configuration-interaction model. Additionally, the spectral data and molecular orbital model identify the energies of charge-transfer configurations and delineate their contribution to the electronic states of these strongly-coupled π systems. These results provide insights into the interactions that can take place between other pairs of chromophores brought within van der Waals contact, such as the bacteriochlorophyll dimer of the photosynthetic reaction center.