Electronic states and (π, π*) absorption and emission characteristics of strongly coupled porphyrin dimers: sandwich complexes of Hf^IV and Zr^IV

Time-resolved and steady state optical measurements are reported for the sandwhich complexes consisting of two tetraphenylporphyrin macrocycles held within 3 Å by either a Zr^IV or Hf^IV ion. Compared to monoporphyrins and weakly coupled bisporphyrins, the transition-metal sandwich complexes show new features in both the ground and excited state absorption spectra as well as a broad fluorescence red-shifted to ≈ 1 μm. A model is presented that describes the electronic states responsible for these optical features. The states arise from strong interactions between the porphyrin macrocycles and contain significant charge resonance character.