Time-dependent density functional calculations of the Q-like bands of phenylene-linked free-base and zinc porphyrin dimers

Time‐dependent density functional theory (TDDFT) calculations have been performed on the excitation energies and oscillator strengths of the Q‐like bands of three structural isomers of phenylene‐linked free‐base (FBP) and zinc (ZnP) porphyrin dimers. The TDDFT calculated results on the low‐lying excited states of the reference monomers, FBP and ZnP, are in excellent agreement with previously calculated and experimental results. It is found that the 1,3‐ and 1,4‐phenylene‐linked dimers have monomerlike Q bands that are slightly red‐shifted compared to the monomers and new Q′ bands comprised of the cross‐linked excitations from the FBP (ZnP) ring to the ZnP (FBP) ring at considerably lower energies than the monomer Q bands. For the 1,2‐phenylene‐linked dimer, the direct π–π interaction between porphyrin rings caused by the van der Waals repulsion between them provides strong mixing of the Q′ bands with the Q bands, which causes its minimum excitation energy to be red‐shifted by 0.05 eV compared to the other isomers. The oscillator strengths of the Q′ bands are also unexpectedly found to be as strong as those of the Q bands in the dimers. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 338–347, 2001