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.
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