Strong coupling of the single excitations in the Q-like bands of phenylene-linked free-base and zinc bacteriochlorin dimers: A time-dependent density functional theory study

Time-dependent density-functional theory (TDDFT) calculations were carried out to predict the Q-like bands of the two structural isomers of the phenylene-linked free-base (FBBC) and zinc (ZnBC) bacteriochlorin dimers. The calculated singlet excitation energies and oscillator strengths for the low-lying excited states of the reference monomers, FBBC and ZnBC, are quite consistent with recent ab initio calculations. The 1,3 and 1,4-phenylene-linked dimers have monomer-like Q bands and new Q′ bands comprised of the cross-linked (i.e., charge transfer) excitations from the FBBC (ZnBC) ring to the ZnBC (FBBC) ring, whose excitation energies are slightly and considerably red-shifted by about 0.03 eV and 0.5–0.7 eV compared to the monomers, respectively. The monomer-like Qx bands are of the mixed highest occupied molecular orbital (HOMO)→lowest unoccupied molecular orbital (LUMO) excitations in the Qx bands from the different bacteriochlorin rings, whose origin is the coupling of the Qx transition dipole moments through the π–π interaction between the unreduced pyrroles situated across the phenylene group. This mixing character is still maintained in the monomers separated by the same distance in the dimer, which is quite different from previous TDDFT calculations for the phenylene-linked free-base and zinc porphyrin dimers. The present dimers may show a more efficient Qx excitation energy transfer between the bacteriochlorins via each’s through-space overlapped LUMOs than the conventional porphyrin dimers.