Communication between porphyrin rings in the butadiyne-bridged dimer Ni(OEP)(mu-C-4)Ni(OEP): A density functional study

The electronic structure and spectra of a family of nickel porphyrin complexes are analyzed using approximate density functional theory (DFT). The three complexes, Ni(OEP), Ni(OEP-C4SiMe3), and Ni(OEP)(mu-C-4)Ni(OEP) represent a logical increase in complexity starting from a simple metalloporphyrin. The spectra of the three complexes all show similar features: an intense Soret band above 20 000 cm(-1) with a weaker Q band at lower energy. Relative to the unsubstituted monomer, the Q and Soret bands of Ni(OEP-C4SiMe3) are shifted to lower energy and also broadened. In the dimer, the Soret band shows a further red shift and is also much broader, showing several distinct components. The intensity of the Q band is also enhanced in the dimer. The observed spectra of all three species are dominated by intense pi --> pi* transitions, and so emphasis throughout is placed on the ligand-based pi levels. The simple model porphyrin Ni(P) has two closely spaced pi levels in the occupied manifold and a vacant doubly degenerate pi* orbital. The butadiyne substituent selectively stabilizes one component of this degenerate unoccupied pi* orbital, and also destabilizes one of the occupied pi orbitals. The net result is a reduction of the HOMO-LUMO gap in Ni(P-C4H), consistent with the observed red shift of both Q and Soret bands. In the dimer Ni(P)Cu-C-4)Ni(P), symmetric and antisymmetric pairs of orbitals with significant amplitude on the bridging C-4 fragment are split by some 4000 cm(-1) when the porphyrin rings are coplanar. These splittings vanish if the porphyrin rings are rotated into a staggered conformation, which lies approximately 15 kcal mol(-1) higher than the coplanar ground state, indicating the presence of a considerable barrier to free rotation about the butadiyne axis. Both the broadening and red-shift of the Soret band in the dimer can be rationalized on the basis of the calculated one-electron transition energies, which in turn are determined by porphyrin-porphyrin coupling mediated by the butadiyne bridge.