Dimers and model monomers of nickel(II) octaethylporphyrin substituted by conjugated groups comprising combinations of triple bonds with double bonds and arenes .1. Synthesis and electronic spectra

Porphyrin dimers linked in the meso-positions by bridges containing alkynes conjugated with the porphyrin pi-systems display highly perturbed electronic absorption spectra. We have prepared a series of such dimers using meso-ethynyl nickel(II) octaethylporphyrin as the primary building block. The bridging units include diethynylbenzenes, 2,5-diethynylthiophene, hexenediyne, octatetrayne, butenyne, hexenediyne, and octadienediyne. Some meso-to-beta dimers were also made. The dimers are complemented by a set of monomers containing the bridging units as mesosubstituents. The compounds were prepared by a combination of palladium-catalyzed couplings of alkynes with unsaturated halides and oxidative alkyne couplings. The separate effects of the substituents and the presence of the second porphyrin chromophore are delineated by comparisons of the visible absorption spectra for the various monomers and dimers. The dimers fit into three spectral classes: (i) those displaying strongly split Soret bands indicative of inter-porphyrin conjugation (diyne, tetrayne, enediyne, enyne, diethynylthiophene), (ii) those in which this conjugation is interrupted (diethynylbenzenes), and (iii) those with double bonds attached to the meso-carbons, which exhibit broad, low-energy bands, but small splittings of the Soret band. These illustrate some of the promising light-harvesting properties which can be achieved by tailoring the composition of the conjugating bridges linking the units in multi-porphyrin arrays.