Pyridylporphyrin Metallacycles with a Slipped Cofacial Geometry: Spectroscopic, X-ray, and Photophysical Characterization

Treatment of the octahedral Ru(II) complex [trans,cis,cis-RuCl2(DMSO-O)2(CO)2] with an equimolar amount of 5,10-bis(3‘-pyridyl)-15,20-diphenylporphyrin (3‘-cis-DPyP) yielded, upon selective replacement of the DMSO ligands, the
neutral 2 + 2 metallacycle 2. NMR spectroscopy provided unambiguous evidence that only one highly symmetrical
species, in which the two chromophores are held in a slipped cofacial arrangement by the external Ru(II) metal
fragments, exists in solution. The unprecedented geometry of 2, and of the fully zincated analogue 2a, were confirmed
in the solid state by X-ray structural investigations. The spatial arrangement of the two parallel chromophores in
2, with an interplanar distance of 4.18 Å and a lateral offset (center-to-center distance) of 9.82 Å, is reminiscent
of those of the special pair of bacteriophylls in the reaction centers and of adjacent B850 units in the LH2 light-harvesting antenna systems of photosynthetic bacteria. For comparison, the X-ray structure of the corresponding
metallacycle with 4‘-cis-DPyP, 1a, is also reported. In 1a, the two porphyrins have an almost perfect coplanar
arrangement. The semi-zincated metallacycles 1b and 2b, in which only one of the two chromophores bears an
inner zinc atom, were prepared from 1 and 2, respectively, and isolated in pure form. Detailed photophysical
investigations of the above porphyrin assemblies were performed. In particular, very fast photoinduced intercomponent
energy transfer processes from the zinc porphyrin to the free-base unit were detected in the semi-metalated derivatives
1b and 2b (time constants:  14 and 12 ps, respectively).