Pyridylporphyrin metallacycles with a slipped cofacial geometry: spectroscopic, X-ray, and photophysical characterization.

Treatment of the octahedral Ru(II) complex [trans,cis,cis-RuCl(2)(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 A and a lateral offset (center-to-center distance) of 9.82 A, 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).