Construction of Sn(IV) porphyrin/trinuclear ruthenium cluster dyads linked by pyridine carboxylates: photoinduced electron transfer in the Marcus inverted region.

Novel conglomerates consisting of saddle-distorted Sn(IV)(DPP) (H(2)DPP = dodecaphenylporphyrin) complexes and mu(3)-O-centered and carboxylato-bridged trinuclear Ru(III) clusters linked by pyridine carboxylates were synthesized and characterized. Sn(IV)-DPP complexes with Cl(-), OH(-), and 3- and 4-pyridine carboxylates ligands were characterized by spectroscopic methods and X-ray crystallography. Reactions of [Sn(DPP)(pyridinecarboxylato)(2)] with trinuclear Ru(III) clusters gave novel conglomerates in moderate yields. The conglomerates are stable in solution as demonstrated by (1)H NMR and electrospray ionization mass spectrometry (ESI-MS) measurements, which show consistent spectra with those expected from their structures, and also by electrochemical measurements, which exhibit reversible multistep redox processes. This stability stems from the saddle distortion of the DPP(2-) ligand to enhance the Lewis acidity of the Sn(IV) center that strengthens the axial coordination of the linker. The fast intramolecular photoinduced electron transfer from the Sn(IV)(DPP) unit to trinuclear Ru(III) clusters, affording the electron-transfer (ET) state {Sn(DPP(*+))-Ru(II)Ru(III)(2)}, was observed by femtosecond laser flash photolysis. The lifetimes of ET states of the conglomerates were determined to be in the range 98-446 ps, depending on the clusters and energies of the ET states. The reorganization energy of the electron transfer was determined to be 0.58+/-0.08 eV in light of the Marcus theory of electron transfer. The rate constants of both the photoinduced electron transfer and the back electron transfer in the conglomerates fall in the Marcus inverted region due to the small reorganization energy of electron transfer.