The Synthesis and Photophysical Properties of New Polymetallic Complexes Designed for Use in Photoinitiated Electron Collection

Abstract

The goal of this research was to develop stereochemically defined multimetallic systems for use as light absorbers and electron donor / light absorber dyads in photoinitiated electron transfer and electron collection. The basis for the stereochemical control was provided by the symmetric bridging ligands 2,3,5,6-tetra(pyridyl)pyrazine (tpp) and 2,2'-bipyrimidine (bpm). The symmetric bidentate ligand 4N-perylene was designed and the majority of the synthesis was completed.

The bimetallic complexes [(tpy)M(tpp)Ru(LLL)]²⁺, where M = Ru or Os and LLL = Cl₃, (CH₃CN)₃, tpp, or (dpq)Cl, and the model monometallic complexes [(tpy)M(tpp)]²⁺, where M = Ru or Os, and [(tpy)Ru(CH₃CN)₃]²⁺ were synthesized and characterized using electrochemistry, UV-vis spectroscopy, and UV-vis spectroelectrochemistry. The bimetallic complexes were investigated as potential light-absorber / electron-donor complexes to be used in larger supramolecular devices for photoinitiated electron collection and electron transfer. The electrochemistry showed that the relative energy of the metal orbitals is suitable for the metal in the (tpy)M(tpp) coordination environment to act as an electron donor. These bimetallic complexes possess extremely complicated UV-vis spectroscopy due to the number of possible transitions. The assignment of the UV-vis spectroscopy and the electrochemistry of these complexes was greatly facilitated by the UV-vis spectroelectrochemistry. The metal-to-metal charge transfer spectra of the mixed-valence species of the bimetallic complexes were obtained using NIR spectroelectrochemistry and indicate a significant degree of metal-metal communication through the bridging tpp. The bimetallic complexes [(tpy)Ru(tpp)Ru(tpy)]⁴⁺, [(tpy)Ru(tpp)Ru(tpp)]⁴⁺ [(tpy)Os(tpp)Ru(tpp)]⁴⁺, and [(tpy)Ru(tpp)Ru(CH₃CN)₃]⁴⁺ were found to have emission lifetimes on the order of 100 ns.

The complexes [(bpy)₂Ru(bpm)]²⁺, [(bpy)₂Ru(bpm)Ru(bpy)₂]⁴⁺, [(bpm)₂IrCl₂]⁺, and {[(bpy)₂Ru(bpm)]₂IrCl₂}⁵⁺ were synthesized and characterized using electrochemistry, UV-vis spectroscopy, and UV-vis spectroelectrochemistry. The complex {[(bpy)₂Ru(bpm)]₂IrCl₂}⁵⁺ is a LA-EC-LA device for photoinitiated electron collection. The UV-vis spectroelectrochemistry of these complexes facilitated the assignment of the UV-vis spectroscopy as well as the electrochemistry. The UV-vis spectrum of the electrochemically generated two electron reduced form of {[(bpy)₂Ru(bpm)]₂IrCl₂}⁵⁺ was obtained. This spectrum is critical in the understanding of future studies of the photochemically generated two electron reduced species.

The symmetric, planar, bidentate bridging ligand 4N-perylene was designed. This ligand would eliminate some of the isomers associated with multimetallic complexes bridged by unsymmetric bidentate bridging ligands. The large π system of 4N-perylene would likely result in a low energy π* orbital compared to dpp, dpq, or bpm. The ligand 4N-perylene would hold bridged metals at a greater distance than 2,2'-bipyrimidine and should facilitate the formation of multimetallic complexes. The synthesis of 1,8-dichloro-2,7-naphthyridine has been completed. 1,8-dichloro-2,7-naphthyridine is a possible reactant in the homo-coupling reaction of a substituted 2,7-naphthyridine to form 4N-perylene.

The stereochemically defined molecular systems developed in this work show great promise for use in larger supramolecular complexes designed for photoinitiated electron transfer and electron collection.