Synthesis of selected electron donors and acceptors and study of inter/intramolecular electron transfer from ruthenium(II) complexes in solution, frozen matrix and silica aerogels as the basis for optical devices

The possible effect of a localized versus a delocalized 3MLCT excited state on photoinduced electron transfer (PET) was investigated by measuring the quenching rate constant, k_q, for the photoluminescence from Ru(bpy)₃]2+, [Ru(bpy) ₂phen]2+, [Ru(phen)₂bpy]2+ and [Ru(phen)₃]2+ by a large series of electrically neutral electron transfer quenchers. It was found that for any given quencher, the k_q values follow the sequence [Ru(phen) ₃]2+ > [Ru(phen)₂bpy]2+ > [Ru(bpy) ₂phen]2+ > [Ru(bpy)₃]2+. The data were treated according to the Marcus theory and the differences were traced to different reorganization energies.; Along these studies it was necessary to find a neutral quencher much easier to oxidize than anything available. Thus dodecahydro-3a,9a-diazaperylene (DHDAP) was designed and synthesized in one step from p-phenylenediamine and 1-bromo-3-chloropropane. Its first redox potential is 292 mV more negative than the redox potential of N,N,N′, N′-tetramethyl-p-phenylenediamine (TMPD). With DHDAP available, the 3a,9a-diazaperylenium dication (DAP) was synthesized for the first time, via oxidation of DHDAP with HgO. DAP is strongly photoluminescent (Φ = 0.75 ± 0.01 in CH₃CN) and ab initio calculations yield a ground state with a 6.4° tilt between the two quinolizinium building blocks. Other quenchers considered were the 4-benzoyl-N-methylpyridinium cation, and N-benzyl-N′-methyl viologen. The two systems have comparable redox properties, and undergo two successive, well-separated 1-e reductions. A linear free energy study for the former system was undertaken, and along the way it was discovered that the 1-e reduced form of the nitro group is an extremely strong electron donor ($sp-NO\dot{}-2$ = −0.97). The two quenchers were linked to [Ru(phen)₃] 2+ via ethynyl bridges and the resulting Ru complex/electron acceptor dyads were evaluated as dopands of silica aerogels for optical switching and chemical sensor applications.