Photosensitized Isomerization of Norbornadiene to Quadricyclane with (Arylphosphine)copper(I) Halides

Article abstract of DOI:10.1021/ja00310a053

The (arylphosphine)copper(I) halides <(Ph3P)3CuX, (MePh2P)3CuX, (Ph2PCH2CH2PPh2)CuCl; X=Cl, Br, I> were studied as sensitizers in the photoisomerization side of a solar energy storage cycle based on the interconversion of norbornadiene (NBD) and quadricyclane (Q).The lowest electronic excited state of these compounds can be effective triplet energy sensitizers as evidenced by a maximum quantum yield of 1.0 with (MePh2P)3CuX.The efficiency of the (MePh2P)3CuX sensitizers is unprecedented compared to other reported inorganic complexes used as sensitizers for the NBD/Q system.The (MePh2P)3CuCl sensitizer has an intersystem crossing triplet yield of 1.0 with a triplet lifetime in benzene at room temperature of 3.3 μs.The triplet energy transfer rate constant with NBD is 8x10⁷ M^-1 s^-1.While the L3CuX (L=Ph3P, MePh2P) species are the most efficient sensitizers in solution, dissociation of the ligand from the metal in dilute solution detracts from their otherwise attractive features.The much lower quantum efficiency observed for the (Ph3P)3CuX compared to the MePh2P sensitizer is attributed to this extensive ligand dissociation.The coordinately unsaturated (arylphosphine)copper(I) halides produced by ligand dissociation appear to have extremely short lifetimes while the free ligand undergoes photodecomposition.Ground-state complexation of NBD with the coordinately unsaturated (arylphosphine)copper(I) halides is shown to occur in the presence of a large excess of NBD.Photoexcitation of NBD-(MePh2P)CuCl (C1) and NBD-(MePh2P)2CuCl (C2) complexes can produce Q, but with quantum efficiencies less than one.