Photoinduced charge separation in zinc-porphyrin/tungsten-alkylidyne dyads: generation of reactive porphyrin and metallo radical states.

The luminescent tungsten-alkylidyne metalloligand [WCl(≡C-4,4'-C6H4CC-py)(dppe)2] (1; dppe=1,2-bis(diphenylphosphino)ethane) and the zinc-tetraarylporphyrins ZnTPP and ZnTP(Cl)P (TPP=tetraphenylporphyrin, TP(Cl)P=tetra(p-chlorophenyl)porphyrin) self-assemble in fluorobenzene solution to form the dyads ZnTPP(1) and ZnTP(Cl)P(1), in which the metalloligand is axially coordinated to the porphyrin. Excitation of the porphyrin-centered S1 excited states of these dyads initiates intramolecular energy-transfer (ZnPor→1) and electron-transfer (1→ZnPor) processes, which together efficiently quench the S1 state (~90%). Transient-absorption spectroscopy and an associated kinetic analysis reveal that the net product of the energy-transfer process is the (3)[dπ*] state of coordinated 1, which is formed by S1→(1)[dπ*] singlet-singlet (Förster) energy transfer followed by (1)[dπ*]→(3)[dπ*] intersystem crossing. The data also demonstrate that coordinated 1 reductively quenches the porphyrin S1 state to produce the [ZnPor(-)][1(+)] charge-separated state. This is a rare example of the reductive quenching of zinc porphyrin chromophores. The presence in the [ZnPor(-)][1(+)] charge-separated states of powerfully reducing zinc-porphyrin radical anions, which are capable of sensitizing a wide range of reductive electrocatalysts, and the 1(+) ion, which can initiate the oxidation of H2, produces an integrated photochemical system with the thermodynamic capability of driving photoredox processes that result in the transfer of renewable reducing equivalents instead of the consumption of conventional sacrificial donors.