Premium
Photoinduced Charge Separation in Zinc–Porphyrin/Tungsten–Alkylidyne Dyads: Generation of Reactive Porphyrin and Metallo Radical States
Author(s) -
Moravec Davis B.,
Hopkins Michael D.
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201303118
Subject(s) - porphyrin , photochemistry , chemistry , intersystem crossing , singlet state , quenching (fluorescence) , intramolecular force , zinc , electron transfer , chromophore , excited state , fluorescence , stereochemistry , organic chemistry , physics , quantum mechanics , nuclear physics
Abstract The luminescent tungsten–alkylidyne metalloligand [WCl(≡C‐4,4′‐C 6 H 4 CC‐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 S 1 excited states of these dyads initiates intramolecular energy‐transfer (ZnPor→ 1 ) and electron‐transfer ( 1 →ZnPor) processes, which together efficiently quench the S 1 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 S 1 → 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 S 1 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 H 2 , 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.