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Long‐Lived, Directional Photoinduced Charge Separation in Ru II Complexes Bearing Laminate Polypyridyl Ligands
Author(s) -
Majewski Marek B.,
de Tacconi Norma R.,
MacDonnell Frederick M.,
Wolf Michael O.
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.201203786
Subject(s) - phenazine , excited state , acceptor , ligand (biochemistry) , photochemistry , photoinduced charge separation , moiety , electron acceptor , ultrafast laser spectroscopy , ground state , materials science , crystallography , chemistry , spectroscopy , stereochemistry , atomic physics , catalysis , artificial photosynthesis , biochemistry , physics , receptor , photocatalysis , condensed matter physics , quantum mechanics
Ru II complexes incorporating both amide‐linked bithiophene donor ancillary ligands and laminate acceptor ligands; dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz), tetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐h:2′′′,3′′′‐j]phenazine (tpphz), and 9,11,20,22‐tetraazatetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐l:2′′′,3′′′]‐pentacene (tatpp) exhibit long‐lived charge separated (CS) states, which have been analyzed using time‐resolved transient absorption (TA), fluorescence, and electronic absorption spectroscopy in addition to ground state electrochemical and spectroelectrochemical measurements. These complexes possess two electronically relevant 3 MLCT states related to electron occupation of MOs localized predominantly on the proximal “bpy‐like” portion and central (or distal) “phenazine‐like” portion of the acceptor ligand as well as energetically similar 3 LC and 3 ILCT states. The unusually long excited state lifetimes ( τ up to 7 μs) observed in these complexes reflect an equilibration of the 3 MLCT prox or 3 MLCT dist states with additional triplet states, including a 3 LC state and a 3 ILCT state that formally localizes a hole on the bithiophene moiety and an electron on the laminate acceptor ligand. Coordination of a Zn II ion to the open coordination site of the laminate acceptor ligand is observed to significantly lower the energy of the 3 MLCT dist state by decreasing the magnitude of the excited state dipole and resulting in much shorter excited state lifetimes. The presence of the bithiophene donor group is reported to substantially extend the lifetime of these Zn adducts via formation of a 3 ILCT state that can equilibrate with the 3 MLCT dist state. In tpphz complexes, Zn II coordination can reorder the energy of the 3 MLCT prox and 3 MLCT dist states such that there is a distinct switch from one state to the other. The net result is a series of complexes that are capable of forming CS states with electron–hole spatial separation of up to 14 Å and possess exceptionally long lifetimes by equilibration with other triplet states.