“…In this equation, φ and φ 0 are the emission quantum yield of 8 and the reference 9 , respectively, and τ 0 is the emission lifetime of 9 . With τ 0 = 867 ns (obtained by single-photon-counting), the value found for k ET is 2.6 × 10 7 s -1 , which is comparable to that obtained for pseudorotaxane-type Ru−tris(bipyridine)−BXV 4+ assemblies 8l. It is expected that association of BXV 4+ in 8 may extend the bridging chains due to electrostatic repulsion and as a result the photogenerated redox product can be stabilized against back electron transfer.…”
supporting
confidence: 72%
“…With regard to the design of model systems for the photosynthetic reaction center, many transition-metal complexes including rotaxanes and catenanes have been investigated during the past few years 8a-d In recent years we have developed a novel approach for organizing chromophore-electron acceptor diad assemblies (especially pseudorotaxane-type assemblies) by the application of donor-modified chromophores that form the supramolecular noncovalently linked diads with the electron acceptor via donor−acceptor π−π interaction. 8e-i Time-resolved studies have been carried out in a collaboration project with Willner. 8j-l In this paper, we report for the first time (1) the template-directed assembly of a novel [2]catenane ( 8 ) incorporating a ruthenium−tris(2,2‘-bipyridine) complex (sensitizer) and a cyclobis(paraquat- p -phenylene) (BXV 4+ , acceptor) (Scheme ) and (2) photoinduced intramolecular electron transfer in this catenane complex. 1 Sequence of Reactions Leading to [2]Catenane 7 and Complex 8 (L = 4,4‘-Dimethyl-2,2‘-bipyridine) …”
“…In this equation, φ and φ 0 are the emission quantum yield of 8 and the reference 9 , respectively, and τ 0 is the emission lifetime of 9 . With τ 0 = 867 ns (obtained by single-photon-counting), the value found for k ET is 2.6 × 10 7 s -1 , which is comparable to that obtained for pseudorotaxane-type Ru−tris(bipyridine) −BXV 4+ assemblies 8l. It is expected that association of BXV 4+ in 8 may extend the bridging chains due to electrostatic repulsion and as a result the photogenerated redox product can be stabilized against back electron transfer.…”
supporting
confidence: 72%
“…With regard to the design of model systems for the photosynthetic reaction center, many transition-metal complexes including rotaxanes and catenanes have been investigated during the past few years 8a-d In recent years we have developed a novel approach for organizing chromophore-electron acceptor diad assemblies (especially pseudorotaxane-type assemblies) by the application of donor-modified chromophores that form the supramolecular noncovalently linked diads with the electron acceptor via donor−acceptor π−π interaction. 8e-i Time-resolved studies have been carried out in a collaboration project with Willner. 8j-l In this paper, we report for the first time (1) the template-directed assembly of a novel [2]catenane ( 8 ) incorporating a ruthenium−tris(2,2‘-bipyridine) complex (sensitizer) and a cyclobis(paraquat- p -phenylene) (BXV 4+ , acceptor) (Scheme ) and (2) photoinduced intramolecular electron transfer in this catenane complex. 1 Sequence of Reactions Leading to [2]Catenane 7 and Complex 8 (L = 4,4‘-Dimethyl-2,2‘-bipyridine) …”
“…In these systems, structured alignment and rigidification of the molecular systems result in the stabilization of the redox products against back electron transfer. A further method to organize photosensitizer−acceptor diads involved intermolecular non-covalent linkage of the diad components by complementary H bonds or a molecular receptor unit. , Recently, we reported on a novel approach to organize chromophore−electron acceptor diad assemblies by the application of electron donor-modified chromophores that form the supramolecular non-covalently-linked diads with the electron acceptor via donor−acceptor interactions …”
Photoinduced electron transfer in photosystems consisting of
bis(6,6‘-dimethoxy-3,3‘-bipyridazine)(6,6‘-bis[8-((4-methoxyphenyl)oxy)-3,6-dioxaoctyl-1-oxy]-3,3‘-bipyridazine)ruthenium(II)
dichloride (1), tris(6,6‘-bis[8-((4-methoxyphenyl)oxy)-3,6-dioxaoctyl-1-oxy]-3,3‘-bipyridazine)ruthenium(II)
dichloride (2a), tris(6,6‘-bis[11-(4-methoxyphenyl)-3,6,9-trioxa-undecyl-1-oxy]-3,3‘-bipyridazine)ruthenium(II)
dichloride (2b), and tris(6-(8-hydroxy-3,6-dioxa-octane-1-oxy)-6‘-[8-((4-methoxyphenyl)oxy)-3,6-dioxaoctyl-1-oxy]-3,3‘-bipyridazine)-1,3,5-benzenetricarboxylate-ruthenium(II) dichloride (3), with
bis(N,N‘-p-xylylene-4,4‘-bipyridinium)
(BXV4+, 4) were
examined. The series of photosensitizers include alkoxyanisyl
donor components tethered to the photosensitizer
sites, capable of generating donor−acceptor supramolecular complexes
with BXV4+ (4). Detailed analyses of
the
steady-state and time-resolved electron transfer quenching reveal a
rapid intramolecular electron transfer quenching,
k
sq, within the supramolecular assemblies formed
between the photosensitizers and BXV4+ (4) and
a diffusional
quenching, k
dq, of the free photosensitizers by
BXV4+ (4). A comprehensive model that
describes the electron
transfer in the different photosystems and assumes the formation of
supramolecular assemblies of variable
stoichiometries, SA
n
, is formulated.
Analysis of the experimental results according to the formulated
model indicates
that supramolecular complexes between 1−3 and
BXV4+ of variable stoichiometries exist in the different
photosystems.
Maximal supramolecular stoichiometries between 1,
2a and 3, and BXV4+ (4),
corresponding to N = 2, 6, and 3,
respectively, contribute to the electron transfer quenching paths.
The derived association constants of BXV2+ to
a
single binding site in the photosensitizers 1,
2a, 2b, and 3 are 240, 100, 100, and
140 M-1, respectively. The
back
electron transfer of the photogenerated redox products was followed in
the different photosystems. Back electron
transfer proceeds via two routes that include the intramolecular
recombination, k
sr, within the supramolecular
diads
and diffusional recombination, k
dr, of free
redox photoproducts. Detailed analysis of the back electron
transfer in
the different photosystems revealed that the non-covalently linked
supramolecular assemblies, SA
n
, act as
static
diads where electron-transfer quenching and recombination occurs in
intact supramolecular structures despite the
dynamic nature of the systems. The lifetime of the redox
photoproducts Ru3+−BXV•3+
in the various systems is
relatively long as compared to diad assemblies (0.56−1.20 μs).
This originates from electrostatic repulsive
interactions
of the photoproducts within the supramolecular assemblies resulting in
stretched conformations of the diads and
spatial separation of the redox products.
“…In the case of the CO 2 /CH 4 system (reaction 2) only few studies all investigating ruthenium−polypyridines as sensitizers have been carried out. ,, A reason may be the kinetic barrier for the reaction, where the reduction of CO 2 to CH 4 requires the accumulation of eight electrons on the catalyst. Nevertheless, the use of supramolecular covalently linked sensitizer−acceptor systems should increase the efficiency of the system compared to the use of an external electron relay (acceptor). , …”
Mimicking the fundamental processes of the photosynthetic reaction center is a large field of research over
the past decade. We present as biomimetic model systems for electron transfer the mono- and heteroleptic
ruthenium complexes 2−9 containing differently substituted bipyridazine−glycol ligands. Depending on the
number of glycol chains of the complexes 2−9, they are divided into three classes: A (2 branches, 2−4); B
(3 branches, 5−6) and C (6 branches, 7−9). UV, fluorescence, single-photon counting, and laser flash
photolysis were employed as techniques for photophysical characterization. Redox properties were determined
using cyclic voltammetry. Detailed steady-state quenching studies of 2−9 with MV2+, OV2+, and MPVS
have shown different supramolecular interactions between sensitizer and acceptor in the case of MV2+. These
effects may be explained by π−π donor−acceptor attractions as well as hydrophobic interactions. Supporting
molecular modeling studies have been done. The suitability as biomimetic model systems was shown by
testing the ruthenium complexes 2−9 in artificial photosynthesis systems. Two typical reactions were
selected: the sacrificial reduction of water and the reduction of CO2 to CH4. The use of 2−9 leads to
satisfactory hydrogen production from the reduction of water. In the CH4/CO2 system, complexes 7−9 worked
efficiently.
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