Photoinduced electron transfer in supramolecular assemblies composed of alkoxyanisyl-tethered ruthenium(II)-tris(bipyridazine) complexes and a bipyridinium cyclophane electron acceptor

Article abstract of DOI:10.1021/ja952070p

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.

Full text of DOI:10.1021/ja952070p

J. Am. Chem. Soc. 1996, 118, 655-665
655
Photoinduced Electron Transfer in Supramolecular Assemblies
Composed of Alkoxyanisyl-Tethered
Ruthenium(II)-Tris(bipyridazine) Complexes and a
Bipyridinium Cyclophane Electron Acceptor
1
2
1
,2
Marlene Kropf, Ernesto Joselevich, Heinz D u1 rr, and Itamar Willner*
Contribution from the UniVersit a¨ t des Saarlandes, Organische Chemie, Fachbereich 11.2,
6
6041 Saarbr u¨ cken, Germany, and The Institute of Chemistry and the Farkas Center for Light
Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel
X
ReceiVed June 26, 1995
Abstract: 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-
,6-dioxa-octane-1-oxy)-6′-[8-((4-methoxyphenyl)oxy)-3,6-dioxaoctyl-1-oxy]-3,3′-bipyridazine)-1,3,5-
3
4
+
benzenetricarboxylate-ruthenium(II) dichloride (3), with bis(N,N′-p-xylylene-4,4′-bipyridinium) (BXV , 4) were
examined. The series of photosensitizers include alkoxyanisyl donor components tethered to the photosensitizer
4
+
sites, capable of generating donor-acceptor supramolecular complexes with BXV (4). Detailed analyses of the
steady-state and time-resolved electron transfer quenching reveal a rapid intramolecular electron transfer quenching,
4
+
ksq, within the supramolecular assemblies formed between the photosensitizers and BXV (4) and a diffusional
4
+
quenching, kdq, of the free photosensitizers by BXV (4). A comprehensive model that describes the electron
transfer in the different photosystems and assumes the formation of supramolecular assemblies of variable
stoichiometries, SAn, is formulated. Analysis of the experimental results according to the formulated model indicates
4+
that supramolecular complexes between 1-3 and BXV of variable stoichiometries exist in the different photosystems.
4
+
Maximal supramolecular stoichiometries between 1, 2a and 3, and BXV (4), corresponding to N ) 2, 6, and 3,
2
+
respectively, contribute to the electron transfer quenching paths. The derived association constants of BXV to a
-
1
single binding site in the photosensitizers 1, 2a, 2b, and 3 are 240, 100, 100, and 140 M , 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, ksr, within the supramolecular diads
and diffusional recombination, kdr, of free redox photoproducts. Detailed analysis of the back electron transfer in
the different photosystems revealed that the non-covalently linked supramolecular assemblies, SAn, act as static
diads where electron-transfer quenching and recombination occurs in intact supramolecular structures despite the
3
+
•3+
dynamic nature of the systems. The lifetime of the redox photoproducts Ru -BXV 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.
Introduction
redox products in molecular triads was achieved by coupling
8
the molecular assemblies to heterogeneous matrices, i.e.
Substantial efforts are directed toward mimicking the vectorial
electron-transfer and charge separation in the photosynthetic
reaction center. Electron transfer in covalently linked donor-
acceptor diads, triads, and pentads was extensively studied,
and effective stabilization of the electron transfer products was
accomplished by the vectorial spatial separation of the redox
products in the molecular arrays. Further stabilization of the
8
a
8b
zeolites or layered phosphates. In these systems, structured
alignment and rigidification of the molecular systems result in
3
4
5,6
7
(5) (a) Gust, D.; Moore, T. A. Top. Curr. Chem. 1991, 159, 103. (b)
Gust, D.; Moore, T. A. Science 1989, 244, 35. (c) Moore, T. A.; Gust, D.;
Moore, A. L.; Bensasson, R. V.; Seta, P.; Bienvenue, E. In Supramolecular
Photochemistry; Balzani, V., Ed.; D. Reidel: Boston, 1987; p 283. (d) Gust,
D.; Moore, T. A. In Supramolecular Photochemistry; Balzani, V., Ed.; D.
Reidel: Boston, 1987; p 267. (e) Wasielewski, M. R.; Gains, G. L., III;
Wiederrecht, G. P.; Svec, W. A.; Niemczyk, M. P. J. Am. Chem. Soc. 1993,
115, 10442.
(6) (a) Sanders, G. M.; van Dijk, M.; van Veldhuizen, A.; van der Plas,
M. J. J. Chem. Soc., Chem. Commun. 1986, 1311. (b) Wasielewski, M. R.;
Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J. Am. Chem. Soc. 1985, 107,
5562. (c) Mecklenburg, S. L.; Peek, B. M.; Erickson, B. W.; Meyer, T. J.
J. Am. Chem. Soc. 1987, 109, 3297. (d) Danielson, E.; Elliott, C. M.; Mesket,
J. W.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 2519. (e) Johnson, D. G.;
Niemczyk, M. P.; Minsek, D. W.; Widerrecht, G. P.; Svec, W. A.; Gaines,
G. L., III; Wasielewski, M. R. J. Am. Chem. Soc. 1993, 115, 5692.
(7) (a) Kurreck, H.; Huber, M. Angew. Chem., Int. Ed. Engl. 1995, 34,
849. (b) Gust, D.; Moore, T. A.; Moore, A. L.; Lee, S.-J.; Bittersmann, E.;
Luttrull, D. K.; Rehms, A. A.; DeGraziano, J. M.; Ma, X. C.; Gao, F.;
Belfored, R. E.; Trier, T. T. Science 1990, 248, 199.
*
To whom correspondence should be addressed.
Abstract published in AdVance ACS Abstracts, January 1, 1996.
X
(1) Universit a¨ t des Saarlandes, Organische Chemie, Fachbereich 11.2,
6
6041 Saarbr u¨ cken, Germany.
2) Institute of Chemistry, The Hebrew University of Jerusalem, Jerusa-
lem 91904, Israel. Fax: 972-2-6527715
3) (a) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol.
(
(
Biol. 1984, 180, 385. (b) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.;
Michel, H. Nature 1985, 318, 618. (c) Chang, C. H.; Tiede, D.; Tang, J.;
Smith, U.; Norris, J.; Schiffer, M. FEBS Lett. 1986, 205, 82.
(
4) (a) Connolly, J. S.; Bolton, J. R. In Photoinduced Electron Transfer,
Part D; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, Section 6.2.
b) Cooley, L. F.; Headford, C. E. L.; Elliott, C. M.; Kelley, D. F. J. Am.
Chem. Soc. 1988, 110, 6673.
(
0
002-7863/96/1518-0655$12.00/0 © 1996 American Chemical Society

6
56 J. Am. Chem. Soc., Vol. 118, No. 3, 1996
Kropf et al.
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
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 (2), and tris(6-(8-hydroxy-
3,6-dioxaoctyl-1-oxy)-6′-[8-((4-methoxyphenyl)oxy)-3,6-
dioxaoctyl-1-oxy]-3,3′-bipyridazine)-1,3,5-benzene-tricarboxylate-
ruthenium(II) dichloride (3), with the bipyridinium cyclophane
9,10
unit.
Recently, we reported on a novel approach to organize
chromophore-electron acceptor diad assemblies by the applica-
tion of electron donor-modified chromophores that form the
supramolecular non-covalently-linked diads with the electron
4
+
(BXV , 4). We examine the electron-transfer quenching
pathways in the resulting supramolecular assemblies and
characterize the resulting photogenerated redox products and
their recombination in the supramolecular assemblies. We find
that effective electron-transfer quenching of the excited chro-
mophores proceeds in the supramolecular systems and that the
resulting supramolecular complex of photoproducts is preserved.
We reveal that the lifetime of the photogenerated redox products
in the supramolecular asemblies is long as compared to other
molecular diad systems. This is attributed to the fact that the
alkoxyanisyl binding sites are tethered to the Ru(II) chro-
mophores by poly(ethylene glycol) chains. Association of
11
acceptor via donor-acceptor interactions.
Similarly, we
showed that the formation of donor-acceptor supramolecular
complexes between a molecular triad and electron donor results
in steric rigidification of the triad that results in photoinduced
vectorial electron transfer and stabilization of the redox products
against back electron transfer.12
It is established that N,N′-dialkylbipyridinium salts form
1
3,14
donor-acceptor complexes with different electron donors.
Recently, the intermolecular complexes between dialkoxyben-
4+
zenes and cyclo[bis(N,N′-p-xylylene-4,4′-bipyridinium)], BXV ,
1
5,16
were extensively studied by Stoddart and co-workers.
It
4
+
was found that dialkoxybenzene intercalates into the bipyri-
dinium cyclophane via donor-acceptor interaction, and the
resulting stable supramolecular assemblies were applied to
BXV to the binding sites results in electrostatic repulsions
between the electron-acceptor and the Ru(II)-chromophore
component that induce the steric spatial separation of the redox
products which are stabilized against back electron transfer.
17,18
synthesize ingenious catenane macromolecules.
Chemical
modification of Ru(II)-polypyridine complexes with dialkoxy-
4
+
benzene units could provide binding sites for the BXV
Experimental Section
electron acceptor. Here we wish to report on the supramolecular
complexes formed between the series of alkoxyanisyl-tethered
Ru(II)-tris(bipyridazine) complexes: bis(6,6′-dimethoxy-3,3′-
Absorption spectra were recorded with a Uvikon-860 (Kontron)
spectrophotometer. Fluorescence spectra were recorded with a SFM-
2
5 (Kontron) spectrofluorometer. Flash photolysis experiments were
carried out with a Nd-YAG laser (Model GCR-150, Spectra Physics)
coupled to a detection system (Applied Photophysics K-347) that
included a monochromator and photomultiplier linked to a digitizer
(Tektronix 2430 A) and computer for data storage and processing. This
flash-photolysis setup has a time resolution of >20 ns. For shorter
time-scale transients (>0.5 ns) a flash photolysis system consisting of
(
8) (a) Yonemoto, E. H.; Kim, Y. I.; Schmehl, R. H.; Wallis, J. O.;
Shoulders, B. A.; Richardson, B. R.; Haw, J. F.; Mallouk, T. E. J. Am.
Chem. Soc. 1994, 116, 10557. (b) Vermuelen, L. A.; Thompson, M. E.
Nature 1992, 358, 656. (c) Ungashe, S. B.; Wilson, W. L.; Katz, H. E.;
Scheller, G. R.; Putrinski, T. M. J. Am. Chem. Soc. 1992, 114, 8717.
(9) (a) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1993,
1
1
15, 10418. (b) Harriman, A.; Kubo, Y.; Sessler, J. L. J. Am. Chem. Soc.
992, 114, 388. (c) Turr o´ , C.; Chang, C. K.; Leroi, G. E.; Cukier, R. I.;
a N
2
laser (PRA, LN-1000) coupled to a dye laser (Laser Photonics,
Coumarin 440) was employed. These lasers were coupled to a detection
system consisting of a monochromator and photomultiplier (Applied
Photophysics) linked to a digitizer (Tektronix 7912 AD) and a computer
for data storage and analysis.
All materials and solvents were of highest purity from commercial
sources (Aldrich, Sigma). The ligands (6,6′-bis[8-((4-methoxyphenyl)-
oxy)-3,6-dioxaoctyl-1-oxy]-3,3′-bipyridazine and (6,6′-bis[8-((4-meth-
Nocera, D. G. J. Am. Chem. Soc. 1992, 114, 4013.
10) (a) Sun, L.; von Gersdorff, J.; Niethammer, D.; Tian, P.; Kurreck,
(
H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2318. (b) Sun, L.; von Gersdorff,
J.; Sobek, J.; Kurreck, H. Tetrahedron 1995, 51, 3535. (c) D u¨ rr, H.;
Bossmann, S.; Schwarz, R. J. Inf. Rec. Mater. 1994, 21, 471. (d) D u¨ rr, H.;
Bossmann, S.; Kropf, M.; Hayo, R.; Turro, N. J. J. Photochem. Photobiol.
A: Chemistry 1994, 80, 341.
(
11) Seiler, M.; D u¨ rr, H.; Willner, I.; Joselevich, E.; Doron, A.; Stoddart,
J. F. J. Am. Chem. Soc. 1994, 116, 3399.
12) Zahavy, E.; Seiler, M.; Marx-Tibbon, S.; Joselevich, E.; Willner,
I.; D u¨ rr, H.; O’Connor, D.; Harriman, A. Angew. Chem., Int. Ed. Engl.
995, 34, 1005.
13) (a) Willner, I.; Eichen, Y.; Rabinovitz, M.; Hoffman, R.; Cohen, S.
oxyphenyl)oxy)-3,6,9-trioxaundecyl-1-oxy]-3,3′-bipyridazine were pre-
(
19,20
pared by coupling
of 6-[8-((4-methoxyphenyl)oxy)-3,6-dioxaoctyl-
1
-oxy)-3-chloropyridazine or 6-[11-((4-methoxyphenyl)oxy)-3,6-
trioxaundecyl-1-oxy]-3-chloropyridazine, respectively, in the presence
of Ni(PPh (DMF, 8h) followed by chromatographic purification
(SiO , CH -CH OH 95:5 (v/v) as eluent). The photosensitizer bis-
(6,6′-dimethoxy-3,3′-bipyridazine)(6,6′-bis[8-((4-methoxyphenyl)oxy)-
,6-dioxaoctyl-1-oxy)-3,3′-bipyridazine)ruthenium(II) dichloride (1) was
prepared by the reaction of Ru(II)-bis(6,6′-dimethoxy-3,3′-bipyridazine)
100 mmol) and the respective ligand (150 mmol) (ethylene glycol,
80 °C, 4 h) followed by chromatographic separation (SiO , CH Cl
CH OH 80:20 (v/v) as eluent). The photosensitizers, tris(6,6′-bis[8-
1
(
3 4
)
J. Am. Chem. Soc. 1992, 114, 637. (b) Kisch, H.; Fernandez, A.; Wakatsuki,
Y.; Yamazaki, H. Z. Naturforsch. 1985, 406, 292. (c) Nakamura, K.; Kai,
Y.; Yasuoka, N.; Kasai, N. Bull. Chem. Soc. Jpn. 1981, 54, 3300.
2
Cl
2 2
3
(14) (a) Usui, Y.; Misawa, H.; Sakuragi, H.; Tokumara, U. Bull. Chem.
3
Soc. Jpn. 1987, 60, 1573. (b) Willner, I.; Eichen, Y.; Joselevich, E. J. Phys.
Chem. 1990, 94, 3092.
(
(15) Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado,
1
2
2
2
-
M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philip, D.; Pietrasz-
kiewicz, M.; Prodi, L.; Reddington, M. V.; Slavin, A. M. Z.; Spencer, N.;
Stoddart, J. F.; Vincent, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114,
2
((4-methoxyphenyl)oxy)-3,6-dioxaoctyl-1-oxy]-3,3′-bipyridazine)ru-
thenium(II) dichloride (2a) and tris(6,6′-bis[11-((4-methoxyphenyl)oxy)-
1
93.
16) (a) Anelli, P. L.; Ashton, P. R.; Spencer, N.; Slawin, A. M. Z.;
Stoddart, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1991, 30,
036. (b) Ashton, P. R.; Odell, B.; Reddington, M. V.; Slavin, A. M. Z.;
Stoddart, J. F.; Williams, D. J. Angew. Chem. 1988, 27, 1550.
17) (a) Anelli, P. L.; Spencer, N.; Stoddart, J. F. J. Am. Chem. Soc.
991, 113, 5131. (b) Ashton, P. R.; Brown, C. L.; Chrystal, E. J. T.;
(
3
,6,9-trioxaundecyl-1-oxy]-3,3′-bipyridazine)ruthenium(II) dichloride
(
2b), were prepared by the reaction of Ru(DMSO)
4
Cl
2
(100 mmol)
O, 3:1 (v/v)
1
and the respective ligands (400 mmol) in ethanol-H
2
(
(reflux, 24 h), followed by chromatographic purification (SiO
Cl -CH OH, 80:20 (v/v) as eluent).
The ligand (6-[3,6-dioxaoctyl-1-oxy]-6′-[8-((4-methoxyphenyl)oxy)-
,6-dioxaoctyl-1-oxy]-3,3′-bipyridazine)-1,3,5-benzene-tricarboxylate
2 2
, CH -
1
2
3
Goodnow, T. T.; Kaifer, A. E.; Parry, K. P.; Slawin, A. M. Z.; Spencer,
N.; Stoddart, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1991, 30,
3
1
039. (c) Ashton, P. R.; Philip, D.; Spencer, N.; Stoddart, J. F. J. Chem.
Soc., Chem. Commun. 1992, 1124.
18) (a) Bissell, R. A.; Cordova, E.; Kaifer, A. E.; Stoddart, J. F. Nature
994, 369, 133. (b) Ashton, P. R.; Ballardini, R.; Balzani, V.; Gandolfi, M.
was prepared by the reaction between 6-(8-hydroxy-3,6-dioxaoctyl-1-
oxy)-6′-[8-((4-methoxyphenyl)oxy)-3,6-dioxaoctane and benzene tri-
(
1
T.; Marquis, D. J.-F.; Per e´ z-Garci a´ , L.; Prodi, L.; Stoddart, J. F.; Venturi,
M. J. Chem. Soc., Chem. Commun. 1994, 177. (c) Amabilino, D. B.; Ashton,
P. R.; Reder, A. S.; Spencer, N.; Stoddart, J. F. Angew. Chem., Int. Ed.
Engl. 1994, 33, 433.
(19) (a) Tiecco, M. Bull. Soc. Chim. Belg. 1986, 95, 1009. (b) Kendo,
A.; Liebeskind, L.; Braitsch, D. Tetrahedron Lett. 1975, 3375.
(20) Detailed synthesis of the complexes will be described elsewhere:
D u¨ rr, H.; Kropf, M. In preparation.

Alkoxyanisyl-Tethered Ru(II)-Trisbipyridazine Complexes
J. Am. Chem. Soc., Vol. 118, No. 3, 1996 657
Chart 1
carboxychloride.18 The photosensitizer tris(6-(8-hydroxy-3,6-dioxaoctyl-
ments were performed in 0.4 × 1 cm glass cuvettes that included an
-5
1
-oxy)-6′-[8-((4-methoxyphenyl)oxy)-3,6-dioxaoctyl-1-oxy]-3,3′-
aqueous solution of the respective photosensitizer, 4.0 × 10 M (O.D.
4+
bipyrazidine)-1,3,5-benzene-tricarboxylate-ruthenium(II) dichloride (3)
was prepared by the reaction of the latter ligand (100 mmol) with Ru-
≈ 1.0), and the appropriate concentration of the BXV . In steady-
state experiments λex ) 460 nm, and in the transient experiments using
(
DMSO)
4
Cl
2
(100 mmol) in ethanol-H
2
O, 3:1 (v/v) (reflux, 24 h)
O as
the Nd-Yag laser λex ) 532 nm and for the N -dye laser λex ) 437 nm.
2
followed by chromatographic purification (Sephadex G-15, H
2
The resulting emission was recorded at λem ) 600 nm. The recombina-
eluent). The ligands 6,6′-bis[8-hydroxy-3,6-dioxaocttl-1-oxy]-3,3′-
bipyridazine and 6,6′-bis[11-hydroxy-3,6,9-trioxaundecyl-1-oxy]-3,3′-
tion processes were characterized in aqueous solution that included the
-
5
4+
-3
respective photosensitizer, 4.0 × 10 M, and BXV , 4.36 × 10
19
bipyridazine were prepared by coupling of 6-[8-hydroxy-3,6-dioxaoctyl-
M. All transient measurements were performed under an oxygen-free
Ar atmosphere.
1
3
-oxy]-3-chloropyridazine or 6-[11-hydroxy-3,6,9-trioxaundecyl-1-oxy]-
-chloropyridazine (DMF, 8 h) followed by chromatographic purification
(
SiO
bis(6,6′-dimethoxy-3,3′-bipyridazine)(6,6′-bis[8-hydroxy-3,6-dioxaoctyl-
-oxy]-3,3′-bipyridazine)ruthenium(II) dichloride (5) was prepared by
2
, CH
2
Cl
2
-CH
3
OH, 90:10 (v/v) as eluent). The photosensitizer
Results and Discussion
The electron-transfer quenching and charge separation in a
series of supramolecular systems consisting of polyethylene
oxyanisyl Ru(II)-bipyridazine complexes, acting as photosen-
sitizers, and N,N′-bipyridinium salts acting as electron acceptors
were examined. The photosensitizer series include bis(6,6′-
dimethoxy-3,3′-bipyridazine)(6,6′-bis[8-((4-methoxyphenyl)-
oxy)-3,6-dioxaoctyl-1-oxy]-3,3′-bipyridazine)ruthenium(II) dichlo-
ride (1), tris(6,6′-bis[8-((4-methoxyphenyl)oxy)-3,6-dioxaoctyl-
1
the reaction of Ru(II)-bis(6,6′-bis(methoxy))-3,3′-bipyridazine (100
mmol) and the latter ligand (150 mmol) (ethylene glycol, 180 °C, 4 h)
followed by chromatographic separation (Sephadex G10, water as
eluent). Cyclo[bis(N,N′-p-xylylene-4,4′-bipyridinium)] tetrachloride (4)
was prepared according to the literature.
satisfactory elementary analyses and H-NMR spectra.
2
1
All compounds gave
1
All photochemical measurements were performed in triply distilled
water.All steady-state fluorescence and time-resolved quenching experi-
1-oxy]-3,3′-bipyridazine)ruthenium(II) dichloride (2a), tris(6,6′-
bis[11-((4-methoxyphenyl)oxy)-3,6,9-trioxaundecyl-1-oxy]-3,3′-
bipyridazine)ruthenium(II) dichloride (2b), and tris(6-(8-hydroxy-
3,6-dioxaoctyl-1-oxy)-6′-[8-((4-methoxyphenyl)oxy)-3,6-
(
21) Odell, B.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.;
Stoddart, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27,
547.
1

6
58 J. Am. Chem. Soc., Vol. 118, No. 3, 1996
Kropf et al.
a
Figure 1. Steady-state luminescence quenching of (a) 2a, (b) 3, (c) 1,
and (d) 6. All photosensitizers are at a concentration corresponding to
b
-
5
4
× 10 M.
dioxaoctyl-1-oxy]-3,3′-bipyridazine)-1,3,5-benzene-tricarboxy-
late-ruthenium(II) dichloride (3). All of the Ru(II)-bipyridazine
complexes include p-methoxyanisyl units that form charge-
transfer complexes with bis(N,N′-(p-xylylene)-4,4′-bipyridinium
4+
(
BXV , 4). The formation of charge-transfer supramolecular
complexes between 1,4-dimethoxybenzene and the bis-bipyri-
dinium cyclophane, BXV4 (4), has been specifically demon-
strated by Stoddart and co-workers in recent years. Photoin-
duced electron transfer from Ru(II)-polypyridine complexes
to bipyridinium electron acceptors has been studied extensively.
Thus, formation of photosensitizer/electron acceptor complexes
between the p-methoxyanisyl units linked to the Ru(II)-
bipyridazine complexes (1-3) is anticipated to control the
photoinduced electron transfer in the resulting supramolecular
assemblies.
+
Figure 2. Transient luminescence intensities of (a) 2a and (b) 6 in
the presence and absence of BXV4 . Upper transients correspond to
+
4+
the photosensitizer luminescence without BXV . All other transients
4+
represent the system with added BXV at consecutive increments
corresponding to 7 × 10-4 M.
Figure 1 shows the steady-state luminescence quenching of
the series of complexes 1-3 by BXV4 (4). Nonlinear Stern-
Volmer plots are obtained for all of the photosensitizers and
the highest deviation from linearity is observed for 2. As control
experiments, the luminescence quenching processes of the
photosensitizers bis(6,6′-dimethoxy-3,3′-bipyridazine)(6,6′-bis-
+
4+
photosensitizers 1-3 upon addition of BXV . The extent of
decrease in the initial luminescence intensities and the shortening
of the lifetimes is variable and depends on the structure of the
complex and the nature of electron acceptor. Figure 2b shows
the luminescence transients obtained upon addition of BXV4
to the reference complex, 6. The initial luminescence intensity
+
[8-hydroxy-3,6-dioxaoctyl-1-oxy]-3,3′-bipyridazine)ruthenium-
(
1
II) dichloride (5) and tris(6,6′-bis[8-hydroxy-3,6-dioxaoctyl-
-oxy]-3,3′-bipyridazine)ruthenium(II) dichloride (6) by BXV
remains nearly constant, but the luminescence lifetime is
4+
4+
shortened upon increase of BXV
concentration. These
were examined. The latter two photosensitizers lack the
alkoxyanisyl binding sites and are not capable of forming
supramolecular assemblies. For both photosensitizers 5 and 6,
linear Stern-Volmer luminescence plots are observed. This is
exemplified in Figure 1 (curve d) with the luminescence
features are also characteristic of the luminescence quenching
4
+
of 5 by BXV . The latter behavior, where only the lumines-
4+
cence lifetime is shortened upon addition of BXV , is
characteristic of a diffusional electron-transfer quenching. Thus,
the unique features observed for the luminescence decay of the
4+
4+
quenching plot of 6 by BXV . These results clearly imply
that the electron-transfer quenching of the Ru(II)-bipyridazines
photosensitizers 1-3 by BXV and reflected by the decrease
of luminescence intensity and shortening of the lifetime are
attributed to the participation of two complementary electron-
transfer quenching pathways. Photosensitizers 1-3 including
the alkoxyanisyl binding sites form supramolecular complexes
1
-3 by bipyridinium salt 4 exhibits a complex route, where
the electron-transfer quenching of 5 and 6 is diffusionally
controlled. As the complexes 1-3 include the alkoxyanisyl
4+
4+
binding sites for BXV , the nonlinear luminescence quenching
could be assigned to the formation of intermolecular photosen-
sitizer/electron acceptor complexes. Luminescence quenching
within the supramolecular assembly and via diffusional quench-
ing of noncomplexed photosensitizer units could give rise to
the nonlinear features of the Stern-Volmer plots (Vide infra).
Further insight into the electron transfer quenching of the
with BXV . Intramolecular electron-transfer quenching in the
supramolecular assembly is fast, and reflected in the decrease
of luminescence intensity (this rapid decay will be discussed
and analyzed later, Vide infra). The transient luminescence and
its lifetime shortening are attributed to free photosensitizer which
4
+
is quenched by the electron acceptor (BXV ) via a diffusional
route, similar to that occurring in the reference compounds 5
and 6.
4+
Ru(II)-bipyridazine complexes 1-3 by BXV is gained by
time-resolved laser flash photolysis experiments. Figure 2a
To analyze quantitatively the electron-transfer quenching of
photosensitizers 1-3 by the bipyridinium electron acceptor and
to determine the association constants of the resulting supramo-
lecular assemblies, we formulate a comprehensive model. The
Ru(II)-bipyridazine complexes 1, 2, and 3 differ in the number
of alkoxyanisyl sites (N ) 2, 6, and 3, respectively). The kinetic
model takes into consideration the photoprocesses of the
4+
shows the transients of the luminescence decay of 2a by BXV .
The luminescence transients reveal two important features: (i)
the initial luminescence intensity decreases as the concentration
of BXV4 increases, and (ii) the luminescence lifetime is
+
4+
shortened as the concentration of BXV increases. These
features appear to be general for the luminescence transients of

Alkoxyanisyl-Tethered Ru(II)-Trisbipyridazine Complexes
J. Am. Chem. Soc., Vol. 118, No. 3, 1996 659
multireceptor photosensitizer, S, in the presence of the electron
acceptor, A, within the resulting supramolecular assemblies
exhibiting all the possible stoichiometries: SA, SA2, ..., SAN
populations of excited photosensitizers have the same diffusional
quenching rate constant, kdqn ) kdq. (g) The static quenching
rate constant is proportional to the number of bound acceptor
units, that is, ksqn ) nksq, where ksq is the static quenching rate
constant attributed to one acceptor unit. (h) The concentration
of the acceptor is much higher as compared to the concentration
of the photosensitizer, [A] >> [S]. Assumptions (a) and (b)
imply that any interactions between acceptor units bound to
different binding sites are neglected. Assumptions (c) and (d)
imply that electron transfer is faster than the dynamics of
formation or dissociation of the supramolecular complexes. That
is, there is no exchange between populations of excited
photosensitizer (eq 3), implying that each population of excited
photosensitizer decays at a specific rate, regardless of the decays
of the other populations. Assumptions (e) and (f) are less
important, since the natural decay and the diffusional quenching
are much slower than static quenching and, hence, are only
relevant to the unbound photosensitizer. Assumption (h) means
that the concentration of the acceptor is not significantly changed
in the presence of the photosensitizer, and can be regarded as
a constant. As a result of these assumptions (see detailed
development in the Appendix), we obtain that all the concentra-
tions of the ground-state photosensitizer and photosensitizer-
acceptor assemblies, SAn, which are found in equilibrium before
photoexcitation, are given by eq 10, where So is the analytical
concentration of the photosensitizer and K ) k+/k- is an
equilibrium association constant attributed to the binding of one
acceptor unit to a single binding site associated with the
photosensitizer.
(N is the number of binding sites on the photosensitizer), as
well as the quenching of the photosensitizer by a diffusional
route. The photosensitizer configurations are generally repre-
sented as SAn (n ) 0, 1, 2, ..., N), where the case n ) 0
designates the unbound photosensitizer. The relevant processes,
species, and kinetic constants involved in the photoreactions of
the different supramolecular photosensitizer states are sum-
marized in eqs 1-9. These equations represent the following
processes: (1) association and dissociation of the photosensitizer
and electron acceptor in the ground state; (2) photoexcitation;
(3) association and dissociation of the photosensitizer and
acceptor in the excited state; (4) natural decay of the photo-
sensitizer; (5) diffusional electron-transfer quenching; (6) static
electron-transfer quenching within the supramolecular as-
semblies; (7) diffusional back electron transfer of the photoge-
nerated redox products; (8) static back electron transfer recom-
bination of the redox products within the supramolecular
assemblies; and (9) escape or dissociation of the reduced
acceptor from the supramolecular assemblies.22
kn
SAn-1 + A y z SAn
(n ) 1, 2, ..., N)
(1)
(2)
(3)
k-n
hν
SAn
9
8 S*An
(n ) 0, 1, 2, ..., N)
k*n
S*An-1 + A yk*-nz S*An
(n ) 1, 2, ..., N)
kDn
S0
N!
n
n
[
SA] )
N‚
K [A]
S*An
8 SAn (n ) 0, 1, 2, ..., N)
(4)
n
n!(N - n)!
1 + K[A])
(
kdqn
(n ) 0, 1, 2, ..., N) (10)
+
-
S*A + A
8 S A + A
(n ) 0, 1, 2, ..., N) (5)
n
n
This is a binomial distribution which can be represented by the
typical form of eq 11, where Pn is the probability of a
photosensitizer to be associated to n quencher units, and p )
K[A]/(1 + K[A]) is the probability of one binding site to be
bound to an acceptor unit.
ksqn
+
-
S*An 8 S A A
(n ) 1, 2, ..., N)
(6)
n-1
kdr
+
-
S A + A
9
8 SA + A (n ) 0, 1, 2, ..., N) (7)
n
n
ksr
+
-
N!
n
N-n
S A A
9
8 SAn
(n ) 1, 2, ..., N)
(8)
n-1
Pn(p) )
p (1 - p)
(11)
n!(N - n)!
kesc
+
-
+
-
S A A
8 S A + A
(n ) 1, 2, ..., N) (9)
n-1
n-1
Upon pulse irradiation of the system, equal fractions of every
population are photoexcited to the excited states, S*An, and
immediately after excitation their population is similar to the
ground-state distribution. As each population decays at a
different rate, the overall decay of the luminescence of the
photosensitizer is multiexponential. The transient emission
decay according to this model is given by eq 12.
The main assumptions adopted in the kinetic model can be
summarized as follows: (a) The association rate constant is
proportional to the number of free binding sites. That is, kn )
(
N + 1 - n)k+, where k+ is the association constant attributed
to one binding site. (b) The dissociation rate constant is
proportional to the number of bound acceptor units. That is,
k-n ) nk-, where k- is the dissociation rate constant of a single
bound acceptor unit. (c) Diffusional quenching is faster than
association; that is, kdq >> k*n. (d) Static quenching is faster
than dissociation, that is, ksq >> k*-n. (e) All the populations
of excited photosensitizer, S*An (n ) 0, 1, 2, ..., N), have the
same natural decay rate constant, kDn ) kD. (f) All the
-
ksqt N
1
+ K[A]e
-(kD+kdq[A])t
I(t) ) I(0)e
(12)
(
)
1 + K[A]
Since the static quenching is much faster than the diffusional
quenching (ksq >> kdq[A]), the transient luminescence is
composed of a fast decay which is attributed to the static
quenching in the supramolecular assemblies, SAn (n ) 1, 2, ...,
N), and a slow decay is attributed mainly to natural decay and
diffusional quenching of the unbound photosensitizer, S. The
fast decay ends when the term K[A] exp(-ksqt) in eq 12 becomes
negligible, and then the slow emission, Islow(t), is expressed by
eq 13.
(
22) One reviewer suggested an electron exchange mechanism as an
additional possible pathway for the formation of separated photoproducts:
kex
+
-
+
-
S A A + A
98 S A + A
n
n-1
The major conclusion is, however, that no unbound photoproducts origi-
nating from the supramolecular assembly are detected (even at high BXV4
concentrations). Hence, this process has little or no physical contribution
to the electron transfer in the systems.
+

6
60 J. Am. Chem. Soc., Vol. 118, No. 3, 1996
Kropf et al.
1
D
-(k +kdq[A])t
Islow(t) ) I(0)
e
(13)
N
(
1 + K[A])
The emission transients shown in Figure 2a are assigned to
the slow emission decay, Islow(t), of the free photosensitizer (eq
3). The apparent decrease in the initial luminescence as the
1
4+
concentration at the acceptor BXV is increased is attributed
to the fast static quenching (not seen on this time scale) of higher
fractions of bound photosensitizer, and the accompanying
shortening of the luminescence lifetime is interpreted as a result
of the diffusional quenching of the free photosensitizer (the fast
decay due to static quenching was observed in complementary
experiments performed at shorter time scales, which we shall
describe in detail later in this paper). The emission transients
corresponding to the luminescence decay of the photosensitizer
Figure 3. Shortening of the lifetime of the slow luminescence decay
at different concentrations of BXV4 : (a) 2a, (b) 6. Data of lifetimes
were extracted from Figure 2.
+
4
+
including 6 alkoxyanisyl groups (2a) with addition of BXV
Figure 2a) is characterized by this apparent decrease in the
(
Table 1. Diffusional and Static Electron-Transfer Quenching Rate
4+
Constants in the Photosensitizer-BXV Assemblies and the
initial intensity, whereas the emission transients corresponding
to the luminescence decay of the reference photosensitizer
Association Constants of the Resulting Supramolecular Systems
6
10-8k
-7
sq
4
+
10 τ
10
k
c
lacking the alkoxyanisyl groups (6) with addition of BXV
Figure 2b) are only characterized by a shortening of the lifetime
0
dq
-
1
(M ‚s )a
-1 -1
K (M-1)b
(s-1)
complex
(s )
(
with almost no change in the initial intensity. This is attributed
1
2.3
2.5
2.4
2.4
2.1
1.9
1.2
1.1
1.2
1.2
0.27
0.32
240 ( 15
2
a
100 ( 10 (85)
100 ( 10 (90)
140 ( 15
25.0
4.5
to a significant value of the association constant K for the
binding of BXV4+ to the alkoxyanisyl group, and an almost nil
2b
3
5
4+
value of the constant K for the binding of BXV to the
triethylene glycol chain lacking the alkoxyanisyl group.
The steady-state luminescence intensity of the photosensitizer
at any concentration of electron acceptor is proportional to the
integration over time of the transient luminescence intensity (eq
6
a
Deduced from the shortening of the slow-decay luminescence
lifetimes. b Derived from the modified Stern-Volmer plots, according
to eq 18. The association constants derived by analysis of the fast
luminescence decay, according to eq 12, are in parentheses. Derived
c
1
2) as given by eq 14, where R is a proportionality constant.
by fitting the fast luminescence decay according to eq 12.
-
ksqt N
∞
1 + K[A]e
1
I ) R
∫
I(0)e-(kD+kdq[A])t
dt
(14)
0
(
)
photosensitizers 2a and 6 upon addition of different concentra-
+ K[A]
4
+
tions of BXV (4) obtained by exponential regression of the
transients shown in Figure 2. According to eq 16, the diffusional
4
+
Since the term K[A] exp(-ksqt) very quickly becomes null, it
does not contribute significantly to the integration. That is, the
steady state luminescence comes almost exclusively from the
free photosensitizer. Then eq 14 can be approximated to eq
quenching rate constants of 2a and 6 by BXV obtained from
the slope of the linear regression correspond to k ) 1.1 ×
dq
8
-1 -1
7
-1 -1
10 M ‚s and kdq ) 3.2 × 10 M ‚s , respectively. Similar
analyses were performed for 1 and 3 and the reference
compound 5. Table 1 summarizes the natural decay lifetimes
of the series of photosensitizers and the respective values for
the diffusional electron-transfer quenching rate constant by
15, where τ is the lifetime of the slow luminescence decay given
by eq 16.
RI(0)τ
4+
BXV , kdq. The diffusional quenching rate constants of 1 and
I )
(15)
(16)
N
2
6
a are comparable to those of the reference compounds 5 and
, respectively, albeit the values for the alkoxyanisyl complexes
(
1 + K[A])
1
/τ ) k + k [A]
are slightly higher. The differences are attributed to different
diffusion coefficients of the alkoxyanisyl photosensitizers, as
compared to the hydroxyl-substituted reference compounds.
The association parameters and the nature of supramolecular
assemblies formed between the series of photosensitizers 1-3
D
dq
The nonlinear Stern-Volmer plots in Figure 1 (curves a, b, and
c) can be rationalized by eq 17, where Io and τo are the steady
state luminescence intensity and the slow luminescence decay,
respectively, of the photosensitizer in the absence of electron
acceptor.
4+
and BXV can be analyzed by rearrangement of eq 17 in the
form of a modified Stern-Volmer equation, eq 18, where Io/I
is the normal Stern-Volmer plot (as in Figure 1) obtained from
steady-state measurements, and τo/τ is derived from the
experimental values of the lifetimes, as shown in Figure 3.
Figure 4 shows the modified Stern-Volmer plot of the
experimental data corresponding to the electron-transfer quench-
Io
τo
τ
N
)
(1 + K[A])
(17)
I
The Stern-Volmer plot in Figure 1 corresponding to the
reference compound (6a) which lacks the alkoxyanisyl groups
is linear since 1/τ is linear (eq 16) and K is virtually null.
Figure 3 shows the shortening of the lifetimes of the
4+
ing of the photosensitizer (2a) by BXV , assuming maximal
stoichiometries N ) 1 (Figure 4a) and N ) 2, 3, and 6 (Figure
4
b). The best linear fitting (R ) 0.990) is obtained for the
(
23) For the calculation of the second-order back electron transfer rate
maximal stoichiometry N ) 6. The meaining of the plot
corresponding to N ) 1 (Figure 4a) is the molar ratio between
the band and free photosensitizer. Figure 5 shows the modified
Stern-Volmer plots corresponding to the electron-transfer
quenching of the series of photosensitizers 1, 2a, and 3 by
3
+
constants, the extinction coefficient of BXV was assumed to be similar
-1
-1
to that of benzyl viologen radical cation (ꢀ ) 12 700 M ‚cm ). The direct
•
3+
determination of the BXV extinction coefficient is difficult due to the
intramolecular dimerization of the radical cations of the two bipyridinium
units of 4.

Alkoxyanisyl-Tethered Ru(II)-Trisbipyridazine Complexes
J. Am. Chem. Soc., Vol. 118, No. 3, 1996 661
1
/N
Io
τ
a
‚
) 1 + K[A]
(18)
(
)
I τ
o
The optimized association constants for the binding of the
4
+
electron acceptor BXV to a single alkoxyanisyl site in each
of the complexes, deduced from the modified Stern-Volmer
plots, are summarized in Table 1. It can be seen that the
association constants are comparable but their values decrease
as the number of binding sites in the photosensitizer increases.
This might be attributed to the steric crowdedness of the binding
sites in the photosensitizers containing enriched alkoxyanisyl
binding sites. The analysis reveals, however, an unexpected
feature of the resulting supramolecular complexes between the
4
+
series of photosensitizers 1-3 and BXV , where maximum
binding of the quencher to the alkoxyanisyl sites is feasible.
b
4
+
Association of BXV to a single photosensitizer site is expected
to yield electrostatic repulsive interactions for binding of the
subsequent BXV electron acceptor. The excellent fitting
between the experimental data and the suggested model for
electron transfer quenching in the supramolecular assemblies
implies that maximum occupation of the alkoxyanisyl binding
sites by BXV is feasible. That is, the photosensitizers that
include two, three and six binding sites, 1, 3, and 2, form
supramolecular complexes exhibiting stoichiometries of up to
N ) 2, 3, and 6, respectively.
4
+
4
+
Our discussion attributed the decrease in the initial lumines-
cence intensities of the transients of 1-3 upon addition of
4
+
BXV to a fast electron-transfer quenching of the excited
photosensitizer in the supramolecular assembly, S*An, eq 6.
Using a short-pulse laser, the fast decay of the excited
photosensitizer in the supramolecular complex could be re-
solved. Figure 6A (curve b) shows the transient corresponding
Figure 4. Modified Stern-Volmer plots for the luminescence quench-
4
+
ing of 2a by BXV assuming different supramolecular maximal
stoichiometries (a) N ) 1 and (b) 2, 3, or 6. Linear fitting is observed
only for N ) 6 (R ) 0.990).
4+
to the fast decay of photosensitizer 2a upon addition of BXV ,
where Figure 6B (curve b) shows the fast decay of photosen-
4
+
sitizer 2b upon addition of BXV . For comparison, the
emission profiles of 2a and 2b in the absence of BXV4 are
also provided in the respective figures (curves (a) in Figures
+
6
A and 6B). On this short time scale no decay of the emission
4
+
is observed in the absence of added BXV , implying that the
fast decay corresponds to effective electron-transfer quenching
within the supramolecular assemblies. It should be noted that
the fast emission intensity does not decay to zero, but to a
constant finite value. This residual luminescence, which appears
constant at this time scale, represents the diffusional electron-
transfer quenching of the free photosensitizer by BXV4 and
decays to zero at a longer time window (Vide supra). It should
+
Figure 5. Modified Stern-Volmer plots for the luminescence quench-
4+
be noted that addition of BXV to the reference photosensitizer
did not result in any fast decay at this time scale and the
luminescence intensity of 6 is almost identical in the absence
4+
ing of the different photosensitizers by BXV assuming the appropriate
supramolecular maximal stoichiometries: (a) 1, N ) 2; (b) 3, N ) 3;
and (c) 2a, N ) 6.
6
4
+
or presence of BXV . This indicates that no supramolecular
complexes between 6 and BXV are formed, and no intramo-
BXV4 assuming the maximal supramolecular stoichiometries
N ) 2, 6, and 3, respectively (R ) 0.989, 0.990, and 0.986,
respectively). Thus, the best linear fittings of the modified
Stern-Volmer plots for the photosensitizers 1, 2a, 2b, and 3
+
4+
lecular electron transfer takes place. Photosensitizer 6 is
quenched by a diffusional pathway that is observable only at a
longer time scale.
4
+
are obtained for the supramolecular photosensitizer-BXV
The fast emission transients shown in Figure 6, corresponding
to the static electron-transfer quenching within the supramo-
lecular assemblies, were analyzed by least-squares fitting to eq
12, where the term exp{-(kD + kdq[A])t} was obviated (as there
is no significant natural decay nor diffusional quenching at this
short time scale). The parameters I(0), K[A], and ksq were
optimized. The fitted curves (overlaid in Figure 6) were in
excellent agreement with the experimental points. Table 1
summarizes the resulting values of ksq and K (in parentheses)
derived from the least-squares fitting. The values obtained for
K from these analyses are also in good agreement with the values
obtained from the modified Stern-Volmer plots.
stoichiometries corresponding to 2, 6, 6, and 3, respectively. It
should be noted that our model and results do not imply that
these are the only stoichiometries of the supramolecular
photosensitizer-BXV4 complexes, but the experimental results
fit best upon assuming a contribution of a population of maximal
binding occupancy of the photosensitizer. For example, for
+
4
+
complex 2a the supramolecular assembly consisting of BXV
associated to all six binding sites of the photosensitizer
participates in the decay of the excited state, and other
supramolecular complexes of lower stoichiometries simulta-
neously contribute to the decay of the excited state.

6
62 J. Am. Chem. Soc., Vol. 118, No. 3, 1996
Kropf et al.
decay of the reduced product corresponds to the recombination
process. The decay does not fit a first- or second-order reaction
over the entire time domain. The transient, however, can be
separated into a fast recombination which follows a first-order
kinetics and to a slow recombination that follows a second-
order kinetics. The fast first-order decay is attributed to back
electron transfer from the bound reduced acceptor, BXV• , to
the oxidized chromophore, within the supramolecular assembly
3+
(eq 8). The slow second-order decay is attributed to the
diffusional back electron transfer between the reduced acceptor
and an oxidized photosensitizer which are not associated (eq
7). The rate constants of the static and diffusional back-electron-
transfer recombination of the photoproducts formed upon
4+
quenching of 2a by BXV , which are obtained from first-order
analysis of the fast decay and second-order analysis of the slow
•
3+
6 -1
decay of the absorption of BXV , are ksr ) 1.8 × 10 s and
9
-1 -1
kdr ) 3.5 × 10 M ‚s , respectively.
A similar behavior is observed for the series of Ru(II)-
bipyridazine complexes that include the alkoxyanisyl units, 1-3,
Table 2. That is, for all these Ru(II) photosensitizers, the
formation of the supramolecular complexes with BXV4 results
in two distinct populations of photoproducts: the supramolecular
complex composed of the reduced and oxidized products, where
back electron transfer proceeds by a first-order kinetics, ksr, eq
+
8. The second population of photoproducts corresponds to free
redox intermediates, eq 7, that recombine via a second-order
process, kdr. Table 2 summarizes the intramolecular and
diffusional back electron transfer rates in the series of complexes
Figure 6. Transient luminescence decay corresponding to intramo-
lecular electron-transfer quenching within the supramolecular as-
1
-3. The back electron transfer of the photogenerated redox
products formed by quenching of the reference photosensitizer
6) lacking the alkoxyanisyl binding groups was exclusively
-
5
semblies: (A) (a) Luminescence decay of 2a, 4.0 × 10 M; and (b)
-
5
4+
luminescence decay of 2a, 4.0 × 10 M, in the presence of BXV
,
(
-
3
-5
4
.36 × 10 M. (B) (a) Luminescence decay of 2b, 4.0 × 10 M and
-5 4+
second order, a fact that provides further evidence that the
alkoxyanisyl groups are responsible for the formation of the
supramolecular assemblies.
(
b) luminescence decay of 2b, 4.0 × 10 M in the presence of BXV
,
-
3
4
.36 × 10 M.
It is interesting to note that the intramolecular electron-
Another important aspect to note about the analysis of the
back electron transfer is the relative contribution of the fast first-
order static component and the slow second-order diffusional
component. If the reduced bound acceptor, formed upon static
quenching within the supramolecular assembly, does not dis-
sociate (eq 9) before the back electron transfer occurs, then we
should expect the ratio between the photoproduct populations
undergoing static and diffusional back electron transfer to be
equal to the ratio between the photosensitizer populations
undergoing static and diffusional quenching. If, otherwise, the
reduced acceptor formed upon static quenching escapes from
the supramolecular assembly, then the ratio between photo-
product populations undergoing static and diffusional back
electron transfer should decrease as compared to the respective
populations of quenched photosensitizer. Table 2 shows the
fraction of photosensitizer undergoing static quenching (θsq) and
the fraction of photoproducts undergoing static back electron-
transfer recombination (θsr). We see that for photosensitizers
1 and 3 there is no significant difference between θsr and θsq.
For complexes 2a and 2b, θsr is slightly higher than θsq. Since
the difference is not much larger than the error range, we can
only give an upper limit to the escape efficiency, θesc ) kesc/
(ksr + kesc), or (θsr - θsq)/θsq ) 0.08. That is, not more than
8% of the reduced acceptor formed upon electron transfer within
the supramolecular assembly succeeds in escaping from back
electron transfer within the supramolecular assemblies. We
could expect, in principle, that the dissociation of a reduced
acceptor unit (which has lesser π-acceptor character) should be
faster than the dissociation of an unreduced acceptor unit (kesc
> k-n, k*-n), and hence the escape of the reduced photoproducts
should be feasible. We note, however, that the acceptor BXV4
transfer-quenching rate constant of 2a by BXV4 is ca. 5 times
faster than that of 2b. The two photosensitizers differ by a
single ethylene glycol unit in the tether chain bridging the
alkoxyanisyl binding site to the photosensitizer. We can assume
that formation of the supramolecular assemblies between the
+
4
+
BXV electron acceptor and the photosensitizers results in
electrostatic repulsive interaction as well as steric hindrance that
stretch the diad assemblies. The longer electron transfer distance
in photosensitizer 2b, as a result of the longer tether bridge,
decreases the intramolecular quenching rate constant as com-
pared to 2a. A similar effect of the tether length is observed in
the back electron transfer of the redox photoproducts (Vide
infra).
We thus conclude that the series of photosensitizers 1-3 form
an equilibrium mixture of free photosensitizer and mono- and
polydentate supramolecular photosensitizer-acceptor diads.
Figure 7 shows schematically the various supramolecular
4+
assemblies formed between 3 and BXV . Quantitative analysis
of the electron-transfer quenching of the series of photosensi-
tizers implies that multisubstitution of the alkoxyanisyl sites by
4
+
BXV occurs despite the electrostatic repulsions between
4
+
BXV units. We believe that stabilizing donor-acceptor
interactions and the length of the flexible tether bridging chains
compensate for the electrostatic repulsive interactions.
Finally, we examined the electron-transfer products formed
upon quenching via the intramolecular pathway in the supramo-
lecular complexes and by the diffusional route where the free
photosensitizer is quenched. Figure 8 shows the transient of
•
3+
the photoreduced product BXV
formed upon excitation of
+
2
a. It exhibits a characteristic absorbance at λ ) 600 nm. The

Alkoxyanisyl-Tethered Ru(II)-Trisbipyridazine Complexes
J. Am. Chem. Soc., Vol. 118, No. 3, 1996 663
Figure 7. Scheme for the equilibrated supramolecular assemblies formed between 3 and BXV4+.
is composed of two non-conjugated bipyridinium moieties, and
only one of them is reduced, while the other one is still capable
of forming a π-donor-acceptor complex with the alkoxyanisyl
group. The relatively stable supramolecular structures even after
electron transfer are attributed to the stabilization of π-donor-
acceptor complexes between the alkoxyanisyl units and the
oxidized bipyridinium sites of BXV•3+. Thus, the non-co-
valently linked supramolecular assemblies SAn act as static diads
where electron-transfer quenching and recombination occurs in
intact supramolecular structures despite the dynamic nature of
the systems.
The reference compounds 5 and 6 that do not form supra-

6
64 J. Am. Chem. Soc., Vol. 118, No. 3, 1996
Kropf et al.
transfer rate constants, ksr, in the supramolecular systems
consisting of photosensitizers 2a and 2b. The two photosen-
sitizers differ only by one ethylene glycol unit that links the
alkoxyanisyl binding site to the Ru(II) photosensitizer. The
intramolecular back electron transfer rate constant of the
photogenerated redox products is ca. 2-fold slower in the
complex formed with 2b (see Table 2). The slower recombina-
4
+
tion rate in the supramolecular assembly 2b-BXV
as
4+
compared to 2a-BXV is attributed to the longer tether linking
BXV to the photosensitizer that assists the spatial separation
of the redox products.
4
+
Conclusions
Figure 8. Transient decay of the reduced photoproduct, BXV•3+ formed
upon excitation of 2a, as a result of back electron transfer. Reduced
photoproduct was followed at λ ) 600 nm.
We have demonstrated a novel means to organize photosen-
sitizer-electron acceptor diad arrays by structural tailoring of
the photosensitizer with electron donor groups capable of
generating supramolecular diads via donor-acceptor interac-
tions. Effective intramolecular electron transfer quenching
proceeds in the resulting supramolecular assemblies. Mecha-
nistic analysis of the intramolecular electron-transfer quenching
and of the back electron transfer of the photogenerated redox
products within the supramolecular systems revealed several
important features: (i) The electron transfer quenching proceeds
in two distinct populations of the photosensitizer that include
supramolecular assemblies of the photosensitizer-acceptor
components and free photosensitizer that is quenched via a
diffusional pathway. (ii) For multireceptor photosensitizers,
which include several binding sites for the acceptor, supramo-
lecular assemblies of variable stoichiometries SAn up to
complete occupation of all binding sites are formed. For
example, for the hexadentate photosensitizer, 2, formation of
complexes of stoichiometries SA, SA2, ..., SA6 is supported by
mechanistic analysis of the electron-transfer quenching. Func-
tionalization of the supramolecular photosensitizer-acceptor
assembly by a high degree of electron acceptor components
enhances the intramolecular electron transfer quenching. (iii)
The electron transfer products formed in the systems reveal two
distinct populations consisting of the redox products within the
supramolecular assembly and redox products formed via dif-
fusional quenching of free photosensitizers. The two groups
of redox products are non-exchangeable within the lifetime of
back electron transfer. (iv) The lifetime of the redox products
in the resulting supramolecular assemblies is relatively long as
compared to covalently-linked diad systems. This is attributed
to the fact that the electron-acceptor binding sites are tethered
to the photosensitizer by long-chain spacing bridges. Electro-
static repulsion between the electron acceptor units and the
photosensitizer site results in stretched conformations of the
diads. The resulting spatial separation of the redox products
stabilizes them against back electron transfer.
Table 2. Diffusional and Static Back Electron Transfer Rate
Constants and Fraction of Photosensitizer or Redox Photoproducts
Being Quenched or Undergoing Recombination by the
Intramolecular Pathways
photosensitizer 10-6
sr (s-1)a
10-9
dr (M-1‚s-1)b
c
d
k
k
θ
sq
θ
sr
1
2
2
3
5
6
0.83
1.76
0.87
0.96
5.9
3.5
3.4
5.6
2.2
1.2
0.71
0.85
0.84
0.71
0.70
0.78
0.78
0.74
a
b
≈0
≈0
≈0
≈0
a
Determined by analysis of the fast decay absorption transient of
BXV•
3+ b
.
Determined by analysis of the slow decay absorption transient
of BXV•3+.
c
θsq correspsonds to the fraction of photosensitized being
quenched by intramolecular electron transfer in the supramolecular
assemblies. Values determined from the modified Stern-Volmer plots
correspond to systems where [BXV ] ) 4.36 × 10 M. d
4
+
-3
θ
sr
corresponds to the fraction of reduced photogenerated product that
recombines via intramolecular back electron transfer in the supramo-
lecular assemblies.
molecular complexes with BXV4 form upon photoexcitation
the respective photoproducts that recombine via a second-order
diffusional back electron transfer. The recombination rate
constants for these photosensitizers are also included in Table
+
2
.
Figure 7 shows schematically the supramolecular assemblies
4
+
formed by photosensitizer 3 and the electron acceptor, BXV .
We have shown that photoinduced electron transfer occurs in
the supramolecular assemblies and that the electron-transfer
products are stable in the resulting structure and do not dissociate
within their lifetime. The formation of the intermolecular
complexes between the alkoxyanisyl Ru(II) photosensitizer and
4
+
BXV represents a novel means to generate non-covalently-
linked photosensitizer-acceptor diads. It should be noted that
3+
•3+
the lifetime of the redox photoproducts Ru -BXV in the
various systems is relatively long, 0.56-1.20 µs. This value is
substantially longer than the lifetime observed in covalently
linked porphyrin-quinone or Ru(II)-tris-bipyridine-bipyri-
dinium diads. In the present systems, the alkoxyanisyl binding
units, acting as the active site for assembling the photosensi-
We thus conclude that although the series of photosensitizers
4
+
1
-3 and the electron acceptor BXV (4) represent dynamic
systems, the electron transfer quenching and the recombination
of the photogenerated redox products proceed in static supramo-
4+
lecular assemblies consisting of photosensitizer-BXV . Fur-
4
+
ther modification of the primary BXV electron acceptor with
secondary electron acceptors is anticipated to yield more
complex supramolecular triad assemblies where vectorial elec-
tron transfer would enhance the lifetime of the resulting redox
products.
4+
tizer-BXV complexes, is tethered to the photosensitizer unit
by a relatively long polyethylene bridging chain. Association
of BXV4 to the binding site results in electrostatic repulsive
interaction between the Ru(II) photosensitizer unit and the
acceptor component. These repulsive interactions stretch the
bridging chain, and as a result the photogenerated redox products
are spatially separated. The spatial (distance) separation
imposed by the electrostatic interactions stabilizes the photo-
products against back electron transfer. This phenomenon is
supported by comparison of the intramolecular back electron
+
Appendix: Quenching of a Multireceptor Photosensitizer
by a Quencher Substrate
We supply here the algebraic paths that lead from assumptions
(a)-(h) of the kinetic model stated in the section of Results

Alkoxyanisyl-Tethered Ru(II)-Trisbipyridazine Complexes
J. Am. Chem. Soc., Vol. 118, No. 3, 1996 665
S*o
and Discussion, to the derivation of eq 10 expressing the
distribution of the electron acceptor units over the multireceptor
photosensitizer units, and to eq 12 describing the after-pulse
decay of the luminescence intensity in the systems.
The ground-state species SAn (n ) 0, 1, 2, ..., N) are found
in equilibrium prior to the application of the laser pulse.
According to eq 1 and to assumptions (a) and (b), the
equilibrium constants can be defined by eq 19 which gives the
N!
n
n
[S*A ](0) )
K [A]
(24)
n
N
n!(N - n)!
1 + K[A])
(
According to eq 4, 5, and 6, and to assumptions (c)-(g),
each population of the excited states S*An follows a first-order
exponential decay, given by eq 25.
-
(kD+kdq[A]+nksq)t
[S*A ](t) ) [S*A ](0)e
(25)
n
n
[
SAn]
kn
(N + 1 - n)k+
)
)
(19)
The transient emission intensity is proportional to the sum of
the concentration of all the excited states, eq 26.
[
SAn-1][A] k-n
nk-1
concentration of each population, SAn, in an inductive way with
respect to its precursor, SAn-1. This can be cumulatively
expressed by eq 20, which can be converted to eq 21, where K
N
I(t)
1
)
[S*A ](t)
(26)
∑
n
I(0) S*on)0
)
k+/k-.
Combination of eqs 24, 25, and 26 leads to eq 27, which can
be reorganized into the form of eq 28. The latter is brought,
by the expression of the binomium, to the final form of eq 12.
n
(
N + 1 - i)k+
[
SA ] ) [S]
[A]
(20)
(21)
n
∏
i)1
ik
-
N
I(0)
N!
n
n -(kD+kdq[A]+nksq)t
I(t) )
N‚
K [A] e
N!
n!(N - n)!
n
n
∑
[
SA ] ) [S]
K [A]
n)0
n!(N - n)!
n
(1 + K[A])
(27)
The analytical concentration of the photosensitizer, So, is the
sum of the concentrations of all the populations, as given by
eq 22
-(k +kdq[A])t
D
N
e
N!
-ksqt n
I(t) ) I(0)
(K[A]e
)
(28)
∑
N
1 + K[A]) n)0
n!(N - n)!
(
N
N
N!
n
n
So )
[SA ] ) [S]
K [A]
(22)
∑
n
∑ n!(N - n)!
n)0
n)0
Acknowledgment. The research project is supported by the
Volkswagen Stiftung. The support of the laser laboratory at
The Hebrew University by the James Frank Foundation is
gratefully acknowledged. The support of the DAAD program
(M.K.) is acknowledged. We thank Prof. M. Ottolenghi for
enabling us to use the N2-laser system. We also express our
gratitude to Mrs. V. Heleg-Shabtai for her assistance and Mr.
S. Ravid (Rabinovitz) and Ms. M. Armony for their helpful
discussion on the mathematical analysis of the experimental
results.
which can be converted by the expression of the binomium to
eq 23.
N
So ) [S](1 + K[A])
(23)
Combining eqs 21 and 23 yields eq 10.
The distribution of the excited state immediately after
photoexcitation of [S*An](0) is similar to that of the ground
state, as given by eq 24, where S*o represents the overall
concentration of excited state species.
JA952070P