Excited state distortion in photochromic ruthenium sulfoxide complexes

Article abstract of DOI:10.1021/ja9099399

A series of photochromic ruthenium sulfoxide complexes of the form [Ru(bpy)₂(OSOR)]⁺, where bpy is 2,2-bipyridine and OSOR is 2-(benzylsulfinyl)benzoate (OSOBn), 2-(napthalen-2-yl-methylsulfinyl)-benzoate (OSONap), or 2-(pentafluorophenylmethanesulfinyl)benzoate (OSOBnF₅), have been synthesized and characterized. In aggregate, the data are consistent with phototriggered isomerization of the sulfoxide from S-bonded to O-bonded. The S-bonded complexes feature ³MLCT absorption maxima at 388 nm (R = BnF₅), 396 nm (R = Bn), and 400 nm (R = Nap). Upon charge transfer excitation the S-bonded peak diminishes concomitant with new peaks growing in at ～350 and ～495 nm. Spectroscopic and electrochemical data suggest that the electronic character of the substituent on the sulfur affects the properties of the S-bonded complexes, but not the O-bonded complexes. The isomerization is reversible in methanol solutions and, in the absence of light, thermally reverts to the S-bonded isomer with biexponential kinetics. The quantum yields of isomerization (φ_s→o) were found to be 0.32, 0.22, and 0.16 for the R = BnF₅, Bn, and Nap complexes, respectively. Kinetic analyses of femtosecond transient absorption data were consistent with a nonadiabatic mechanism in which isomerization occurs from a thermally relaxed ³MLCT state of S-bonded (or n²-sulfoxide) character directly to the singlet O-bonded ground state. The time constants of isomerization (τ_s→o) were found to be 84, 291, and 427 ps for the R = BnF₅, Bn, and Nap complexes, respectively. Analysis of room temperature absorption and 77 K emission spectra reveal significant distortion between the S-bonded ground state (¹GS_S) and singlet metal-to-ligand charge transfer state (¹MLCT_S) and thermally relaxed ³MLCT, respectively. The distortion is primarily attributed to low frequency metal-ligand and S=O vibrational modes, which are intrinsically involved in the isomerization pathway.

Full text of DOI:10.1021/ja9099399

Published on Web 03/29/2010
Excited State Distortion in Photochromic Ruthenium
Sulfoxide Complexes
Beth Anne McClure, Eric R. Abrams, and Jeffrey J. Rack*
Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute,
Ohio UniVersity, Athens, Ohio 45701
Received November 24, 2009; E-mail: rackj@ohio.edu
Abstract: A series of photochromic ruthenium sulfoxide complexes of the form [Ru(bpy)2(OSOR)]+, where
bpy is 2,2′-bipyridine and OSOR is 2-(benzylsulfinyl)benzoate (OSOBn), 2-(napthalen-2-yl-methylsulfinyl)-
benzoate (OSONap), or 2-(pentafluorophenylmethanesulfinyl)benzoate (OSOBnF5), have been synthesized
and characterized. In aggregate, the data are consistent with phototriggered isomerization of the sulfoxide
3
from S-bonded to O-bonded. The S-bonded complexes feature MLCT absorption maxima at 388 nm (R
) BnF5), 396 nm (R ) Bn), and 400 nm (R ) Nap). Upon charge transfer excitation the S-bonded peak
diminishes concomitant with new peaks growing in at ∼350 and ∼495 nm. Spectroscopic and electrochemi-
cal data suggest that the electronic character of the substituent on the sulfur affects the properties of the
S-bonded complexes, but not the O-bonded complexes. The isomerization is reversible in methanol solutions
and, in the absence of light, thermally reverts to the S-bonded isomer with biexponential kinetics. The
quantum yields of isomerization (Φsfo) were found to be 0.32, 0.22, and 0.16 for the R ) BnF5, Bn, and
Nap complexes, respectively. Kinetic analyses of femtosecond transient absorption data were consistent
with a nonadiabatic mechanism in which isomerization occurs from a thermally relaxed 3MLCT state of
S-bonded (or η2-sulfoxide) character directly to the singlet O-bonded ground state. The time constants of
isomerization (τsfo) were found to be 84, 291, and 427 ps for the R ) BnF5, Bn, and Nap complexes,
respectively. Analysis of room temperature absorption and 77 K emission spectra reveal significant distortion
between the S-bonded ground state (1GSS) and singlet metal-to-ligand charge transfer state (1MLCTS) and
thermally relaxed 3MLCT, respectively. The distortion is primarily attributed to low frequency metal-ligand
and SdO vibrational modes, which are intrinsically involved in the isomerization pathway.
ligands21-24 may be triggered electrochemically by oxidation
or reduction, light is a preferred trigger to induce rapid switching
between states. Ideal characteristics of these complexes include
efficient conversion of photonic energy into potential energy
and a sufficiently long-lived metastable state. Our investigations
of photoswitchable complexes have focused on a class of
simultaneously electrochromic and photochromic ruthenium
Introduction
Photoswitchable molecules which can be converted between
two or more optically and electronically distinct states may be
used for applications in logic gates,1-3 molecular switches,4-10
and other photoresponsive materials.11-20 Although rotaxanes4-9
and certain transition metal complexes featuring ambidentate
(1) Andreasson, J.; Straight, S. D.; Bandyopdhyay, S.; Mitchell, R. H.;
Moore, T. A.; Moore, A. L.; Gust, D. Angew. Chem., Int. Ed. 2007,
46, 958–961.
(12) Schaniel, D.; Imlau, M.; Weisemoeller, T.; Woike, T.; Kramer, K. W.;
Gudel, H.-U. AdV. Mater. 2007, 19, 723–726.
(13) Yildiz, I.; Deniz, E.; Raymo, F. M. Chem. Soc. ReV. 2009, 38, 1859–
1867.
(14) Imlau, M.; Schieder, R.; Rupp, R. A.; Woike, T. Appl. Phys. Lett.
1999, 75, 16–18.
(15) Imlau, M.; Woike, T.; Schieder, R.; Rupp, R. A. Europhys. Lett. 2001,
53, 471–477.
(16) Schefer, J.; Schaniel, D.; Petricek, V.; Woike, T. Z. Kristallogr. 2008,
223, 259–264.
(17) Woike, T.; Schaniel, D. Z. Kristallogr. 2008, 223, IV.
(18) Roberts, M. N.; Carling, C.-J.; Nagle, J. K.; Branda, N. R.; Wolf,
M. O. J. Am. Chem. Soc. 2009, 131, 16644–16645.
(19) Roberts, M. N.; Nagle, J. K.; Finden, J. G.; Branda, N. R.; Wolf, M. O.
Inorg. Chem. 2009, 48, 19–21.
(2) Andreasson, J.; Straight, S. D.; Moore, T. A.; Moore, A. L.; Gust, D.
J. Am. Chem. Soc. 2008, 130, 11122–11128.
(3) Straight, S. D.; Liddell, P. A.; Terazono, Y.; Moore, T. A.; Moore,
A. L.; Gust, D. AdV. Funct. Mater. 2007, 17, 777–785.
(4) McNitt, K. A.; Parimal, K.; Share, A. I.; Fahrenbach, A. C.; Witlicki,
E. H.; Pink, M.; Bediako, D. K.; Plaiser, C. L.; Le, N.; Heeringa,
L. P.; Vander Griend, D. A.; Flood, A. H. J. Am. Chem. Soc. 2009,
131, 1305–1313.
(5) Periyasamy, G.; Sour, A.; Collin, J.-P.; Sauvage, J.-P.; Remacle, F. J.
Phys. Chem. B 2009, 113, 6219–6229.
(6) Silvi, S.; Venturi, M.; Credi, A. J. Mater. Chem. 2009, 19, 2279–
2294.
(7) Spruell, J. M.; Paxton, W. F.; Olsen, J.-C.; Benitez, D.; Tkatchouk,
E.; Stern, C. L.; Trabolsi, A.; Friedman, D. C.; Goddard, W. A.;
Stoddart, J. F. J. Am. Chem. Soc. 2009, 131, 11571–11580.
(8) Stoddart, J. F. Chem. Soc. ReV. 2009, 38, 1802–1820.
(9) Bonnet, S.; Collin, J.-P.; Koizumi, M.; Mobian, P.; Sauvage, J.-P. AdV.
Mater. 2006, 18, 1239–1250.
(20) Irie, M.; Kobatake, S.; Horichi, M. Science 2001, 291, 1769–1772.
(21) Hamaguchi, T.; Inoue, Y.; Ujimoto, K.; Ando, I. Polyhedron 2008,
27, 2031–2034.
(22) Johansson, O.; Johannissen, L. O.; Lomoth, R. Chem.sEur. J. 2009,
15, 1195–1204.
(10) Irie, M. Chem. ReV. 2000, 100, 1685–1716.
(23) Bitterwolf, T. E. Coord. Chem. ReV. 2006, 250, 1196–1207.
(24) Coppens, P.; Novozhilova, I.; Kovalevsky, A. Chem. ReV. 2002, 102,
861–883.
(11) Dieckmann, V.; Eicke, S.; Rack, J. J.; Woike, T.; Imlau, M. Opt.
Express 2009, 17, 15052–15060.
9
5428 J. AM. CHEM. SOC. 2010, 132, 5428–5436
10.1021/ja9099399  2010 American Chemical Society

Photochromic Ruthenium Sulfoxide Complexes
A R T I C L E S
Scheme 1. S-Bonded [Ru(bpy)2(OSOR)]+ Isomerizes to the Metastable O-Bonded Isomer upon Charge Transfer Irradiationa
a The O-bonded metastable isomer reverts thermally to the S-bonded isomer.
polypyridine sulfoxide complexes, which exhibit phototriggered
switching on the nanosecond to picosecond time scale.25-38 The
sulfoxide is S-bonded in the ground state and is switched to a
metastable O-bonded state upon either charge transfer irradiation
or oxidation of the ruthenium metal (Scheme 1).36 The dramatic
change in color arises from the difference in Ru-S and Ru-O
bonding.
As exemplified by rhodopsin, which undergoes cis-trans
isomerization with a time constant of 200 fs, a fast rate of
isomerization is due to efficient relaxation via vibrational modes
that lead directly to the photoproduct.39-42 Many such reactions,
including the conversion of [RuCl5(NO)]2- to one of two
metastable states (O-bonded or η2-NO), have been proposed to
occur via rapid nonradiative decay through conical intersections
involving two or more reaction coordinates between adjacent
potential energy surfaces.43,44 Although these transitions are
traditionally considered forbidden, higher order (e.g., spin orbit,
rotational modes) and nonadiabatic coupling mechanisms in-
crease the transition probability at the nuclear coordinates of
the conical intersection, allowing for transitions to occur within
a few vibrations.45-49 Furthermore, dynamics are more ef-
fectively described by the shape and properties of the potential
energy surfaces,45,50 suggesting that they are strongly influenced
by subtle structural modifications, solvent, or temperature.
As suggested by the work of McCusker and Damrauer,51-53
[Ru(bpy)3]2+ (bpy ) 2,2′-bipyridine) type complexes exhibit
rapid localization from the singlet metal-to-ligand charge transfer
(1MLCT) state to the 3MLCT excited state within ∼300 fs with
unit quantum efficiency due to a large spin orbit coupling
constant. Thus intramolecular vibrational relaxation, internal
conversion, and intersystem crossing occur simultaneously.
However, unlike photochromic complexes which feature large
excited state displacement of nuclear coordinates, complexes
like [Ru(bpy)3]2+ exhibit minimal displacement between the
Franck-Condon and thermally relaxed 3MLCT state geometries.
Thus we raise the question of whether similar electronic factors,
such as a large spin-orbit coupling constant, increase the
probability of transition to a metastable state through conical
intersections, thereby increasing the efficiency of phototriggered
isomerization in ruthenium polypyridine complexes. The present
work aims to analyze spectral properties of a series of photo-
chromic complexes in an attempt to establish a relationship
between the distortion energy of the 3MLCT and isomerization
reactivity. Furthermore, we hope to probe the effect of changing
the electronic and steric factors contributing to excited state
isomerization by variation of the substituent on the sulfur.
(25) Butcher, D. P.; Rachford, A. A.; Petersen, J. L.; Rack, J. J. Inorg.
Chem. 2006, 45, 9178–9180.
(26) McClure, B. A.; Mockus, N. V.; Butcher, D. P.; Lutterman, D. A.;
Turro, C.; Petersen, J. L.; Rack, J. J. Inorg. Chem. 2009, 48, 8084–
8091.
(27) Mockus, N. V.; Petersen, J. L.; Rack, J. J. Inorg. Chem. 2006, 45,
8–10.
Experimental Section
Materials. The compound cis-[Ru(bpy)2Cl2]·2H2O was synthe-
sized according to a published method.54 The reagents RuCl3 ·xH2O
and silver hexafluorophosphate (AgPF6) were purchased from Strem
and used as received. The reagents [Ru(bpy)3]Cl2 ·6H2O, 2,2′-
bipyridine (bpy), thiosalycylic acid, m-chloroperoxybenzoic acid,
triethylamine, benzyl bromide, 2,3,4,5,6-pentafluorobenzyl bromide,
2-(bromomethyl)-napthalene, sodium hydroxide, and concentrated
hydrochloric acid were purchased from Aldrich and used as
received. All solvents such as methanol, ethanol, diethyl ether,
(28) Mockus, N. V.; Rabinovich, D.; Petersen, J. L.; Rack, J. J. Angew.
Chem., Int. Ed. 2008, 47, 1458–1461.
(29) Rachford, A. A.; Petersen, J. L.; Rack, J. J. Inorg. Chem. 2005, 44,
8065–8075.
(30) Rachford, A. A.; Petersen, J. L.; Rack, J. J. Inorg. Chem. 2006, 45,
5953–5960.
(31) Rachford, A. A.; Rack, J. J. J. Am. Chem. Soc. 2006, 128, 14318–
14324.
(32) Rack, J. J.; Mockus, N. V. Inorg. Chem. 2003, 42, 5792–5794.
(33) Rack, J. J.; Rachford, A. A.; Shelker, A. M. Inorg. Chem. 2003, 42,
7357–7359.
(34) Rack, J. J.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2001, 123,
2432–2433.
(45) Domcke, W.; Yarkony, D. R.; Koppel, H. Conical Intersections:
Electronic Structure, Dynamics and Spectroscopy; World Scientific:
Singapore, 2004; Vol. 15.
(35) Rack, J. J. Z. Kristallogr. 2008, 223, 356–362.
(36) Rack, J. J. Coord. Chem. ReV. 2009, 253, 78–85.
(37) Mockus, N. V.; Marquard, S.; Rack, J. J. J. Photochem. Photobiol. A
2008, 200, 39–43.
(46) Barckholtz, T. A.; Miller, T. A. Int. ReV. Phys. Chem. 1998, 17, 435–
524.
(38) McClure, B. A.; Rack, J. J. Angew. Chem., Int. Ed. 2009, 48, 8556–
8558.
(47) Matsika, S.; Yarkony, D. R. J. Chem. Phys. 2002, 116, 2825–2835.
(48) Mayer, M.; Cederbaum, L. S. J. Chem. Phys. 1996, 105, 4938–4963.
(49) Worth, G. A.; Cederbaum, L. S. Annu. ReV. Phys. Chem. 2004, 55,
127–58.
(39) Kim, J. E.; McCamant, D. W.; Zhu, L.; Mathies, R. A. J. Phys. Chem.
B 2001, 105, 1240–1249.
(40) Mathies, R. A.; Cruz, C. H. B.; Pollard, W. T.; Shank, C. V. Science
1988, 240, 777–779.
(50) Klessinger, M.; Michl, J. Excited States and Photochemistry of Organic
Molecules; VCH: New York, 1995.
(41) Peteanu, L. A.; Schoenlein, R. W.; Wang, Q.; Mathies, R. A.; Shank,
C. V. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 11762–11766.
(42) Schoenlein, R. W.; Peteanu, L. A.; Mathies, R. A.; Shank, C. V.
Science 1991, 254, 412–415.
(51) Damrauer, N. H.; Cerullo, G.; Yeh, A.; Boussie, T. R.; Shank, C. V.;
McCusker, J. K. Science 1997, 275, 54–57.
(52) Damrauer, N. H.; McCusker, J. K. J. Phys. Chem. A 1999, 103, 8440–
8446.
(43) Baranovskii, V. I.; Sizova, O. V. J. Struct. Chem. 2008, 49, 803–809.
(44) Levine, B. G.; Martinez, T. J. Annu. ReV. Phys. Chem. 2007, 58, 613–
634.
(53) McCusker, J. K. Acc. Chem. Res. 2003, 36, 876–887.
(54) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17,
3334–3341.
9
J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010 5429

A R T I C L E S
McClure et al.
dichloromethane, chloroform, acetonitrile, and hexanes were pur-
chased from VWR and used without further purification. Tetrabu-
tylammonium hexafluorophosphate (TBAPF6) was purchased from
Aldrich and recrystallized three times from hot ethanol. Acetonitrile
for electrochemical experiments was HPLC grade (Burdick and
Jackson) and used without further purification. The synthesis and
isolation of all ruthenium sulfoxide complexes were carried out in
the dark or under red light.
[Ru(bpy)2(OSOBn)](PF6)·0.75H2O (OSOBn is 2-Benzylsulfi-
nyl-benzoic Acid). The complex [Ru(bpy)2(OSBn)](PF6) (101 mg,
0.124 mmol) and 60% m-chloroperoxybenzoic acid (57 mg, 0.199
mmol) were dissolved in 50 mL of dichloromethane and stirred at
room temperature for 25 h. The solvent was removed by rotary
evaporation, and the resulting residue was dissolved in a minimum
amount of methanol. Ether was added to precipitate the ruthenium
solid. The solid was isolated by vacuum filtration, rinsed with ether,
and air-dried. Yield: 86 mg (83%). UV-vis (MeOH) λmax ) 396
nm (6200 M-1 cm-1) S-bonded, 350 nm (8000 M-1 cm-1) and 496
nm (7800 M-1 cm-1) O-bonded. ν(SdO) ) 1097 cm-1 (S-bonded),
1022 cm-1 (O-bonded). E°′ Ru3+/2+ vs Ag/Ag+ (CH3CN) ) 0.90
V S-bonded, 0.53 and 0.34 V O-bonded. 1H NMR (d6-acetone, 300
MHz) δ: 9.61 (d, 1 H), 8.82 (d, 1 H), 8.68 (d, 1 H), 8.55 (m, 3 H),
8.37 (t, 1 H), 8.24 (d, 1 H), 8.00 (m, 4 H), 7.76 (d, 1 H), 7.56 (d,
1 H), 7.40 (m, 3 H), 7.25 (m, 3 H), 7.13 (t, 1 H), 7.00 (t, 2 H),
6.67 (d, 2 H), 3.92 (d, 1 H), 3.40 (d, 1 H) ppm. Elemental analysis,
calculated for [Ru(C10H8N2)2(C14H11O2S)]PF6 ·0.75H2O: C, 49.12%;
H, 3.46%; O, 7.22%; N, 6.74%. Found: C, 48.75%; H, 3.41%; O,
7.17%; N, 6.72%.
2-Benzylsulfanyl-benzoic Acid (OSBn). Thiosalicylic acid
(2.992 g, 19.4 mmol) and 2 equiv of sodium hydroxide (1.578 g,
39.4 mmol) were combined in methanol (75 mL). The solution was
stirred at room temperature for 10 min until a yellow solution
formed and the solids were dissolved. The methanol was removed
by rotorary evaporation yielding a light yellow solid. Acetone (100
mL) and benzyl bromide (2.53 mL, 21.3 mmol) were added to the
resulting solid. A white precipitate immediately formed upon
mixing, and the solution was sonicated for 5 min and then placed
in a -4 °C freezer for 1 h. The white precipitate was collected and
washed three times with cold diethyl ether. The resulting white
solid was dissolved in water (100 mL), and concentrated HCl was
added (1.8 mL) to yield a white precipitate. After being placed in
a -4 °C freezer for 1 h, the precipitate was collected by vacuum
filtration and washed with cold water. The white solid was then
[Ru(bpy)2(OSNap)](PF6). The complex cis-[Ru(bpy)2Cl2] (151
mg, 0.291 mmol), OSNap (105 mg, 0.357 mmol), silver hexafluo-
rophosphate (159 mg, 0.629 mmol), and triethylamine (100 µL)
were dissolved in 50 mL of ethanol and refluxed under nitrogen
for 18 h. The solution was cooled to -30 °C to ensure complete
precipitation of AgCl. The AgCl was filtered off and rinsed with
acetone until the filtrate was colorless. Solvent was removed by
rotary evaporation. The resulting residue was dissolved in a minimal
amount of ethanol. Hexanes were added to precipitate a red-orange
solid. The solid was isolated in via vacuum filtration and washed
with hexanes and air-dried. Yield: 241 mg (97%). UV-vis (MeOH)
λmax ) 456 nm (6700 M-1 cm-1). E°′ Ru3+/2+ vs Ag/Ag+ (CH3CN)
1
dried under vacuum overnight. Yield: 4.260 g (89.9%). H NMR
(d6-acetone, 300 MHz) δ: 8.05 (d, 1H), 7.52 (m, 4H), 7.27 (m,
4H), 4.27 (s, 2H) ppm.
2-(Naphthalen-2-ylmethylsulfanyl)-benzoic Acid (OSNap).
OSNap was synthesized by following a similar procedure as OSBn,
starting with thiosalicylic acid (0.701 g, 4.55 mmol) and using
2-(bromomethyl)naphthalene (1.067 g, 4.83 mmol) in place of the
benzyl bromide. Yield: 0.565 g (42.2%). 1H NMR (d6-acetone, 300
MHz) δ: 8.00 (d, 1 H), 7.98 (t, 1 H), 7.87 (m, 3 H), 7.60 (m, 2 H),
7.48 (m, 3 H), 7.21 (m, 1 H), 4.40 (s, 2 H) ppm.
1
) 0.61 V. H NMR (d6-acetone, 300 MHz) δ: 9.64 (d, 1 H), 8.87
(d, 1 H), 8.58 (m, 2 H), 8.46 (d, 1 H), 8.27 (m, 3 H), 8.05 (m, 3
H), 7.81 (m, 3 H), 7.40 (m, 10 H), 7.17 (t, 1 H), 7.07 (s, 1 H), 6.90
(d, 1 H), 4.15 (d, 1 H), 3.68 (d, 1 H) ppm.
2-Pentafluorophenylmethylsulfanyl-benzoic Acid (OSBnF5).
OSBnF5 was synthesized in a similar procedure to that for OSBn,
starting with thiosalicylic acid (1.015 g, 6.583 mmol) and substitut-
ing 2,3,4,5,6-pentafluorobenzyl bromide for the benzyl bromide.
[Ru(bpy)2(OSONap)](PF6)·H2O (OSONap is 2-(Naphthalen-
2-ylmethylsulfinyl)-benzoic Acid). The complex [Ru(bpy)2(OSNap)]-
(PF6) (88 mg, 0.101 mmol) and 60% m-chloroperoxybenzoic acid
(45 mg, 0.155 mmol) were dissolved in 30 mL of dichloromethane
and stirred at room temperature for 24 h. The solvent was removed
by rotary evaporation, and the resulting residue was dissolved in a
minimum amount of methanol. Ether was added to induce
precipitation. The solid was isolated by vacuum filtration, rinsed
with ether, and air-dried. Yield: 84 mg (94%). UV-vis (MeOH)
λmax ) 400 nm (6900 M-1 cm-1) S-bonded, 351 nm (8600 M-1
cm-1) and 496 nm (8600 M-1 cm-1) O-bonded. ν(SdO) ) 1093
cm-1 (S-bonded), 1022 cm-1 (O-bonded). E°′ Ru3+/2+ vs Ag/Ag+
1
Yield: 1.434 g (65.4%). H NMR (d6-acetone, 300 MHz) δ: 7.98
(d, 1 H), 7.60 (m, 2 H), 7.35 (m, 1 H), 4.33 (s, 2 H) ppm.
2-Pentafluorophenylmethanesulfinyl-benzoic Acid (OSOB-
nF5). The compound OSBnF5 (172 mg, 0.515 mmol) was dissolved
in 35 mL of chloroform. In a separate 35 mL of chloroform,
1
m-chloroperoxybenzoic acid (150 mg 60% peroxo reagent by H
NMR, 0.521 mmol) was dissolved and added slowly to the OSBnF5
solution. The combined solutions were stirred at room temperature
for 10 min. The solvent was removed by rotary evaporation.
Approximately 2 mL of diethyl ether were added to the resulting
residue, and the solid was isolated by vacuum filtration and air-
1
1
dried. Yield: 111 mg (62%). H NMR (d6-acetone, 300 MHz) δ:
(CH3CN) ) 0.90 V S-bonded, 0.53 V and 0.34 V O-bonded. H
NMR (d6-acetone, 300 MHz) δ: 9.52 (d, 1 H), 9.03 (d, 1 H), 8.71
(m, 2 H), 8.61 (d, 1 H), 8.53 (d, 1 H), 8.44 (t, 1 H), 8.22 (m, 1 H),
8.10 (m, 4 H), 7.95 (d, 1 H), 7.80 (d, 1 H), 7.54 (m, 8 H), 7.37 (t,
1 H), 7.27 (d, 2 H), 7.17 (s, 1 H), 6.74 (d, 1 H), 4.51 (d, 1 H), 4.20
(d, 1 H) ppm. Elemental analysis, calculated for [Ru(C10H8N2)2-
(C18H13O3S)]PF6 ·H2O: C, 51.50%; H, 3.53%; O, 7.22%; N, 6.33%.
Found: C, 51.42%; H, 3.21%; O, 6.93%; N, 6.34%.
8.21 (d, 1 H), 7.66-7.80 (m, 3 H), 4.68 (d, 1 H), 4.40 (d, 1 H)
ppm.
[Ru(bpy)2(OSBn)](PF6). The complex cis-[Ru(bpy)2Cl2] (197
mg, 0.379 mmol), OSBn (142 mg, 0.580 mmol), silver hexafluo-
rophosphate (211 mg, 0.835 mmol), and triethylamine (150 µL)
were dissolved in 100 mL of ethanol and refluxed for 6 h under
nitrogen. The solution changed from a deep purple to a red color.
The solution was cooled to -30 °C overnight to ensure complete
precipitation of AgCl. The AgCl was filtered off and rinsed with
acetone until the filtrate was colorless. Solvent was removed by
rotary evaporation. The resulting residue was dissolved in a minimal
amount of ethanol and acetone. Hexanes were added to precipitate
a red-orange solid. The solid was isolated in an approximately
quantitative yield via vacuum filtration and washed with hexanes
and air-dried. UV-vis (MeOH) λmax ) 458 nm (5900 M-1 cm-1).
E°′ Ru3+/2+ vs Ag/Ag+ (CH3CN) ) 0.61 V. 1H NMR (d6-acetone,
300 MHz) δ: 9.61 (d, 1 H), 8.82 (d, 1 H), 8.68 (d, 1 H), 8.55 (m,
3 H), 8.37 (t, 1 H), 8.24 (d, 1 H), 8.00 (m, 4 H), 7.76 (d, 1 H), 7.56
(d, 1 H), 7.40 (m, 3 H), 7.25 (m, 3 H), 7.13 (t, 1 H), 7.00 (t, 2 H),
6.67 (d, 2 H), 3.92 (d, 1 H), 3.40 (d, 1 H) ppm.
[Ru(bpy)2(OSOBnF5)](PF6)·1.2H2O. The complex cis-
[Ru(bpy)2Cl2] (56 mg, 0.108 mmol), OSOBnF5 (45 mg, 0.129
mmol), silver hexafluorophosphate (59 mg, 0.234 mmol), and
triethylamine (50 µL) were dissolved in 25 mL of ethanol and
refluxed under nitrogen for 19 h. The solution was cooled to -30
°C overnight to ensure complete precipitation of AgCl. The AgCl
was filtered off by vacuum filtration and rinsed with acetone.
Solvent was removed by rotary evaporation. The resulting solid
was dissolved in dichloromethane and extracted with a 10 mL
aqueous solution of 22 mg of LiOH · H2O to remove NEt3HPF6
formed during reaction. The dichloromethane layer was dried with
magnesium sulfate, and the solvent was removed by rotary
evaporation. The solid was dissolved in ∼2 mL ethanol. Ether was
9
5430 J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010

Photochromic Ruthenium Sulfoxide Complexes
A R T I C L E S
added to precipitate a yellow solid. The product was isolated via
vacuum filtration, washed with ether (3 × 15 mL), and air-dried.
Yield: 80 mg (82%). UV-vis (MeOH) λmax ) 388 nm (5970 M-1
cm-1) S-bonded, 350 nm (7800 M-1 cm-1) and 495 nm (7900 M-1
cm-1) O-bonded. ν(SdO) ) 1105 cm-1 (S-bonded), 1022 cm-1
(O-bonded). E°′ Ru3+/2+ vs Ag/Ag+ (CH3CN) ) 0.97 V S-bonded,
employed to generate the white light continuum probe source by
focusing the light through either a CaF2 or 3 mm sapphire plate.
Detection was achieved with a double CCD spectrograph over the
range ∼360-750 nm for CaF2 or ∼450-750 nm for sapphire.
Transient spectra at a particular delay time represent the average
of 4000 excitation pulses. The instrument is operated through an
in-house (BGSU) LabVIEW software routine. Kinetic analysis of
the data was performed at Ohio University (OU) with the SPECFIT
(version 3.0.37, Spectrum Software Associates) program, a global
analysis routine based on single value decomposition. Goodness-
of-fit was evaluated qualitatively by inspection of the residual plots.
For the S-bonded complex, samples were prepared in bulk methanol
solution and analyzed using a flow through 2 mm path length cell
with a flow rate of approximately 15-23 mL/s. For the O-bonded
isomer, a 2 mm path length steady state cell containing a small
portion of the same solution was used.
Quantum Yield of Isomerization. Quantum yield of isomer-
ization measurements were obtained by irradiating solutions of the
complexes [Ru(bpy)2(OSO-R)]+ in methanol at room temperature.
Photolysis was achieved using a PTI C-60 fluorimeter. The
wavelength of light was set to the lower energy isosbestic point
between S-bonded and O-bonded isomers. The incident radiation
intensity, Io, was determined by potassium ferrioxalate actinometry.
The quantum yield was determined according to eq 1 in a manner
similartothatpreviouslydescribedforphotosubstitutionreactions.29,58-61
In eq 1, CT and Cp are the total concentration and concentration of
O-bonded formed respectively, V is the volume of solution (3 mL),
ꢀ is the molar extinction coefficient at the isosbestic point, Io is the
incident radiation intensity, and l is the path length (1 cm). The
concentration of O-bonded isomer was determined using multilinear
regression analysis.
1
0.53 V O-bonded. H NMR (d6-acetone, 300 MHz) δ: 9.50 (d, 1
H), 9.02 (d, 1 H), 8.88 (d, 1 H), 8.79 (d, 1 H), 8.72 (d, 1 H), 8.60
(m, 2 H), 8.22 (t, 1 H), 8.12 (m, 4 H), 7.98 (d, 1 H), 7.73 (d, 1 H),
7.61 (m, 3 H), 7.49 (m, 2 H), 7.38 (t, 1 H), 4.40 (d, 1 H),
3.98 (d,
1 H) ppm. Elemental analysis, calculated for
[Ru(C10H8N2)2(C14H7O3SF5)]PF6 ·1.2 H2O: C, 43.94%; H, 2.65%;
N, 6.03%; S, 3.45%. Found: C, 43.58%; H, 2.84%; N, 6.38%; S,
3.61%.
[Ru(bpy)2(OSBnF5)](PF6). The synthesis and isolation of this
complex were similar to those for [Ru(bpy)2(OSOBnF5)](PF6). The
complex cis-[Ru(bpy)2Cl2] (224 mg, 0.431 mmol), OSBnF5 (160
mg, 0.479 mmol), silver hexafluorophosphate (220 mg, 0.870
mmol), and triethylamine (150 µL) were dissolved in 100 mL of
ethanol and refluxed under nitrogen for 4.5 h. The isolation and
purification of the product were carried out as described above.
Yield: 87 mg (23%). UV-vis (MeOH) λmax ) 448 nm (6500 M-1
1
cm-1). E°′ Ru3+/2+ vs Ag/Ag+ (CH3CN) ) 0.67 V. H NMR (d6-
acetone, 300 MHz) δ: 9.66 (d, 1 H), 8.92 (d, 1 H), 8.72 (t, 2 H),
8.62 (t, 2 H), 8.48 (t, 1 H), 8.26 (d, 1 H), 8.10 (m, 4 H), 7.87 (d,
1 H), 7.70 (d, 1 H), 7.46 (m, 3 H), 7.29 (m, 3 H), 4.08 (d, 1 H),
2.94 (d, 1 H) ppm.
Instrumentation. Cyclic voltammetry was performed on a CH
Instruments CHI 730A Electrochemical Analyzer. This workstation
contains a digital simulation package as part of the software package
to operate the workstation (CHI version 2.06). The working
electrode was either a glassy carbon disk (Cypress Systems, 1.5
mm) or platinum (BAS, 2.0 mm) electrode. The counter and
reference electrodes were Pt wire and Ag/Ag+, respectively.
Electrochemical measurements were performed in acetonitrile
solutions containing 0.1 M TBAPF6 electrolyte in a one-compart-
ment cell.
C V ln(C ) - ln(C - C )
[
]
T
T
T
p
Φ )
(1)
tIo(1 - 10-εC l
)
T
Results and Discussion
Electronic absorption spectra were collected on an Agilent 8453
spectrophotometer. Bulk photolysis experiments were conducted
using a 75 W xenon-arc lamp (Oriel) fitted with a Canon standard
camera UV filter. Corrected emission measurements were collected
at 77 K in 4:1 EtOH/MeOH solutions on a PTI-C60 fluorimeter
equipped with a Hamamatsu R928 PMT(185-900 nm). Quantum
yields of emission were determined using [Ru(bpy)3]Cl2 as a
reference. Proton nuclear magnetic resonance (1H NMR) spectra
were collected on a 300 MHz Bruker spectrometer in deuterated
chloroform (CDCl3) or deuterated acetone (d6-acetone). Infrared
spectroscopic measurements were collected on a Nicolet 380 FT-
IR spectrometer. Samples were prepared by evaporating a solution
of dichloromethane onto a 25 mm × 4 mm KBr plate. The ν(SdO)
stretch was assigned in the S-bonded isomer by comparison between
the thioether and sulfoxide complexes. The O-bonded spectrum was
achieved by irradiating the dichloromethane solution as described
above before evaporating the solution onto a KBr plate.
Picosecond transient absorption spectra were acquired at the Ohio
Laboratory for Kinetic Spectrometry (OLKS) as part of Bowling
Green State University’s (BGSU) Center for Photochemical Studies.
The experimental details were described previously, and only a brief
discussion is provided here.55-57 Spectra-Physics Hurricane Evolu-
tion and Ti:sapphire were combined to yield 800 nm pulses of 130
fs in duration at a rate of 1 kHz. The excitation wavelength of 400
nm (∼2 µJ/pulse) was obtained from frequency-doubling the Ti:
sapphire fundamental. A portion of the 800 nm fundamental was
We have previously reported the synthesis and spectral
properties of [Ru(bpy)2(OSO)]+, where OSO is 2-methanesul-
finylbenzoate.25,26 Analogous benzoate ligands (OS-R) have
been synthesized from the common starting material thiosalicylic
acid with benzyl bromide or 2-(bromomethyl)naphthalene. These
ligands were subsequently allowed to react with [Ru(bpy)2Cl2]
in the presence of triethylamine and 2 equiv of AgPF6 to yield
the corresponding thioether coordination complexes in high
yields. The thioether complexes were then oxidized to the
corresponding sulfoxide complexes by reaction with m-chlo-
roperoxybenzoic acid. The exception was the complex
[Ru(bpy)2(OSOBnF5)]+ which did not readily undergo oxidation
from the thioether to the sulfoxide. Alternatively, the OSBnF5
ligand was oxidized using m-chloroperoxybenzoic acid to yield
the sulfoxide ligand, OSOBnF5, which was subsequently allowed
to react with [Ru(bpy)2Cl2], triethylamine, and 2 equiv of AgPF6
to yield [Ru(bpy)2(OSOBnF5)]+. The synthesis of this complex
also required an extraction with an aqueous LiOH solution to
remove NEt3HPF6, which was not necessary with the complexes
[Ru(bpy)2(OSBn)]+ or [Ru(bpy)2(OSNap)]+. The identity and
1
formulation of the complexes was confirmed by H NMR and
IR spectroscopy and elemental analysis.
(58) Bonnet, S.; Collin, J.-P.; Sauvage, J.-P.; Schofield, E. Inorg. Chem.
2004, 43, 8346–8354.
(55) Nikolaitchik, A. V.; Korth, O.; Rodgers, M. A. J. J. Phys. Chem. A
1999, 103, 7587–7596.
(59) Hecker, C. R.; Fanwick, P. E.; McMillin, D. R. Inorg. Chem. 1991,
30, 659–666.
(56) Okhrimenko, A. N.; Gusev, A. V.; Rodgers, M. A. J. J. Phys. Chem.
A 2005, 109, 7653–7656.
(60) Kirchhoff, J. R.; McMillin, D. R.; Marnot, P. A.; Sauvage, J.-P. J. Am.
Chem. Soc. 1985, 107, 1138–1141.
(57) Pelliccioli, A. P.; Henbest, K.; Kwag, G.; Carvagno, T. R.; Kenney,
M. E.; Rodgers, M. A. J. J. Phys. Chem. A 2001, 105, 1757–1766.
(61) Suen, H.-F.; Wilson, S. W.; Pomerantz, M.; Walsh, J. L. Inorg. Chem.
1989, 28, 786–791.
9
J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010 5431

A R T I C L E S
McClure et al.
Table 1. Spectroscopic and Electrochemical Characterization for
[Ru(bpy)2(OSOR)]+
R
Bn
Nap
BnF5
λmax (S-bonded),
nm (ꢀ, M-1 cm-1
λmax (O-bonded),
nm (ꢀ, M-1 cm-1
396 (6220)
400 (6870)
388 (5970)
)
496 (7820)
7360
496 (8640)
6380
495 (7890)
7250
)
fwhm, cm-1, S-bonded
λmax,em (S-bonded),
nm (Φem), 77 K
576 (0.076) 580 (0.090) 572 (0.034)
E°′ (S-bonded), V
0.896
0.525
0.340
1097
0.896
0.528
0.336
1093
1022
0.974
0.533
E°′ (O-bonded), Va
ν(SdO), cm-1 (S-bonded)
1105
1022
ν(SdO), cm-1 (O-bonded) 1022
a We have observed what appears to be two O-bonded isomers by
cyclic voltammetry for the R ) Bn and Nap complexes, but not for the
R ) BnF5 complex. We are still investigating this result.
Figure 1. Absorption spectra of S-[Ru(bpy)2(OSOBnF5)]+ (black) and
[Ru(bpy)3]2+ (red) in methanol and normalized corrected 77 K emission of
S-[Ru(bpy)2(OSOBnF5)]+ in 4:1 EtOH/MeOH glass (experimental, blue;
calculated fit, red dashed). Experimental and calculated emission of R )
Bn and Nap and [Ru(bpy)3]2+ in Supporting Information.
The ν(SdO) stretches for the S-bonded complexes (Table 1)
agree well with literature values for other transition metal
dimethylsulfoxide (dmso) complexes.62,63 The electronic char-
acter of the substituent on sulfur appears to affect the ν(SdO)
stretch with higher frequencies corresponding to the more
electron-withdrawing groups. This is likely a through bond or
inductive effect since the opposite trend would be expected for
an enhancement of a π-back bonding interaction between the
sulfoxide group and the ruthenium metal center. The O-bonded
ν(SdO) stretches are somewhat higher than those listed in
literature for O-bonded dmso complexes but do not show any
variation with different substituents.
The absorption maxima of the lowest energy transition occur
at 388 nm (R ) BnF5), 396 nm (R ) Bn), and 400 nm (R )
Nap) (Table 1) for the S-bonded isomers, which are in close
agreement with the S-bonded isomer of the previously reported
[Ru(bpy)2(OSO)]+ complex (λmax ) 396 nm). Therefore, this
transition is assigned as an MLCT band of the S-bonded isomer
for these complexes. As for [Ru(bpy)2(OSO)]+, there is a
significant blue shift in the absorption maximum of the S-bonded
sulfoxide complexes relative to the thioether complexes as well
as [Ru(bpy)3]2+, indicative of stabilization of the ruthenium dπ
orbitals.64 This stabilization is mirrored in the shift in Ru3+/2+
reduction potentials (Table 1). As evidenced by the similar
absorption maxima and Ru3+/2+ reduction potentials, the stabi-
lization conferred in both the R ) Bn and Nap complexes
matches closely with that of [Ru(bpy)2(OSO)]+ in which the
substituent on the sulfur is a methyl group (R ) Me). This
suggests that despite the steric bulk of these groups, the effect
on the extent of Ru-S mixing, and therefore on the ground
state electronic structure, is negligible. In contrast, the R ) BnF5
complex exhibits a dramatic blue shift in the absorption
maximum for both the sulfoxide and thioether complexes (388
and 448 nm, respectively) relative to the corresponding R )
Bn complexes (396 and 458 nm, respectively). This indicates
that the electron-withdrawing fluorine atoms have an electronic
effect at the metal center through the sulfur, despite the aliphatic
methylene carbon connecting them. This electronic effect is
consistent with the relatively more positive Ru3+/2+ potentials
observed for this complex as well.
The absorption spectra of these complexes are notably broader
than the model complex [Ru(bpy)3]2+ (fwhm ) 3550 cm-1
,
MeOH) with full width at half-maximum (fwhm) values of 7360,
6380, and 7250 cm-1 for R ) Bn, Nap, and BnF5 (Figure 1),
respectively. The fwhm values were obtained by doubling the
difference (in cm-1) between the peak maximum and the point
at half of the maximum intensity on the low energy side.
Previous studies have described the MLCT manifold of
[Ru(bpy)3]2+ as a composite of four distinct transitions
(dπa1fπ*1a2, dπa1fπ*1e, dπefπ*1a2, dπefπ*1e).65,66 Due to
the pseudo-C2 reduced symmetry of our complexes, the
dπefπ*e transitions will be further split, yielding more transi-
tions. Despite this, TD-DFT calculations of [Ru(bpy)2(OSO)]+
have found closely spaced transitions at 422 nm (f ) 0.0625)
and 379 nm (f ) 0.0623) which overlap in solution to give a
single band with a peak at 396 nm, which we have assigned as
a dπfπ* transition.26 For simplicity we have chosen to treat
the spectra of [Ru(bpy)3]2+ and [Ru(bpy)2(OSOR)]+ as a single
transition (see below). While this approach may overestimate
the width of the bands, it will at least provide a qualitative
comparison of the compounds.
The fwhm is proportional to the nuclear displacement (∆Q)
between the equilibrium geometries of the ground state (1GS)
and the 1MLCT state.67 The distortion or reorganization energy
(λ*, Scheme 2) is proportional to fwhm squared and is typically
classified into outer sphere or solvent (λo), intraligand (λIL), and
metal-ligand (λML) contributions as described in eq 2, in which
hνIL and hνML are intraligand (bpy ring modes) and metal-ligand
vibrational (Ru-N, Ru-S and Ru-O) modes, respectively.67,68
The values for λo and λIL are expected to be similar to those for
[Ru(bpy)3]2+, whereas λML should be greater due to the change
in bonding ligation in isomerizable compounds. Assuming
approximate values of 500, 1350, 1300, and 500 cm-1 for λo,
λIL, hνIL, and hνML, respectively, based on results for
[Ru(bpy)3]2+ and other ruthenium polypyridine complexes,65,68
(65) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1984, 23, 3877–3886.
(66) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967–3977.
(67) Solomon, E. I.; Lever, A. B. P. Inorganic Electronic Structure and
Spectroscopy; John Wiley & Sons: New York, 1999.
(68) Winkler, J. R.; Netzel, T. L.; Creutz, C.; Sutin, N. J. Am. Chem. Soc.
1987, 109, 2381–2392.
(62) Alessio, E. Chem. ReV. 2004, 104, 4203–4242.
(63) Calligaris, M. Coord. Chem. ReV. 2004, 248, 351–375.
(64) Lutterman, D. A.; Rachford, A. A.; Rack, J. J.; Turro, C. J. Phys.
Chem. A 2009, 113, 11002–11006.
9
5432 J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010

Photochromic Ruthenium Sulfoxide Complexes
A R T I C L E S
Scheme 2. Potential Energy Surfaces for Ground State (1GS),
indicating the formation of the O-bonded isomer.25,26,69 The
reaction is depicted in Scheme 1. What is notable is that despite
differences in the S-bonded absorption maxima, the O-bonded
maxima are essentially the same for all three complexes. This
suggests that the electronic structure of the S-bonded isomer is
more sensitive to substituent effects than the O-bonded isomer.
The minimal variation of the O-bonded Ru3+/2+ reduction
potentials (Table 1) further support this conclusion.
a
3
1MLCT, and MLCT
Quantum yields of isomerization were collected and were
relatively large for all complexes indicating a rapid rate of
isomerization (Table 2).31 Comparing the R ) Bn, Nap, and
Me complexes (R ) Me, Φsfo) 0.45), there is a trend of
decreasing quantum yield with bulkier substituents. Comparing
the R ) BnF5 and Bn complexes, the more electron-withdrawing
BnF5 group results in a higher quantum yield of isomerization.
These results indicate that the efficiency of isomerization is
affected by both steric and electronic factors.
Transient absorption spectroscopy was employed to determine
the kinetics of phototriggered isomerization. Selected time traces
from the transient absorption spectra for S-[Ru(bpy)2(OSOBnF5)]+
are shown in Figure 2. The initial traces (red) feature a bleach
near 450 nm corresponding to the depletion of the ground state
MLCT absorption, concomitant with a broad absorption at
longer wavelengths which are attributed to the ligand-to-metal
charge transfer (LMCT) transition from the neutral bpy to the
Ru3+ center and a π*fπ* transition arising from the reduced
bpy. These features are characteristic of a thermally relaxed
3MLCT excited state.51-53,70-75 This state is achieved within
approximately 1 ps. Subsequent traces show a decay in these
features as well as a positive absorbance growing in at 500 nm.
In the last time trace (blue), the broad long wavelength
absorbance has decayed back to zero and only the absorbance
feature centered at 500 nm remains. This spectrum qualitatively
agrees with the difference between the S-bonded and O-bonded
ground state absorption spectra (Figure 2, inset). Due to an
approximate isosbestic point as well as a lack of a bleach feature
near 500 nm, which would be indicative of an O-bonded 3MLCT
excited state, the transient absorption data suggest that isomer-
a Eop is optical energy for MLCT transition, E0 is the difference between
zeroth vibrational levels of 1GS and 3MLCT, and ∆ST is singlet-triplet
1
1
energy splitting, λ* is distortion energy between GS and MLCT.
eq 2 predicts the metal-ligand contribution λML to be 10 800,
14 400, and 14 800 cm-1 for R ) Nap, BnF5, and Bn,
respectively. By this analysis the metal-ligand contribution is
clearly the dominant mode of distortion. The values of E0, the
zero-zero energy, may be calculated from E0 ) Eop - λ* -
∆ST, where Eop is the optical absorption maximum; λ* is the
sum of λo, λIL, and λML; and ∆ST is the triplet-singlet energy
gap (3000 cm-1) as reported for [Ru(bpy)3]2+,66,68 to yield values
of 9350, 6520, and 5600 cm-1 for R ) Nap, BnF5, and Bn,
respectively. The E0 values correspond to approximate emission
maxima at wavelengths of 1070, 1530, and 1780 nm for R )
Nap, BnF5, and Bn, respectively. No emission was observed
for these complexes at room temperature over the range
500-850 nm, and further measurements are needed to see if
there is emission in the NIR. However, the calculated values
are significantly red-shifted from the emission maxima (λem)
observed at 77 K with values of 580, 576, and 572 nm for R )
Nap, Bn, and BnF5, respectively. For comparison, our analysis
of [Ru(bpy)3]2+ yields values of λML ) 510 cm-1, E0 ) 16 860
cm-1, and λem ) 590 nm, which agree reasonably well with
previously reported values of λML ) 500 cm-1 and λem ) 620
nm (16 130 cm-1), thus validating our single transition ap-
proximation employed in this approach.68,69 This analysis shows
that the low frequency metal-ligand vibrations strongly affect
the appearance of the absorption spectrum and that the potential
energy surfaces are sensitive to temperature.
3
ization proceeds directly from a S-bonded MLCT state to the
O-bonded 1GS. Transient absorption data for R ) Bn and Nap
complexes are qualitatively similar (Supporting Information).
Such a transition requires not only significant changes in nuclear
coordinates but also a change in spin state. These multidimen-
sional reaction coordinates typically occur at conical intersec-
tions and are thus termed nonadiabatic.45
Kinetic analysis of the transient absorption data for the R )
BnF5 complex (Figure 2b) produces a rate constant k1 of 3.7
((0.5) × 1010 s-1. In accord with a nonadiabatic mechanism,
this rate corresponds to the decay from the thermally relaxed
3
S-bonded MLCT state to both the S-bonded and O-bonded
ground states, ks and ksfo, respectively. Thus, for the R ) BnF5
complex with a quantum yield of 0.32, the rate constant of
(fwhm)2
5.55
hνML
2kT
) λo(2kT) + λML hν coth
+ λ (hν )
IL IL
(
(
))
ML
(2)
(70) Bhasikuttan, A. C.; Suzuki, M.; Nakashima, S.; Okada, T. J. Am. Chem.
Soc. 2002, 124, 8398–8405.
(71) Curtright, A. E.; McCusker, J. K. J. Phys. Chem. A 1999, 103, 7032–
7041.
Upon charge transfer irradiation, the absorption peak near
400 nm diminishes in intensity concomitant with a growth in
absorption near ∼350 and ∼495 nm (Table 1). These features
are consistent with other Ru(bpy)2 complexes with two O
donors, such as [Ru(bpy)2(OH2)2]2+ and O-[Ru(bpy)2(OSO)]+,
(72) McFarland, S. A.; Lee, F. S.; Cheng, K. A. W. Y.; Cozens, F. L.;
Schepp, N. P. J. Am. Chem. Soc. 2005, 127, 7065–7070.
(73) Ramakrishna, G.; Jose, D. A.; Kumar, D. K.; Das, A.; Palit, D. K.;
Ghosh, H. N. J. Phys. Chem. B 2006, 110, 10197–10203.
(74) Wallin, S.; Davidsson, J.; Modin, J.; Hammarstrom, L. J. Phys. Chem.
A 2005, 109, 4697–4704.
(69) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von
(75) Yeh, A. T.; Shank, C. V.; McCusker, J. K. Science 2000, 289, 935–
938.
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85–277.
9
J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010 5433

A R T I C L E S
McClure et al.
Table 2. Kinetic Results for [Ru(bpy)2(OSOR)]+
R
Bn
Nap
BnF5
Φsfo
0.22
0.16
0.32
k1 (S-bonded), s-1
ksfo, s-1 (τsfo, ps)
k2, k3 (O-bonded), s-1
1.56 × 1010
1.42 × 1010
3.7 × 1010
3.43 × 109 (291)
2.27 × 109 (427)
1.2 × 1010 (84)
8.6 × 1010
8.3 × 1010
1.2 × 1011
5.0 × 109
4.8 × 109
5.9 × 109
k4, k5 (thermal, 30 °C), s-1
5.74 ((0.06) × 10-4
3.08 ((0.10) × 10-6
5.98 ((0.04) × 10-4
4.66 ((0.09) × 10-6
3.32 ((0.12) × 10-4
4.13 ((0.19) × 10-5
Figure 2.
[Ru(bpy)2(OSOBnF5)]+ in methanol. Starting in red time traces are: 5.0,
10.2, 20.3, 50.7, 100.3, 495.3 ps. Inset: Absorption spectra, ꢀ (M-1 cm-1
vs wavelength (nm) of S-bonded (red), O-bonded (blue) and O-S (black).
(b) Kinetic trace at 500 nm yields τsfo ) 84 ps.
(a) Picosecond transient absorption of S-bonded
)
Figure 3. (a) Picosecond transient absorption of O-bonded [Ru(bpy)2-
(OSOBnF5)]+ in methanol. Starting in red time traces are: 1.0, 2.0, 5.0,
10.3, 20.4, 50.4, 101.1, 203.0, 508.0, 1008 ps. (b) Kinetic trace at 500 nm
yields biexponential decay with time constants of 8.4 and 170 ps.
isomerization (ksfo) is 1.2 × 1010 s-1 yielding a time constant
for isomerization (τsfo) of 84 ((11) ps. The isomerization time
constants for the R ) Bn and Nap complexes are 291 ((5) ps
and 427 ((24) ps, respectively. The k1 values of both of the R
) Bn and Nap complexes are similar (Table 2) and agree well
with that of the R ) Me complex (1.48 × 1010 s-1).26 Thus the
slower rate of isomerization arises mainly from the smaller
quantum yields of isomerization. Comparatively, the k1 value
for the R ) BnF5 complex is faster by a factor of ∼2.5 in
addition to a quantum yield that is greater than both of the R )
Bn and Nap complexes. Despite the greater quantum yield for
the R ) Me complex (0.45), the k1 value is sufficiently fast
enough for R ) BnF5 that τsfo is faster (84 ps compared to
150 ps). The rate of isomerization for R ) BnF5 is the fastest
we have observed for any ruthenium sulfoxide complex and is
comparable to the rate of isomerization in stilbene, which has
a time constant of 42 ps76 in methanol and a rate constant of
∼1010 s-1 in other solvents.77 Fast isomerization rates indicate
an efficient conversion of photonic energy into potential energy
for photochemical bond cleavage and formation. This is
presumably due in part to the large nuclear displacement
exhibited by these complexes.
The O-bonded transient absorption spectra for the R ) BnF5
complex are shown in Figure 3. The initial traces exhibit a
bleach feature centered at 500 nm as well as a broad positive
absorption at longer wavelengths to the limit of detection. We
(76) Courtney, S. H.; Balk, M. W.; Philips, L. A.; Webb, S. P.; Yang, D.;
Levy, D. H.; Fleming, G. R. J. Chem. Phys. 1988, 89, 6697–6707.
(77) Waldeck, D. H. Chem. ReV. 1991, 91, 415–436.
9
5434 J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010

Photochromic Ruthenium Sulfoxide Complexes
A R T I C L E S
Table 3. 77 K Corrected Emission Spectra Fitting Parameters
E0,
cm-1
hνm,
hνl,
fwhm,
cm-1
Complex
cm-1
cm-1
Sm
Sl
[Ru(bpy)3]2+
17 500 1400
17 200 1150
17 200 1150
400
-
0.8 1.0
600
1450
1450
1600
[Ru(bpy)2(OSOBn)]+
[Ru(bpy)2(OSONap)]+
1.4
1.4
1.2
-
-
-
-
[Ru(bpy)2(OSOBnF5)]+ 17 300 1300
-
in Table 2. As for other chelating sulfoxide complexes25,28 the
rate of thermal reversion for these complexes is much slower
than previously studied dmso complexes29,30 and is attributed
to fewer degrees of freedom imposed by the chelating sulfoxide
ligand. Comparing R ) Bn and Nap complexes, k4 values are
similar whereas for k5 values the R ) Nap complex is faster.
This is counterintuitive since the sterically bulkier group would
be expected to inhibit the thermal reversion and proceed at a
slower rate. Comparison of R ) Bn and BnF5 complexes is not
straightforward. The k4 value is greater in the R ) Bn complex,
but the k5 value is greater in the R ) BnF5 complex. This
suggests there is a more complicated electronic effect on the
rate of thermal reversion parameters. Further studies will
elucidate these effects.
The 77 K corrected emission spectra provide further evidence
of a large displacement between ground state and thermally
relaxed excited state structures. The sulfoxide complexes all
exhibit particularly large Stokes shifts (7890 cm-1 R ) Bn, 7760
cm-1 R ) Nap, 8290 cm-1 R ) BnF5). These shifts are
significantly larger than those observed for the model complex
[Ru(bpy)3]2+ (∼ 6000 cm-1, H2O)81 as well as the corresponding
thioether complexes (5360 cm-1 R ) Bn, 5460 cm-1 R ) Nap,
5680 cm-1 R ) BnF5).
Figure 4. Thermal reversion of [Ru(bpy)2(OSOBnF5)]+ in methanol at 30
°C. Arrows indicate reaction progression from O-bonded (red) to S-bonded
(blue). Inset: Kinetics fit to biexponential decay with rate constants of 3.32
((0.12) × 10-4 s-1 and 4.13 ((0.19) × 10-5 s-1
.
have interpreted this as the formation of the thermally relaxed
O-bonded 3MLCT within the first ∼1 ps (red trace). Unlike the
S-bonded complex, further time traces show a spectrum
indistinguishable from the zero line (blue trace), which is
indicative of decay to only the O-bonded ground state surface.
The decay is biexponential with time constants of τ2 ) ∼10 ps
and τ3) ∼ 200 ps (Figure 3b). As we have described previously,
the longer time constant is consistent with the decay of the
O-bonded 3MLCT to the ground state for other complexes, and
the shorter time constant may be attributed to some minor
rearrangement prior to decay to the ground state.26
Corrected emission spectra were normalized and fit using eq
382 (Figure 1; Supporting Information) to obtain values of E0,
fwhm, Sm, and Sl, in which E0 and fwhm are defined as in eq 2,
and Sm and Sl are the electron-vibrational coupling constants or
Huang-Rhys parameters for the medium frequency, hνm, and
low frequency, hνl, modes, respectively. Values for [Ru(bpy)3]2+
were found to be in close agreement with previously reported
values.83 The spectrum was fit to two modes, one medium
frequency intraligand mode (1400 cm-1) and one low frequency
metal-ligand mode (400 cm-1). Unlike the 200 K spectrum,
the 77 K spectrum of [Ru(bpy)3]2+ requires the inclusion of a
low energy vibrational mode to adequately model the data.83,84
In contrast, the 77 K spectra for [Ru(bpy)2(OSOR)]+ complexes
were fit using only one vibrational mode and Huang-Rhys
parameter (Table 3). The most notable feature is that the fwhm
of these complexes are significantly larger than that for
[Ru(bpy)3]2+. In accord with the absorption spectra, a large fwhm
value is indicative of substantial nuclear displacement between
the equilibrium ground state and 3MLCT state geometries. The
medium frequency mode (hνm) has shifted to lower energy from
1400 cm-1 to 1150-1300 cm-1, which is in relatively close
agreement with the ν(SdO) stretch obtained by IR spectroscopy
(Table 1). We interpret the shift to lower energy of the medium
frequency mode as inclusion of the SdO stretch as a significant
The proposed nonadiabatic isomerization mechanism requires
that the thermally relaxed S-bonded excited state geometry is
significantly displaced from the ground state. This is consistent
with the broad absorption spectra for these complexes. A
reasonable structure is an η2-type or asymmetric η2 structure.
To our knowledge, this bonding has not been observed in any
sulfoxide complex, although it has been observed in the
metastable states of complexes containing SO2.78-80 Briefly,
irradiation of single crystals of [Ru(NH3)4(L)(SO2)]2+ (L ) NH3
or H2O) yields both η2-SO2 and O-bonded SO2 metastable
coordination modes. Clearly, the transition states for these
reactions must involve some asymmetric η2-type bonding.
Consistent with other complexes of this class, the metastable
O-bonded isomers were found to thermally revert in methanol
solution at room temperature (Figure 4). In accord with
[Ru(bpy)2(OSO)]+, the reversion kinetics exhibit biexponential
behavior (rate constants: k4 ∼10-4 s-1, k5 ∼10-6 s-1, inset Figure
2); however, unlike this complex, there are two sets of isosbestic
points. These results are consistent with the interpretation of
two O-bonded isomers found electrochemically. While these
may be diastereomers, it is difficult to explain the disparity in
rate constants. Thus we propose that the fast rate is some
molecular rearrangement preceding OfS isomerization. The rate
constants of thermal reversion at 30 °C in methanol are listed
(81) Yoon, S.; Kukura, P.; Stuart, C. M.; Mathies, R. A. Mol. Phys. 2006,
104, 1275–1282.
(78) Bowes, K. F.; Cole, J. M.; Husheer, S. L. G.; Raithby, P. R.; Savarese,
T. L.; Sparkes, H. A.; Teat, S. J.; Warren, J. E. Chem. Commun. 2006,
2448–2450.
(82) Barqawi, K. R.; Murtaza, Z.; Meyer, T. J. J. Phys. Chem. 1991, 95,
47–50.
(79) Kovalevsky, A. Y.; Bagley, K. A.; Cole, J. M.; Coppens, P. Inorg.
Chem. 2003, 42, 140–147.
(83) Caspar, J. V.; Westmoreland, T. D.; Allen, G. H.; Bradley, P. G.;
Meyer, T. J.; Woodruff, W. H. J. Am. Chem. Soc. 1984, 106, 3492–
3500.
(80) Kovalevsky, A. Y.; Bagley, K. A.; Coppens, P. J. Am. Chem. Soc.
2002, 124, 9241–9248.
(84) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444–2453.
9
J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010 5435

A R T I C L E S
McClure et al.
The results presented herein are summarized in Figure 5. The
difference between 1GSS and 1GSO is determined electrochemi-
cally. Following absorption of a photon by 1GSS to form
1MLCTS, internal conversion, intersystem conversion, and
3
intramolecular vibrational relaxation occur to yield a MLCT-
1
η2 SO state. Relaxation from this state proceeds back to GSS
with a rate constant of ks and to 1GSO with the rate constant of
isomerization, ksfo. The sum of these two terms is k1 as listed
in Table 2. Following excitation to 1MLCTO from 1GSO,
relaxation occurs biexponentially with rate constants k2 and k3
1
1
(Table 2). GSO thermally reverts to GSS with rate constants
k4 and k5 (Table 2).
Figure 5. Modified Jablonksi type diagram for [Ru(bpy)2(OSOR)]+. 1GSS
and 1GSO are the S-bonded and O-bonded ground states. 1MLCTS and
1MLCTO are S-bonded and O-bonded excited states. The O-bonded 3MLCT
state is denoted as a dashed line since the relative energy is not known.
3
The MLCT state is formed within ∼1 ps with substantial
nuclear displacement from the initially produced 1MLCT state.
Our kinetic analysis demonstrates that SfO isomerization
occurs rapidly and efficiently on a picosecond time scale. While
our spectroscopic analysis does not reveal a clear relationship
between distortion energy and isomerization reactivity, it does
show significant excited state mixing from low energy
metal-ligand and SdO vibrational modes with the electronic
transitions. We propose that SfO isomerization or surface
hopping is enhanced by this distortion and occurs through a
conical intersection formed from a multidimensional reaction
coordinate. Future studies will further investigate these
relationships.
component of excited state deactivation. Due to the poor
vibrational resolution in the spectrum, we cannot distinguish
the relative contributions of these modes. The Sm values are
also increased relative to [Ru(bpy)3]2+. This suggests greater
coupling between the electronic states and medium frequency
vibrational mode, again indicating that the SdO stretching mode
is strongly coupled to excited state relaxation.
5
15
3
E0 - mhνm - lhνl
Iν )
∑ ∑
(
)
E0
m)0 l)0
Acknowledgment. We thank Dr. Aaron Rachford for helpful
discussions in preparing this manuscript. We are grateful to Dr.
Evgeny Danilov for experimental assistance at the Ohio Laboratory
for Kinetic Spectrometry at BGSU and for access to the femto-
second transient absorption spectrometer employed in these studies.
J.J.R. acknowledges NSF (CHE 0809669), Ohio University,
Condensed Matter and Surface Science (CMSS), and the Nano-
BioTechnology Initiative (NBTI) for funding. B.A.M. recognizes
NDSEG for a fellowship.
2
ν - E0 + mhνm + lhνl
SmSl exp -4 ln 2 ·
(
)
m
l
[
]
fwhm
(3)
m! · l!
However, the results of the emission and absorption spectral
data are not directly comparable because the absorption spectrum
1
1
quantifies the distortion energy between the GS and MLCT
potential energy surfaces, whereas the emission spectrum
quantifies the distortion energy between the thermally relaxed
1
3MLCT and GS potential energy surfaces. Furthermore, there
Supporting Information Available: Transient absorption
spectra of S-bonded and O-bonded [Ru(bpy)2(OSOBn)]+ and
[Ru(bpy)2(OSONap)]+ and associated kinetic traces, 77 K
emission spectra with calculated fits of [Ru(bpy)3]2+, S-[Ru(bpy)2-
(OSOBn)]+, and S-[Ru(bpy)2(OSONap)]+. This material is
available free of charge via the Internet at http://pubs.acs.org.
are significant differences due to solvent/matrix and temperature
(300 K compared to 77 K). The fwhm values of the room
temperature absorption spectra are a factor of 4-5 larger than
those for the corresponding 77 K emission spectra. Thus it
appears that the potential energy surfaces and distortion energy
between the ground state and 3MLCT are strongly temperature
and solvent dependent, as is expected for a nonadiabatic
isomerization.
JA9099399
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5436 J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010