A new electron-transfer donor for photoinduced electron transfer in polypyridyl molecular assemblies

Article abstract of DOI:10.1021/ic980544q

A synthetic procedure has been devised for the preparation of the reductive quencher ligand 4-methyl-4′-(N-methyl-p-tolylaminomethyl)-2,2′-bipyridine (dmb-tol), which contains toluidine covalently bound to 2,2′-bipyridine. When bound to Re^I in [Re^I(dmb-tol)(CO)₃Cl], laser flash Re^I → dmb metal-to-ligand charge-transfer (MLCT) excitation at 355 nm in CH₃CN at 298 ± 2 K is followed by efficient, rapid (<5 ns) appearance of a transient with an absorption feature at 470 nm. The transient spectrum is consistent with formation of the redox-separated state, [Re^I(dmb^--tol⁺)(CO)₃Cl], which returns to the ground state by back electron transfer with k_ET = (1.05 ± 0.01) × 10⁷ s^-1 (τ = 95 ± 1 ns) at 298 ± 2 K. Rapid, efficient quenching is also observed in the Ru^II complex [Ru(4,4′-(C(O)NEt₂)₂bpy)₂(dmb-tol)] ²⁺. Based on transient absorption measurements, a rapid equilibrium appears to exist between the initial metal-to-ligand charge-transfer excited state and the redox-separated state, which lies at higher energy. Decay to the ground state is dominated by back electron transfer within the redox-separated state which occurs with k > 4 × 10⁸ s^-1 at 298 ± 2 K.

Full text of DOI:10.1021/ic980544q

Inorg. Chem. 1999, 38, 1193-1198
1193
A New Electron-Transfer Donor for Photoinduced Electron Transfer in Polypyridyl
Molecular Assemblies
Colleen M. Partigianoni, Sandrine Chodorowski-Kimmes, Joseph A. Treadway,
Durwin Striplin, Scott A. Trammell, and Thomas J. Meyer*
Department of Chemistry, University of North Carolina at Chapel Hill, CB# 3290,
Chapel Hill, North Carolina 27599-3290
ReceiVed May 14, 1998
A synthetic procedure has been devised for the preparation of the reductive quencher ligand 4-methyl-4′-(N-
methyl-p-tolylaminomethyl)-2,2′-bipyridine (dmb-tol), which contains toluidine covalently bound to 2,2′-bipyridine.
When bound to ReI in [ReI(dmb-tol)(CO)3Cl], laser flash ReI f dmb metal-to-ligand charge-transfer (MLCT)
excitation at 355 nm in CH3CN at 298 ( 2 K is followed by efficient, rapid (<5 ns) appearance of a transient
with an absorption feature at 470 nm. The transient spectrum is consistent with formation of the redox-separated
state, [ReI(dmb--tol+)(CO)3Cl], which returns to the ground state by back electron transfer with kET ) (1.05 (
0.01) × 107 s-1 (τ ) 95 ( 1 ns) at 298 ( 2 K. Rapid, efficient quenching is also observed in the RuII complex
[Ru(4,4′-(C(O)NEt2)2bpy)2(dmb-tol)]2+. Based on transient absorption measurements, a rapid equilibrium appears
to exist between the initial metal-to-ligand charge-transfer excited state and the redox-separated state, which lies
at higher energy. Decay to the ground state is dominated by back electron transfer within the redox-separated
state which occurs with k > 4 × 108 s-1 at 298 ( 2 K.
Introduction
(4-picolyl)phenothiazine (py-PTZ) and 4-(10-methylphenothi-
azine)-4′-methyl-2,2′-bipyridine (bpy-PTZ),1,4,9,10 in which E1/2
Pyridyl and bipyridyl ligands derivatized by addition of
chemically linked electron-transfer donors1-3 and acceptors4-6
have been used extensively in studies of photoinduced electron
transfer.1-10 Commonly used electron-transfer donors are 10-
for the PTZ+/0 couple is 0.85 V vs SSCE in 1,2-dichloroethane.10d
In [(bpy)Re(CO)3(py-PTZ)]+, metal-to-ligand charge transfer
(MLCT), ReI f bpy, excitation is followed by intramolecular
(1) (a) Chen, P.; Westmoreland, D. T.; Danielson, E.; Schanze, K. S.;
Anthon, D.; Neveux, P. E.; Meyer, T. J. Inorg. Chem. 1987, 26, 1116.
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Peek, B. M.; Schoonover, J. R.; McCafferty, D. G.; Wall, C. G.;
Erickson, B. W.; Meyer, T. J. J. Am. Chem. Soc. 1993, 115, 5479. (e)
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(5) (a) Ryu, Chong, K.; Wang, R.; Schmehl, R. H.; Ferrere, S.; Ludwikow,
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M.; Kelley, D. F. J. Am. Chem. Soc. 1988, 110, 6673. (c) Danielson,
E.; Elliott, C. M.; Merkert, J. M.; Meyer, T. J. J. Am. Chem. Soc.
1987, 109, 2519. (d) Elliott, C. M.; Freitag, R. A.; Blaney, D. D. J.
Am. Chem. Soc. 1985, 107, 4647.
(6) (a) Claude, J. P.; Williams, D. S.; Meyer, T. J. J. Am. Chem. Soc.
1996, 118, 9782. (b) Katz, N. E.; Mecklenburg, S. L.; Meyer, T. J.
Inorg. Chem. 1995, 34, 1282. (c) Yonemoto, E. H.; Saupe, G. B.;
Schmehl, R. H.; Hubig, S. M.; Riley, R. L.; Iverson, B. L.; Mallouk,
T. E. J. Am. Chem. Soc. 1994, 116, 4786. (d) Goulle, V.; Harriman,
A.; Lehn, J. M. J. Chem. Soc., Chem. Commun. 1993, 1034. (e)
Berthon, R. A.; Colbran, S. B.; Moran, G. M. Inorg. Chim. Acta 1993,
204, 3. (f) Deronzier, A.; Essakalli, M. J. Chem. Soc., Chem. Commun.
1990, 242. (g) Schanze, K. S.; Sauer, K. J. Am. Chem. Soc. 1988,
110, 1180. (h) Matsuo, T.; Sakamoto, T.; Takuma, K.; Sakura, K.;
Ohsako, T. J. Phys, Chem. 1981, 85, 1277.
(7) (a) Kalyanasundaram, K. Photochemistry of Polypyridine and Por-
phyrin Complexes; Academic Press: New York, 1992. (b) Sauvage,
J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, C.; Balzani,
V.; Barigelletti, F.; DeCola, L.; Flamigni, L. Chem. ReV. 1994, 94,
993. (c) Peterson, J. D.; Gahan, S. L.; Rasmussen, S. C.; Ronco, S. E.
Coord. Chem. ReV. 1994, 132, 15. (d) Cabana, L. A.; Schanze, K. S.
AdV. Chem. Ser. 1989, 226, 101. (e) Balzani, V.; Scandola, F. In
Photoinduced Electron-Transfer Part D, Inorganic Substrates and
Applications; Fox, M. A., Chanon, M., Eds.; Elsevier: New York,
1988; pp 148-78. (f) Schmehl, R. H.; Ryu, C. K.; Elliott, C. M.;
Headford, C. L. E.; Ferrere, S. AdV. Chem. Ser. 1989, 228, 211.
(8) (a) Harriman, A.; Obodel, F.; Sauvage, J.-P. J. Am. Chem. Soc. 1994,
116, 5481. (b) Collin, J.-P.; Harriman, A.; Heitz, V.; Odobel, F.;
Sauvage, J.-P. J. Am. Chem. Soc. 1994, 116, 5679.
(9) (a) Rutherford, T. J.; Keene, F. R. Inorg. Chem. 1997, 36, 2872. (b)
Treadway, J. A.; Chen, P.; Rutherford, T. J.; Keene, F. R.; Meyer, T.
J. J. Phys. Chem. 1997, 101, 6824. (c) Shaver, R. J.; Perkovic, M.
W.; Rillema, D. P.; Woods, C. Inorg. Chem. 1995, 34, 5446. (d)
Strouse, G. F.; Schoonover, J. R.; Duesing, R.; Meyer, T. J. Inorg.
Chem. 1995, 34, 2725. (e) Jones, W. E., Jr.; Bignozzi, C. A.; Chen,
P.; Meyer, T. J. Inorg. Chem. 1993, 32, 1167. (f) Collin, J.-P.;
Guillerez, S.; Sauvage, J.-P.; Barigelleti, F.; DeCola, L.; Flamigni,
L.; Balzani, V. Inorg. Chem. 1992, 31, 4112. (g) Larson, S. L.; Cooley,
L. F.; Elliott, C. M.; Kelley, D. F. J. Am. Chem. Soc. 1992, 114, 9504.
(h) Duesing, R.; Tapolksy, G.; Meyer, T. J. J. Am. Chem. Soc. 1990,
112, 5378.
10.1021/ic980544q CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/20/1999

1194 Inorganic Chemistry, Vol. 38, No. 6, 1999
Partigianoni et al.
4-carboxy-4-methyl-2,2′-bipyridine (1 g, 1 equiv), and N-methylmor-
pholine (3 mL) were combined in 50 mL of dry THF. The argon-
sparged solution was stirred at room temperature for 6 h. The suspension
was filtered and THF removed from the filtrate by rotary evaporation.
The residue was extracted into ether and washed three times with acidic
aqueous solutions at pH ∼ 5, five times with saturated aqueous
NaHCO3, and twice with saturated NaCl. The organic layer was dried
over anhydrous MgSO4 and freed of solvent by rotary evaporation.
The product, 4-CH3-4′-[C(O)N(CH3)-p-C6H4CH3]-2,2′-bipyridine, was
purified by column chromatography with silica as the solid support.
Excess N-methyltoluidine was removed by eluting with neat CH2Cl2,
and the desired product (0.9 g) collected with 3% MeOH/CH2Cl2 as
the eluant. The product was characterized by 1H NMR in CDCl3. The
amide-linked toluidine shows distinguishing resonances vs TMS at 6.95
ppm (4H, m), 2.21 ppm (3H, s) and amino methyl resonances at 3.45
ppm (3H, s). Bipyridine resonances appear at 2.42 ppm (3H, s), 7.13
ppm (2H, d), 8.11 ppm (1H, s), 8.3 (1H, s), and 8.45 ppm (2H, m).
The borane reduction was carried out by using a method comparable
to that previously described, but with some significant modifications.15
The amide was dissolved in 50 mL of dried THF, and 20 mL of a 1.0
M THF solution of BH3SMe2 (10 equiv) was added slowly to the
deaerated solution. The solution was stirred under Ar at room
temperature for 72 h. Precipitation of the borane adduct of the reduced
ligand was induced by slow addition of dry ether.15c The borane adduct
was collected by filtration and then hydrolyzed to yield the free amine
and boric acid by stirring in MeOH for 3 h. The methanol was
completely removed in vacuo, leaving the borate acidic salt of the
amine. Attempts to extract the neutral amine from basic aqueous media
resulted in decomposition and poor yields. Alternatively, the acidic salt
was suspended in toluene and neutralized by addition of excess
isopropylamine. The toluene suspension was filtered and the solvent
completely removed from the filtrate by rotary evaporation. The final
product (0.33 g, 24%) was purified by using column chromatography
with silica gel as the solid support and 1% MeOH/CH2Cl2 as the eluant.
PTZ f Re(II) electron transfer, to give the redox-separated state,
[(bpy-)Re(CO)3(py-PTZ+)]+, which decays by bpy•- f PTZ•+
back electron transfer. Assemblies of this kind have played an
important role in the study of electron transfer in the inverted
region,10 and there is a continuing need for the development of
new electron-transfer units for incorporation into molecular
assemblies.
Schanze and co-workers have utilized aromatic amine deriva-
tives covalently bound to pyridine.2 As an extension of their
work, we have developed a general synthetic route for covalently
attaching aniline derivatives to bipyridine, which, in principle,
can provide a wide range of reduction potentials by varying
substituents on the aniline.11 We report here the preparation of
the first member of this series, 4-methyl-4′-(N-methyl-p-
tolylaminomethyl)-2,2′-bipyridine (dmb-tol), which has a reduc-
tion potential comparable to the PTZ+/0 couple, and its use in
the study of photoinduced electron transfer.
1
The appearance of the methylene H resonance at 4.55 ppm (2H, s)
Experimental Section
and the chemical shifts vs TMS of both the toluidine aromatic
resonances of the final product, 6.65 ppm (2H, d) and 7.01 ppm (2H,
d), and the amino methyl resonance, 3.05 (3H, s), distinguish the final
product from the amide-linked presursor. An additional toluidine
resonance appears at 2.21 (3H, s), along with bipyridine resonances at
2.42 ppm (s, 3H), 7.15 ppm (m, 2H), 8.26 ppm (s, 1H), 8.31 ppm (s,
1H), 8.46 ppm (d, 1H), and 8.56 ppm (d, 1H). Anal. Calcd for
C20H21N3O3‚1.5H2O: C, 72.70; H, 7.32; N, 12.72. Found: C, 72.17;
H, 6.64; N, 12.00.
Materials. The ligand 4,4′-bis(diethylcarbamoyl)-2,2′-bipyridine
(4,4′-(C(O)NEt2)2bpy) and the complexes [Re(dmb)(CO)3Cl] and Ru-
[4,4′-(C(O)NEt2)2bpy]2Cl2 were prepared as described previously.12-14
The reagents 4,4′-dimethyl-2,2′-bipyridine (GFS Chemicals), N-meth-
yltoluidine (ACROS Chemicals), BOP ) 1-benzotriazoleoxy)tris-
(dimethylamino)phophonium hexafluorophosphate (Nova Biochem),
Re(CO)5Cl (Pressure Chemicals), NH4PF6 (Aldrich), and BH3SMe2/
1.0 M in THF (Aldrich) were used as received. N-Methylmorpholine
was purchased from Aldrich and distilled from CaH2 prior to use.
Reagent grade solvents ether, toluene, and THF, used in the prepara-
tions, were dried by distillation from Na/benzophenone under nitrogen.
Spectral grade CH3CN (Burdick & Jackson) was used for electrochemi-
cal and spectroscopic measurements. The supporting electrolyte tetra-
n-butylammonium hexafluorophosphate (TBAH) was purchased from
Aldrich and recrystallized twice from a H2O/ethanol (2:1 v/v) mixture.
Solutions for transient absorption studies were prepared by using
spectral grade solvents which had been heated at reflux over and
distilled from CaH2 and then stored under vacuum over CaH2.
Preparations. 4-Methyl-4′-(N-methyl-p-tolylaminomethyl)-2,2′-
bipyridine (dmb-tol). Covalent attachment of the N-methyltoluidine
donor to bipyridine was achieved by amide coupling of the amine
followed by borane reduction. In a typical amide coupling reaction,
N-methyltoluidine (0.55 mL, 1.2 equiv), BOP (3.0 g, 1.1 equiv),
[Re(dmb-tol)(CO)3Cl] (1). This complex was prepared by using a
standard procedure utilizing carbonyl displacement from Re(CO)5Cl.13
In a typical preparation, Re(CO)5Cl (0.21 g, 1.0 equiv) and dmb-tol
(0.20 g, 1.2 equiv) were combined in 20 mL of dry toluene. The
deaerated mixture was heated at reflux under Ar for 1.5 h. The yellow
precipitate (0.34 g, 96%) was filtered and washed with ether. Anal.
Calcd for C23H21N3O3ClRe: C, 45.35; H, 3.48; N, 6.82. Found: C,
45.04; H, 3.40; N, 6.90. IR spectrum: ν(CO) 2025, 1907, 1859 cm-1
.
1H NMR (CD3CN, ppm, vs TMS): δ 2.15 (s, 3H), δ 2.53 (s, 3H),
δ 3.07 (s, 3H), δ 4.64 (s, 2H), δ 6.67 (d, 2H), δ 7.01 (d, 2H) δ 7.42
(m, 2H) δ 8.25 (s, 1H), δ 8.30 (s, 1H), δ 8.82 (d, 2H) δ 8.89 (d, 2H).
[Ru(4,4′-(C(O)NEt2)2bpy)2(dmb-tol)](PF6)2 (2). The complex Ru-
[4,4′-(C(O)NEt2)2bpy]2Cl2 (40 mg, 4.3 × 10-5 mol) and dmb-tol (12
mg, 4 × 10-5 mol) were dissolved in an argon-sparged mixture of
ethanol (5 mL) and H2O (0.25 mL). The solution was heated at reflux
under argon for 6 h, cooled to room temperature, and concentrated
under vacuum. A saturated aqueous solution of KPF6 was added, and
the resulting orange precipitate was filtered and washed with H2O and
diethyl ether. The salt was purified initially with column chromatog-
raphy (alumina, CH2Cl2/0.5% MeOH); yield ) 30 mg, 53%. The
product was eluted from the column as quickly as possible in order to
avoid decomposition. Final purification was accomplished by using
preparatory HPLC chromatography by utilizing linear gradient elution
(10) (a) Chen, P. Y.; Mecklenburg, S. L.; Meyer, T. J. J. Phys. Chem.
1993, 97, 13126. (b) Chen, P.; Mecklenburg, S. L.; Duesing, R.; Meyer,
T. J. J. Phys. Chem. 1993, 97, 6811. (c) Chen, P.; Duesing, R.; Graff,
D. K.; Meyer, T. J. J. Phys. Chem. 1991, 95, 5850. (d) Chen, P.;
Duesing, R.; Tapolsky, G.; Meyer, T. J. J. Am. Chem. Soc. 1989, 111,
8305.
(11) Mann; Barnes. Electrochemical Reactions in Nonaqueous Systems;
Bard, Ed.; Marcel Dekker Inc.: New York , 1970; pp 272-278.
(12) Elliott, C. M.; Hershenhart, E. J. J. Am. Chem. Soc. 1982, 104, 7519.
(13) Worl, L. A.; Duesing, R.; Chen, P.; Ciana, L. D.; Meyer, T. J. J. Chem.
Soc., Dalton Trans. 1991, 849.
(15) (a) Brown, H. C.; Narasimhan, S.; Choi, Y. M. Synthesis 1981, 996.
(b) Krishnamurthy, S. Tetrahedron Lett. 1982, 23, 3315. (c) The borane
adduct is described in ref 15a.
(14) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17,
3334.

New Electron-Transfer Donor
Inorganic Chemistry, Vol. 38, No. 6, 1999 1195
with 0-200 mM KBr in 1:3 (v/v) CH3CN/H2O. After elution, the
complex was precipitated as the hexafluorophosphate salt by addition
of a saturated aqueous KPF6 solution followed by slow removal of
acetonitrile under vacuum. Even after repeated HPLC purification, a
small amount (<3%) of impurity was detected in the analytical HPLC.
The amount of the impuritiy slightly increased after the second
purification cycle. The observation that the impurity is formed during
purification cycles indicates that the impurity is most likely an
irreversible oxidative decomposition product of the toluidine. 1H NMR
(CD2Cl2, ppm, vs TMS): δ 1.18 (m, 24H), δ 2.2 (s, 3H), δ 2.51 (s,
3H), δ 3.04 (s, 3H), δ 3.42 (m, 16H), δ 4.67 (s, 2H), δ 6.64 (d, 2H, J
) 8.6 Hz), δ 7.01 (d, 2H, J ) 8.2 Hz), 7.34 (m, 2H), δ 7.4 (m, 4H),
δ 7.55 (d, 1H, 5.8 Hz), δ 7.57 (d, 1H, J ) 5.7 Hz), δ 7.78 (d, 2H, J
) 5.7 Hz), δ 7.88 (d, 1H, J ) 5.7 Hz), δ 7.9 (d, 1H, J ) 5.7 Hz), δ
8.15 (s, 1H), δ 8.26 (s, 1H), δ 8.36 (s, 4H).
Emission lifetimes were measured by using a PRA nitrogen laser
model LN 1000 and a PRA dye laser model LN102 combination for
sample excitation. The emission was monitored at a right angle to the
excitation source with a McPherson 272 grating monochromator and a
Hamamatsu R928 PMT operating at 700 V. Transient absorption
difference spectra were measured by using a Quanta Ray PDL-2 pulsed
laser dye (Coumarin 460 dye) pumped by a Quanta-Ray DCR-2A Nd:
YAG laser with an excitation pulse of ∼6 ns at <5 mJ/ pulse. The
excitation beam was coincident to an Applied Photophysics laser kinetic
spectrometer, which utilized at 150 W pulsed Xe arc lamp as a probe
beam, quartz optics, f/3.4 grating system, and a five-stage, cooled PMT.
The outputs for both time-resolved experiments were collected with a
LeCroy 7200A digital oscilloscope interfaced to a IBM PC. Time-
resolved data were fit by utilizing a global minimization routine based
on a modified Levenburg-Marquardt nonlinear, least-squares iterative
procedure.
FABMS: M - 2PF6 (theoretical) ) 1113.48, M - 2PF6 (sample) )
1113.4852 ∆M ) 3.4 ppm. Mass-to-charge ratio: m/z (theoretical) )
556.74, m/z (sample) ) 556.7
Results
Measurements and Instrumentation. UV-visible spectra were
measured in CH3CN on a Hewlett-Packard 8452D diode array spec-
trophotometer. FT-IR spectra were recorded in KBr disks with a
Mattson Galaxy Series FTIR 5020. 1H NMR spectra were recorded on
a Bruker WM250 spectrometer. Chemical shifts are reported as ppm
vs TMS at 20 °C. FAB-MS measurements were obtained by using a
JEOL HX110HF mass spectrometer at North Carolina State University.
The resolving power was 10 000, the accelerating power was 10 keV,
and the ion source temperature was 40 °C. HPLC purification was
carried out by using cation exchange chromatography on a Brownlee
CX-300 Prep 10 column. The elutions were controlled by a Rainin
Dynamax SD-300 solvent delivery system equipped with 25 mL/min
pump heads and monitored by a Shimadzu SPD-M10AV diode array
UV-visible spectrophotometer fitted with a 4.5 mm path length flow
cell. Elemental analyses were performed by Oneida Research Services.
The dmb-tol•+ radical cation was prepared in situ by stopped-flow
mixing of separate CH3CN solutions containing dmb-tol (1.2 × 10-4
M) and (NH4)2Ce(NO3)6 (2.4 × 10-4 M) at 25 °C, in a Hi-Tech
Scientific Double Mixing stopped-flow mixing apparatus. Constant
temperature was maintained by use of a Lauda RM6 circulating water
bath. UV-visible kinetic measurements of the radical cation were made
by use of a Hi-Tech CU-61 rapid scanning spectrophotometer attached
to the stopped-flow mixing apparatus by using the KinetAsyst 2.0
software program. A total of 200 scans were collected over 1.5 s. An
increase in absorption due to the radical cation (λmax ) 478 nm) appears
within the first 15 ms. It disappeared completely within 1.5 s. The molar
absorptivity of the radical cation was determined from the initial
concentration of dmb-tol, assuming complete conversion. To test the
latter assumption, a 10-fold excess of Ce(IV) was used, which gave no
increase in the calculated concentration of radical cation.
Cyclic voltammograms collected at 100 mV/s were obtained in 0.1
M TBAH/CH3CN solutions with a Princeton Applied Research 273
potentiostat/galvanostat interfaced to a personal computer. A silver/
silver nitrate (0.1 M) reference electrode, a platinum wire auxiliary
electrode, and a BAS MF-2013 platinum disk working electrode were
used in a three-compartment cell. Solutions were deaerated by bubbling
with N2 for 10 min. Potentials were measured relative to the ferrocene
couple (0.40 V vs SCE) and are reported vs SCE.16
All photophysical measurements were obtained in 1 cm path length
quartz cells at 298 ( 2 K. Solutions for steady-state and time-resolved
emission spectra were sparged with argon for 1 h prior to data collection.
Solutions for transient absorption studies were subjected to six freeze-
pump-thaw cycles under high vacuum.
Covalent attachment of N,N-dimethyltoluidine to 2,2′-bipyr-
idine leads to a significant increase in the reduction potential,
E1/2 (D+/0) ) 0.86 V vs SCE in CH3CN, relative to free N,N-
dimethyltoluidine, E1/2 (D+/0) ) 0.65 V. As has been previously
reported in the literature, the radical cation is not sufficiently
stable to prepare quantitatively by bulk electrolysis,19 but cyclic
voltammograms were chemically reversible at 100 mV/s with
the ratio of anodic-to-cathodic peak currents ∼1. Quantitative,
in situ preparation was achieved by chemical oxidation with
Ce(IV) by using rapid stopped-flow mixing. The absorption
spectrum of the resulting dmb-tol•+ radical cation in CH3CN
has a visible absorption at λmax ) 478 nm, ꢀ ) 4200 M-1 cm-1
,
and is similar to the literature spectrum for the N,N-dimethyl-
aniline radical cation generated at low temperatures in an ethanol
glass.20 The ligand is stable in the solid state in air and in the
absence of light for extended periods. Problems with irreversible
decomposition of the RuII complex were encountered over
extended periods in water.
Relevant ground-state and photophysical properties obtained
in CH3CN solution at 298 ( 2 K for [Re(dmb-tol)(CO)Cl], 1,
and [Ru(4,4′-(C(O)NEt2)2bpy)2(dmb-tol)]2+, 2, are summarized
in Table 1. ∆G° , the free energy of the excited state above the
ES
ground state, was calculated from the emission spectra of [Re-
(dmb)(CO)3Cl] and [Ru(4,4′-(C(O)NEt2)2bpy)2(dmb)]2+ by ap-
plication of a standard Franck-Condon analysis,21 which gave
∆G° ) 2.58 ( 0.08 eV for the ReII(dmb-) excited state and
ES
∆G° ) 2.07 ( 0.06 eV for the RuIII(4,4′-(C(O)NEt2)2bpy-)
ES
excited state.21 From these values and the ground-state reduction
potentials for [Re(dmb)(CO)3Cl]0/- and [Ru(4,4′-(C(O)NEt2)2-
bpy)2(dmb-tol)]2+/+ couples in Table 1, E°′ ) 1.12 ( 0.1 V for
the excited-state couple [ReII(dmb-)(CO)Cl]* + e- f [ReI-
(dmb-)(CO)Cl]- and E°′ ) 0.88 ( 0.1 V for the excited-state
couple [RuIII(4,4′-(C(O)NEt2)2bpy-)(4,4′-(C(O)NEt2)2bpy)-
(dmb)]2+* + e- f [RuII(4,4′-(C(O)NEt2)2bpy-)(4,4′-(C(O)-
NEt2)2bpy)(dmb)]+.
(17) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583.
(18) Emission quantum yields were calculated by the equation, φem
)
φ′em(I/I′)(A′/A). I and I′ are integrated emission intensities for the
sample and a standard. A and A′ are the absorbances at the excitation
wavelength. φ′em is the quantum yield of the standard, [Ru(bpy)3]-
(PF6)2, φ′em ) 0.062.17
Corrected steady-state emission spectra were recorded on a SPEX
fluorolog photon-counting fluorimeter interfaced to a SPEX DM1B
computer. The spectra were corrected for instrument response by the
(19) (a) Seo, E. D.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy,
D. W.; Adams, R. N. J. Am. Chem. Soc. 1966, 88, 3448. (b) Seo, E.
T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy, O. W.; Adams,
R. N. J. Am. Chem. Soc. 1967, 89, 447.
(20) (a) Williams, K. P. J.; Hester, R. E. J. Raman Spectrosc. 1981, 10,
161. (b) Chow, Y. L.; Danen, W. C.; Nelson, S. F.; Rosenblatt, D. H.
Chem. ReV. 1978, 78, 243. (c) Kimura, K.; Yoshinaga, K.; Tsubomura,
H. J. Phys. Chem 1967, 71, 4485.
procedure supplied by the manufacturer. Emission quantum yields (φem
)
of optically dilute solutions (OD ∼ 0.1 at the excitation wavelength)
were measured relative to [Ru(bpy)3](PF6)2 (φ′em ) 0.062) in CH3-
CN.17,18
(16) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.

1196 Inorganic Chemistry, Vol. 38, No. 6, 1999
Partigianoni et al.
Table 1. Electrochemical and Spectral Data in CH3CN at 298 ( 2 K
E1/2(V)a
UV-visible spectra λmax, nm
d
complex or compound
(ReII/I or RuIII/II
)
D+/0,b
bpy0/-,c
(ꢀ × 10-3, M-1 cm-1
)
dmb-tol
[Re(dmb)(CO)3Cl]
0.86
1.35
1.45
1.39
-1.44
-1.42
-1.21
-1.33
-1.19
-1.37
364 (3.63); 290 (13.2); 254 (22.9)
366 (2.99); 292 (13.3); 254 (23.0)
464 (12.8); 293 (64.6); 250 (37.8)
[Re(CO)3(dmb-tol)Cl]
0.91
[Ru[4,4′-(C(O)NEt2)2bpy]2(dmb)]2+
[Ru[4,4′-(C(O)NEt2)2bpy]2(dmb-tol)]2+
1.35
0.89
466 (14.8); 294 (68.0); 250 (49.8)
a In 0.1 M TBAH vs SCE; (0.05 V. b Potential for the D+/0 couple localized on the toluidine donor. c Potential for the bpy0/- dmb-based couple
for the Re complexes and 4,4′-(C(O)NEt2)2bpy-based for the Ru complexes. d (2 nm, (200 M-1 cm-1
.
Figure 1. Transient absorption difference spectrum of [Re(dmb)-
(CO)3Cl] in CH3CN at room temperature collected 10 ns after laser
flash excitation at 355 nm at 2 mJ/pulse. Inset shows the ground-state
absorption spectrum of the radical cation, dmb-tol+, in CH3CN
generated chemically by oxidation with CeIV (see text).
Figure 2. Transient absorption difference spectrum of [Ru[4,4′-(C(O)-
NEt2)2bpy]2(dmb)]2+ in CH3CN at 298 ( 2 K; excitation at 460 nm,
2.5 mJ/pulse, collected 10, 20, 30, and 60 ns after the laser pulse.
Compared to [Re(dmb)(CO)3Cl], for which φem ) 0.0058,
and k(298 ( 2 K) ) 2.0 × 107 s-1 (τ ) 49 ns) for excited-state
decay,13 emission from [Re(dmb-tol)(CO)3Cl], 1, is completely
quenched with φem < 10-5. Transient absorption measurements
following laser flash photolysis in CH3CN (λexc ) 355 nm, 2
mJ/pulse) reveal the appearance of a transient formed within
the laser pulse (<5 ns) which is characterized by a new ab-
sorption feature centered at 480 nm, Figure 1. Time resolution
of the transient decay based on measurements from 370 to 640
nm gave k(298 ( 2 K) ) (1.05 ( 0.01) × 107 s-1 (τ ) 95 (
1 ns), independent of monitoring wavelength.
sample purification procedures, but typically with φem ∼ 0.006.
The decay of this emission can be fit to the biexponential
function, I(t) ) a exp(k1t) + (1 - a) exp(k2t), with k1 ) (9.1 (
0.2) × 105 s-1 and k2 ) (3.8 ( 0.2) × 107 s-1. I0 is the emission
irradiance at t ) 0. Transient absorption measurements con-
ducted in CH3CN (λexc ) 460 nm, 2.5 mJ/pulse) gave the
difference spectrum shown in Figure 2. The decay kinetics
monitored at 380 and 480 nm were exponential and gave k(298
( 2 K) ) (3.8 ( 0.2) × 107 s-1, τ ) 26 ( 2 ns. In the difference
spectrum at 10 ns there appears to be a slight distortion at 450-
420 nm due to light scattering from the 460 nm excitation
pulse.
For [Ru(4,4′-(C(O)NEt2)2bpy)2(dmb)]2+ in CH3CN, emission
is observed at 653 nm, which decays with k(298 ( 2 K) ) 1.0
× 106 s-1 (τ ) 997 ns). In contrast to 1, some emission is
observed from [Ru(4,4′-(C(O)NEt2)2bpy)2(dmb-tol)]2+ (2),
centered at 664 nm, whose intensity varies somewhat with
Discussion
Based on the photophysical data, ReI f dmb metal-to-ligand
charge transfer in [ReI (dmb-tol)(CO)3Cl] (1) is followed by
rapid, essentially complete toluidine f ReII intramolecular
electron-transfer quenching. Quenching occurs within the time
resolution of the apparatus used (<5 ns). Evidence for the
formation of the ligand-based, redox-separated state, [ReI-
(dmb--tol+)(CO)Cl], appears in the transient absorption differ-
ence spectrum in the absorption feature at λmax ) 478 nm. This
feature coincides with the absorption maximum for dmb-tol•+
generated by CeIV oxidation (Figure 1).
These observations are consistent with the sequence of
reactions in Scheme 1 which occur following MLCT excitation.
In the scheme, ReI f dmb flash photolysis is followed by rapid
(k > 2 × 108 s-1) tol f ReII electron transfer to give the redox-
separated state, [ReI(dmb--tol+)(CO)Cl]. Based on the excited-
state reduction potential for [ReI(dmb-)(CO)Cl]*, estimated by
emission spectral fitting,21 this reaction is favored by ∼0.25
eV. The approximate free energy of the redox-separated state,
(21) A standard Franck-Condon analysis was applied to the emission
spectra as described elsewhere.13,22 The single mode approximation
was used in the fits. For the Ru complex, pω was fixed at 1300 cm-1 22
.
For the Re complex, pω was fixed at 1450 cm-1 13
. These are average
values reflecting the contributions of a series of ν(bpy) ring stretching
modes from 1000 to 1650 cm-1 with pω ) ∑j Sj pωj/∑jSj. The Sj and
pωj are the electron-vibrational coupling constants and quantum
spacings for the contributing vibrations, j.22,23 The other parameters
in the fits were S ) ∑jSj, E0, the energy difference between the ground
and excited states in their lowest vibrational levels, and ∆νj1/2, the
bandwidth at half-maximum. The term ∆νj1/2 includes the solvent
reorganizational energy (λo) and coupled low-frequency modes treated
2
classically (λi,L) with (∆νj1/2
)
) (16kBT ln 2)λo,L, λo,L ) λo + λi,L
.
The free energy of the excited state above the ground state, ∆G° ,
ES
was calculated from ∆GE°S ) E0 + (∆νj1/2)2/(16kBT ln 2). For [Re-
(dmb)(CO)Cl], E0 ) 16 400 cm-1, SM ) 1.2, and ∆νj1/2 ) 3200 cm-1
.
For [Ru[4,4′-(C(O)NEt2)2bpy]2(dmb)]2+, E0 ) 15 500 cm-1, S ) 1.1,
and ∆νj1/2 ) 1700 cm-1. Estimated errors are E0 (( 5%), S ((10%),
and ∆νj1/2 ((15%). These parameters are strongly correlated in their
influence on the calculated band shapes. Appropriate combinations
of parameters calculated in the ranges cited gave reasonable fits, but
∆GE°S ((3%) was relatively well-defined.

New Electron-Transfer Donor
Inorganic Chemistry, Vol. 38, No. 6, 1999 1197
Scheme 1
to a significant degree, its overlap with the MLCT bleach would
cause a blue shift in the maximum of the bleach, relative to the
ground-state absorption maximum. The absence of these features
in the transient absorption difference spectrum indicates that
relatively little of the redox-separated state is present at
equilibrium (<10%).
The dominance of the MLCT state and relatively low
concentration of the redox-separated state are not surprising from
a thermodynamic view. From emission spectral fitting on [Ru-
(4,4′-(C(O)NEt2)2bpy)2(dmb)]2+*, the energy of the RuIII(4,4′-
(CO2Et2)2bpy-) MLCT state for 2 in CH3CN is 2.07 eV. From
the results of the electrochemical studies in Table 1, the energy
of the ligand-based, redox-separated state, (4,4′-(CO2Et2)2bpy-)-
(dmb-tol+), is ∼2.08 eV, neglecting work terms. On the basis
of this analysis, ∆G° ∼ 0 for the interconversion between the
MLCT and the redox-separated state (RSS). Given these values,
it is reasonable to expect that the two states coexist or that the
RSS lies slightly higher in energy. This is consistent with the
transient absorption results, from which it is estimated that
[RSS]/[MLCT] < 0.1.
A summary of the photophysical events that occur in [Ru-
(4,4′-(C(O)NEt2)2bpy)2(dmb-tol)]2+ following RuII f (4,4′-
(C(O)NEt2)2bpy) excitation is given in Scheme 2. On the basis
of the available data the following comments can be made: (1)
The lifetime of the initially formed MLCT excited state
decreases from 995 ns (k ) 1.0 × 106 s-1) for the model
complex [Ru(4,4′-(C(O)NEt2)2bpy)2(dmb)]2+* to τem ) 26 ns
(k ) 3.8 × 107 s-1) in [Ru(4,4′-(C(O)NEt2)2bpy)2(dmb-tol)]2+*.
The decrease is because of intramolecular toluidine f RuIII
reductive quenching. (2) The MLCT state lies lower in energy,
and after initial quenching, an equilibrium is established between
the redox-separated and MLCT states, which favors the latter.
(3) The kinetics of excited-state decay are independent of
monitoring λ, which demonstrates that the MLCT a RSS
equilibrium is maintained rapidly compared to k1, k3. This is
consistent with the kinetic limit k2, k-2 . k1, k3 in Scheme 2.
(4) Decay is dominated by the RSS and k3 . k1. In this limit,
estimated from the ground-state redox potentials (Table 1)
neglecting work terms, is 2.33 eV.
Photophysical events that occur following 456 nm MLCT
excitation of [Ru(4,4′-(C(O)NEt2)2bpy)2(dmb-tol)]2+ (2) are
more complex. In 2, a small amount of residual emission is
observed which could be fit to biexponential decay kinetics. In
these fits there is evidence for a small amount of a long-lived
(τ ) 1.06 µs) emitting impurity which is present in varying
amounts depending on sample purification. The remaining
emission is intrinsic with τ ) 26 ns (k ) 3.8 × 107 s-1) and
coincides with the kinetics obtained for decay of the transient
observed by transient absorption measurements. The greatly
decreased MLCT lifetime is consistent with intramolecular
quenching analogous to that for the Re complex. The transient
absorption difference spectrum in Figure 2 reveals the formation
of a MLCT excited state having a bleach at ∼470 nm, which
coincides with the ground-state absorption maximum, and a
characteristic absorption in the π f π* region at 380 nm.24
Given the electron-withdrawing character of the amide ligand
and transient infrared results on the diethylester analogue, [Ru-
(4,4′-(CO2Et2)2bpy)2(dmb)]2+, this is a RuIII(4,4′-(CO2Et2)2-
bpy-)-based MLCT state.25
The bleach for the MLCT state at ∼470 nm overlaps the
absorption maximum for dmb-tol•+ at 476 nm (ꢀ ) 4200 M-1
cm-1), is far more intense, and would mask the appearance of
the radical cation. In addition to absorption by the radical cation,
the redox-separated state, [RuII(4,4′-(CO2Et2)2bpy)(4,4′-(CO2-
Et2)2bpy-)(dmb-tol+)]2+, would also exhibit a more intense
(ꢀ ∼ 16 000 M-1 cm-1) RuII f (4,4′-(C(O)NEt2)2bpy) absorp-
tion that would be significantly red-shifted (λmax > 530 nm)
relative to the ground state, owing to the electron-donating
ability of the reduced bipyridine.26 If this feature were present
k
obs for excited-state decay becomes τ-1 ) kobs ) k3(k2/k-2) )
k3Keq, with Keq ) [RSS]/[MLCT]. On the basis of this analysis
and Keq < 0.1, k3 > 4 × 108 s-1, and k2, k-2 > k3.
With the new donor ligand dmb-tol, facile reductive quench-
ing of the lowest MLCT excited states of [Re(dmb-tol)(CO)3Cl]
and [Ru(4,4′-(C(O)NEt2)2bpy)2(dmb-tol)]2+ occurs. The radical
cation has a characteristic spectroscopic signature at 478 nm
(ꢀ ) 4200 M-1 cm-1). Aniline derivatives as electron donors
offer greater flexibility in the design of polypyridyl-based
electron-transfer molecular assemblies, as compared, for ex-
ample, to more commonly used PTZ derivatives. With the
aniline derivatives, fine-tuning of the D+/0 reduction potential
in molecular assemblies can be achieved by varying the aniline
substituents. Thus, the synthetic procedure for preparation of
dmb-tol, generalized to phenyl-substituted anilines, opens the
possibility of preparing a family of electron-transfer donors
having a wide range of reduction potentials and systematic
variation in the driving force for electron transfer.
(22) (a) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.
Chem. 1986, 90, 3722. (b) Caspar, J. V.; Meyer, T. J. Inorg. Chem.
1983, 22, 2444.
(23) (a) Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. Soc. 1979, 101,
4391. (b) McClanahan, J.; Dallinger, R. F.; Holler, F. J.; Kincaid, J.
R. J. Am. Chem. Soc. 1985, 107, 4853. (c) Danzer, G. D.; Janusz, A.
G.; Strommen, D. P.; Kincaid, J. R. J. Raman Spectrosc. 1990, 21, 3.
(24) Lachish, U.; Infelta, P. P.; Gratzel, M. Chem. Phys. Lett. 1979, 62,
317.
The rate constant for dmb--tol+ f dmb-tol back electron
transfer in [Re(dmb-tol)(CO)3Cl], which interconverts the RSS
and ground states, is enhanced relative to analogous PTZ
electron transfers. For example, the rate constant for back
electron transfer in the RSS of 1 is 1.05 × 107 s-1 with ∆G° )
-2.33 eV. For the PTZ analogue, [Re(dmb-PTZ)(CO)3Cl], in
CH3CN under the same conditions, the rate constant for dmb--
PTZ+ f dmb-PTZ back electron transfer in the redox-separated
state is slower (kb ) 7.0 × 106 s-1), with ∆G° ) -2.21 eV.10a
(25) Chen, P.; Omberg, K. M.; Kavaliunas, D. A.; Treadway, J. A.; Palmer,
P. A.; Meyer, T. J. Inorg. Chem. 1997, 36, 954.
(26) (a) For example, [Ru(bpy)3]+ has a RuII f bpy, MLCT absorption
centered at 520 nm, with ꢀ ) 16 800 M-1 cm-1 (Maestri, M.; Gratzel,
M. Ber. Bunsen-Ges. Phys. Chem. 1977, 81, 504).

1198 Inorganic Chemistry, Vol. 38, No. 6, 1999
Partigianoni et al.
Scheme 2
These reactions occur deeply in the inverted region, and all other
things being equal, kET is expected to decrease with increasing
-∆G°. A detailed investigation of factors governing rate
constant differences between aniline and phenothiazine deriva-
tives is currently under way.
Foundation for a Career Advancement Award and a Visiting
Professorship Award for Women (Grant HRD-9627180) and
Ithaca College for a leave of absence. Mass spectra were
obtained at the Mass Spectrometry Laboratory for Biotechnology
at North Carolina State University. Partial funding for the
Facility was obtained from the North Carolina Biotechnology
Center and the National Science Foundation.
Acknowledgment. T.J.M. acknowledges the National Sci-
ence Foundation for financial support under Grant CHE
-9705724. C.M.P. also acknowledges the National Science
IC980544Q