Calculation of electron transfer rate constants from emission spectra in Re(I) chromophore-quencher complexes

Article abstract of DOI:10.1021/jp021216l

The photophysical properties of the chromophore-quencher complexes, fac-[(4,4′-R₂bpy)Re^I(CO)₃(LA)]ⁿ⁺ (4,4′-R₂bpy = 4,4′-R₂-2,2′-bipyridine, R = Me or ^tBu, and LA = the quinone a

Full text of DOI:10.1021/jp021216l

J. Phys. Chem. A 2002, 106, 7795-7806
7795
Calculation of Electron Transfer Rate Constants from Emission Spectra in Re(I)
Chromophore-Quencher Complexes
Juan Pablo Claude,† Kristin M. Omberg,‡ Darryl S. Williams,§ and Thomas J. Meyer*
Department of Chemistry, CB# 3290, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290
and Office of the Associate Director for Strategic Research, Mail Stop A127, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545
ReceiVed: May 15, 2002
The photophysical properties of the chromophore-quencher complexes, fac-[(4,4′-R2bpy)ReI(CO)3(LA)]n+
t
(4,4′-R2bpy ) 4,4′-R2-2,2′-bipyridine, R ) Me or Bu, and LA ) the quinone acceptor ligands, benz[g]-
isoquinoline-5,10-dione (BIQD), 2-oxy-1,4-naphthoquinone anion (ONQ-), 1/2 2,6-dihydroxyanthraquinone
dianion (AFA2-), or the pyridinium acceptor, 1-methyl-6-oxyquinoline (OQD)) in 1,2-dichloroethane are
described. Following ReI f 4,4′-R2bpy metal-to-ligand charge transfer (MLCT) excitation, intramolecular
electron transfer leads to the transients, fac-[(4,4′-R2bpy)ReII(CO)3(LA•-)]n+. They have been characterized
by emission spectral fitting and transient absorption measurements and, for LA ) OQD, by transient infrared
measurements. As shown by analysis of excited-state emission, there is weak-to-moderate electronic coupling
between the electron donor and acceptor sites in the transients with HDA varying from 153 cm-1 for the
BIQD complex to 3.9 cm-1 for the OQD complex. The transients are redox-separated (RS) states with the
electronic configurations dπ5πLA*1 for BIQD or σ(Re-O)1πLA*1 for ONQ-, AFA2-, and OQD. They are
weak emitters and return to the ground state largely by nonradiative decay which occurs by back electron
transfer (kET). Reasonable agreement has been reached between kET and values calculated from kinetic
parameters derived by emission spectral fitting and excited-state decay. The RS states for the AFA2- and
OQD complexes are remarkably long-lived (τ ) 4 µs for LA ) AFA2- in DCE at 296 K and 16 µs for LA
) OQD in DCE at 296 K) due to orbital and spin restrictions on back electron transfer.
Introduction
groups are chemically linked to a chromophore unit, have been
useful in these studies. Porphyrin-based chromophores linked
to carotenoids30,31 and anilines32-34 as donors and quinones as
acceptors have been studied extensively, as have chromophore-
quencher complexes based on polypyridyl complexes of
Ru(II), Os(II), or Re(I), which serve as metal-to-ligand charge
transfer (MLCT) chromophores.35-41 A variety of donors and
acceptors have been used in MLCT-based assemblies, including
quinones.42-45 Quinones are attractive as acceptors because they
are available in a range of redox potentials by making substituent
changes. They are also important components in natural
photosynthetic reaction centers.46
In the study of electron transfer, energy transfer, and
nonradiative decay, an important development has been the
recognition of the close relationship between spectroscopy and
reaction dynamics.1-13 Theoretical expressions have been
derived which relate rate constants to molecular quantities such
as reorganizational energies, driving forces, and the coupling
matrix elements that mix states, all of which can be measured
in favorable cases. The application of these ideas has led to the
use of emission, absorption, and resonance Raman spectra14 to
evaluate reaction barriers and rate constants for electron and
energy transfer in the normal and inverted regions, and for
nonradiative decay.15-29
This approach requires evaluation of the solvent reorganiza-
tional energy, the quantum spacings and changes in equilibrium
displacements for the coupled vibrations, and the interaction
energy arising from electronic coupling. The first two can be
evaluated from absorption and/or emission band shapes and the
latter from oscillator strengths by integrating absorption bands,
or by evaluating the Einstein coefficient for spontaneous
emission by emission quantum yield and lifetime measurements.
Chromophore-quencher complexes, which are molecular
assemblies in which electron transfer donor and/or acceptor
In this manuscript, we report the photophysical properties of
the series of chromophore-quencher complexes, fac-[(4,4′-R2-
bpy)ReI(CO)3(LA)]n+ (4,4′-R2bpy ) 4,4′-R2-2,2′-bipyridine, R
t
) Me or Bu, and LA ) the quinone acceptor ligands, benz-
[g]isoquinoline-5,10-dione (BIQD), 2-oxy-1,4-naphthoquinone
anion (ONQ-), 1/2 2,6-dihydroxyanthraquinone dianion (AFA2-),
or the pyridinium acceptor, 1-methyl-6-oxyquinoline (OQD)).
The structures of the ligands and of the complex fac-[(4,4′-
Me2bpy)ReI(CO)3(BIQD)]+ are illustrated below.
These complexes were designed to study the relationship
between electron transfer and spectroscopy. Given the nature
of the chemical links between units, electronic coupling between
the metal and acceptor ligand is expected to be moderate for
BIQD and weak for the aryloxide ligands. Following ReI f
4,4′-R2bpy excitation and 4,4′-R2bpy f LA electron transfer
to give fac-[(4,4′-R2bpy)ReII(CO)3(LA•-)]n+, LA•- f ReII back
electron transfer occurs and can be time-resolved by transient
absorption measurements because the one-electron-reduced
* To whom correspondence should be addressed.
† Current address: Department of Chemistry, University of Alabama at
Birmingham, Birmingham, AL 35294-1240.
‡ Current address: Systems Engineering & Integration Group, Decision
Applications Division, Mail Stop F607, Los Alamos National Laboratory,
Los Alamos, NM 87545.
§ Current address: Research & Development, Texas Eastman Division,
Eastman Chemical Company, P.O. Box 7444, Longview, TX 75607-7444.
10.1021/jp021216l CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/07/2002

7796 J. Phys. Chem. A, Vol. 106, No. 34, 2002
Claude et al.
Syntheses. fac-[(4,4′-tBu2bpy)Re(CO)3Cl]. Re(CO)5Cl (2.00
g, 5.53 mmol) was slurried in toluene (50 mL) with 4,4′-tBu2-
bpy (1.48 g, 5.53 mmol) and heated at reflux under nitrogen
for 1 h. The resulting yellow solution was cooled to room
temperature and filtered. The yellow precipitate was washed
with diethyl ether and dried to yield 2.8 g (88%) of pure fac-
1
[(4,4′-tBu2bpy)Re(CO)3Cl]. H NMR (CD2Cl2, ppm): δ 8.92
(d, 2H), δ 8.10 (d, 2H), δ 7.51 (dd, 2H), δ 1.43 (s, 18H).
fac-[(4,4′-tBu2bpy)Re(CO)3(OTf)]. AgOTf (470 mg, 1.83
mmol) was added to a THF (20 mL) solution of fac-[(4,4′-
tBu2bpy)Re(CO)3Cl] (1.00 g, 1.742 mmol). The solution was
stirred for 2 h, and the white precipitate of silver chloride was
removed by filtration through a pad of Celite. The precipitate
was washed with diethyl ether, the solution volume was doubled
with hexane and half of the solvent volume was removed under
vacuum. The resulting precipitate was filtered, washed with
hexane, and dried to yield 1.10 g (91%) of a lemon yellow
1
powder. H NMR (CD2Cl2, ppm): δ 8.97 (d, 2H), δ 8.09 (d,
2H), δ 7.59 (dd, 2H), δ 1.45 (s, 18H).
fac-[(4,4′-tBu2bpy)Re(CO)3(OPh)]. A solution containing fac-
[(4,4′-tBu2bpy)Re(CO)3(OTf)] (100 mg, 0.14 mmol) and phenol
(14 mg, 0.14 mmol) in CH2Cl2 (20 mL) was stirred over
powdered K2CO3 for 24 h. The solution color changed from
pale yellow to red-orange over the course of the reaction. The
solution was filtered and concentrated under vacuum. Dissolu-
tion of the residue in a minimum amount of CH2Cl2 and vapor
diffusion of diethyl ether into this solution yielded 75 mg (82%)
of product as orange needles. 1H NMR (CD2Cl2, ppm): δ 8.86
(d, 2H), δ 8.15 (d, 2H), δ 7.49 (dd, 2H), δ 6.87 (dd, 1H), δ
6.36 (t, 1H), δ 6.24 (dd, 2H), δ 1.45 (s, 18H). Anal. Calcd. for
C27H29N2O4Re: C, 51.33%; H, 4.63%; N, 4.43%. Found: C,
51.03%; H, 4.52%; N, 4.40%.
quinone and pyridinium ligands have characteristic absorption
features in the near-UV and visible. Most importantly, all of
these complexes emit, and analysis of the emission gives the
parameters required to calculate both the electron-transfer barrier
and electron-transfer coupling matrix elements.20-23 A prelimi-
nary account of this work has appeared.47
fac-[(4,4′-tBu2bpy)Re(CO)3(ONQ)]. A solution containing fac-
[(4,4′-tBu2bpy)Re(CO)3(OTf)] (100 mg, 0.14 mmol) and 2-hy-
droxy-1,4-naphthoquinone (25 mg, 0.14 mmol) in CH2Cl2 (20
mL) was stirred for 30 min. Powdered K2CO3 was added and
stirring continued for 24 h. The solution color changed from
pale yellow to red over the course of the reaction. The solution
was filtered and concentrated under vacuum. Dissolution of the
residue in a minimum amount of CH2Cl2 and vapor diffusion
of diethyl ether into this solution yielded 88 mg (85%) of cherry
Experimental Section
Materials. Benz[g]isoquinoline-5,10-dione, 2-hydroxy-1,4-
naphthoquinone, 2,6-dihydroxyanthraquinone, 6-hydroxyquino-
line, 4,4′-dimethyl-2,2′-bipyridine, tetrabutyl-n-ammonium hy-
droxide (1 M solution in H2O), ammonium hexafluorophosphate,
silver triflate, and phenol were purchased from Aldrich and used
as received. Re(CO)5Cl was obtained from Pressure Chemicals
and methyl iodide from Fisher. The ligand 4,4′-tert-butyl-2,2′-
bipyridine48 and the complexes fac-[(4,4′-Me2bpy)Re(CO)3-
(OTf)] (bpy ) 2,2′-bipyridine; OTf ) CF3SO3-), fac-[(bpy)-
Re(CO)3(4-Etpy)](PF6) (4-Etpy ) 4-ethylpyridine), and fac-
[(4,4′-Me2bpy)Re(CO)3(4-Etpy)](PF6) were prepared according
to literature procedures.49,50 HPLC grade 1,2-dichloroethane
(DCE) was purchased from Aldrich, dried over CaH2, and
distilled under an inert atmosphere before use. All other solvents
were reagent grade and used as received.
1
red needles. H NMR (CD2Cl2, ppm): δ 8.91 (d, 2H), δ 8.11
(d, 2H), δ 7.97 (dd, 1H), δ 7.73 (dd, 1H), δ 7.6-7.4 (m, 4H),
δ 6.03 (s, 1H), δ 1.43 (s, 18H). Anal. Calcd. for C31H29N2O6-
Re: C, 52.31%; H, 4.11%; N, 3.94%. Found: C, 52.63%; H,
4.34%; N, 3.44%.
fac-[(4,4′-Me2bpy)Re(CO)3]2(AFA). The conjugated base of
anthraflavic acid (2,6-dihydroxyanthraquinone dianion) was
prepared by dissolving the compound in a minimum volume of
50% (v:v) THF:MeOH and adding 2 equiv of [(nBu)4N](OH)
(1 M solution in H2O). The deep red salt [(nBu)4N](AFA) rapidly
precipitated and was collected by filtration. A solution of the
AFA salt (240 mg, 0.33 mmol) in 50 mL of 50% (v:v) THF:
MeOH was slowly added to a solution of fac-[(4,4′-Me2bpy)-
Re(CO)3(OTf)] (100 mg, 0.17 mmol) in 30 mL of the same
solvent mixture, resulting in the slow, quantitative precipitation
of the orange product. Because this compound is sparingly
soluble, no 1H NMR data are available. Anal. Calcd. for
C44H30N4O10Re2: C, 46.07%; H, 2.64%; N, 4.88%. Found: C,
45.52%; H, 2.75%; N, 4.50%.
General Methods. 1H NMR spectra were acquired on Bruker
AML300 or WM250 spectrometers, and chemical shifts are
reported as ppm vs TMS at 20 °C in CD2Cl2 unless otherwise
specified. Deuterated solvents were purchased from Cambridge
Isotope Labs and used as received. FTIR spectra were collected
in KBr disks on a Mattson Galaxy 5020 spectrometer. UV-vis
spectra were recorded on an OLIS-modified Cary 14 spectro-
photometer in 1-cm path length quartz cuvettes. Analyses were
performed by Oneida Research Services.
fac-[(4,4′-tBu2bpy)Re(CO)3(OQD)](OTf). 6-Hydroxyquinoline
was methylated with methyl iodide and deprotonated with
[(nBu)4N](OH) in THF, resulting in precipitation of the zwit-
terion. A solution of fac-[(4,4′-tBu2bpy)Re(CO)3(OTf)] (100 mg,

Constants in Re(I) Chromophore-Quencher Complexes
J. Phys. Chem. A, Vol. 106, No. 34, 2002 7797
0.14 mmol) and 1-methyl-6-oxyquinoline (zwitterion) (23 mg,
0.14 mmol) in CH2Cl2 (15 mL) was stirred for 12 h. The solution
color changed from pale yellow to pale orange over the course
of the reaction. The solution was filtered and concentrated under
vacuum. Dissolution of the residue in a minimum amount of
CH2Cl2 and vapor diffusion of diethyl ether into this solution
fac-[(bpy)Re(CO)3(4-Etpy)](PF6) (φem ) 0.13 in DCE at 296
K)38 or [Os(bpy)3](PF6)2 (φem ) 0.005 in CH3CN at 296 K).16
Nanosecond emission lifetimes were measured following laser
flash excitation with a PRA LN1000 nitrogen laser and a PRA
LN102 dye laser as described previously.38 Emission was
monitored at 90° relative to the excitation with a McPherson
272 grating monochromator and a Hammamatsu R928 photo-
multiplier operating within the range -600 to -750 V. The
output was recorded on a LeCroy 7200A digital oscilloscope,
and the digitized traces were fit to either single- or double-
exponential decays by using a Levenberg-Marquardt routine.
Samples had an absorbance of ∼0.1 at the excitation wavelength.
1
yielded 65 mg (56%) of a yellow-orange powder. H NMR
(CD2Cl2, ppm): δ 8.87 (d, 2H), δ 8.57 (d, 1H), δ 8.42 (d, 1H),
δ 8.22 (d, 2H), δ 7.75 (d, 1H), δ 7.61 (dd, 1H), δ 7.56 (dd,
2H), δ 7.19 (dd, 1H), δ 7.03 (s, 1H), δ 4.44 (s, 3H), δ 1.48 (s,
18H). Anal. Calcd. for C32H33N3O7SF3Re: C, 45.38%; H,
3.93%; N, 4.96%. Found: C, 45.32%; H, 3.66%; N, 4.86%.
Nanosecond transient absorption spectra and kinetic traces
were obtained for samples with an absorbance of 0.3-0.5 at
the excitation wavelength. The third harmonic of a Quanta-Ray
DCR-2A Nd:YAG laser was used to pump a Quanta-Ray PDL-2
dye laser to produce excitation pulses of ∼2 mJ/pulse. The
excitation beam was expanded with a Gallilean telescope and
then vertically compressed with a cylindrical lens to provide a
rectangular cross-section at the sample cell. The probe beam
was at an angle of 90° relative to excitation and was coincident
with the excited volume of sample along the major axis of the
excitation cross-section. The probe beam was provided by a
150 W pulsed Xe arc lamp, and the signal was measured with
an Applied Photophysics f/3.4 grating monochromator and a
five-stage photomultiplier. The resulting output was collected
with a LeCroy 7200A digital oscilloscope interfaced to a
personal computer. Electronic control and synchronization of
the laser, probe, and oscilloscope were provided by electronics
of our own design. Appropriate Oriel cutoff filters were used
to exclude high-energy probe and scattered excitation light.
Transient absorption decays were fit to single- or double-
exponential decays by using a Levenberg-Marquardt routine.
Time-Resolved Infrared Measurements. Infrared measure-
ments utilized a BioRad FTS 60A/896 step-scan interferometer
as previously described.51 Samples were excited at 355 nm by
using the third harmonic of a Nd:YAG laser (Spectra Physics
GCR-11, 7 ns pulse width, operated at 10 Hz). Data acquisition
was gated between the laser pulse and 500 ns for TRIR spectra.
Ground-state spectra were acquired as an average of 64 scans,
and excited-state spectra an average of 32 scans.
Time-resolved infrared (TRIR) spectra were measured in
DCE. Sample concentrations were adjusted to give an IR
absorbance of approximately 1.0 for the ground-state CO bands.
The sample cell and sample solutions were deoxygenated by
sparging with argon for 15 min. Solutions were transferred to
the cell under an inert atmosphere. Spectra were acquired in
blocks of 16 scans to ensure sample integrity. No sample
decomposition was observed during the acquisition periods.
Raman Measurements. Resonance Raman (RR) spectra
were acquired by using continuous wave excitation from a
Coherent Innova 400 Ar+ laser. The scattered radiation was
collected in a 135° backscattering geometry and passed into a
SPEX 1877D triple monochromator and detected with a liquid
nitrogen cooled Photometrics Model CH210 CCD.
Time-resolved resonance Raman (TR3) spectra were measured
by using the third harmonic (354.7 nm) of a Quanta-Ray DCR-
2A pulsed Nd:YAG laser both to create the excited state and
as a source for the Raman scattering. The scattered radiation
was collected in a 135° backscattering geometry into a SPEX
1877 Triplemate spectrometer equipped with an 1800 grooves/
mm grating. The Raman signal was detected by a Princeton
Instruments IRY-700G optical multichannel analyzer operating
in the gated mode with a ST-110 OSMA detector-controller.
fac-[(4,4′-Me2bpy)Re(CO)3(BIQD)](OTf). A solution of fac-
[(4,4′-Me2bpy)Re(CO)3(OTf)] (100 mg, 0.17 mmol) and benz-
[g]isoquinoline-5,10-dione (35 mg, 0.17 mmol) in CH2Cl2 (15
mL) was stirred for 90 min. The solution was filtered and
concentrated under vacuum. Dissolution of the residue in a
minimum amount of CH2Cl2 and vapor diffusion of diethyl ether
into this solution yielded 100 mg (77%) of a pale yellow solid.
The triflate anion can be exchanged for PF6- by dissolving the
complex in a concentrated solution of NH4PF6 in H2O and
1
extracting with CH2Cl2. H NMR (CDCl3, ppm): δ 8.94 (s,
1H), δ 8.90 (d, 2H), δ 8.51 (d, 1H), δ 8.42 (m, 2H), δ 8.40 (d,
2H), δ 7.99 (d, 1H), δ 7.72 (m, 2H), δ 7.60 (d, 2H), δ 1.35 (s,
18H). Anal. Calcd. for C29H19N3O8SF3Re: C, 42.86%; H,
2.36%; N, 5.17%. Found: C, 42.41%; H, 2.50%; N, 4.85%.
Electrochemistry. Tetrabutyl-n-ammonium hexafluorophos-
phate (TBAH), [N(n-C4H9)4](PF6), was recrystallized twice from
ethanol and dried under vacuum for 10 h. Cyclic voltammo-
grams were obtained in 0.1M TBAH/DCE solutions with a
Princeton Applied Research 273 potentiostat/galvanostat inter-
faced to a personal computer. A silver/silver nitrate (0.1 M)
reference electrode (0.314 V vs SSCE), platinum-wire auxiliary
electrode, and BAS MF-2013 platinum-disk working electrode
(0.31 cm2 electrode area) were used in a standard three-
compartment cell equipped for sparging with argon. Data were
collected at a scan rate of 200 mV/s, and potentials are reported
versus SSCE.
Spectroelectrochemistry. Absorption spectra of electro-
chemically reduced species were obtained in a three-compart-
ment cell attached to a 1-cm path length quartz cuvette mounted
in a Hewlett-Packard 8452D diode array spectrometer. The
complex in solution (0.1M TBAH/DCE) was reduced at a
platinum-mesh working electrode above the cuvette. Constant
bubbling with high-purity argon provided the transport of the
reduced species into the probe beam. The counter electrode was
a platinum wire separated from the cathodic compartment by a
glass frit, and a silver wire was used as a pseudo-reference
electrode. A Princeton Applied Research 273 potentiostat/
galvanostat interfaced to a personal computer was used to control
the electrolysis. The charge passed through the working
electrode was constantly monitored during the experiment.
Photophysical Measurements. All photophysical measure-
ments were conducted on filtered solutions in 1-cm path length
quartz cells. Solutions were degassed by sparging with solvent-
saturated, high-purity argon for a minimum of 20 min. Steady-
state emission and excitation spectra were collected on a SPEX
Fluorolog 212 photon-counting fluorimeter interfaced to a SPEX
DM1B computer. The spectra were corrected for instrument
response by the procedure supplied by the manufacturer.
Emission quantum yields were measured in optically dilute
solutions (OD ≈ 0.1 at the excitation wavelength) by using a
previously described procedure,38 and are reported relative to

7798 J. Phys. Chem. A, Vol. 106, No. 34, 2002
Claude et al.
TABLE 1: Crystallographic Data for
TABLE 2: Important Bond Lengths (Å) and Angles (deg) in
fac-[(4,4′-tBu2bpy)Re(CO)3(ONQ)] According to the
Numbering Scheme in Figure 1
fac-[(4,4′-tBu2bpy)Re(CO)3(ONQ)]
formula
space group
FW
a, Å
b, Å
ReC35H39N2O7
P21/n
Re(1)-C(1)
1.887 (18) C(3)-Re(1)-N(21)
1.851 (23) C(3)-Re(1)-N(32)
1.885 (20) O(4)-Re(1)-N(21)
2.133 (11) O(4)-Re(1)-N(32)
93.8 (7)
92.7 (7)
78.4 (5)
81.2 (5)
Re(1)-C(2)
785.90
11.3422 (11)
10.9911 (14)
27.316 (5)
95.605 (11)
3389.1 (8)
4
3.67
1.540
0.061
0.073
Re(1)-C(3)
Re(1)-O(4)
Re(1)-N(21)
Re(1)-N(32)
C(1)-O(1)
2.150 (12) N(21)-Re(1)-N(32) 74.4 (5)
c, Å
2.183 (13) Re(1)-C(1)-O(1)
1.176 (23) Re(1)-C(2)-O(2)
176.5 (16)
178.0 (16)
176.4 (17)
133.2 (10)
114.7 (17)
127.7 (16)
â, deg
V, Å3
Z
C(2)-O(2)
C(3)-O(3)
1.18 (3)
1.14 (3)
Re(1)-C(3)-O(3)
Re(1)-O(4)-C(11)
µ (mm-1
)
O(4)-C(11)
C(11)-C(12)
C(11)-C(20)
C(12)-O(12)
C(12)-C(13)
C(13)-C(14)
C(13)-C(18)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
C(17)-C(18)
C(18)-C(19)
C(19)-O(19)
C(19)-C(20)
C(1)-Re(1)-C(2)
C(1)-Re(1)-C(3)
C(1)-Re(1)-O(4)
1.302 (24) O(4)-C(11)-C(12)
D
calc, Mg m-3
1.45 (3)
1.35 (3)
1.23 (3)
1.47 (3)
1.35 (4)
1.40 (4)
1.37 (5)
1.37 (5)
1.37 (4)
1.36 (4)
1.51 (3)
1.22 (3)
1.39 (3)
88.3 (7)
88.3 (8)
97.3 (6)
O(4)-C(11)-C(20)
Rf
Rw
C(12)-C(11)-C(20) 117.6 (19)
C(11)-C(12)-O(12) 119.8 (22)
C(11)-C(12)-C(13) 123.0 (20)
O(12)-C(12)-C(13) 117.2 (20)
C(12)-C(13)-C(14) 121.3 (23)
C(12)-C(13)-C(18) 115.1 (20)
C(14)-C(13)-C(18) 123.5 (22)
C(13)-C(14)-C(15) 116 (3)
C(14)-C(15)-C(16) 123 (3)
C(15)-C(16)-C(17) 118 (3)
C(16)-C(17)-C(18) 121 (3)
C(13)-C(18)-C(17) 117.6 (23)
C(13)-C(18)-C(19) 121.8 (20)
C(17)-C(18)-C(19) 120.6 (23)
C(18)-C(19)-O(19) 119.1 (20)
C(18)-C(19)-C(20) 117.3 (20)
O(19)-C(19)-C(20) 123.6 (20)
C(11)-C(20)-C(19) 124.7 (19)
Re(1)-N(21)-C(22) 125.8 (11)
Re(1)-N(21)-C(26) 117.9 (10)
Re(1)-N(32)-C(27) 117.1 (11)
Re(1)-N(32)-C(31) 124.9 (12)
NO[I > 2σ(I)]a
NO
2815
4377
1.89
0.015
quality of fit indicator
largest shift/esd
a NO ) number of observations.
C(1)-Re(1)-N(21) 100.9 (6)
C(1)-Re(1)-N(32) 175.2 (6)
C(2)-Re(1)-C(3)
C(2)-Re(1)-O(4)
90.1 (9)
96.9 (7)
C(2)-Re(1)-N(21) 170.1 (6)
C(2)-Re(1)-N(32) 96.3 (6)
C(3)-Re(1)-O(4)
171.2 (7)
away from a true octahedron. The bite angle of the bound (4,4′-
tBu2bpy) ligand is N21-Re-N32 ) 74.4(5)° with the in-plane
C-Re-N angles varying from 100.9(6)° (C1-Re-N32) to
96.3(6)° (C2-Re-N32). The aryloxide ligand is bent away from
the in-plane CO ligands with C3-Re-O4 ) 120.1(6)°. There
is no evidence for a structural trans effect exerted by the
aryloxide ligand because the trans Re-CO, Re-C3 bond
distance of 1.885(20) Å is the same within experimental error
as Re-C1 at 1.887(18) Å. The Re-O bond length is 2.133(11)
Å. Crystallographic data are listed in Table 1 and bond lengths
and angles in Table 2.
Figure 1. ORTEP diagram for fac-[(4,4′-tBu2bpy)Re(CO)3(ONQ)].
Bond lengths and angles are listed in Table 2.
Timing was controlled by a Princeton Instruments FG-100 pulse
generator. The final spectra were the result of 9 min of total
integration time. Laser power was between 3 and 5 mJ per pulse.
Data collection and storage were controlled by an IBM AT with
Princeton Instruments SMA software. All Raman spectra were
acquired at ∼4 mM in DCE. Samples for time-resolved
resonance Raman measurements were degassed by sparging with
argon.
Spectral and Electrochemical Data. UV-vis spectra of the
complexes are shown in Figure 2, and λmax values for the lowest-
energy absorption bands are summarized in Table 3. Dry DCE
was the solvent of choice for these and other measurements
due to the sensitivity of the aryloxide ligands to solvolysis in
the presence of even trace amounts of water.
X-ray Crystal Structure. Crystals of fac-[(4,4′-tBu2bpy)-
Re(CO)3(ONQ)] suitable for X-ray diffractometry were grown
by vapor diffusion of diethyl ether into a CH2Cl2 solution of
the complex. Data were collected on a Rigaku AFC6/S
diffractometer with Mo KR radiation, a graphite monochromator,
and the θ/2θ scan mode. The software package used was
NRCVAX.50 The data collection and analysis parameters are
listed in Table 1.
There is evidence in the spectrum of fac-[(4,4′-Me2bpy)Re-
(CO)3(BIQD)]+ for low-energy dπ(ReI) f π*(BIQD) bands,
convoluted with dπ(ReI) f π*(4,4′-Me2bpy) bands in the same
region. Related dπ(ReI) f π*(LA) bands do not appear for the
other complexes. These bands were investigated further for the
BIQD complex by excitation-dependent resonance Raman
spectroscopy. Raman scattering with excitation at 364 nm
resulted in resonantly enhanced bands at 1035, 1201, 1285,
1320, 1494, and 1555 cm-1, characteristic of 4,4′-Me2bpy as
the acceptor ligand and consistent with resonance enhancement
by a ReI f 4,4′-Me2bpy transition. Bands at 1612 and 1683
cm-1 are also enhanced and of roughly comparable intensity.
These are mixed CdC and quinone CdO stretches on the BIQD
ligand.52 The same bands were enhanced with excitation at 458
and 476 nm but with the quinone bands enhanced by a factor
of 5 compared to the bipyridine bands.
Analysis. Emission spectra were fit by a one-mode Franck-
Condon analysis as previously described.47
Results
Structure of fac-[(4,4′-tBu2bpy)Re(CO)3(ONQ)]. The struc-
ture of fac-[(4,4′-tBu2bpy)Re(CO)3(ONQ)] is shown in Figure
1. It illustrates the expected facial coordination geometry of an
octahedral Re(I) tricarbonyl complex with ONQ bound as a
typical aryloxide ligand. The coordination geometry is distorted

Constants in Re(I) Chromophore-Quencher Complexes
J. Phys. Chem. A, Vol. 106, No. 34, 2002 7799
component, ∆νj1/2, the quantum spacing for the average acceptor
mode, pω, and its electron-vibrational coupling constant, S.
Results are summarized in Table 5, and the emission spectrum
and fit for fac-[(4,4′-Me2bpy)Re(CO)3(BIQD)]+ are shown in
Figure 4. The quantum spacing, pω, was varied for each
complex in order to obtain the best fit. Some of the fits were
difficult due to the low quantum yields for emission, and this
is reflected in the standard deviations of the fitting parameters.
Emission from fac-[(4,4′-tBu2bpy)Re(CO)3(OPh)] is weak and
poorly defined, and the fit slightly dependent on pω. The slight
contribution to emission from the 550 nm shoulder in fac-[(4,4′-
Me2bpy)Re(CO)3]2(AFA) was deconvoluted by discarding the
data points from 16 500 to 22 000 cm-1. This increased the inter-
parameter correlations making the fits weakly dependent on pω.
Transient Absorption. Transient absorption difference spec-
tra were acquired in DCE following laser flash excitation at
364 nm. The difference spectra for fac-[(4,4′-Me2bpy)Re(CO)3-
(BIQD)]+ and fac-[(4,4′-tBu2bpy)ReI(CO)3(OQD)]+ are shown
in Figure 5, with spectra for the ONQ and AFA complexes
available as supplementary information. In all cases, the
transients that formed appeared during the ∼4 ns laser pulse.
The transient absorption decays were fit to the exponential
decay function
At ) A0 exp(-kt)
(1)
with At and A0 the absorbances at times t and 0, respectively,
and k the decay rate constant. Rate constants are listed in Table
4. Decays for the ONQ complex were nonexponential but could
be fit to the biexponential function
At ) A0,1 exp(-k1t) + A0,2 exp(-k2t)
(2)
7
5
with k1 ) 4.0 × 10 s-1 and k2 ) 9.4 × 10 s-1 in DCE at
Figure 2. Top: UV-vis absorption spectra in 1,2 -dichloroethane for
fac-[(4,4′-Me2bpy)Re(CO)3(BIQD)](CF3SO3) (s) and fac-[(4,4′-Me2-
bpy)Re(CO)3(4-Etpy)](PF6) (- - -). Bottom: UV-vis absorption
spectra in 1,2 -dichloroethane for fac-[(4,4′-tBu2bpy)Re(CO)3(OPh)]
(‚‚‚), fac-[(4,4′-tBu2bpy)Re(CO)3(ONQ)] (- - -), fac-[(4,4′-Me2bpy)-
Re(CO)3]2(AFA) (-‚-) and fac-[(4,4′-tBu2bpy)Re(CO)3(OQD)](PF6)
(s). The inserts show enlarged views of the low energy portions of
the spectra.
296 K.
Spectroelectrochemistry. Reduction of the BIQD complex
in 0.1 M TBAH/DCE occurred with n ) 1.0. In the spectrum
of the reduced complex, absorption features appeared at ∼580
and ∼640 nm, near the features observed in the transient
absorption difference spectrum of the BIQD complex in Figure
5. A shoulder also appeared at ∼390 nm. For the other
chromophore-quencher complexes, reduction was chemically
irreversible.
Ground-state infrared ν(CO) band energies and electrochemi-
cal results obtained by cyclic voltammetry in 0.1 M TBAH/
DCE are summarized in Table 3.
Time-Resolved Infrared (TRIR). In the ground-state infra-
red spectrum of the OQD complex in DCE, ν(CO) bands appear
at 2019, 1909, and 1900 cm-1 arising from the A′(1), A′(2),
and A′′ modes in Cs symmetry.53 In the TRIR spectrum, three
bands also appear, shifted to slightly higher energies at 2023,
Emission. Emission spectra in DCE are shown in Figure 3
and λmax values, emission quantum yields (φem), and lifetimes
(τ) are listed in Table 4. The slight shoulder at 550 nm in the
emission spectrum of the AFA dimer was observed for several
different preparations of the complex, and appears to be intrinsic.
Emission decays were fit to the exponential function, It/I0 )
exp(-kt) (with It and I0 the emission intensity at times t and 0,
respectively) and fit by using a Levenberg-Marquardt routine
as previously described.47
There is a good match between absorption and excitation
spectra for fac-[(4,4′-Me2bpy)Re(CO)3(BIQD)]+, fac-[(4,4′-tBu2-
bpy)Re(CO)3(OQD)], and fac-[(4,4′-Me2bpy)Re(CO)3]2(AFA)
above 350 nm. For fac-[(4,4′-tBu2bpy)Re(CO)3(ONQ)], the
excitation spectrum failed to reveal the low-energy absorption
feature at ∼400 nm in Figure 2. There was a better match with
the absorption spectrum for the -OPh model complex.
Emission spectra were fit by using a one-mode Franck-
Condon analysis described previously.47 The parameters that
result from the analysis are the V* ) 0 f V ) 0 energy gap,
E0, the full bandwidth at half-height for a single vibronic
1936, and 1908 cm-1
.
Discussion
Before analyzing the electron-transfer data, it is necessary to
analyze the spectroscopic and photophysical properties of the
chromophore-quencher complexes. They are revealing as to
the electronic and molecular structures of the transients reached
following laser flash photolysis.
Electronic Structure of the Photochemical Transients. The
electrochemical data in Table 3 establish that the quinone ligands
are better acceptors than the 4,4′-R2bpy ligands. There is
evidence for direct dπ(Re) f π*(LA) absorptions for the BIQD
complex in the low-energy part of the spectrum in Figure 2, as
shown by the resonance Raman results.
Emission from fac-[(4,4′-Me2bpy)Re(CO)3(BIQD)]+ is sig-
nificantly red-shifted and decreased in intensity compared to
the 4-Etpy model complex. This can be explained by ReI f

7800 J. Phys. Chem. A, Vol. 106, No. 34, 2002
Claude et al.
TABLE 3: Spectral and Electrochemical Data in 1,2-Dichloroethane at 296 K
complex
λ
max, nmc (ꢀ, M-1 cm-1
)
ν(CO), cm-1
E1/2(ReII/I), V
E1/2(LA0/-), V
-0.39
E1/2(4,4′-R2bpy0/-), V
[(4,4′-Me2bpy)Re(CO)3(4-Etpy)]+ a
[(4,4′-Me2bpy)Re(CO)3(BIQD)]+ b
[(4,4′-tBu2bpy)Re(CO)3(OPh)]
351 (6.0 × 104)
339 (1.3 × 105)
384 (3.2 × 104)
470 (9.0 × 103)
387 (5.3 × 104)
435 (4.9 × 104)
404 (3.0 × 104)
421 (3.0 × 104)
480 (7.0 × 103)
449 (4.5 × 104)
2034; 1928
2035; 1921
2011; 1900; 1878
1.78d
e
-1.16d
-1.31
-1.46
0.88f
[(4,4′-tBu2bpy)Re(CO)3(ONQ)]
[(4,4′-Me2bpy)(CO)3Re]2(AFA)
2017; 1909; 1896
2017; 1884
1.29f
-1.13f
-1.48
g
g
g
[(4,4′-tBu2bpy)Re(CO)3(OQD)]+b
2019; 1909; 1900
1.36f
-1.13
-1.47
a PF6 salt. b CF3SO3 salt. c Maximum for the lowest-energy, intense absorptions. Numbers in parentheses are molar absorptivities. d From
reference.38 Irreversible wave, Ep,a is reported. e Beyond the electrochemical window. f Irreversible wave, Ep,a is reported. g Not sufficiently soluble
for electrochemical measurement.
-
-
4), corroborating it as the origin of the short-lived emission.
The good match between excitation and absorption spectra
shows that efficient population of the emissive state occurs both
upon dπ(Re) f π*(4,4′-Me2bpy) excitation followed by in-
tramolecular electron transfer, and by direct dπ(Re) f π*(LA)
absorption in the low-energy visible.
A kinetic scheme is illustrated in Scheme 1 which includes
the electronic configurations of the states involved. MLCT
absorption in these complexes is dominated by transitions which
give states of electronic configuration (dπ5π*bpy1). They are
1
largely singlet in character. For the model complex, emission
occurs from the corresponding triplets 3(dπ5π*bpy1). These
“triplet” and “singlet” states are of mixed spin character because
of spin-orbit coupling at d5 ReII (ú ≈ 2000 cm-1). The emitting
“triplet” is actually a manifold of three closely lying states in
rapid Boltzmann equilibrium which behave dynamically as a
single state at or near room temperature.54
The emitting state for the BIQD complex is probably largely
1
triplet in character as well. Conversion from (dπ5π*bpy1) to
3(dπ5π*bpy1) is known to be rapid in related Ru and Os
polypyridyl complexes and the triplet(s) appear to be the states
that undergo electron-transfer quenching. For the ONQ- com-
plex, the quenching step was observed by direct measurement
and occurs with k ≈ 4 × 107 s-1
.
Spin is conserved in the quenching step because the operator
that mixes the initial and final states and induces electron transfer
does not include spin. The spin state wave function before
quenching is given approximately by, Ψs ) 3Ψs + R1Ψs, with
3Ψs the pure triplet spin wave function and 1Ψs the singlet spin
wave function for the lowest energy singlet excited state. The
mixing coefficient R depends on the spin-orbit coupling
constant and the triplet-singlet energy gap. The percentage of
singlet character in the initial MLCT state is 10-20%.54 The
percent of singlet character in the electron-transfer product
depends on R2 and should be less than 5%.
On the basis of an analysis presented in the following section,
the magnitude of the electron-transfer matrix element arising
from the ReII(BIQD•-) electronic interaction in the transient is
only 153 cm-1, far less than the reorganizational energy
associated with BIQD•- f ReII electron transfer. Because of
this, the transient is best described as a “redox-separated” (RS)
state in which the electron transfer donor and acceptor sites are
only moderately coupled electronically and back electron
transfer occurs in the inverted region.
Figure 3. Top: Corrected emission spectra at room temperature in
1,2-dichloroethane for fac-[(4,4′-Me2bpy)Re(CO)3(BIQD)](CF3SO3) (s)
(420 nm excit.), fac-[(4,4′-tBu2bpy)Re(CO)3(ONQ)] (- ‚ ‚ -) (377 nm
excit.), fac-[(4,4′-Me2bpy)Re(CO)3]2(AFA) (‚‚‚) (420 nm excit.), and
fac-[(4,4′-tBu2bpy)Re(CO)3(OQD)](CF3SO3) (- - -) (480 nm excit.).
The spectra have been normalized to a constant intensity maximum
for ease of comparison. Bottom: Excitation (s) and absorption
(- - -) spectra for fac-[(4,4′-Me2bpy)Re(CO)3(BIQD)](CF3SO3) in
1,2-dichloroethane at room temperature monitored at 670 nm. The
spectra are arbitrarily scaled for ease of comparison.
4,4′-Me2bpy excitation followed by intramolecular electron
transfer and emission from a ReII(BIQD•-)-based state. This
conclusion is consistent with the transient absorption difference
spectrum in Figure 5 and the enhanced intensity of the BIQD
quinone bands in the ground-state resonance Raman spectrum
with low-energy excitation. The ∼25 ns decay time for the
transient is close to the observed 32 ns emission lifetime (Table
Emission from the model complex fac-[(4,4′-tBu2bpy)Re-
(CO)3(OPh)] is also red-shifted, of lower intensity, and occurs
with a shorter lifetime than fac-[(bpy)Re(CO)3(4-Etpy)]+*.
These are the expected consequences of replacing 4-Etpy with
the σ- and π-donating -OPh ligand. Emission from the AFA2-
and OQD complexes is red-shifted even further, but the lifetimes

Constants in Re(I) Chromophore-Quencher Complexes
J. Phys. Chem. A, Vol. 106, No. 34, 2002 7801
TABLE 4: Spectroscopic and Photophysical Properties in DCE at 296 K
λemmax, nm
φem
τ
em, nsf
τ )
abs, ns (k, s-1
d
g
complex
[(4,4′-Me2bpy)Re(CO)3(4-Etpy)]+ a
[(4,4′-Me2bpy)Re(CO)3(BIQD)]+ b
[(4,4′-tBu2bpy)Re(CO)3(OPh)]
[(4,4′-tBu2bpy)Re(CO)3(ONQ)]
568
688
628c
602
0.129
510 ( 20
32 ( 1
0.0018
0.0006
0.0005
25 ( 1 (4.0 × 107)
84 ( 10
79 ( 7
25 (4 × 107),h
1.0 × 103 (1 × 106)
5.6 ( 0.3 × 103
(1.8 × 105)
[(4,4′-Me2bpy)(CO)3Re]2(AFA)
[(4,4′-tBu2bpy)Re(CO)3(OQD)]+ b
664
748
0.001
4.1 ( 1.5 × 103
(16 ( 2) × 103
0.0005e
(15.2 ( 0.7) × 103
(6.6 × 104)
a PF6- salt. b CF3SO3- salt. c Shoulder. d Emission quantum yields relative to fac-[(bpy)Re(CO)3(4-Etpy)](PF6) in DCE at 296 K.38 e Relative to
[Os(bpy)3](PF6)2 in CH3CN at 296 K. f Lifetimes as determined from emission decays measured at a minimum of three monitoring wavelengths.
Comparable values were obtained with excitation at 376, 420, and 460 nm for BIQD. For OQD, the average value obtained with excitation at 420,
460, and 480 nm excitation is reported. g Lifetimes and rate constants for transient absorption decays following laser flash excitation at 364 nm. See
text. h Decay of the ReII(4,4′-tBu2bpy•-) MLCT excited state.
TABLE 5: Emission Spectral Fitting Parameters in DCE at 296 K
complex
E0, cm-1 c
S
∆νj1/2, cm-1
pω, cm-1 d
[(4,4′-Me2bpy)Re(CO)3(4-Etpy)]+ a
[(4,4′-Me2bpy)Re(CO)3(BIQD)]+ b
[(4,4′-tBu2bpy)Re(CO)3(OPh)]
[(4,4′-tBu2bpy)Re(CO)3(ONQ)]
[(4,4′-Me2bpy)(CO)3Re]2(AFA)
[(4,4′-tBu2bpy)Re(CO)3(OQD)]+ b
18900 (20)
14900 (40)
16700 (150)
17200 (100)
15500 (100)
13600 (10)
2.14 (0.02)
0.81 (0.05)
1.45 (0.18)
1.25 (0.10)
0.95 (0.12)
1.11 (0.01)
2050 (27)
2900 (67)
2850 (214)
3130 (155)
2830 (192)
1770 (17)
1186
1625
1300e
1606
1500e
1376
a PF6 salt. b CF3SO3 salt. c Numbers in parentheses are standard deviations. d Values cited give the smallest standard deviation for the fit.
e Parameter fixed arbitrarily; the overall fit was weakly dependent on the value chosen.
-
-
For the ONQ- complex, there is evidence for both short- and
long-lived transients. Emission is dominated by the short-lived
transient and the excitation spectrum largely matches the
absorption spectrum of the -OPh model, rather than the
chromophore-quencher complex. Transient absorption decay
data were fit to eq 2 with τ1 ) 25 ns (k ) 4.0 × 107 s-1) and
τ2 ) 1 µs (k ) 1 × 106 s-1), Table 4. In the transient absorption
difference spectra for the long-lived transient obtained at 90
and 300 ns after the laser pulse, a bleach appears at ∼500 nm
and absorptions at 380 nm 600 nm. The more intense, short-
lived emission appears to mask any red-shifted, long-lived
emission from this complex. It may contribute to the difference
between emission and absorption lifetimes in Table 4.
Figure 4. Normalized emission spectrum of fac-[(4,4′-Me2bpy)Re-
(CO)3(BIQD)]+ in 1,2-dichloroethane at room temperature (O) and one-
mode fit (s) with the parameters E0 ) 14 920 cm-1, S ) 0.81, ∆νj1/2
The transient reached following MLCT excitation of the OQD
complex was also investigated by transient infrared measure-
ments. Bands appear at 2019, 1909, and 1900 cm-1 in the
ground-state spectrum arising from the A′(1), A′′, and A′(2)
modes in Cs symmetry. In the transient, the band at 2019 cm-1
is shifted to 2023 cm-1, the band at 1909 cm-1 is shifted to
) 2900 cm-1 and pω ) 1625 cm-1
.
are remarkably long, Table 4. Excitation and absorption spectra
match, showing that the transients are produced efficiently over
a wide wavelength range by dπ(Re) f π*(4,4′-R2bpy) excita-
tion.
1936 cm-1 and the band at 1916 cm-1 is shifted to 1908 cm-1
.
These shifts of 4, 37, and 8 cm-1 are in contrast to the
ReII(bpy•-) MLCT excited state of fac-[(bpy)ReI(CO)3(4-Etpy)]+
For the AFA2- complex, there is evidence for a high-energy
component in the emission spectrum in the appearance of a
shoulder at 550 nm (Figure 3). Emission decay at this
wavelength was convoluted with the instrument response
function (τ < 20 ns). An excitation spectrum acquired at 550
nm coincided with the absorption maximum at 370 nm.
For the OQD complex, a bleach appears at 470 nm in the
transient absorption difference spectrum consistent with oxida-
tion at Re(I) and loss of the dπ(Re) f π*(4,4′-tBu2bpy)
absorption. There are also narrow and broad features at 360
and 600 nm, respectively, consistent with formation of the
reduced pyridinium form of the complex.55-57 For the AFA2-
complex, there is a bleach at 410 nm, and new absorptions at
370-380 and 500 nm arising from the reduced AFA3- radical
anion. The radical anion of anthraquinone generated by pulse
radiolysis has broad absorptions at 400 and 540 nm in CH3-
CN,58 and coordinated 4,4′-R2bpy anion at ∼370 nm.59
for which the corresponding excited-to-ground-state shifts are
60,61
35, 84, and 27 cm-1
.
Large, positive ν(CO) shifts are expected for MLCT excited
states because partial oxidation decreases dπ(Re)-π*(CO) back-
bonding, which increases CO bond order. Small positive shifts
are a characteristic feature of σπ* excited states.62-64 In these
excited states the “hole” resides in a metal-ligand σ bond, in
this case the Re-O bond, to give the configuration σ(Re-
O)1π*(LA)1, rather than the dπ(Re)5π*(LA)1 MLCT configu-
ration of the BIQD assembly.
This explains the smaller ν(CO) shifts. Even though there is
charge-transfer character in the transient, the site of oxidation
is σ(Re-O). To zero order, this orbital is orthogonal to the dπ
orbitals and to the dπ(Re)-π*(CO) back-bonding interaction.
This interpretation also explains the chemical irreversibility of
the AFA2-, ONQ-, and OQD-based ReII/I couples. They occur

7802 J. Phys. Chem. A, Vol. 106, No. 34, 2002
Claude et al.
SCHEME 2
The suggested sequence of events that occurs following ReI f
π*(4,4′-R2bpy) MLCT excitation in these cases is illustrated in
Scheme 2.
In this case, initial dπ(ReI) f π*(4,4′-R2bpy) excitation is
followed by rapid (<5 ns) formation of the ReII(4,4′-R2bpy•-)
“triplet” state. For the AFA2- and ONQ- complexes, there is
evidence for the intermediacy of this state by the appearance
of short-lived emissions at high energies. The initial MLCT state
undergoes rapid interconversion to the final σ-π* states with
k ≈ 4.0 × 107 for the ONQ- complex and k > 2 × 108 s-1 for
the AFA2- complex. This may occur by initial π*(bpy) f
π*(LA) electron transfer to give 3(dπ5π*LA1) followed by σRe-O
f dπ(Re) electron transfer, or by electron transfer in the reverse
order.
Analysis Based on Spectral Fitting Parameters. Application
of a Franck-Condon analysis to emission spectral profiles with
the average mode approximation gave the fitting parameters in
Table 5. The bandwidth at half-height for each vibronic
component, ∆νj1/2, is related to λo,L, the sum of the solvent
reorganizational energy, λo, and the reorganzational energy
contributed by low-frequency modes treated classically λi,L, by
eq 316,65
Figure 5. Top: Time-resolved absorption difference spectra for
fac-[(4,4′-Me2bpy)Re(CO)3(BIQD)]+ in 1,2-dichloroethane at room
temperature following 355 nm, 4 ns pulsed, 2 mJ/pulse excitation.
Bottom: Time-resolved absorption difference spectra for fac-
[(4,4′-tBu2bpy)Re(CO)3(OQD)]+ in 1,2-dichloroethane at room tem-
perature following 420 nm, 4 ns pulsed, 2 mJ/pulse excitation.
SCHEME 1
(∆νj1/2)2 ) 16kΒΤλï,L ln 2
(3)
λi,L is given by
λi,L ) Σλi,l ) ΣSlpωl
(4)
where the sum is over the coupled vibrations, l (those with Sl
* 0).
These are inherently multi-mode processes. Based on reso-
nance Raman measurements, there are 11 totally symmetrical
ν(bpy) modes coupled to nonradiative decay of the MLCT
state(s) of [Ru(bpy)3]2+*, for example.66,67 In the average mode
approximation, S is the sum of Sj values for the coupled high-
and medium-frequency vibrations
S ) ΣSj
(5)
at relatively low potentials but, since oxidation occurs at the
Re-O bond, oxidation leads to decomposition. The orthogonal-
ity of the dσ(Re-O) donor and π*(LA) acceptor orbitals also
contributes to the very weak electronic coupling between ReII
and the reduced AFA and OQD ligands (next section). Weak
electronic coupling is a major contributor to slow back electron
transfer and the extended lifetimes of the transients.
and pω is the weighted average of the quantum spacings
pω ) ΣSjpωj/ΣSj
(6)
It is also possible to calculate the electron-transfer matrix
element arising from dπ(ReII) (or σ(Re-O)) electronic coupling
with π*(LA), HDA, from the rate constant for radiative decay,
kr, and the emission spectral fitting parameters. kr is related to
The transients reached in this case are also best described as
RS states, but with the hole at σ(Re-O) rather than dπ(Re).

Constants in Re(I) Chromophore-Quencher Complexes
J. Phys. Chem. A, Vol. 106, No. 34, 2002 7803
TABLE 6: Exited State Parameters in DCE at 296 K
complex
kr, s-1 b
ν-3 -1, cm3 c
d (Å)
λ
o,L, cm-1 d
E
abs, cm-1 e
∆GoES, cm-1 f
[(4,4′-Me2bpy)Re(CO)3(BIQD)]+ a
[(4,4′-Me2bpy)(CO)3Re]2(AFA)
[(4,4′-tBu2bpy)Re(CO)3(OQD)]+ a
5.6 × 104
2.4 × 102
3.2 × 101
2.48 × 1012
2.86 × 1012
1.80 × 1012
6
6
5
3660
3500
1360
23600
23900
17900
18600
19000
15000
a CF3SO3- salt. b Calculated from kr ) φem/τem
.
c Calculated from the emission spectra by using eq 10. d Calculated from eq 3 and parameters in
Table 5. e Calculated from eq 11 and the parameters in Table 5. f Calculated with eq 13 and the parameters in Table 5.
the emission quantum yield and lifetime by eq 7. Values are
listed in Table 6. There is no entry for the ONQ complex
because the low-energy emission from the σ-σ* RS state is
masked by residual MLCT emission.
with d the electron-transfer distance
Eabs
1/2
HDA
)
[(1.39 × 105)k νj-3
]
(12)
(
)
r
nd
The free energy content of the emitting state above the ground
kr ) φem/τ
(7)
state, ∆GoES, is given by
There are two approaches to calculating HDA from kr, both based
on perturbation theory and application of the Strickler-Berg
equation.68
2
(∆νj1/2,s
)
∆GoES ) E0 + λo,L ) E0 +
(13)
16kBT ln 2
The transition moment is given by M ) Ψe|Σ rn|Ψg , with
Ψe and Ψg the excited- and ground-state vibronic wave
functions including spin, and rn the electronic coordinates. The
summation is over n. By assuming significant mixing only
between the excited (Ψe) and ground-state (Ψg) electronic wave
functions and the involvement of only two states, HDA is related
to kr as shown in eq 8
For back electron transfer, ∆Go = -∆GoES. The relationships
in eqs 3-13 and the data in Tables 4-6 allow λo,L to be
calculated from eq 3, Eabs from eq 11, HDA from eq 12, ꢀmax
from eq 9, and ∆Go from eq 13. The results are tabulated in
ES
Tables 6 and 7.
Molecular and Electronic Structure. The results of the
analysis in the previous section give further insight into the
electronic and molecular structures of the transients.
For the aryloxy complexes, the electronic coupling matrix
element is small, 9.4 cm-1 for the AFA2- complex and 3.9 cm-1
for the OQD complex. This is consistent with the orthogonal
orbital relationship between the donor and acceptor orbitals in
the σπ* RS states and their anticipated triplet character.71 On
the basis of this analysis, formation of these states by direct
excitation is unimportant. The corresponding absorptions are
predicted to occur at 418 nm with ꢀ ) 0.078 M-1 cm-1 for the
AFA2- complex, and at Eabs ≈ 560 nm with ꢀ ) 0.016 M-1
cm-1 for the OQD complex.
34π2n3
3p
kr )
(HDA∆µ)2 νj
(8)
∆µ is the vector difference in permanent dipole moments
between the ground and excited states, νj is the average
emission energy which is usually taken as the emission
maximum, and n is the solvent index of refraction.21,22,24,69
Application of eq 8 requires knowledge of the excited- and
ground-state dipole moments.
It is also possible to calculate HDA by using the emission
band shape parameters to calculate the corresponding absorption
band. HDA can then be calculated by applying an analysis given
by Hush for intervalence transfer in electronically weakly
coupled mixed-valence complexes.65,70 This involves relating
the transition moment to the oscillator strength of the absorption
band.
Electronic coupling is modest even for the BIQD complex
(153 cm-1). The calculated absorption for direct excitation is
predicted to occur at 424 nm with ꢀ ) 22 M-1 cm-1, which
would appear in the tail of the absorption spectrum shown in
Figure 2. Enhancement of the quinone-based BIQD modes in
excitation-dependent resonance Raman spectra and evidence for
ReI f BIQD absorption can be attributed to transitions to the
corresponding singlet states which are close in energy.
The transient RS states return nonradiatively to the ground
state by LA- f ReII electron transfer. These reactions occur in
the inverted region, since, based on the parameters in Table 6,
Assuming a Gaussian band shape, kr is related to the molar
extinction coefficient ꢀmax in M-1 cm-1, the absorption band
energy, Eabs in cm-1, and the spectral bandwidth ∆νj1/2,s
()∆νj1/2,s for emission) in cm-1 as in eq 9
ꢀmax∆νj1/2,s
kr ) (3.05 × 10-9) n2 νj-3 -1
(9)
-∆Go ) ∆Go > λ ()λo,L + Spω).
ES
Eabs
Although the electronic configurations of the transients are
those of σπ* or MLCT excited states, they are not excited states.
The relationship between the two is analogous to that between
localized and delocalized forms of mixed-valence com-
plexes.70,77 For example, in the series [(NH3)5Ru(4,4′-bpyridine)-
Ru(NH3)5]3+ (1),72-74 [(NH3)5Ru(pz)Ru(NH3)5]5+ (2, pz is
pyrazine),75 and [(NH3)5Os(pz)Os(NH3)5]5+(3),76 there is a
transition from electronically localized to delocalized behavior.
Electronic coupling in 1 is relatively weak, which localizes the
exchanging electron. This maximizes structural differences and
solvent polarization. Electronic coupling is promoted by dπ-
π,π*-dπ mixing across the ligand bridge. Increased electronic
coupling in 2 partly delocalizes the exchanging electron,
decreasing structural differences and averaging the solvent.77
In 3, the odd electron is delocalized in a dπ-dπ molecular
orbital with considerable pyrazine mixing.
ν-3 -1 is defined as
I(ν) dν
∫
νj-3
)
(10)
ν-3I(ν) dν
∫
I(νj) is the emitted intensity at frequency νj.
Eabs is related to the corresponding emission maximum, Eem
and the spectral fitting parameters ∆νj1/2, S, and pω as in eq 11
,
2(∆νj1/2,s
16kBT ln 2
)
Eabs ≈ Eem + 2λo,L + 2Spω ) Eem
+
+ 2Spω
(11)
The final relationship between kr and HDA is given in eq 12,

7804 J. Phys. Chem. A, Vol. 106, No. 34, 2002
Claude et al.
TABLE 7: Comparison of Calculated and Experimental Back Electron Transfer Rate Constants in DCE at 296 K
complex
ꢀmax, M-1 cm-1 b
H
DA, cm-1 c
ln[F(calc)]d
k
obs, s-1 e
k
b,calc, s-1 f
[(4,4′-Me2bpy)Re(CO)3(BIQD)]+ a
[(4,4′-Me2bpy)(CO)3Re]2(AFA)
[(4,4′-tBu2bpy)Re(CO)3(OQD)]+ a
22
0.078
0.016
153
9.4
3.9
-12.2
-13.5
-12.1
4.0 × 107
1.78 × 105
6.58 × 104
1.1 × 107
1.2 × 104
9.2 × 103
a CF3SO3- salt. b Calculated from eq 9 and the parameters in Tables 5 and 6. c Calculated from eq 12 and the parameters in this table and Tables
4 and 5. d Calculated with eq 16a and the parameters in Table 5. e From Table 4. f Calculated from eq 14 or 16.
consistent with the excitation-dependent resonance Raman
results on the BIQD complex.
The values for ∆νj1/2 are also larger for the quinone
complexes, ∼3000 cm-1 compared to ∼1800 cm-1. This may
be a consequence of specific solvent interactions with the
quinone oxygen atoms in the semiquinone-based electron-
transfer intermediates, which increases solvent coupling to
electron transfer.
Calculation of Electron-Transfer Rate Constants. From
electron transfer theory for a single coupled medium- or high-
frequency mode, the rate constant for electron transfer is given
by eq 149-13,78,83,84
Figure 6. Schematic energy-coordinate diagrams illustrating the effect
of increasing electronic coupling and the transition between electron
2
(|∆Go| + V′pω + λo,L
4λo,LkBT
)
transfer in the inverted region (A) to nonradiative decay (C). The energy
SV′
curves were calculated by using the following equations: E1 ) [λ(2X2
kET ) νET exp(-S)
exp -
- 2X + 1 + ∆E0]/2 - {[λ(2X - 1) - ∆E0]2 + 4HDA
}
1/2/2; E2 )
2
∑
(
)
V′!
V′
[λ(2X2 - 2X + 1 + ∆E0]/2 + {[λ(2X - 1) - ∆E0]2 + 4HDA
}
1/2/2 and
2
the parameters λ ) 5000 cm-1, ∆E0 ) 19 000 cm-1 and (A) HDA ) 0
cm-1, (B) HDA ) 1000 cm-1, and (C) HDA ) 2500 cm-1
(14)
.
This equation utilizes the average-mode approximation and is
valid in the limit pω . kBT. The summation is from the V ) 0
vibrational level in the initial state to all V′ levels in the final
state. The remaining parameters were defined previously. νET
is the frequency factor for electron transfer, which in the
nonadiabatic limit is given by
Figure 6 illustrates the influence of increasing electronic
coupling on the energy-coordinate curves for electron transfer
in the inverted region. The curves illustrate the variation in
energy along the normal mode of a coupled vibration as HDA
increases.78,79 For the case in Figure 6A, HDA ≈ 0, which is the
case for the AFA2- and OQD complexes. Electron transfer in
these complexes occurs between electronically weakly coupled
sites with a large free energy change, ∆Go. As noted above,
they are “redox-separated states” and conversion to the ground
state occurs by a combination of radiative decay and nonradia-
tive decay by electron transfer. Electron transfer is dominated
by nuclear tunneling through vibrational overlap in the coupled
medium- and high-frequency ν(4,4′-X2bpy) and ν(LA•-) modes
rather than by classical barrier-crossing at the intersection
between energy curves.80,81
2
2πHDA
1/2
1
νET
)
(15)
(
)
p
2πp ωE0
In the limit E0 . Spω, the sum in eq 14 is accurately given by
the “energy gap law” result in eq 16, with F(calc) the Franck-
Condon weighted density of states.7,16,81,82,85
γE0
γ + 1
pω
kET ) νET exp(-S) exp -
exp
2kBTλo,L (16a)
(
)
{
(
}
)
πω
The effect of increasing HDA, as in the BIQD complex, is
illustrated in Figure 6B. As in mixed-valence complexes,
enhanced electronic coupling mixes the electronic character of
the donor and acceptor which decreases the classical barrier to
electron transfer.
E0
γ ) ln
- 1
(16b)
(16c)
(
)
Spω
ln kET ) ln νET + ln[F(calc)]
The equivalent to the delocalized, mixed-valence case is
shown in Figure 6C. In this limit, there is strong electronic
coupling and the zero-order energy surfaces are highly mixed
to give well separated excited- and ground-state surfaces. This
All the parameters that appear in eqs 14-16 are available from
the emission spectral fitting results and kr. Calculated values of
kET are compared with experimental values in Table 7. The same
is the case for the MLCT excited state of [Ru(bpy)3]2+
,
[RuIII(bpy•-)(bpy)2]2+*. In this limit, excited-state decay occurs
by a combination of emission and nonradiative decay (knr). The
nonradiative transition is vibronically induced by coupling with
a “promoting” mode or modes. The interconversion between
states involves a significant change in radial electronic distribu-
tion but not electron transfer.12,16,81,82
There is also information in the spectroscopic data about the
structural changes that accompany electron transfer. Values of
pω for the quinone-based BIQD and AFA2- acceptors are
significantly higher (1500-1600 cm-1) than in typical polypy-
ridyl complexes (∼1300 cm-1). This is a consequence of the
addition to the average mode approximation, eq 6, of the coupled
quinone-based modes at 1612 and 1683 cm-1. It is also
values were obtained by using either eq 14 or eq 16. The ONQ
complex is not included in the comparison because the emission
from the RS state is masked by the more intense MLCT
emission.
Inspection of the data in Table 7 shows that there is agreement
between calculated and experimental rate constants that ranges
from a factor of ∼4 for the BIQD complex and to within a
factor of ∼15 for the AFA2- complex. These examples join a
limited number of others based on organic donor-acceptor
complexes, and nonradiative decay of MLCT excited states for
which it has been possible to calculate rate constants with
reasonable accuracy by using spectroscopically derived
parameters.1,14,16,21-23,69

Constants in Re(I) Chromophore-Quencher Complexes
J. Phys. Chem. A, Vol. 106, No. 34, 2002 7805
Given the approximations involved, the agreement between
calculated and experimental values is reasonable. However, it
is notable that the calculated values are lower than the
experimental values in all three cases. There may be contribu-
tions to these discrepancies from the low emission intensities
with a concomitant uncertainty in the emission spectral fitting
parameters, and the use of the calculated absorption band to
Re(CO)3(ONQ)]. Supplementary Figure 1, with time-resolved
nanosecond transient absorption difference spectra of fac-[(4,4′-
tBu2bpy)Re(CO)3(ONQ)]. Supplementary Figure 2, with time-
resolved nanosecond transient absorption difference spectra of
fac-[(4,4′-Me2bpy)Re(CO)3]2(AFA). This material is available
free of charge via the Internet at http://pubs.acs.org.
References and Notes
evaluate HDA
.
There may be another explanation. The RS states are largely
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Acknowledgment. This work was supported by the National
Science foundation under Grant No. CHE-9705724 to T.J.M.
Supporting Information Available: Supplementary Table
1, with atomic positional parameters for fac-[(4,4′-tBu2bpy)-

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