Tuning charge recombination rate constants through inner-sphere coordination in a copper(I) donor-acceptor compound

Article abstract of DOI:10.1021/ic000197w

The coordination compounds [Cu(bpy-MV²⁺)(PPh₃)₂](PF₆)₃, where bpy-MV²⁺ is the 1-(4-(4'-methyl-2,2'-bipyridin-4-yl)butyl)-1'-methyl-4,4'-bipyridinediium(2+) cation, and [Cu(dmb)(PPh₃)₂](PF₆), where dmb is 4,4'-dimethyl-2,2'-bipyridine, have been prepared and characterized. Visible light (417 nm) excitation of [Cu(bpy-MV²⁺)(PPh₃)₂]³⁺ at room temperature leads to rapid intramolecular electron transfer, k(cs) > 1 x 10⁸ s^-1, to form a charge-separated state with an electron localized on the pendant viologen group and a copper(II) metal center, abbreviated [Cu(II)-bpy-MV·⁺]. This state recombines to ground-state products with first-order rate constants that can be tuned with solvent over a ~10⁷ - 10⁵ s^-1 range. The activation parameters were determined from temperature-dependent electron-transfer data with Arrhenius analysis. A model is proposed wherein a solvent molecule is coordinated to Cu(II) in the charge-separated state, [(S)Cu(II)-bpy-MV·⁺]. Visible light excitation of [Cu(dmb)(PPh₃)₂](PF₆) in argon-saturated dichloromethane produces long-lived photoluminescent excited states, τ = 80 ns, that are dynamically quenched by the addition of Lewis basic solvents. The measured quenching constants each correlate well with the lifetime of the charge-separated state measured after excitation of [Cu(bpy-MV²⁺)(PPh₃)₂]³⁺ in the corresponding solvent.

Full text of DOI:10.1021/ic000197w

Inorg. Chem. 2000, 39, 3765-3770
3765
Tuning Charge Recombination Rate Constants through Inner-Sphere Coordination in a
Copper(I) Donor-Acceptor Compound
Donald V. Scaltrito, Craig A. Kelly, Mark Ruthkosky, Mark C. Zaros, and Gerald J. Meyer*
Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218
ReceiVed February 23, 2000
The coordination compounds [Cu(bpy-MV2+)(PPh3)2](PF6)3, where bpy-MV2+ is the 1-(4-(4′-methyl-2,2′-bipyridin-
4-yl)butyl)-1′-methyl-4,4′-bipyridinediium(2+) cation, and [Cu(dmb)(PPh3)2](PF6), where dmb is 4,4′-dimethyl-
2,2′-bipyridine, have been prepared and characterized. Visible light (417 nm) excitation of [Cu(bpy-MV2+)(PPh3)2]3+
at room temperature leads to rapid intramolecular electron transfer, kcs > 1 × 108 s-1, to form a charge-separated
state with an electron localized on the pendant viologen group and a copper(II) metal center, abbreviated [CuII-
bpy-MV•+]. This state recombines to ground-state products with first-order rate constants that can be tuned with
solvent over a ∼107-105 s-1 range. The activation parameters were determined from temperature-dependent
electron-transfer data with Arrhenius analysis. A model is proposed wherein a solvent molecule is coordinated to
Cu(II) in the charge-separated state, [(S)CuII-bpy-MV•+]. Visible light excitation of [Cu(dmb)(PPh3)2](PF6) in
argon-saturated dichloromethane produces long-lived photoluminescent excited states, τ ) 80 ns, that are
dynamically quenched by the addition of Lewis basic solvents. The measured quenching constants each correlate
well with the lifetime of the charge-separated state measured after excitation of [Cu(bpy-MV2+)(PPh3)2]3+ in the
corresponding solvent.
Introduction
We recently communicated a strategy for quantifying these
important processes that circumvents the diffusion limitations
Electron-transfer processes that are accompanied by large
conformational changes represent an important class of redox
events in biology and chemistry.1,2 In principle, a conformational
change can be the rate-limiting step, and in such cases, “gated”
or “directional” electron transfer is predicted to occur.3-5 The
extent to which nature utilizes structural changes to modulate
electron-transfer dynamics represents an important area of
ongoing research.6,7 Rate constants for electron transfer or
structural reorganization have been experimentally obtained on
millisecond and longer time scales by NMR, EPR, electro-
chemical, and stopped-flow techniques, but these measurements
are often frustrated by diffusional encounters of the electron
donor and acceptor that can be rate limiting. Nevertheless,
examples of fully gated electron transfer have been realized in
model systems.8
of bimolecular redox chemistry and opens up new time windows
for characterization.9 The key ideas were to (1) exploit the
known geometric changes that are novel to Cu(II/I) redox
chemistry and (2) photoinitiate the process such that transient
spectroscopic techniques could be employed to gain insight into
mechanistic details of structural reorganization and electron
transfer. This strategy was successful. Visible light excitation
of the Cu(I) donor-acceptor compound where L is triphen-
(1) Bolton, J. R.; Mataga, N.; McLendon, G. Electron Transfer in
Inorganic, Organic, and Biological Systems; Advances in Chemistry
Series 228; American Chemical Society: Washington, DC, 1991.
(2) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265-
322.
ylphosphine, in fluid solution at room temperature led to rapid
intramolecular charge separation, kcs > 1 × 108 s-1, to form a
charge-separated state with an electron localized on the pendant
methyl viologen and an oxidized copper metal center. Back
(3) Hoffman, B. M.; Ratner, M. A. J. Am. Chem. Soc. 1987, 109, 6243-
6257.
electron transfer to generate ground-state products, MV•+
f
(4) Brunschwig, B. S.; Sutin, N. J. Am. Chem. Soc. 1989, 111, 7454-
7465.
(5) (a) Lumry, R.; Eyring, H. J. Phys. Chem. 1954, 58, 110-120. (b)
Vallee, B. L.; Williams, R. J. P. Proc. Natl. Acad. Sci. U.S.A. 1968,
59, 498-505.
(6) (a) Graige, M. S.; Paddock, M. L.; Bruce, J. M.; Feher, G.; Okamura,
M. K. J. Am. Chem. Soc. 1996, 118, 9005-9016. (b) Stowell, M. H.
B.; McPhillips, T. M.; Rees, D. C.; Soltis, S. M.; Abresch, E.; Feher,
G. Science 1997, 276, 812-816.
(7) Williams, R. J. P. Eur. J. Biochem. 1995, 234, 363.
(8) (a) Meagher, N. E.; Juntunen, K. L.; Salhi, C. A.; Ochrymowycz, L.
A.; Rorabacher, D. B. J. Am. Chem. Soc. 1992, 114, 10411-10420.
(b) Flanagan, S.; Dong, J.; Haller, K.; Wang, S.; Scheidt, W. R.; Scott,
R. A.; Webb, T. R.; Stanbury, D. M.; Wilson, L. J. J. Am. Chem. Soc.
1997, 119, 8857-8864.
Cu(II), was first order with rate constants that could be tuned
with solvent over a kcr ∼ 107-105 s-1 range. To account for
this remarkable solvent dependence, a model was proposed
wherein the coordination number at Cu(II) increased to 5 in
the charge-separated state.9 In support of this model, it was noted
that Cu(II) compounds are subject to Jahn-Teller distortions
and thus Cu(II) generally adopts higher coordination numbers
than does Cu(I).10
Here we report a more detailed study of the photodriven
electron transfer in this first-generation copper(I) dyad including
temperature-dependent electron-transfer data acquired in four
10.1021/ic000197w CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/21/2000

3766 Inorganic Chemistry, Vol. 39, No. 17, 2000
Scaltrito et al.
Chart 1. Atom Numbering for [Cu(bpy-MV)(PPh3)]3+
(c) Transient Absorption. Transient absorption measurements were
different solvents. These data are contrasted with excited-state
properties of [Cu(dmb)(PPh3)2]+ in the presence and absence
of Lewis basic solvents that quench the excited state. The
excited-state-quenching constants are found to be correlated with
the charge-separated-state lifetimes observed after excitation of
the dyad. The results support the hypothesis that inner-sphere
reorganization at copper underlies the remarkable solvent tuning
of electron-transfer dynamics.
carried out on an apparatus previously described.13 Briefly, excitation
was carried out using 355 nm pulses, ca. 8 ns and 1-30 mJ cm-2
,
from a Continuum Surelite II Nd:YAG laser or 417 nm pulses obtained
by focusing the 355 nm line in an H2-pressurized Raman shifter. The
5 mm diameter beam was expanded to ensure homogeneous illumina-
tion of the cuvette. The sample was protected from the probe light
using a fast shutter and appropriate UV- and heat-absorbing glass and
solution filter combinations. In general, kinetic traces represent the
average of 10-50 laser shots.
(d) Photoluminescence. Corrected photoluminescence (PL) spectra
were obtained with a Spex Fluorolog instrument that had been calibrated
with a standard NBS tungsten-halogen lamp. PL lifetimes were
obtained with pulsed 336 nm light excitation from a nitrogen laser and
with a detection apparatus that was previously described.13
Electrochemistry. Cyclic voltammetry experiments were performed
in a 0.1 M tetrabutylammonium hexafluorophosphate (TBAH)/aceto-
nitrile electrolyte. A BAS Model CV27 potentiostat was used in a
standard three-electrode arrangement consisting of a 1 cm2 Pt working
electrode, a Pt gauze counter electrode, and an SCE reference electrode.
Approximately millimolar concentrations of the compounds were
dissolved in the electrolyte and the solutions were subsequently purged
with argon.
Experimental Section
Materials. Spectroscopic grade solvents were purchased from
Burdick & Jackson and used as received. [Cu(CH3CN)4](PF6) and bpy-
MV2+ were prepared by published methods.11,12
Synthesis. The cuprous phosphine compounds were prepared by
stirring 1 equiv each of [Cu(CH3CN)4](PF6) and [1-(4-(4′-methyl-2,2′-
bipyridin-4-yl)butyl)-1′-methyl-4,4′-bipyridinediium](PF6)2 (abbreviated
bpy-MV2+) or 4,4′-dimethyl-2,2′-bipyridine (abbreviated dmb; Aldrich)
with 2.5 equiv of triphenylphosphine (Aldrich; recrystallized from
ethanol) under an argon atmosphere in acetonitrile for 10 min. The
solution was evaporated to dryness under vacuum at room temperature,
the residue was dissolved in acetone, and the yellow product was
precipitated by layering diethyl ether over the acetone solution. Typical
yields were 80-90% for [Cu(dmb)(PPh3)](PF6) and 65-75% for [Cu-
(bpy-MV2+)(PPh3)](PF6)3.
Results
[Cu(4,4′-dimethyl-2,2′-bipyridine)(PPh3)2](PF6): 1H NMR (CD3-
CN) δ CH3- 2.5 (singlet), PPh3 7.0 (doublet), 7.2 (triplet), 7.35 (triplet),
dmb 7.12 (doublet), 8.11 (doublet), 8.30 (singlet). Anal. Calc (found):
C, 62.85 (62.83); H, 4.61 (4.68); N, 3.05 (3.00).
The ground-state absorption spectra of [Cu(bpy-MV2+)-
(PPh3)2](PF6)3 and [Cu(dmb)(PPh3)2](PF6) display a broad
charge-transfer band centered at ∼360 nm in all the solvents
examined. The absorption band tails into the visible region,
giving the solutions a yellow color. Photoluminescence, PL, is
observed for [Cu(dmb)(PPh3)2](PF6) in dichloromethane solution
with a maximum centered around 680 nm. The PL is red-shifted
significantly from the absorption band and extends beyond the
sensitivity of our fluorimeter, as shown in the Supporting
Information. No detectable PL is observed for [Cu(bpy-MV2+)-
(PPh3)2](PF6)3 under the same conditions.
[Cu(4-(4-(1′-methyl-4,4′-bipyridinediium-1-yl)butyl)-4′-methyl-
1
2,2′-bipyridine)(PPh3)2](PF6)3•H2O: H NMR (CD3CN) (atom num-
bering: Chart 1) δ -CH2- 1.9 (2H, H9, quintet), -CH2- 2.1 (2H,
H8, quintet), -CH3 2.5 (3H, H7′, singlet), -CH2- 2.8 (2H, H7, triplet),
N+CH3 4.4 (3H, H15, singlet), N+CH2- 4.7 (2H, H10, triplet), 7.14 (12H,
H16, doublet), 7.27 (14H, H17, H5, multiplet), 7.40 (6H, H18, triplet),
8.28 (2H, H3, singlet), 8.38-8.44 (5H, H3′, H12, H13, m), 8.60-8.67
(2H, H6, dd), 8.85-8.95 (4H, H11, H14, dd). Anal. Calc (found): C,
51.80 (50.96); H, 4.21 (4.24); N, 3.90 (4.05).
Time-resolved PL data for [Cu(dmb)(PPh3)2](PF6) in argon-
saturated dichloromethane display nonexponential kinetics. The
PL decays comprise a fast component that can not be time-
resolved, τ < 10 ns, and a slower exponential component with
an 80 ns lifetime. The fast component is present when a 10-
fold increase in triphenylphosphine is added to the dichlo-
romethane solution. Pulsed laser excitation of [Cu(dmb)(PPh3)2]-
(PF6) in argon-saturated dichloromethane at room temperature
results in the transient absorption spectra shown in Figure 1.
Spectroscopy. (a) UV-Vis. All UV-visible ground-state absorption
spectra were acquired at ambient temperature using a Hewlett-Packard
8453 diode array spectrometer.
(b) NMR. H NMR spectra were obtained on a Bruker 300AMX
FT-NMR spectrometer.
1
(9) Ruthkosky, M.; Kelly, C. A.; Zaros, M. C.; Meyer, G. J. J. Am. Chem.
Soc. 1997, 119, 12004-12005.
(10) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. ReV. 1996, 96,
2239-2314 and references therein.
(11) Shriver, D. F. Inorg. Synth. 1979, 19, 90.
(12) Yonemoto, E. H.; Riley, R. L.; Kim, Y.; Atherton, S. J.; Schmehl, R.
H.; Mallouk, T. E. J. Am. Chem. Soc. 1992, 114, 8081-8087.
(13) Ruthkosky, M.; Castellano, F. N.; Meyer, G. J. Inorg. Chem. 1996,
35, 6406-6412.

Tuning Charge Recombination Rate Constants
Inorganic Chemistry, Vol. 39, No. 17, 2000 3767
Figure 1. Absorption difference spectra after pulsed 417 nm light excitation (∼3 mJ/cm2, 8 ns fwhm) of [Cu(dmb)(PPh3)2]+ in argon-saturated
dichloromethane at room temperature. The spectra are shown at the following delay times: 0 ns (squares); 10 ns (circles); 25 ns (up triangles); 50
ns (stars); 100 ns (diamonds); 200 ns (down triangles); 400 ns (crosses); and at infinite time (circles). The inset displays single-wavelength absorption
transients monitored at 390 nm with a superimposed first-order fit to the data.
concentration. The decrease in lifetime is well described by the
Stern-Volmer model, eq 1. The inset of Figure 2 displays
τo/τ ) 1 + KSV[Q] ) 1 + (kq[Q]τo)
(1)
Stern-Volmer analysis of the data shown, from which a Stern-
Volmer constant, KSV, and an apparent quenching rate constant,
kq ) 8.3 × 108 M-1 s-1, were abstracted. The KSV and kq values
for a number of Lewis bases found to dynamically quench the
excited state are given in the Supporting Information. No
evidence for static quenching was found over the concentration
range 0.0-0.2 M. [Cu(dmb)(PPh3)2](PF6) is nonemissive in neat
CH3CN and DMSO with excited-state lifetimes that could not
be time-resolved with our instrumentation, τ < 10 ns.
Pulsed laser excitation (λ ) 355 or 416 nm) of the donor-
acceptor complex, [Cu(bpy-MV2+)(PPh3)2]3+, leads to the
immediate appearance of the absorption difference spectra
shown in Figure 3. Charge recombination follows first-order
kinetics in argon-saturated DMSO, 1,2-dichloroethane (DCE),
DMF, and CH3CN solvents (Table 1) and in argon-saturated
CH2Cl2. The inset in Figure 3 displays typical time-resolved
absorption data monitored at 400 nm following excitation with
416 nm light. Superimposed on these data is the fit to a first-
order kinetic model. The recombination kinetics are independent
of the monitoring wavelength (λ ) 330-650 nm), the sample
concentration (0.1-20 mM), and the excitation irradiance (355
or 417 nm, 3-30 mJ/cm2). Ground-state absorption measure-
ments before and after laser excitation reveal negligible sample
decomposition in DMSO and acetonitrile. In dichloromethane,
a decrease in the absorption band centered near 350 nm was
occasionally observed after prolonged laser irradiation. The
extent of the photochemistry could be minimized by employing
417 nm excitation light and was sufficiently small that it did
not represent an obstacle to these experimental studies.
Figure 2. Single-wavelength absorption transients monitored at 390
nm after pulsed 417 nm excitation (∼5 mJ/cm2, 8 ns fwhm) of [Cu-
(dmb)(PPh3)2]+ in dichloromethane at room temperature as a function
of added DMSO. From top to bottom the transients were recorded in
the following DMSO concentrations: 0, 8, 20, 35, 50 mM. The inset
displays a Stern-Volmer plot of these data from which a Stern-Volmer
constant was abstracted, KSV ) 70 M-1
.
The kinetics are first order, independent of the monitoring
wavelength within experimental error, and the transients return
to pre-excitation levels cleanly. The measured lifetime is 80 (
5 ns. No evidence is found for a second component. Ground-
state absorption measurements before and after pulsed laser
excitation reveal negligible photochemistry.
The [Cu(dmb)(PPh3)2]+* excited state is dynamically quenched
by the addition of Lewis bases to the dichloromethane solution.
Shown in Figure 2 are typical time-resolved absorption traces
recorded at 380 nm as a function of dimethyl sulfoxide, DMSO,
The temperature dependence of charge recombination was

3768 Inorganic Chemistry, Vol. 39, No. 17, 2000
Scaltrito et al.
Figure 3. Absorption difference spectra observed after laser excitation (417 nm; 8 ns fwhm, 4 mJ/pulse) of [Cu(bpy-MV2+)(PPh3)2](PF6)3 in
argon-saturated dichloromethane at the following delay times: 0 ns (squares); 10 ns (circles); 25 ns (up triangles); 50 ns (diamonds); 100 ns (down
triangles); 200 ns (squares); and at infinite time (circles). The inset displays a single-wavelength transient monitored at 400 nm with a superimposed
fit to a first-order kinetic model.
Table 1. Kinetic Data and Activation Parametersa
b
c
solvent
∆Gq (kJ/mol)
∆Hq (kJ/mol)
∆Sq (J/(K mol))
kcr (s-1
)
kq (M-1 s-1
)
CH2ClCH2Cl
CH3CN
DMF
31 ( 4
32 ( 4
40 ( 3
39 ( 3
2 ( 0.5
4 ( 1
-90 ( 10
-90 ( 10
-70 ( 6
-60 ( 6
4.9 × 107
3.1 × 107
9.7 × 105
5.6 × 105
-
1.5 × 108
6.2 × 108
8.3 × 108
18 ( 2
25 ( 3
DMSO
a Thermodynamic activation parameters for charge recombination measured from Arrhenius analysis of the temperature-dependent first-order
b
rate constants for charge recombination. kcr is the first-order electron-transfer rate constant for charge recombination measured at 25 °C. The
estimated error is (10%. c Rate constants for the excited-state quenching of [Cu(dmb)(PPh3)2]+* by CH3CN, DMF, and DMSO in CH2Cl2, from
Table S1 (Supporting Information) measured at 25 °C.
monitored at 400 nm after 417 nm excitation of [Cu(bpy-MV2+)-
(PPh3)2]3+ in DMSO, DMF, CH3CN, and DCE. DCE was
chosen for its more favorable temperature window when
compared to CH2Cl2. At all temperatures, the rate of charge
separation could not be time-resolved. Arrhenius plots of the
observed first-order rate constants for charge recombination are
given in the Supporting Information, and the abstracted ther-
modynamic data are given in Table 1. The rate constants were
independent of the temperature change direction and were found
to be reversible.
The electrochemical properties were measured by cyclic
voltammetry in 0.1 M TBAH/acetonitrile solutions. [Cu(bpy-
MV2+)(PPh3)2](PF6)3 and [Cu(dmb)(PPh3)2](PF6) both displayed
an irreversible oxidation wave at ∼0.7 V vs SCE with scan
rates from 20 to 200 mV/s. Irreversible Cu(II/I) redox chemistry
was also observed for [Cu(dmb)(PPh3)2]+ in dichloromethane
and dimethyl sulfoxide electrolytes. For [Cu(bpy-MV2+)-
(PPh3)2](PF6)3, a reversible viologen reduction at -0.39 vs SCE
was observed in acetonitrile electrolyte.
charge-transfer excited state, MLCT, eq 2.15 The positive
hν
+
[Cu(dmb)(PPh3)2]+ 98 [CuII(dmb-)(PPh3)2 * ≡
Cu(dmb)(PPh3)2]+* (2)
absorption features at 390 and 550 nm are expected for the
reduced dmb ligand.16 The high-energy absorption obscures the
expected bleach of the ground-state MLCT band. The positive
feature at ∼475 nm is tentatively assigned to a PR3 f Cu(II)
ligand-to-metal charge-transfer (LMCT) band.17 A similar band
is observed at lower energy, ∼500 nm, in the charge-separated
state, [CuII(bpy-MV•+)(PPh3)2]3+. The shift to lower energy is
expected for an LMCT transition and a fully oxidized Cu(II)
(14) (a) Del Paggio, A. A.; McMillin, D. R. Inorg. Chem. 1983, 22, 691-
692. (b) Palmer, C. E. A.; McMillin, D. R. Inorg. Chem. 1987, 26,
3837-3840.
(15) For recent reviews of copper(I) excited states, see: (a) Kutal, C. Coord.
Chem. ReV. 1990, 99, 213. (b) Kalyanasundaram, K. Photochemistry
of Polypyridine and Porphyrin Complexes; Academic Press: London,
1992; Chapter 9. (c) Horvath, O. Coord. Chem. ReV. 1994, 135, 303-
324.
Discussion
(16) (a) Nakumura, T.; Soma, M.; Onishi, T.; Tamura, K. Z. Phys. Chem.
(Munich) 1974, 89, 122. (b) Kato, T.; Shida, T. J. Am. Chem. Soc.
1979, 101, 6869.
(17) Hong, B.; Woodcock, S. R.; Saito, S. K.; Ortega, J. V. J. Chem. Soc.,
Dalton Trans. 1998, 2615-2623.
Excited-State Behavior. Room-temperature photolumines-
cence is observed after light excitation of [Cu(dmb)(PPh3)2]+*
in dichloromethane solution.14 The transient absorption spectra
of [Cu(dmb)(PPh3)2]+* are consistent with a metal-to-ligand

Tuning Charge Recombination Rate Constants
Inorganic Chemistry, Vol. 39, No. 17, 2000 3769
center. Charge-transfer transitions have also been previously
observed in the excited-state absorption spectra of Os(II)
coordination compounds with phosphine ligands.17
We consider two possible kinetic limits based on eqs 3 and
4 that are consistent with the observed results. The first is that
eq 3 represents a rapid preequilibrium, ka + kd . k4. In this
case, the measured quenching rate constant is approximately
equal to the product of the equilibrium constant and the exciplex
quenching rate; i.e., kq ∼ k4K3. Alternatively, if solvent addition
to the excited state is slow such that ka + kd , k4, then
equilibrium is not achieved and kq ∼ ka.
[Cu(dmb)(PPh3)2]+* excited-state relaxation measured by
transient absorption is first order with a lifetime of 80 ns at
room temperature, while the time-resolved photoluminescence
displays this component and a fast component (or component(s))
that could not be time-resolved. The discrepancy between the
transient photoluminescence and absorption data indicates either
is an emissive impurity present in low concentrations or that
the change in extinction coefficient between the ground state
and the fast component(s) excited state(s) is too small to be
detected by absorption. We note that the fast component is
present in rigorously purified [Cu(dmb)(PPh3)2]+* and in other
substituted bpy complexes but is not observed for [Cu(dmp)-
(PPh3)2]+*, where dmp is 2,9-dimethyl-1,10-phenanthroline.18
The origin of the short-lived luminescent component is the
subject of ongoing studies.19
Table 1 shows that the Lewis base induced excited-state-
quenching rate constant, kq, of [Cu(dmb)(PPh3)2]+* is inversely
related to the first-order rate constant, kcr, and activation
enthalpy, ∆Hq, for charge recombination observed after pulsed
light excitation of [Cu(bpy-MV2+)(PPh3)2]3+. The proposed
mechanism for charge recombination and solvent coordination
is shown in eqs 5 and 6. In contrast to the excited-state chemistry
ka′
[CuII(bpy-MV•+)(PPh3)2]3+ + S y
\
z
kd′
Solvent Tuning. The [Cu(dmb)(PPh3)2]+* MLCT excited
state is dynamically quenched by the addition of Lewis bases
to the dichloromethane solution. The quenching follows the
Stern-Volmer model, and the quenching rate constants vary
from ∼8 × 107 M-1 s-1 for tetrahydrofuran to 8.3 × 108 M-1
s-1 for DMSO. McMillin and co-workers have observed similar
behavior for [Cu(dmp)2]+* and other emissive Cu(I) diimines
and have explored the phenomena in considerable detail.20 Their
results and interpretation are consistent with quenching via a
5-coordinate Lewis base excited state adduct, or “exciplex”. An
exciplex, by definition, is unstable with respect to ground-state
products, and formation promotes nonradiative decay. In support
of this mechanism being operative for [Cu(dmb)(PPh3)2]+*
quenching, trends in measured quenching constants, where
comparisons are possible, are in agreement with this previous
work.20 Furthermore, at the irradiances utilized here, only a
shortened excited state is observed with no spectroscopic
evidence for 5-coordinate Cu(I) compounds, ground-state ad-
ducts, or other chemical products.
[(S)CuII(bpy-MV•+)(PPh3)2]3+ (5)
kcr
[(S)CuII(bpy-MV•+)(PPh3)2]3+ 9
8
[CuI(bpy-MV2+)(PPh3)2]3+ + S (6)
of [Cu(dmb)(PPh3)2]+, “full” electron transfer to the viologen
moiety yields a spectroscopically distinguishable intermediate
attributed to the solvent adduct, [(S)CuII(bpy-MV•+)(PPh3)2]3+
;
that is, K5 . 1 and the reaction is rate-limited by eq 6. This is
consistent with eqs 3 and 5, both being rapid equilibria with K3
, 1 and K5 . 1 due to the stronger Lewis acidity of the CuII
center in the charge-separated state compared to the more
electron delocalized d f π* excited state. The charge recom-
bination reaction is therefore rate-limited by kcr and kobs ∼ kcr.
The solvent donor strength correlations with excited-state-
quenching rate constants, charge recombination rates, and
enthalpies of activation support the notion that inner-sphere
coordination controls the charge recombination rate: the stronger
the CuII-S bond strength, the longer-lived the charge-separated
state.
The quenching rate constants abstracted from Stern-Volmer
analysis are below what one would expect for a diffusion-limited
process in dichloromethane, kdiff ) 1.6 × 1010 M-1 s-1 on the
basis of the Smoluchowski-Stokes-Einstein equations.21 To
account for the relatively low quenching constants measured, a
simplified mechanism depicted by eqs 3 and 4 is proposed where
Conclusion
Two Cu(I) coordination compounds, [Cu(bpy-MV2+)(PPh3)2]-
(PF6)3 and [Cu(dmb)(PPh3)2](PF6), have been prepared and
characterized. An interesting solvent correlation was discovered
that relates the excited-state-quenching efficiency of the latter
compound to the electron-transfer dynamics of the former. This
correlation is consistent with the hypothesis that inner-sphere
coordination underlies the remarkable solvent tuning of the
electron-transfer dynamics. The data supporting this hypothesis
are compelling yet indirect. Future studies will be directed
toward utilizing time-resolved vibrational spectroscopies of
ka
[CuII(dmb-)(PPh3)2]+* + S y
\
kz
d
[(S)CuII(dmb-)(PPh3)2]+* (3)
k4
[(S)CuII(dmb-)(PPh3)2]+*
9
8 [CuI(dmb)(PPh3)2]+ + S (4)
S is a solvent molecule that acts as a Lewis base quencher. An
important consequence of invoking a composite mechanism is
that the measured quenching constants, kq, may not be elemen-
tary rate constants.20
(18) Ruthkosky, M. Ph.D. Thesis, The Johns Hopkins University, 1998.
(19) Scaltrito, D. V.; O’Callaghan, J. A.; Meyer, G. J. Work in progress.
(20) (a) McMillin, D. R.; Kirchoff, J. R.; Goodwin, K. V. Coord. Chem.
ReV. 1985, 64, 83. (b) Palmer, C. E. A.; McMillin, D. R.; Kirmaier,
C.; Holten, D. Inorg. Chem. 1987, 26, 3167. (c) Stacy, E. M.;
McMillin, D. R. Inorg. Chem. 1990, 29, 393-396.
The adduct shown as the product of eq 3 is not a simple
encounter complex but must be thought of as an excited-state
complex, or “exciplex”. The exciplex has never been directly
observed, but the indirect evidence for its presence is compel-
ling.20 Since both the transient absorption and PL spectra are
essentially independent of solvent and since the spectral
properties of the 5-coordinate solvent adducts are expected to
differ significantly from those of the 4-coordinate complex,22
it seems reasonable to conclude that the transient species
observed is the uncomplexed MLCT excited state [Cu(dmb)-
(PPh3)2]+*.
(21) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photrochemistry,
2nd ed.; Marcel Dekker: New York, 1993; pp 207-208.
(22) (a) Stumpf, H. O.; Pei, Y.; Ouahab, L.; Le Berre, F.; Codjovi, E.;
Kahn, O. Inorg. Chem. 1993, 32, 5687. (b) Persson, I.; Penner-Hahn,
J. E.; Hodgson, K. O. Inorg. Chem. 1993, 32, 2497. (c) Tezuka, Y.;
Hashimoto, A.; Ushizaka, K.; Imai, K. J. Org. Chem. 1990, 55, 329.
(d) Bencini, A.; Gatteschi, D.; Zanchini, C. Inorg. Chem. 1986, 25,
2211. (e) Stibrany, R. T.; Knapp, S.; Potenza, J. A.; Schugar, H. J.
Inorg. Chem. 1999, 38, 132.

3770 Inorganic Chemistry, Vol. 39, No. 17, 2000
Scaltrito et al.
related copper donor-acceptor compounds to directly probe the
coordination number of copper.
administered by the American Chemical Society, for support
of this research.
Supporting Information Available: Ground-state absorption and
emission spectra, Arrhenius plots, and a table of Stern-Volmer
quenching constants. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. The authors thank Professor Thomas
Mallouk and Dr. Edward Yonemoto for helpful discussions and
an initial sample of the bpy-MV2+ ligand. Acknowledgment is
also made to the donors of the Petroleum Research Fund,
IC000197W