On the quenching of MLCTRe-bpy luminescence by Cu(II) species in Re(I) polymer micelles

Article abstract of DOI:10.1021/jp053758j

Transmission electron microscopy (TEM) and dynamic light scattering (DLS) studies on acetonitrile solutions of the polymer {[(vpy)₂-vpyRe(CO) ₃bpy] CF₃SO₃}₂₀₀ demonstrated that the Re(I) polymer molecules aggregate to form spherical micelles of radius R = 156 nm. Coordination of Cu(II) species to the Re (I) polymer causes a decrease in the micelle radius and a distortion from the spherical shape. Besides, the coordination of Cu(II) species to the {[(vpy)₂-vpyRe(CO) ₃bpy] CF₃SO₃}₂₀₀ polymer produces the quenching of the metal to ligand charge transfer (MLCT) excited state by energy transfer processes that are more efficient than those in the quenching of the monomer pyRe(CO)₃bpy⁺ luminescence by Cu(II). Moreover, the kinetics of the quenching by Cu(II) do not follow a Stern-Volmer behavior. Conversely, the quenching of the MLCT luminescence of the Re(I) polymer by the sacrificial electron donor 2,2′,2″-nitrilotriethanol, TEOA, follows a Stern-Volmer kinetics. A comparison is made between the quenching by CuX₂ (X = Cl or CF₃SO₃) and TEOA.

Full text of DOI:10.1021/jp053758j

22890
J. Phys. Chem. B 2005, 109, 22890-22898
On the Quenching of MLCTRefbpy Luminescence by Cu(II) Species in Re(I)
Polymer Micelles
Ezequiel Wolcan,*,† Jose´ L. Alessandrini,‡ and Mario R. Fe´liz*,§
INIFTA, Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata,
Casilla de Correo 16, Sucursal 4, (1900) La Plata, Repu´blica Argentina
ReceiVed: July 8, 2005; In Final Form: September 22, 2005
Transmission electron microscopy (TEM) and dynamic light scattering (DLS) studies on acetonitrile solutions
of the polymer {[(vpy)2-vpyRe(CO)3bpy] CF3SO3}200 demonstrated that the Re(I) polymer molecules aggregate
to form spherical micelles of radius R ) 156 nm. Coordination of Cu(II) species to the Re (I) polymer causes
a decrease in the micelle radius and a distortion from the spherical shape. Besides, the coordination of Cu(II)
species to the {[(vpy)2-vpyRe(CO)3bpy] CF3SO3}200 polymer produces the quenching of the metal to ligand
charge transfer (MLCT) excited state by energy transfer processes that are more efficient than those in the
quenching of the monomer pyRe(CO)3bpy+ luminescence by Cu(II). Moreover, the kinetics of the quenching
by Cu(II) do not follow a Stern-Volmer behavior. Conversely, the quenching of the MLCT luminescence of
the Re(I) polymer by the sacrificial electron donor 2,2′,2′′-nitrilotriethanol, TEOA, follows a Stern-Volmer
kinetics. A comparison is made between the quenching by CuX2 (X ) Cl or CF3SO3) and TEOA.
Introduction
al. dealt with the photophysics of π-conjugated Re(I) oligomers
of aryleneethynylenes that contain the (bpy)Re(CO)3Cl chro-
mophore (bpy ) 2,2′-bipyridine).27 Photophysical properties in
those Re(I) compounds were dominated by the intense π f π*
transitions of the oligomeric aryleneethynylenes, with the
exception of the shorter Re-oligomer of the series, where the
MLCT was the lowest excited state.
We have previously investigated solvent and temperature
effects on the photophysical properties of the polymer {[(vpy)2-
vpyRe(CO)3bpy]CF3SO3}n∼200 and the related monomer CF3-
SO3[pyRe(CO)3bpy]. Differences between the polymer and the
monomer photophysical behavior were rationalized in terms of
solvent and thermal effects on the transition between rigid rod
and coil structures of the Re(I)-polymer.28
During the past three decades, numerous studies have been
concerned with luminescent Re(I) tricarbonyl complexes coor-
dinated to mono or bidentate azines. The accessible excited
states, Re(I) to azine metal-to-ligand charge transfer (MLCT),
ligand-to-ligand charge transfer (LLCT), and/or intraligand (IL)
excited states, are generally involved with the observed lumi-
nescence at room temperature.1-5 A rational design in the
synthesis of the azine ligands was used to tune the photophysical
and photochemical properties of the metal complexes in order
to obtain photosensitizers that might be utilized in areas such
as electron-transfer studies,6 solar energy conversion,7-9 and
potential technical applications to catalysis.10
Another approach, however, involved the synthesis of com-
pounds where the metal complex containing the chromophore
unit was attached to an organic polymeric backbone. Much of
the literature examples involve transition metal ions metalating
poly(p-phenylenevinylene) polymers incorporating 2,2′-bipyri-
dines,11-13 polypyridil Ru(II) and/or Os(II) derivatized polysty-
rene,14-24 and some multimetallic oligomeric complexes con-
taining Ru(II) and Os(II) coordinated to 1,10-phenanthroline
(phen).25 However, little attention has been paid to Re(I)
polymeric complexes. A previous work has shown marked
differences between the photochemical and the photophysical
properties of the monomer pyRe(CO)3(phen)+ and the related
polymer [(vpy)2-vpyRe(CO)3phen+]n∼200 (vpy) vinylpyridine,
py) pyridine), revealing the presence of intrastrand electron
and energy transfer processes.26 A recent work by Schanze et
The formation of micelles has been reported in polystyrene-
block-poly(4-vinylpyridine) (PS-b-PVP) functionalized with
pendants -Re(CO)3(bpy)+ groups. Two PS-b-PVP- Re(CO)3-
(bpy)+ polymers with different Re(I) content exhibited the
formation of rodlike and/or spherical micelles, as shown by
transmission electron microscopy and light scattering experi-
ments.29, 30
Coordination of Cu(II) ions to poly-4-vinylpyridine has been
reported in the literature.31-35 In this paper, we report the
aggregation of single polymer molecules to form spherical
micelles in acetonitrile solutions of the {[(vpy)2-vpyRe(CO)3bpy]-
CF3SO3}n∼200 polymer and in the polymers formed after
coordination of Cu(II) species to {[(vpy)2-vpyRe(CO)3bpy]CF3-
SO3}n∼200. We also explore the quenching of the Re(I) polymer’s
MLCT excited state by the coordination of CuX2 (X ) Cl or
CF3SO3) to the free pyridines of the polymer backbone. A
comparison is made between the quenching by CuX2 and the
quenching by the sacrificial electron donor TEOA.
* To whom correspondence should be addressed. Phone: 54-221 425
7430/425 7291. Fax: 54-221 425 4642. E-mail: ewolcan@inifta.unlp.edu.ar
(E.W.); mfeliz@inifta.unlp.edu.ar (M.R.F.).
† Member of CONICET. Departamento de Qu´ımica, Facultad de Ciencias
Exactas, UNLP.
Experimental Section
‡ Member of CICPBA. Departamento de F´ısica, Facultad de Ciencias
Exactas, UNLP.
Materials. CF3SO3[pyRe(CO)3bpy] and the polymer {[(vpy)2-
vpyRe(CO)3bpy] CF3SO3}n∼200 (Re-P4VP) were available from
§ Member of CICPBA. Departamento de Qu´ımica, Facultad de Ciencias
Exactas, UNLP.
10.1021/jp053758j CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/08/2005

On the Quenching of MLCTRefbpy Luminescence
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22891
SCHEME 1
TEM. Transmission electron micrographs were recorded on
a JEOL 100 CX electron microscope at an electron acceleration
voltage of 80 kV with a nominal point-to-point resolution of 3
Å. The photos were taken at an amplification of 20 000×.
Flash Fluorescence Procedures. Luminescence lifetime
measurements were performed at room temperature by excitation
with a Laseroptics nitrogen laser (7 ns fwhm and 2mJ/pulse at
337 nm). A modified 1P28 photomultiplier tube circuit with
ca. 1 ns response time was used as the detector for the green
emission dispersed through a monochromator. Decays typically
represented the average of 10-30 pulses and were collected
on a HP-54600B digital oscilloscope interfaced with a PC
computer. Solutions for the photochemical work were deaerated
with streams of ultrahigh-purity N2 before and during the
irradiations.
a previous work.28 The polymer can be assigned the formal
structure, Scheme 1, where pendants -Re(CO)3bpy groups are
randomly distributed along the strand of the polymer with an
average of two uncoordinated 4-vinylpyridine groups for each
one coordinated to a -Re(CO)3bpy chromophore.
CuCl2‚2H2O (Riedel-de-Hae¨n) was dried under vacuum at
80 °C until constant weight to obtain anhydrous CuCl2, which
was then mantained in a dissecator. Cu(CF3SO3)2 was obtained
by a displacement reaction between CuCl2‚2H2O and AgCF3-
SO3 in acetonitrile after removing the precipitated AgCl by
filtration. The acetonitrile solution containing Cu(CF3SO3)2 was
rotoevaporated to dryness, and then the remaining solid was
dried under vacuum at 80 °C until constant weight to obtain
anhydrous Cu(CF3SO3)2. Acetonitrile (J. T. Baker) and TEOA
(J. T. Baker) were reagent grade and were used without further
purifications.
Light Scattering Intensity Measurements. The time cor-
relation G(q,t) of the light scattering intensity was measured at
90° with a goniometer ALV/CGS-5022F with Multiple Tau
digital correlator ALV-5000/EPP covering a wide time range
(10-6-103 s). The light source was an helium/neon laser with
a wavelength 633 nm operating at 22 mW. All of the
measurements were carried out at room temperature.
Under homodyne conditions, valid for the present system,
the measured G(q,t) at a given wave vector q ) (4πn/λ) sin-
(θ/2) (n being the refractive index of the medium, λ is the
wavelength in a vacuum and θ is the scattering angle) is related
to the desired normalized electric field correlation function g(q,t)
by
Steady-State Irradiations. The luminescence of the Re(I)
complexes was investigated in a Perkin-Elmer 50-B spectro-
fluorimeter interfaced to a microcomputer. The spectra were
corrected for differences in instrumental response and light
scattering. Solutions were deaerated with O2-free nitrogen in a
gastight apparatus before recording the spectra.
General methods. UV-vis spectra were recorded on a Cary
3 spectrophotometer.
Results
Binding of Cu(II) to the Re(I)-Polymer. The addition of
an acetonitrile solution of CuX2 (X ) Cl or CF3SO3) to an
acetonitrile solution of the polymer Re-P4VP ([Re(I)] ∼ 8 ×
10-4 M, [py]uncoordinated ∼ 1.5 × 10-3M) produces a rapid
coordination of the Cu(II) species to the uncoordinated pyridines
of the Re-P4VP polymer. Indeed, after the mixing of the
solutions there is an instantaneous change in the solution color
from yellow to green, due to the appearance of a new absorption
band. The coordination of the Cu(II) species to the Re(I)
polyelectrolyte was followed by the UV-vis spectral changes
of the weak ligand field d-d bands of Cu(II) in the range 500-
900 nm, where the polymer Re-P4VP has no significant
absorption. The addition of CuCl2 to the Re(I) polymer produces
the growth of a new absorption band, centered at λmax ∼ 725
nm, which is blue shifted respect to the d-d bands of the “free”
CuCl2 (λmax ∼ 850 nm). After the coordination of Cu(II) is
complete, the solution spectrum maximum shifts from λmax
∼
725 to 850 nm due to the presence of increasing amounts of
uncoordinated CuCl2 in the solution. From a linear fit analysis
of the initial and final slopes of the plot A725 nm vs [Cu(II)] it
could be calculated that the maximum amount of Cu(II) bound
to the polymer was [Cu(II)]b,max ) 6 × 10-4 M, i.e., between
one-half and one-third of the total initial free pyridine concen-
tration. Similar spectral changes were observed after the addition
of a solution of Cu(CF3SO3)2 to a solution of the polymer Re-
P4VP in the same experimental conditions ([Re(I)] ∼ 8 × 10-4
M, [py]uncoordinated ∼ 1.5 × 10-3M). The addition of Cu(CF3-
SO3)2 to the Re(I) polymer generates a new absorption band
centered at λmax ∼ 610 nm which is blue shifted respect to the
d-d bands of the “free” Cu(CF3SO3)2 (λmax ∼ 750 nm). After
the saturation of the polymer with Cu(CF3SO3)2, there is a
progressive shifting of the absorption maximum from 610 to
750 nm due to the presence of increasing amounts of uncoor-
dinated Cu(CF3SO3)2 in the solution. From a linear fit analysis
of the initial and final slopes of the plot A600 nm vs [Cu(II)] it
could be calculated that the maximum amount of Cu(II) bound
to the polymer was [Cu(II)]b,max ) 4 × 10-4 M, i.e., less than
one-third of the total initial free pyridine concentration. It is
worthy to note that the coordination of Cu(II) to the polymer
2
G(q,t) ) A∞[1 + f*|R g(q,t)| ]
(1)
where f* is an instrumental factor, R is the fraction of the total
scattered intensity I(q) arising from fluctuations in the optical
polarizability with correlation times longer than about 10-6 s,
and A∞ is the baseline measured at long delay times.
The incident laser beam was polarized perpendicular to the
scattering plane. I(q) is the isotropic component of the light
scattering intensity due to density and concentration fluctuations
with the former being, in principle, much faster.
The characteristic relaxational parameters are extracted by
carrying out the inverse Laplace transformation (ILT) of the
measured G(q,t) assuming a superposition of exponentials36
1/2
G(q,t)
- 1
[
]
A∞
∞
) R g(q,t) ) -∞ L(ln τ)e-t/τ d(ln τ)
{
}
∫
f*
(2)
Static light scattering (SLS) measurements were performed at
two scattering angles to get estimations of the radius of gyration
through the dissymmetry method.37

22892 J. Phys. Chem. B, Vol. 109, No. 48, 2005
Wolcan et al.
n ) 3.0 were obtained for the binding of CuCl2 or Cu(CF3-
SO3)2 to the Re(I) polymer respectively (see Figure 1).
Dynamic Light Scattering (DLS). Poly-4-Vinylpyridine
(P4VP). Dilute solutions of P4VP in dichloromethane (DCM)
at concentrations between 0.2 and 0.5 mg mL-1 were analyzed
at room temperature. The spectrum of the relaxation times L(ln
τ ) is a single peak centered at τ0 ) 39 µs in all cases. The
corresponding hydrodynamic radius Rh for the highest concen-
tration is around 7.2 nm, as obtained from the Stokes-Einstein
relation (see below). No significant changes were obtained from
measurements at lower concentrations.
Re-P4VP Polymer. Solutions of Re-P4VP in acetonitrile
at the same polymer concentration display a bimodal spectrum
of relaxation times. Besides a short time peak centered at τshort
∼ 34 µs, a very slow relaxation mode well separated from it
was seen in all the data sets at τlong ∼ 0.72 ms. Figure 2 shows
a typical intensity weighted spectrum, where the maximum at
the short-time mode represents one tenth of the contribution of
the long-time mode.
Figure 1. McGhee-von Hippel plot for the binding between Cu(II)
species and {[(vpy)2-vpyRe(CO)3bpy] CF3SO3}n∼200. The solid lines
represent the best fit of the experimental data according to eq 3.
Re-P4VP-CuCl2 Polymer. A similar pattern is displayed
by the solutions of Re-P4VP-CuCl2 polymer. The two
relaxation modes are centered at times τshort ∼ 29 µs and τlong
∼ 0.46 ms, respectively. Differences with the precedent system
arise in the height and width of the short time contribution. In
fact, the ratio of the amplitudes L(ln τ) at τshort and τlong is around
1.1, much higher than in the Re-P4VP polymer. The short-
time peak also exhibits a noticeable broadening. Moreover, a
closer inspection of the spectra shows a shift of the long-time
peak toward smaller time values. These features are also shown
in Figure 2, for the system analyzed at a polymer concentration
0.5 mg mL-1. This pattern is similar for lower concentrations.
Henceforth, we refer all our results to that polymer concentra-
tion.
Re-P4VP-Cu(CF3SO3)2 Polymer. The time distribution
spectra contain two peaks at 29 µs and 0.56 ms, with the short-
time mode contributing around 50% of the long-time one. The
broadening of the first peak is similar to that displayed by the
Re-P4VP-CuCl2 polymer system.
Figure 2. Representative relaxation time distribution L(ln τ) versus
time at polymer concentrations 0.5 mg mL-1. Full line: Re-P4VP
polymer; dot-dash: Re-P4VP-CuCl2 polymer; dot: Re-P4VP-Cu-
(CF3SO3)2 polymer.
Static Light Scattering. The apparent radius of gyration of
the solutes were estimated by measuring the time integrated
light intensity at complementary scattering angles (θ ) 45°,
Re-P4VP produces an increase in the extinction coefficient of
the d-d transitions of the CuX2 (X ) Cl or CF3SO3).
Quantitative treatment of UV-vis spectral changes allows the
determination of the binding constants between Cu(II) and the
pyridines in the Re(I) polyelectrolyte according to different
theories.38-39 We chose that of McGhee and von Hippel,39 which
describes random noncooperative binding to a lattice
135°) and calculating the dissymmetry factor Zθ ) Iexc(θ)/Iexc
-
(π-θ), where Iexc(θ) is the excess scattering of the solution ()
Isolution (θ) - Isolvent(θ)). The results obtained for the systems
Re-P4VP polymer, Re-P4VP-CuCl2 polymer, and Re-
P4VP-Cu(CF3SO3)2 polymer were 5.2, 4.8, and 2.1, respec-
tively. The high value of Z45 for the first two systems excludes
the possibility of having rodlike or coiled structures and is only
compatible with the elastic scattering from spherical objects.
The last one allows the existence of coiled structures in
addition.37
(1 - nν)n
[1 - (n - 1)ν]n-1
ν
Cf
) Kb
(3)
where ν is the binding ratio Cb/[py], Kb is the binding constant,
and n is the average size of a binding site (expressed in number
of pyridines per Cu(II) species). To use this equation, the
concentration of Cu(II) bound (Cb) and Cu(II) free (Cf) for each
total concentration of Cu(II) (CT ) Cb + Cf) have to be
determined. Therefore, Cb was calculated from the UV-vis
absorption data as follows: for each total Cu(II) concentration,
the difference Apolymer+Cu - ACu must be proportional to Cb and
eventually reaches a limiting value for CT . Cb. From this we
can obtain the ratio Cb/Cb,max for each CT. Finally, with the
known values of Cb,max (see above), we determined Cb for each
value of CT. From a curve fit analysis of ν/Cf vs ν (eq 3) values
of Kb ) 2 × 104 M-1 and n ) 1.8 and Kb ) 1 × 105 M-1 and
Solvent-Cast Films of the Polymers. The morphologies of
the polymers Re-P4VP and Re-P4VP-CuCl2 were studied
by transmission electron microscopy (TEM). The polymer films
were obtained by room temperature solvent evaporation of
acetonitrile solutions of the Re-P4VP and Re-P4VP-CuCl2
polymers, respectively. When taking photos, the polymer films
were not stained with any chemicals, and the contrast of the
image in the TEM photos can only originate from the rhenium
complexes incorporated to the polymers. The Re(I) complexes
in the polymer Re-P4VP aggregate and form isolated nano-
domains that are dispersed in the polymer matrix film. The
dimensions of the nanodomains are between 90 and 430 nm
and are mainly spherical in shape. However, the polymer Re-

On the Quenching of MLCTRefbpy Luminescence
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22893
Figure 3. Transmission electron micrographs of the solvent cast films
of the polymers (a) Re-P4VP and (b) Re-P4VP-CuCl2.
Figure 5. Normalized luminescence spectra of {[(vpy)2-vpyRe-
(CO)3bpy] CF3SO3}200 at different Cu(II) concentrations in the quench-
ing by (a) CuCl2 and (b) Cu(CF3SO3)2.
Figure 4. Steady-state quenching of {[(vpy)2-vpyRe(CO)3bpy] CF3-
SO3}200’s luminescence by Cu(II) species: (9) Φ0/Φ (right y axis),
quencher ) CuCl2, (4) Φ0/Φ (left y axis), quencher ) Cu(CF3SO3)2.
See text for details.
with respect to the linear Stern-Volmer usual behavior with
no limiting value of Φ0/Φ. Moreover, at [CuCl2] ∼ 3 × 10-4
M, Φ0/Φ ∼ 40 (corresponding to 97% of the total emission
quenched). Figure 5 shows the normalized emission spectra at
different [Cu(II)] for the quenching by CuCl2 (Figure 5a) and
by Cu(CF3SO3)2 (Figure 5b). As it can be observed from Figure
5a, the Re(I)-polymer emission spectra band shapes are not
altered by CuCl2 addition up to [CuCl2] < 6 × 10-5 M. When
the [CuCl2] > 8 × 10-5 M there is a progressive shifting of the
emission maximum from 589 to 555 nm. Figure 5b shows that
there are hardly any changes in the normalized emission spectra
of the Re(I)-polymer after the addition of Cu(CF3SO3)2 in the
same concentration range as with CuCl2. However, the lumi-
nescence quenching of the monomer CF3SO3[pyRe(CO)3bpy]
by CuX2 (X ) Cl or CF3SO3) follows a typical Stern-Volmer
P4VP-CuCl2 aggregate to form nanodomains that are distorted
from the spherical shape and whose dimensions are smaller than
those nanodomains formed by the polymer Re-P4VP (see
Figure 3). It should be noted that the dimensions of the two
kinds of nanodomains are considerably larger than the full
stretch length of the polymers. As a result, it is likely that the
nanodomains contain a considerable number of polymer mol-
ecules.
Luminescence Quenching by Cu(II). The emission spectra
of CF3SO3[pyRe(CO)3bpy] and Re-P4VP in deoxygenated
CH3CN at room temperature exhibited unstructured bands both
centered at 589 nm with identical band shapes. The quenching
of the luminescence of the Re(I) polymer by CuCl2 and/or
Cu(CF3SO3)2 was studied using steady state and time-resolved
techniques under similar experimental conditions ([Re(I)] ∼ 1
× 10-4 M) to those utilized in the investigation of the binding
of Cu(II) species to the Re(I) polyelectrolyte. As it can be
observed from Figure 4, the quenching does not follow a Stern-
Volmer kinetics. For instance, in the quenching by Cu(CF3-
SO3)2, the ratio Φ0/Φ follows a sigmoid curve with a limiting
value of (Φ0/Φ)max ) 6.3 at [Cu(CF3SO3)2] ∼ 3 × 10-4 M,
which corresponds to 84% of the total emission quenched.
Conversely, Φ0/Φ in the quenching by CuCl2 deviates upward
kinetics with Ksv,CuCl ) 4.6 × 103 M-1 and Ksv,Cu(CF SO )
)
2
3
3 2
2.1 × 102 M-1 respectively (see Figure 6). It is noteworthy
that no spectral changes occur in the emission spectra of CF3-
SO3[pyRe(CO)3bpy] in the quenching of its luminescence by
CuX2 (X ) Cl or CF3SO3).
After excitation with a N2 laser, the luminescence decay of
the polymer Re-P4VP in acetonitrile is monoexponential with
a lifetime of τem ) 203 ns.28 However, after the addition of

22894 J. Phys. Chem. B, Vol. 109, No. 48, 2005
Wolcan et al.
Figure 7. UV-vis absorption spectra of CuCl2 and Cu(CF3SO3)2.
Comparison with pyRe(CO)3bpy+ luminescence spectrum.
Figure 6. Stern-Volmer plots for the luminescence quenching of
pyRe(CO)3bpy+ by (0) CuCl2 and (O) Cu(CF3SO3)2.
TABLE 1: Parameters Obtained in the Curve-Fit Analysis
of the Experimental Luminescence Intensity, Iem(t),
According to a Biexponential Decay (Eq 4)a
[Cu(CF3SO3)2]/
M
τ1/
ns
τ2/
ns
[CuCl2]/
M
τ1/
ns
τ2/
ns
I1/I2
I1/I2
0
203b
195b
0
203b
166b
9.8 × 10-6
2.0 × 10-5
3.0 × 10-5
3.9 × 10-5
4.9 × 10-5
5.8 × 10-5
6.8 × 10-5
7.8 × 10-5
8.7 × 10-5
1.2 × 10-5
1.47 190 44 2.4 × 10-5 1.44 169 41
0.63 148 34 3.6 × 10-5 0.69 146 33
0.39 126 30 4.8 × 10-5 0.37 122 27
0.29 119 29 6.0 × 10-5 0.20 112 24
0.27 114 27
0.25 116 27
0.21 131 27
0.17 131 27
a See text for details. b Monoexponential decays.
CuX2 (X ) Cl or CF3SO3), the luminescence decay of the Re-
Figure 8. Stern Volmer plots for the luminescence quenching of
{[(vpy)2-vpyRe(CO)3bpy] CF3SO3}200 (polymer) and pyRe(CO)3bpy+
(monomer) by TEOA: (4) polymer’s luminescence quenched by
TEOA, (3) monomer’s luminescence quenched by TEOA.
(I) polymer became biexponential, eq 4
Iem(t) ) I1 exp(-t/τ1) + I2 exp(-t/τ2)
(4)
Table 1 shows the values of the preexponential factors (I1 and
I2) and the lifetimes (τ1 and τ2) obtained by a curve fit analysis
of Iem according to eq 4 in the quenching of the luminescence
of the Re(I)-polymer by CuX2 (X ) Cl and CF3SO3).
Inspection of Table 1 shows that, as the Cu(CF3SO3)2 concen-
tration is increased, the ratio I1/I2 decreases. Besides, the longer
lifetime τ1 decreases from 203 ns (in the absence of quencher)
to ∼120 ns and eventually remains constant. The shorter lifetime
also decreases from τ2 ∼ 40 to 30 ns. At concentrations of Cu-
(CF3SO3)2 > 6 × 10-5 M, both lifetimes τ1 and τ2 remain
constant. A similar behavior is followed by the curve fit
parameters I1, I2, τ1, and τ2 in the quenching by CuCl2. Figure
7 shows a comparison of CuCl2 and Cu(CF3SO3)2 absorption
spectra and the CF3SO3[pyRe(CO)3bpy] luminescence spectrum.
Reductive Quenching by TEOA. The reductive quenching
of the MLCT excited-state luminescence of the monomer CF3-
SO3[pyRe(CO)3bpy] and polymer Re-P4VP by TEOA followed
Stern-Volmer kinetics with Ksv values of 54 and 77 M-1 for
the monomer and the polymer respectively (Figure 8).
coordination of CuX2 to the Re-P4VP polymer have λmax
)
725 and 610 nm when X ) Cl and CF3SO3, respectively. These
facts can be compared with the similar shift of the absorption
maximum from 770 to 610 nm and an increase of the molar
extinction coefficient observed in the stepwise formation of
CuLi2+ complexes, (i ) 1-4) with L ) ethylpyridine,32 in water
as a solvent. This is because the pyridine produces a stronger
ligand field, which causes the absorption band to move from
the far red to the middle of the red region of the spectrum.
Besides, an increase in the d-d extinction coefficient after
coordination of pyridine to Cu(II) should be related to a decrease
in the symmetry around the metal center. It is reasonable to
assume then that the complexation of CuCl2 to the Re-P4VP
polymer to form the polymer Re-P4VP-CuCl2 proceeds with
a lower average coordination number than in the complexation
of Cu(CF3SO3)2 to the Re(I) polymer to form the polymer Re-
P4VP-Cu(CF3SO3)2. Indeed, approximate coordination num-
bers being 2 and 3 were calculated from UV-vis changes for
CuCl2 and Cu(CF3SO3)2 respectively (see above).
Discussion
The coordination of CuCl2 to poly-4-vinylpyridine (P4VP)33
or to the partially methyl quaternized P4VP32 has been reported
in the literature. The new absorption bands generated after the
The single peak in the P4VP/DCM spectrum at 39 µs is the
fingerprint of the translational diffusion behavior of the single
polymer molecules in dilute solutions. The Stokes-Einstein

On the Quenching of MLCTRefbpy Luminescence
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22895
TABLE 2: Relaxation Times τ1 and τ2 (in Milliseconds), Relative Contributions, and Hydrodynamic Radii R1 and R2 (in
Nanometers) at the Peaks of the Corresponding Distributionsa
system
τ1/ms
L(ln τ1)
τ2/ms
L(ln τ2)
R1/nm
R2/nm
P4VP
Re-P4VP
Re-P4VP-CuCl2
Re-P4VP-Cu(CF3SO3)2
0.039 (0.005)
0.034 (0.005)
0.029 (0.004)
0.029 (0.003)
1.00
0.10
1.0
7.2 (0.6)
7.4 (1.0)
6.2 (0.8)
6.9 (0.7)
0.72 (0.03)
0.46 (0.06)
0.56 (0.03)
1.0
0.90
1.0
156 (6)
96 (11)
137 (8)
0.50
a Here, the label 1 (2) replaces the name “short” (“long”) used in the text. Figures in parentheses represent standard deviations.
equation relates the hydrodynamics radius of the polymer Rh
with the relaxation time τ
with Cu(II) compounds at polymer scales broaden and increase
in height, demonstrating an increase of the relative “population”
of Re-P4VP-CuX2 polymers with respect to that of the
corresponding micelles.
Rh ) kBTq2τ/6πη
(5)
The radii of gyration revealed by SLS measurements were
obtained assuming spherical solutes using tabulated values of
their scattering form factor.37 We obtained 91, 89, and 67 nm
for the three Re-P4VP polymer, Re-P4VP-CuCl2 polymer,
and Re-P4VP-Cu(CF3SO3)2 polymer, respectively. If a coiled
structure is assumed for the latter system, the corresponding
radius of gyration would be a little higher, between 76 and 80
nm depending on the (unknown) polydispersity of the sample.
where kB is the Boltzmann constant, T is the absolute temper-
ature, and η is the shear viscosity of the solvent in the dilute
solution regime. The maximum of the unweighted radius
distribution is at Rh ) 7.2 ( 0.6 nm. Here and below we report
the mean value of relaxation times and radii at the maxima of
the corresponding distributions and the standard deviation of
the mean, from the statistics of at least 10 independent
experiments. The literature value of the unperturbed radius of
gyration RG of monodisperse P4VP with a molecular weight 6
× 104 g mol-1(which corresponds to the Mw of P4VP polymer
used in the preparation of Re-P4VP polymer26) in solvents such
as water and ethanol at 25 °C is 7.1 nm.37 Hence, we find a
good agreement between the polymer size determined from DLS
and published data in other solvents. Exact determination of
the radius of gyration was out of the scope of this work and
will be reported elsewhere.
Clearly, these figures correspond to some weighted average
of the coexisting structures detected by DLS. Size differences
of the three systems are less noticeable here, but they are
consistent with the picture provided by DLS, i.e., two indepen-
dent structures at equilibrium, polymer and micelles, with
relative compositions depending on the particular system.
Assuming that the mean density of a given micelle is the
same as that of an isolated Re-P4VP random coil polymer and
neglecting solvation contributions to Rh, a ratio
The bimodal relaxation time spectra registered in the systems
Re-P4VP polymer, Re-P4VP-CuCl2 polymer and Re-
P4VP-Cu(CF3SO3)2 polymer reveals the existence of bigger
structures with diffusion times τlong > 10 τshort coexisting with
smaller ones associated to τshort. In fact, these latter structures
are identified as Re-P4VP macromolecules in the scale of 6-8
nm say, of the same size as that of P4VP in DCM. Since the
molecular weight of Re-P4VP polymers is around 1.8 × 105
g mol -1, it appears that these polymers would adopt in
acetonitrile more compact conformations than their parents
P4VP in DCM.
On the other hand, the mean size of the bigger micelles can
be obtained from the value of the relaxation time at the
corresponding maximum of the distribution L(ln τlong) assuming
the same hydrodynamic model. We note that the wide distribu-
tion of relaxation times between τshort and τlong implies a wide
distribution of sizes of spherical objects, ranging from a few
nanometers at polymer scales to few hundreds of nanometers
covering micelle scales.
In Table 2, we reproduce mean values and standard deviations
of the mean of relaxation times and hydrodynamic radius at
the peaks of the time and radius distributions for the three
systems Re-P4VP polymer, Re-P4VP-CuCl2 polymer, and
Re-P4VP-Cu(CF3SO3)2 polymer. The system P4VP/DCM was
also included for comparison.
Average polymer sizes are not modified by the incorporation
of CuX2 to Re-P4VP, but significant changes are evident in
the size of the companying micelles. The reduction in size of
the Re-P4VP-CuX2 micelles is more important for Re-
P4VP-CuCl2 than Re-P4VP-Cu(CF3SO3)2.
2
Mw(micelle)
Mw(polymer)
R2
≈
(6)
( )
R1
can roughly be estimated giving 4.4 × 102, 3.9 × 102, and 2.4
× 102 for Re-P4VP, Re-P4VP-Cu(CF3SO3)2, and Re-
P4VP-CuCl2 polymers, respectively. Those figures represent,
roughly speaking, the number of single polymers that could
aggregate to form the micelles.
TEM images obtained for Re-P4VP and Re-P4VP-CuCl2
polymers are in good agreement with the calculated R2 ) 156
( 6 nm and R2 ) 96 ( 11 nm from DLS experiments for the
micelles of the Re-P4VP and Re-P4VP-CuCl2 polymers,
respectively. The ratio L(ln τ1)/L(ln τ2) is 0.1, 1.1, and 0.5 for
the Re-P4VP, Re-P4VP-CuCl2, and Re-P4VP-Cu(CF3-
SO3)2 polymers, respectively. This fact suggests that coordina-
tion of Cu(II) to Re-P4VP breaks the original micelle reducing
its radius and increasing the number of polymers that are not
forming micelles. This effect is more important (both in
augmenting L(ln τ1)/L(ln τ2) and reducing R2) when the copper
salt used was CuCl2 than Cu(CF3SO3)2. TEM images show that
the reduction of the micelle radius is accompanied by a distortion
from the spherical shape toward some elongated structure. The
short-time peak for Re-P4VP-CuCl2 and Re-P4VP-Cu(CF3-
SO3)2 polymers in Figure 2 also exhibits a noticeable broadening
when compared to the one for Re-P4VP polymer. This means
that after coordinating by Cu(II) to the free pyridines the
resulting Re(I)-P4VP-Cu(II) polymers that are now not forming
micelles may show a huge variety of structures (and a
concomitant broadening of the distribution around R1) since
intramolecular coordination between adjacent vpy units on the
polymer chain is not possible in P4VP33 and all P4VP complexes
should be perforce cross linked by Cu(II). The distortion of the
The size distributions are similar to time distributions, shifted
by the factor given by eq 5. We note that the shape of the three
distributions at micelles scales are the same, only a shift to lower
sizes (times) is evident. Instead, the distributions of the systems

22896 J. Phys. Chem. B, Vol. 109, No. 48, 2005
Wolcan et al.
SCHEME 2
concentration. For instance, in the luminescence (steady state
or time resolved) quenching by Cu(CF3SO3)2, we have [Re(I)]
∼ 1 × 10-4 M and [py]uncoordinated ∼ 2 × 10-4 M and hence
Cb,max ∼ 7 × 10-5 M. From Table 1, it can be observed that at
[Cu(CF3SO3)2] ∼ 7 × 10-5 M both lifetimes τ1 and τ2 achieve
constant values. However, Figure 4 shows that the quenching
measured by steady-state luminescence techniques (increase in
the ratio Φ0/Φ vs Cu(II) concentration) is still significant at
[Cu(CF3SO3)2] > 7 × 10-5 M. A limiting plateau in Φ0/Φ is
achieved only at [Cu(CF3SO3)2] > 1.5 × 10-4 M. Moreover,
results from steady-state experiments (Figure 5a) with CuCl2
must be associated too with more than one quenching process.
For instance, when 0 < [CuCl2] < Cb,max ∼ 7 × 10-5 M, the
quenching proceeds with no spectral changes in the lumines-
cence spectra. However, when [CuCl2] > Cb,max there is a
progressive shifting of the emission maximum to shorter
Re-P4VP micelles in the presence of CuCl2 as shown by TEM
is also supporting the existence of cross linked Re-P4VP
polymers by Cu(II). Because CF3SO3- is a weak ligand, it can
explain the apparent higher coordination number. Indeed,
wavelengths. This behavior could be attributed to some kind
(2-x)
of interaction between CuClX
species and the Re(I)-
polymer. This effect was not observed in the quenching by Cu-
(CF3SO3)2 (compare panels a and b of Figure 5). On the other
hand, the monomer CF3SO3[pyRe(CO)3bpy] luminescence is
quenched by CuCl2 and by Cu(CF3SO3)2 following typical
Stern-Volmer kinetics with no spectral changes in the lumi-
nescence spectrum even at higher [Cu(II)] than those used when
quenching the luminescence of the Re-P4VP polymer.
CF3SO3 is a poor ligand (compared to Cl-), and it does not
-
compete with pyridine ligands.28 In fact, in Cu(CF3SO3)2
acetonitrile solutions, the predominant species should be Cu-
(CH3CN)4+2; however, in CuCl2 acetonitrile solutions, a sig-
nificant presence of other species should be considered. Binding
constants of Kb ) 2 × 104 M-1 and 1 × 105 M-1 were
calculated for the interaction of Re-P4VP polymer with CuCl2
and Cu (CF3SO3)2, respectively. Probably, the lower value of
Kb in the case of CuCl2 is due to the Cl- in the sphere of
coordination of Cu(II). On the other hand, the 10-100 fold
lower values for Kb reported in the literature for the interaction
of partially methyl quaternized P4VP with Cu(NO3)2 in aqueous
solutions32 may be explained by a reduction of the pyridine
basicity due to protonation of the pyridinic nitrogen in water
solutions. Scheme 2 depicts the distribution of species that may
be encountered in solution after the binding of Cu(II) to the
Re-P4VP polymer. Our experiments show that after excitation
with a N2 laser the luminescence decay of the polymer Re-
P4VP in acetonitrile is monoexponential, τem ) 203 ns. It is so
although the Re-P4VP polymer molecules are aggregated
forming micelles in acetonitrile. We do not have experimental
evidence about the structures of those micelles. However, some
speculations can be made. For instance, P4VP is nearly insoluble
in acetonitrile, but this solvent is a good one for the Re-P4VP
polymer. Then it is plausible to imagine that the inner core of
the micelles will be formed mainly by the free pyridines of the
Re-P4VP polymers and the outer part will be constituted mainly
by the Re(I) chromophores. The observation of a monoexpo-
nential decay of the MLCT luminescence means that the
majority of these chromophores are in approximately equivalent
environments on the surface of the micelles. However, a second-
order process was observed in addition to the first-order decay
of the Re-P4VP MLCT luminescence when the sample was
excited by higher power laser sources, such as Nd:YAG (355
nm) and XeF (351 nm) excimer lasers.28 The second-order
processes observed in the MLCT luminescence decay of the
Re-P4VP polymer could be a consequence of the production
of a high number of MLCT excited states in close proximity in
the micelles due to the intense Nd:YAG (355 nm) and XeF
(351 nm) laser pulses. In these conditions, excited-state self-
annihilation can occur.
The luminescence quenching by Cu(CF3SO3)2 in the Re(I)
polymer is far more efficient than in the monomer (compare
Figures 4 and 6). There are two different reasons that can explain
such a difference. First, there should be an enhancement of the
energy transfer rate constant due to a close vicinity between
the Re(I) chromophore and the Cu(II) bound to a near pyridine
in the polymer. Second, an enhancement of the spectral overlap
integral (J) between the emission spectrum of the Re(I)
chromophore and the new absorption band which appears after
the coordination of the Cu(II) ion to the Re-P4VP polymer
(note that the Dexter’s exchange mechanism for energy transfer
predicts that ketR exp(-R), whereas the Forster’s dipole-induced
energy transfer theory predicts that ketRR-6; that is, both theories
predict an increase of ket as R decreases. Besides, in both
Dexter’s and Forter’s theories ketRJ).40 The quenching of the
monomer CF3SO3[pyRe(CO)3bpy] luminescence by CuCl2 fol-
lows a typical Stern-Volmer kinetics and taking into account
its luminescence lifetime28 a bimolecular rate constant kq,CuCl
2
) 2 × 1010 M-1 s-1 can be calculated, the value of which is
close to the diffusional limit in acetonitrile.41 A far lower
bimolecular rate constant kq,Cu(CF SO ) ) 8 × 108 M-1 s-1 can
3
3 2
be calculated from the quenching by Cu(CF3SO3)2 (Figure 6).
The poor spectral overlap integral (J) between the absorption
spectrum of Cu(CF3SO3)2 and the CF3SO3[pyRe(CO)3bpy]
emission spectrum (see Figure 7) should be responsible for this
distinct photophysical behavior.
The binding of Cu(II) to the Re(I) polymer produces the
breakage of the micelles and yields a wide distribution of par-
ticle sizes ranging from a few nanometers at polymer scales
to a few hundreds of nanometers covering micelle scales. In
fitting the luminescence decay by a biexponential function,
the parameters τ1 and τ2 may then be viewed as “mean” life-
times representing average contributions from the different
species.
The quenching of the Re-P4VP polymer’s luminescence by
CuX2 (X ) Cl or CF3SO3) proceeds via both dynamic and static
mechanisms as it can be observed from the comparison of Φ0/Φ
(Figure 4) and τ functional dependences (Table 1) on Cu(II)
The reductive quenching of the {[(vpy)2-vpyRe(CO)3bpy]CF3-
SO3}200 and CF3SO3[pyRe(CO)3bpy] MLCT excited states by
TEOA26,41 are represented in eqs 7-10

On the Quenching of MLCTRefbpy Luminescence
[(vpy)2-vpyRe(+)(CO)3bpy]m+n
hν8 [(vpy)2-vpyRe(+)(CO)3-
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22897
transfer reaction between the Re(I)-polymer’s MLCT and
TEOA. The fact that the polymer’s kq is nearly 2 times higher
than that of the monomer may be reflecting a contribution from
the polymer backbone to a decrease of the Marcus inner sphere
reorganization energy (λi) of the electron-transfer process via
vibrational modes of the uncomplexed pyridines in the polymer
backbone. However, a higher value of kq in the polymer than
in the monomer may also be explained due to the fact that the
diffusion of the TEOA molecules to form the encounter complex
with the Re(I) chromophore is favored-compared to the mono-
mer, due to the fact that TEOA might have a tendency to be
close to the polymer by hydrogen bonding interactions with the
free pyridine groups and may thus more often encounter Re(I)
chromophores than in the case for the monomer pyRe-
(CO)3bpy+.
9
bpy(GS)]n[(vpy)2-vpyRe(2+)(CO)3(bpy-)(MLCT)]m (7)
[(vpy)2-vpyRe(+)(CO)3bpy(GS)]n[(vpy)2-vpyRe(2+)(CO)3
(bpy-)(MLCT)]m + mTEOA f [(vpy)2-vpyRe(+)(CO)3bpy
(GS)]n[(vpy)2-vpyRe(+)(CO)3(bpy-)]m + mTEOA+ (8)
pyRe(+)(CO)3(bpy)(GS)
9
hν8 pyRe(2+)(CO)3(bpy-)(MLCT)
(9)
pyRe(2+)(CO)3(bpy-)(MLCT) + TEOA f
pyRe(+)(CO)3(bpy-) + TEOA+ (10)
Concluding Remarks
where m + n ) 200 in the polymer, -vpyRe(2+)(CO)3(bpy-)
TEM images and DLS experiments have demonstrated that
Re-P4VP polymers aggregate to form mainly spherical micelles
whose dimensions are in the range of 90-430 nm. After the
coordination of Cu(II) species to the Re(I) polymer, the polymer
Re-P4VP-CuCl2 aggregates to form micelles that are distorted
from the spherical shape and whose dimensions are smaller than
those micelles formed by the polymer Re-P4VP.
stands for the MLCT(Re f bpy) excited state, and -vpyRe(+)
-
(CO)3(bpy-) is the reduced radical produced after the reductive
quenching by the sacrificial reductant TEOA, eq 8.
The Re-P4VP polymer’s luminescence reductive quenching
by TEOA follows a typical Stern-Volmer kinetics (see Figure
8). From the Stern-Volmer constant Ksv ) 77 M-1 and the
luminescence lifetime28 in acetonitrile, a bimolecular quenching
constant kq ) 3.8 × 108 M-1 s-1 can be calculated. The
quenching of the monomer CF3SO3[pyRe(CO)3bpy] lumines-
cence by TEOA also follows a Stern-Volmer kinetics with a
lower Ksv ) 54 M-1. Taking into account its luminescence
lifetime, a bimolecular quenching constant kq ) 2.2 × 108 M-1
s-1 can be calculated.
The coordination of Cu(II) species to the polymer {[(vpy)2-
vpyRe(CO)3bpy] CF3SO3}200 produces the quenching of the
MLCT excited state by energy transfer processes that are more
efficient than those in the quenching of the monomer’s
luminescence by Cu(II). Besides, the kinetics of the quenching
by Cu(II) does not follow a Stern-Volmer behavior. Moreover,
although in the quenching of the polymer {[(vpy)2-vpyRe-
(CO)3bpy] CF3SO3}200 luminescence by Cu(CF3SO3)2 the ratio
Φ0/Φ shows a sigmoid dependence on Cu(CF3SO3)2 concentra-
tion with a limiting value of Φ0/Φ ∼ 6, the quenching by CuCl2
does not show a plateau on Φ0/Φ. Conversely, the reductive
redox quenching of the Re(I) polymer’s MLCT excited state
by TEOA follows a Stern-Volmer kinetics. The striking
differences found in the quenching mechanisms with Cu(II) or
TEOA are a consequence of the strong chemical interaction
(binding) of Cu(II) to the poly-4-vynilpyridine backbone of the
Re(I)-polymer. All of the quenching processes either by Cu(II)
or TEOA are more efficient in the polymer than in the monomer.
According to Marcus, the outer sphere reorganization energy
depends on the size of reactants and the separation distance, as
well as on the dielectric properties of the embedding solvent.
The theory, in its original formulation, was developed for a
homogeneous solution. Thus, to describe the electron transfer
data on micelle surfaces, we should properly account for the
heterogeneous local structure of micelles. However, Marcus
theory of electron transfer has already been extended to
micelles.42 In this regards, micelles are supposed to provide a
good reaction media to observe the Marcus inverted region for
bimolecular electron-transfer reactions. In fact, the Marcus
inverted region has been observed in electron-transfer reactions
between coumarin dyes and amines in micellar solution.43
Because of the rigid structures of the micelles, reactants will
be entangled within the micellar chains and thus their move-
ments will be highly restricted. In other words, the high viscosity
of the micellar media prevents the diffusion of the reactants,
and if the electron transfer reactions are relatively faster than
diffusion, then the bimolecular reactions in micellar solutions
can be envisaged as the effective intramolecular reactions. Thus,
the reaction dynamics in the micellar media is expected to differ
largely as compared to that in the homogeneous media.
Taking into account the oxidation redox potential of TEOA,41
the reduction potential of the complex pyRe(CO)3bpy+,44 and
the energy of the excited state which can be estimated from the
emission spectrum as E0-0 ∼ 2.37 eV, we can calculate a ∆G
∼ -0.4 eV for the forward electron-transfer reaction between
pyRe(CO)3bpy+’s MLCT excited state and TEOA. The polymer
should have a similar ∆G value. This ∆G value lies in the
Marcus normal region.41 It is well-known that the motion of
the solvent molecules in the restricted media, i.e., in micelles,
is retarded by several orders of magnitude compared to that in
homogeneous solvents. Thus, the solvent reorganization may
not contribute completely within the time scale of the electron-
Acknowledgment. This work was supported in part by a
project grant from Agencia de Promocio´n Cient´ıfica y Tecno-
lo´gica de Argentina (ANPCyT Grant No. PICT 06-12610),
Consejo Nacional de Investigaciones Cientificas y Tecnolo´gicas
(CONICET-PIP 02470/00), Universidad Nacional de La Plata,
and the Comisio´n de Investigaciones Cient´ıficas de la Provincia
de Buenos Aires (CICPBA). E.W. thanks CONICET for a
personal research grant.
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