Probing the presence of hydrophobic sites in anionic polyelectrolytes with tris(polypyridine)chromium(III) complexes

Article abstract of DOI:10.1016/S0020-1693(00)00231-0

The emission properties of tris(polypyridine)chromium(III) complexes, [Cr(LL)₃]³⁺ (LL = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 4,7-dimethylphenanthroline (dim), and 4,7-diphenylphenanthroline (dip)), in air-saturated aqueous solutions, are dramatically modified in the presence of anionic polyelectrolytes. Investigated polyions included poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), and sodium poly(styrenesulfonate) (PSS). All four chromium complexes displayed tunable emission properties in the polyelectrolyte microenvironment, in a degree that depended on the hydrophobic character of both the macromolecule and the polyamine. The largest effect was observed for 2.5 x 10^-5 M aqueous solutions of [Cr(dip)₃]³⁺ in 4 x 10^-3 M PMA at approximately pH 5 and t = 15°C, where I(PMA)/I_W = 25.66 (I(PMA) = emission intensity in aqueous solutions containing PMA; I_W = emission intensity in water). Decay lifetimes revealed that [Cr(dip)₃]³⁺ units are distributed in hydrophobic and hydrophilic sites of the PMA, giving rise to short and long-lived intermediates. Distribution of [Cr(dip)₃]³⁺ units in PAA and PSS is homogeneous throughout the polyelectrolyte, and the excited-state intermediates decay following a first order exponential rate law. All other investigated chromium polypyridines showed a homogeneous distribution in solutions containing PMA, PAA or PSS. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00231-0

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Inorganica Chimica Acta 308 (2000) 107–115
Probing the presence of hydrophobic sites in anionic
polyelectrolytes with tris(polypyridine)chromium(III) complexes
Silvia Ronco *, Brian Persing, Roy Mortinsen, Jackie Barber, Sabrina Shotwell
Chemistry Department, Uni6ersity of South Dakota, Vermillion, SD 57069, USA
Received 4 February 2000; accepted 21 June 2000
Abstract
The emission properties of tris(polypyridine)chromium(III) complexes, [Cr(LL)3]3+ (LL=2,2%-bipyridine (bpy), 1,10-phenan-
throline (phen), 4,7-dimethylphenanthroline (dim), and 4,7-diphenylphenanthroline (dip)), in air-saturated aqueous solutions, are
dramatically modified in the presence of anionic polyelectrolytes. Investigated polyions included poly(acrylic acid) (PAA),
poly(methacrylic acid) (PMA), and sodium poly(styrenesulfonate) (PSS). All four chromium complexes displayed tunable emission
properties in the polyelectrolyte microenvironment, in a degree that depended on the hydrophobic character of both the
macromolecule and the polyamine. The largest effect was observed for 2.5×10−5 M aqueous solutions of [Cr(dip)3]3+ in
4×10−3 M PMA at approximately pH 5 and t=15°C, where IPMA/Iw=25.66 (IPMA=emission intensity in aqueous solutions
containing PMA; Iw=emission intensity in water). Decay lifetimes revealed that [Cr(dip)3]3+ units are distributed in hydrophobic
and hydrophilic sites of the PMA, giving rise to short and long-lived intermediates. Distribution of [Cr(dip)3]3+ units in PAA and
PSS is homogeneous throughout the polyelectrolyte, and the excited-state intermediates decay following a first order exponential
rate law. All other investigated chromium polypyridines showed a homogeneous distribution in solutions containing PMA, PAA
or PSS. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Photophysics; Luminescence probes; Chromium complexes; Polyelectrolytes; Hydrophobic sites
sic lifetime of 2T1/2E, ~o, given by 1/(2knr+2krx) [3].
Lifetime measurements and absorption decays showed
1. Introduction
that the excited-state lifetimes have a strong depen-
dence on the nature of the ligands, the pH of the
solution, and the presence of added salts [4]. Chrom-
ium(III) polypyridines are good target compounds for
solar energy conversion for their long lifetimes and
strong oxidizing properties (*E°=1.2–1.5 V) [1]. A
recent report described the ability of [Cr(LL)3]3+ to
interact with DNA and the potential use of chromiu-
m(III) polyamines to serve as probes of DNA binding
[5].
Polypyridyl complexes of Cr(III) display low activity
toward photoaquation upon excitation with light
(uexc=313–464 nm) [1b,3], being [Cr(bpy)3]3+ the most
reactive species with a quantum yield, b=0.11 (pH 9.6,
in air-saturated, 1 M NaCl at 11°C) [4b]. The low
photochemical reactivity of these complexes is ex-
plained via deactivation mechanisms that involve com-
peting reactive, radiative and non-radiative modes. The
The photophysical and photochemical properties of
chromium(III) polypyridyl complexes have been the
subject of extensive investigation [1]. Luminescence
from the lowest-energy excited states consists of two
bands from the thermally equilibrated states 2E/2T1,
and has been identified as of metal-centered ligand-field
character, d–d or LF [2,3]. Values for decay lifetimes
(~) for (2E/2T1)[Cr(LL)3]3+ result from contribution of
different modes of decay, 1/~=krad+knr+krx+
kg[4A2]+kq[Q] (krad, rate constant for luminescence de-
cay; knr, rate constant for non-radiative decay; krx, rate
constant for reactive decay, such as photoaquation; kg,
rate constant for ground-state quenching; kq, rate con-
stant for other quenching modes) [3]. Infinite diluted
solutions in the absence of quenchers display an intrin-
* Corresponding author. Tel.: +1-605-677 5487; fax: +1-605-677
6397.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00231-0

108
S. Ronco et al. / Inorganica Chimica Acta 308 (2000) 107–115
probe the presence of hydrophobic sites in aqueous
solutions containing polyelectrolyte microenviron-
ments. In order to gain a better understanding on
hydrophobic and hydrophilic interactions between
guest and host molecules, we compared the emission
changes observed in [Cr(dip)3]3+ with luminescence
changes in solutions containing chromium(III) com-
plexes with less hydrophobic ligands, such as 2,2%-
bipyridine (bpy), 1,10-phenanthroline (phen), and
4,7-dimethylphenanthroline (dim) (see Scheme 1)1.
proposed seven-coordinate intermediate, [Cr(bpy)3-
(H2O)]3+, is believed to undergo deprotonation fol-
lowed by loss of bpy, to yield the final product,
[Cr(bpy)2(OH)2]+ [6]. Biphotonic studies proved that
visible excitation of the long-lived doublet states of
[Cr(phen)3]3+ promotes overall ligand labilization [7].
These photochemical processes depend strongly on the
pH, ionic strength and temperature of the reaction
medium. Quantum yields for photoaquation decrease at
lower temperature and increasing ionic strength.
Polyelectrolyte microenvironments have been re-
ported to modify different steps associated with pho-
toinduced processes, such as: (i) rates of forward and
back reactions; (ii) luminescence quantum yields; and
(iii) decay lifetimes [8]. Electrostatic interactions be-
tween positively charged transition metal complexes
and anionic polyelectrolytes have been used to enhance
the rate of photoinduced electron transfer processes,
and retard recombination reactions [8]. Binding affini-
ties for chromium(III) complexes with the polyelec-
trolytes depend on a series of experimental parameters,
in which the hydrophobic character of both the ligand
and the macromolecule plays a major role [9]. In this
context, we investigated binding properties between
guest and host molecules and compared them with
emission intensities and decay lifetimes of a series of
[Cr(LL)3]3+ containing ligands with a varying degree of
hydrophobicity. Lifetime and intensity measurements
were conducted as a function of the concentration of
chromium units and polyelectrolytes, ionic strength, pH
and temperature.
2. Experimental
2.1. Materials
All materials were reagent grade and used without
further purification. [Cr(LL)3](ClO4)3 complexes were
prepared by literature procedures. [Cr(bpy)3]3+ and
[Cr(phen)3]3+ were prepared by reducing CrCl3 with
Zn/Hg in acidic aqueous solution under an Ar atmo-
sphere [10]. A flurry containing the ligand in deaerated
ethanol was added to the Cr(II) solution, and allowed
to react for ten minutes. Solution handling was con-
ducted with Hamilton air-tight syringes. Oxidation of
[Cr(LL)3]2+ was achieved by bubbling chlorine or oxy-
gen. [Cr(dim)3]3+ and [Cr(dip)3]3+ were prepared by
dissolving anhydrous CrCl2 in deoxygenated methanol
[11]. A flurry containing the ligand in air-free methanol
was added to this solution, and was allowed to react for
ten minutes. Oxidation of the Cr(II) complex was
achieved by gently bubbling oxygen. The complexes
were precipitated as the perchlorate salts by dissolving
them in the minimum amount of methanol followed by
a dropwise addition of HClO4. Purity of the chromium
complexes was verified by their UV–Vis spectra [12].
Caution: perchlorate salts of transition metal complexes
are potentially explosive and should be handled with
extreme care!
In the work described herein, we report the ability of
[Cr(dip)3]3+ (dip=4,7-diphenylphenanthroline) to
Poly(sodium 4-styrenesulfonate) (Mr=70 000) and
poly(acrylic acid) (Mr=250 000) were purchased from
Aldrich and used without further purification. Poly(-
methacrylic acid) sodium salt (25% aqueous solution;
Mr=15 000) was purchased from Polysciences.
Polyelectrolyte concentrations were calculated on the
basis of the monomeric unit formula weight. Solutions
of [Cr(LL)3]3+ –polyelectrolyte were prepared by
adding appropriate amounts of chromium complex to
aqueous solutions containing a known concentration of
1 Abbreviations used in this paper: bpy=2,2%-bipyridine; phen=
1,10-phenanthroline; dim=4,7-dimethylphenanthroline; dip=4,7-
diphenylphenanthroline;PAA=poly(acrylic acid);PMA=poly(meth-
acrylic acid); PSS=poly(sodium 4-styrenesulfonate).
Scheme 1.

S. Ronco et al. / Inorganica Chimica Acta 308 (2000) 107–115
109
polyelectrolyte. A known amount of acid or base, de-
2.4. Binding constant measurements
(M−1), for chromium
termined from a titration curve of the corresponding
[Cr(LL)3]3+ –polyelectrolyte system, was added to the
solution to adjust the pH. For O2 quenching experi-
ments the solutions were air-saturated and concentra-
tions were calculated using Ostwald solubility
coefficients [13]. Bimolecular rate constants for quench-
ing with O2 were calculated by measuring emission
lifetimes and intensities in the presence and absence of
dissolved O2, and assuming an oxygen concentration of
2.5×10−4 M at 25°C in solutions of ionic strength,
I=0.01 M (KH2PO4). Unless otherwise noted, experi-
ments were carried out at 25°C. Steady temperatures
were maintained with a circulating bath and measured
inside the cuvettes with a high precision digital ther-
mometer (Fischer Scientific) (t90.01°C) prior to inten-
sity and lifetime measurements.
Binding constants,
K
polypyridines bound to polyelectrolyte were measured
by following emission changes with increasing polyelec-
trolyte concentration, and calculated from log { fb/(1−
fb)}=log K+n log ([polyelectrolyte]/3), where fb is the
fraction of complex bound to the polyelectrolyte [15].
As a control method, some binding constants were also
measured by a reported method using dialysis mem-
branes [9].
3. Results and discussion
3.1. Binding and emission properties in the
polyelectrolyte architecture
2.2. Photophysical measurements
The binding affinities of a series of chromium(III)
polypyridines with polyelectrolytes were investigated in
aqueous solutions by following emission intensities at
increasing polyelectrolyte concentrations. The polyelec-
trolyte architecture modifies the photophysical proper-
ties of these complexes due to a strong association
between guest and host units, and to an increase in the
medium viscosity. All investigated solutions displayed
‘tunable’ luminescence properties that varied as a result
of the association with the macromolecules, and leveled
off at large polyelectrolyte concentrations. All four
studied Cr(III) polypyridines showed emission changes
upon addition of polyelectrolytes, indicative of good
binding affinity between guest and host components.
Chromium polyamines interact electrostatically with
carboxylate and sulfonate groups from the polyions.
No changes in the UV–Vis spectra of the chromium
complexes proved no complexation with the polyelec-
trolytes. Binding constants follow the order bpyB
phenBdipBdim. Emission changes are also reflected
in the decay lifetimes, which follow a similar trend than
emission intensities. Fig. 1 and Table 1 exemplify the
effect of PMA in all four complexes. These changes
depend on experimental conditions, such as pH of the
solution, ionic strength, and nature of the polyelec-
trolyte, ligands and added salts. Chromium polypyrid-
ines containing ligands with significant hydrophobic
character, such as dim and dip, displayed large changes
in the emission properties when bound to PMA, PSS or
UV–Vis spectra were obtained on a Hewlett-Packard
8452 A diode array spectrophotometer. Luminescence
spectra were measured with a Spex Fluoromax spec-
trofluorometer equipped with a Hamamatsu R928 pho-
tomultiplier tube. Emission lifetimes were measured
with a laser flash photolysis instrument that uses a
Surelite II Nd/YAG laser (10 Hz) as the excitation
source (uexc=355 nm). Emission was monitored at 90°
to the excitation beam with a Bausch & Lomb
monochromator and a Hamamatsu R928 photomulti-
plier tube powered by a Betran high voltage power
supply. The output of the detector was digitized with a
400 MHz Hewlett–Packard 54504A digital scope (200
M samples s−1) and sent to a PC through an IEEE
card where it was analyzed. Each decay trace represents
the average of 64 experiments.
2.3. Quantum yield measurements
A medium-pressure Hg spectral lamp was used, in
conjunction with fused silica optics, for continuous
photolysis of [Cr(LL)3]3+ at 313 nm. Photoaquation
quantum yields, baq, were calculated by following ab-
sorbance changes in the UV–Vis spectra as a function
of irradiation time. The absorption spectrum of the
photolyte was recorded after each irradiation up to
ꢀ10% change in absorbance at the monitoring wave-
length (umon=344 nm). A sample of non-irradiated
solution was kept as a blank and analyzed in the same
PAA. The largest effect observed was for 2.5×10−5
M
way. Light intensity (ꢀ1×10−6 einstein dm−3 s−1
)
aqueous solutions of [Cr(dip)3]3+in 4×10−3 M PMA
(pH 5.0) with maximum emission intensities, IPMA/Iw=
25.66 at t=15°C (IPMA=emission intensity in PMA;
Iw=emission intensity in pure water; ~PMA=decay life-
time in PMA; ~w=decay lifetime in water). The effect
of the PMA architecture on the photophysics of transi-
tion metal complexes has been previously explained
using a micelle-like model. In PMA aqueous solutions,
was determined by ferrioxalate actinometry [14]. Pho-
tolyte concentrations were adjusted to absorb 99.99%
or more of the incident light at the excitation wave-
length. Quantum yields brx were determined from ex-
trapolation of the linear portions of plots of ƒrx(t)
versus time. Each quantum yield value represents the
average of five experiments.

110
S. Ronco et al. / Inorganica Chimica Acta 308 (2000) 107–115
Table 1
3+
Emission lifetimes a and binding constants b of [Cr(LL)3]
in PMA
Ligand
~ (ms)
~
(ms)
~/~w
K (M−1
)
0
bpy
30
118
158
140, 510
43
233
570
1010
0.69
2.01
3.95
9.33, 34
212
phen
dim c
dip c
1.18×103
2.21×105
1.00×104
a [Cr(III)]=2.5×10−5 M, [PMA]=4×10−3 M, non-controlled
ionic strength, pH 4.5; ~, decay lifetimes of air-saturated PMA
solutions; ~0, decay lifetimes of Ar-saturated PMA solutions; ~w,
decay lifetimes of air-saturated solutions in pure water.
b K, binding constants for [Cr(LL)3]3+ with PMA.
c Sequential first-order decays (see text).
ture of the functional group. Association of PSS with
metal complexes has been described as a combination of
electrostatic and p-electron overlap interactions in a
microdomain, in which the number of monomer units
does not depend on the concentration of the polymer or
the ionic strength [17].
Fig. 1. (a) pH-dependence of the emission intensity of 2.5×10−5
M
(ii)
[Cr(LL)3]3+ in 1.25×10−3
M
PMA: (i) [Cr(dip)3]3+
;
[Cr(dim)3]3+; (iii) [Cr(phen)3]3+; (iv) [Cr(bpy)3]3+. (b) Changes in
the relative emission intensity of 2.5×10−5 M [Cr(dip)3]3+ by the
addition of PMA (pH 4.5).
In contrast with complexes containing hydrophobic
ligands, bpy and phen complexes showed smaller effects
for all three polyelectrolytes under our experimental
conditions, see Table 1 and Fig. 1. These observations
suggest that the most predominant interactions between
these chromium units and carboxylate or sulfate groups
are electrostatic. A systematic study conducted with
the methyl groups are located inside of the polymer coil
and the ꢀCOO− and ꢀCOOH functional groups are
oriented towards the water interface [16]. Binding has
been explained as electrostatic in nature, where the
molecules are severely restricted in a hydrophobic envi-
ronment. PMA undergoes conformational changes from
a tight coil at approximately pH 2.0 to an extended
chain at approximately pH 7.0. At pH 4–5 the polymer
ionizes and swells, allowing some water molecules to
penetrate into PMA. At pH 5.0 a significant fraction of
the carboxylate functional groups is deprotonated (ꢀ
24%). At pH\9, the degree of ionization is \97% and
the polymer exists as uncoiled species with a strong
negative charge [16]. PMA shows much sharper transi-
tions with pH than PAA. These transitions are associ-
ated with a stronger hydrophobic character leading to
large emission changes in the species attached to it. It
has been reported that hydrophobic Ru(II) complexes
have appreciable affinity to PAA at low pH and induce
conformational changes of the polymer upon binding
[9]. Our experimental observations showed that Cr(III)
complexes containing hydrophobic ligands, dim and
dip, in PAA display increased luminescence properties
that are a function of pH with maximum intensities at
pH 5.5 and 6.00, respectively. Similar experiments with
[Cr(bpy)3]3+ and [Cr(phen)3]3+ failed to show this pH
effect. These observations support the view that PAA
might be in a compact conformation when is attached to
dip or dim complexes.
2.5×10−5 M solutions of [Cr(LL)3]3+in 4×10−3
M
polyelectrolyte revealed that bpy complexes display
smaller emission intensities and shorter decay lifetimes
in PMA (IPMA/Iw=0.69, ~PMA/~w=0.81, pH 4.5), PSS
(IPSS/Iw=0.66, ~PSS/~w=0.82, pH 6.00) and in PAA
(IPAA/Iw=0.63, ~PAA/~w=1.03, pH 6.00) than in water.
Phen complexes show a similar trend than for the two
substituted phenanthrolines, in which the emission
properties are enhanced upon addition of polyelec-
trolytes (IPAA/Iw=2.5, ~PAA/~w=1.93, IPMA/Iw=3.06,
~PMA/~ꢀ=3.55, and IPSS/Iw=1.29, ~PSS/~w=1.44).
Decay lifetimes were measured under different exper-
imental conditions (see Tables 1 and 2). Air-saturated
aqueous solutions, 2.5×10−5 M in Cr(III) and 4×
10−3 M in PMA, PSS or PAA, exhibit emission life-
times that are significantly longer than those measured
in pure water except for bpy. Once again, a larger effect
was observed for complexes containing the more hydro-
phobic ligands, such as dip and dim. Emission lifetimes
for bpy, phen and dim complexes in all three polyelec-
trolytes followed a first-order rate law, suggesting a
uniform chromium distribution throughout the macro-
molecule. [Cr(dip)3]3+ showed decay lifetimes that fit
an exponential first-order rate law in PAA and PSS.
The emission lifetimes of [Cr(dip)3]3+ upon binding to
PMA followed a double-exponential behavior that can
be fit to a decay with two clear first-order components,
It=I0[h exp (−k1t)+(1−h) exp (−k2t)] (h is the frac-
Lifetime and intensity measurements in PSS do not
show a strong pH-dependence, as expected by the na-

S. Ronco et al. / Inorganica Chimica Acta 308 (2000) 107–115
111
Table 2
Effect of the polyelectrolytes on the luminescence properties of [Cr(dip)3]3+ in aqueous solutions a
Medium
~
0
(ms)
~ (ms)
~/~0
I/I0
K (M−1
)
Water, pH 4.91
PAA, pH 6.26
PMA b, pH 4.91
PSS, pH 5.00
345
145
1 010
240
15
36
0.04
0.25
0.14, 0.51
0.18
0.10
0.21
0.48
0.21
5.45×106
1.00×104
2.94×106
140 (~1), 510 (~2)
43
a [Cr(dip)33+]=2.5×10−5 M; [polyelectrolyte]=4×10−3 M, non-controlled ionic strength; 25°C; ~0=decay lifetime in air-free solutions;
~=decay lifetime in air-saturated solutions; I and I0=emission intensities in air-saturated and air-free solutions, respectively.
b Sequential first-order decays.
tion of chromium units that decays with rate constant
k1, and k2 are exponential decays of excited chromium
species, and I0 and It are emission intensities at time 0
and t, respectively). h values were calculated from the
double exponential equation as described by Thomas et
al.[16]. The short-lived component, ~1ꢀ140 ms and
h=0.46, may be assigned to chromium units located in
the polyelectrolyte-water interface that are attracted to
carboxylate groups by electrostatic interactions, and to
free forms. The long-lived component, ~2ꢀ510 ms, may
be assigned to chromium bound to PMA sites by
hydrophobic interactions, see Table 2. In the absence of
O2, exponential decays for PMA solutions follow a
first-order rate law with lifetimes, ~0=1.01 ms (1/~0=
knr+kr)2. This observation suggests that hydrophobic
interactions between metal complex and polyelectrolyte
lead to a significant shielding from oxygen quenching.
Also, the fact that the emission intensities and decay
lifetimes are enhanced in the presence of PMA for both
aerated and deaerated solutions suggests that the rate
of nonradiative decay is decreased in the polyelectrolyte
microenvironment [9]. This can be interpreted in terms
of a more rigid environment created by the polyelec-
trolyte that restricts the vibrational freedom of the
ligands around the metal center, and that replaces some
of the water molecules in the solvation sphere [3].The
slight decrease in the emission properties of
[Cr(bpy)3]3+ in the polyelectrolyte was investigated by
evaluating knr and krx values at 25°C. This decrease,
observed in emission intensities and lifetimes, ap-
proaches a limiting value at higher polyelectrolyte con-
centrations. It has been reported that [Cr(bpy)3]3+ is
PMA (baqB0.013). Photoaquation quantum yields in
PMA, reach a lower limit at acidic pHs while show an
upper limit plateau in alkaline solutions (baq(H2O)/
baq(PMA)=1.00 (pH 4.5); baq(H2O)/baq(PMA)=1.31
(pH 9)) describing a ‘titration curve’, as previously
observed for aqueous solutions [3]. Both rate constants,
(knr=1.44×104; krx=9.43×103
s
−1), are slightly
higher for [Cr(bpy)3]3+ in PMA (pH 4.5, t=25°C)
than in pure water (knr=1.29×104; krx=7.05×103
s
−1). These observations seem to suggest that both
non-radiative and reactive pathways contribute to de-
creased emission lifetimes and intensities under our
experimental conditions4. Photoaquation quantum
yields of [Cr(bpy)3]3+ in PAA and PSS, 0.0155 and
0.0107, lead to non-significant contributions of the re-
active pathway in the overall decay scheme with rate
constants krx=449 and 328 s−1, respectively, and leave
knr as the major contribution to the intrinsic decay of
the excited state. The assumption that knr is increased in
the polyelectrolyte microenvironment agrees well with
luminescence lifetimes for [Cr(dip)3]3+ in PAA or PSS
in the absence of O2 (see Table 2). Only hydrophobic
complexes, [Cr(dip)3]3+ and [Cr(dim)3]3+, bound to
PMA showed a decrease in knr values, probably due to
strong binding of Cr(III) to sterically restricted hydro-
phobic sites inside the polymer coil.
The temperature dependence of the emission proper-
ties of [Cr(dip)3]3+ in the polyions was investigated in a
temperature range between 280 and 310 K, see Fig. 2.
Static and dynamic measurements in PSS and PAA
show the same trend for emission intensity and decay
lifetime changes in the temperature range under investi-
gation. For PMA solutions the temperature dependence
reflects the chromium distribution throughout the
polyion and was found that I/Iw=(h~1+(1−h)~2)/~w.
photochemically active with photoaquation quantum
yields (baq) defined as baq=krx.~ , and that ~−0 1
−1
=
air
3
knr+krx
.
All other chromium(III) polypyridines
showed non-significant photochemical reactivity in
2 In these assumptions we are not considering a non-radiative
photoaquation pathway. Previous reports in aqueous solutions indi-
cated that photoaquation quantum yields for phen, dim, and dip
complexes are non-significant and krx negligible in the overall decay
scheme.
4 A full investigation of the effect of the electrostatic interactions in
the emission properties of [Cr(bpy)3]3+ attached to anionic polyelec-
trolytes is currently under investigation. baq(H2O)=0.31;
baq(PMA)=0.33; ~air(H2O)=44 ms; ~air(PMA)=35 ms; ~0 (H2O)=
50 ms; ~0(PMA)=42 ms.
3 No ground-state quenching was observed for our experimental
conditions. See Ref. [1a].

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S. Ronco et al. / Inorganica Chimica Acta 308 (2000) 107–115
3.2. Excited-state quenching
Bimolecular rate constants for the quenching of the
excited-states of all four complexes with I−, O2, and
Fe2+ ions were measured in pure water (pH 4.5 and 6),
PMA (pH 4.5), PAA (pH 6.0), and PSS (pH 6.0) (see
Fig. 3. Stern–Volmer plots for the bimolecular reaction between
[Cr(LL)3]3+ and I− in the presence of PMA: (i) phen; (ii) bpy; (iii)
dim; (iv) dip. [Cr(LL)33+]=1×10−5 M; [PMA]=1×10−4 M;
[KH2PO4]=0.01 M.
Table 3). All quenching reactions were measured in
air-saturated solutions at the pH of the most pro-
nounced emission change. Static quenching was evalu-
ated by following emission intensity changes at the
wavelength of maximum intensity in the emission spec-
tra. Stern–Volmer plots showed a perfectly linear be-
havior in the concentration range under study for all
four complexes with PSS, PMA and PAA5 [18]. Fig. 3
exemplifies the behavior of all four chromium
polypyridines with I− in the presence of PMA. The
hydrophobic nature of both ligand and macromolecule
played a major role in the quenching reactions for all
tested solutions, despite of the charge of the quencher.
The more hydrophobic ligand, dip, has the largest
affinity for the hydrophobic pockets of PSS and PMA,
and hence feels the steric shielding effect of the
polyelectrolyte backbone in a larger extent. On the
other hand, smaller effects observed for bpy complexes
indicate smaller binding affinities with the polyelec-
trolyte and coulombic interactions between chromium
and polyelectrolyte. The data collected with PMA and
Fig. 2. Temperature-dependence of emission properties of 2.5×10−5
M [Cr(dip)3]3+ in 4×10−3 M polyelectrolyte; (a) PMA; (b)(i) PSS;
(ii) PAA. Solid and open circles are emission intensity and decay
lifetime measurements, respectively.
Table 3
Quenching of [Cr(LL)3]3+ in polyelectrolyte solutions at 25°C a
Medium kq(polyelectrolyte)/kq(water)
Bpy
phen
dim
dip
(i )I−
PAA
PSS
0.874
0.202
0.930
0.359
0.0147
0.773
0.00957
0.00425
0.0765
0.0165
0.00105
0.0197,
0.00460
b
PMA
(ii )Fe2+
PAA
1.64
2.87
3.40×102
97.2
PSS
4.42×103 2.52×103 1.42×103 1.82×102
PMA b
1.36
1.13
1.64×102 3.20×102, 37.5
(iii )O2
PAA
0.967
1.03
1.00
0.324
0.280
1.00
0.449
0.453
0.744
0.970 (0.327)
0.176 (0.299)
0.463, 0.0611
(0.152, 0.0152)
PSS
PMA b
5 Stern–Volmer parameters were determined by plotting: I0/I=
1+KSV[Q]. I0/I, ratio of the emission intensity in the absence of
quencher to that in the presence of quencher; KSV, Stern–Volmer
coefficient. Bimolecular rate constants for the quenching reactions,
kq, were calculated as: 1/~=kobsd=k0+kq[Q] where ~o=1/k0 is the
decay time without the quencher, and [Q] is the quencher concentra-
tion [8a].
a [Cr(III)]=1×10−5
M;
[polyelectrolyte]=1×10−4
M;
[KH2PO4]=0.01 M unless stated. Results in parentheses correspond
to [Cr(III)=2.5×10−5 M; [polyelectrolyte]=4×10−3 M with no
added salt.
b Results indicate both short and long-lived intermediates with
h=0.79 for these experimental conditions.

S. Ronco et al. / Inorganica Chimica Acta 308 (2000) 107–115
113
PSS solutions indicates that the overall polyelectrolyte
effect follows a trend that is in agreement with the
hydrophobic character of the polyamines, bpyB
phenBdimBdip.
in the polyelectrolyte chains. The same general trend
was observed for PAA and PMA solutions. While bpy
and phen complexes do not show much of an effect in
the presence of either polycarboxylate, the differences
between dip and dim complexes can be interpreted in
terms of an unequal distribution of Cr(III) and Fe2+
ions throughout the macromolecule with a stronger
shielding effect for dip. Fe2+ ions are known to form
complexes with carboxylate and sulfonate groups [19]
and expected to be located near the water interface.
Complexation makes Fe2+ less accessible to the Cr(III)
units. Preliminary results showed that bimolecular rate
constants measured with [Ru(bpy)3]2+ as an electron
transfer quencher follow the same trend, in which for
dip the small value observed for PMA can be explained
in terms of location of chromium and ruthenium units
in different sites within the polyelectrolyte (kq(PMA)/
kq(water)=0.24). One site is restricted and removed
away from the water interface and the other is in close
proximity to the water.
Anionic polyelectrolytes are expected to decrease the
interaction between cationic species, the Cr(III) com-
plex, and a negatively charged quencher, I−, due to
coulombic repulsions. Hydrophobic polyelectrolytes
make this effect even more pronounced by isolating the
chromium units from the water interface. PSS solutions
show the largest effect among all the polyelectrolytes
for all tested compounds with rate constant values
between 4.74 (bpy) and 950 (dip) times smaller than in
pure water. PMA solutions revealed strong hydropho-
bic interactions between the polyelectrolyte and
chromium units, kq(water)/kq(PMA)=217 (dip) (kq=
bimolecular quenching rate constant, M−1 −1).
s
A similar trend on both the polyelectrolyte and the
nature of the coordinating ligand is observed in
quenching experiments conducted with a positively
charged quencher, Fe2+. In this case, the presence of
the anionic polyelectrolyte increases the rate constant
values, by bringing the reactants together due to a
strong electrostatic interaction [8,15]. This effect was
proved to be mainly a volume effect in which the
reactants are condensed in the negative potential field
of the microenvironment [8]. PSS solutions display a
substantial increase in the bimolecular rate constants
for all four complexes (kq(PSS)/kq(water)=4.42×103
(bpy), 2.52×103 (phen), 1.42×103 (dim) and 1.82×
102 (dip)). This trend shows a decreased rate constant
for dip due to a strong shielding association with the
polyelectrolyte and absorption of the cationic quencher
Bimolecular rate constants for the quenching reac-
tion of the excited states of Cr(III) with dissolved O2
were evaluated by assuming relationships such as
~
−1=krad+knr+kq[O2]. The inhibition of O2 quench-
ing is greater for [Cr(dip)3]3+ in the presence of PMA
at a pH of a compact conformation (pH 4.5). At this
pH the macromolecules offer a rigid microenvironment
in which the metal complexes are protected against
quenching by oxygen and other nonradiative decay
modes of the excited state [8].
3.3. Effect of added salts
As mentioned above, the photophysical properties of
chromium(III) polyamines in polyelectrolytes are
closely related to experimental parameters that influ-
ence binding affinities between guest and host compo-
nents. Addition of salts modifies coulombic interactions
with the macromolecules due to extensive ion-pairing
between chromium units and added anions6 [20,21].
Ion-pairing decreases the overall charge of the
chromium species bound to macromolecules, and there-
fore it modifies the electrostatic interactions between
guest and host components. Additional positively
charged ions ‘screen’ the interactions between these
chromium species and host molecules [22]. Cr(III) units
hydrophobically bound inside the polymer coils are
expected to be sterically shielded and hence, to feel the
ionic strength effect in a lesser extent. Fig. 4 and Table
4 exemplify the effect of added salts on the emission
6 The ion-pairing constant (Kip) can be calculated as a function of
Fig. 4. Effect of added salts on the relative emission intensity of
1×10−5 M [Cr(dip)3]3+ in 1×10−4 M PMA at various pHs,
[I]=0.01 M: (i) ꢀ NaClO4, ꢁ LiCF3SO3; (ii) KH2PO4.
the ionic strength by using the Fuoss equation. For {[Cr(bpy)3]3+
,
ClO−4 } the ion-pairing equilibrium constant is expected to be larger
than 5.73.

114
S. Ronco et al. / Inorganica Chimica Acta 308 (2000) 107–115
Table 4
Effect of added salts on the emission lifetimes of [Cr(dip)3]3+ attached to PMA a
b
c
Salt
h
~(ms)
~/~w
~
~/~0
0
NaClO4
LiCF3SO3
KH2PO4
0.46
0.53
0.77
110(~1), 410(~2)
135(~1), 490(~2)
25(~1), 103(~2)
11, 41
12.2, 44.5
2.4, 9.8
1160
1130
289
0.10, 0.35
0.12, 0.43
0.09, 0.36
a [Cr(dip)33+]=2.5×10−5 M; [PMA]=4×10−3 M; I=0.1 M.
b Sequential first-order decays. h is the fraction of chromium units that decays with lifetime ~1.
c First-order decays.
intensities and decay lifetimes of [Cr(dip)3]3+ in PMA.
While the addition of 0.01 M perchlorate or triflate
salts does not change emission patterns substantially,
KH2PO4 favors an overall decrease in luminescence
suggesting dominant coulombic interactions rather than
hydrophobic7. This sharp decrease in emission intensity,
together with shorter decay lifetimes in the presence of
phosphate, indicates ion-pair formation and displace-
ment of chromium units from inside the hydrophobic
polymer coils. Perchlorate and triflate solutions display
h values and ~/~o ratios comparable to those measured
in the absence of added salts. Inspection of emission
data in PMA solutions with KH2PO4 suggests that the
chromium units are pushed out of the hydrophobic
sites and placed near the water interface, where they are
more exposed to O2 quenching. Time-resolved experi-
ments showed short-lived (~1=25 ms) and long-lived
(~2=103 ms) transients with decay lifetimes in the
neighborhood of those measured for pure water (~=15
ms) and for electrostatically bound phenanthroline com-
plexes (~ꢀ118 ms), respectively.
The ionic strength effect on the emission decays was
investigated by adding increasing amounts of NaClO4
to PMA solutions of bpy and dip complexes, see Fig. 5.
Both complexes showed opposite effects that reflect the
different nature of the chromium/polyelectrolyte inter-
action. Addition of salt to [Cr(dip)3]3+ shortens emis-
sion lifetimes of both short-lived (~1) and long-lived (~2)
components of the emission (~1=119 ms and 69 ms for
[NaClO4]=0 and 0.6 M respectively; ~2=510 ms and
395 ms for [NaClO4]=0 and 0.6 M respectively) as
expected for a screened interaction between guest and
host molecules. Decays for [Cr(bpy)3]3+ units in PMA
become longer at increasing concentrations of the salt,
and level off at [NaClO4]ꢀ0.4 M with ~=67 ms. This
result matches decay lifetimes measured in aqueous
solutions with no polyelectrolyte (I=0.4 M). Thus,
[Cr(bpy)3]3+ ions are surrounded by perchlorate anions
that occupy interligand pockets and provide additional
rigidity to the metal surrounding making emission life-
times longer than those observed with no added salts.
Longer lifetimes in a highly ionic medium have been
explained by invoking ionic interferences of the ligand
vibrations that promote non-radiative processes, and a
change in the microstructure of the medium surround-
ing the metal complex [3].
4. Conclusions
The experimental data described above show that
chromium(III) polypyridines can be used as sensitive
probes for hydrophilic and hydrophobic sites in
polyelectrolyte microenvironments. While the emission
properties of [Cr(dip)3]3+ show the effect of a combina-
tion of hydrophobic and electrostatic interactions,
[Cr(bpy)3]3+ solutions display the effect of only cou-
lombic interactions with the polyelectrolyte backbone.
From here we can conclude that electrostatic interac-
tions between Cr(III) and anionic polyelectrolytes con-
tribute to reduced emission properties, apparently due
to an increase in knr. On the other hand, strong hydro-
phobic interactions place chromium units in restricted
microenvironments, in which the excited state proper-
7 The anion effect has been reported to result from a combination
of phenomena such as ion-pairing, molecular events in interligand
pockets, vibrational freedom of the ligands and the structure of the
solvent [11].
Fig. 5. Ionic strength effect on the decay lifetimes of 2.5×10−5
M
[Cr(LL)3]3+ in 4×10−3 M PMA. (a) bpy; (b) dip; (i) no NaClO4; (ii)
[NaClO4]=0.6 M.

S. Ronco et al. / Inorganica Chimica Acta 308 (2000) 107–115
115
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Acknowledgements
The authors acknowledge NSF-EPSCoR for partial
support of this research. B.P. acknowledges the
Howard Hughes Medical Institute for summer and
academic year student stipends. J.B. and S.S. acknowl-
edge NSF-EPSCoR for summer support.
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