Folding of a poly(oxyethylene) chain as probed by photoinduced energy transfer between Ru- and Os-polypyridine termini

Article abstract of DOI:10.1039/b104637n

Photoinduced energy transfer involving Ru- and Os-polypyridyl complexes was studied. The folding of a poly(oxyethylene) chain was probed by the extent of energy transfer between the unit of complexes attached to either end of the chain. The distribution of end-to-end distances in the ensemble of conformation changes in different solvents was analyzed using luminescence studies. Results showed that the distribution of Ru···Os separations was solvent dependent.

Full text of DOI:10.1039/b104637n

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Folding of a poly(oxyethylene) chain as probed by photoinduced
energy transfer between Ru– and Os–polypyridine termini
Simon J. A. Pope,a Craig R. Rice,a Michael D. Ward,*a Angeles Farran Morales,b
Gianluca Accorsi,b Nicola Armarolib and Francesco Barigelletti*b
a School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS.
E-mail: mike.ward@bristol.ac.uk
b Istituto FRAE-CNR, Via P. Gobetti 101, I-40129, Bologna, Italy. E-mail: franz@frae.bo.cnr.it
Received 29th May 2001, Accepted 22nd June 2001
First published as an Advance Article on the web 6th July 2001
The extent of photoinduced energy-transfer between
[Ru(bipy)3]2ϩ and [Os(bipy)3]2ϩ units attached to either end
of a deca(oxoethylene) 33-atom chain can be used to show
how the distribution of end-to-end distances in the
ensemble of conformations changes in different solvents.
Photoinduced energy transfer involving polypyridyl complexes
of Ru(), Os() and Re() has been extensively studied in
relation to development of artificial systems for solar energy
harvesting,1 the construction of ‘molecular wires’ allowing
Scheme 1
long-range information transfer,2 and anion/cation sensing3
and switching devices.4 Accordingly, very many dinuclear
heterometallic complexes have been designed and investigated
for the assessment of the role played by structural and
electronic factors of the bridging ligand (BL) – which provides
the covalent link between the photoactive termini – in control-
ling the energy-transfer process.5–8 In general BLs containing
unsaturated components are likely to mediate the transfer of
excitation energy via a Dexter-type electron-exchange process,9
whereas the use of saturated components within the BL only
allows for the dipole-type, Förster mechanism,10 which is
typical of organic chromophores but may also occur for
polypyridine complexes of Ru() and Os() and other metal
centers.
In most dinuclear complexes where photoinduced energy-
transfer is studied, the BLs are rigid such that energy-transfer is
occurring over a well-defined distance, and the efficiency of
energy-transfer can then be related to the particular properties
of the bridging ligand.1,2 In contrast, the study of photoinduced
energy-transfer between chromophore and quencher units
attached to either end of highly flexible molecules (such as
polyalkanes11 or polypeptides12) has been used to provide
information on their conformational behaviour under different
conditions, which is of value for comparison with results of
conformational models and molecular dynamics simulations. In
this vein we recently described the complex Ru-L5-Os (Scheme
1) in which [Ru(bipy)3]2ϩ and [Os(bipy)3]2ϩ termini are linked by
a flexible 18-atom bridge derived from attachment of bipyridine
units to either end of a penta(ethyleneglycol) chain.13 The
conformations of oligomeric chains containing –(CH2CH2O)–
units are sensitive to the polarity of the solvent,14,15 and we
found that for Ru-L5-Os the efficiency of photoinduced
Ru→Os energy transfer decreased smoothly as the solvent
polarity increased, due to a solvent-induced conformational
change which increases the Ru ؒ ؒ ؒ Os separation in more
polar solvents. We could accordingly use the Ru→Os energy
transfer rate as a ‘ruler’ to determine the variation in Ru ؒ ؒ ؒ Os
separation as a consequence of the conformational changes in
the chain.
(bipy)3]2ϩ energy-acceptor termini. We have found that Ru→Os
energy transfer is again a useful probe of the conformational
behaviour of this system in solution, and that the con-
formational behaviour of Ru-L10-Os in different solvents is
significantly different from that of Ru-L5-Os.
Whereas the bridging ligand in Ru-L5-Os was prepared in a
single step from 5-(bromomethyl)-2,2Ј-bipyridine and com-
mercially available penta(ethyleneglycol), for Ru-L10-Os
we had to devise a more elaborate stepwise synthesis as deca-
(ethyleneglycol) is not readily available. The synthesis is sum-
marised in Scheme 2,† which shows how the 10-mer chain is
constructed in two steps from shorter commerically available
starting materials. Addition of the metal termini by stepwise
reaction with [Ru(bipy)2Cl2] and then [Os(bipy)2Cl2], followed
by isolation of the complex as its hexafluorophosphate salt,
follows the previously-published method.13,†
Our study of the solvent effect on the conformation of
Ru-L10-Os follows a spectroscopic approach that has been
described before.16,‡ We have employed CH2Cl2 as a low
polarity, low viscosity solvent [relative permittivity ε = 9.08,
viscosity η = 0.449 cp where 1 centipoise (cp) = 10Ϫ3 Pa s]; as a
highly polar and viscous solvent, we used an ethylene glycol/
MeOH mixture (0.8 : 0.2 v/v, with resulting bulk ε = 36.7 and
η = 16.04 cp; solubility factors prevented the use of higher
viscosity solvents), which we abbreviate hereafter as EG.17
Table 1 collects absorption and luminescence properties for
Ru-L10-Os in both solvent systems. Fig. 1 compares absorption
and luminescence spectra for Ru-L5 (a mononuclear model
complex available from previous work)13 and Ru-L10-Os as
obtained in CH2Cl2; similar results were obtained in EG. The
luminescence spectra were obtained by excitation at 415 nm;
from the absorption spectral properties (see Fig. 1), excitation
at this wavelength results in equal excitation of both the
Ru-based and Os-based 1MLCT transitions. The Os-based
luminescence occurs at λ > 700 nm and is obscured by the tail
of the much more intense emission from the Ru-based lumino-
phore. For reference purposes, data for the mononuclear
complex Ru-L5 in the same solvents are also reported.13
Since the study of the conformational properties of poly-
(oxoethylene) chains is a topic of considerable importance,14,15
we have extended this work by preparing and studying the
complex Ru-L10-Os (Scheme 1), in which a flexible 33-atom
chain separates the [Ru(bipy)3]2ϩ energy-donor and [Os-
The bridging ligand L10 does not include low-energy con-
ducting units; actually the energy level of the HOMOs of the
L10 chain is lower by ca. 3 eV than that of the bpy ligand to
which it is attached, and for the corresponding LUMOs the
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J. Chem. Soc., Dalton Trans., 2001, 2228–2231
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104637n

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Table 1 Spectroscopic and photophysical dataa
Absorption
Emissionb
f (r)c
µ/Å
λmax/nm (10Ϫ3ε/dm3 molϪ1 cmϪ1
)
λmax/nm
102 Φ
τd/ns
a/Å
CH2Cl2
Ru-L5
Ru-L10-Os
EG
Ru-L5
Ru-L10-Os
287 (91.0)
289 (149.7)
453 (14.4)
452 (26.6)
608
602
1.6
ca. 0.5
310
256d
14.1
14.7
4.2
2.7
287 (90.0)
291 (155.0)
450 (16.1)
450 (25.5)
608
610
2.4
ca. 0.8
540
430d
a Room temperature, in air-equilibrated solvents; sample concentrations were 2 × 10Ϫ5 M. b Ru-based luminescence properties: excitation was at
415 nm. c µ and a are parameters for the intermetal distance distribution, as obtained from the least squares analysis according to eqns. (3) and (4);
see text. d This is what the emission lifetime of the Ru chromophore of Ru-L10-Os would be in the absence of any quenching by the Os component;
calculated from eqn. (3), see text.
Scheme 2
equally balanced. The parameters ꢀd and τd are the intrinsic
luminescence quantum yield and lifetime of the [Ru(bipy)3]2ϩ
donor, respectively; κ2 is a geometric factor taken as 2/3 for
statistical reasons; the integral in eqn. (2) is the overlap between
the emission spectrum of the donor and the absorption
spectrum of the acceptor and is evaluated on an energy scale
(cmϪ1). By using eqns. (1) and (2) and the available spectro-
scopic parameters (Table 1) we find that Ro = 24 Å for both
CH2Cl2 and EG solvents.
The long and flexible poly(oxyethylene) chain in L10 is
expected to undergo conformational rearrangements, arising
from the change in the preferential conformation about each
C–C bond from anti in low-polarity solvents to gauche in
Fig. 1 Absorption spectra for Ru-L10-Os (solid line), Ru-L5 (----),
high-polarity solvents.14,15 We have probed these changes as
and [Os(bpy)3]2ϩ (- - - - ) in CH2Cl2. The inset shows the luminescence
. . . .
illustrated below. From Fig. 1, we see that the Ru-based
luminescence intensity for Ru-L10-Os, as detected at 610 nm, is
ca. 1/3 that of the reference mononuclear complex Ru-L5. This
is due to Ru→Os energy transfer, which is thermodynamically
allowed by ca. Ϫ0.3 eV.5–8,13 For the extensively studied cases of
rigid dinuclear Ru/Os complexes, where the photoactive units
are separated by a fixed distance, the Ru-based luminescence
spectra for Ru-L10-Os (solid line) and Ru-L5 (----) as observed for
excitation of isoabsorbing solutions at 415 nm.
energy gap is >20 eV, as evaluated from EHMO calculations.18
On this basis, we expect no significant orbital interactions
between the donor and acceptor metal complex units and the
bridging poly(oxyethylene) chain,19 such that Ru→Os photo-
induced energy transfer step over the intermetal distance dMM
can only take place via the dipole–dipole (Förster) mechan-
ism,10 eqns. (1) and (2).
decay is describable by a single exponential law, I(t)Ru
=
A exp(Ϫt/τ), and ken = 1/τ Ϫ 1/τd, with τd being the lifetime of
a suitable reference Ru-based complex (A is a preexponential
factor). We also found this behaviour in Ru-L5-Os, with a single
Ru ؒ ؒ ؒ Os separation being measured at each solvent com-
position.13 For Ru-L10-Os however, the Ru-based decay shows
a more complicated behaviour, arising from the fact that the
greater conformational flexibility of the chain allows a range
of Ru ؒ ؒ ؒ Os separations even at a fixed solvent composition.
We have found that the following expression, in accord with the
spectroscopic approach described by Lakowicz,16 fits our results
well.
(1)
(2)
In eqns. (1) and (2), Ro is the critical transfer distance, i.e. the
distance at which the deactivation rate of the [Ru(bipy)3]2ϩ
donor is equal to ken, the energy-transfer rate, such that the
luminescence and energy-transfer deactivation pathways are
(3)
J. Chem. Soc., Dalton Trans., 2001, 2228–2231
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overall distribution of different chain conformations in these
flexible oligomers.
Acknowledgements
We thank the European Community (A. F. M., TMR contract
no. CT98–0226 and COST Programme D11/0004/98), and the
EPSRC (S. J. A. P. and C. R. R.) for financial support. M. D. W.
is the Royal Society of Chemistry Sir Edward Frankland Fellow
for 2000/2001.
Notes and references
‡ 2,2Ј-Bipy-5-(CH2OCH2)4CH2OTs. To a solution of 5-hydroxymethyl-
2,2Ј-bipyridine21 (0.10 g, 0.54 mmol) in dry THF (20 cm3) was added
NaH (0.021 g of 60% dispersion in oil, 0.54 mmol) and the mixture was
stirred under N2 for 15 min. To this was added a solution of tetra(ethyl-
ene gylcol) ditosylate (0.50 g, 1.0 mmol) in THF (10 cm3). The reaction
was then monitored by TLC (Silica; MeOH/CH2Cl2, 1 : 9 v/v) until
most of the 5-hydroxymethyl-2,2Ј-bipyridine had disappeared. The
solvent was then removed and the resulting oil dissolved in CH2Cl2
(50 cm3) and washed with water (2 × 20 cm3). The crude material was
purified by repeated column chromatography (silica; MeOH/CH2Cl2,
1
1 : 9 v/v). H NMR (300 MHz, CDCl3): δ 8.68 (1 H, d; bipy H6Ј), 8.64
Fig. 2 Luminescence decays of Ru-L10-Os as observed at 610 nm. The
full curves are the fit to the experimental points according to eqns. (3)
and (4) of the text. The insets show the f(r) gaussian distribution of the
intermetal distances. Channel of time axis was 1.02 ns.
(1 H, br s; bipy H6), 8.38 (2 H, m; bipy), 7.78–7.82, [4 H, m; bipy (×2)
and phenyl H2/H6]; 7.30–7.35 [3 H, m; bipy (×1) and phenyl H3/H5],
4.64 (2 H, s; bipy-CH2O), 4.15 (2 H, m; CH2), 3.60–3.70 (14 H, m;
CH2), 2.35 (3 H, s; CH3). EIMS: m/z 516 (5%, Mϩ).
2,2Ј-Bipy-5-(CH2OCH2)10CH2OH. To a solution of NaH (0.013 g
of 60% dispersion in oil, 0.32 mmol) in THF (20 cm3) was added
hexa(ethylene gylcol) (0.09 g, 0.32 mmol) and the reaction stirred under
N2 for 1 h. After this time 2,2Ј-bipy-5-(CH2OCH2)4CH2OTs (0.082 g,
0.16 mmol) was added and the reaction was refluxed overnight. The
reaction was then quenched with water, and extracted with CH2Cl2; the
extract was evaporated to dryness to give a crude oil which was purified
by column chromatography (silica; MeOH/CH2Cl2, 1 : 9 v/v) to give the
product as a pale yellow oil (yield: 46%). 1H NMR (300 MHz, CDCl3):
δ 8.66 (2 H, m; bipy); 8.38 (2 H, m; bipy); 7.82, (2 H, m; bipy); 7.31 (1 H,
m; bipy); 4.65 (2 H, s; bipy-CH2O); 3.65 (40 H, m; CH2). EIMS: m/z 626
(50%, Mϩ).
L10. To a solution of 2,2Ј-bipy-5-(CH2OCH2)10CH2OH in dry,
degassed THF (30 cm3) was added NaH (0.026 g of 60% dispersion in
oil, 0.64 mmol) and the mixture was stirred for 1 h. To this was added
a solution of 5-bromomethyl-2,2Ј-bipyridine22 (0.060 g, 0.24 mmol) in
dry THF (5 cm3) and the mixture was stirred at reflux for 24 h. Removal
of the solvent afforded a brown oil which was purified by column
chromatography (silica; MeOH/CH2Cl2, 1 : 9 v/v) to give L10 as a vis-
cous oil (yield: 32%). 1H NMR (300 MHz, CDCl3): δ 8.68 (4 H, m;
bipy); 8.45 (4 H, m; bipy); 7.85, (4 H, m; bipy); 7.35 (2 H, m; bipy); 4.65
(4 H, s; bipy-CH2O); 3.66 (40 H, m; CH2). EIMS: m/z 794 (3%, Mϩ).
[Ru(bipy)2(L10)][PF6]2 (Ru-L10). Reaction of L10 (0.035 g, 0.044
mmol) and [Ru(bipy)2Cl2]ؒ2H2O (ref. 23) (0.023 g, 0.044 mmol) in
EtOH (30 cm3) at reflux for 6 h afforded a red solution. The solvent was
removed in vacuo and the product chromatographed on Sephadex-
SP25, eluting with 0.3 M aqueous NaCl. The major red band yielded
Ru-L10 after precipitation with aqueous NH4PF6, extraction with
CH2Cl2, and evaporation to dryness. Yield: 42%. FAB-MS: m/z 1208
({M ϩ H Ϫ 2PF6}ϩ). Found: C, 46.0; H, 4.1; N, 6.4%. Required for
[Ru(bipy)2(L10)][PF6]2ؒ2CH2Cl2: C, 46.1; H, 4.7; N, 6.7%. In addition a
small amount of the dinuclear complex [{Ru(bipy)2}2(L10)][PF6]4 was
isolated (11% yield) after further elution with 0.5 M NaCl.
[(bipy)2Ru(L10)Os(bipy)2][PF6]4 (Ru-L10-Os). Ru-L10 (0.040 g, 0.027
mmol) was added to a slight excess of [Os(bipy)2Cl2] (ref. 24) (0.017 g,
0.030 mmol) in ethylene glycol (15 cm3) and heated at 120 ЊC for 24 h.
The dark green reaction mixture was allowed to cool, added to distilled
water (100 cm3) and introduced onto Sephadex-SP25. Firstly, the
mixture was eluted with a further 100 cm3 of water, and then with
0.5 M aqueous NaCl which afforded a primary green band which was
collected. Addition of aqueous NH4PF6, extraction with CH2Cl2, and
evaporation to dryness afforded Ru-L10-Os in 63% yield. FAB-MS:
m/z 2148 ({M ϩ H Ϫ PF6}ϩ), 1999 ({M ϩ H Ϫ 2PF6}ϩ), 1707 ({M ϩ
H Ϫ 4PF6}ϩ). Found: C, 41.6; H, 3.7; N, 6.6%. Required for
[(bipy)2Ru(L10)Os(bipy)2][PF6]4ؒCH2Cl2: C, 42.0; H, 3.9; N, 7.1%.
§ Absorption spectra of dilute solutions (2 × 10Ϫ5 M) of the complexes
were measured in the indicated solvents at room temperature with
Perkin-Elmer Lambda 5, Lambda 9 or Lambda 19 UV/Vis spectro-
photometers. For the luminescence experiments, air-equilibrated solu-
tions of the samples were used. Luminescence spectra were obtained
from solutions whose absorbance values were ≤0.2 at the employed
(4)
Here, τd is the unquenched luminescence lifetime of the
Ru-based donor unit of Ru-L10-Os (to be compared with that
for the reference complex Ru-L5); Ro is the critical transfer
radius [from eqns. (1) and (2)]; and f(r) describes a gaussian
distribution centred around µ, the average Ru ؒ ؒ ؒ Os separation
in Ru-L10-Os, with a representing the width of the distribution.
Analyses of the Ru-based luminescence decays of Ru-L10-Os,
as monitored at 610 nm in the employed solvents, were there-
fore performed according to eqn. (3) by using an iterative
least-squares non-linear approach;20 results are collected in
Table 1. Fig. 2 displays representative decays observed in the
two solvents; the derived f(r) distributions are illustrated in the
insets.
From the detailed analysis of the time resolved luminescence
we can draw the following conclusions. The average intermetal
distance in Ru-L10-Os is evaluated as 14–15 Å in both solvents
employed, which does not correspond to a full extension of
the connecting chain (estimated as 38 Å).18 According to the
obtained f(r) distributions (insets of Fig. 2) this indicates that
in most cases the Ru→Os energy transfer takes place within
the “critical sphere”, with Ro = 24 Å. The conformational
picture emerging from this result is consistent with the chain
of L10 undergoing a high degree of folding whose average
extent (based on the Ru ؒ ؒ ؒ Os average separation) is not
solvent dependent. The distribution of Ru ؒ ؒ ؒ Os separations
[parameter a in eqns. (3) and (4)] is however significantly solvent
dependent, with the f(r) distribution (Fig. 2) being remarkably
narrower in EG than in CH2Cl2, suggesting that in the EG
solvent Ru-L10-Os behaves like a more rigidified dinuclear
system compared to the behaviour in CH2Cl2. This behaviour
contrasts with that shown by the shorter complex Ru-L5-Os,
where a single exponential decay for the Ru-based emission
was observed at each solvent composition, corresponding to a
Ru ؒ ؒ ؒ Os separation of 12 Å in CH2Cl2 and 15 Å in MeOH.
The majority of recent studies on the conformation of poly-
(oxoethylenes)14 (and simple model complexes such as dimeth-
oxyethane15) have concentrated on how the proportion of anti
and gauche conformers varies in the ensemble under different
conditions, using a wide variety of techniques such as 13C
NMR, infrared, Raman, and neutron- and electron-diffraction
methods. It is clear from our new work that use of luminescence
methods can play an important rôle in helping to determine the
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J. Chem. Soc., Dalton Trans., 2001, 2228–2231

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excitation wavelength (415 nm) using Perkin-Elmer LS50B or Spex
Fluorolog II spectrofluorimeters. While uncorrected band maxima are
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quantum yields we have employed corrected spectra and used as
a reference [Ru(bpy)3]2ϩ (Φ = 0.028 in air-equilibrated water) as a
reference standard.25 Band maxima and relative luminescence inten-
sities are subject to uncertainties of ca. 2 nm and 20%, respectively.
Luminescence lifetimes were obtained using an IBH single-photon
counting equipment equipped with a nitrogen-filled thyratron gated
lamp. The uncertainty in the lifetime values is within 8%.
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