Photoinduced energy transfer within hydrogen-bonded multi-component assemblies based on a ruthenium-polypyridyl donor and an osmium-polypyridyl or ferrocenyl acceptor

Article abstract of DOI:10.1039/b006503j

Formation of multipartite associates driven by the complementary H-bonding abilities of cytosine and guanine appended to [Ru(bipy)₃]²⁺, [Os(bipy)₃]²⁺, and ferrocenyl chromophores is probed by exploiting the luminescence properties of the various building blocks and the energy transfer processes occurring between them.

Full text of DOI:10.1039/b006503j

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Photoinduced energy transfer within hydrogen-bonded
multi-component assemblies based on a ruthenium–polypyridyl donor
and an osmium–polypyridyl or ferrocenyl acceptor
Susana Encinas,a Naomi R. M. Simpson,b Paul Andrews,b Michael D. Ward,*b
Claire M. White,b Nicola Armaroli,a Francesco Barigelletti*a and Andrew Houltonc
a Istituto FRAEÈCNR, V ia P. Gobetti 101, 40129 Bologna, Italy. E-mail: franz=frae.bo.cnr.it
b School of Chemistry, University of Bristol, CantockÏs Close, Bristol, UK BS8 1T S.
E-mail: mike.ward=bristol.ac.uk
c Department of Chemistry, University of Newcastle upon T yne, Bedson Building,
Newcastle-upon-T yne, UK NE1 7RU
Received (in Cambridge, UK) 5th August 2000, Accepted 31st August 2000
First published as an Advance Article on the web 23rd November 2000
Formation of multipartite associates driven by the complementary H-bonding abilities of cytosine and guanine
appended to [Ru(bipy) ]2`, [Os(bipy) ]2`, and ferrocenyl chromophores is probed by exploiting the luminescence
3
3
properties of the various building blocks and the energy transfer processes occurring between them.
Within the Ðeld of polypyridine transition metal complexes,
we have been interested in the study of photoinduced energy
Results and discussion
Of the compounds used in this study, [Ru(tBu bipy) (bipy-G)]
transfer occurring in multichromophoric systems based on
luminescent Ru(II)-, Os(II)- and Re(I) complexes.1h5 The design
and building up of such systems is aimed at realising precise
structural and energy patterns. For instance, compounds
incorporating both Ru- and Os-based components may allow
the achievement of long-distance excitation energy trans-
port.6,7 This process is driven by the di†erent energy content
of the Ru- and Os-based 3MLCT luminescent levels (for
Ru ] Os transfer, [*G is in the range 0.2 to 0.4 eV) and is
mediated by the electronic and structural properties of the
interposed spacer.7
2
2
[PF ] (hereafter Ru-G) was prepared from [Ru(tBu bipy)
6 2
2
2
2
Cl ] and bipy-G in the usual way.3,9 The new ligand bipy-C
2
was prepared by alkylation of 5,5@-bis(bromomethyl)-2,2@-bipy-
ridine with two equivalents of cytosine, using the same general
procedure as for bipy-C.3 Reaction of this with
[Os(bipy@) Cl ]
[bipy@ \ 4,4@-bis(5-nonyl)-2,2@-bipyridine]
2
2
There are numerous examples of multicomponent systems
incorporating polypyridine transition metal complexes whose
components are assembled via covalent linkages. Cases
include complexes featuring interesting geometries, like wires
(mostly dinuclear cases, which are particularly apt for probing
switching conditions),6,7 dendrimers (for artiÐcial mimicking
of light energy harvesting processes),8 and so forth. The use of
covalent linkages between components generally guarantees a
high degree of control of the topological properties, but may
prove difficult because of the multistep synthetic procedures
frequently needed.
We have been interested in an alternative approach for the
construction of multi-component systems, based on
[Ru(bipy) ]2` and [Os(bipy) ]2` building blocks which bear
3
3
pendant nucleobase groups appended to one of the coordi-
nated bpy ligands (see Scheme 1).3,9 Other groups have also
used this approach to link together porhpyrin-based
chromophore/quencher assemblies by hydrogen-bonding
between peripheral substituents.10 In an earlier report, we
described the syntheses of the ligands bipy-C and bipy-G, and
showed how a mixture of [Ru(tBu bipy) (bipy-C)][PF ] and
2
2
6 2
[Os(tBu bipy) (bipy-G)][PF ] associated strongly in CH Cl
2
2
6 2
2 2
solution such that Ru ] Os photoinduced energy transfer
occurred across the hydrogen-bonded bridge linking the com-
ponents, with a rate constant k \ 9.3 ] 107 s~1.3 We now
en
describe our investigations into the formation of higher
nuclearity hydrogen-bonded assemblies, and also the use of an
alternative energy acceptor (the ferrocenyl unit).
Scheme 1
DOI: 10.1039/b006503j
New J. Chem., 2000, 24, 987È991
987
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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a†orded [Os(bipy@) (bipy-C )][PF ]
(hereafter C-Os-C),
Table 1 collects spectroscopic and photophysical properties
for C-Os-C and Ru-G; the absorption spectral properties of
Fc-C are also reported. Fc-C is non-luminescent at wave-
lengths shorter than 900 nm, and the ferrocene unit is report-
ed to act both as an energy acceptor (its lowest-lying 3MC
excited level is estimated at ca. 1.7 eV) and an electron donor
(the reduction potential for Fc`/Fc is ]0.39 V vs. SCE in
MeCN).16 Thus, we use the couple Ru-G/Fc-C for compari-
son purposes with respect to the behaviour exhibited by the
couple Ru-G/C-Os-C.
2
2
6 2
which contains two pendant cytosine units; the nonyl substit-
uents on the ancillary bipy@ ligands are necessary to maintain
solubility in CH Cl . 1-Ferrocenylmethylcytosine (Fc-C) was
2
2
available from earlier work.11
The complexes Ru-G and C-Os-C are expected to self-
assemble because of complementary three-point H-bonding
interactions yielding 1 : 1 and 2 : 1 associates (see eqn. 1 and 2
and Scheme 2).
Ru-G + C-Os-C H Ru-G : C-Os-C
(1)
(2)
Fig. 1 and 2 summarise the approach we have employed,
which is based on the use of luminescence data obtained from
CH Cl solutions containing mixtures of Ru-G and C-Os-C,
Ru-G + Ru-G : C-Os-C H Ru-G : C-Os-C : G-Ru
2
2
and of Ru-G and Fc-C, respectively. For all the cases in Fig. 1
and 2 the total concentration of the complexes was 4 ] 10~5
M, with variable Ru-G to C-Os-C and Ru-G to Fc-C ratios;
In the case of the 1 : 1 associate, Ru-G : C-Os-C, the two pho-
toactive termini can interact thanks to the structural role
played by the G : C triple bond, [see Scheme 2(a)]. This inter-
action can provide a sizeable association constant; a value of
j
was 450 nm, corresponding either to an isoabsorptive
exc
point for all the solutions (Fig. 1), or to predominant absorp-
tion by Ru-G (Fig. 2).
For the mixture of Ru-G and C-Os-C, the Ru-based lumi-
K
P 5 ] 103 M~1 was found in CH Cl for the similar Ru-
ass
2 2
C : G-Os associate.3 Though 1 : 1 association is an interesting
step in itself, the new complex C-Os-C was designed to
provide more complex assemblies, being potentially capable of
simultaneous association with two Ru-G units (eqn. 2). Within
the desired three-component Ru-G : C-Os-C : G-Ru associate,
provided the Ru ] Os energy transfer step is e†ective,12h15
light absorption by both of the peripheral Ru-based com-
ponents would contribute to the emission from the central Os-
based component [Scheme 2(b)]; i.e. an antenna e†ect by
which the excitation energy from the two peripheral Ru units
is conveyed to a single central point.
nescence intensity of Ru-G (j \ 626 nm) is stronger by one
max
order of magnitude than that of C-Os-C (j \ 750 nm,
max
Table 1), with the latter being hidden by the tail of the former.
This is typical behaviour for [Ru(bipy) ]2` and [Os(bipy) ]2`
3
3
chromophores and, given the predominance of unassociated
over associated species, prevents observation of any sensitisa-
tion of the Os-based luminescence; our approach was there-
fore based on the observation of the quenching of the
Ru-based luminescence. At the concentrations used, any quen-
ching of the emission of Ru-G can only occur within the
Scheme 2 Possible energy transfer steps in the investigated Ru-G/C-Oc-C couple.
Table 1 Spectroscopic and photophysical dataa
Absorption
Emission
j
/nm (10~3 e/dm3 mol~1 cm~1)
j
/nm
102 U
q/ns
energyb/eV
max
max
C-Os-C
Ru-G
Fc-C
291(85.6)
288(76.1)
261(12.6)
446(13.7)
459(12.3)
440(0.1)
590(4.2)
750
626
0.3
4
33
450
1.75
2.1
a Room temperature, air-equilibrated CH Cl solvent. b Estimated from the onset of the emission spectra.
2
2
988
New J. Chem., 2000, 24, 987È991

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Fig. 1 Luminescence intensities, as observed at 626 nm, for the
various Ru-G/C-Os-C mixtures (total concentration was 4 ] 10~5 M)
in neat CH Cl (I , =), and after EtOH addition (IEtOH, >); the
2
2
Ru
Ru
dashed line indicates the luminescence intensities expected based
solely on the Ru-G molar fractions. The inset shows *I (…), see
Ru
text; the full line is the best Ðt according to a Gaussian distribution.
hydrogen-bonded associates, with di†usional quenching being
negligible.3,9 For the various proportions of the Ru-G/C-Os-C
Fig. 3 Luminescence decays, observed at 626 nm, for a CH Cl solu-
2
2
mixture (Fig. 1), the Ru-based luminescence intensity, I
,
tion of Ru-G (9.2 ] 10~5 M) and C-Os-C (4 ] 10~5 M), before (a)
and after (b) EtOH addition. Excitation was at 337 nm, and the time
axis was 0.511 ns channel~1; the full line is the result of the Ðt.
Ru
does not lie on the straight line corresponding to the molar
fraction of Ru-G; instead a bowed, downward trend is regis-
tered. A parallel observation is that the time-resolved lumines-
cence, as detected at 626 nm, exhibits a dual behaviour [Fig.
3(a)]; for example, with a Ru-G molar fraction of 0.7, q and
1
q were 12.5 and 422 ns, respectively, with relative amplitudes
2
In the inset of Fig. 1, *I (\IEtOH [ I ) is plotted against
Ru
Ru
Ru
of 1.3 and 98.7%. Addition of a few drops of EtOH to the
solution in each case caused an increase of the Ru-based lumi-
the Ru-G/(Ru-G + C-Os-C) molar ratio; as mentioned above,
I
and IEtOH are luminescence intensities registered in the
Ru
Ru
nescence (Fig. 1), and the luminescence intensity (IEtOH)
absence and the presence of EtOH, respectively. Thus, the
inset of Fig. 1 corresponds to a Job plot and it can be seen
Ru
became close to what was expected based on the molar ratios
for Ru-G. In agreement with this, after EtOH addition, a
that *I peaks at a mol fraction of Ru-G of ca. 0.5. This
Ru
single exponential decay is detected [Fig. 3(b)] with q \ 390
indicates that Ru-G and C-Os-C associate to form a 1 : 1
ns (j \ 626 nm).
complex (eqn. 1), while no clear-cut evidence for a 2 : 1 associ-
ate is available from the luminescence behaviour. From the
em
The rationale for these observations is based on formation,
in neat CH Cl , of an associate between Ru-G and C-Os-C
luminescence data of Fig. 1, an association constant of K
1.1 ] 104 M~1 is obtained by following the method pre-
viously described.3
\
2
2
ass
(eqn. 1 and 2). Within the associate, the Ru-based lumines-
cence is nearly completely quenched, as indicated by the fact
that the lifetime q is only 12.5 ns, as opposed to 450 ns for
Regarding Fig. 2, it can be seen that the luminescence of
Ru-G is not much a†ected by the presence of Fc-C; it turns
out that addition of EtOH has, likewise, little e†ect. For the
Ru-G/Fc-C mixtures, the time-dependent properties of the
Ru-based luminescence are always describable in terms of a
single exponential decay. Possible explanations for this
outcome may be traced back either to (i) a low association
constant for this couple or (ii) a low efficiency for the quen-
ching of the Ru-based luminescence by the Fc unit within the
associate. We note that interaction between C and G proved
1
free Ru-G. On this basis the measured Ru ] Os inter-
component energy transfer rate constant is k \ 8.0 ] 107
en
s~1. Addition of EtOH breaks the associate, causing virtually
complete restoration of the Ru-based luminescence properties
(intensity and lifetime) of free Ru-G.
to result in K values of P5 ] 103 M~1 for both Ru-C/Os-G
ass
(studied earlier)3 and for the Ru-G/C-Os-C pair (this work, see
above). It is therefore more likely that Fc is poor at both
energy-transfer and electron-transfer quenching across the
hydrogen-bonded interface of the Ru-G : C-Fc associate, com-
pared to the situation where much stronger electronic coup-
ling is provided by covalent linkages between the two
centres.7
In order to better understand the nature of the photoin-
duced processes we have performed some calculations regard-
ing the energy transfer steps occurring within the mixtures
Ru-G/C-Os-C and Ru-G/Fc-C, where Ru-G always acts as an
energy donor (D) towards the accepting partners (A), either
C-Os-C or Fc-C. In general, two mechanisms are available for
discussing this process. One may be viewed as a dipoleÈdipole
e†ect, according to a treatment due to Forster,17 and the
other is based on a description in terms of double electron
exchange, dealt with by Dexter.18 The relevant energy transfer
Fig. 2 Luminescence intensities, observed at 626 nm, for the various
Ru-G/Fc-C mixtures (total concentration was 4 ] 10~5 M) in neat
CH Cl (I , =), and after EtOH addition (IEtOH, >); the dashed line
2
2
Ru
Ru
indicates the luminescence intensities expected based solely on the
Ru-G molar fractions.
New J. Chem., 2000, 24, 987È991
989

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rate constants kF and kD can be evaluated provided some
electronic mediation by the H-bonds (which is weak, as sug-
en
en
parameters are known (eqn. 3 and 4).
gested by the results of Fig. 2).20,21 In contrast, for the Ru-
G/C-Os-C couple, the calculated value of J is large (Table 2)
F
8.8 ] 10~25K2U
kF \
which happens because of the sizeable overlap between donor
É J
(3)
(4)
en
F
n4qd6
luminescence and acceptor absorption (Fig. 4) and the evalu-
MM
ated R \ 23.2 A is much larger than the estimated RuÈOs
4n2H2
0
kD \
É J
intercenter distance of ca. 13È14 A. Thus, for the couple Ru-
en
D
h
G/C-Os-C, energy transfer is likely to take place via the
Forster mechanism, i.e. without electronic mediation by H-
bonds, which would therefore fulÐl the purely structural role
of holding the Ru and Os centres close together.
Within the Forster approach (eqn. 3), K2 is a geometric
factor (taken to be 2/3 for statistical reasons), U and q are the
luminescence quantum yield and lifetime of Ru-G (Table 1),
In conclusion, with the help of luminescence techniques we
have investigated formation of associates driven by H-bonds
involving complementary C and G units appended to some
chromophore and quencher complexes. For the Ru-G/C-Os-C
case, our approach provided no evidence for a trinuclear
assembly, which had been expected to form based on the H-
bonding capability of the C-Os-C complex (Scheme 2). We
respectively, n is the refractive index of the solvent, and d is
MM
the distance between the interacting centres; the critical trans-
fer radius R (the intercomponent distance for which the
0
intrinsic deactivation of the donor is equal to kF ) can also be
en
evaluated (Table 2). For evaluation of the Dexter rate con-
stant (eqn. 4), an electronic interaction (represented by H, not
available here) between D and A is provided by interposed
molecular fragments.7 J and J are overlap integrals (Table
note that the derived K \ 1.1 ] 104 M~1 for the 1 : 1
ass
F
D
association is about double the value previously found for the
2) between the luminescence spectrum of the donor, F(l), and
the absorption spectrum of the acceptor, e(l), on an energy
scale (cm~1). Fig. 4 shows luminescence and absorption
spectra for the couples Ru-G/C-Os-C and Ru-G/Fc-C.
related Ru-C/Os-G couple, K \ 5 ] 103 M~1.3 A simple
ass
explanation for this outcome may be based on the fact the the
“concentrationÏ of cytosine units for C-Os-C is e†ectively
double that provided by an equimolar solution of Os-C.
Use of the evaluated parameters in Table 2 provides inter-
esting hints as to the type of energy transfer mechanism,
Forster or Dexter, involved. For the couple Ru-G/Fc-C, the
Experimental
Forster treatment yields such a low value for J (Table 2 and
F
Fig. 4) that the critical transfer radius is very small, R \ 6.6
o
Syntheses
A, compared to an RuÈFc intercentre distance of at least
Bipy-C . To a solution of cytosine (111 mg, 1 mmol) in dry
13 A (estimated from molecular modelling).19 Thus, either a
Dexter mechanism provides for any Ru ] Fc energy transfer,
or Ru ^ Fc electron transfer occurs,16 both of which imply
2
dmf (10 cm3) at 0 ¡C under N was added NaH (1 mmol) and
2
the mixture was stirred for 10 min. Then KI (30 mg) was
added followed by addition of a suspension of 5,5@-bis(bromo-
methyl)-2,2@-bipyridine (170 mg, 0.5 mmol) in dry dmf (10
cm3). The mixture was stirred for 5 h at 50 ¡C under N . After
2
cooling to room temperature and slow addition of water (50
cm3), bipy-C precipitated as a pale brown solid which was
2
Ðltered o†, washed with MeOH, and dried. Yield: 28%. EI-
MS: m/z 402 [M`]. The ligand is not sufficiently soluble for
NMR spectroscopy, even in dmso.
Bipyº. 4,4@-Bis(5-nonyl)-2,2@-bipyridine was prepared (60%
yield) by reaction of 4-[(1-butyl)pentyl]pyridine with Raney
nickel at 150 ¡C for 2 days, followed by chromatographic puri-
Ðcation (Al O , CH Cl ), according to the method described
2
3
2 2
for conversion of 4-tert-butylpyridine to 4,4@-bis(tert-butyl)-
2,2@-bipyridine.22
[Os(bipyº) Cl ]. [Os(bipy@) Cl ] was prepared (58% yield)
2
2
2 2
by reaction of bipy@ with K OsCl (1 : 1 molar ratio) in dmf
Fig. 4 Luminescence spectrum of Ru-G and absorption spectra of
C-Os-C and Fc-C on an energy scale.
2
6
under N at reÑux for 24 h, according to the method of ref. 23.
2
Table 2 Energy transfer parametersa
Forster
Dexter
J b/cm6 mol~1
R c/A
dd/A
J e/cm
F
0
D
D/A Couple
Ru-G/C-Os-C
Ru-G/Fc-C
2.7 ] 10~14
1.5 ] 10~17
23.2
6.6
12.7
\4
1.8 ] 10~4
1.1 ] 10~4
P
F(l)e(l)/l4 dl
a Room temperature, CH Cl solvent, measured rate constant k \ 8.0 ] 107 s~1. b Forster integral, J \
en
cm6 mol~1. c Critical
2
2
F
P
F(l) dl
transfer radius, R \ 9.79 ] 10~3 (K2n~4U
J )1@6 A. d Intercomponent distance (metal to metal) for which the calculated rate constant,
0
RuhG
f
P
F(l)e(l) dl
kF \ k . e Dexter integral, J
\
cm.
en
en
D
P P
F(l) dl e(l) dl
990
New J. Chem., 2000, 24, 987È991

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3
4
5
N. Armaroli, F. Barigelletti, G. Calogero, L. Flamigni, C. M.
White and M. D. Ward, Chem. Commun., 1997, 2181.
N. C. Fletcher, M. D. Ward, S. Encinas, N. Armaroli, L. Flamigni
and F. Barigelletti, Chem. Commun., 1999, 2089.
S. Encinas, K. L. Bushell, S. M. Couchman, J. C. Je†ery, M. D.
Ward, L. Flamigni and F. Barigelletti, J. Chem. Soc., Dalton
T rans., 2000, 1783.
(a) B. Schlicke, P. Belser, L. De Cola, E. Sabbioni and V. Balzani,
J. Am. Chem. Soc., 1999, 121, 4207; (b) F. Barigelletti, L. Fla-
migni, M. Guardigli, A. Juris, M. Beley, S. Chodorowski-
Kimmes, J.-P. Collin and J.-P. Sauvage, Inorg. Chem., 1996, 35,
136.
(a) R. Ziessel, M. Hissler, A. El-ghayoury and A. Harriman,
Coord. Chem. Rev., 1998, 177, 1251; (b) L. De Cola and P. Belser,
Coord. Chem. Rev., 1998, 177, 301; (c) P. T. Gulyas, T. A. Smith
and M. N. Paddon-Row, J. Chem. Soc., Dalton T rans., 1999,
1325; (d) F. Barigelletti and L. Flamigni, Chem. Soc. Rev., 2000,
29, 1.
(a) V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni and M.
Venturi, Acc. Chem. Res., 1998, 31, 26; (b) E. C. Constable, Chem.
Commun., 1997, 1073.
(a) C. M. White, M. F. Gonzalez, D. A. Bardwell, L. H. Rees, J. C.
Je†ery, M. D. Ward, N. Armaroli, G. Calogero and F. Barigel-
letti, J. Chem. Soc., Dalton T rans., 1997, 727; (b) M. D. Ward,
C. M. White, F. Barigelletti, N. Armaroli, G. Calogero and L.
Flamigni, Coord. Chem. Rev., 1998, 171, 481.
Os-C . [Os(bipy@) (bipy-C )][PF ] was prepared by reac-
6 2
2
2
2
tion of [Os(bipy@) Cl ] with bipy-C (1 : 1 molar ratio) in
2
2
2
ethylene glycol at reÑux using the same general method as
described previously.9 Yield: 20%. ES-MS: m/z 1701
[M ] H]`, 1554 [M [ PF ]`, 705 [M [ 2PF ]2`, 470 [M
6
6
[ 2PF ] H]3`. 1H NMR (300 MHz, CD CN): d 0.80 (24
6
3
6
H, m, CH of alkyl substituents on bipy@), 1.25 (32 H, m,
CH CH of alkyl substituents), 1.68 (16 H, m, CH of alkyl
substituents), 2.78 (4 H, m, CH of alkyl substituents), 4.65 (4 H,
3
2
2
2
s, CH of bipy-C ), 5.72 (2 H, d, J 7.5, cytosine CH2CH), 6.11
2
2
(4 H, br s, cytosine NH ), 7.08 (2 H, dd, J 1.6, 6.1, bipy), 7.15
7
2
(2 H, dd, J 1.6, 6.1, bipy), 7.37 (2 H, d, J 7.5, cytosine CH2CH),
7.43 (4 H, m, bipy), 7.57 (2 H, br s, bipy), 7.64 (2 H, d, J 8.4 Hz,
bipy), 8.31 (6 H, m, bipy). Found: C, 50.1; H, 5.8; N, 8.7%.
Required for [Os(bipy@) (bipy-C )][PF ] É HPF : C, 49.5; H,
2
2
6 2
6
5.8; N 9.1%.
8
9
Ru-G. [Ru(tBu bipy) (bipy-G)][PF ] was prepared by
6 2
2
2
reaction of [Ru(tBu bipy) Cl ] and bipy-G (1 : 1 molar ratio)
2
2 2
in ethylene glycol at reÑux using the same general method as
described previously.9 Yield: 30%. ES-MS: m/z 1102 [M
[ PF ]`, 478 [M [ 2PF ]2`. 1H NMR [300 MHz,
6
6
10 (a) M. D. Ward, Chem. Soc. Rev., 1997, 26, 365; (b) J. L. Sessler, B.
Wang and A. Harriman, J. Am. Chem. Soc., 1995, 117, 704; (c) P.
Tecilla, R. P. Dixon, G. Slobodkin, D. S. Alavi, D. H. Waldeck
and A. D. Hamilton, J. Am. Chem. Soc., 1990, 112, 9408; (d) M.
Asano-Someda, H. Levanon, J. L. Sessler and R. Z. Wang, Mol.
Phys., 1998, 95, 935.
11 A. Houlton, C. J. Isaac, A. E. Gibson, B. R. Horrocks, W. Clegg
amd and M. R. J. Elsegood, J. Chem. Soc., Dalton T rans., 1999,
3229.
12 For the present and past (see ref. 3 and 9) investigations per-
formed by us on this topic, photoinduced electron transfer
involving the pendant nucleobases was not observed. Indeed, for
the easiest oxidizable base (guanine) the G`/G reduction poten-
tial is ]1.49 V vs. NHE [MeCN solvent (ref. 13) corresponding
(CD ) CO]: d 1.39 (36 H, m, tBu), 5.33 (2 H, s, CH of
3 2
2
bipy-G), 6.16 (2 H, br s, guanine NH ), 7.36 (1 H, s, guanine
2
CH), 7.44 (1 H, m, bipy), 7.55 (4 H, m, bipy), 7.66 (2 H, m,
bipy), 7.79 (3 H, m, bipy), 7.95 (1 H, m, bipy), 8.21 (2 H, m,
bipy), 8.83 (6 H, m, bipy), 10.32 (1 H, br s, guanine NH).
Found: C, 48.2; H, 4.6; N, 11.6%. Required for
[Ru(tBu bipy) (bipy-G)][PF ] É 2H O: C, 48.6; H, 5.1; N,
2
2
6 2
2
12.0%.
NMR and mass spectra, ground state absorption spectra,
steady state luminescence data and time resolved and photo-
physical results were obtained as described in ref. 5. Calcu-
lations for the energy transfer steps were performed with the
help of Matlab 5.2 (MatWorks).24 Molecular modelling calcu-
lations were performed with the program CS Chem-3D
(version 5.0).25
to ]1.25 vs. SCE]. Thus, for electron-transfer from
G to
[*Ru(bipy) ]2`, *G is ca. ]0.4 eV because the potential for the
3
couple [*Ru(bipy) ]2`/[Ru(bipy) ]` is ]0.84 V (ref. 14). Pho-
3
3
toinduced electron transfer between RuII- and OsII-polypyridine
centres can also be disregarded because of the lack of favourable
driving force (ref. 15).
13 C. A. M. Seidel, A. Schulz and M. H. M. Sauer, J. Phys. Chem.,
Acknowledgements
1996, 100, 5541.
14 A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and
A. Von Zelewsky, Coord. Chem. Rev., 1988, 84, 85.
15 L. De Cola, V. Balzani, F. Barigelletti, L. Flamigni, P. Belser, A.
von Zelewsky, M. Frank and F. Vogtle, Inorg. Chem., 1993, 32,
5228.
16 E. J. Lee and M. S. Wrighton, J. Am. Chem. Soc., 1991, 113, 8262.
17 Th. Forster, Discuss. Faraday Soc., 1959, 27, 7.
18 D. L. Dexter, J. Chem. Phys., 1953, 21, 836.
19 Due to possible rearrangements about single bonds, the Ru É É É Fe
distance spans a range of ca. 13 to 20 A.
S. E. thanks the EU for a post-doctoral TMR fellowship
(contract number 980226). This research was also supported
by the European Commission COST Programme D11/0004/
98 “New Aspects of Supramolecular Photochemistry: from
Light-harvesting Arrays to Molecular MachinesÏ, and by the
EPSRC (UK).
20 For cases where many H-bonds are involved, the electronic medi-
ation may be rather e†ective and allow electron transfer between
MII and MIII centers (M \ Ru, Os), see ref. 21.
References and notes
1
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24 Matlab 5.2, The Mathworks, Inc., Nabick, MA, 1999.
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2
R. L. Cleary, K. J. Byrom, D. A. Bardwell, J. C. Je†ery, M. D.
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991