Very large acceleration of the photoinduced electron transfer in a Ru(bpy)3-naphthalene bisimide dyad bridged on the naphthyl core

Article abstract of DOI:10.1039/b615085c

By linking a naphthalenebisimide (NBI) unit to [Ru(bpy)₃] ²⁺ on the naphthyl core the rate of photoinduced Ru-to-NBI electron transfer was 1000-fold increased compared to the case with a conventional linking on the nitrogen. The Royal Society of Chemistry.

Full text of DOI:10.1039/b615085c

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Very large acceleration of the photoinduced electron transfer in a
Ru(bpy)3–naphthalene bisimide dyad bridged on the naphthyl core{
Fre´de´rique Chaignon,a Magnus Falkenstro¨m,b Susanne Karlsson,b Errol Blart,a Fabrice Odobel*a and
Leif Hammarstro¨m*b
Received (in Cambridge, UK) 17th October 2006, Accepted 8th November 2006
First published as an Advance Article on the web 22nd November 2006
DOI: 10.1039/b615085c
By linking a naphthalenebisimide (NBI) unit to [Ru(bpy)3]2+ on
the naphthyl core the rate of photoinduced Ru-to-NBI electron
transfer was 1000-fold increased compared to the case with a
conventional linking on the nitrogen.
There is considerable interest in the development of molecular
arrays for long range electron transfer in view of applications in
molecular electronics and solar energy conversion.1,2 In this
context, electronic communication via the bridging unit is essential
for achieving fast and efficient electron transport over a long
distance. Naphthalenebisimide (NBI) has many favorable proper-
Chart 1 Structures of the complexes.
ties as electron acceptor in such arrays, with a suitable reduction
potential (20.5 V vs. SCE) and a characteristic radical anion
absorption spectrum.3 Moreover, a relatively high-lying triplet
preparation of 1 calls for the key intermediate 2,6-dibromo-
naphthalene tetracarboxylic dianhydride 11. The preparation of
the dichloro analogue of 11 was published, but requires many steps
and the utilization of noxious reagents such as chlorine gas.9,10 We
found that the bromination of naphthalene tetracarboxylic
dianhydride 9 could be performed with dibromoisocyanuric acid
10 in hot sulfuric acid solution in a quantitative yield. The poorly
state (2.03 eV) makes triplet–triplet energy transfer from an
appended chromophore less prone to compete with electron
transfer. The latter is a particular problem for arrays based on
transition metal chromophores, as has been noted in e.g.
RuII(polypyridine)–fullerene arrays, where the fullerene triplet lies
at only 1.25 eV.4,5 The utilization of NBI acceptors in molecular
arrays has been reported previously by several groups,1,3,6,7 but the
NBI was exclusively attached to the neighboring unit through the
nitrogen atom of the bisimide group. We noted that the electron
transfer rate has often been surprisingly slow. In e.g. a series of
RuII(polypyridine)–NBI dyads7 the rate was up to three orders of
magnitude slower than for corresponding dyads based on
methylviologen as acceptor.8 Most of this difference can probably
be attributed to a weaker electronic coupling. Indeed, the NBI
frontier orbitals show a nodal plane through both the nitrogen
atoms, while the main density is on the 2,3,6,7 positions of the
naphthyl core.9 To increase the electronic coupling, we report
herein a new type of connectivity in dyad 1 (Chart 1) by linking on
the naphthyl core of NBI, which has a remarkable effect on the
electron transfer rate.
The preparation of dyads 1 and 2 is depicted in Scheme 1.
Although the synthesis of dyad 2 is quite straightforward
from starting materials already published in the literature, the
aLaboratoire de Synthe`se Organique, UMR 6513 CNRS & FR CNRS
2465, Universite´ de Nantes, Faculte´ des Sciences et des Techniques de
Nantes, BP 92208; 2, rue de la Houssinie`re, 44322, Nantes Cedex 03,
France
Scheme 1 Synthetic route to prepare the dyads 1 and 2. Reagents and
conditions: (i) DMF, reflux, 48%; (ii) PdCl2, Cu(OAc)2, P(Ph)3, Et3N,
THF, 70 uC, 15 h, 90%; (iii) Bu4NF, THF, R.T., 1 h, 100%; (iv) H2SO4,
130 uC, 15 h, 100%; (v) octylamine, acetic acid, 130 uC, 1 h, 28%; (vi)
Pd(PPh3)4, toluene, 120 uC, 15 h, 76%; (vii) Bu4NF, THF, R.T., 1 h, 100%;
(viii) Pd(dppf)Cl2, CuI, Et3N, DMF, 45–70 uC, 15 h, 40% for 1 and 47%
for 2.
bChemical Physics, Department of Photochemistry and Molecular
Science, Uppsala University, Box 523, SE-751 20 Uppsala, Sweden.
E-mail: Leif@fotomol.uu.se
{ Electronic supplementary information (ESI) available: Full synthetic
procedures for the preparation of new compounds, UV-Vis absorption
spectra of 1–2 and transient absorption spectra for 2. See DOI: 10.1039/
b615085c
64 | Chem. Commun., 2007, 64–66
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soluble 2,6-dibromonaphthalene-1,4:5,8-tetracarboxylic dianhy-
dride 11 was then reacted with octylamine in acetic acid and
the expected soluble N,N9-dioctyl-2,6-dibromonaphthalene-1,4,5,8-
tetracarboxylic acid bisimide 12 was subjected to a Stille cross-
coupling reaction with an excess of 4-trimethylsilylethynyl
trimethyltin benzene 13 in the presence of a catalytic amount of
Pd(PPh3)4 to give the bis coupled compound 14 in 76% yield.
The last step of the synthesis of 1, 2 and 3 consists of a
Sonogashira cross-coupling reaction which was directly performed
on the ruthenium trisbipyridine complex 16 with the NBI
derivatives 8 or 15. The compounds 1, 2 and 3 were thus
respectively obtained with 47%, 40% and 70% yield. All the new
1
compounds gave satisfactory H NMR and mass analyses.
Fig. 1 (left): Transient absorption spectra for 1 at 1 ps, 25 ps, 50 ps,
100 ps and 250 ps after excitation with a y 100 fs laser pulse at 450 nm.
The inset shows the kinetic traces recorded at 475 nm (squares), 500 nm
(triangles; signal enlarged) and 605 nm (circles). The solid lines are
biexponential fits with time constants of 14 ps and 52 ps. (right): Transient
absorption spectrum after 25 ps when the contribution from the remaining
excited state has been subtracted (see text); (CH3CN, 298 K).
The UV-Vis absorption spectra of the dyads 1 and 2 were simple
sums of the spectra of their components: the reference ruthenium
complex 3 and the NBI unit (see Supporting Information). The
lowest energy transition is the MLCT band of the Ru-complex
around 450 nm, while the NBI unit absorption is seen around
400 nm, allowing for selective Ru-excitation. The reduction
potential of the [Ru(bpy)3]3+/2+-couple, measured by cyclic
voltammetry (in DMF with Bu4NPF6), was 1.21 V and 1.28 V
vs. SCE, for 1 and 2 respectively, and for the NBI0/2 couple it was
20.48 V and 20.56 V, respectively.
the back reaction explains the small transient signals for the
charge-separated state. Nevertheless, the absorption peak around
605 nm from NBI 2 can be discerned in the spectra at 25–100 ps.
?
The 3MLCT emission of 3 in 298 K, deaerated acetonitrile
showed a maximum at 666 nm and a lifetime of 1.5 ms. The
emission intensity of the dyads was instead only 4 and 8% in 1 and
2, relative to 3. The emission lifetime for 2 was reduced to 63 ns,
and in 1 it decreased dramatically to only 30 ps. Thus, the
quenching rate increases by three orders of magnitude when the
phenylacetylene spacer is attached to NBI through the naphthyl
core compared to when it is attached through the nitrogen.
The appended NBI unit could in principle quench the 3MLCT
state through both triplet energy transfer and electron transfer.
Energy transfer would be isoergonic, with E00 = 2.04 eV4 for the
3MLCT state and E00 = 2.03 eV5 for 3NBI. The 3NBI lifetime is ca.
20 ms.11 Thus, if energy transfer was the main reaction one would
expect an excited state equilibration and a more long-lived
‘‘delayed’’ Ru-emission, as opposed to the observed lifetime
quenching. Instead, electron transfer to the NBI is the
probable quenching mechanism, with a moderately high driving
force: DG0 = 20.35 and 20.20 eV for 1 and 2, respectively.
For 2 the transient absorption data only showed the features of
the 3MLCT decay, with the same time constant (63 ns) as obtained
in the emission experiments (Supporting Information). This
suggests that the electron transfer products recombine very rapidly
A difference spectrum at 25 ps, in which the contribution of
excited states remaining was subtracted, showed very clearly the
features of NBI 2 (Fig. 1, right panel): absorption peaks at 474 nm
?
(e = 2.3 6 104 M21 cm21) and 605 nm (e = 6.4 6 103 M21
cm21),12 while the broad RuIII–RuII bleach around 480 nm (De #
1 6 104 M21 cm21) only reduced the net signal in this region.
A long-lived transient with the same spectral shape as the
3
initial MLCT state remained after . 1 ns and was attributed
to the product of
measurement.
a partial photo-decomposition during
The three orders of magnitude increase in electron transfer for 1
compared to 2 shows that our design was successful. The distance
between the components is nearly identical. The 0.15 eV extra
driving force for electron transfer in 1 would, according to Marcus
theory, only increase the rate by a factor of ca. 20.13 The main
effect, accounting for the remaining two orders of magnitude
increase in rate, must be attributed to the much better electronic
coupling obtained by linking the spacer onto the naphthyl core. A
similar enhancement of triplet energy transfer by linking a
peryleneimide acceptor to the aromatic core instead of on the
nitrogen has been reported.14
The quick and efficient preparation of 2,6-dibromonaphthalene-
1,4,5,8-tetracarboxylic acid bisanhydride we report for the first
time allows for the functionalization of NBI directly on the
naphthyl core. This gives a much better electronic coupling as
shown by the very large increase in rate of photoinduced electron
transfer in dyad 1. This approach can be used to design new
molecular systems using NBI as electron acceptor for rapid, long
distance electron transfer.
2
?
to the ground state reactants. In 1, however, the NBI radical
could be traced by its absorption around 500 nm, although this is
very close to the MLCT bleaching maximum, and its contribution
around 605 nm (Fig. 1). The initial transient absorption (1 ps) is
3
identical to that for the reference 3, and shows the MLCT state,
with a bleaching maximum at 480 nm. Subsequently the isosbestic
point around 500 nm blueshifts during the first tens of
picoseconds, and the trace at 500 nm shows a clear rise-and-decay
behavior (see inset), which is not seen in 3. This can be attributed
We thank Prof. Dan G. Nocera (MIT) for discussions. This
work was supported by the Swedish Foundation for Strategic
Research, The Swedish Research Council, the Swedish Energy
Agency, the European Commission (NEST-STRP ‘‘SOLAR-H’’,
contract no. 516510), and the French Ministry of Research for the
‘‘Action Concerte´e Incitative’’ (ACI) ‘‘Jeune Chercheur’’ 4057 and
the ANR program ‘‘PhotoCumElec’’.
to the NBI 2 absorption. A biexponential, global fit to the data at
?
four different wavelengths gave t = 52 ps for the forward electron
II
transfer (*Ru –NBI A Ru –NBI 2), in fair agreement with the
III
?
time resolved emission results, and t = 14 ps for the back reaction
III
(Ru –NBI A RuII–NBI) (Fig. 1). The short time constant for
2
?
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