Towards molecular T-junction relays

Article abstract of DOI:10.1016/j.tetlet.2003.09.078

The synthesis of a ditopic 2,2′:6′,2″-terpyridine ligand and bis-ruthenium(II) complex is described which incorporates in its molecular backbone a light-activated spiropyran moiety.

Full text of DOI:10.1016/j.tetlet.2003.09.078

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 8245–8247
Towards molecular T-junction relays
Ata Amini, Katie Bates, Andrew C. Benniston,* Donald J. Lawrie and Estelle Soubeyrand-Lenoir
Molecular Photonics Laboratory, School of Natural Sciences (Chemistry), University of Newcastle, Newcastle upon Tyne,
NE1 7RU, UK
Received 7 July 2003; revised 2 September 2003; accepted 11 September 2003
Abstract—The synthesis of a ditopic 2,2%:6%,2%%-terpyridine ligand and bis-ruthenium(II) complex is described which incorporates in
its molecular backbone a light-activated spiropyran moiety.
© 2003 Elsevier Ltd. All rights reserved.
Effective methods of controlling rate and directionality
of electron migration along molecular-scale wires will
need to be addressed if the technology these systems
promise is to be realized.1 It is recognized that efficient
and fast electron migration could be maintained in an
organic wire if there is proficient orbital overlap
between connecting subunits.2 Mastering this challenge
will hopefully see the emergence in the near future of
molecular-scale assemblies capable of manipulating
3
,
information over distances of 100–200 A. Real wires,
however, contain junctions along their path which
when selectively activated alter the direction of electron
flow. To perform such a task at the molecular level
requires the design and synthesis of sophisticated
assemblies. So far we have observed the emergence of
‘molecular switches’ capable of performing on/off
tasks,4 but the added sophistication of coupling this to
electron directionality is yet to be fully realized. Hence,
we have taken on the challenge of creating molecular-
scale assemblies capable of directing electron migration
along disparate pathways. To ensure fast gating a pho-
ton initiated process has been deemed crucial if we are
to achieve this goal. The basic design idea is conceptu-
ally illustrated in Figure 1, and the term T-junction
relay is coined to describe the mode of action. Electron
flow via D to A1 is the dominant photoinitiated process
which is switched off upon activation of the T-junction
arm via a second photon controlled event. Charge
migration then proceeds via the alternative pathway of
D to A2. In designing such a T-junction assembly we
have identified linear ditopic 2,2%:6%,2%%-terpyridine
complexes5 as critical, which incorporate in the bridge a
light-activated spiropyran moiety. Spiropyrans have
Figure 1. Pictorial representation of a T-junction relay.
found wide applications in switching devices since they
readily undergo ring opening into their coloured mero-
cyanine-based form which display disparate photophys-
ical properties.6,7
The strategy selected for functionalization of the
spiropyran is outlined in Scheme 1 for the particular
case of equipping the system with terminal ruthenium(II)
bis(2,2%:6%,2%%-terpyridine) complexes. It should be noted
that prior work has shown that such metal complexes
are photoactive only if substituted with an electron-
withdrawing group like an alkyne.8 It is known that the
extent of electron delocalization along the molecular
axis at the triplet level in such photoactive dyads is
controlled by the degree of p-electron conjugation over
the bridging polytopic ligand. It is our conjecture that
the extent of electron delocalization and direction will
be controlled, at least to some degree, by the open or
closed state of the spiropyran group.
The ditopic ligand L1 was prepared in six steps starting
from the readily prepared 2,5-dibromobenzene-1,4-diol
1.9 Although it has been reported that 1 is selectively
converted to the mono-aldehdye10 using Duff’s reac-
* Corresponding author. Tel.: +44 (0) 191 222 5706; fax: +44 (0) 191
222 6929; e-mail: a.c.benniston@ncl.ac.uk
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.078

8246
A. Amini et al. / Tetrahedron Letters 44 (2003) 8245–8247
Irradiation of an acetonitrile N2–purged solution of L1
at room temperature with UV-visible light (l > 350 nm)
resulted in the appearance of a blue solution that
rapidly converted back to its original yellow colour.
The colour changes are fully consistent with ring open-
ing to generate the merocyanine form, and thermal
reformation of the spiropyran. Unfortunately, the rapid
colour change prohibited collection of the room tem-
perature UV-visible spectrum of the open form. How-
ever, at lower temperatures the colouration of the
solution lasted much longer. Illustrated in Figure 2 is
the UV-visible spectrum of L1 recorded at 288 K in
acetonitrile before and after irradiation.16 At this tem-
perature the ring closure reaction is slow enough to
permit collection of the merocyanine form spectrum.
Reformation of the spiropyran closed form was moni-
tored by decay of the signal at u=665 nm over time,
and least-squares fitted to a single exponential decay to
afford a rate constant k of 9×10−3 s−1. Rate constants
were also collected over a modest temperature range
(263–293 K) to afford an activation energy Ea=91 kJ
mol−1 and a pre-exponential factor A=3.2×1014. These
results are particularly encouraging since the switching
action of the ligand is fast and reversible.
Detailed molecular orbital calculations were performed
on L1 in the open and closed forms to ascertain the
relative energies and sites of the HOMO and LUMO
orbitals.17 The molecular orbital diagrams illustrated in
Figure 3 clearly show that there is significant perturba-
tion of the molecular orbitals. In the closed form (Fig.
3A) the HOMO/LUMO orbitals clearly reside on the
linear portion of the bridge unit and span across the
two 2,2%:6%,2%%-terpyridyl ligands. In contrast the
HOMO/LUMO orbitals of the open form (Fig. 3B) are
more localized on the merocyanine portion of the lig-
and. Thus, we can speculate that for the complex
Ru(L1)Ru the MLCT state, which is known to involve
the LUMO of the ligand, will span across both termi-
nals in the closed form, but delocalise onto the T-junc-
tion relay in the open form. Current detailed photo-
physical studies are underway on complex Ru(L1)Ru in
the two different states to ascertain if this is the case,
and will be reported elsewhere at a later date.
Scheme 1. Synthetic methods used in the preparation of L1
and Ru(L1)Ru.
tion, in our hands the yields were extremely low. Since
we required for solubility reasons a lipophilic alkyl
group attached to L1, it was decided to carry this out
before introducing the aldehyde functional group.
Hence, compound 1 was firstly monoprotected using
benzyl bromide to afford 2 in a modest, yet unopti-
mised, 59% yield. Reaction of 2 with NaH in DMF and
cetyl bromide produced 3 in 80% yield. Deprotection of
the benzyl ether was carried out by the H-atom transfer
method using cyclohexene/Pd(OH)2 to afford 4 in
almost quantitative yield. Reaction of 4 using standard
Duff chemical conditions proceeded smoothly (60%
yield), and introduced the carbonyl functional group
into the aromatic ring with the required regioselectivity.
Spiropyran 7 was prepared by reaction of 5 with 6 in
good yield (60%) despite the existence of the bulky
groups. Coupling of
7 to 4%-ethenyl- [2,2%:6%,2%%]-
terpyridine 811 was carried out using Sonogashira cross-
coupling conditions12 to yield after careful column
chromatography (silica gel, petrol ether:ethyl acetate
(17:3)) ditopic ligand L1 in 13% yield.13 It was noticed
that the reaction did not go to completion and that
starting materials were recovered from the reaction
mixture. We can only surmise that the catalyst gets
poisoned during the coupling reaction, and so this
reaction will require further study. Reaction of L1 with
14
[Ru(terpy)(CH3CN)3](PF6)2
produced the complex
Ru(L1)Ru which was purified by column chromatogra-
Figure 2. UV-visible spectrum of L1 (conc.=0.097 mmol) in
acetonitrile before (red) and after (blue) irradiation.
phy (basic alumina: CH3CN/0.1M KNO3(19:1)).15

A. Amini et al. / Tetrahedron Letters 44 (2003) 8245–8247
8247
12. Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron
Lett. 1975, 50, 4467.
1
13. Analytical data L1: H NMR (500 MHz, CDCl3) l=0.79
(m, 3H, CH3), 0.97–1.37 (m, 26H, C13H26), 1.51 (s, 6H,
2×CH3gem), 1.88 (m, 2H, CH2alkyl), 2.72 (s, 3H, N-CH3),
3.99 (t, 2H, J=6.4 Hz, CH2alkyl), 5.89 (d, 1H, J=10.35
Hz, CHdb), 6.50 (d, 1H, J=8.0 Hz, CHph,), 6.60 (t, 1H,
J=7.5 Hz, CHph), 6.80 (s, 1H, CHph), 6.97 (t, 1H, J=7.6
Hz, CHph), 7.04 (d, 1H, J=7.3 Hz, CHph,), 7.31 (m, 4H,
4 x CHterpy), 7.40 (d, 1H, J=10.40 Hz, CHph), 7.81 (m,
4H, 4×CHterpy), 8.04 (s, 2H, 2×CHterpy), 8.46 (d, 2H,
J=8.0 Hz, 2×CHterpy), 8.56 (s, 2H, 2×CHterpy), 8.58 (d,
2H, J=8.4 Hz, 2×CHterpy), 8.68 (d, 2H, J=4.3 Hz,
2×CHterpy), 8.70 (d, 2H, J=4.4 Hz, 2×CHterpy). 13C NMR
(125.65 MHz, CDCl3) l=14.11, 20.44, 22.68, 25.14,
26.20, 29.27, 29.35, 29.48, 29.67 (8C), 31.92 (2C), 51.82
(2C), 69.76 (2C), 87.67, 89.07, 93.37, 97.42, 105.14,
107.04, 110.47, 115.57, 119.30, 121.27 (2C), 121.31 (2C),
122.45, 122.77 (4C), 123.84 (2C), 124.01 (2C), 127.37,
133.31, 133.43, 136.75 (2C), 136.89 (2C), 148.17, 149.10
(2C), 149.20 (2C), 149.73, 153.40, 155.52, 155.61, 155.74,
155.93. (Note: 7 quaternary carbons are coincidental with
other aromatic resonances). HR-MS calculated for
C69H69N7O2: 1027.5513. Found: 1027.5450.
Figure 3. Representation of the HOMO/LUMO molecular
orbitals of a methyl chain modified version of L1 in the closed
form (A) and open form (B).
14. Suen, H. F.; Wilson, S. W.; Pomerantz, M.; Walsh, J. L.
Inorg. Chem. 1989, 28, 786.
15. Analytical data Ru(L1)Ru: 1H NMR (500 MHz,
CD3CN): l=0.54 (t, 3H, J=7.2 Hz, CH3alkyl), 0.64–0.88
(m, 26H, CH13H26), 0.89 (s, 3H, CH3gem), 1.02 (s, 3H,
CH3gem), 1.10 (m, 2H, CH2alkyl), 3.03 (s, 3H, N-CH3),
3.81 (t, 2H, J=6.1 Hz, CH2alkyl), 5.80 (d, 1H, J=10.4 Hz,
CHdb), 6.30 (d, 1H, J=7.6 Hz, CHph), 6.53 (td, 1H,
J=7.2 Hz, J%=0.9 Hz, CHph), 6.74 (td, 1H, J=6.5 Hz,
J%=1.3 Hz, CHph), 6.85 (m, 8H, 6×CHterpy, 2×CHph),
7.04 (d, 2H, J=4.8 Hz, 2×CHterpy), 7.07 (d, 2H, J=
5.2Hz, 2×CHterpy), 7.31 (s, 2H, 2×CHterpy), 7.35 (m, 5H, 4
x CHterpy, CHdb), 7.40 (d, 2H, J=4.0 Hz, 2×CHterpy),
7.41 (d, 2H, J=3.7 Hz, 2×CHterpy), 7.43 (d, 2H, J=3.6
Hz, 2×CHterpy), 7.61 (m, 8H, 8 x CHterpy), 8.11 (t, 2H,
J=8.2 Hz, 2×CHterpy), 8.17 (d, 2H, J=7.7 Hz, 2×
CHterpy), 8.21 (d, 2H, J=7.9 Hz, 2×CHterpy), 8.26 (d, 2H,
J=8.3 Hz, 2×CHterpy), 8.44 (d, 2H, J=8.2 Hz, 2×
CHterpy), 8.52 (s, 2H, 2×CHterpy). ESI-MS results showed
multiple fragment ions from breakdown of the cetyl
chain. Calcd. for C99H90N13O2Ru2P4F24 m/z=339.17 find
Acknowledgements
This work was supported by the Engineering and Phys-
ical Sciences Research Council and the Nuffield Foun-
dation. We thank the EPSRC Mass Spectrometry
Facility at Swansea, Mr Alan Rolfe for preliminary
work and Professor A. Harriman for many helpful
discussions.
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