Three-state photochromic switching in a silyl bridged diarylethene dimer

Article abstract of DOI:10.1039/b702417g

The synthesis and photochemical characterization of bicomponent molecular switches based on covalently tethered dithienylethene photochromes is described. Both photochromic units undergo complete switching between the fully cyclised and fully non-cyclised

Full text of DOI:10.1039/b702417g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Three-state photochromic switching in a silyl bridged diarylethene dimer†
Jetsuda Areephong, Wesley R. Browne and Ben L. Feringa*
Received 15th February 2007, Accepted 8th March 2007
First published as an Advance Article on the web 19th March 2007
DOI: 10.1039/b702417g
The synthesis and photochemical characterization of bi-
component molecular switches based on covalently tethered
dithienylethene photochromes is described. Both photo-
chromic units undergo complete switching between the fully
cyclised and fully non-cyclised states despite a significant level
of electronic communication between the individual units and
their proximity.
The photochromic properties of diarylethenes continue to receive
considerable attention,1 in particular those based on heterocyclic
aryl rings, due to their efficient switching between a coloured
(closed) and non-coloured (open) molecular state with high fatigue
resistance and excellent thermal stability over multiple cycles.2
These properties make them excellent candidates for application
in optical memory,3 photoswitching,4 molecular electronics5 and
display devices.6 Recently efforts have been directed towards the
construction of multicomponent molecular systems for multi-
mode switching based on the diarylethene photochromic unit
by connecting two or more switches covalently. These multi-
functional systems offer the possibility of increasing the number
of photochromic states available from two {i.e., open (o) and
closed (c)} to three {i.e., o–o; c–o; c–c} or more, allowing
for the possibility of achieving higher logic operations than
possible with a two state system (Fig. 1). Ultimately multicolour
materials or multifrequency optical memories might be developed.
However, to achieve operations of a higher order of logic,
the systems should behave in a supramolecular manner. That
is, both photochromic units must show a significant level of
intercomponent interaction i.e. that the first ring closing event
is ‘felt’ by the remaining open photochrome, while still retaining
the photochemical reactivity of the individual units.
Several bi- and tri-component organic photochromic based
systems have been reported, which employ a bridging single
bond,7 phenyl,8 phenylene,9 ethynylene,10 or diyne,11 as well
as a combination thereof.12 The photochemical behaviour of
these multicomponent systems has proven to be sensitive to
the nature of the bridging unit. In the case of dithienylethene
based photochromic switches, substitution of the thienyl rings
with conjugated units (e.g., phenyl, thienyl and pyridine rings,
alkenes and alkynes)13 can result in stabilisation of the LUMO
of the dithienylethene photochromic unit in the closed state. This
stabilisation has the effect of reducing the quantum efficiency of
the ring opening reaction14 and, in multicomponent systems, can
Fig. 1 Photochromic reactions of diarylethene dimer.
lead to more efficient energy transfer from an open component
to a closed component, inhibiting and even preventing full ring
closure of the multicomponent system.15
Indeed, in the majority of multicomponent dithienylethene
systems reported to date only one of the diarylethene units can
convert to the closed-ring form upon irradiation with ultraviolet
light. When the c–o form of the dimer is irradiated with UV light,
intramolecular energy transfer from the open dithienylethene unit
to the closed unit prevents the formation of the c–c state.
A number of diarylethene dimers have been prepared which
can, upon UV irradiation, convert to the c–c closed state,
using phenyl13 or fluorene16 based spacer units to separate the
photochromic units. The absence of communication between the
photochromic units in these systems is either complete and the c–
o intermediate state cannot be accessed independently of the c–c
state or the first ring closing step results in a significant perturba-
tion of the electronic structure of the second chromophoric unit.8,13
In this report we focus on examining the communication
between two proximately connected diarylethene units, which are
bridged via a single silicon atom {e.g., 2H(o–o), Fig. 2}. Of special
interest is how close the two photochromic units can be brought to-
gether to allow for intercomponent communication without a loss
of the photochromic properties of each component. A key aspect
of through bond communication between photochromic units is
the overlap integral of the orbitals of the bridging unit with the
two photochromic units. If this overlap can be reduced (as in the
case of a dimethylsilyl bridge) then through bond interaction can
Laboratory of Organic Chemistry, Stratingh Institute for Chemistry &
Zernike Institute for Advanced Materials, University of Groningen, Nijen-
borgh 4, 9747 AG, Groningen, The Netherlands. E-mail: b.l.Feringa@rug.nl;
Fax: +31 50 3634296; Tel: +31 50 3634235
† Electronic supplementary information (ESI) available: Synthesis, an-
alytical data and UV–vis spectra changes upon photolysis. See DOI:
10.1039/b702417g
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Fig. 2 Monomeric (1) and dimeric (2) and (3) photochromic switches.
be minimised resulting in through space interactions dominating
any communication between the components observed.
The diarylethene dimer 2H(o–o) was prepared as outlined in
Scheme 1. Starting with the dichloro compound 5Ho, 6Ho was
prepared by a Suzuki coupling reaction of the boronic ester17 of
5Ho with 4-iodoanisole. Lithiation of 6Ho with t-BuLi and sub-
sequent silylation18 by dichlorodimethylsilane yielded compound
2H(o–o). 1Ho, 2F(o–o), 3H(o–o) and 3F(o–o) were prepared by a
similar route (see electronic supplementary information†).
Photolysis of the monomeric switch 1Ho at 312 nm resulted
in formation of the ring closed forms of the compounds with a
photostationary state (PSS, by HPLC) of 37 : 63 (1Ho : 1Hc, see
ESI†).
Fig. 3 a) UV–vis spectral changes of 2H(o–o) upon irradiation with
312 nm light. b) Photochromic bleaching of 2H(c–c) (isolated by thin
layer chromatography) upon irradiation at >455 nm.
Photolysis of 2H(o–o) at 312 nm in hexane converts the colour-
less solution to a coloured solution, with the appearance of
new absorption bands in the visible region (Table 1, Fig. 3). The
Scheme
1
Syntheses of monomeric and dimeric dithienylethene based switches. (i) n-BuLi/THF, (ii) B(OBu)3, (iii)
4-iodoanisole/Pd(PPh3)4/Na2CO3/THF, (iv) t-BuLi/THF, (v) Me3SiCl, (vi) t-BuLi/THF, (vii) Me2SiCl2.
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Table 1 UV–vis absorption dataa
(O) kmax (e)
isolation and characterisation†). Fig. 4 shows the 1H NMR spectra
in the range of 3.6 to 8.0 ppm of the three isomers. For 2H(c–o),
the heterocyclic proton absorptions of 2H(o–o) at 6.84 and
6.96 ppm are replaced by a new set of four singlets at 6.04,
6.25, 6.87, and 7.03 ppm (Fig. 4b), two of which appear close
to the absorptions of 2H(o–o). The absorptions at 6.04 and
6.25 ppm are shifted significantly upfield as expected for the ring-
closed state.20 The asymmetric nature of this product confirms
that the singly-closed ring isomer 2H(c–o) has formed. Moreover,
Fig. 4c shows only two heterocyclic proton signals at 6.17 and
6.25 ppm confirming that both diarylethene units are in the closed
state i.e., 2H(c–c).
(C) kmax (e)
PSS312 nm
1H
2H
2F
3H
3F
275 (24.5)
274 (50.0)
256 (45.0), 294 (50.7)
233 (44.0), 270 (45.0)
256 (53.0), 282 (48.0)
296 (25.5), 500 (10.43)
300, 506 (21.8)
340 (37.7), 578 (26.5)
345 (15.6), 518 (21.9)
369 (13.0), 568 (21.0)
37 : 63b
7 : 37 : 56c
1 : 99d
8 : 36 : 56c
1 : 99d
a k in nm, e in mM−1 cm−1. In heptane solution. b Ratio by HPLC of 1Ho :
1Hc. c Ratio by HPLC of o–o : c–o : c–c isomers. d Ratio by HPLC of o–o
: (c–o + c–c) isomers. (O) = open form; (C) = closed form.
1
In contrast to the H NMR spectra of the three isomers of
2H, the UV–vis spectrum of the singly closed form 2H(c–o) is
identical to a 1 : 1 mixture of the fully open {2H(o–o)} and closed
{2H(c–c)} forms. The visible absorption spectra (i.e. >400 nm)
of both 2H(c–o) and 2H(c–c) are identical except with respect
to the molar absorptivity (e 11.0 and 21.8 mM−1 cm−1 at kmax
absorption bands are typical of the formation of the closed-
ring forms, 2H(c–o) and 2H(c–c). Irradiation with >455 nm
light leads to a complete reformation to the colourless 2H(o–o)
state. A single isosbestic point at 292 nm was maintained during
both ring closing and opening. The photostationary state (PSS)
was determined by HPLC to be a mixture of all three forms
in the ratio of 7 : 37 : 56 {2H(o–o) : 2H(c–o) : 2H(c–c)}.‡
Similar results were obtained for 2F, 3H and 3F, however, as
chromatographic separation of all three isomers was achieved only
for 2H, the remainder of the discussion will focus on this com-
pound. The lower PSS of 2Ho compared with 2Fo is as expected
by comparison with related symmetrically substituted systems
(Table 1).19
1
506 nm, respectively). Hence, although it is clear from H NMR
spectroscopy that the two dithienylethene photochromic units
show considerable through space steric interaction (as evidenced
by the difference in the chemical shift of the thienyl protons
of the open and closed units in the three isomers), UV–vis
spectroscopy shows that the extent of electronic communication
between the units is, at most, minimal.
The PSS of 2H indicates that the efficiency of photochemical
ring closing to the c–o state is good (>93%), and it is higher than
observed for the monomeric model compound 1Ho. However,
2H(c–o) and 2H(c–c) were generated photochemically and
isolated by preparative thin-layer chromatography (see ESI for
Fig. 4 1H NMR (400 MHz) spectra of a) 2H(o–o), b) 2H(c–o) and c) 2H(c–c) in CDCl3.
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the efficiency of the second ring closing step is considerably
reduced, presumably due to through space quenching of the
excited state of the open photochromic unit by the closed unit,
reducing the quantum yield of the photochemical ring closing
reaction and hence the equilibrium point reached with the reverse
photochemical ring opening reaction.
electronic communication. It is apparent that the silicon atom
spacer22 precludes effective through bond interaction presumably
due to poor orbital overlap with those of the dithienylethene
components.
Acknowledgements
Although photochemical ring closure is an essentially temper-
ature independent process above 80 K, the reverse process, that
of ring opening, is a thermally activated process and typically
below 130 K ring opening of dithienylethenes is not observed even
under prolonged visible irradiation.14a,21 In order to confirm that
the low photostationary state between the c–o and c–c forms at
298 K (Table 1) is due to impedance of the second photochemical
ring closing step, photochemical ring closure of the o–o form was
carried out under conditions designed to avoid photochemical ring
opening. Hence the photochemical ring closure of both 2H(o–o)
and 2H(c–o) was followed by visible spectroscopy at 120 K. For
both, complete conversion to 2H(c–c) was observed, and although
the ring closure is monoexponential for 2H(c–o), it is distinctly
biexponential for 2H(o–o) ring closure, with the initial ring closing
step (o–o to c–o) being three times faster than the second step
(c–o to c–c, Fig. 5). In addition to this, the photostationary state
reached at 120 K was >98% in favour of 2H(c–c). Thus the
lower PSS achieved at room temperature (Table 1) provides strong
evidence that although the two photochromic units do not show
through bond interaction,i.e. delocalisation, they nevertheless
show modest through space electronic communication.
The authors thank the University of Groningen (Ubbo Emmius
scholarship J.A.) and the NRSC-C program (W.R.B.) for financial
support.
Notes and references
‡ Although the fully open form of all compounds could be separated by
HPLC from the partial and fully closed forms, with the exception of 2H,
separation of the single and the doubly closed forms was not achieved.
1 B. L. Feringa, Molecular Switches, Wiley-VCH, Weinheim, Germany,
2001; J. C. Crano and R. Guglielmetti, Organic Photochromic and
Thermochromic Compounds, Plenum Press, New York, NY, 1999; M. A.
Ratner and J. Jortner, Molecular Electronics, Blackwell, Oxford, UK,
1997; B. L. Feringa, R. A. van Delden, N. Koumura and E. M.
Geertsema, Chem. Rev., 2000, 100, 1789–1816; Y. Yokoyama, Chem.
Rev., 2000, 100, 1717–1740; F. M. Raymo and M. Tomasulo, Chem.
Soc. Rev., 2005, 34, 327–336.
2 M. Irie, Chem. Rev., 2000, 100, 1685; H. Tian and S. Yang, Chem. Soc.
Rev., 2004, 33, 85–94.
3 K. Uchida, M. Saito, A. Murakami, T. Kobayashi, S. Nakamura and
M. Irie, Chem.–Eur. J., 2005, 11, 534–542.
4 X.-H. Zhou, F. Wei, F.-S. Zhang, F.-Q. Zhao, X.-D. Liu, F.-Y. Xu and
C.-H. Tung, J. Photochem. Photobiol., A, 2005, 171, 205–208; C. Okabe,
N. Tanaka, T. Fukuminato, T. Kawai, M. Irie, Y. Nibu, H. Shimada, A.
Goldberg, S. Nakamura and H. Sekiya, Chem. Phys. Lett., 2002, 357,
113–118; C. Bertarelli, M. C. Gallazzi, A. Lucotti and G. Zerbi, Synth.
Met., 2003, 139, 933–935.
5 (a) J. E. Green, J. W. Choi, A. Boukai, Y. Bunimovich, E. Johnston-
Halperin, E. Delonno, Y. Luo, B. A. Sheriff, K. Xu, Y. S. Shin, H. R.
Tseng, J. F. Stoddart and J. R. Heath, Nature, 2007, 445, 414–417; (b) T.
Kudernac, S. J. van der Molen, B. J. van Wees and B. L. Feringa, Chem.
Commun., 2006, 3597–3599; (c) N. Katsonis, T. Kudernac, M. Walko,
S. J. van der Molen, B. J. van Wees and B. L. Feringa, Adv. Mater.,
2006, 18, 1397.
6 R. Eelkema, M. M. Pollard, N. Katsonis, J. Vicario, D. J. Broer and
B. L. Feringa, J. Am. Chem. Soc., 2006, 128, 14397–14407.
7 A. Peters and N. R. Branda, Adv. Mater. Opt. Electron., 2000, 10,
245–249.
8 W. L. Zhao and E. M. Carreira, Org. Lett., 2006, 8, 99–102.
9 K. Matsuda and M. Irie, J. Am. Chem. Soc., 2001, 123, 9896–
9897.
10 T. Kaieda, S. Kobatake, H. Miyasaka, M. Marukami, N. Iwai, Y.
Nagata, A. Itaya and M. Irie, J. Am. Chem. Soc., 2002, 124, 2015–
2024.
Fig. 5 Formation of 2H(c–c) from 2H(o–o) at 120 K (kexc 312 nm)
in isomethylpentane monitored at 510 nm (data: circles, bi-exponential
growth; fit: solid line).
11 R. H. Mitchell and S. Bandyopadhyay, Org. Lett., 2004, 6, 1729–1732;
K. Yagi and M. Irie, Chem. Lett., 2003, 32, 848–849.
12 S. Saita, T. Yamaguchi, T. Kawai and M. Irie, ChemPhysChem, 2005,
6, 2300–2306; I. Jung, H. Choi, E. Kim, C.-H. Lee, S. O. Kang and J.
Ko, Tetrahedron, 2005, 61, 12256–12263.
13 S. Kobatake and M. Irie, Tetrahedron, 2003, 59, 8359–8364.
14 (a) M. Irie, T. Eriguchi, T. Takada and K. Uchida, Tetrahedron, 1997,
53, 12263–12271; (b) T. Tsujioka, Y. Hamada and K. Shibata, Appl.
Phys. Lett., 2001, 78, 2282–2284.
15 T. Kawai, T. Sasaki and M. Irie, Chem. Commun., 2001, 711–712.
16 S. Kobatake, S. Kuma and M. Irie, Bull. Chem. Soc. Jpn., 2004, 77,
945–951.
17 J. Kuhni, V. Adamo and P. Belser, Synthesis, 2006, 12, 1946–1948.
18 E. Lukevics, P. Arsenyan, J. Popelis and O. Pudova, Phosphorus, Sulfur
Silicon Relat. Elem., 2003, 178, 639–647.
19 J. J. D. de Jong, L. N. Lucas, R. M. Kellogg, B. L. Feringa and J. H.
Van Esch, Eur. J. Org. Chem., 2003, 10, 1887–1893; L. N. Lucas, J. J. D.
De Jong, R. M. Kellogg, J. H. Van Esch and B. L. Feringa, Eur. J. Org.
In summary, we have demonstrated that multicomponent
dithienylethene based switches in which the components are con-
nected via a very short Me2Si spacer unit can undergo ring closing
and opening between three distinct photochromic states even
with a small but significant electronic communication between
the photochromic units present. By introducing asymmetry in the
bicomponent system, this approach may allow for the develop-
ment of functional systems for incorporation into molecular logic
devices. Furthermore the present system demonstrates that spatial
proximity between components can be achieved independently of
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Chem., 2003, 1, 155–166; W. R. Browne, Chem.–Eur. J., 2005, 11, 6414–
6429; J. J. D. Jong, T. Kudernac, M. Walko, L. N. Lucas, K. Uchida,
J. H. Van Esch, B. L. Feringa, W. R. Browne, J. J. D. Jong, T. Kudernac,
M. Walko, L. N. Lucas, K. Uchida, J. H. Van Esch and B. L. Feringa,
Chem.–Eur. J., 2005, 11, 6430–6441; L. N. Lucus, J. H. Van Esch, R. M.
Kellogg and B. L. Feringa, Chem. Commun., 2001, 759–760.
20 S. H. Kawai, S. L. Gilat, R. Ponsinet and J. L. Lehn, Chem.–Eur. J.,
1995, 1, 285–293.
21 D. Dulic, T. Kudernac, A. Pugzlys, B. L. Feringa and B. van Wees, Adv.
Mater., 2007, in press.
22 N. W. Duffy, B. H. Robinson and J. Simpson, J. Organomet. Chem.,
1999, 573, 36–46.
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