The enhancement of photoswitching in a diarylethene derivative by the incorporation of cyanobiphenyl groups

Article abstract of DOI:10.1039/b316146c

A photochromic liquid crystal system based on a diarylethene functionalised in the 2,2′-positions and linked to 4-cyano-4′-hydroxybiphenyl groups via alkyl spacers as chromophores was synthesised and its photochromic and mesomorphic behaviour was investig

Full text of DOI:10.1039/b316146c

The enhancement of photoswitching in a diarylethene derivative by the
incorporation of cyanobiphenyl groups
Michel Frigoli and Georg H. Mehl*
Department of Chemistry, University of Hull, Hull, UK. E-mail: g.h.mehl@hull.ac.uk; Fax: (44) 1482
466411; Tel: (44) 1482 465590
Received (in Cambridge, UK) 11th December 2003, Accepted 10th February 2004
First published as an Advance Article on the web 23rd February 2004
A photochromic liquid crystal system based on a diarylethene
functionalised in the 2,2A-positions and linked to 4-cyano-4A-
hydroxybiphenyl groups via alkyl spacers as chromophores was
synthesised and its photochromic and mesomorphic behaviour
was investigated.
the lithiated derivative with 2-(11-bromoundecyloxy)tetrahydro-
pyran (78%), containing the protected alcohol function. The
photochromic diol 2 was prepared by reacting the lithiated
derivative of 5 obtained by an bromide/lithium exchange using n-
BuLi in THF at 278 °C with a semi-equivalent of octa-
fluorocyclopentene, followed by the removal of the protective
group, carried out in ethanol at reflux in the presence of a catalytic
amount of PPTS (overall yield 58%). Diol functions of photo-
chromic compound 2 were reacted with 4-cyano-4A-hydrox-
ybiphenyl by a Mitsunobu reaction in the presence of DIAD and
PPh3 in dichloromethane yielding the target molecule 1 (29%). The
molecular structure and purity of 1a were confirmed by 1H-NMR,
elemental analysis and HPLC.7
The photochromic properties of 1 and 2 are compared with those
obtained for bis(2-methyl-1-benzothiophen-3-yl)hexafluorocyclo-
pentene 3.6 The introduction of cyanobiphenyl groups that absorb
strongly in the UV region increases considerably the extinction
coefficient at the wavelength of irradiation, shown in Fig. 1. At 313
nm, the extinction coefficient for 1a (25000) is more than seven
times higher than that of 2a (3500). When photochromic solutions
of 1a and 2a were irradiated with 313 nm light, all the compounds
turned from colourless to red-purple. The change of colour is due to
the formation of the closed forms 1b and 2b. The presence of the
cyanobiphenyl groups does not affect the maximum absorption in
the visible region. For both molecules 1b and 2b, the visible band
is centred at 538 nm. A bathochromic shift of 21 nm is observed
when compared with the maximum absorption value of molecule
3a (517 nm). Similar results have been also reported for structurally
related dithienylethene derivatives when methyl groups are re-
placed by hexyl groups.8 The original absorption spectrum can be
recovered by irradiation with visible light at 546 nm. Fig. 1 shows
also the calculated absorption spectra of the pure closed-ring forms
1b and 2b.3 The value of the extinction coefficient (e) at 538 nm for
1b is 8600 and 8400 mol21 L cm21 for 2b, similar to the value
obtained at 517 nm for 3b (9100). The photochromic properties are
listed in Table 1.
The combination of self-assembly and photochromic behaviour, the
modulation of chemical structure and physical properties in
molecular systems by light, promises to be very useful in optical
technological devices.1 Diarylethene based photochromes, notably
bis(dibenzothien-3-yl) systems, are very suitable for this purpose,
due to the remarkable thermal stability of both colourless and
coloured forms and their high photo-resistance.2 Functionalisation
of bis(dibenzothien-3-yl) or bis(dithien-3-yl) systems with meso-
gens has so far been in positions at the long axis of the molecules.3,4
However the functionalisation of the 2,2A-positions of a diaryle-
thene unit allows for a controlled modulation of the absorption
spectra and of the electrochemical properties, the introduction of
chirality and the addressing of the equilibrium between open and
closed forms.5,6 Also, the substituents introduced in 2,2A-positions
influence strongly the distribution between the parallel (p) and
antiparallel (ap) conformers (active form for electrocyclisation)
present in diarylethene derivatives (Scheme 1).6
We report the first photochromic liquid crystal 1, which is
connected to two mesogens via a spacer of eleven methylene
groups in 2,2A-positions, based on the novel photochromic diol 2
shown in Scheme 1. The reactive alcohol functions are separated
from the photochromic core by an alkyl spacer, decoupling of the
aromatic groups is required for the formation of LC behaviour.
Cyanobiphenyl groups were used as mesogens, due to their
fluorescence properties and for comparison with other functional
hybrid oligomeric liquid crystal systems.3,4
The photochromic liquid crystal 1 was prepared following the
synthetic route shown in Scheme 2. The approach takes advantage
of the acidity of the hydrogen in the a-position to the sulfur atom of
the benzothiophene core prefunctionalised in the 3-position by a
bromo group 4. The lithiation in the 2-position using LDA in THF
at 260 °C allowed the conversion of 4 to the product 5 by reacting
The replacement of the methyl group by the undecyloxyl group
in the 2,2A-positions reduces the quantum yield of cyclisation from
0.35 (3) to 0.23 (2). This decrease is directly correlated to the ratio
Scheme 2 Reagents and conditions: i, LDA/THF, 260 °C, 1.5 h,
2-(11-bromoundecyloxy)tetrahydropyran, 278 °C to r.t., 78%; ii, BuLi/
1
THF, 30 min, 278 °C, eq of octafluorocyclopentene, 278 °C to r.t.,
2
ethanol, PPTS, reflux, 4 h, 58%; iii, 4-cyano-4A-hydroxybiphenyl, DIAD,
Scheme 1 Photochromic interconversion in diarylethene series.
PPh3, CH2Cl2, r.t., 3 h, 29%.
818
C h e m . C o m m u n . , 2 0 0 4 , 8 1 8 – 8 1 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Fig. 1 (a) and (b) Absorption spectral changes of 1 and 2 in cyclohexane solution. (c) Rate of photocolouration (absorbance (538 nm) vs. time of irradiation)
for 1 and 2. (d) Fluorescence spectra (excitation wavelength 313 nm) of 1 and 1PS in cyclohexane solution.
Table 2 Transition temperatures (°C) as determined by DSCa
between the active antiparallel (ap) and parallel (p) conformers. 1H
NMR signals for the CH2 groups in a-position (2.21 ppm for ap and
2.55 ppm for p) at 25 °C of 2a indicate that the ratio between the
active antiparallel and parallel conformers is 44/56. For 3a, the
relative population is 65/35. As expected, this decrease in the
relative population of the antiparallel conformer reduces the
quantum yield of electrocyclisation (colouration reaction).6 The
value for the conversion (the concentration of open or closed
molecules) obtained at the photostationary (PS) state (equilibrium
conditions) determined by HPLC9 is 0.60 for 2, higher than the
conversion value obtained for the parent molecule 3 (0.43). The
conformer ratio of 1 (43/57 ap/p)7 is similar to that of 2, the
quantum yield of photocyclisation for 1 (1a to 1b) is 0.12, lower
than for 2 (0.23). However, on irradiation (313 nm), the
photostationary state is reached at least two times faster for 1 than
for 2 and 3, shown in Fig. 1c, and a conversion of 62% at the PS
state was detected. The enhancement in the rate can be explained by
active participation of the cyanobiphenyl groups in this process.
Comparing the emission quantum yield of the fluorescence found
for the cyanobiphenyl group (0.75)10 with those for 1a (0.13) and
for system 1 at the PS state (0.125), Fig. 1(d), indicates that strong
quenching of the fluorescence of the cyanobiphenyl groups (donor)
takes place.11 This process of quenching of fluorescence and the
energy transfer through space to a functional group with a lower
excited state energy (Förster or FRET effect) has been used in
diarylethene series to modulate the fluorescence of the donor
without its participation in the photochromic process.12 For 1,
following excitation with light (313 nm), fluorescence resonance
energy transfer (FRET) from the cyanobiphenyl groups to the
photochromic core occurs, and additionally the rate of the
photoswitching is increased. This result demonstrates for the first
time that suitably selected mesogens can be employed as donors for
the optimisation of FRET assisted photoswitching. For all of the
investigated systems the quantum yield values of cycloreversion by
irradiation with 546 nm light are broadly similar, ranging from 0.35
to 0.30.
Compound
Transition temperature/°C
1a
1PS
Cr 140.1 (96.1 N) Iso
Cr 122.2 (62.1 N) Iso
a () denotes a monotropic transition; Cr: crystalline; N: nematic; Iso:
isotropic liquid.
System 1a melts at 140.1 °C and on cooling from the isotropic
liquid exhibits a nematic phase at 96.1 °C (monotropic LC
behaviour, not thermodynamically stable). Irradiation with UV
light alters these properties significantly. For 1PS, the reduction in
flexibility due to ring closure of the system reduces the stability of
the nematic phase by 34 °C to 62.1 °C while the melting point is less
altered and decreases by 17.9 °C to 122.2 °C.
In conclusion, we have demonstrated that the introduction of
cyanobiphenyl groups in 2,2A-positions via alkyl spacers in a
diarylethene derivative enhances the rate of photoswitching
without altering the other photochromic properties via a Förster
type process. This also affords a new alternative in the design of
photochromic liquid crystals based on diarylethene derivatives.
Notes and references
1 Molecular Switches, ed. B. L. Feringa, Wiley-VCH, Weinheim, 2001;
K. E. Maly, M. D. Wand and R. P. Lemieux, J. Am. Chem. Soc., 2003,
12, 3397.
2 M. Irie, Chem. Rev., 2000, 100, 1685 and refs. therein.
3 M. Frigoli and G. H. Mehl, ChemPhysChem, 2003, 4, 101; M. Frigoli
and G. H. Mehl, Eur. J. Org. Chem., 2004, 636; M. Frigoli and G. H.
Mehl, Chem. Eur. J., 2004, in press.
4 S. H. Chen, H. M. P. Chen, Y. Geng, S. D. Jacobs and K. L. Marshall,
Adv. Mater., 2003, 15, 1061.
5 K. Morimistsu, S. Kobatake, S. Nakamura and M. Irie, Chem. Lett.,
2003, 32, 858; A. Peters and N. R. Branda, Chem. Commun., 2003, 954;
Y. Yokoyama, H. Shiraishi, Y. Tani, Y. Yokoyama and Y. Yamaguchi,
J. Am. Chem. Soc., 2003, 125, 7194; K. Morimitsu, K. Shibata, S.
Kobatake and M. Irie, J. Org. Chem., 2002, 67, 4574.
6 K. Uchida, E. Tsuchida, Y. Aoi, S. Nakamura and M. Irie, Chem. Lett.,
1999, 63.
The LC behaviour of the open-ring isomers 1a and the system at
photostationary conditions 1PS was investigated using differential
scanning calorimetry (DSC) and optical polarising miscoscopy.
The results are given in Table 2.
7 1a 1H NMR (400 MHz, CDCl3): d = 1.2–1.65 (28H, m, CH2); 1.72 (4H,
m, CH2); 2.21 (1.72H, m, CH2 ap); 2.55 (2.28H, m, CH2 p); 3.91 (4H,
t, J = 6.8, OCH2); 6.91 (4H, m, 4H-arom); 7.08 (20H, m, 20H-arom).
Elemental analysis calcd (%) for C69H68F6N2O2S2 (1135.41): C 72.99,
H 6.04, N 2.47; found: C 73.04, H 5.99, N 2.44%.
Table 1 Photochromic properties of compounds 1, 2 and 3a
Cycloreversionb
Cyclisation (313 nm)
(546 nm)
8 S. L. Gilat, S. H. Kawai and J-M. Lehn, Chem. Eur. J., 1995, 5, 275.
9 Silica gel column, 2 mL min21, cyclohexane/ethyl acetate, 60/40 for 2
and 90/10 for 1.
d
Fa?b
Fa?b ea
Conversion
Fb?a
1
2
3c
0.12
0.23
0.35
3000
800
1000
0.62
0.60
0.43
0.31
0.30
0.35
10 M. Van Damme, J. Hofkens, F. C. De Schryver, T. G. Ryan, W. Rettig
and A. Klock, Tetrahedron, 1989, 45, 4693.
11 Fluorescence measurements: emission spectra were performed in
cyclohexane solutions of spectrometric grade at 25 °C (to ±0.2 °C) by
using a Shimadzu RF-1501 spectrofluorophotometer.
12 L. Giordano, T. M. Jovin, M. Irie and E. A. Jares-Erijman, J. Am. Chem.
Soc., 2002, 124, 7481; T. Kawai, T. Sasaki and M. Irie, Chem.
Commun., 2001, 711.
a Photochromic reactions were performed in cyclohexane solution. b For all
the compounds, the conversion of cycloreversion were 1. c Taken from ref.
6. d This product characterises the efficiency of the colouration photo-
reaction.
C h e m . C o m m u n . , 2 0 0 4 , 8 1 8 – 8 1 9
819