Observation of photochromic γ-cyclodextrin host-guest inclusion complexes

Article abstract of DOI:10.1039/b204322j

Substituted photochromic spiropyrans were included in γ-CD inclusion complexes and the photochromic behavior was monitored optically.

Full text of DOI:10.1039/b204322j

Observation of photochromic g-cyclodextrin host–guest inclusion
complexes
Suman Iyengar and Michael C. Biewer*
Department of Chemistry, University of Texas at Dallas, Box 830688, Richardson, TX 75083-0688, USA
Received (in Columbia, MO, USA) 26th April 2002, Accepted 13th May 2002
First published as an Advance Article on the web 31st May 2002
Substituted photochromic spiropyrans were included in g-
CD inclusion complexes and the photochromic behavior was
monitored optically.
Photochromic molecules can switch reversibly between two
1
forms by excitation with the appropriate wavelength of light. If
the switching process can be controlled, photochromic com-
pounds can potentially be used as active components for
2
3
4
molecular electronics, nanoscale machines or sensors. One
class of organic photochromics that has been studied are
Scheme 2 Synthesis of spiropyran compounds
5
spiropyrans. Upon UV absorption the spiropyran (SP) converts
to the photomerocyanine (PM) form. The photomerocyanine
form can either be photochemically or thermally switched back
to the spiropyran. The thermal decay rate of the PM to the SP
form is dependent upon the nature of the substituents on the
pyran ring and the medium of the photochromic compound. The
complex and observed photochromism of the spiropyran
presumably due to the sufficient free volume of the cavity to
allow the interconversion to occur.13 Photodimerizations have
also been observed within the cyclodextrin cavity with
coumarins14 and diaryl ethenes.
15
2
nitro substituted SP structure (X = NO in Scheme 1) has been
In this study we are interested in studying the effect of a
crystalline CD host environment on photochromic SPs with
different substituents (1–3). The spiropyrans were prepared by
reacting the appropriate halogen substituted salicylaldehyde
with 1,3,3-trimethylindolenine. Crystals were grown by slow
evaporation of a 1+2 mixture of g-CD and photochromic
studied extensively due to the longer lifetime of the PM and the
relatively greater fatigue resistance compared to other sub-
stituents. In contrast, the halogen substituted spiropyrans (1 and
2
) have received little attention due to the fact that in solution at
room temperature there is no permanent color, since the half-life
of the PM is generally too short.6
compound in a DMF–H
2
O–ethanol solution.16
In order to study the effect of substituents upon the
photochromic properties of a series of spiropyrans this study
attempted to include the spiropyran into a host matrix.
Presumably the host matrix will offer stability to the photo-
generated PM to increase the lifetime and to hinder photo-
degradation pathways. Previously spiropyrans have been in-
cluded in polymer matrices, self-assembled monolayers,8
Once crystals were obtained the host–guest complex was
demonstrated by powder X-ray diffraction patterns. As seen in
Fig. 1 the host–guest complexes of compounds 2–4 have a
different powder pattern than either the pure host or guest
compounds. The powder diffraction is nearly identical for 1–3
regardless of the substituent which is typical for g-CD inclusion
complexes.17 The inclusion ratio was determined to be 2+1
7
9
10
11
sol–gels, clays and zeolites in order to improve the thermal
half-life of the photogenerated PM. In this study we wanted to
test the effect of an organized organic crystalline environment
upon the properties of these organic photochromic com-
pounds.
1
host+guest by the ratio of the corresponding peaks in the H
NMR of dissolved crystals.
Upon photolysis with 500 W Hg arc lamp with a 365 nm line
filter the host–guest crystals of 1–3 turned deeply colored. The
The host matrix studied was g-cyclodextrin (g-CD).12 With a
suitably sized guest, CD’s are known to form host–guest
crystals in which the guest resides within the cavity of the CD
ring. The size of the inner diameter of the CD complexes is
controlled by the number of -glucose units in the ring (a-CD
D
has 6, b-CD 7 and g-CD 8). Since the width of a typical
spiropyran is ~ 6–7 Å, the g-CD was chosen because the
smallest inner diameter ( ~ 7.5 Å) was wide enough to include
the photochromic guest.
Photochromic guest complexation inside the CD ring has
been observed previously in solid state studies. Arima et al.
studied the powders of g-CD–nitro spiropyran inclusion
Fig. 1 Powder XRD data spectra obtained on a Scintag XRD 2000
diffractometer with a graphite monochromated Cu-Ka radiation: (a) pure 3,
(
b) g-CD–3 host–guest crystal, (c) g-CD–1 host–guest crystal, (d) g-CD–2
Scheme 1 Spiropyran–photomerocyanine interconversion.
host–guest crystal, (e) pure g-CD.
1
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l
max of the photogenerated peaks were 604, 583 and 555 nm for
Table 1 Decay data for photochromic complexes
compound 1–3 in g-CD, respectively. After allowing the PM to
decay in the host cavity to the spiropyran the PM could be
regenerated by subsequent photolysis.
l
max (nm)
l
max (nm)
t1/2 (s)
methanol
Compound
host–guest
t
1/2 (s) host–guest methanol
As seen in Fig. 2 this color change for the photolyzed host–
guest crystals was dramatic. Single crystals of pure 1–3,
however, showed no change in color upon photolysis, pre-
sumably due to the steric constraints of the crystalline lattice on
the molecular reorganization required for this transformation.
The decay kinetics for the three host–guest crystals were fitted
to a first order exponential as seen in Fig. 3 and the half-lives are
listed in Table 1.18 As can be seen the inclusion complex offers
stability for the PM form as evidenced by the observation of
photochromism for the halogen substituted SP (1 and 2) which
was not observed in solution and also the increased lifetime of
the aldehyde substituted SP (3) by an order of magnitude in the
nonpolar g-CD cavity compared to the polar methanol solu-
tion.
1
2
3
604
583
555
320 ± 20
530 ± 20
1500 ±130
*
*
530
*
*
186 ± 2
In conclusion, photochromic host–guest crystals were ob-
tained with differently substituted SPs in a g-CD complex. The
photogenerated PM form was longer lived in the inclusion
complex compared to polar solvents. In the case of the bromo-
and iodo- substituted spiropyrans inclusion within the host–
guest complex allowed observation of photochromism which
was not observed in solution.
Funding for this project was supplied by the Robert A. Welch
Foundation.
Notes and references
1
Organic Photochromic and Thermochromic Compounds, eds. J. C.
Crano and R. Guglielmetti, Plenum Press, NY, 1999.
2
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7
8
9
R. Guglielmetti, Photochromism, Molecules and Systems, Elsevier,
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A. K. Chibisov and H. Gorner, Phys. Chem. Chem. Phys., 2001, 3(3),
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L. De Leon and M. C. Biewer, Tetrahedron Lett., 2000, 41(19),
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3
(a) D. Preston, J.-P. Pouxiel, T. Norinson, W. D. C. Kaska, B. Dunn and
J. I. Zink, J. Phys. Chem., 1990, 94, 4167–4172; (b) D. Levy, Chem
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1
1
1
0 H. Tomioka and T. Itoh, J. Chem. Soc., Chem. Commun., 1991, 8,
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32–533.
Fig. 2 Optical pictures of photochromic host–guest complexes: (a) host–
guest complex g-CD–1, before photolysis, crystal size is 0.10 3 0.05 mm,
1 I. Casades, S. Constantine, D. Cardin, H. Garcia, A. Gilbert and F.
Marquez, Tetrahedron, 2000, 56(36), 6951–6956.
2 See Comprehensive Supramolecular Chemistry, Volume 3, Cyclodex-
trins, 1996, eds. J. L. Atwood, J. E. D. Davies, D. D. Macnicol and F.
Vögtle, Elsevier Science Ltd, New York, USA.
3 T. Tamaki, M. Sakuragi, K. Ichimura, K. Aoki and I. Arima, Polymer
Bulletin, 1990, 24, 559–564.
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J. J. Stezowski, Chem. Commun., 1999, 14, 1275–1276.
5 M. Yamada, M. Takeshita and M. Irie, Mol. Cryst. Liq. Cryst., Sect. A,
(
b) g-CD–1 after 365 nm photolysis, (c) g-CD–2 before photolysis, crystal
size is 0.10 3 0.20 mm, smaller crystal to right is pure 2, (d) both g-CD–2
and pure 2 crystals after 365 nm photolysis, (e) g-CD–3 before photolysis,
crystal size is 0.20 3 0.20 mm, (f) g-CD–3 after photolysis.
1
1
1
1
2
000, 345, 107–112.
6 Typical growing conditions: 1+2 mole ratio of host and guest were
dissolved by heating in 3 mL of 1+2 v/v H O–DMF. Upon dissolution,
mL of ethanol was added and the crystals were obtained after slow
evaporation of the solvent.
2
3
1
7 See L. Szente, Comprehensive Supramolecular Chemistry, Volume 3,
Cyclodextrins, 1996, eds. J. L. Atwood, J. E. D. Davies, D. D. Macnicol
and F. Vögtle, Elsevier Science Ltd., New York, USA, ch. 8, pp.
2
61–262.
1
8 Half-life values were obtained by absorbance measurement of lmax over
time by mounting crystals on a glass slide and photolyzing with a 500 W
Hg arc lamp with a 365 nm line filter inside an HP 8453 UV–Vis
instrument.
0
Fig. 3 Plot of ln (A/A ) versus time absorbance decay of photochromic peak
for the host–guest complexes: (a) g-CD–1, (b) g-CD–2, (c) g-CD–3.
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