CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 6 2043-2045
Communications Solid-State Interactions in Photochromic Host-Guest Inclusion Complexes† Suman Iyengar and Michael C. Biewer* Department of Chemistry, University of Texas at Dallas, Box 830688, Richardson, Texas 75083-0688 Received July 1, 2005;
Revised Manuscript Received August 22, 2005
ABSTRACT: Photochromic host-guest inclusion compounds were obtained by growing crystals of γ-cyclodextrin in the presence of substituted photochromic spiropyrans. The relative stabilities of the photogenerated photomerocyanine guests inside the host structure were determined. Interactions in the solid state selectively enhanced the aldehyde-substituted photomerocyanine relative to other substituents in the host-guest crystals. A photochromic compound is characterized by its ability to interconvert between two different forms, each having different absorption spectra, in response to light irradiation of the appropriate wavelength.1 Studies have been directed toward controlling the thermal lifetime of the two forms in order to use photochromism as a molecular-based switch. Many photochromic systems, including diarylethenes, Nsalicylidene anilines, and aziridines, have been shown to undergo photochromism in the crystalline phase. Spiropyrans (SP) are one of the more widely studied organic photochromic systems, which upon excitation with ultraviolet light convert to an extended conjugated photomerocyanine (PM) form (see Scheme 1).2 Due to the large molecular rearrangement involved in a SP to PM interconversion, however, photochromism is not observed for this system in a single-crystal solid-state reaction. Previously we have shown that the inclusion of halogensubstituted spiropyrans as a guest in a solid-state host with a large cavity allows photochromism to occur in the solid state.3 In solution the halogen-substituted spiropyrans display very low lifetimes for the photogenerated state and photodegradation pathways prevent the cycling of these compounds. The solid host cage dramatically increased the thermal lifetime and hindered photodegradation pathways to allow repeated cycles to occur. In this paper we describe how intermolecular interactions in the crystalline form between the host and photochromic guest can also lead to selectively enhanced stability. Due to the difference in electronic properties of the SP and PM forms, the thermal interconversion rate between the two forms has been found to be dependent upon the medium in which the PM is generated. Previously researchers have observed an increase in the lifetime of the PM form by including spiropyran molecules in polymer mem† This paper is dedicated to J. Michael McBride on the occasion of his 65th birthday. * To whom correspondence should be addressed. E-mail:
[email protected].
Scheme 1.
Spiropyran to Photomerocyanine Interconversion
branes,4 liquid crystal phases,5 and a host of solid-state structures.6 γ-CD has been shown to include guests of an appropriate size in host-guest single crystals.7 γ-CDs are truncated cone-shaped cyclic oligosaccharides consisting of eight D-glucose units. As seen in Figure 1, the wider rim in CDs is formed by the C-2 and C-3 secondary hydroxyl groups and the narrower rim by the primary C-6 hydroxyl group. The hydrogen atoms and the glycosidic oxygen bridges line the cavity. Due to this arrangement, the cavity is relatively hydrophobic compared to water. The rigidity of the CD arises due to the formation of intra- and intermolecular hydrogen bonds between C-2 and C-3 hydroxyls of adjacent glucose units. With larger guests the CD units form channel-type structures, where the CD units are stacked like a roll of coins with finite channels formed by the cavities. In γ-CD the inner width of the cavity at its narrowest point is 7.5-8.3 Å and the height of one CD unit is 7.9 Å. Crystallographic studies have deduced the struc-
Figure 1. Structure of the γ-cyclodextrin host.
10.1021/cg050313b CCC: $30.25 © 2005 American Chemical Society Published on Web 09/21/2005
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Scheme 2.
Communications
Synthesis of Spiropyran Derivatives
tures of CD-guest geometries in the solid state. R-CD (six glucose units) and β-CD (seven glucose units) host-guest crystal structures are relatively abundant,8 while those of γ-CD inclusion complexes are relatively few. X-ray studies on some γ-CD inclusion complexes have indicated that guest molecules often tend to be disordered within the large host cavity.9 Spiropyrans with different substituents were either commercially available (1) or were synthesized by the condensation of 1,3,3-trimethylindolenine with an appropriate salicylaldehyde (see Scheme 2).2 Host-guest crystals of 1-3 inside γ-CD were grown by slow evaporation of a 2:1 mixture of SP and γ-CD dissolved in H2O/ DMF/ethanol.10 Solid-state complexes of γ-CD and a substituted spiropyran were shown previously by us3 and others.11 The inclusion of the SP inside the host cavity can be determined by the change in powder X-ray diffraction for the host-guest complex compared to that of either the pure host or guest.12 1H NMR spectroscopy of the dissolved inclusion crystals indicates a ratio of SP to γ-CD of 1:2. Thus, one spiropyran molecule, with an approximate dimension of 6.5-13 Å, is included in the channel encompassing two γ-CD units. Figure 2 demonstrates a model of the packing of one photomerocyanine molecule in the channel of two γ-CD molecules. It can also be observed in this figure that the aldehyde substituent is near the primary C-6 hydroxyl groups of the γ-CD host. The crystals obtained were often slightly colored, indicating that some of the photochromic guests were already in the PM form. Upon photolysis with 365 nm light, however, the host-guest crystals would noticeably darken in color, as seen in Figure 3. As expected for the PM generated in a hydrophobic cavity, the λmax values of all the crystals were red-shifted versus the PMs generated in a polar solvent such as methanol. Figure 4 demonstrates the λmax change for the PM inside the hydrophobic γ-CD cage versus methanol. To study the relative effect of the substituent on the SP versus the lifetime of the photolytically generated PM, the
Figure 3. Photochromism of spiropyran-γ-cyclodextrin crystals: (top left) 1-γ-CD before photolysis; (top right) 1-γ-CD after 365 nm photolysis; (middle left) 2-γ-CD before photolysis; (middle right) 2-γ-CD after photolysis; (bottom left) 3-γ-CD before photolysis; (bottom right) 3-γ-CD after photolysis.
Figure 4. UV-vis spectra of compound 2: (dark line) in methanol; (shaded line) in host-guest crystal.
Figure 2. Schematic of 3 inside the γ-cyclodextrin channel.
thermal decay rates of these colored crystals after photolysis were determined through UV-vis studies of the crystals over time (Table 1).13 The half-lives of the PM in the hostguest crystals were determined to be 48, 23, and 25 min for complexes formed from 1-3, respectively. This trend
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Table 1. Decay Data for Photochromic Complexes compd
t1/2 (s)a methanolb host-guest
1 2 3
3330 ( 70 (528) 2880 ( 50 (560) 720 ( 30 (532) 1400 ( 130 (570) 186 ( 2 (530) 1500 ( 25 (555)
γ-CD/ methanol rel rate 0.86 1.9 8.1
1 2.2 9.4
a Numbers in parentheses represent λ max values (nm) of photomerocyanine. b The concentration was 2 × 10-4 M.
In conclusion, a series of substituted spiropyrans were grown inside the cavity of γ-CD. The crystals were photochromic, and the half-life of the photogenerated PM was determined. Inside the host lattice the aldehyde functionality was stabilized by nearly an order of magnitude compared to solution relative to other substituents. This stabilization is at least partly due to differences in hydrogen bonding between the hydrogen bond acceptors on the photochromic materials and the host lattice. The specific solid-state orientation of the guest inside the γ-CD host cavity thus stabilizes the photochromic compounds, depending upon the substituents present, in a trend different from that in solution studies. Acknowledgment. This work was supported by the Welch Foundation (Grant No. AT-1388).
References
Figure 5. IR spectra of 2: (dark line) pure 2; (shaded line) hostguest crystal.
of lifetimes is different from those of any of the solution studies where the PM of 1 had the longest half-life, often by an order of magnitude compared to either 2 or 3, due to the stability of the PM imparted by the electron-withdrawing nitro group. In addition, all solution studies of these compounds had 3 with the shortest lifetime for the colored PM. In the host-guest crystal of 3, however, the lifetime of the PM form was relatively greater in the γ-CD complex compared to the other substituents than in any solution study. Clearly in the crystalline lattice of the γ-CD structure, the aldehyde group is stabilized to a greater extent than either the nitro or cyano group. Other CD host-guest systems have been shown to involve hydrogen bonding between the primary C-6 hydroxy groups and included guests.14 In addition, it is known that hydrogen bonding can stabilize the PM form of photochromic spiropyrans.15 To determine if hydrogen bonding was occurring between the γ-CD host and the photochromic guest, IR spectra of crystals of the photochromic host-guest complex of 2 were taken. As seen in Figure 5, a shift of the cyano stretch was observed in the host-guest complex compared to that in the pure photochromic guest. The shift to higher wavenumbers for the cyano group inside the host-guest complex is consistent with hydrogen bonding observed in previous studies for cyano groups.16 These data indicate that hydrogen bonding is present between the γ-CD host and the photochromic spiropyrans. Due to the orientational and spatial differences required for hydrogen bonding to carbonyl, cyano, and nitro groups in the host-guest γ-CD crystals, the amount of stabilization imparted by the solid host lattice is dependent upon the nature of the substituent on the photochromic compound. The aldehyde substituent has a relative enhancement in the solid-state host structure versus a polar solution such as methanol of nearly an order of magnitude relative to the nitro substituent.
(1) Organic Photochromic and Thermochromic Compounds; Crano, J. C., Guglielmetti, R., Eds.; Plenum Press: New York, 1999. (2) Gugglielmetti, R. Photochromism, Molecules and Systems; Elsevier: Amsterdam, 1990; Chapter 8. (3) Iyengar, S.; Biewer, M. C. Chem. Commun. 2002, 13981399. (4) (a) Allcock, H. R.; Kim, C. Macromolecules 1991, 8, 532533. (b) Abe, S.; Nishimura, Y.; Yamazaki, I.; Ohta, N. Chem. Lett. 1999, 2, 165-166. (5) Ramesh, V.; Labes, M. M. J. Am. Chem. Soc. 1987, 109, 3769-3775. (6) (a) Tomioka, H.; Itoh, T. Chem. Commun. 1991, 8, 532533. (b) Casades, I.; Constantine, S.; Cardin, D.; Garcia, H.; Gilbert, A.; Marquez, F. Tetrahedron 2000, 56(36), 69516956. (7) Szejtli, J. Chem. Rev. 1998, 98, 1743-1753. (8) (a) Harata, K. Chem. Rev. 1998, 98, 1803-1827. (b) Rekharsky, M. V.; Inoue, Y. Chem. Rev. 1998, 98, 1875-1917. (9) Ding, J.; Steiner, T.; Saenger, W. Acta Crystallogr. 1991, B47, 731-738. (10) Typical growing conditions for the host-guest crystals were to dissolve a 1:2 mole ratio of host and guest with heating in 3 mL of 1:2 v/v H2O-DMF. Upon dissolution, 3 mL of ethanol was added and the crystals were obtained after slow evaporation of the solvent. (11) Tamaki, T.; Sakuragi, M.; Ichibure, K.; Aoki, K.; Arima, I. Polym. Bull. 1990, 24, 559-564. (12) A characteristic peak near 8° 2θ can be observed for channel complexes which is not present in either the pure host or guest; see: Harade, A.; Suzuki, S.; Okada, M.; Kamachi, M. Macromolecules 1996, 29, 5611-5614. (13) Half-life values were obtained by absorbance measurements of λmax 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. (14) (a) Saenger, W., Steiner, T. Acta Crystallogr. 1998, A54, 798-805. (b) Abderrazzak, D. Chem. Rev. 2004, 104(4), 1955-1976. (15) Suzuki, T.; Lin, T.; Priyadashy, S.; Weber, S. G. Chem. Commun. 1998, 24, 2685-2686. (16) Besseau, I.; Lucon, M.; Laurence, C.; Berthelot, M. J. Chem. Soc., Perkin Trans. 2 1998, 101-107.
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