© Copyright 2003 American Chemical Society
MARCH 18, 2003 VOLUME 19, NUMBER 6
Letters UV-Assisted Formation of Nanoaggregates from Photochromic Spiropyrans in Nonpolar Solvents Pawel Uznanski* Center of Molecular and Macromolecular Studies, PAS, Sienkiewicza 112, 90-363 Lodz, Poland Received December 10, 2002. In Final Form: January 22, 2003 The merocyanine isomer of 1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline] spontaneously forms colored nano-objects on the substrate when cast from an aliphatic solvent solution at room temperature. During solvent evaporation and simultaneous exposition to UV irradiation, the blue substance abundantly precipitates from a photochromic solution within several seconds. Depending on the substrate pretreatment, the photochromic material either appears as a dark blue powder composed of globules ∼200 nm in diameter or forms a blue film consisting of small plates of ∼100 nm in size and larger flakes. Absorption spectra of globules are hypsochromically shifted as compared to monomer, while platelike structures exhibit a red shift indicating the presence of J-aggregates. Molecular association is strongly influenced by the presence of a nitro group in the 6-position in the pyran part and a bulky group at the nitrogen atom in an indolyl moiety. Solid samples of pure merocyanine stuff are thermally stable at room temperature for months after preparation, as was confirmed by Fourier transform infrared spectroscopy and 13C NMR with crosspolarization and magic angle spinning. The method allows selective control of the formation of molecular assemblies by changing the solvent, the substituent position, or preparation conditions (i.e., temperature, concentration). Furthermore, it provides new opportunities in fundamental research on UV-mediated aggregate formation and for practical application of the photochromic spiropyrans. The material has been characterized by spectroscopic methods in order to derive structural information on the aggregates. A mechanism for a photomerocyanine association is proposed.
Photochromic phenomena, associated with reversible changes in the chemical structure between two species, even for well-recognized systems such as spiropyrans, still arouse interest due to the light-induced drastic changes of various physical and chemical properties.1,2 Success in the practical application of photochromic compounds, among other things, requires control of the time of residence in the metastable state. An aggregation phenomenon seems to be very effective in controlling the kinetics of thermal or photochemical transformation in * E-mail:
[email protected]. (1) Organic Photochromic and Thermochromic Compounds; Crano, J. C., Guglielmetti, R., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; Vols. I and II. (2) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev. 2000, 100, 17411753.
photochromic systems. The nitro-substituted spiropyrans (Sp) have a particularly strong tendency to aggregate in solution because the photogenerated merocyanine form (Mc) has a polar structure (Scheme 1). The first report on photoinduced spiropyran associates concerned globules (∼350 nm in diameter) of 6-nitrospiropyran formed in aliphatic solution on intense UV irradiation. These colloidal particles consisted of a crystalline core of Spn+/Mc- charge-transfer complexes surrounded by an amorphous outer layer of Sp/Mc dimers.3 The electric field applied aligns the globules, which form threads of “quasi-crystals”. Quasi-crystals exhibit mac(3) (a) Krongauz, V. A.; Parshutkin, A. A. Photochem. Photobiol. 1972, 15, 503. (b) Krongauz, V. A.; Goldburt, E. S. Nature 1978, 271, 43-45. (c) Krongauz, V. A. Isr. J. Chem. 1979, 18, 304. (d) Krongauz, V. A.; Fishman, S. N.; Goldburt, E. S. J. Phys. Chem. 1978, 82, 2469-2474.
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Scheme 1. Scheme of Photochromism
roscopic electric dipoles and show optical nonlinearity.4 Larger crystalline aggregates of Mc isomer (10-100 µm) were also observed as precipitants in nonpolar5 and polar6 solutions under long UV irradiation. While quasi-crystals are rather thermally unstable, the “large-sized” aggregates become stabilized by intermolecular interactions such as coordination of a nitro substituent to a charged nitrogen atom of an indoline moiety. Another type of association occurs in Langmuir-Blodgett films7-9 or in bilayer matrixes10 where spiropyrans with long alkyl substituents are mixed with different amphiphilic compounds. The deposition process with subsequent heating leads to the arrangement of the chromophores in more precisely defined H- or J-stacks with sharp absorption bands shifted to the blue (493 nm) or to the red (609 nm), respectively, with respect to monomeric peak position of the Mc form (∼585 nm).8b,11 Molecular J-aggregates (JAs) of merocyanine molecules of the chloro-derivative of nitrospiropyran consisting of thin fibers were also observed in polymer films on simultaneous irradiation and solvent evaporation of a cast polymeric solution.12 It was found recently that long-chain spiropyrans could form spontaneously neat films of J-aggregates when deposited from n-hexane solutions of the parent Sp isomer on a substrate under UV illumination (from a low-intensity luminescence lamp) and evaporation of the solvent.13 The arrangement of merocyanine molecules within such aggregates can be further improved by rubbing of the same substrate with the Mc product. In the dark, J-aggregates slowly dissociate for a few days at room temperature, probably due to a strong tendency toward crystallization of the aliphatic substituents surpassing intermolecular interactions between chromophoric parts, and the metastable Mc isomer thermally changes to the Sp form. Additionally, upon irradiation with visible light, a photochemical back closure reaction competitively destroys JAs. This work reports on the formation of a new type of aggregates of photomerocyanine14 of 1′,3′,3′-trimethyl-6nitrospiro[2H-1-benzopyran-2,2′-indoline] (1) having no bulky substituents. Such structures have never been (4) (a) Kalisky, Y.; Orlowski, T. E.; Williams, D. J. J. Phys. Chem. 1983, 87, 5333-5338. (b) Meredith, G. R.; Krongauz, V.; Williams, D. J. Chem. Phys. Lett. 1982, 87, 289-294. (5) Onai, Y.; Mamiya, M.; Kiyokawa, T.; Okuwa, K.; Kobayashi, M.; Shinohara, H.; Sato, H. J. Phys. Chem. 1993, 97, 9499-9505. (6) Hirano, M.; Osakada, K.; Nohira, H.; Miyashita, A. J. Org. Chem. 2002, 67, 533-540. (7) Hibino, J.; Moriyama, K.; Suzuki, M.; Kishimoto, Y. Thin Solid Films 1992, 210/211, 562-564. (8) (a) Miyata, A.; Unuma, Y.; Higashigaki, Y. Bull. Chem. Soc. Jpn. 1991, 64, 1719-1725. (b) Miyata, A.; Unuma, Y.; Higashigaki, Y. Bull. Chem. Soc. Jpn. 1993, 66, 993. (9) (a) Terrettaz, S.; Tachibana, H.; Matsumoto, M. Langmuir 1998, 14, 7511-7518. (b) Tachibana, H.; Yamanaka, Y.; Matsumoto, M. J. Mater. Chem. 2002, 12, 938-942. (10) Seki, T.; Ichimura, K. J. Phys. Chem. 1990, 94, 3769-3775. (11) Ando, E.; Miyazaki, J.; Morimoto, K.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 21-28. (12) Eckhart, H.; Bose, A.; Krongauz, V. A. Polymer 1987, 28, 19591964. (13) Uznanski, P. Synth. Met. 2000, 109, 281-285. (14) Photomerocyanine is a colored form of spiropyran. In the literature, the term “photomerocyanine” is used interchangeably with “merocyanine” to emphasize that the metastable merocyanine form is a product of light irradiation.
Figure 1. Scanning electron micrographs illustrating the merocyanine material formed from n-hexane under UV irradiation on a clean (a) and on a precoated surface (b).
observed before and show clear contrast to quasi-crystals and large-sized aggregates. The studies provide the first direct measurement of structural, spectroscopic, and thermal properties of the metastable Mc entity within aggregates. Continuing the work on molecular assemblies of photomerocyanines, that is, colored forms of spiropyran, and utilizing the same methodology for sample preparation, we have found that UV-irradiated nitro-spiropyran 1 efficiently aggregates and gives pure and thermally stable merocyanine product as evidenced by Fourier transform infrared (FTIR) and 13C NMR studies. The physical state of the surface of the substrate significantly influences the morphology of the precipitated aggregates. However, instead of a continuous material as obtained for the longchain analogue, the merocyanine aggregates of 1 precipitated from n-hexane solution under low-intensity UV irradiation (typically 0.5 mL of ∼1 mM solution was spread on a 10 cm2 glass plate) form heaps of dark blue powder on a fresh surface. The second type of samples was prepared in a two-step deposition process; that is, another spread of the UV-irradiated photochromic solution of 1 was performed on the support rubbed with the Mc product of 1 obtained at the first photoaggregation development. These samples display a large area of blue layer, as observed under the optical microscope. Precise inspection of the Mc powder using scanning electron microscopy (SEM) reveals that it consists of globules of uniform size distribution of ∼200 nm in diameter (Figure 1a) while the blue film is composed mainly of small platelets, which can further convert to the larger plates (Figure 1b). The Mc globules have a strong tendency to flocculate, indicating their high polarity contrary to the plates, which are distributed statistically on the substrate. Molecular assemblies forming globules are referred to here as spontaneous aggregates (SAs), while optically more homogeneous films of plates are conglomerates of a particular
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Figure 2. Solid-state UV-vis absorption spectra of spontaneous aggregates (a) and J-aggregates (b). Spectrum c corresponds to the uncolored Sp form. The inset shows the absorption spectra of 1 in n-hexane solution (a) and just after UV irradiation (b). (The spectra were obtained using diode array spectrometer HP 8453.)
type of aggregates, J-aggregates, as was established based on the results of UV-vis spectroscopic studies. Thermal scanning of the dark blue powder of SAs by differential scanning calorimetry (DSC) shows a variety of nonequivalent structures within globules. The sample exhibits a broad endotherm beginning at Tm ) 21 °C with a peak temperature at Tp ) 93 °C. It involves a thermal rearrangement of variously fixed merocyanine molecules present within the aggregates, as prepared by UV irradiation, aggregate decomposition, and the enthalpy of thermal ring closure reaction of the Mc isomer to the Sp form. For samples containing JAs, the endotherm curve is shifted to a lower temperature (Tm ) 20 °C, Tp ) 76 °C). This indicates a contribution of different molecular packing as compared to the globules. The subsequent exotherm observed with the maximum in the range of 125-132 °C corresponds to a cold crystallization of the colorless sample. Such crystallization occurs by heating from a noncrystalline state.15 The second narrow endotherm at 175 °C is responsible for the melting temperature of the spiropyran crystals. The differences between the two types of samples observed by SEM and DSC experiments are also reflected in the UV-vis absorption spectra. The absorption of a shortly (1 s) UV-irradiated solution of 1 in n-hexane has two bands in the near UV at 367 and 392 nm and one broad band in the visible with a maximum at 576 nm and with two shoulders at 541 and 612 nm (inset in Figure 2). Upon increase of the irradiation time, the shape of the absorption becomes broader and less structured and its maximum shifts slightly hypsochromically. The changes appearing at the shorter wavelength are similar to those observed for nonpolar fluids16 and are connected with photomerocyanine dimer formation. The dimerization mechanism and stacking are probably responsible for the formation of the larger associates. Indeed, the same tendency in the spectral evolution is observed in the absorption spectrum of the precipitated merocyanine product (Figure 2). Thus, globules of SAs created on a clean surface give bands at 375 and 395 nm and an intense one in the visible at 584 nm with a clearly visible shoulder at ∼538 nm. On the precoated substrate, the blue shoulder of the 584-nm band fades while molecular arrangements (15) Wunderlich, B. In Thermal Characterization of Polymeric Materials; Turi, E. A., Ed.; Academic Press: San Diego, 1997; Vol. 1. (16) Kimura, Y.; Takebayashi, Y.; Hirota, N. J. Phys. Chem. 1996, 100, 11009-11013.
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of Mc develop new peaks at 435 and at 626 nm.17 Aggregation of dye molecules is known to affect spectroscopic behavior due to the change in an environment from the monomer to the oligomer and due to the interactions between transition moments.18 When chromophores are arranged parallel to each other, the coupling energy responsible for the position of the absorption band within the aggregate depends on the slip angle R between the center-to-center line of neighboring chromophores and the transition moment. The samples of photomerocyanine with the predominant 626-nm peak, that is, produced on precoated glass, contain a particular type of aggregates, J-aggregates, where chromophores are parallel to each other and the slip angle R is
