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Solid State Photochromism and Thermochromism in Nitroso Monomer-Dimer Equilibrium H. Vancˇ ik,*,† V. Sˇ imunic´ -Mezˇ naric´ , I. C Ä aleta, E. Mesˇtrovic´ , and S. Milovac Department of Chemistry, Faculty of Science and Mathematics, UniVersity of Zagreb, StrossmayeroV trg 14, 10000 Zagreb, Croatia
K. Mlinaric´ -Majerski* and J. Veljkovic´ Department of Organic Chemistry and Biochemistry, Ruder BosˇkoVic´ Institute, Bijenicˇ ka 54, 10000 Zagreb, Croatia ReceiVed: April 23, 2001; In Final Form: September 4, 2001
Monomer-dimer equilibrium of nitroso compounds was investigated in the solid state under cryogenic photochemical conditions. It was found that nitroso dimers can be UV-photolytically converted to nitroso monomers at 12 K, and reverted to dimers by visible light irradiation or warming above 170 K. Such a photothermal “chemical switch”, by which it is possible to break the chemical bond between two nitrogen atoms and bind them again, could eventually be used in supramolecular self-assembly systems. The reaction is very efficient and controlled by topochemical factors. This phenomenon was studied spectroscopically in a series of cyclic and polycyclic nitroso compounds.
Introduction A search for new photochromic systems is of great importance in the design of intelligent materials and molecular electronic devices.1 However, only a limited number of reaction types is known to afford photochromism2 and/or thermochromism. In this paper we report a new very simple chemical system, the nitroso dimer-monomer equilibrium, which produces such photochromic and thermochromic effects in the solid state. The advantage of this novel photochromic system is the formation and/or breaking of the atom-to-atom chemical bond, i.e., the chemical “off-on switch”. Most of the previously described photochromic reactions include bond formation or bond breaking between three or more atoms (for instance, cycloadditions or cyclizations etc.). It is known and well documented that nitroso monomers such as 1a-4a and the corresponding azodioxides 1-4 (Scheme 1) are in equilibrium in solution.3 The large color difference between the monomer (blue, absorbs near 660 nm) and the dimer (colorless, absorbs in the 290 nm region) satisfies the main condition required for photochromism by which the reactant and product should absorb at wavelenghts that are as different as possible.2c,4 Coloration of the dissolved nitroso compounds has been routinely used, even as a visual indication of the equilibrium.5 Although stable crystalline monomers have been known for more than 100 years,3d,e most of the nitroso compounds appear as dimers in the solid state. Since thermal dissociation of azodioxide to nitroso compound is forbidden by symmetry rules3b,6 (if the reaction occurs by the retention of the coplanar C2N2O2 framework and stretching of the NN bond up to the transition state), it could be expected that the reaction in the solid state proceeds only under photochemical conditions. However, it is known from the available literature that UV irradiation of azodioxides at room temperature causes their decomposition, in most cases by * Corresponding authors. † E-mail:
[email protected] extrusion of NO.7 Photorearrangement has been observed in o-dinitrosobenzene, which was converted to benzofuroxan.8a There are also some indications that UV light in solution at -60 °C could induce dissociation of nitroso dimers.8b However, these observations are not quite conclusive because they are based exclusively on UV spectra (without the visible part of the spectrum). For this reason we decided to study the photolysis of azodioxide 1-5 in the solid state at cryogenic temperatures and to characterize the products by FTIR spectroscopy. As it was found by Orrell et al.9 that this method has been used very successfully for distinguishing nitroso monomers and dimers. Results and Discussion Compounds 1, 2, 3, and 5 were obtained by standard procedures,11,15-17 and the preparation of 4 is described in this paper. Starting dimers were photolyzed by a low and/or by a high-pressure Hg lamp in KBr pellets cooled in vacuo to 12 K. Corresponding FT-IR spectra were recorded before and after the photolysis. The differential FTIR spectrum of 1 (Figure 1) shows the signals of monomer (directed upward), which appeared after the photolysis at 12 K, and the absorbances of the starting dimer (directed downward), which simultaneously disappeared. The most intensive “positive” peak of the monomer is the NO stretching absorbances. In 1a, the NO group absorbs at 1548 cm-1,9a,10 and the “negative” signals at 1226, 1215, and 1194 cm-1 are known as characteristic absorbances for Eisomers 111 (Figure 1a). The monomer signal grows always simultaneously with the appearance of blue color. By warming the sample, the blue color disappeared and the spectra of 1 and 1a grow in opposite directions: the “positive” signals belong to 1, and the “negative” signals belong to 1a. The obtained differential spectrum (Figure 1b) is a mirror picture of the differential spectrum recorded after photolysis of 1. The reverse reaction was also observed if the monomers were photolyzed on 12 K by the visible light, but the reaction is not very efficient because of the very low absorbance in the visible spectral region. Dimers were obtained even after 15 min irradiation by a high-
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pressure Hg lamp equipped with the 320 nm cutoff UV filter. However, if the sample is photolyzed at the room temperature, no blue color appears. IR frequencies of dimers 2, 3, and 4 and of the corresponding monomers 2a, 3a, and 4a are shown in Table 1. The spectra are available in the Supporting Information. The multiplet at 1295 cm-1 assigned to the azodioxide group stretching vibrations in 4 is due to the mixture of isomers. Accordingly, two NO stretching signals at 1536 and 1565 cm-1 could be attributed either to two isomers of 4a or to different couplings of NO groups in the crystal lattice. The presented solid state reaction is a photochromic/thermochromic “off-on” switch by which we can break and reconnect the chemical bond between two nitrogen atoms that are close to each other in spatial positions in the crystal lattice, especially if we take into account that the sample is cooled to the temperature as low as 12 K.4b,12 This statement can be confirmed by analysis of the efficiency and yield of the reaction. The change in the IR intensity of the ONdNO stretching signal (1258 cm-1) of azodioxide 2, obtained after consecutively repeated photolysis and thermal reactions, was measured on the same sample (Figure 2). Since the intensity remains almost unchanged for six off-on cycles, the yield of these reactions can be estimated to be higher than 95% (because after six cycles, the 95% reaction yield is expected to diminish the signal absorbance to below 80% of its starting intensity). For an additional test of this phenomenon we prepared an “internal nitroso dimer”, cage-azodioxide 5, a compound with
very short distance between nitrogen atoms. At room temperature, the derivative 5, once formed, is stable even in solution. No 5 vs 5a equilibrium was found (Scheme 2). However, it is known that photolysis of cyclic azodioxides in solution yields nitroxide radicals5b such as 5b. It is believed that this reaction proceeds through the formation of nitroso compounds (such as 5a) followed by loss of an NO group. However, spectral characterizations of the dinitroso intermediate have never been reported. Irradiation of a KBr pellet doped with 5 at 12 K (10 min, 254 nm) produced a small amount of the product that affords a spectrum with two new signals at 1527 and 1566 cm-1, which could be assigned to the asymmetric and symmetric stretching of two NO groups in 5a. These absorbances can unequivocally be recognized in the differential FTIR spectrum (Figure 3). Moreover, the signal at 1268 cm-1, which is characteristic for the azodioxide stretching vibration (see Table 1) of the precursor 5, decreases during the photolysis. Although further photolysis does not increase the intensities of NO stretching signals, it is probable that the photoproduct is the intermediate 5a, especially because its spectrum disappeared after warming the sample above 170 K. Obviously, a small percentage of the conversion of 5 to 5a is a consequence of the photosensitivity of 5a, which can rearrange to 5b by loosing NO. To investigate this reaction, we have photolyzed 5 in the solid state at 77 K by a low-pressure mercury lamp in the cavity of the EPR spectrometer. Slow warming of the irradiated sample with simultaneous recording the EPR spectra resulted in the appearance of a signal at g )
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Figure 2. Reversible changes in IR intensity of ONdNO stretching vibration of 2 (signal at 1258 cm-1) obtained by consecutive photolyses (solid line) and thermal reactions (doted line). In all six cycles, the sample was irradiated 45 min by a 250 W high-pressure Hg lamp, and the thermal reaction was obtained by warming to room temperature.
Figure 1. (a) Differential FT-IR spectrum obtained by photolysis of 1 in KBr pellet at 12 K by a high-pressure Hg lamp (250 W). Positive signals belong to the monomer 1a and negative to the dimer 1. (b) Differential spectrum obtained after warming the photolyzed sample to 170 K. Positive signals belong to dimer 1 and negative to the monomer 1a. Characteristic signals of 111a at 1226, 1215, and 1194 cm-1 are labeled with asterisks.
TABLE 1: Characteristic Experimental IR Frequencies of NO and ONdNO Stretching Vibrations. ν/cm-1 compound 1 1a 2 2a 3 3a 4 4a 5 5a
NO
ONdNO 1226, 1215, 1194
1548 1258 1500 1250 1545 1295 (multiplet) 1536, 1565 1269 1527, 1566
2.0061. This signal remained intensive even after leaving the sample at room temperature. By comparing the g value with the literature data of nitroxide radicals (for example g ) 2.0067 for the CF3N(O)R1R3OH radical),13 we assigned the spectrum to the structure 5b. Topochemistry: Open Questions. Our search for promising sterical motifs for molecular self-assembly based on the use of nitroso groups as chemical receptors stimulated us to start the study of the crystal structure of 3. The structure of 3 was determined by X-ray diffraction analysis.14a However, irradiation of the single crystal of 3 at 100 K afforded no photochromic effect, which we observed with the polychrystalline sample. This difference in photochemical behavior could be explained either as a consequence of the surface reaction or as a reaction controlled by topochemical factors. From the recent studies about the single-crystal-to-single-crystal phototransformations, it follows that a photoreaction can proceed inhomogeneously, i.e., the products appear first on the surface and protect the inner bulk of the crystal to be exposed to the UV light.14b,c Consequently, the polycrystalline sample with much larger surface is expected to be photochemically more reactive. Indications that could support the explanation on the basis of
the topochemical control follow from the study of the polymorphism. The powder diffractogram of the photoactive polycrystalline sample of 3 is shown in Figure 4. Details about calculations are described in the Experimental section. Evidently, it differs from the simulated “powder” diffractogram calculated from the experimental crystal data of the photoinactive single crystal of 3. Such a difference implies that the photoactive (polycrystalline) and photoinactive (single crystal) samples exist in different polymorphic modifications. The exact knowledge about the solid-state mechanism included in this photochromic reaction will become more available after the determination of the crystal structure of this reactive polymorph. Work in these directions is in progress in our group. Conclusion A novel photochromic and thermochromic phenomenon in the solid state, a chemical “off-on” switch that includes bond breaking and bond formation between two nitrogen atoms, is described. The reaction in trans-substituted azodioxides 1-4 is photothermally reversible and probably controlled by topochemical factors. The “intramolecular nitroso dimer”, the cis isomer 5, also undergoes photochemical breaking of the NdN bond and the formation of dinitroso product 5a, which decomposes at temperatures higher than 77 K to nitroxide radical 5b by the extrusion of NO. The use of this photochromic and thermochromic phenomenon in the study of similar polycyclic systems as novel potential devices in supramolecular selfassembly is under further investigation. Experimental Section 1H and 13C NMR spectra were recorded on a Varian Geminni 300 spectrometer, and EPR spectra were recorded on the Varian E 109 spectrometer. FT-IR spectra were recorded on a PerkinElmer 1725× spectrometer under 2 cm-1 resolution. Normally 50 scans were recorded. Cooling equipment consisted of a Leybold-Heraeus ROK 10-300 closed cycle helium cryostat connected on a vacuum line pumped to 10-5 Torr. UV spectra were recorded on Cary 3 UV-vis spectrophotometer. Melting points were determined on a Kofler apparatus and are uncorrected. Elemental microanalyses were performed at the Central Analytical Laboratory of Rudjer Bosˇkovic´ Institute, Zagreb, Croatia. Preparation of Azodioxide 1-5. Among the methods of preparation of 1 and 3, we choose the direct reaction of NOCl
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Figure 3. (a) Differential spectrum of 5 (down) and 5a (up) obtained by photolysis of 5 in KBr pellet at 12 K with a low pressure Hg lamp. Signals tentatively assigned to the NO stretching vibration of 5a at 1527 and 1566 cm-1 are labeled with asterisks.
with the corresponding alkene at -78 °C.11 The obtained products precipitate from the blue solution (λmax near 660 nm) of monomers as colorless or yellowish crystal substances. From MS, 1H NMR, and 13C NMR spectra of 1, it follows that the product is a mixture of E-enantiomers, as it was found previously.15 Compound 3 is obtained in its exo-cis stereochemistry. Azodioxide 2 was prepared by a mild zink reduction of p-bromonitrobenzene.16 Polycyclic azodioxides 4 and 5 were prepared by addition of Cl2 to the corresponding oximes. Spectroscopic data for cisazodioxide 5 are in accord with the published data.17 Preparation of 4 started from pentacyclo[5.4.0.0.2,60.3,1005,9]undecan-8-one.18 Ketone (320 mg, 2.0 mmol), hydroxylamine hydrochloride (560 mg, 8 mmol), and K2CO3 (552 mg, 4 mmol) were dissolved in a 2:1 mixture of EtOH/water (20 mL) and heated under reflux for 4.5 h. The reaction mixture was then cooled and concentrated in vacuum (to 1/2 of volume), then water was added (5 mL). The white solid was filtered, washed with water, and dried. The corresponding oxime19 (255 mg) was dissolved in CH2Cl2 (12 mL), the reaction mixture was cooled to 0 °C (ice-water bath), and chlorine gas was gently bubbled until a blue color appeared. The reaction mixture was then
Figure 4. Powder diffraction patterns of 3. (a) Obtained experimentally for the polycrystallinic photoactive sample. (b) Patterm calculated from photoinactive single-crystal structure data (CuKR radiation). Calculation details are described in the Experimental section.
1580 J. Phys. Chem. B, Vol. 106, No. 7, 2002 warmed to room temperature, the solvent was evaporated, and a mixture of isomers of chloroazodioxide 4 (304 mg, 100%) was obtained as a white crystalline solid: mp ) 118-120 °C. IR (KBr) 2960 (s), 2930 (s), 2860 (m), 1450 (w), 1290 (s), 870 (m), 740 (m), cm-1. 1HNMR (CDCl3) δ: 1.10-1.95 (m, 16H), 2.32-3.10 (m, 28H), 3.15-3.25 (m, 1H), 3.25-3.30 (m, 1H), 3.65-3.75 (m, 1H), 3.75-3.90 (m, 1H). Anal. calcd for C11H12NOCl: C, 63.01; H, 5.77; N, 6.68. Found: C, 62.99; H, 5.76; N, 6.41. Cryogenic Experiments. KBr pellets doped with crystals of azodioxides (1-5) were cooled in a closed cycle helium cryostat to 12 K then photolyzed by a high or low pressure Hg lamp until the color of the pellet turned to blue (10 min to 2.5 h), then left to warm gradually to room temperature. The pellet remained blue until warming above 170 K when the color disappeared. For all the samples, the procedure can be repeated many times with the same sample (recooling to 12 K followed by irradiation turns again to blue color, and warming causes disappearance of the color). The reactions were followed also with recording the FTIR spectra simultaneously. Diffraction Measurement. Powder X-ray diffraction data were collected on Philips PW 1700 automated diffractometer using the scanning method with the following parameters: monochromatic Cu KR1 radiation (λ ) 1.5406 Å), observation range 5°-2θ-60°, step scan with 0.02°, counting time per step 10 s. The data collection was preformed by X’Pert Software suite 1.2 Program package for measuring and analysis diffraction data on Philips X-ray diffraction equipment, Philips Analytical, Almelo, The Netherlands, 1999. Pattern calculation from single-crystal structure data was obtained with X’Pert Plus 1.0 program package (with identical parameters as collect pattern) Program for Crystallography and Rietveld Analysis, Philips Analytical, Almelo, The Netherlands, 1999. Acknowledgment. We thank the Ministry of Science and Technology of the Republic of Croatia for financial support of this study (Grant 0119611). In addition we thank Professor Z. Veksli (Ruder Bosˇkovic´ Institute) for the EPR spectra and Professor D. E. Sunko, Professor Z. Mihalic´, Professor B. Gowenlock, and Professor K. Orrell for helpful discussion. Supporting Information Available: Spectra representing the IR frequencies of dimers 2, 3, and 4 and of the corresponding monomers 2a, 3a, and 4a. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Fox, M. A. Acc. Chem. Res. 1999, 32, 201. (b) Irie, M., Ed. Photochromism: Memories and Switches, Chem. ReV. (special issue) 2000, 100(5). (2) . (a) Du¨rr, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 413. (b) Du¨rr, H. Pure Appl. Chem. I 1990, 62, 1477. (c) Irie, M.; Lifka, T.; Kobatake, S.; Kato, N. J. Am. Chem. Soc. 2000, 122, 4871. (3) (a) Gowenlock, B. G.; Lu¨tke, W.; Quart. ReV. 1958, 12, 321. (b) Snyder, J. P.; Heyman, M. H.; Suciu, E. J. Org. Chem. 1975, 40, 1395. (c)
Vancˇik et al. Singh, P. J. Org. Chem. 1975, 40, 1405. (d) Fletcher, D. A.; Gowenlock, B. G.; Orrell, K. G. J. Chem. Soc., Perkin Trans 2, 1998, 797. (e) Talberg, H. J. Acta Chem. Scand., 1977, A33, 289. (4) (a) Becker, H. G. O.; Bo¨ttcher, H.; Dietz, F.; Rehorek, D.; Roewer, G.; Schiller, K.; Timpe, H.-J. Einfu¨ hrung in die Photochemie, Deutscher Verlag der Wissenschaft: Berlin, 1991; p 419. (b) Schmidt, G. M. J. et al. Solid State Photochemistry; Ginsburg, D., Ed.; Verlag Chemie: Weinheim, New York, 1976; p 80. (5) (a) Wajer, Th. A. J.; De Boer, Th. J. Recueil 1972, 91, 565. (b) Greene, F. D.; Gilbert, K. E. J. Org. Chem. 1975, 40, 1409. (c) Greer, M. L.; Sarker, H.; Medicino, M. E.; Blackstock, S. C. J. Am. Chem. Soc. 1995, 117, 10460. (d) Bamberger, E.; Seligman, Chem. Ber. 1903, 36, 685. (6) Hoffmann, R.; Gleiter, R.; Mallory, F. B. J. Am. Chem. Soc. 1970, 92, 1460. (7) (a) Rassat, A.; Rey, P. J. Chem. Soc., Chem. Commun. 1971, 1161. (b) Ullman, E.; Call, L.; Tseng, S. S. J. Am. Chem. Soc. 1973, 95, 1677. (c) Ullman, E. F.; Singh, P. J. Am. Chem. Soc. 1972, 94, 5077. (8) (a) Dunkin, I. R.; Lynch, M. A.; Boulton, A. J.; Henderson, N. J. Chem. Soc., Chem. Commun. 1991, 1178. (b) Azoulay, M.; Fischer, E. J. Chem. Soc., Perkin Trans. 2 1982, 637. (9) (a) Fletcher, D. A.; Gowenlock, B. G.; Orrell, K. G.; Sˇ ik, V.; Hibbs, D. E.; Hursthouse, M. B.; Abdul Malik, K. M. J. Chem. Soc., Perkin Trans 2 1996, 191. (b) Fletcher, D. A.; Gowenlock, B. G.; Orrell, K. G. J. Chem. Soc., Perkin Trans 2 1997, 2201. (c) Luttke, W. Z. Elektrochem 1957, 61, 302. (10) Yukawa, Y., Ed., Handbook of Organic Structural Analysis; W. A. Benjamin, Inc.: New York, Amsterdam, 1965; p 423. (11) (a) Ciattoni, P.; Lorenzini, A.; Gallinella, E. Chim. Ind. 1964, 46, 286 (Chem. Abstr. 1960, 60, 13155). (b) Metzger, H.; Meier, H. Methoden der Organischen Chemie; Houben-Weyl, 1971; Band 11, pp 926-938. (c) ibid. E16a, 1990; pp 958-959.S. (12) Diminishing molecular movements (by cooling the crystal) was observed even at 200 K by CP/MAS study of the restricted internal rotation of the NO group in nitrosobenzenes. Consequently, the reverse dimerization could occur starting from nitroso groups that are no longer coplanar. In this case the reaction is not restricted by Woodward-Hoffmann rules because the requirement for the conservation of symmetry is no longer satisfied. See also refs 3d, 6, and 9a,b. (13) Chatgilialoglu, C.; Ingold, K. U. J. Am. Chem. Soc. 1981, 103, 4833. (14) (a) Milovac, S.; Sˇ imunic´-Mezˇnaric´, V.; Vancˇik, H.; Visˇnjevac, A.; Kojic´-Prodic´, B. Acta Crystallogr. 2001, E57, 218. (b) Enkelmann, V.; Wegner, G.; Novak, K.; Wagener, K. B.. J. Am. Chem. Soc. 1993, 115, 10390. (c) Brett, T. J.; Alexander, J. M.; Stezowski, J. J. J. Chem. Soc., Perkin Trans. 2 2000, 1105. (15) (a) 13C δ/ppm: 23.5, 25.0, 28.8, 35.2, 57.6, 70.6. 1H δ/ppm: 1.84 (m, 8H); 4.30 (1H); 5.49 (1H). The spectrum agrees with the previously recorded one. See: Ponder, B. D.; Walton, T. E.; Pollock, N. Y. J. Org. Chem. 1968, 33, 3957. (b) Rogic´, M.; Demmin, T. R.; Fuhrmann, R.; Koff, F. W. J. Am. Chem. Soc. 1975, 97, 3241. MS: (m/e: 148 M(monomer)+1 and 296 M(dimer). (16) Furniss, B. S.; Hannaford, A. J.; Rogers, V.; Smith, P. W. G. Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry, 4th. ed.; Longman: London, New York, 1978, pp 627. (17) (a) Snyder, J. P.; Hyman, M. L.; Suciu, E. N. J. Org. Chm. 1975, 40, 1395. (b) Singh, P. J. Org. Chem. 1975, 40, 1405. (18) (a) Lerman, B. M.; Galin, F. Z.; Umanskaya, L. I.; Tolstikov, G. A. Zh. Org. Khim. 1978, 14, 2536. (b) Dekker, T. G.; Oliver, D. W. S. Af. J. Chem. 1979, 32, 44. (19) Pentacyclo[5.4.0.0.2,60.3,1005,9]undecan-8-one oxime was obtained as a mixture of two isomers. An analytical sample was obtained by slow sublimation at 95 °C and 5 mm Hg: white solid, mp ) 111-113 °C. IR (KBr): 3240 (s), 2960 (s), 2860 (s), 1690 (w), 960 (m), 925 (m), cm-1. 1H NMR (CDCl3) δ: 1.25-1.45 (m, 6H), 1.75-1.85 (m, 2H), 2.4-3.00 (m, 14H), 3.42-3.50 (m, 1H), 3.56-3.65 (m, 1H), 7.40 (br. s, 2H). 13C NMR (CDCl3) δ: 30.7 (t), 30.9 (t), 34.6 (d), 35.9 (t), 36.2 (t), 38.2 (d), 39.2 (d), 39.5 (d), 40.1 (d, 2C), 41.7 (d), 42.8 (d), 42.9 (d), 44.6 (d), 45.3 (d), 45.4 (d), 46.5 (d), 46.6 (d), 46.9 (d), 47.1 (d), 167.6 (s), 167.9 (s). Anal. calcd for C11H13NO: C, 75.40; H, 7.48; N, 7.99. Found: C, 75.67; H, 7.42; N, 8.09.
