Reversible Thermal Gating of the Photochromic Properties of 3-Methyl-2-(2′,4′-dinitrobenzyl)pyridine in a Single Crystal Pancˇe Naumov*,† and Kenji Sakurai‡ International Center for Young Scientists and Materials Engineering Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received July 12, 2005;
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1699-1701
Revised Manuscript Received August 4, 2005
ABSTRACT: By cooperative twisting of alternating molecules from the adjacent columns, the structure and reaction kinetics of the photochromic compound 3-methyl-2-(2′,4′-dinitrobenzyl)pyridine can be reversibly gated in a single crystal over a hysteresis gap of 10-11 K close to ambient temperature. Polymorphism is a very convenient probe to ascertain energetic and steric requirements of the supramolecular structure for molecular reactivity, as it enables the comparison of properties of the same molecule in various welldefined scaffolds.1,2 3-Methyl-2-(2′,4′-dinitrobenzyl)pyridine (3MeDNBP) can exist as three polymorphs, two of which (forms A and B) are photochromic and have different reaction kinetics.3-5 The phase bistability (A T B) with hysteresis close to room temperature,4 photoactivity of both A and B with small structural change,6 and enhanced preservation of the crystallinity during the reactions are some of the reasons why 3MeDNBP is considered a potentially important material for applications in photonbased electronics.7 The photochromism of forms A and B is mainly due to thermally activated intramolecular transfer of the benzylic proton in the form CH to the pyridyl nitrogen (NH) via an aci-nitro intermediate (OH, Scheme 1).6,8 In this communication, we report on direct observations of the phase change A T B by an accurate variabletemperature X-ray diffraction technique and also investigate utilization of the molecular bistability for reversible thermal gating of the photochromic properties in a single crystal. The crystal structure of the CH tautomer close to the transition point was studied by X-ray diffraction on a temperature-controlled9 single crystal of 3MeDNBP10 under dark conditions. Complete datasets were collected at five different temperatures during the process of heating from 316.1(3) to 337.5(1) K (points 1-5, Figure 1) and again after stabilization for 17 h at 337.5(1) K (6). The crystal was subsequently cooled to 316.2(3) K, and the lattice parameters were refined at seven discrete temperatures (7-13, Figure 1). No decrease of crystallinity was observed during the thermal cycle. The progress of structural conversion was monitored by changes along the a axis (Figure 1a). The largest part of the heated crystal undergoes A f B transition between 324.0(1) and 328.9(2) K (3 and 4), accompanied by an expansion of the cell by 1%. At this point, the structural transformation is not yet completed and, with small yield, it continues beyond 328.9(2) K (4 and 5), even at constant temperature (5 and 6). Upon cooling of the sample, the reverse-conversion B f A occurs in the interval 319.1(5)-315.9(3) K (12 and 13) and continues below 315.9 K. The hysteresis width obtained by single-crystal diffraction of 10-11 K agrees very well with the value 8-9 K obtained by powder NMR and calorimetric measurements.2 The structure of the crystal * To whom correspondence should be addressed. E-mail: naumov.pance@ nims.go.jp. † International Center for Young Scientists. ‡ Materials Engineering Laboratory.
Scheme 1.
a
Simplified Mechanism of Photoreactions of Forms A and B of 3MeDNBPa
The low-yield radical reactions are not shown.
after one complete thermal cycle is identical to the initial structure (form A). Monitoring of the crystal structure during the transition A f B showed significant alteration of the intensity of almost all reflections. Within the experimental uncertainties, the intensity changes between 324 and 337 K of most of the reflections agree qualitatively well with the predictions based on the powder patterns (Mo KR radiation) simulated from A and B structures determined4 from different crystals far from the transition (Table S1, Supporting Information). The systematic absences and structure refinement confirmed that the monoclinic P21/c symmetry of the crystal is retained within the studied temperature range. The most significant differences between the molecular structures of A(CH) and B(CH) are the slightly different phenylpyridyl (R1) and o-nitrophenyl (R2) dihedral angles (Figure 1B). The series 1-6 confirmed that during the transition A f B the molecule is twisted by 11°, increasing R1 from 56.44(7)° (1-3) to 67.87(5)° (4) and 68.06(8)° (5, 6). Simultaneously, R2 decreases from 40.8(4)° (1-3) to 30.2(3)° (4) and 30.1(3)° (5, 6). Close to the transition temperature, the structure of the composite crystal is a weighted average of A and B. The change of the a-axis indicates that at 328.9(2) K (4) at least 81% of the molecules are converted into the form B.11 The larger amount of B is due to an increase of the B/A ratio during the time (4 h) needed for data collection. Therefore, the structure at 4 is a weighted average of the structure of the two phases.
10.1021/cg050328+ CCC: $30.25 © 2005 American Chemical Society Published on Web 08/19/2005
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Crystal Growth & Design, Vol. 5, No. 5, 2005
Figure 1. (a) Change of length of the a axis of 3MeDNBP during the phase transition A f B. The points represent the structural configurations at 316.0(3) (1), 319.7(2) (2), 324.0(1) (3), 328.9(2) (4), and 337.5(1) K (5 and 6, two datasets for the last temperature). (b) Change of the phenyl-pyridine dihedral angle (R1) around the phase transition.
Communications eventually stopped after the absorbed quantum of thermal excitation energy has been exhausted or at the crystal defects/boundaries, which agrees well with the coexistence of two phases in the hysteresis region, as evidenced by NMR.3 In the high-temperature phase, the dinitrophenyl fragments from adjacent columns interpenetrate better, while their pyridyl tails are forced into zigzag channels that run between the slabs parallel to the b axis. The volume of the reactive nitrobenzylpyridine fragments increases from 22.63 to 24.31 Å3,2 accounting for the decrease of their activation energy of NH thermal decay reaction from 27.4(8) to 18.9(2) kcal mol-1.5 Therefore, the photochromic 3MeDNBP molecule can operate as a switch in a single crystal between the two temperature domains of the structure and the proton-transfer kinetics close to ambient temperature over a hysteresis gap of 10-11 K, without apparent crystal deterioration. The small structural change, narrow hysteresis gap, and absence of strong intermolecular interactions indicate a sufficiently small energy barrier between A and B. As both A and B are photoactive, their interconversion might be triggered by photoinduced structural perturbation. To test this hypothesis, and to ensure that the effect is not due to heating, a colorless single crystal of B(CH) 3MeDNBP annealed in the high-temperature regime was photoexcited in the hysteresis region by UV irradiation in its absorption maximum whereupon it turned deep blue.12 The photoexcitation induced notable changes in the diffraction pattern, although the pattern of the photoirradiated crystal was qualitatively different from that of the low-temperature phase A. Therefore, the macroscopic structural changes which can be induced by photoexcitation are insufficient to switch the structure back to the form A. Acknowledgment. This study was performed through Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. Supporting Information Available: Crystallographic data (tabular and CIF format) and predicted and observed change of the reflection intensities (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 2. Overlapped representation of the molecular packing of 3MeDNBP in the low-temperature phase (A, stick model) and high-temperature phase (B, ball-and-stick model) as viewed perpendicular to (a) and along (b) the c axis.
Inspection of the crystal packing (Figure 2) shows that the molecular twisting is caused by a change of the axial distance among the columns of interleaved dinitrophenyl rings parallel to the c axis. Because of the partial molecular stacking between the columns, twisting of one molecule in a column by heating above 324 K triggers twisting of a neighboring molecule of the adjacent column, and this process is repeated by alternate twisting of the molecules from the two columns. Therefore, it can be suggested that the structural transition of 3MeDNBP has a cooperative character and that the discrete structural domains are transformed by a kind of “domino” effect. The process is
(1) Bernstein, J. Polymorphism in Molecular Crystals; Oxford University Press: New York, 2002; pp 213-223. (2) Naumov, P.; Ohashi, Y. Acta Crystallogr. 2004, B60, 343. (3) Eichen, Y.; Botoshansky, M.; Peskin, U.; Scherl, M.; Haarer, D.; Khatib, S. J. Am. Chem. Soc. 1997, 119, 7167. (4) Schmidt, A.; Kababya, S.; Appel, M.; Khatib, S.; Botoshansky, M.; Eichen, Y. J. Am. Chem. Soc. 1999, 121, 11291. (5) Khatib, S.; Tal, S.; Godsi, O.; Peskin, U.; Eichen, Y. Tetrahedron 2000, 56, 6753. (6) Naumov, P.; Sekine, A.; Uekusa, H.; Ohashi, Y. J. Am. Chem. Soc. 2002, 124, 8540. (7) Irie, M. Chem Rev. 2000, 100, 1685. (8) At cryogenic temperatures, the largest amount of the product (NH) is created by proton tunneling (Scherl, M.; Haarer, D.; Fischer, J.; DeCian, A.; Lehn, J.-M.; Eichen, Y. J. Phys. Chem. 1996, 100, 16175) and/or open-shell reactions (Naumov, P.; Ohashi, Y. J. Phys. Org. Chem. 2004, 17, 865). (9) The temperature was corrected for differences between the monitored and real temperature at the crystal position. Temperature variations during the data collection were typically 0.6 K and did not exceed 1.5 K. (10) 3MeDNBP was synthesized according to the previously described method (Scherl, M.; Haarer, D.; Fischer, J.; DeCian, A.; Lehn, J.-M.; Eichen, Y. J. Phys. Chem. 1996, 100, 16175) and purified with column chromatography (Al2O3, gradient elution with CH2Cl2 in hexane). Good quality colorless crystals of forms A and C, which can be
Communications visually discerned by the crystal shape and by their photoactivity, were obtained concomitantly by slow evaporation of ethanol solutions or by seeding at room temperature in the dark. Data for form A were collected (Mo KR) from a 360 × 240 × 200 µm crystal protected from light with Siemens SMART diffractometer (SAINT-Siemens Area Detector Integration and SMART-Siemens Molecular Analysis Research Tool; Siemens Analytical X-ray Instruments Inc.: Madison, WI, USA, 1996) equipped with CCD detector and open-flow low-temperature system with cold nitrogen gas. For each dataset, the crystal was kept for about 4 h at constant temperature. The structures were solved with direct methods (Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435) and refined (Sheldrick, G.
Crystal Growth & Design, Vol. 5, No. 5, 2005 1701 M. E. SHELXL-97, Structure refinement program; University of Go¨ttingen, Germany, 1997) with all non-H atoms anisotropic and the H atoms as riding bodies to the respective non-H atoms. (11) Calculated from the change of a axis. (12) Colorless 3MeDNBP crystal annealed at 333 K was illuminated for 1 s with double output from a high-pressure Hg lamp (λmax ) 365 nm, SP-7, 250 W, Ushio) at 320 K placed 1-2 cm from the crystal, using internal heat filter, whereupon it turned deep blue-violet. Irradiation for less than 1 s or with smaller power did not result in observable changes of the diffraction pattern.
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