J. Phys. Chem. A 2010, 114, 7377–7381
7377
Kinetics of Solid State Photodimerization of 1,4-Dimethyl-2-pyridinone in its Molecular Compound Deng-Ke Cao,†,‡ Thekku Veedu Sreevidya,† Mark Botoshansky,† Gilad Golden,† Jason Brown Benedict,§ and Menahem Kaftory*,†,§ Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel, State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing UniVersity, Nanjing 210093, People’s Republic of China, and Department of Chemistry, UniVersity at Buffalo, State UniVersity of New York, Buffalo, New York, 14260-3000, USA ReceiVed: February 25, 2010; ReVised Manuscript ReceiVed: June 1, 2010
The [4 + 4] photodimerization of 1,4-dimethyl-2-pyridinone (A) in its molecular compound with 1,1,6,6tetraphenyl-2,4-hexadiyne-1,6-diol (I) was investigated by irradiation of a single crystal of the compound. The conversion to the product in a single-crystal to single-crystal transformation enabled the determination of its crystal structure after each exposure cycle of 10-20 min with a pulsed 355 nm laser light. The unit cell changes as a function of the conversion were monitored and showed monotonic changes. The kinetics of the reaction was studied at two different temperatures (230 and 280 K) and revealed a sigmoidal behavior that could be explained by the JMAK model for crystal growth with a mechanism that is intermediate between random distribution of products in the crystal and the existence of growing nuclei. The Arrhenius plot provided calculated activation energy of 32.5 kJ/mol. SCHEME 1
Introduction Although the majority of synthetic chemistry is carried out in solution, there are situations in which conducting the experiment in the solid state brings advantages that do not exist in solution. In the solid, as opposed to solution, the reactant molecules are fixed in a specific conformation, and sometimes their proximity to neighbor molecules of the same kind may lead to a photochemical bimolecular reaction. Therefore, photochemistry in the solid state provides a mean by which one can synthesize pure and unique compounds. An important advantage of carrying out photochemistry in the solid state is the ability to determine precisely the structure of the reacting molecule, in the case of bimolecular photoreaction, the intermolecular distances and relative geometry can be calculated. These relevant data can be used for the study of the mechanism and the course of the reaction. It would also be interesting to study the kinetics of the reaction. For the study of the kinetics of the reaction one needs to measure the conversion rate as a function of the exposure time. This can be done by irradiating a powder sample for different period of times followed by quantitative analysis of the resultants by X-ray powder diffraction,1 and\or by spectroscopic methods. Studying the kinetics in systems that undergo single-crystal to single-crystal transformation, on the other hand, is much more effective because of the additional information provided by the crystal structure determination, that includes the molecular structure of the product and the precursor. The convergence can be deduced from the crystal structure refinement, using the occupancy factor, after each period of exposure. Unfortunately, most photochemical reactions of crystalline material of neat compounds do not undergo single-crystal to single-crystal transformation. Never* To whom correspondence should be addressed. † Technion-Israel Institute of Technology. ‡ Nanjing University. § State University of New York.
theless, in some of the rare cases where the neat compound undergoes single-crystal to single-crystal transformation, kinetic studies have been performed. For example, the kinetics of photodimerization of 2-benzyl-5-benzylidencyclopentanone by X-ray diffraction,2 the kinetics of the [2 + 2] photodimerization of various derivatives of cinnamic acid was studied by infrared microspectroscopy,3 by solid state NMR,4,5 by optical spectroscopic methods,6 and by X-ray diffraction.6,7 It was recently demonstrated that in supramolecular systems composed of light stable host molecules and light-sensitive guest molecules the photochemical reaction may proceed to completion with the preservation of the single-crystal integrity.8-16 Such systems enable kinetic investigation of compounds that do not undergo photochemical reactions in the single-crystal to singlecrystal transformation in their neat phase. It also enables the study of the effects of the environment on the reaction product.17 In the next sections we describe the crystal structure of the molecular compound including 1,4-dimethyl-2-pyridinone (A) as the light-sensitive guest molecule and 1,1,6,6-tetraphenyl2,4-hexadiyne-1,6-diol (I) as the light-stable host molecule (Scheme 1). The absorption spectrum of the light-sensitive molecule consists of three major peaks: at 297.0 nm (abs. 0.251), at 230.0 nm (abs. 0.284) and at 204.0 nm (abs. 0.958). The
10.1021/jp101703q 2010 American Chemical Society Published on Web 06/17/2010
7378
J. Phys. Chem. A, Vol. 114, No. 27, 2010
Cao et al.
excitation used was the S1 r S0 transition.18 The effect of irradiation on the crystal structure and the time-dependent conversion of (I-A) to (I-B) is followed by the structural changes description. The kinetics of the photochemical reactions as well as the activation energy is also given below. Experimental Details Synthesis. 1,4-Dimethyl-2-pyridinone (A). The mixture of 4-methyl-2-pyridinone (purchased from Sigma-Aldrich) (9.16 mmol, 1.0 g), potassium carbonate (35 mmol, 4.82 g), methyl iodide (35 mmol, 2.2 mL), and acetone (100 mL) was refluxed for 4 h in oil bath. After cooling, potassium carbonate was removed. The filtrate was evaporated, and the resulting residue was mixed with 12 mL water, and then extracted with chloroform (45 mL × 3). The chloroform solutions were dried with MgSO4. After removal of chloroform, the resulting residue was distilled (92 °C, 0.5 mbar) and was crystallized from dry ether.19 1,1,6,6-Tetraphenyl-2,4-hexadiyne-1,6-diol (I). Cu(OAc)2 (5 mmol, 0.9 g) was dissolved in 60 mL of methanol-pyridine (1:1), and then the solution of 1,1-diphenylpropyn-1-ol (purchased from Sigma-Aldrich) (10 mmol, 2.08 g) in 60 mL of methanol-pyridine (purchased from Sigma-Aldrich) (1:1) was added dropwise with stirring in an oil bath (63 °C). After the mixture was stirred for 10 h, it was cooled to room temperature. The solvent was evaporated, and the resulting residue was mixed with 100 mL of ether-CS2 (v/v ) 4/1) and 250 mL of 2 M HCl. The organic phase of the mixture was separated, and the aqueous phase was further extracted with 2 × 100 mL of ether-CS2 (v/v ) 4/1). Finally, the combined organic phase was washed with 3 × 100 mL of 2 M HCl, 3 × 100 mL of water, and 3 × 100 mL of NaCl solution, and dried with MgSO4. After removal of solvent, the pale yellow residue was refluxed in hexane (40 mL) to yield powder. After recrystallization in CH2Cl2-hexane, the colorless needle-like crystals were collected with 89% yield.20 Molecular Compound (I-A). The mixture of 1,1,6,6tetraphenyl-2,4-hexadiyne-1,6-diol (I) (0.05 mmol/0.0207 g) and 1,4-Dimethyl-2-pyridinone (A) (0.1 mmol/0.0123 g) in mixed solvent of ethyl acetate (2 mL) and methanol (2 mL) was stirred at room temperature for 1 h. The solution was evaporated in air for a few days, and colorless blocky crystals were obtained. Laser Exposure. Single crystals were irradiated with the third (355 nm) harmonics of a circularly polarized pulsed Nd:VO4 laser. The laser was set to pulse width of 35 ns and 10 kHz. The average power was less than 1 mW for a spot with a diameter of ∼1 mm. The crystal revolved while it was exposed to the light. The photochemical reaction also takes place with irradiation of a UV LED at 365 nm under continuous wave conditions. X-ray Data Collection. X-ray diffraction data were collected at 280 and 230 K, using a Bruker Smart APEX2 CCD diffractometer installed at a rotating anode source (Mo KR, λ ) 0.71073 Å) and equipped with an Oxford Cryosystems nitrogen gas flow apparatus. Diffraction intensity data were measured before irradiation and after each cycle of exposure. The data were measured to different degrees of resolution in the range of 0.680-0.79 Å in all stages of the reaction. The transformation from a single-crystal to single-crystal took place with no significant affect on the quality of the crystal and on the diffraction intensities and mosaicities. Other crystallographic and refinement data are given in Table 1 as Supporting Information.
Figure 1. The structure of (I-A) before irradiation (at 280 K).
Refinement of the Crystal Structure before Exposure. The crystal structure at the two temperatures were solved by direct methods (SHELX97).21 All non-hydrogen atoms were refined anisotropically. The positions of the hydrogen atoms were located in difference Fourier maps and refined by riding on their parent atoms. The R values at the end of the refinement are 0.045 and 0.044 for the crystal structure at 230 and 280 K respectively. Refinement of the Crystal Structure after Each Cycle of Exposure. The refinement of the crystal structure that includes the unreacted monomers of the guest molecule (A) and of the partially produced dimer (B) was carried in a similar procedure as used above, however the occupancy factor of the atoms of the unreacted molecules and the product molecules was refined. At lower and higher conversion, the atoms of the minor species were refined isotropically, at conversion closer to 50% both unreacted molecules and the product molecules were refined anisotropically. The positions of the methyl hydrogen atoms were disordered and their positions were calculated based on the methyl carbon position prior to dimerization. The R values at the end of the refinement are 0.043, 0.051 for the crystal structures at ca. 50% conversion, and 0.042 and 0.050 for the crystal structures at full conversion at 230 and 280 K, respectively. Results Monitoring the Photodimerization. The photodimerization of (I-A) to (I-B) took place in a single-crystal to single-crystal transformation. The unit cell dimensions and the structural variation have been monitored at two different temperatures: 230 and 280 K. The crystal structure before irradiation, after ca. 50% conversion and at full conversion is shown in Figures 1-3 (at 280 K). As a result of the irradiation of a single crystal (Figure 1), bonds are being formed betweem atoms C17 and C20 of one molecule to a second molecule related by inversion center. Figure 2 shows the crystal structure of the mix crystal including the monomer and the dimer. At full conversion there is no trace of the monomer, and only the dimer is found (Figure 3). The crystal structure before irradiation consists of pairs of guest molecules that are hydrogen bonded to the host molecules (O1-H1 · · · O2 distance is 1.88 Å, O1.. .O2 distance is 2.738(2) Å, and O1-H1 · · · O2 angle is 167.6° at 280 K). The isolated pairs of guest molecules are related to each other by inversion centers. The distances between the potentially reacting atoms (C17 and C20 in Figure 1) are 3.767(3) and 3.753(3) Å at 280
Photodimerization of 1,4-Dimethyl-2-pyridinone
Figure 2. The structure showing the monomer (in yellow) the dimer (in green) after 46% conversion (at 280 K).
J. Phys. Chem. A, Vol. 114, No. 27, 2010 7379 increases slightly toward the end of the conversion. Two of the unit cell angles (R and γ) closed-up while the third (β) opensup during the photodimerization until ca. 50% conversion and then slightly decreases. The unit cell volume decreases monotonically by 2.2% as a result of the reaction, and the data points can be fitted to a linear equation. Kinetics of the Reaction. Figure 5 shows the fraction of the photodimer produced (expressed in term of conversion) by the irradiation of a single crystal by 355 nm pulsed laser light as a function of the exposure time at 230 and 280 K. The time scale in Figures 4 and 5 are based simply on fluence of the laser and are not an absolute property of the sample. The fraction of the dimer was extracted from a least-squares refinement of the crystal structure (the occupancy factor). The conversion is calculated from the occupancy factor used in the crystal structure refinement. During the refinement procedure the sum of the occupancy factor of atoms of the monomer and the occupancy factor of the atoms of the dimer was kept to 1.0. The occupancy factor was free to refine until the residual electron density, and the R value was minimal. The sigmoidal shape of the data points suggested that they are indicative of JMAK22-25 kinetics used in the description of the [2 + 2] photodimerization of R-cinnamic acid.4 The fit of the data points was done using the equation:22-25
conversion ) 1 - e-kt (where conversion ) 1 - occupancy factor) n
Figure 3. The structure at full conversion (at 280 K) of (I-B).
and 230 K, respectively. The lateral shift of the p-orbitals at C17 and C20 (of the symmetry-related molecule) is 0.612(3) Å at 280 K, and 0.685(2) Å at 230 K. It is interesting to note that although at lower temperatures the distance between the mean planes of the molecules related by inversion center get shorter (3.717(4), 3.690(4), and 3.669(2) Å at 280, 230, and 180 K, respectively), the lateral shift of C17 with respect to C20 (of the symmetry-related molecule) increases (0.612(2), 0.685(2), and 0.751(2) Å, respectively). This implies that upon lowering the temperature, the molecules slide away from each other. This fact might explain why we had difficulties to impose photodimerization at 180 K in a reasonable exposure time to enable additional set of measurements. At the end of the conversion the hydrogen bonding between the host molecules and the guest molecules (dimers) remain (O1-H1 · · · O2 distance is 1.91 Å, O1 · · · O2 distance is 2.753(2) Å, and O1-H1 · · · O2 angle is 166.1° at 280 K). The oxygen and the methyl carbon atoms serve as anchors during the dimerization and do not move. During the irradiation the unit cell dimensions undergo changes that reflect the movements of the guest molecules toward each other. The variations of the unit cell axes and angles as a function of the conversion at 280 K are shown in Figures 4a and 4b. The change of the unit cell volume at 280 K is shown in Figure 4c. Although the a axis does not change, the b axis increases and the c axis decreases until 50% conversion and
This equation was used as a model for the kinetics of phase transition involving nucleation and growth mechanism.7 In the equation, “conversion” is the fraction of the dimer, t is the exposure time, k is the constant of the growth rate, and n is the Avrami exponent that shows the dimension of the growth. When n ) 2, 3, or 4 the dimensionality of the growth is 1, 2, and 3, respectively. When n ) 1 it means that the reaction is homogeneous with equal probability to occur in any region of the sample. Bertmer et al. have used the Avrami model to fit the sigmoidal curve observed in the kinetic study of the [2 + 2] photodimerization of R-trans-cinnamic acid.4 The Avrami exponent was found to be 1.66(10), Benedict and Coppens7 have found 1.43(8) for the same reaction using a two-photon absorption experiment. The former explained that the deviation from an Avrami exponent of exactly 2, which describes a perfect one-dimensional growth, is due to the decreasing rate of nucleation. The later pointing out that this deviation might be a result of a hybrid mechanism composed of homogeneously occurring reaction and a growth of nuclei once formed. This will lead to a modified Avrami equation. It was found that different substituents at the aromatic ring of the cinnamic acid influence the reaction mechanism. For example, it was found5 that the Avrami exponent is 0.98(11) for the kinetics curve of o-methoxy cinnamic acid, and it was 2.22(11) for the kinetic curve of o-ethoxy cinnamic acid. The first is indicative of a heterogeneous one-dimensional linear growth, and the second is indicative of heterogeneous two-dimensional growth, with a decreasing nucleation rate over time. The chemical system in the present work consists of light sensitive guest molecules that are almost isolated within cavities formed by light stable host molecules, therefore we expected different behavior. We have found that n ) 1.6(1) and 1.5(1) at 230 and 280 K, respectively, indicating that the growth mechanism is similar to the two examples described above.4,7 A very different Avrami exponent of 0.55(8) was recently reported for the [4 + 4] photodimerization of 9-anthracene-carboxylic acid.26 An Avrami exponent
7380
J. Phys. Chem. A, Vol. 114, No. 27, 2010
Cao et al.
Figure 4. Variation of the unit cell dimensions at 280 K. (a) Unit cell axes (in Å), (b) unit cell angles (in °), and (c) unit cell volume (in Å3). The time scale is based simply on fluence of the laser and are not an absolute property of the sample.
Figure 5. Plot of the fraction of the guest molecule as a function of the irradiation time at 230 (a) and 280 K (b). The data points (black squares) are the refined occupancy factors, and the lines are a fit using the equation and the parameters shown. The time scale is based simply on fluence of the laser and are not an absolute property of the sample.
of 0.55 would lead to a negative dimensionality, which might suggest an autoinhibition step in the reaction. It is important to note that in this particular case the two monomers are related by crystallographic translation leading to head-to-head orientation (β-type packing), therefore the carboxylic acid groups have to adjust their conformations to enable dimerization with minimal interference between them. This might lead to additional disorder in the crystal, as suggested by the authors. Lowering the temperature by 50 K decreases the rate of the reaction by almost 2 orders of magnitude. We estimated that lowering the temperature to 180 K will further decrease the rate by an order of magnitude. Therefore, there was no surprise that lowering the temperature to 180 K did not result in traces of photodimerization, even after illumination of 4 h. The rate constants at the two temperatures were used for estimating the activation energy for the photodimerization in the solid state. From the Avrami plot a value of 32.5 kJ/ mol was calculated. Unfortunately, we could not carry out the experiment at more temperatures, therefore we have no estimate of the standard error of the calculated activation
energy. The activation energy for the [2 + 2] photodimerization of the neat compound 2-benzyl-5-benzylidenecyclopentanone2 was calculated to be ca. 13 kJ/mol. Activation energy of 104.6 kJ/mol was calculated1 from 1H NMR spectroscopy using a first-order reaction for the [2 + 4] photoreaction between anthracene and bis-ethylimino-1,4dithiin, a value of 92.0 kJ/mol was calculated using the Avrami equation. The activation energy calculated from a solution reaction was found to be 51.2 J/mol. Concluding Remarks It was shown that molecular crystals composed of lightstable host molecule and light-sensitive guest molecule can undergo single-crystal to single-crystal solid state photoreaction, which enables monitoring of the structural variation during the reaction and provides the data necessary for the study of the reaction kinetics. It was shown that the kinetics of the photoreaction fits the JMAK expression with calculated Avrami exponent of an intermediate value between a one-
Photodimerization of 1,4-Dimethyl-2-pyridinone dimensional growth in a constant rate throughout the reaction (n ) 2) and a homogeneous transformation that can start at any region of the crystal (n ) 1). The rate of the photodimerization of A is slowed down by 2 orders of magnitude upon lowering the temperature by 50 K. Acknowledgment. The work was supported by the Israel Science Foundation, No. 499/08. We would like to thank Professor Philip Coppens from the Department of Chemistry, University at Buffalo for the hospitality, for enabling us to use the equipment to perform this research, and for his helpful discussions. Partial support of this work by the US National Science Foundation (CHE0843922) is gratefully acknowledged. Supporting Information Available: Table of crystallographic data, data collections, and refinement, as well as crystallographic information files (CIF) are available. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Kim, J. H.; Jaung, J. Y.; Jeong, S. H. Optoc. Mater. 2003, 21, 395– 400. (2) Honda, K.; Nakanishi, F.; Feeder, N. J. Am. Chem. Soc. 1999, 121, 8246–8250. (3) Jenkins, S. L.; Almond, M. J.; Atkinson, D. M.; Drew, G. B.; Hollins, P.; Mortimore, J. L.; Tobin, M. J. J. Mol. Struct. 2006, 786, 220– 226. (4) Bertmer, M.; Nieuwendaal, R. C.; Barnes, A. B.; Hayes, S. E. J. Phys. Chem. B 2006, 110, 6270–6273. (5) Fonseca, I.; Hayes, S. E.; Bertmer, M. Phys. Chem. Chem. Phys. 2009, 11, 10211–10218. (6) Davaasambuu, J.; Busse, G.; Techert, S. J. Phys. Chem. A 2006, 110, 3261–3265. (7) Benedict, J.; Coppens, P. J. Phys. Chem. A 2009, 113, 3116–3120. (8) Lavy, T.; Sheynin, Y.; Sparkes, H. A.; Howard, J. A. K.; Kaftory, M. Cryst. Eng. Comm. 2008, 10, 734–739.
J. Phys. Chem. A, Vol. 114, No. 27, 2010 7381 (9) Lavy, T.; Sheinin, Y.; Kaftory, M. Eur. J. Org. Chem. 2004, 4802– 4808. (10) Zouev, I.; Lavy, T.; Kaftory, M. Eur. J. Org. Chem. 2006, 4164– 4169. (11) Lavy, T.; Kaftory, M. Cryst. Eng. Comm. 2007, 9, 123–127. (12) (a) Amirsakis, D. G.; Elizarov, A. M.; Garcia-Garibay, M. A.; Glink, P. T.; Stoddart, J. F.; White, A. J. P.; William, D. J. Angew. Chem., Int. Ed. 2003, 42, 1126–1132. (b) Amirsakis, D. G.; Garcia-Garibay, M. A.; Rowan, S. J.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. 2001, 40, 4256–4261, and references therein. . (13) Toda, F.; Bishop, B. Separation and Reactions in Organic Supramolecular Chemistry: PerspectiVes in Supramolecular Chemistry; John Wiley and Sons: New York, 2004; Vol. 8. (14) Ananchenko, G. S.; Udachin, K. A.; Ripmeester, J. A.; Perrier, T.; Coleman, A. W. Chem.sEur. J. 2006, 12, 2441–2447. (15) Halder, G.; Kepert, C. Aust. J. Chem. 2006, 59, 597–604. (16) Coppens, P.; Zheng, S.-L.; Gembicky, M.; Messerschmidt, M.; Dominiak, P. M. CrystEngComm 2006, 8, 735–741. (17) (a) Chong, K. C. W.; Sivaguru, J.; Shichi, T.; Yoshimi, Y.; Ramamurthy, V.; Scheffer, J. R. J. Am. Chem. Soc. 2002, 124, 2858–2859. (b) Joy, A.; Uppili, S.; Netherton, M. R.; Scheffer, J. R.; Ramamurthy, V. J. Am. Chem. Soc. 2000, 122, 728–729. (c) Leibovitch, M.; Olovsson, G.; Sundarababu, G.; Ramamurthy, V.; Scheffer, J. R.; Trotter, J. J. Am. Chem. Soc. 1996, 118, 1219–1220. (18) Similarly to those in other 2-pyridones:(a) Fujimoto, A.; Inuzuka, K.; Shiba, R. Bull. Chem. Soc. Jpn. 1981, 54, 2802–2806. (b) Matsuda, Y.; Ebata, T.; Mikami, N. J. Chem. Phys. 1999, 110, 8397–8407. (c) Matsuda, Y.; Ebata, T.; Mikami, N. J. Chem. Phys. 2000, 113, 573–580. (19) (a) Adams, R.; Schrecker, A. W. J. Am. Chem. Soc. 1949, 71, 1186– 1195. (b) Cook, D. J.; Bowen, R. E.; Sorter, P.; Daniels, E. J. Org. Chem. 1961, 26, 4949–4955. (20) Kuwatani, Y.; Yamamoto, G.; Oda, M.; Iyoda, M. Bull. Chem. Soc. Jpn. 2005, 78 (12), 2188–2208. (21) Sheldrick, G. M. SHELXS97; University of Gottingen: Germany, 1997. (22) Avrami, M. J. Chem. Phys. 1939, 7, 1103–1112. (23) Avrami, M. J. Chem. Phys. 1940, 8, 212–224. (24) Avrami, M. J. Chem. Phys. 1941, 9, 177–184. (25) Christian, J. W. The Theory of Transformations in Metals and Alloys, Part I; Elsevier Science Ltd.: Oxford, UK, 2002; Vol. 1. (26) More, R.; Busse, G.; Hallmann, J.; Paulmann, C.; Scholz, M.; Techert, S. J. Phys. Chem. C 2010, 114, 4142–4148.
JP101703Q
