An in situ time-dependent study of the photodimerisation of chloro-derivatives of trans-cinnamic acid using infrared microspectroscopy with a synchrotron radiation source

Abstract

The photodimerisation of single crystals of substituted cinnamic acid has been monitored continuously by infrared microscopy using a synchrotron source. The β-form of 2,4-dichloro-trans-cinnamic acid dimerises under ultraviolet irradiation to form the corresponding β-truxinic acid derivative in a reaction which follows strictly first order kinetics. By contrast the corresponding reactions in single crystals of β-2-chloro-trans-cinnamic acid and β-4-chloro-trans-cinnamic acid deviate somewhat from first order kinetics as a result of solid-state effects. In all three cases the reactions proceed smoothly from monomer to dimer with no hint of any reaction intermediate.

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Interest in reactions of organic single crystals has recently been rekindled because of the possibility of “crystal engineering” in which the crystal lattice is utilized to direct the course of the reaction and to determine the stereochemistry of products.^1,2 Proper utilization of such methods is held back, however, because of a lack of understanding of reaction mechanism within single crystals. In part this deficiency results from the lack of suitable methods to provide the necessary information. Single crystal X-ray methods are often not appropriate because the crystals may degrade during the course of the reaction.³ Atomic force microscopy has been used to monitor some reactions and while it allows the reaction progress to be monitored it gives no information concerning chemical structure.⁴ In recent publications we have demonstrated the use of infrared and Raman microspectroscopy to investigate the structures of products of the photodimerisation of derivatives of trans-cinnamic acid.⁵ Further use could clearly be made of these methods to monitor reactions in situ in order to obtain information about reaction intermediates and to provide kinetic data. There has never before been reported any experiment in which vibrational spectroscopy has been used to follow the time dependency of a reaction within a single organic crystal. In order to obtain meaningful kinetic data it is, of course, necessary to follow the reaction in one single crystal. This eliminates differences in reaction rate, which will arise from variations in the sizes of crystallites, in their orientation and distance from the light source and from overlapping such that some crystallites lie in the shadow of others if a powder sample is studied.

We now report that, in a single crystal, the β-form of 2,4-dichloro-trans-cinnamic acid dimerises under ultraviolet irradiation to form the corresponding β-truxinic acid derivative in a reaction (see Scheme 1) which follows strictly first order kinetics. By contrast the corresponding reactions in single crystals of β-2-chloro-trans-cinnamic acid and β-4-chloro-trans-cinnamic acid deviate somewhat from first order kinetics as a result of solid-state effects. In all three cases the reactions proceed smoothly from monomer to dimer with no hint of any reaction intermediate.

Scheme 1 The photodimerisation of β-2,4-dichloro-trans-cinnamic acid.

We selected derivatives of trans-cinnamic acid as our preliminary subjects for study because these reactions are well-documented and may be taken as a paradigm of single crystal organic reactions.⁶ The specific choice of the chloro-derivatives was made because it is known that bigger distortions of the crystal structure are possible during the dimerisation of trans-cinnamic acid derivatives when large atoms are coordinated to the phenyl ring.⁶ Thus a smooth conversion from monomer to dimer with no fragmentation of the crystal was anticipated.

A single crystal (length around 30 μm) of the reagent under investigation⁷ was placed on a BaF₂ window on the stage of a Nicolet Nic-Plan microscope attached to a Nicolet model 730 spectrometer located at beamline 13.3 of the Daresbury Laboratory Synchrotron Radiation Source. Details of the design of this line have been reported elsewhere.⁸ The crystal was subjected to UV–visible irradiation (λ = 260–570 nm) using an Oriel 100 W high pressure mercury lamp (model 6281) via a fused silica fibre-optic bundle (Oriel 77578). It has been shown elsewhere that photodimerisation proceeds by irradiation into the long-wavelength broad absorption band shown by both solid and dissolved (methanolic solution) 2,4-dichloro-trans-cinnamic acid in the range 240–300 nm.^5,6 It was not practical to use filters to limit the wavelength range of the incident light further because the loss of light intensity led to unacceptably long reaction times. Infrared spectra were recorded at approximately 2-minute intervals until the reaction had proceeded almost to completion. It was noted that small cracks typically developed in the crystal during the course of the reaction but that the crystal remained intact.

In Fig. 1 the conversion of monomer to dimer is clearly shown by the decay in the band arising from ν(C=C) (1619 cm⁻¹) and the shift to higher frequency of the ν(C=O) band (1686 to 1708 cm⁻¹) as conjugation is lost. The presence of clear isosbestic points demonstrates a reaction that proceeds from reactant to product with no detectable intermediate. Very small shifts in the crossover of these points may be accounted for by small variations in the baseline. Similar results were obtained for single crystals of β-2-chloro-trans-cinnamic acid and β-4-chloro-trans-cinnamic acid.

Fig. 1 Infrared spectra (1550–1800 cm⁻¹) of a single crystal of β-2,4-dichloro-trans-cinnamic acid monitored over a period of 6.5 h.

The spectra were subjected to multivariate quantitative analysis using the Nicolet TQ Analyst program with authentic samples of the monomer and dimer as standards. It may be seen from Fig. 2 that the conversion of β-2,4-dichloro-trans-cinnamic acid to the corresponding β-truxinic acid derivative follows strictly first order kinetics over the conversion range 0–97%. (The linear relationship is maintained at even higher conversions, but the scatter in the data becomes greater when the monomer concentration is very low and small measurement errors in this concentration lead to large errors in its logarithm.) Repeat experiments were carried out on crystals of slightly different sizes (range around 30–100 μm) and it was found that the order of reaction was unaffected by crystal size although the reaction proceeded more slowly for larger crystals. First order kinetics reflects the fact that reaction occurs when one molecule of the monomer in the first excited state reacts with the other monomer molecule occupying the same unit cell. Our findings demonstrate that the reaction also certainly proceeds via an exciton which may permeate through the crystal, allowing dimerisation of molecules within the crystal when the surface is irradiated. This mechanism explains how the reaction may proceed by first order kinetics even though the extinction coefficient of the monomer is so high (ε = 20 000 at λ_max = 270 nm in methanolic solution). Under these conditions the absorption of photons from the lamp is almost total even at very high conversions of monomer to dimer; extrapolation of the solution data indicates that even when conversion to the dimer reaches 99%, more than 99.99% of the incident photons of the appropriate wavelength and polarization will be absorbed within the thickness of the crystal. In the absence of an exciton mechanism, one would thus expect the photon flux to be rate-determining and the reaction would be zero order. A reaction mechanism of this type has previously been proposed in the photodimerisation of 4-bromo-trans-cinnamic acid.⁹

Fig. 2 First order fit of β-2,4-dichloro-trans-cinnamic acid.

As Fig. 3 shows, the photodimerisation of β-2-chloro-trans-cinnamic acid and β-4-chloro-trans-cinnamic acid deviate somewhat from first order kinetics. In the case of the 4-chloro derivative, there is an initial period during which the slope of the graph increases before it stabilizes to give a first-order plot during the later stages of conversion. The slope of the graph for the 2-chloro derivative continues to increase throughout the reaction: the kinetics do in fact fit closely to the contracting cube model¹⁰ over the conversion range studied, although this may be coincidental. The existence of an induction phase during which the effective rate constant increases may be associated with increased ease of reaction when the crystal structure is distorted or the dislocation density is high. The absence of such a phase in the dimerisation of the 2,4-dichloro derivative may then be explained by the smaller distortion needed to produce the dimer when two chlorine atoms, rather than one, are present on the phenyl ring. It is entirely in line with earlier observations regarding the ability of crystal structures of trans-cinnamic acid derivatives to distort.⁶

Fig. 3 First order fits of β-2-chloro-trans-cinnamic acid (■) and β-4-chloro-trans-cinnamic acid (△).

Our findings suggest an important role for vibrational microscopy in studying the kinetics and mechanisms of reactions in organic single crystals.

Acknowledgements

We thank EPSRC for studentships for SDMA and KSW and The University of Reading for a studentship to SLJ.

References

1. G. Kaupp, H. Frey and G. Behmann, Chem. Ber., 1988, 121, 2135–2145.
2. M. A. Garcia-Garibay, S. Shin and C. N. Sanrame, Tetrahedron, 2000, 56, 6729–6737.
3. V. Enklemann, G. Wegner, K. Novak and J. Wagener, J. Am. Chem. Soc., 1993, 115, 10 390–10 391.
4. (a) G. Kaupp, in Advances in Photochemistry, ed. D. C. Neckers, D. H. Volman and G. J. von Bònau, Wiley, New York, vol. 19, 119–177; (b) G. Kaupp and M. Haak, Angew. Chem., Int. Ed. Engl., 1996, 35, 2774–2777; (c) G. Kaupp, J. Schmeyers, A. Kuse and A. Atfeh, Angew. Chem., Int. Ed. Engl., 1999, 38, 2896–2899.
5. (a) S. D. M. Atkinson, M. J. Almond, G. A. Bowmaker, M. G. B. Drew, E. J. Feltham, P. Hollins, S. L. Jenkins and K. S. Wiltshire, J. Chem. Soc., Perkin Trans. 2, 2002, 1533–1537; (b) S. D. M. Atkinson, M. J. Almond, P. Hollins and S. L. Jenkins, Spectrochim. Acta Part A, 2003, 59, 629–635; (c) S. D. M. Allen, M. J. Almond, J.-L. Bruneel, A. Gilbert, P. Hollins and J. Mascetti, Spectrochim. Acta Part A, 2000, 56, 2423–2430.
6. (a) V. Ramamurthy and K. Venkatesan, Chem. Rev., 1987, 87, 433–481; (b) G. M. J. Schmidt, J. Chem. Soc., 1964, 2014; (c) M. D. Cohen, G. M. J. Schmidt and F. I. Sonntag, J. Chem. Soc., 1964, 2000.
7. Samples of 2,4-dichloro-trans-cinnamic acid, 4-chloro-trans-cinnamic acid and 2-chloro-trans-cinnamic with stated purities of 98.0%, 99.0% and 99.0% were supplied by Aldrich. Crystals of all three compounds were obtained by recrystallization at room temperature from acetone (2,4-dichloro- and 4-chloro-trans-cinnamic acid) or from an acetone/water mixture (2-chloro-trans-cinnamic acid). This procedure gives the β (head-to-head) form. See ref. 6.
8. M. A. Chesters, E. C. Hargreaves, M. Pearson, P. Hollins, D. A. Slater, J. M. Chalmers, B. Ruzicka, M. Surman and M. J. Tobin, Nuovo Cimento Soc. Ital. Fis., D, 1998, 20, 439–448.
9. M. Ghosh, A. K. Maity, S. Chakrabarti and T. N. Misra, Proc. Indian Acad. Sci., 1995, 107, 149–155.
10. C. N. R. Rao and J. Gopolakrishnan, New Directions in Solid State Chemistry, Cambridge University Press, Cambridge, 2nd edn., 1997, ch. 8.

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