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. 2015 Oct 16;2(Pt 6):653-60.
doi: 10.1107/S2052252515017297. eCollection 2015 Nov 1.

Crystal chemistry and photomechanical behavior of 3,4-dimethoxycinnamic acid: correlation between maximum yield in the solid-state topochemical reaction and cooperative molecular motion

Manish Kumar Mishra  1 , Arijit Mukherjee  2 , Upadrasta Ramamurty  3 , Gautam R Desiraju  1
Affiliations

Crystal chemistry and photomechanical behavior of 3,4-dimethoxycinnamic acid: correlation between maximum yield in the solid-state topochemical reaction and cooperative molecular motion

Manish Kumar Mishra et al. IUCrJ. .

Abstract

A new monoclinic polymorph, form II (P21/c, Z = 4), has been isolated for 3,4-dimethoxycinnamic acid (DMCA). Its solid-state 2 + 2 photoreaction to the corresponding α-truxillic acid is different from that of the first polymorph, the triclinic form I ([Formula: see text], Z = 4) that was reported in 1984. The crystal structures of the two forms are rather different. The two polymorphs also exhibit different photomechanical properties. Form I exhibits photosalient behavior but this effect is absent in form II. These properties can be explained on the basis of the crystal packing in the two forms. The nanoindentation technique is used to shed further insights into these structure-property relationships. A faster photoreaction in form I and a higher yield in form II are rationalized on the basis of the mechanical properties of the individual crystal forms. It is suggested that both Schmidt-type and Kaupp-type topochemistry are applicable for the solid-state trans-cinnamic acid photodimerization reaction. Form I of DMCA is more plastic and seems to react under Kaupp-type conditions with maximum molecular movements. Form II is more brittle, and its interlocked structure seems to favor Schmidt-type topochemistry with minimum molecular movement.

Keywords: cinnamic acid; crystal engineering; nanoindentation; photosalient; polymorphism.

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Figures

Figure 1
Figure 1
Crystal packing of form I of DMCA. Note that molecules 1 and 4 are in a γ-type environment, while molecules 2 and 3 (translationally related to 4) are in the α-type environment and potentially reactive upon irradiation.
Figure 2
Figure 2
Form II of DMCA: (a) potentially photoreactive sites; (b) interlocked structure to show the absence of slip planes.
Figure 3
Figure 3
Packing diagram of truxillic acid dimer. Note the O—H⋯O hydrogen-bonded carboxylic acid dimer synthons are shaded in light gray and C—H⋯O interactions are shaded in dark gray.
Figure 4
Figure 4
Snapshots (a)–(f) of form I crystals to a shown kinematic effect during UV irradiation. Snapshots (d)–(f) shows visible parallel cracks generated on the crystal surface.
Figure 5
Figure 5
An overlap diagram of form II (blue color) and truxillic acid dimer (red color).
Figure 6
Figure 6
A pictorial depiction of the probable mechanism of photoreaction in (a) form I and (b) form II. Arrows represent the directions of movement of molecules during photochemical reaction.
Figure 7
Figure 7
Molecular packing of DMCA form I. Blue dotted lines in form I represent slip planes.
Figure 8
Figure 8
Representative P–h curves of the DMCA forms.
See this image and copyright information in PMC

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