Abstract
Three different polymorphs of o-ethoxy-trans-cinnamic acid (OETCA) are known to behave quite differently when irradiated with ultraviolet light. While the α and β polymorphs photodimerise to yield ethoxy-truxillic acid and ethoxy-truxinic acid respectively, the γ polymorph is unreactive. A fourth polymorph, α′, can be obtained by heating the α polymorph to 333 K and stabilising it by partially reacting it. This polymorph contains two distinctly different reaction sites in a 2 : 1 ratio. Irradiation of the stabilised α′ polymorph at 293 K yielded an ordered single crystal product that went to 67% completion (corresponding to the ratio of the different reaction sites). Here we report on the reaction of the α′-polymorph at a higher temperature, 343 K, which now occurs in two separate stages. In stage 1, only molecules in one of the sites react (in a single-crystal-to-single-crystal manner in a similar way to that of the 293 K reaction). In stage 2 the crystal undergoes a phase transition to a polycrystalline phase in which molecules in the second site react, driving the reaction to 100% completion. The powder X-ray diffraction pattern of the final product corresponds to the calculated pattern of the recrystallized ethoxy-truxillic acid photodimer. Examination of close contacts in the crystal structure after stage 1 of the reaction, and using lattice energy calculations to evaluate different arrangements of molecules in the crystal after stage 1, reveals that the reaction occurs in an ordered and cooperative manner even though the product solves as a disordered structure.
Highlights
Organic solid-state reactions, including the first observation of the photodimerisation of cinnamic acid into α-truxillic acid in 1895, were investigated by a number of eminent chemists in the second half of the 19th century, as elegantly documented by Roth in his review of the history of organic photochemistry.[1]
In this work we investigate the role of increased temperature on the solid-state reactivity of the fourth polymorph[15] of o-ethoxy-trans-cinnamic acid (OETCA), α′, in terms of rate, yield and reaction mechanism
The photodimerisation of the α′ polymorph at 343 K initially (6 to 24 hours of irradiation, 12 to 67% monomer conversion in the AB site) follows a similar path as at 293 K in that the photodimerisation occurs only at the AB site and only the DE photoproduct is obtained at this point
Summary
Organic solid-state reactions, including the first observation of the photodimerisation of cinnamic acid into α-truxillic acid in 1895, were investigated by a number of eminent chemists in the second half of the 19th century, as elegantly documented by Roth in his review of the history of organic photochemistry.[1]. The photodimerisation of the α′ polymorph at 343 K initially (6 to 24 hours of irradiation, 12 to 67% monomer conversion in the AB site) follows a similar path as at 293 K in that the photodimerisation occurs only at the AB site and only the DE (or direct) photoproduct is obtained at this point (reaction at 293 K effectively only involves the processes shown in Fig. 3a and b with the product crystal being composed of an ordered arrangement of DE and C molecules; the reaction occurs in one stage and there are no obvious side reactions).
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