title: Crystal chemistry and photomechanical behavior of 3,4-dimethoxycinnamic acid: correlation between maximum yield in the solid-state topochemical reaction and cooperative molecular motion.
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.
authors.affiliation: Solid State and Structural Chemistry Unit, Indian Institute of Science , Bangalore 560 012, India.
authors.forename: Manish Kumar
authors.initials: MK
authors.lastname: Mishra
authors.affiliation: Solid State and Structural Chemistry Unit, Indian Institute of Science , Bangalore 560 012, India ; Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven , Celestijnenlaan 200 F, B-3001 Leuven, Belgium.
authors.forename: Arijit
authors.initials: A
authors.lastname: Mukherjee
authors.affiliation: Department of Materials Engineering, Indian Institute of Science , Bangalore 560 012, India ; Centre for Excellence for Advanced Materials Research, King Abdulaziz University , Jeddah 21589, Saudi Arabia.
authors.forename: Upadrasta
authors.initials: U
authors.lastname: Ramamurty
authors.affiliation: Solid State and Structural Chemistry Unit, Indian Institute of Science , Bangalore 560 012, India.
authors.forename: Gautam R
authors.initials: GR
authors.lastname: Desiraju
keywords: cinnamic acid; crystal engineering; nanoindentation; photosalient; polymorphism