Understanding of structure and property correlation of organic solids is pivotal to manufacture functional devices using organic solids. The reactivity of these materials in solid state, particularly those that undergo photo-induced single-crystal-to-single-crystal (SCSC) transformation, has potential application in pharmaceutical and material science. One of the factors that controls SCSC reactivity in solids is their mechanical properties, which are poorly understood.  Herein, we present softening and hardening of a series of cocrystals of substituted resorsicinols with trans-1,2-bis(4-pyridyl)ethylene (4,4'-bpe) that undergo 2+2 photocycloaddition SCSC reaction. Using atomic force microscopy (AFM) nanoindentation technique, millimeter- and nanometer-sized cocrystals were studied to determine their mechanical properties and how they change during the SCSC transformation.  The mechanical properties (stiffness) were found to correlate with the extent of the reaction and depend on crystal size and composition.  Our studies open new possibilities to tune mechanical properties of organic materials without involving rigorous synthetic methodology and hence create opportunities for device application with materials that have tunable mechanical properties.

Hardening and softening of nanometer and millimeter sized organic cocrystals in a single-crystal transformation

243rd National Meeting (2012)

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