Molecular Pedal Motion Influences Thermal Expansion Properties within Isostructural Hydrogen-Bonded Co-crystals

The influence of molecular pedal motion on the thermal expansion properties of three isostructural hydrogen-bonded co-crystals based upon resorcinol is reported. The resulting co-crystals all exhibit discrete four-component assemblies held together by O-H···N hydrogen bonds and comprise resorcinol (res) with a series of isosteric bipyridines, namely, 4,4′-azopyridine (4,4′-AP), trans-1,2-bis(4-pyridyl)ethylene (4,4′-BPE), and 1,2-bis(4-pyridyl)acetylene (4,4′-BPA). The ability to change the core of the hydrogen bond acceptor molecules from an azo (N=N) to an ethylene (C=C) and finally an acetylene (C≡C) group affords co-crystals that differ in their tendency to undergo dynamic pedal motion in the organic solid state. All three co-crystals, 2(res)•2(4,4′-AP), 2(res) •2(4,4′-BPE), and 2(res) •2(4,4′-BPA), exhibit thermal expansions that correlate with the strength of the noncovalent interactions, as well as the propensity of the core to undergo pedal motion.