Functional organic molecular materials

Pentachloronitrobenzene, C6Cl5NO2, (PCNB) is one of a series of chloronitrobenzene derivatives that are currently being studied because their disordered nature and propensity to undergo phase transitions are thought to be responsible for their anomalous dielectric properties. Disorder often strongly affects the electronic environment in such materials and it is considered that this may be fundamental to physical properties such as, for example, second harmonic generation. Our goal is to quantify the disorder and short-range order as fully as possible with a view to being able to control or harness such disorder and aid the design of new functional molecular materials.

Monte Carlo computer simulation has been used to interpret and model observed single-crystal diffuse X-ray scattering data for PCNB. Each site in the crystal contains a molecule in one of six different basic orientations with equal probability. However, no short-range order amongst these different orientations has been detected. The strong, detailed and very distinctive diffraction patterns can be accounted for almost entirely on the assumption of random occupancy of each molecular site, but with very large local relaxation displacements that tend to increase the neighbouring distances for contacts involving NO2—NO2 and NO2—Cl with a corresponding reduction for those involving Cl—Cl. The results show that the mean NO2—NO2 distance is increased by ~0.6Å, compared with that given by the average structure determination.