Crystal structures and thermal expansion properties have been studied for hexamethylbenzene (HMB), picric acid (PIC), tetracyanobenzene (TCB), HMB–PIC complex, and HMB–TCB complex. HMB–PIC and HMB–TCB form charge transfer complexes in the solid state as well as in solution. From the UV–vis spectroscopy study, it has been found that HMB–TCB forms a stronger π···π complex than HMB–PIC. On the other hand, HMB, in its crystal structure, forms a very weak π···π stacking interaction. A thermal expansion study shows that thermal expansion along the stacking direction is highest in HMB, which is followed by HMB–PIC and then by HMB–TCB complexes. Therefore, this study shows that stronger π···π stacking interaction leads to a weaker thermal expansion in the materials.

Three different approaches of statistical analyses, after surface area corrections, on the C–X···X–C (X = halogen) and C–H···X–C interactions have been performed. The analyses suggest that with respect to the accessible surface area on H and X, the C–H···X–C interactions are preferable over C–X···X–C interactions for all the halogens in the organic crystals. This is in contrast to the previous statistical analysis which stated that lighter halogens prefer C–H···X–C interactions whereas heavier halogens prefer C–X···X–C interactions. We have shown here the origin of the difference from the previous analysis. We also have shown here why it gives the impression that lighter halogens generally do not go for C–X···X–C interactions, but when they are replaced with the heavier halogens in the same molecule, the probability of formation of the C–X···X–C contacts increases. The preferred geometry of the C–X···X–C interactions with respect to the two ∠CXX angles and X···X distance have been shown. This study suggests that the high frequency of the C–X···X–C interactions for the heavier halogens is mainly due to their large surface area rather than their interaction strength.

The refcode list for the structures containing C, H, F, and O elements and some relevant tables (PDF)

It has been shown that the thermal expansion would be higher in a direction along which the molecular width is shorter and it would be smaller if the molecular width is longer along that direction.

Charge transfer complexes, made of suitably chosen electron-rich and electron-deficient components, can perform as vanishing colour, disappearing–reappearing colour and colour changing materials.