Stronger π•••π Interaction Leads to a Smaller Thermal Expansion in Some Charge Transfer Complexes

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 complex than HMB-PIC. On the other hand, HMB, in its crystal structure, forms a very weak pi center dot center dot center dot pi 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 pi center dot center dot center dot pi stacking interaction leads to a weaker thermal expansion in the materials.