Molecular complexes of 4, 4'-dinitrobiphenyl

Abstract

This study attempted to correlate the chemical and physical properties of the para disubstituted and para monosubstituted biphenyl complexes of 4,4'-DNBP with their structural and electronic properties, using a variety of physical and spectroscopic techniques, and X-ray diffraction methods. These complexes drew considerable attention in the past due to their capacity to demonstrate intense colours from yellow to dark red, which are dissimilar to the colour combination of their parent compounds. This prompted a series of investigations in the past through which the full structure of only Complex of 4,4'-Dinitrobiphenyl with 4- Hydroxybiphenyl (C-4OHBP) by single crystal diffraction was determined and the infrared and Raman spectra of C-4OHBP, Complex of 4,4'-Dinitrobiphenyl with Biphenyl (C-BP) and Complex of 4,4'-Dinitrobiphenyl with 4-Bromobiphenyl (C-BrBP) were obtained. In this study, other para-disubstituted biphenyl complexes with 4,4'-DNBP were investigated to add to the existing knowledge base and to study the effects of varying the substitution on the aromatic rings of the donor. Because the complete solid state structures of the complexes in this study (except C-4OHBP) and the donor compound BZ are not known, their single crystal determination was attempted. Unfortunately, no suitable diffraction quality crystals could be crystallised. Although the X-ray crystal structure of 4,4'-DNBP was determined by photographic methods by Van Niekerk and Boonstra, the R-factor of this solution was relatively high at 15%, which is outside the current internationally accepted standard. Because the current technology using automated diffractometers is superior to photographic techniques used by Van Niekerk and Boonstra, it was decided to repeat the crystallographic analysis for 4,4’-DNBP. However, despite repeated efforts with a large variety of solvents used during the crystallisation process in this study, the crystals formed were of a relatively poor quality, resulting in a solution with a final R-factor of also only 15%. Therefore, the only crystallographic data available for correlation with the physical properties of the complexes have been that of the uncomplexed compounds (except BZ) and C-4OHBP from the previous studies. Based on these results, the comparison of the dihedral angles between the two aromatic rings of the parent compounds with their infrared and Raman spectra yielded that, unlike the conventional compounds the centrosymmetricity of the biphenyl derivatives cannot be uniquely determined by their infrared and Raman spectra. Using thermogravimetric measurements, melting points and phase transitions of each pure component and as well as the complexes were obtained. Packing energy in the complex seems to be relatively more favourable than in the parent components alone. Otherwise, two phase transitions could be expected, one resulting from 4,4'-DNBP and one resulting from the other component in the complex. Complex of 4,4'-Dinitrobiphenyl with 4,4'-Dihydroxybiphenyl (C-44DiOHBP) and Complex of 4,4'-Dinitrobiphenyl with Benzidine (C-BZ) showed some unexpected high melting points in their thermogravimetric studies. These elevated melting points are interpreted as resulting from hydrogen bonding, and infrared studies of these complexes confirmed this interpretation. Conductivity measurements in the solid state revealed that only tmethylbz exhibits a measure of charge transfer. The absence of current in both its complexes (1:1 and 1:4 ratio) with 4,4'-DNBP was attributed to the close-packing and twisting of the components in the complex which prevented the electron flow through the complexes. Because ultraviolet-visible, infrared and Raman spectra showed only small shifts in the pure compounds after complexation in solution, and their conductivity measurements revealed no current flow, the interactions in those complexes are ascribed mainly to Van der Waals forces. The previously assigned molecular ratios are incorrect. This study has reassigned these molecular ratios using nuclear magnetic resonance techniques. Nuclear magnetic resonance spectroscopy detected only very small chemical shifts in the pure compounds after they formed complexes in solution. As these solutions did not maintain the colour of complexation, this could be seen as supportive proof that all the interactions involved in the formation of these complexes are very weak. The same conclusion as the previous studies has been reached, notably that the molecular ratios, in which the components of these complexes unite, are caused almost exclusively by packing factors and are stabilised by weak interactions.