The ultraviolet absorption spectra of 2,2′-biphenyls with bridges containing ether, sulphide, and sulphone groups
The u.v. spectra have been measured for a series of 2,2′-bridged biphenyls, with ether, sulphide, or sulphone groups at both ends of a bridge containing up to five methylene groups. The spectra indicate that the dihedral angles between the planes of the phenyl rings are determined not only by the length and rigidity of the bridging group but also by the nature of the functional groups at the ends of the bridge. The tendency to maintain the phenyl rings of the biphenyl system antiplanar increases in the order ether < methylene < sulphide < sulphone. The characteristic biphenyl band is present in the spectra of all the ethers. However, the repulsive forces between the sulphone groups inhibit biphenyl conjugation and are probably sufficient to produce a preference for a transoid conformation when the bridge contains four or five methylene groups.