Synthesis, Structure, and Spectral Behavior of Donor-Acceptor Substituted Biphenyls

Article abstract of DOI:10.1021/jo00172a040

Reaction of the activated halonitrobenzenes 2a-f with the bis- and tris(dialkylamino)benzenes 1a-e affords, via a direct nucleophilic substitution, the highly substituted biphenyls 3a-o; the lesser substituted biphenyls 5 and 6 were prepared by an Ullmann reaction.All these biphenyls are deeply colored; the dark red color can be assigned to an intramolecular charge transfer.A crystal structure determination was carried out for 2,4,6-tripyrrolidino-2',4',6'-trinitrobiphenyl (3a): space group C2/c, a = 16.071 (2) Angstroem, b = 14.545 (1) Angstroem, c = 20.177 (2) Angstroem, β = 91.361 (9)deg, Z = 8 (temperature ca. 120 K).The dihedral angle between the two arene rings was found to be only 52.5 deg, despite the four bulky substituents in the o,o'-positions.With this far-from-orthogonal torsional angle about the biphenyl linkage, the strong intramolecular charge transfer from the ? system of the donor into the ? system of the acceptor arene becomes easily understandable.A PPP calculation with the torsional angles taken from the X-ray structure analysis satisfactorily reproduces the experimental absorption spectrum of 3a.The shift of the long-wavelength absorption between the individual biphenyls 3a-o, 5, and 6 likewise is accounted for satisfactorily by the calculation; it depends primarily on the intrinsic donor strength of the different NR2 moieties (pyrrolidino > dimethylamino > piperidino > morpholino).The 1H NMR spectra (in dilute solution) likewise mirror this gradation in NR2 donor capacity; they also show that steric hindrance at the biphenyl linkage is mainly the result of interaction between the o-dialkylamino groups and the C6 skeleton of the acceptor arene.This is borne out by the crystal structure analysis.