Dipole Parallel-Alignment in the Crystal Structure of a Polar Biphenyl: 4'-Acetyl-4-Methoxybiphenyl (AMB).

4'-Acetyl-4-methoxy-biphenyl (AMB) was synthesized via catalytic Suzuki cross-coupling reaction and the crystal structure was determined. The crystals of AMB feature an orthorhombic unit cell with a = 7.900(6) A, b = 5.429(4) A, and c = 26.80(2) A and the crystals are noncentrosymmetric, space group Pna2(1). The crystal structure of AMB features parallel-alignment of essentially planar molecules in the syn-conformation with molecular dipole moments of about 5.4 Debye and affords about 53% of maximal polar alignment. The polar crystal structure of syn-AMB contrasts with the nonpolar crystal structure of the homologue 4-methoxyacetophenone syn-MAP. The fundamental difference between the packing motives of AMB and MAP is explained on the basis of lateral arene-arene interactions. The lattice architecture of AMB is compared to the motives realized in the only two other parallel-aligned polar biphenyls, 4'-n-butoxy-4-cyanobiphenyl (nBCB) and 4'-dimethylamino-4-cyanobiphenyl (DMACB). The analysis provides conceptual insights to guide the rational design of polar materials and several resulting hypotheses are stated explicitly and can be tested.