CRYSTAL-STRUCTURES, PHASE-TRANSITIONS AND ENERGY CALCULATIONS OF POLY(P-PHENYLENE) OLIGOMERS

The room temperature crystal structures, unit cell dimensions at 110 K and phase transitions of three poly(p-phenylene) oligomers are report ed. The structures of p-quinquephenyl (PQP), C30H22, p-sexiphenyl (PSP ), C36H26, and p-septiphenyl (PSePtiP), C42H30, each belonging to spac e group P2(1)/c, are similar to those of shorter oligomers. The unit c ell dimensions are a=22.056 angstrom, b=5.581 angstrom, c=8.070 angstr om and beta=97.91-degrees for PQP a=26.241 angstrom, b=5.568 angstrom, c=8.091 angstrom and beta=98.17-degrees for PSP, and a=30.577 angstro m, b=5.547 angstrom, c=8.034 angstrom and beta=100.52-degrees for PSep tiP. The a axis increases with molecular length. The molecules are lin ear and planar in all three structures. The herringbone nature of the packing is similar for PQP and PSeptiP, while a considerably greater t ilt occurs in PSP. At 110 K, the unit cell parameters b and c are appr oximately doubled while a remains nearly the same as in the room tempe rature cell. A time-dependent solid state transition is observed for P QP, PSP and PSeptiP when crystals are cooled to 110 K. At elevated tem peratures, thermal measurements indicate the oligomers to be thermotro pic liquid crystals. The crystal-smectic transition temperatures are r eported for PQP, PSP, PSeptiP and p-octiphenyl (POP), C48H34. The resu lts of a molecular mechanics study on the conformation and packing of PSP are also presented. The competition between intramolecular forces (such as ortho hydrogen repulsions) and intermolecular crystal packing forces was examined in particular. Molecular mechanics calculations p redict non-planar conformations in isolated polyphenyls, implying that conjugation between phenyl rings is insufficient to overcome ortho hy drogen repulsions. In a crystalline environment, however, intermolecul ar forces tend to force a planar conformation. Calculations on arrays of PSP molecules show that changing the phenyl-phenyl torsion angles f rom the coplanar value increases the total energy of the structure. Th e most favourable intermolecular interactions between oligomers are ac hieved for conformations having the phenyl rings coplanar.