ROTATIONAL BARRIERS AND CONJUGATION IN POLYPHENYLENE OLIGOMERS

The effective conjugation length in polymers is defined by the degree of pi overlap of adjacent monomer units. Rotations around the carbon-c arbon single bonds that connect monomers cause misalignment of p orbit als, resulting in decreased electrical conductivity and shifts of the polymer absorption and luminescence to higher energies. In polyphenyle nes, steric interactions between ortho protons on adjacent aromatic ri ngs forces the rings to rotate and deviate from planarity. To understa nd the effect of ring rotation on the electronic properties of polyphe nylenes, we prepared exact length polyphenylene oligomers with 1, 2 an d 6 carbon side chains. The barriers to ring rotation were calculated using molecular mechanics programs, and were measured using variable t emperature NMR experiments. The measured barriers, ranging from 18-20 Kcal/mol, were dependent on the length of the side chain but independe nt of the phenylene chain length. The calculated barriers were slightl y lower but followed the same trend. Luminescence spectra obtained for a series of oligomers with increasing chain length show that the effe ctive conjugation length in substituted polyphenylenes is at least 4 r ings and is also dependent on the length of the side chain.