Rotational barriers and conjugation in polyphenylene oligomers

The effective conjugation length in polymers is defined by the degree of π overlap of adjacent monomer units. Rotations around the carbon-carbon single bonds that connect monomers cause misalignment of p Orbitals, resulting in decreased electrical conductivity and shifts of the polymer absorption and luminescence to higher energies. In polyphenylenes, steric interactions between ortho protons on adjacent aromatic rings forces the rings to rotate and deviate from planarity. To understand the effect of ring rotation on the electronic properties of polyphenylenes, we prepared exact length polyphenylene oligomers with 1, 2 and 6 carbon side chains. The barriers to ring rotation were calculated using molecular mechanics programs, and were measured using variable temperature NMR experiments. The measured barriers, ranging from 18–20 Kcal/mol, were dependent on the length of the side chain but independent of the phenylene chain length. The calculated barriers were slightly lower but followed the same trend. Luminescence spectra obtained for a series of oligomers with increasing chain length show that the effective conjugation length in substituted polyphenylenes is at least 4 rings and is also dependent on the length of the side chain.