Using semi-empirical quantum mechanic (AMI, ZINDO/S) and ab-initio Hartree-Fock (HF) calculations we investigated selected structural, electronic and vibrational properties of phenylene-based oligomers from biphenyl P2P to para-sexiphenil P6P. Special attention is paid to the interplay of electrostatic and conjugation effects on torsion barrier, energy gap, charge distribution and selected vibrational modes, and their dependence on torsion angle and oligomer size.