Theoretical study of the structure and torsional potential of substituted biphenylenes and their fluorene derivatives

Conformational analysis was carried out on the methyl and methoxy substituted biphenyls as well as their fluorene derivatives by ab initio Hartree-Fock (HF), second-order Møller-Plesset (MP2) and density functional theory (DFT) methods using the 6-31G* basis set. Both MP2 and B3LYP increase the relative height of the perpendicular energy barriers by electron correlation corrections to the self-consistent field torsional potentials of methyl and methoxy substituted biphenyls. All three methods predict the enhancement of the conjugation effects when going from biphenyls to the corresponding fluorene derivatives. All three methods lead essentially to identical descriptions of torsion potentials, with the MP2 results closer to the HF results. The theoretical results can be well correlated to the shifts in maximum absorption wavelength of related polymers.