Protonation of bipyridines and their vinylene-phenylene-vinylene derivatives: Theoretical analysis of the positive charge effects

Torsion analysis of the bipyridine (BPy) segment was carried out on the free and protonated bipyridines (head-to-head) as well as their vinylene-phenylene-vinylene (VPV) derivatives with semiempirical AM1 calculations to determine the configuration of protonated species. The hydrogen bonding energy has been estimated through comparison of the energy barriers with those of the free base form. Intramolecular hydrogen bonding plays an important role in the ring torsion where one nitrogen atom is protonated. Torsion potentials are prodominated by repulsion between the positive charges where both nitrogen atoms are protonated. Proton bonding to nitrogen has strong electron withdrawing effect. It stabilizes the HOMO and LUMO, with the second being the most affected. Consequently, protonation decreases the HOMO/LUMO energy gap. These protonation effects will be gradually reduced as the counterions come closer to the protons. The first transition energies, were calculated with ZINDO/SCI methods and compared with the AM1 calculated HOMO/LUMO energy gaps at various torsion angles. The theoretical results support the ion-induced as well as protonation induced red-shifting of both the absorption and emission maximum of bipyridine contained polymers.