Homo-lumo energy gap analysis of alkyl viologen with a positively charged aromatic ring

Abstract

In order to confirm the effects of positive charges of alkyl viologens (CnVCn, n = 3, 5, 7 and 9) with aromatic rings, the changes in the HOMO-LUMO energy gap have been studied using the cyclic-voltammetry, the UV-visible spectra, and the calculations of the AM1 method. The electron transport across molecules has been explained by assuming that the incoming electron passes through the lowest unoccupied molecular orbital. The conduction barrier was determined from the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy states. The values of the HOMO-LUMO energy gap of alkyl viologen, which can be obtained from the cyclic-voltammogram characteristics, were found to be in the range of 5.02 ？ 4.86 eV. These values were similar to those obtained from the UV-visible spectra and the AM1 method. As the chain length of alkyl viologen increased, the HOMO-LUMO energy gap decreased. In addition, the positive charge on the aromatic ring lowered the energies of the π levels, while the effect of the positive charge on the alkyl chain decreased with increasing the alkyl chain length