SEMIEMPIRICAL CALCULATIONS ON THE OPTICAL-ABSORPTION OF METHYLVIOLOGEN AND P-CYANOPHENYLVIOLOGEN IN DIFFERENT OXIDATION-STATES

Viologen compounds are known as materials suitable for electrochromic displays based on the electrochemical color change. INDO/S, PM3 and MNDOC calculations have been carried out on methylviologen and p-cyanophenylviologen in order to interpret the spectral changes in the visible region upon electrochemical reduction. The viologens investigated are colorless in the dication state, but they exhibit an intense optical absorption band in the visible region due to the formation of monocation radicals upon reduction. Molecular geometries are optimized by the AM1 and PM3 methods for the initial dication state, monocation radical state and fully-reduced neutral state. Bipyridyl rings in both methylviologen and p-cyanophenylviologen are considerably twisted by about 40-50 degrees in the initial state, but these become almost planar in the first-reduced state and completely planar in the second-reduced state. Optical absorptions are calculated on the basis of the optimized geometry. The results show a satisfactory agreement with experimental values, although the calculated longest-wavelength absorption in the visible region is overestimated towards longer wavelength. The planarity of the bipyridyl rings as well as the resulting delocalization of the unpaired electron of the free monocation radical is found to be important to cause the appearance of the optical absorption band in the visible region.