Theoretical studies on the electronic structure and spectra of the merocyanines

Abstract

Molecular orbital calculations are reported on the structure and electronic properties of the simple merocyanine 4-[(1-methyl-4(1 H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (which can also exist as a zwitterion) using the AM1, CNDOVS and the ab initio STO-3G and 3-21G methods. The AM1 optimisation method invariably produces the lower energy quinone as the favoured structure, but the presence of ionic charges or sparkles positioned above and below the nitrogen and oxygen atoms of the merocyanine are sufficient to force the structure into a higher energy zwitterion form. Spectroscopic calculations on the AM1 structures in the gas phase using the CNDOVS method predict that the zwitterion will absorb at a longer wavelength (587 nm) than the quinone (470 nm), contradicting previous theoretical studies.