A semiempirical molecular orbital study on the reaction of an aminopyrazolinyl azo dye with singlet molecular oxygen

Semiempirical MO (PM3) calculations were carried out to evaluate the photochemical reaction with singlet molecular oxygen at the azo group for an aminopyrazolinyl azo dye (Yellow-I) and for some hydroxy azo dyes. Structure optimization to give the minimum energy of formation showed that Yellow-I exists more predominantly in the azo form than the hydrazone one in vacuum. The azo and hydrazone forms of Yellow-I were shown to have similar reactivity in the reaction with singlet oxygen via ene reaction or 1,2-cycloaddition mechanism. Singlet oxygen adds to the carbon¯nitrogen double bond of the hydrazone tautomer, as well as to the carbon¯carbon double bond in the pyrazoline ring of the azo tautomer to give the same end product, i.e. hydroperoxide. The facility with which a dye is photo-oxidized by singlet oxygen is dependent on the properties of the dye and on the azo-hydrazone tautomerism. The ease of photooxidizability for azo dyes can be estimated by the value of the superdelocalizability at the carbon atoms on which azo groups are substituted for hydroxylated and aminated aromatics.