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 photoc hemical reaction with singlet molecular oxygen at the azo group for an amin opyrazolinyl azo dye (Yellow-I) and for some hydroxy azo dyes. Structure op timization to give the minimum energy of formation showed that Yellow-I exi sts more predominantly in the azo form than the hydrazone one in vacuum. Th e 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 hy drazone tautomer, as well as to the carbon-carbon double bond in the pyrazo line ring of the azo tautomer to give the same end product, i.e. hydroperox ide. The facility with which a dye is photooxidized by singlet oxygen is de pendent 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 o f the superdelocalizability at the carbon atoms on which azo groups are sub stituted for hydroxylated and aminated aromatics. (C) 1999 Elsevier Science Ltd. All rights reserved.