UV-visible absorption spectra and luminescence of the pesticide fenarimol

The UV-visible absorption and emission spectra have been measured of the pesticide fenarimol ((+/-)-2,4'-dichloro-alpha-(pyrimidin-5-yl)-benzhydryl alcohol) in solution. From comparison with the spectra of chlorotoluenes and pyrimidine, and from the effect of solvent polarity on the absorption spectrum, it is shown that the lowest excited singlet state is localized on the pyrimidine ring, and has n,pi* character. Higher excited pi,pi* states are localized on both chlorotoluene and pyrimidine rings. Fenarimol shows a weak, fluorescence from the n,pi* state, with a quantum yield which is strongly dependent on solvent. It is shown that this is due to changes in the nonradiative decay rate, particularly in protic solvents, due to increased intersystem crossing. Phosphorescence is observed in low temperature glasses. Although this shows two decay components, it is suggested that only one triplet state is involved, and that this has predominantly pi,pi* character. (C) 1997 Elsevier Science B.V.