PHOTOISOMERIZATION OF 4-HYDROXYBENZONITRILE INTO 4-HYDROXYBENZOISONITRILE

In deoxygenated water, methanol, and ethanol, 4-hydroxybenzonitrile (4 -HEN) is photoisomerized into 4-hydroxybenzoisonitrile (4-HBIN), which is then hydrolyzed into 4-hydroxyformanilide in acidic medium. In sli ghtly acidic (pH 5.4) or moderately alkaline (pH 9.4) solutions as wel l as in alcohols, the reaction proceeds with a chemical yield exceedin g 85%. The triplet-triplet absorption of 4-HBN (lambda(max)=300 nm) is detected by transient absorption spectroscopy; the intersystem crossi ng quantum yields are phi(isc)=0.14 in neutral water and phi(isc)=0.45 in ethanol. The triplet is converted into long-lived transients absor bing in the far UV. The cyanophenolate ion (lambda=275 nm) is transien tly produced upon excitation of moderately acidic solutions, with a qu antum yield of 0.082; this process is possible because of the high aci dity of the excited singlet. The analysis of the kinetics of 4-HBIN fo rmation as a function of irradiating photon flux shows that the photoi somerization of 4-HBN is a two-stage photoprocess. According to triple t-quenching studies, the first stage proceeds via the 4-HBN triplet to yield an intermediate capable of absorbing a second UV photon, which then gives 4-HBIN in the second stage. Mechanistic considerations indi cate that this intermediate is likely to be an azirine: [GRAPHICS]