An analysis of the photo-reactivity of monoazo reactive dyes derived from H-acid and related naphthalene sulfonic acids using the PM5 method

The reactivities (k0) of 10 reactive azo dyes derived from H-acid (eight) and related naphthalene sulfonic acids (two) on cellulose films toward singlet oxygen (1O2) were determined by exposing the dyed films immersed in aerobic Rose Bengal solution to a carbon arc. The k0 values were analyzed in terms of frontier orbital theory using the semiempirical molecular orbital PM5 method, and were confirmed to correlate to the electrophilic frontier density, fr(E), at the double bonds with high electron density in HOMO for the predominant tautomers of the dyes on cellulose. The azo–hydrazone tautomerism of the dyes examined was analyzed by calculating the standard heat of formation in the gas phase and water using the PM5 method. The modes of reaction between the azo dyes and 1O2 were the ene and/or the [2 + 2] cycloaddition reaction. The reactivities were expressed as the sum of fr(E) at the corresponding double bonds. The plots of log k0 against the sum of fr(E) for all of the dyes examined yielded a close correlation line, and this was considered to be the molecular descriptor of the reactivity of aromatics against 1O2.