The ethylenically unsaturated organic compounds are of the general formula ##STR1## where R is alkyl, aryl or a radical R.sup.1, and R.sup.1 is a radical ##STR2## where R.sup.2 to R.sup.6 are each H, alkyl, OH, Oalkyl, SH, Salkyl, halogen, N(alkyl).sub.2 or N(alkyl)(aryl), and not less than one but not more than three of the radicals R.sup.2 to R.sup.6 are each a radical ##STR3## where X is alkylene or oxaalkylene, each of 2 to 10 carbon atoms, and Y is H or CH.sub.3. These compounds have particularly high photochemical reactivity in the medium-wavelength to relatively long-wavelength UV range.