supplying the light energy a high pressure mercury vapor lamp of 400 watts is used 5 g of sulfurised heterocycle compound are suspended in 500 cm 3 of triethylphosphite by magnetic stirring and a current of nitrogen the period of irradiation is 2 hours during which a cooling flow of water is necessary too long a period of irradiation reduces the yield there is complete dissolution the solvent is evaporated under slight vacuum 20 mm and the evaporation is terminated under more intense vacuum the residue is taken up again with a little acetone 10 ml and it is filtered the residue recovered 3 g is recrystallised in toluene 70 ml 2 2 g of white product is obtained with correct melting point 285 u00b0 c with a nmr spectrum also in accordance with the literature however this product contains an impurity which is fluorescent in uv which is not removed by sublimation its elimination is done by filtration over silica a solution of 2 g in 300 ml of benzene is passed over a column of 2 cm diameter filled with 15 g of silica the impurity does not migrate to recover the whole of the paracyclophane it is necessary to elute with 300 ml of benzene to increase the yields recrystallisation in toluene is done after grouping together from several irradiations from the residues of recrystallisation in toluene.