Photocycloaddition of the title compound 1 which contains two coumarin groups linked by a long-chain ether has been studied in this article. It was found that 1 Shows different photo chemical behaviours from that of monomeric coumarin, i.e., in the concentration less than 10^-^4mol·L^-^1 and in the nonpolar solvent intramolecular photo- dimerization can occur, the steric structure of the product was determined by the spectra of ^1H NMR, ^1^3C NMR and 2D-J, which show it as an intramolecular syn-head to head dimer. Further investigation indicated that the stereospecific fashion of the photodimer is independent of the solvent polarity wa well as triplet sensitizer. The photocycloaddition mechanism of compound 1 was preliminaryly discussed.

Key words: PHOTOSENSITIZATION, COUMARIN, CHROMOPHOR, LONG CHAIN COMPOUND