Synthesis and comparative photoswitching studies of unsymmetrical 2,3-diarylcyclopent-2-en-1-ones

Photochromic diarylethenes (DAEs) based on the unsymmetrical ethene "bridge" bearing heterocycles of the different nature (oxazole and thiophene) as aromatic moieties have been designed and the photoswitching properties have been studied. The comparative studies of the photochromic characteristics of unsymmetrical isomeric 2,3-diarylcyclopent-2-en-1-ones have shown that the isomers have different thermal stability, absorption maxima, and quantum yields. It was found that the unsymmetrical diarylcyclopentenone bearing at second position of the cyclopentenone ring the thiophene unit displays high thermally stability, hypsochromic shift of absorption maxima wavelengths of initial and cyclic forms, and high quantum yields of cyclization and cycloreversion reactions. The replacement of the carbonyl group with oxime leads to a reduction of the difference in the photochromic properties of these isomers and just as the reduction of the carbonyl group to the hydroxy-group negates this difference to zero. The intramolecular hydrogen bond formation in the oxime and hydroxy derivatives was confirmed by IR and (1)H NMR spectral analysis, but the increase of the quantum yields of the cyclization reaction in a nonpolar hexane is observed only in the case of hydroxy derivatives that can be explained by the formation of more rigid six-membered heterocycle in hydrogen bonding.