THE EFFECT OF MOLECULAR ENVIRONMENT ON THE PHOTOCHEMISTRY OF P-METHOXYCINNAMIC ACID AND ITS ESTERS

In addition to rapid, reversible trans-cis photoisomerisation, air-sat urated solutions of p-methoxycinnamic acid and 2-ethylhexyl-p-methoxyc innamate in polar solvents undergo a slower, irreversible photoreactio n when exposed to ultraviolet radiation. However, no such reaction is evident in polar solvents that are deoxygenated or in air-saturated, n on-polar solvents. The fluorescence spectra were also dependent on the solvent polarity with a shift to longer wavelengths in polar solvents . This behaviour suggests a degree of charge transfer character in the lowest excited singlet state responsible for fluorescence in a polar molecular environment. The fluorescence yields of solutions of these c innamoyl molecules were low, less than or equal to 1.6 x 10(-3), indic ating that there are very efficient non-radiative pathway/s for nonrad iative relaxation of the excited singlet states of these molecules. (C ) 1998 Elsevier Science S.A. All rights reserved.