SINGLET-STATE CIS,TRANS PHOTOISOMERIZATION AND INTERSYSTEM CROSSING OF 1-ARYLPROPENES

The temperature dependence of the singlet state lifetime and photoisom erization and fluorescence quantum yields for trans- and cis-1-phenylp ropene have been determined in hexane solution. Calculated barriers fo r twisting about the double bond on the singlet potential energy surfa ce are 8.8 and 4.6 kcal/mol for the trans and cis isomer, respectively . The barrier for the trans isomer is sufficiently high to prevent iso merization on the singlet state surface at or below room temperature. However, isomerization occurs at low temperatures as a consequence of intersystem crossing to the triplet state, which undergoes barrierless isomerization. The quantum yield for intersystem crossing, as determi ned by time-resolved photoacoustic calorimetry, is 0.60 +/- 0.03 and t he rate constant for intersystem crossing is 4.7 x 10(7) s(-1). While internal conversion is not significant at or below room temperature, t hermally activated internal conversion competes with singlet isomeriza tion at high temperatures. The cis isomer undergoes isomerization pred ominantly via the singlet state at room temperature. Both electron-don ating (p-methoxy) and electron-withdrawing (m- and p-cyano, p-carbomet hoxy, and p-trifluormethyl) aromatic substituents are found to lower t he barrier for singlet state isomerization. Increased solvent polarity (acetonitrile vs hexane) results in variable decreases in the barrier for singlet state isomerization. Photoisomerization of the p-cyano de rivative at room temperature occurs predominantly via the triplet stat e in hexane solution and via the singlet state in acetonitrile solutio n. The effects of substituents and solvent are better correlated with the magnitude of the S-2-S-1 energy gap than the stability of either z witterionic or biradical intermediates. Rate constants for intersystem crossing are, in most cases, not highly dependent upon aromatic subst itution or solvent polarity.