Methoxy-substituted stilbenes, styrenes, and 1-arylpropenes: photophysical properties and photoadditions of alcohols

The photochemistry of trans-stilbene and four methoxy-substituted stilbene derivatives has been investigated in a variety of solvents. The fluorescence of all five trans isomers was quenched by 2,2,2-trifluoroethanol (TFE). Upon irradiation of the five substrates in TFE, the products derived from photoaddition of the solvent were detected. Nuclear magnetic resonance spectroscopy of the products formed by irradiation in TFE-OD indicated that the proton and nucleophile are attached to two adjacent atoms of the original alkene double bond. Irradiation of the corresponding methoxy-substituted styrenes and trans-1-arylpropenes in TFE produced the analogous solvent adducts. The photoaddition of TFE proceeded with the general order of reactivity: styrenes > trans-1-arylpropenes > trans-stilbenes. Transient carbocation intermediates were observed following laser flash photolysis of the stilbenes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The results are consistent with a mechanism that involves photoprotonation of the substrates by TFE or HFIP, followed by nucleophilic trapping of short-lived carbocation intermediates. Compared to the other stilbene derivatives, trans-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. The unique photophysical properties of trans-3,5-dimethoxystilbene are attributed to formation of a highly polarized charge-transfer excited state (mu(e) = 13.2 D).