Regioselectice Trans-Cis Photoisomerization of m-Styrylstilbenes

Article abstract of DOI:10.1021/ja00209a031

Trans-cis photoisomerization of m-styrylstilbenes, i.e., 2,4,6-triisopropyl-3'-styrylstilbene (TISS), 2,4,6-trimethyl-3'-styrylstilbene (TMSS), and 3-styrylstilbene (SS), and of stilbenes, i.e., 2,4,6-triisopropylstilbene (TIS), 2,4,6-trimethylstilbene (TMS), and stilbene (S), are studied under direct or benzophenon-sensitized irradiation in hexane.Measurements of quantum yields for isomerization have revealed that although the styrylstilbene molecule bears two styryl groups, the reaction is highly regioselective, depending upon the excitation conditions and reactant structures.For example, isomerizations of trans,trans-TISS and trans,cis-TISS occured either at the 2,4,6-trisubstituted styryl side upon direct excitation or at the unsubstituted styryl side upon sensitized excitation.When the starting material carries an unsubstituted cis-styryl group, the major isomerization always occurred at this moiety by either direct or sensitized axcitation, e.g., cis,trans-TISS ---> trans,trans-TISS, cis,cis-TISS ---> trans,cis-TISS, and cis,trans-SS ---> trans,trans-SS.Furthermore, the photoisomerization of cis-SS was found to be one-way.These results are interpreted in terms of the usual "energy sink" concept: the excited-state energies (E_S and E_T) of the stilbene chromophores depend on molecular distortion in a subtle manner.It seems that an extremely rapid cis ---> trans isomerization rate of the unsubstituted cis-styryl group is also responsible for the observed preferential photoisomerization of this group.Finally, the cis,cis isomers of TISS, TMSS, and SS underwent upon sensitized excitation minor but substantial one-photon two-double-bond isomerization (cis,cis ---> trans,trans) in addition to major one-double-bond isomerization.This reaction is not common, since the two isomerizing double bonds are cross-conjugated.