Photoisomerization of cis,cis- to trans,trans-1,4-diaryl-1,3-butadienes in the solid state: the bicycle-pedal mechanism.

The photoisomerizations of crystalline or powdered cis,cis-1,4-diphenyl- and 1,4-di(o-tolyl)-1,3-butadienes (cc-DPB and cc-DTB) to the trans,trans isomers were studied at room temperature. The progress of the reactions was monitored by fluorescence spectroscopy, powder X-ray diffraction, (1)H NMR, and high-performance liquid chromatography. Conversions to the trans,trans isomers were as high as 90% for cc-DPB and 20% for cc-DTB. Formation of the cis,trans isomers, the sole products obtained in solution and in very viscous glassy media at 77 K, is completely suppressed in the solid state. The observed two-bond photoisomerizations are explained by the bicycle-pedal (BP) photoisomerization mechanism. X-ray structure determinations show that o-methyl substitution causes a widening of the phenyl/diene dihedral angles from 40 degrees to 56 degrees and decreases the number of conformers in the crystal from two in cc-DPB to one in cc-DTB. The two conformers of cc-DPB molecules exist in crystals in edge-to-face alternating arrays, one of which has the two phenyls in parallel planes and the other in roughly perpendicular planes. The edge-to-face relationship is maintained in cc-DTB, but only the conformer with the o-tolyl groups in parallel planes is present. The time evolutions of fluorescence spectra measured in the course of the photoreaction show remarkable similarities, despite the different molecular conformations and crystal packing arrangements. Principal component analyses of the spectral matrices indicate the formation of discrete components, suggesting that the two-bond photoisomerizations proceed in stages involving molecules in different microcrystal environments. The structureless appearances of the initial fluorescence spectra show that the reactions are in part diabatic. The BP mechanism can account for the observations if the bicycle-pedal motion began in the excited state, S(1), and were completed in the ground state, S(0). Analysis of void spaces in the crystal lattice reveals much less compact packing of cc-DPB than of cc-DTB molecules, possibly explaining the much higher conversions to photoproduct from cc-DPB.