Saltiel Jack, Krishna Tallapragada S R, Laohhasurayotin Somchoke, Fort Katy, Clark Ronald J
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, USA.
J Phys Chem A. 2008 Jan 17;112(2):199-209. doi: 10.1021/jp077342c. Epub 2007 Dec 22.
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.
在室温下研究了结晶或粉末状的顺式，顺式-1,4-二苯基-1,3-丁二烯（cc-DPB）和1,4-二（邻甲苯基）-1,3-丁二烯（cc-DTB）向反式，反式异构体的光异构化反应。通过荧光光谱、粉末X射线衍射、¹H NMR和高效液相色谱监测反应进程。cc-DPB向反式，反式异构体的转化率高达90%，cc-DTB的转化率为20%。在固态中，顺式，反式异构体的形成被完全抑制，而顺式，反式异构体是在溶液中和77 K下非常粘稠的玻璃态介质中得到的唯一产物。观察到的双键光异构化反应可通过自行车踏板（BP）光异构化机理来解释。X射线结构测定表明，邻甲基取代导致苯基/二烯二面角从40度扩大到56度，并使晶体中构象异构体的数量从cc-DPB中的两个减少到cc-DTB中的一个。cc-DPB分子的两种构象异构体以面对面交替排列的形式存在于晶体中，其中一种构象异构体的两个苯基在平行平面内，另一种在大致垂直的平面内。在cc-DTB中保持了面对面的关系，但只存在邻甲苯基在平行平面内排列的构象异构体。尽管分子构象和晶体堆积排列不同，但在光反应过程中测得的荧光光谱的时间演变显示出显著的相似性。光谱矩阵的主成分分析表明形成了离散成分，这表明双键光异构化反应分阶段进行，涉及不同微晶环境中的分子。初始荧光光谱无结构的外观表明反应部分是非绝热的。如果自行车踏板运动始于激发态S(1)并在基态S(0)完成，BP机理可以解释这些观察结果。对晶格中空隙空间的分析表明，cc-DPB分子的堆积比cc-DTB分子松散得多，这可能解释了cc-DPB向光产物的转化率高得多的原因。
