Fuciman Marcel, Keşan Gürkan, LaFountain Amy M, Frank Harry A, Polívka Tomáš
Institute of Physics and Biophysics, Faculty of Science, University of South Bohemia , Branišovská 1760, 37005 České Budějovice, Czech Republic.
J Phys Chem B. 2015 Jan 29;119(4):1457-67. doi: 10.1021/jp512354r. Epub 2015 Jan 16.
Two carotenoids with aryl rings were studied by femtosecond transient absorption spectroscopy and theoretical computational methods, and the results were compared with those obtained from their nonaryl counterpart, β-carotene. Although isorenieratene has more conjugated C═C bonds than β-carotene, its effective conjugation length, Neff, is shorter than of β-carotene. This is evidenced by a longer S1 lifetime and higher S1 energy of isorenieratene compared to the values for β-carotene. On the other hand, although isorenieratene and renierapurpurin have the same π-electron conjugated chain structure, Neff is different for these two carotenoids. The S1 lifetime of renierapurpurin is shorter than that of isorenieratene, indicating a longer Neff for renierapurpurin. This conclusion is also consistent with a lower S1 energy of renierapurpurin compared to those of the other carotenoids. Density functional theory (DFT) was used to calculate equilibrium geometries of ground and excited states of all studied carotenoids. The terminal ring torsion in the ground state of isorenieratene (41°) is very close to that of β-carotene (45°), but equilibration of the bond lengths within the aryl rings indicates that the each aryl ring forms its own conjugated system. This results in partial detachment of the aryl rings from the overall conjugation making Neff of isorenieratene shorter than that of β-carotene. The different position of the methyl group at the aryl ring of renierapurpurin diminishes the aryl ring torsion to ∼20°. This planarization results in a longer Neff than that of isorenieratene, rationalizing the observed differences in spectroscopic properties.
利用飞秒瞬态吸收光谱和理论计算方法研究了两种带有芳基环的类胡萝卜素，并将结果与它们不含芳基的对应物β-胡萝卜素的结果进行了比较。尽管异菌绿素比β-胡萝卜素具有更多的共轭C═C键，但其有效共轭长度Neff比β-胡萝卜素短。与β-胡萝卜素的值相比，异菌绿素更长的S1寿命和更高的S1能量证明了这一点。另一方面，尽管异菌绿素和玫红菌素具有相同的π电子共轭链结构，但这两种类胡萝卜素的Neff不同。玫红菌素的S1寿命比异菌绿素短，表明玫红菌素的Neff更长。这一结论也与玫红菌素比其他类胡萝卜素更低的S1能量一致。采用密度泛函理论（DFT）计算了所有研究类胡萝卜素基态和激发态的平衡几何结构。异菌绿素基态的末端环扭转角（41°）与β-胡萝卜素的末端环扭转角（45°）非常接近，但芳基环内键长的平衡表明每个芳基环形成了自己的共轭体系。这导致芳基环与整体共轭部分脱离，使得异菌绿素的Neff比β-胡萝卜素短。玫红菌素芳基环上甲基的不同位置使芳基环扭转角减小至约20°。这种平面化导致玫红菌素的Neff比异菌绿素更长，从而合理解释了观察到的光谱性质差异。
