Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford , South Parks Road, Oxford OX1 3QZ, U.K.
J Am Chem Soc. 2014 Feb 12;136(6):2650-8. doi: 10.1021/ja4121814. Epub 2014 Jan 30.
Understanding how molecular structure and environment control energy flow in molecules is a requirement for the efficient design of tailor-made photochemistry. Here, we investigate the tunability of the photochemical and photophysical properties of the retinal-protonated Schiff base chromophore in solution. Replacing the n-butylamine Schiff base normally chosen to mimic the saturated linkage found in nature by aromatic amines results in the reproduction of the opsin shift and complete suppression of all isomerization channels. Modification of retinal by directed addition or removal of backbone substituents tunes the overall photoisomerization yield from 0 to 0.55 and the excited state lifetime from 0.4 to 7 ps and activates previously inaccessible reaction channels to form 7-cis and 13-cis products. We observed a clear correlation between the presence of polarizable backbone substituents and photochemical reactivity. Structural changes that increase reaction speed were found to decrease quantum yields, and vice versa, so that excited state lifetime and efficiency are inversely correlated in contrast to the trends observed when comparing retinal photochemistry in protein and solution environments. Our results suggest a simple model where backbone modifications and Schiff base substituents control barrier heights on the excited-state potential energy surface and therefore determine speed, product distribution, and overall yield of the photochemical process.
了解分子结构和环境如何控制分子中的能量流动，是定制光化学设计的有效要求。在这里，我们研究了视黄醛质子化席夫碱生色团在溶液中的光化学和光物理性质的可调节性。用芳香胺代替通常用于模拟天然中存在的饱和键的正丁胺席夫碱，可重现视蛋白位移并完全抑制所有异构化通道。通过定向添加或去除主链取代基对视网膜进行修饰，可将整体光异构化产率从 0 调节至 0.55，将激发态寿命从 0.4 调节至 7 ps，并激活以前无法进入的反应通道，形成 7-顺式和 13-顺式产物。我们观察到存在可极化主链取代基与光化学反应性之间存在明显的相关性。发现增加反应速度的结构变化会降低量子产率，反之亦然，因此与在蛋白质和溶液环境中比较视黄醛光化学反应时观察到的趋势相反，激发态寿命和效率呈反比关系。我们的结果表明了一个简单的模型，其中主链修饰和席夫碱取代基控制激发态势能表面上的势垒高度，从而决定光化学过程的速度、产物分布和整体产率。
