Xu Jun-Jia, Sung Robert, Sung Kuangsen
Department of Chemistry , National Cheng Kung University , Tainan , Taiwan.
J Phys Chem A. 2019 Jun 6;123(22):4708-4716. doi: 10.1021/acs.jpca.9b02924. Epub 2019 May 22.
Any method that can enhance the fluorescence of fluorophores is highly desirable. Fluorescence enhancement accomplished by restricted Z/ E photoisomerization through intramolecular steric hindrance or relatively high bond order of a C═C double bond in a S excited state has rarely been studied. In this article, we used green fluorescent protein (GFP) chromophore analogues as a model to get new physical insights into the restricted Z/ E photoisomerization and E/ Z thermoisomerization phenomena. We found that the S and S potential energy surfaces (PESs) of the GFP chromophore analogues experience two dramatically different types of restricted rotation, and 2b can be a representative example. In its S PES, it is not the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group but the relatively high bond order of the I-bond in the S excited state of 2b that makes it have a higher barrier for the Z/ E photoisomerization, a smaller Z/ E photoisomerization quantum yield, and a higher fluorescence quantum yield. In its S PES, it is not the reduced bond order of the I-bond in the S ground state of 2b but the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group that makes it have an extra higher barrier for E/ Z thermoisomerization and a much smaller E/ Z thermoisomerization rate constant.
任何能够增强荧光团荧光的方法都是非常可取的。通过分子内空间位阻限制Z/E光异构化或S激发态下C═C双键相对较高的键级来实现荧光增强的研究很少。在本文中，我们以绿色荧光蛋白（GFP）发色团类似物为模型，以获得对受限Z/E光异构化和E/Z热异构化现象的新物理见解。我们发现，GFP发色团类似物的S和S势能面（PES）经历了两种截然不同类型的受限旋转，2b可以作为一个代表性例子。在其S PES中，不是面外苄基与面内间吡啶鎓基团之间的分子内空间位阻，而是2b的S激发态下I键相对较高的键级，使其具有更高的Z/E光异构化势垒、更小的Z/E光异构化量子产率和更高的荧光量子产率。在其S PES中，不是2b的S基态下I键的键级降低，而是面外苄基与面内间吡啶鎓基团之间的分子内空间位阻，使其具有额外更高的E/Z热异构化势垒和更小得多的E/Z热异构化速率常数。
