Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Gronostajowa 7, 30387 Kraków, Poland.
Faculty of Technical Physics, Poznan University of Technology, Piotrowo 3, 60965 Poznan, Poland.
J R Soc Interface. 2019 Sep 27;16(158):20190191. doi: 10.1098/rsif.2019.0191. Epub 2019 Sep 4.
In carotenoids, by analogy to polyenes, the symmetry of the π-electron system is often invoked to explain their peculiar electronic features, in particular the inactivity of the S → S transition in one-photon excitation. In this review, we verify whether the molecular symmetry of carotenoids and symmetry of their π-electron system are supported in experimental and computational studies. We focus on spectroscopic techniques which are sensitive to the electron density distribution, including the X-ray crystallography, electronic absorption, two-photon techniques, circular dichroism, nuclear magnetic resonance, Stark and vibrational spectroscopies, and on this basis we seek for the origin of inactivity of the S state. We come across no experimental and computational evidence for the symmetry effects and the existence of symmetry restrictions on the electronic states of carotenoids. They do not possess an inversion centre and the C symmetry approximation of carotenoid structure is by no means justified. In effect, the application of symmetry rules (and notification) to the electronic states of carotenoids in this symmetry group may lead to a wrong interpretation of experimental data. This conclusion together with the results summarized in the review allows us to advance a consistent model that explains the inactivity of the S → S transition. Within this model, S is never accessible from S due to the negative synergy of (i) the contributions of double excitations of very low probability, which elevate S energy, and (ii) a non-verticality of the S → S transition, due to the breaking of Born-Oppenheimer approximation. Certainly, our simple model requires a further experimental and theoretical verification.
在类胡萝卜素中，通过类比多烯，可以援引π 电子体系的对称性来解释其特殊的电子特性，特别是在单光子激发中 S→S 跃迁的不活跃性。在这篇综述中，我们验证了类胡萝卜素的分子对称性及其π 电子体系的对称性是否在实验和计算研究中得到支持。我们专注于对电子密度分布敏感的光谱技术，包括 X 射线晶体学、电子吸收、双光子技术、圆二色性、核磁共振、斯塔克和振动光谱，并在此基础上寻找 S 态不活跃的原因。我们没有发现实验和计算证据表明对称性效应和电子态的对称性限制存在于类胡萝卜素中。它们没有反演中心，类胡萝卜素结构的 C 对称性近似是不合理的。实际上，在这个对称群中，将对称规则（和通知）应用于类胡萝卜素的电子态可能导致对实验数据的错误解释。这个结论与综述中总结的结果一起，使我们能够提出一个一致的模型，解释 S→S 跃迁的不活跃性。在这个模型中，由于（i）双激发贡献的负协同作用（概率非常低，会提高 S 能量）和（ii）S→S 跃迁的非垂直性（由于打破了玻恩-奥本海默近似），S 永远无法从 S 中获得。当然，我们的简单模型需要进一步的实验和理论验证。
