Amarie Sergiu, Arefe Kibrom, Starcke Jan Hendrik, Dreuw Andreas, Wachtveitl Josef
Institute of Physical and Theoretical Chemistry, Goethe-University Frankfurt, Frankfurt am Main, Germany.
J Phys Chem B. 2008 Nov 6;112(44):14011-7. doi: 10.1021/jp806030y. Epub 2008 Oct 9.
Carotenoids are the crucial pigments involved in photoprotection and in scavenging harmful free radicals in all living organisms. The underlying chemical processes are charge transfer and free radical reactions, both of them leading to carotenoid radical cation (Car*+) formation. Accurate knowledge of the molecular properties of Car*+ is thus a prerequisite for understanding of their function as photoprotective and antioxidant agents. Despite their fundamental importance in nonphotochemical quenching in green plants, only little is known about the Car*+ excited states and their dynamics. Our combined experimental and theoretical investigation employing femtosecond time-resolved pump-probe spectroscopy and quantum chemical calculations proves the existence of a second low-lying pipi* excited-state energetically below the well-known strongly allowed excited-state responsible for the intense absorption of Car*+ in the near-IR region. Hence, we suggest denoting the latter state as D3 state in the future. Our findings have also implications for nonphotochemical quenching in green plants, since direct quenching of chlorophyll excited states by Forster energy transfer to Car*+ is possible and efficient.
类胡萝卜素是所有生物中参与光保护和清除有害自由基的关键色素。潜在的化学过程是电荷转移和自由基反应，两者都会导致类胡萝卜素自由基阳离子（Car*+）的形成。因此，准确了解Car*+的分子性质是理解其作为光保护剂和抗氧化剂功能的先决条件。尽管它们在绿色植物的非光化学猝灭中具有根本重要性，但关于Car*+激发态及其动力学的了解却很少。我们采用飞秒时间分辨泵浦-探测光谱和量子化学计算相结合的实验和理论研究证明，存在第二个低能ππ激发态，其能量低于众所周知的强允许激发态，该激发态是导致Car+在近红外区域强烈吸收的原因。因此，我们建议今后将后者的状态称为D3态。我们的研究结果对绿色植物的非光化学猝灭也有影响，因为通过福斯特能量转移将叶绿素激发态直接猝灭到Car*+是可能且有效的。
