Department of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.
J Phys Chem A. 2011 Apr 28;115(16):3936-46. doi: 10.1021/jp108317u. Epub 2010 Dec 20.
In plants, the oxidoreductase enzyme POR reduces protochlorophyllide (Pchlide) into chlorophyllide (Chlide), using NADPH as a cofactor. The reduction involves the transfer of two electrons and two protons to the C17═C18 double bond of Pchlide, and the reaction is initiated by the absorption of light by Pchlide itself. In this work we have studied the excited state dynamics of Pchlide dissolved in water, where it forms excitonically coupled aggregates, by ultrafast time-resolved transient absorption and fluorescence experiments performed in the 480-720 nm visible region and in the 1780-1590 cm(-1) mid-IR region. The ground state visible absorption spectrum of aqueous Pchlide red shifts and broadens in comparison to the spectrum of monomeric Pchlide in organic solvents. The population of the one-exciton state occurs at low excitation densities, of <1 photon per aggregate. We characterized the multiexciton manifolds spectra by measuring the absorption difference spectra at increasingly higher photon densities. The multiexciton states are characterized by blue-shifted stimulated emission and red-shifted excited state absorption in comparison to those of the one-exciton manifold. The relaxation dynamics of the multiexciton manifolds into the one-exciton manifold is found to occur in ∼10 ps. This surprisingly slow rate we suggest is due to the intrinsic charge transfer character of the PChlide excited state that leads to solvation, stabilizing the CT state, and subsequent charge recombination, which limits the exciton relaxation.
在植物中，氧化还原酶 POR 使用 NADPH 作为辅助因子将原叶绿素（Pchlide）还原为叶绿素（Chlide）。还原过程涉及将两个电子和两个质子转移到 Pchlide 的 C17═C18 双键，反应由 Pchlide 本身吸收光引发。在这项工作中，我们通过在 480-720nm 可见区和 1780-1590cm(-1) 中红外区进行超快时间分辨瞬态吸收和荧光实验，研究了溶解在水中的 Pchlide 的激发态动力学，在水中它形成激子耦合聚集体。与有机溶剂中单体 Pchlide 的光谱相比，水合 Pchlide 的基态可见吸收光谱发生红移和展宽。单激子态的种群出现在低激发密度下，<1 个光子/聚集体。我们通过在越来越高的光子密度下测量吸收差光谱来表征多激子态谱。与单激子态相比，多激子态的特征是蓝移的受激发射和红移的激发态吸收。多激子态向单激子态的弛豫动力学被发现发生在 ∼10 ps 内。我们认为这种出乎意料的缓慢速率是由于 PChlide 激发态的固有电荷转移特性导致溶剂化，稳定 CT 态，随后发生电荷复合，从而限制了激子弛豫。
