Choudhury Sharmistha Dutta, Kumbhakar Manoj, Nath Sukhendu, Pal Haridas
Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.
J Chem Phys. 2007 Nov 21;127(19):194901. doi: 10.1063/1.2794765.
Photoinduced electron transfer (ET) from N,N-dimethylaniline to some coumarin derivatives has been studied in small unilamellar vesicles (SUVs) of the phospholipid, DL-alpha-dimyristoyl-phosphatidylcholine, using steady-state and time-resolved fluorescence quenching, both below and above the phase transition temperature of the vesicles. The primary interest was to examine whether Marcus inversion [H. Sumi and R. A. Marcus, J. Chem. Phys. 84, 4894 (1986)] could be observed for the present ET systems in these organized assemblies. The influence of the topology of SUVs on the photophysical properties of the reactants and consequently on their ET kinetics has also been investigated. Absorption and fluorescence spectral data of the coumarins in SUVs and the variation of their fluorescence decays with temperature indicate that the dyes are localized in the bilayer of the SUVs. Time-resolved area normalized emission spectra analysis, however, reveals that the dyes are distributed in two different microenvironments in the SUVs, which we attribute to the two leaflets of the bilayer, one toward bulk water and the other toward the inner water pool. The microenvironments in the two leaflets are, however, not indicated to be that significantly different. Time-resolved anisotropy decays were biexponential for all the dyes in SUVs, and this has been interpreted in terms of the compound motion model according to which the dye molecules can experience a fast wobbling-in-cone type of motion as well as a slow overall rotating motion of the cone containing the molecule. The expected bimolecular diffusion-controlled rates in SUVs, as estimated by comparing the microviscosities in SUVs (determined from rotational correlation times) and that in acetonitrile solution, are much slower than the observed fluorescence quenching rates, suggesting that reactant diffusion (translational) does not play any role in the quenching kinetics in the present systems. Accordingly, clear inversions are observed in the correlation of the fluorescence quenching rate constants k(q) with the free energy change, DeltaG(0) of the reactions. However, the coumarin dyes, C152 and C481 (cf. Scheme 1), show unusually high k(q) values and high activation barriers, which is not expected from Marcus ET theory. This unusual behavior is explained on the basis of participation of the twisted intramolecular charge transfer states of these two dyes in the ET kinetics.
利用稳态和时间分辨荧光猝灭技术，在磷脂DL-α-二肉豆蔻酰磷脂酰胆碱的小单层囊泡（SUV）中，研究了N，N-二甲基苯胺向一些香豆素衍生物的光诱导电子转移（ET），实验温度低于和高于囊泡的相变温度。主要目的是检验在这些有序组装体中，当前的ET体系是否能观察到马库斯反转现象[H. Sumi和R. A. Marcus，《化学物理杂志》84，4894（1986）]。还研究了SUV的拓扑结构对反应物光物理性质以及对其ET动力学的影响。SUV中香豆素的吸收光谱和荧光光谱数据，以及它们的荧光衰减随温度的变化表明，染料定位于SUV的双层中。然而，时间分辨面积归一化发射光谱分析表明，染料分布在SUV的两种不同微环境中，我们将其归因于双层的两个小叶，一个朝向大量水，另一个朝向内部水池。然而，两个小叶中的微环境并没有显示出显著差异。对于SUV中的所有染料，时间分辨各向异性衰减都是双指数的，这已根据复合运动模型进行了解释，根据该模型，染料分子可以经历快速的圆锥内摆动型运动以及包含该分子的圆锥的缓慢整体旋转运动。通过比较SUV中的微粘度（由旋转相关时间确定）和乙腈溶液中的微粘度估算出的SUV中预期的双分子扩散控制速率，比观察到的荧光猝灭速率慢得多，这表明反应物扩散（平移）在当前体系的猝灭动力学中不起任何作用。因此，在荧光猝灭速率常数k(q)与反应的自由能变化ΔG(0)的相关性中观察到明显的反转。然而，香豆素染料C152和C481（参见方案1）显示出异常高的k(q)值和高活化能垒，这是马库斯ET理论所预期不到的。这种异常行为是基于这两种染料的扭曲分子内电荷转移态参与ET动力学来解释的。
