Hariharan Mahesh, Neelakandan Prakash P, Ramaiah Danaboyina
Photosciences and Photonics, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology, Council of Scientific and Industrial Research, Trivandrum, India.
J Phys Chem B. 2007 Oct 18;111(41):11940-7. doi: 10.1021/jp074643d. Epub 2007 Sep 25.
Interaction of beta-cyclodextrin (beta-CD) with a few novel electron donor acceptor dyads 1a-c and 2a-c, having aryl and flexible methylene spacer groups, has been investigated through photophysical, chiroptical, electrochemical, NMR, and microscopic techniques. Dyads 1a and 1c, with p-tolyl and biphenyl spacer groups, respectively, exhibited significantly decreased fluorescence quantum yields and lifetimes in the presence of beta-CD, while negligible changes were observed for dyad 1b with an o-tolyl spacer. In contrast, spacer-length-dependent significant enhancement in fluorescence quantum yields and lifetimes was observed for dyads 2a-c, with flexible polymethylene (n = 1, 3, 11) spacer groups. Association constants of beta-CD encapsulated complexes have been determined and the contrast behavior observed in these systems is explained through an electron transfer (kET) mechanism based on calculated favorable change in free energy (DeltaGET = -1.27 eV) and the redox species characterized through laser flash photolysis studies. Rates of kET have been estimated and are found to increase ca. 2-fold in the case of dyads 1a and 1c when encapsulated in beta-CD, while significantly decreased kET values were observed for the dyads 2a-c with flexible spacer (ca. 9-fold for 2c). As characterized through cyclic voltammetry, 2D NMR [correlated (COSY) and nuclear Overhauser enhancement (NOESY) spectroscopy], and laser flash photolysis studies, the beta-CD encapsulation of dyads with aliphatic spacer groups leads to the conformational unfolding of a sandwich type of structure, whereas dyads with rigid aryl spacer groups undergo unusual planarization as compared to the uncomplexed dyads, resulting in enhanced electron-transfer reaction between the donor and acceptor moieties.
通过光物理、圆二色光谱、电化学、核磁共振和显微镜技术研究了β-环糊精（β-CD）与一些具有芳基和柔性亚甲基间隔基团的新型电子供体-受体二元体系1a-c和2a-c的相互作用。二元体系1a和1c分别具有对甲苯基和联苯基间隔基团，在β-CD存在下荧光量子产率和寿命显著降低，而具有邻甲苯基间隔基团的二元体系1b变化可忽略不计。相比之下，对于具有柔性聚亚甲基（n = 1, 3, 11）间隔基团的二元体系2a-c，观察到荧光量子产率和寿命随间隔长度的增加而显著提高。测定了β-CD包封配合物的缔合常数，并通过基于计算的有利自由能变化（ΔGET = -1.27 eV）的电子转移（kET）机制以及通过激光闪光光解研究表征的氧化还原物种来解释这些体系中观察到的对比行为。估算了kET速率，发现当包封在β-CD中时，二元体系1a和1c的kET速率增加约2倍，而具有柔性间隔基团的二元体系2a-c的kET值显著降低（2c约降低9倍）。通过循环伏安法、二维核磁共振[相关（COSY）和核Overhauser增强（NOESY）光谱]以及激光闪光光解研究表征，具有脂肪族间隔基团的二元体系被β-CD包封会导致三明治型结构的构象展开，而与未络合的二元体系相比，具有刚性芳基间隔基团的二元体系会发生异常的平面化，从而导致供体和受体部分之间的电子转移反应增强。
