Ghosh Deborin, Pradhan Anup Kumar, Mondal Samiran, Begum N A, Mandal Debabrata
Department of Chemistry, University College of Science & Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India.
Phys Chem Chem Phys. 2014 May 14;16(18):8594-607. doi: 10.1039/c3cp52209a.
Flavonol 4'-chloro,3-hydroxyflavone (Cl-3HF) has been investigated in solvents of varying polarity and hydrogen-bonding capacity as well as in aqueous micelle solutions. Quantum chemical calculations indicate that although the Cl-atom at the 4'-position of the 2-phenyl ring weakly perturbs the electron distribution of the parent 3-hydroxyflavone, the nuclear framework remains largely intact, and excited state intra-molecular proton-transfer (ESIPT) is feasible. The ESIPT process in both polar solvents and micelles was found to be fast and irreversible, with remarkably long time-constants of several tens of picoseconds. This dramatic inhibition of the ESIPT rate (which is intrinsically a sub-picosecond event) could be rationalized in terms of the emergence of complexes between the solvent and the enol form of Cl-3HF, whose dynamics is coupled to the relatively slow dynamics of inter-molecular hydrogen bonds. In the micelle solutions, spectroscopic data establish that the guest Cl-3HF molecules localized almost exclusively at the polar exterior shell, where they experienced a nearly uniform local environment similar to that in moderately polar solvents. Thus, the Cl-3HF molecules tend to avoid the non-polar core of the micelles, in spite of being strongly hydrophobic themselves. This apparently unusual observation is explained by the formation of inter-molecularly hydrogen-bonded complexes between the guest Cl-3HF and the water molecules tethered to the polar shells of the micelles.
黄酮醇4'-氯-3-羟基黄酮（Cl-3HF）已在具有不同极性和氢键能力的溶剂以及水性胶束溶液中进行了研究。量子化学计算表明，尽管2-苯环4'-位的Cl原子对母体3-羟基黄酮的电子分布有微弱扰动，但核骨架基本保持完整，激发态分子内质子转移（ESIPT）是可行的。发现在极性溶剂和胶束中的ESIPT过程都很快且不可逆，具有数十皮秒的显著长时间常数。ESIPT速率（本质上是亚皮秒级事件）的这种显著抑制可以根据溶剂与Cl-3HF烯醇形式之间形成的络合物的出现来解释，其动力学与分子间氢键的相对较慢动力学相关联。在胶束溶液中，光谱数据表明客体Cl-3HF分子几乎完全定位在极性外壳层，在那里它们经历了与中等极性溶剂中几乎相同的局部环境。因此，尽管Cl-3HF本身具有很强的疏水性，但它们倾向于避开胶束的非极性核心。这种明显不寻常的现象可以通过客体Cl-3HF与连接在胶束极性壳层上的水分子之间形成分子间氢键络合物来解释。
