School of Chemistry, University of Hyderabad, Hyderabad, India.
J Phys Chem B. 2010 Mar 11;114(9):3122-32. doi: 10.1021/jp910808u.
The aggregation behavior of poly(4,4'-diphenylether-5,5'-bibenzimidazole) (OPBI) in polar aprotic (dimethyl acetamide, DMAc) and protic (formic acid, FA) solvents is studied as a function of the polymer concentration and solution temperature. The effects of solvent protic character on the aggregation behavior of OPBI are elucidated. The photophysical studies suggest that the OPBI chains form aggregated structures in both DMAc and FA solutions when the OPBI concentration is increased. The dependences of the emission spectra on the polymer concentrations in two solvents are not similar in nature, indicating that in both of the solvents the aggregations are intermolecular processes, though their mechanisms are different owing to the polyelectrolytic nature of OPBI in FA medium. The triexponential decay profiles obtained from the time-resolved fluorescence study for the concentrated solutions (both in DMAc and FA) display a negative fractional coefficient and longer excited state lifetime, providing support for the aggregations at higher concentration. The temperature dependence emission spectra suggest that the aggregations in both of the solvents destabilize with increasing temperature. The higher activation energy of aggregation (E(A)) in DMAc (5.62 KJ/mol) compared with that in FA (3.07 kJ/mol) reveals that the aggregation formation pathways are different in two solvents and stronger aggregates are formed in the former solvent. The dilute solution viscometry (DSV) studies demonstrate that the OPBI chains adapt a bigger extended conformation in FA compared with DMAc owing to the stronger intramolecular chain repulsion in FA arising due to the polyelectrolyte nature of OPBI in this solvent. A conformation transition of OPBI chains from compact collapsed to extended conformer is observed in DMAc solvent with increasing concentration, whereas any such transition is absent in FA medium. Transmission electron microscope (TEM) images and circular dichroism (CD) spectra are also in agreement with the presence of a conformational transition in DMAc and the absence of it in FA. The temperature dependent DSV studies further support the disruption of aggregated structure with increasing temperature in both of the solvents. DSV studies exhibit that the deaggregation is driven by a conformation transition (extended to compact collapsed) in DMAc, whereas in FA the disruption happens without conformational transition.
聚(4,4'-二苯醚-5,5'-联苯并咪唑)(OPBI)在极性非质子 (二甲基乙酰胺,DMAc) 和质子 (甲酸,FA) 溶剂中的聚集行为作为聚合物浓度和溶液温度的函数进行研究。阐明了溶剂质子特性对 OPBI 聚集行为的影响。光物理研究表明,当 OPBI 浓度增加时,OPBI 链在 DMAc 和 FA 溶液中均形成聚集结构。两种溶剂中发射光谱随聚合物浓度的依赖性在本质上并不相似,表明在两种溶剂中,聚合都是分子间过程,尽管由于 OPBI 在 FA 介质中的聚电解质性质,它们的机制不同。时间分辨荧光研究中从高浓度溶液(在 DMAc 和 FA 中)获得的三指数衰减谱显示出负分数系数和更长的激发态寿命,为在更高浓度下的聚集提供了支持。温度依赖性发射光谱表明,两种溶剂中的聚集随着温度的升高而不稳定。在 DMAc 中聚合的活化能(E(A))(5.62 kJ/mol)高于在 FA 中聚合的活化能(E(A))(3.07 kJ/mol),这表明在两种溶剂中聚合形成途径不同,在前一种溶剂中形成更强的聚集。稀溶液黏度计(DSV)研究表明,与 DMAc 相比,OPBI 链在 FA 中采用更大的扩展构象,这是由于 OPBI 在该溶剂中具有聚电解质性质,导致 FA 中分子内链排斥更强。随着浓度的增加,观察到 OPBI 链从紧密折叠到扩展构象的构象转变在 DMAc 溶剂中发生,而在 FA 介质中不存在这种转变。透射电子显微镜(TEM)图像和圆二色性(CD)谱也与 DMAc 中存在构象转变和 FA 中不存在构象转变的情况一致。温度依赖的 DSV 研究进一步支持了两种溶剂中温度升高时聚集结构的破坏。DSV 研究表明,在 DMAc 中,解聚是由构象转变(从扩展到紧密折叠)驱动的,而在 FA 中,解聚是在没有构象转变的情况下发生的。
