Organic Semiconductors, Experimental Physics II, Department of Physics and Bayreuth Institute of Macromolecular Science, University of Bayreuth, Bayreuth 95440, Germany.
J Am Chem Soc. 2012 Jul 18;134(28):11594-601. doi: 10.1021/ja302408a. Epub 2012 Jul 5.
The poly(p-phenylene vinylene) derivative MEH-PPV is known to exist as two morphologically distinct species, referred to as red phase and blue phase. We show here that the transition from the blue phase to the red phase is a critical phenomenon that can be quantitatively described as a second order phase transition with a critical temperature T(c) of 204 K. The criticality is associated with the trade-off between the gain in the electronic stabilization energy when the π-system of a planarized chain can delocalize and the concomitant loss of entropy. We studied this transition by measuring the absorption and fluorescence in methyltetrahydrofuran (MeTHF) in two different concentrations as a function of temperature. The spectra were analyzed based upon the Kuhn exciton model to extract effective conjugation lengths. At room temperature, the chains have effective conjugation lengths of about five repeat units in the ground state (the blue phase), consistent with a disordered defect cylinder conformation. Upon cooling below the critical temperature T(c), the red phase with increased effective conjugation lengths of about 10 repeat units forms, implying a more extended and better ordered conformation. Whereas aggregation is required for the creation of the red phase, its electronic states have a predominant intrachain character.
聚对苯乙炔衍生物 MEH-PPV 已知存在两种形态上明显不同的物种，分别称为红相和蓝相。我们在这里表明，从蓝相到红相的转变是一种临界现象，可以定量描述为具有临界温度 T(c)为 204 K 的二级相变。这种临界性与当平面化链的π系统可以离域时电子稳定能的增加与熵的相应损失之间的权衡有关。我们通过测量在两种不同浓度的甲基四氢呋喃 (MeTHF) 中的吸收和荧光随温度的变化来研究这种转变。根据库恩激子模型分析光谱以提取有效的共轭长度。在室温下，链在基态（蓝相）中的有效共轭长度约为五个重复单元，与无序缺陷圆柱构象一致。在低于临界温度 T(c)冷却时，形成具有增加的有效共轭长度约 10 个重复单元的红相，这意味着更扩展和更好的有序构象。虽然聚集是形成红相所必需的，但它的电子态具有主要的链内特征。
