Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, OX1 3QZ, United Kingdom.
J Phys Chem A. 2011 Aug 25;115(33):9111-9. doi: 10.1021/jp204822h. Epub 2011 Aug 1.
We consider two types of ultrafast dynamical localization of photoexcited states in conformationally disordered poly(p-phenylenevinylene). First, we discuss nonadiabatic interconversion from higher energy extended exciton states to lower energy more localized local exciton ground states. Second, we calculate the dynamics of local exciton ground states on their Born-Oppenheimer potential energy surfaces. We show that within the first C-C bond oscillation following photoexcitation (∼35 fs) the exciton becomes self-trapped and localized over approximately eight monomers. This process is associated with a Calderia-Leggett type loss of phase coherence owing to the coupling of the polymer to a dissipative environment. Subsequent torsional relaxation (on a time scale of approximately picoseconds) has little effect on the localization. We conclude from this that the initial torsional disorder determines the spatial distribution and localization length of vertical excitations but that electron-phonon coupling is largely responsible for the localization length of self-trapped excitons. We next consider the effect of dynamical localization on fluorescence depolarization. We show that exciting higher energy states causes a larger fluorescence depolarization, because these states have a larger initial delocalization. Using the observation that fluorescence depolarization is a function of excitation wavelength and polymer conformation, we show how the models of exciton localization discussed here can be experimentally investigated.
我们考虑了在构象无序的聚对苯乙炔中光激发态超快动力学局域化的两种类型。首先，我们讨论了从高能扩展激子态到低能更局域的局域激子基态的非绝热转换。其次，我们计算了局域激子基态在 Born-Oppenheimer 势能表面上的动力学。我们表明，在光激发后的第一个 C-C 键振动（约 35 fs）内，激子成为自陷并局域在大约 8 个单体上。这个过程与由于聚合物与耗散环境的耦合而导致的 Caldeira-Leggett 型相位相干性损失有关。随后的扭转弛豫（在大约皮秒的时间尺度上）对定位几乎没有影响。因此，我们得出结论，初始扭转无序决定了垂直激发的空间分布和局域化长度，但电子-声子耦合在很大程度上决定了自陷激子的局域化长度。接下来，我们考虑动力学局域化对荧光各向异性的影响。我们表明，激发更高的能级会导致更大的荧光各向异性，因为这些能级具有更大的初始离域度。利用荧光各向异性是激发波长和聚合物构象的函数这一观察结果，我们展示了如何通过实验研究这里讨论的激子局域化模型。
