Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
Phys Chem Chem Phys. 2010 Jul 21;12(27):7427-33. doi: 10.1039/c003217d. Epub 2010 Jun 14.
Recent single molecule experiments have suggested the existence of a photochemical funnel in the photophysics of conjugated polymers, like poly[2-methoxy-5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] (MEH-PPV). The funnel is believed to be a consequence of the presence of conformational or chemical defects along the polymer chain and efficient non-radiative energy transfer among different chromophore segments. Here we address the effect of the excitation energy dynamics on the photophysics of PPV. The PPV chain is modeled as a polymer with the length distribution of chromophores given either by a Gaussian or by a Poisson distribution. We observe that the Poisson distribution of the segment lengths explains the photophysics of PPV better than the Gaussian distribution. A recently proposed version of an extended 'particle-in-a-box' model is used to calculate the exciton energies and the transition dipole moments of the chromophores, and a master equation to describe the excitation energy transfer among different chromophores. The rate of energy transfer is assumed to be given here, as a first approximation, by the well-known Förster expression. The observed excitation population dynamics confirms the photochemical funneling of excitation energy from shorter to longer chromophores of the polymer chain. The time scale of spectral shift and energy transfer for our model polymer, with realistic values of optical parameters, is in the range of 200-300 ps. We find that the excitation energy may not always migrate towards the longest chromophore segments in the polymer chain as the efficiency of energy transfer between chromophores depends on the separation distance between the two and their relative orientation.
最近的单分子实验表明，在共轭聚合物的光物理中存在光化学漏斗，如聚[2-甲氧基-5-（2'-乙基己基）氧基-1,4-亚苯基乙烯基]（MEH-PPV）。据信，这种漏斗是由于聚合物链上存在构象或化学缺陷以及不同发色团片段之间有效的非辐射能量转移所致。在这里，我们研究了激发能动力学对 PPV 光物理的影响。将 PPV 链建模为具有由高斯或泊松分布给出的发色团长度分布的聚合物。我们观察到，段长度的泊松分布比高斯分布更好地解释了 PPV 的光物理。最近提出的扩展“盒中粒子”模型的版本用于计算激子能量和发色团的跃迁偶极矩，并使用主方程来描述不同发色团之间的激发能转移。这里假设能量转移速率首先由著名的Förster 表达式给出。观察到的激发态群体动力学证实了从聚合物链上较短的发色团到较长的发色团的光化学激发能漏斗效应。对于我们的模型聚合物，具有实际光学参数，其光谱位移和能量转移的时间尺度在 200-300 ps 范围内。我们发现，激发能不一定总是向聚合物链中最长的发色团段迁移，因为发色团之间的能量转移效率取决于两者之间的分离距离及其相对取向。
