Theory of optical transitions in conjugated polymers. II. Real systems
Theory of optical transitions in conjugated polymers. II. Real systems
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DOI：
10.1063/1.4897985
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发表时间：
2014-10-28
影响因子：
4.4
通讯作者：
Barford, William
中科院分区：
文献类型：
--
作者：
Marcus, Max;Tozer, Oliver Robert;Barford, William
Barford和Marcus发展的光学跃迁理论[“共轭聚合物中的光学跃迁理论。I. Ideal systems，”J.Chem.Phys.141，164101（2014）]中描述的线性有序聚合物链，本文将其扩展为构象无序系统的模型。我们的关键结果是在Born-Oppenheimer体系中，发光强度与S（1）/成正比< IPR >，其中S（1）是单体的Huang-Rhys参数。< IPR >是排放物质的平均参与率倒数，即，局域激子基态（LEGS）。由于LEGS的空间相干性决定了发色团的空间范围，因此该结果的意义在于它将实验观测值与发色团大小直接相关（其中< IPR >是单体单元中平均发色团大小的一半）。由于多体波函数的玻恩-奥本海默因式分解，这个结果与发色团的形状无关。我们通过线性无序链和卷曲有序链的绝热极限下Frenkel-Holstein模型的密度矩阵重整化群（DMRG）计算验证了这一预测。我们还模拟了聚（对苯撑）和聚（对苯撑-亚乙烯基）低聚物和聚合物的光谱。对于低聚物，我们通过DMRG方法求解完全量子化的Frenkel-Holstein模型。对于聚合物，我们使用更简单的方法解决单粒子弗伦克尔模型，并采用Born-Oppenheimer表达式的有效的弗兰克-康登因子的发色团，其参与率的倒数。我们发现，增加无序减少生色团的大小和增加的不均匀加宽，但有一个非单调的过渡能量的影响。我们还表明，作为平面化的聚合物链的激子带宽度增加，它会导致发色团的尺寸增加，跃迁能降低，和加宽减少。最后，我们表明，吸收光谱比发射光谱更宽，并且随着链变得更加卷曲，吸收光谱的加宽增加。这主要是因为LEGS和准扩展激子态（QEES）都发生吸收，并且QEES随着发色团弯曲而获得增加的强度，而发射仅从LEGS发生。(c)2014 AIP Publishing LLC.
The theory of optical transitions developed in Barford and Marcus ["Theory of optical transitions in conjugated polymers. I. Ideal systems," J. Chem. Phys. 141, 164101 (2014)] for linear, ordered polymer chains is extended in this paper to model conformationally disordered systems. Our key result is that in the Born-Oppenheimer regime the emission intensities are proportional to S(1)/< IPR >, where S(1) is the Huang-Rhys parameter for a monomer. < IPR > is the average inverse participation ratio for the emitting species, i.e., local exciton ground states (LEGSs). Since the spatial coherence of LEGSs determines the spatial extent of chromophores, the significance of this result is that it directly relates experimental observables to chromophore sizes (where < IPR > is half the mean chromophore size in monomer units). This result is independent of the chromophore shape, because of the Born-Oppenheimer factorization of the many body wavefunction. We verify this prediction by density matrix renormalization group (DMRG) calculations of the Frenkel-Holstein model in the adiabatic limit for both linear, disordered chains and for coiled, ordered chains. We also model optical spectra for poly(p-phenylene) and poly(p-phenylene-vinylene) oligomers and polymers. For oligomers, we solve the fully quantized Frenkel-Holstein model via the DMRG method. For polymers, we use the much simpler method of solving the one-particle Frenkel model and employ the Born-Oppenheimer expressions relating the effective Franck-Condon factor of a chromophore to its inverse participation ratio. We show that increased disorder decreases chromophore sizes and increases the inhomogeneous broadening, but has a non-monotonic effect on transition energies. We also show that as planarizing the polymer chain increases the exciton band width, it causes the chromophore sizes to increase, the transition energies to decrease, and the broadening to decrease. Finally, we show that the absorption spectra are more broadened than the emission spectra and that the broadening of the absorption spectra increases as the chains become more coiled. This is primarily because absorption occurs to both LEGSs and quasi-extended exciton states (QEESs), and QEES acquire increased intensity as chromophores bend, while emission only occurs from LEGSs. (c) 2014 AIP Publishing LLC.