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arXiv 2003.10355physics.chem-ph
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管状分子聚集体中激子局域化:尺寸效应和光学响应

Exciton localization in tubular molecular aggregates: size effects and optical response

Anna S. Bondarenko, Thomas L. C. Jansen, Jasper Knoester

摘要:

我们研究了光学活性分子无序管状聚集体的激子局域化及其光学响应。先前已经表明,由于长程偶极-偶极相互作用和聚集体的更高维(不是真正的一维)性质的组合效应,这种管状结构允许激子在超过一千个分子上离域。这样大的离域尺寸提示的问题,在何种程度上,在实验系统中的离域仍然可以由聚集体的大小(直径和长度),以及这如何影响聚集体的光学响应和动力学。我们进行了系统的研究的尺寸效应的本地化性能,使用数值模拟的激子状态的圆柱形模型结构的启发,由先前导出的几何形状的一个圆柱形聚集的花青染料分子(C8 S3)。为了表征激子局域化,我们计算了激子波函数的参与比和自相关函数。计算了态密度和吸收光谱。我们发现强烈的影响管的半径上的本地化和光学性质的相关实验参数范围内。此外,令人惊讶的是,我们发现,即使对于长达750 nm的管,对于与实验情况相关的无序值,定位尺寸也受到管长度的限制,而对于高达约150 nm的管长度,吸收光谱中的管长度的可观察影响仍然发生。后者可以解释溴取代C8 S3聚集体老化过程中观察到的光谱变化。对于弱无序,激子波函数表现出散射的、分形的性质,类似于二维无序系统中的准粒子。

英文摘要:

We study the exciton localization and resulting optical response for disordered tubular aggregates of optically active molecules. It has been shown previously that such tubular structures allow for excitons delocalized over more than a thousand molecules, owing to the combined effects of long-range dipole-dipole interactions and the higher-dimensional (not truly one-dimensional) nature of the aggregate. Such large delocalization sizes prompt the question to what extent in experimental systems the delocalization may still be determined by the aggregate size (diameter and length) and how this affects the aggregate's optical response and dynamics. We perform a systematic study of the size effects on the localization properties, using numerical simulations of the exciton states in a cylindrical model structure inspired by the previously derived geometry of a cylindrical aggregate of cyanine dye molecules (C8S3). To characterize the exciton localization, we calculate the participation ratio and the autocorrelation function of the exciton wave function. Also, we calculate the density of states and absorption spectrum. We find strong effects of the tube's radius on the localization and optical properties in the range of parameters relevant to experiment. In addition, surprisingly, we find that even for tubes as long as 750 nm, the localization size is limited by the tube's length for disorder values that are relevant to experimental circumstances, while observable effects of the tube's length in the absorption spectrum still occur for tube lengths up to about 150 nm. The latter may explain changes in the optical spectra observed during the aging process of bromine-substituted C8S3 aggregates. For weak disorder, the exciton wave functions exhibit a scattered, fractal-like nature, similar to the quasi-particles in two-dimensional disordered systems.

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