固体效应对推拉分子J聚集体光激发的第一性原理模拟
Solid-State Effects on the Optical Excitation of Push-Pull Molecular J-Aggregates by First-Principles Simulations
摘要：
J-聚集体是一类低维分子晶体，其显示出与光的增强的相互作用。这些系统显示出有趣的光学性质作为一个强烈的和窄的红移吸收峰（J带）相对于相应的单体的光谱。需要从理论上研究J-聚集体中的光激发是双重的：一个彻底的第一性原理描述仍然缺失，最近在理解J-带的性质，特别是关于其形成所涉及的集体机制方面，重新引起了人们的兴趣。在这项工作中，我们研究了J-聚集体分子晶体的电子和光学性质的有序排列的有机推拉生色团。通过使用时间相关的密度泛函理论的方法，我们评估的作用，分子包装的增强和红移的J-带随着限制的影响，在光吸收，当移动到低维晶体结构。我们模拟了不同配置的光吸收（即，单体、二聚体、聚合物链和单层片）。通过分析J带的感应电荷密度，我们得出结论：它是一种纵向激发，沿着平行的线性链离域，它的整体红移是由竞争耦合机制引起的，一些产生红移，另一些产生蓝移，这些耦合机制来自于跃迁密度之间的耦合和单粒子能级的重整化。
英文摘要：
J-aggregates are a class of low-dimensional molecular crystals which display enhanced interaction with light. These systems show interesting optical properties as an intense and narrow red-shifted absorption peak (J-band) with respect to the spectrum of the corresponding monomer. The need to theoretically investigate optical excitations in J-aggregates is twofold: a thorough first-principles description is still missing and a renewed interest is rising recently in understanding the nature of the J-band, in particular regarding the collective mechanisms involved in its formation. In this work, we investigate the electronic and optical properties of a J-aggregate molecular crystal made of ordered arrangements of organic push-pull chromophores. By using a time dependent density functional theory approach, we assess the role of the molecular packing in the enhancement and red shift of the J-band along with the effects of confinement in the optical absorption, when moving from bulk to low-dimensional crystal structures. We simulate the optical absorption of different configurations (i.e., monomer, dimers, a polymer chain, and a monolayer sheet) extracted from the bulk crystal. By analyzing the induced charge density associated with the J-band, we conclude that it is a longitudinal excitation, delocalized along parallel linear chains and that its overall red shift results from competing coupling mechanisms, some giving red shift and others giving blue shift, which derive from both coupling between transition densities and renormalization of the single-particle energy levels.
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