Structures and excited state dynamics of self-assembled photonic structures

西北大学的George Schatz得到了化学系化学理论、模型和计算方法计划颁发的奖项的支持，该奖项旨在开发理论和计算方法来研究分子组件和其他光学设备中的能量流动速率，这些光学设备依赖于能量通过分子阵列的流动。这项研究是化学家开发新的树叶状材料的重要努力的一部分，在这种材料中，分子组件充当天线结构，吸收阳光，然后将这种能量传输到其他分子将其转化为电能的位置。沙茨和他的同事们正在开发一种新的理论方法来描述这一过程，这种方法可以包括光和电子之间的耦合以及分子和环境的振动运动，这样就可以定量地确定比以前可以访问的大得多的结构的传输速度。这项研究还包括将分子组装成优化能量流动的结构的研究，并与制造这些结构的实验小组进行了广泛的合作。这个项目是与本科生、研究生和博士后合作进行的，研究结果被用于开发大学课程的教材。在这项研究项目中，发展了一种新的理论方法来表征光子器件中感兴趣的染料生色团聚集体和集合体的激发态动力学。在这种方法中，用一种时间域方法代替了传统的Förster方法来确定与相互作用激子有关的能量转移速率。在这种方法中，与施主物种有关的辐射用经典的电磁理论来描述，并且在受主位置产生的电场被用来计算描述能量转移速率的响应函数。这种方法是从光学物理界借来的，它包含了几个超越Förster方法的因素，包括结合了与光子结构有关的延迟和复杂的电磁边界条件。这项研究涉及将这种方法推广到描述与分子生色团相关的物理情况，在这种情况下，点偶极发射器被充当天线的振荡电流取代，周围介质的介电响应由洛伦兹振子模型描述，该模型能够描述激子耦合引起的光谱漂移。该理论的扩展包括声子运动、退相和弛豫、非相干和相干能量传递以及非线性效应。数值实现采用了时域有限差分法等方法。对发色团聚集体的应用证明了所提出的理论的能力，发色团聚集体的结构由自组装过程的分子动力学模拟确定。这种类型的例子包括多肽两亲性分子(带有嵌入的生色团)，它们自组装成胶束聚集体，其中生色团堆积成有组织的结构。同样令人感兴趣的是在金属有机骨架材料(MOF)和DNA连接的纳米颗粒超晶格中产生的生色团的周期性阵列。这些研究的重点是在器件应用中有用的激子输运性质。

George Schatz of Northwestern University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry division to develop theoretical and computational approaches to study the rate of energy flow in molecular assemblies and other optical devices that depend on the flow of energy through an array of molecules. This research is part of an important effort by chemists to develop new leaf-like materials in which assemblies of molecules act as antenna structures to absorb sunlight and then transport this energy to locations where other molecules convert it into electrical energy. Schatz and his coworkers are developing a new theoretical approach to describe this process, one which makes it possible to include the coupling between light, and electrons and vibrational motions of the molecules and surroundings, so that the rate of transport can be quantitatively determined for structures that are much larger than have previously been accessible. The research also includes studies of the assembly of the molecules into structures that optimize energy flow, and there are extensive collaboration with experimental groups who are making the structures. This project is carried out in collaboration with undergraduates, graduate students and postdocs, Results from this research are used in developing teaching material for college courses. In this research project, a new theoretical method is developed for characterizing the excited state dynamics of aggregates and assemblies of dye chromophores that are of interest in photonic devices. In this method, the traditional Förster approach for determining the rate of energy transfer associated with interacting excitons is replaced by a time domain approach in which the emitted radiation associated with the donor species is described by classical electromagnetic theory and the resulting electric field at the acceptor position is used to calculate a response function that describes the energy transfer rate. This approach, which is borrowed from the optical physics community, includes several factors that go beyond the Förster approach, including the incorporation of retardation and complex electromagnetic boundary conditions associated with the photonic structure. The research involves generalizing this approach to describe the physical situation associated with molecular chromophores in which point dipole emitters are replaced by oscillating currents that act as antennas, and the dielectric response of the surrounding medium is described by Lorentz oscillator models that enable the description of spectral shifts due to exciton coupling. Extensions of the theory include phonon motion, dephasing and relaxation, both incoherent and coherent energy transfer, and nonlinear effects. Numerical implementation employs the finite-difference time-domain method and other methods. The capabilities of the proposed theory is being demonstrated by applications to chromophore aggregates whose structures are determined from molecular dynamics simulations of the self-assembly process. Examples of this type include peptide amphiphiles (with embedded chromophores) that self-assemble to give micelle aggregates in which the chromophores are stacked into organized structures. Also of interest are periodic arrays of chromophores that are produced in metal organic framework materials (MOFs) and DNA-linked nanoparticle superlattices. Emphasis in these studies is on exciton transport properties that are useful in device applications.