Enhancing Molecular Alignment and Photostability in Organic EO Materials using Single-Molecule Microscopy

单分子光谱技术将用于提高有机电光材料的效率和光稳定性。有机光子材料的最新进展及其在新型器件体系结构中的应用表明，这些材料将在下一代电信和其他应用的EO开关器件中发挥重要作用。虽然与现有材料相比，有机材料具有增强EO活性、更快的开关速度和更低的成本的前景已经被人们认识了一段时间，但有两个问题限制了这些材料的广泛使用：发色团排列和光稳定性。这些研究旨在深入了解这些问题背后的分子细节，并通过这种深入了解可以确定和追求增强材料性能的策略。通过研究单分子的旋转动力学作为外部扰动（特别是电场和温度）的函数，研究了有机EO材料中的排列。EO活性产生于材料磁化率的÷(2)水平，这要求材料非中心对称。这些材料中的非中心顺序是通过极化引入的，在极化过程中，外电场与发色团的永久偶极矩相互作用（理论上）限制发色团的重新定向，从而提供材料对齐。分子水平的极化细节是知之甚少的，我们最近的研究已经确定，只有适度的对齐是实现在这个过程中。此外，极化通常在低于聚合物主体玻璃化转变温度5 ~ 10°C时进行，但是在这些温度下，发色团重定向动力学是什么样子的呢？温度、聚合物弛豫和极化诱导有序之间的相互作用是什么？单分子研究将提供分子水平的洞察到极点的过程，并随后细化这一过程。研究了二元发色团有机玻璃的光极化现象。相对于单独的电场极化，光极化提供了两倍的EO活动增强。理论认为这种增强是由于光场降低了宿主的空间维度。该团队将测量单分子在存在和不存在光极化场的情况下的旋转动力学，以直接验证这一假设。通过测量单分子的随时间发射（闪烁）、光谱扩散和激发态寿命，研究了有机EO材料的光稳定性。时间标记、时间相关的单光子计数技术被用来直接将闪烁行为与潜在的光物理联系起来，从而导致非发射或暗态的种群数量增加和减少。实验与蒙特卡罗模拟相结合，以确定这些状态作为材料光分解的门户。这项工作的一个独特之处在于，该团队将采用单分子晶体分离技术来测试复杂环境中有关分子光物理学的令人烦恼的问题，晶体提供了一个宿主，在那里溶剂化被定义和控制。基础知识的进步将对量子信息和光子学领域产生影响。直接参与这些研究的研究生将接受基础物理、材料科学和纳米制造的多学科教育。这项研究虽然本质上是基础性的，但对本科生来说很容易获得，并且将从本科生的参与中受益。光吸收和发射的基本原理提供了独特的机会，说明纳米科学从幼儿园到高中学生。例如，可见量子点的发射提供了出色的视觉演示，将用于高需求布法罗公立学校的pi的外展活动。在暑假期间，一名中学科学教师将参与pi的研究活动，这将大大加强这种推广。与此同时，这个暑期项目还将使pi能够从教师在初中和高中阶段使用的教育工具开发方面的经验中受益。最后，pi将为高中生组织一个暑期研讨会，介绍纳米科学中令人兴奋的研究。

Single-molecule spectroscopic techniques will be employed to enhance the efficiency and photostability of organic electro-optic (EO) materials. Recent advances in organic photonic materials and their inclusion into novel device architectures suggests that these materials will play an important role in the next generation of EO switching devices for telecommunications and other applications. Although the promise of organics to provide enhanced EO activity, faster switching speeds, and at a lower cost relative to current materials has been recognized for some time, there are two issues that limit the wide-spread use of these materials: chromophore alignment and photostability. The studies are designed to provide insight into the molecular details underlying these issues, and with this insight strategies for enhanced material performance can be identified and pursued.Alignment in organic EO materials is studied by investigating the rotational dynamics of single-molecules as a function of external perturbation (in particular, electric field and temperature). EO activity arises at the ÷(2) level of material susceptibility which requires that the material non-centrosymmetric. Acentric order in these materials is introduced by poling, a process in which an external electric field interacts with the permanent dipole moment of the chromophore to (theoretically) restrict chromophore reorientation thus providing for material alignment. The molecular-level details of poling are poorly understood, and our recent studies have established that only modest alignment is achieved in this process. Furthermore, poling is generally performed 5 to 10° C below the glass transition temperature of the polymer host, but what do chromophore reorientational dynamics look like at these temperatures? What is the interplay between temperature, polymer relaxation, and poling-induced order? The single molecule studies will provide molecular-level insight into the poling process, and subsequently refinement of this process. Optical poling in binary chromophore organic glasses is also studied. Optical poling provides for a two-fold enhancement in EO activity relative to electric-field poling alone. Theory suggests that this enhancement arises from the optical field reducing the spatial dimensionality of the host. The team will measure the rotational dynamics of single molecule in the presence and absence of the optical poling field to directly test this hypothesis. The photostability of organic EO materials is investigated by measuring the time-dependent emission (blinking), spectral diffusion, and excited-state lifetimes of single molecules. Time-tagged, time-correlated single photon counting techniques are used to directly correlate blinking behavior to the underlying photophysics that result in population and depopulation of the non-emissive or dark state. The experiments are combined with Monte-Carlo simulations to identify these states which serve as a gateway to material photodecomposition. A unique aspect of this work is that the team will employ single molecule crystal isolation techniques to test vexing questions concerning molecular photophysics in complex environments, with the crystal providing a host where solvation is well-defined and controlled.The advancement of fundamental knowledge will have impact on the fields of quantum information and of photonics. The graduate students directly involved in these studies will receive a multidisciplinary education in basic physics, materials science, and nanofabrication. The research, while fundamental in nature, is readily accessible to undergraduates and will benefit from the involvement of undergraduate students in the program. The fundamentals of optical absorption and emission provide unique opportunities for illustrating nanoscience to pre-Kindergarten through high school students. For example, the emission from visible quantum dots provides excellent visual demonstrations that will be used in the outreach activities of the PIs in high needs Buffalo Public Schools. This outreach will be enhanced significantly by the incorporation of a middle school science teacher in the PIs research activities during the summer. Simultaneously, this summer program will also enable the PIs to benefit from the experience of the teacher in the development of educational tools for use at the middle and high school levels. Finally, the PIs will organize a summer workshop for high school students to provide an introduction to the exciting research in nanoscience.