NSF- MNW: Structure, Dynamics and Critiacl Phenomena in Biaxial Liquid Crystals

目前，对树突状分子的研究是化学领域最相关的研究课题之一，这一研究活动主要集中在材料科学（主要是纳米科学和纳米技术）以及生物/医学领域。这一研究课题在几年内从边缘兴趣发展成为许多研究学科的核心课题。这是由于树突结构的建筑之美及其潜在应用的魅力。最初的研究主要集中在初始树突结构的设计和构造以及确定基本结构性质的相关性。目前的研究关注的是在树突结构中添加功能，并精确地确定树突结构在哪里增强了特定的性能。尽管已经报道了大量的工作，但令人惊讶的是，很少有人关注树状大分子的光定向形状和性质调制，特别是没有人报道，据申请人所知，对树状大分子分支点定位光致变色基团的影响进行系统和定量的调查。因此，这样的研究是非常及时和必要的，并有望取得重大的科学进步，这是本提案的主题。最终的目标是在光化学控制下实现对树状大分子的定量理解。这是为了未来的树状大分子设计，其中通过使用光，a)树状大分子的整体形状可以改变，b)树状大分子的内部可以暴露（要么释放有效载荷，要么用于催化目的）。c)可以解决自组装行为，d)可以利用光力学行为（例如人造肌肉）。为了进行系统的研究，树突和树状大分子将在分支点上含有光致变色基团，直到第3代。作为光致变色基团，将使用热可逆的铬基和双稳定的二乙烯基。需要定量调查的属性是：数值和/或变化：消光系数、光转换的量子产率、吸收最大值、光转换的速率常数、光致变色基团的反应比例、光转换的顺序（如从外围到中心，或随机）、形成速率常数与树状大分子中光致变色基团位置的关系（如内部与外部基团）、光反应的大小和形状变化（流体动力学半径）、光反应动力学与聚集行为。基于这些研究，将确定树突中空间接近的光致变色基团的协同效应和整体树突效应。相反，在树突结构中包含光致变色基团的影响将被理解，并将制定新功能材料类别的设计策略和规则。

Currently research into dendrimers is one of the most relevant research topics in Chemistry and this research activity is centred on Materials Science (mainly in Nanoscience and Nanotechnology) as well as on Biology/Medicine.This research subject has moved within a few years from a fringe interest to a subject which is central to a large number of research disciplines. This is due to the fascination with the architectural beauty of dendritic constructions as well as their potential applications. Initially research has focused on the design and construction of the initial dendrimer architectures and to determine basic structure properties correlations. At present research is concerned with adding functionality to dendritic architectures and to establish precisely where dendritic architecture enhances specific properties. Though a great deal of work has been reported, surprisingly little attention has been paid to photodirected shape and property modulations of dendrimers, specifically no effort has been reported, to the best of the knowledge of the applicant, of a systematic and quantitative investigation of the effects of positioning photochromic groups in the branching points of dendrimers. Thus, such research is very timely and necessary and promises significant scientific advances and it is the subject matter of this proposal. Ultimately it is the aim to achieve a quantitative understanding of dendrimers under photochemical control. This is for the future design of dendrimers, where by using light, a) the overall shape of the dendrimer can be changed, b) the interior of the dendrimer can be exposed (either to release a payload or for catalytic purposes., c) the self assembly behaviour can be addressed, d) the optomechanical behaviour can be exploited (eg artificial muscles).For a systematic investigation dendrons and dendrimers containing photochromic groups in the branching points will be prepared up to generation 3. As photochromic groups thermally reversible chromenes and bistable dithienylethene groups will be used.Properties to be investigated quantitatively are the values and/or changes of: extinction coefficients, quantum yields of the photoconversions, absorption maxima, rate constants for the photoconversions, proportion of the photochromic groups reacting, sequence of the photoconversions, (eg from periphery to centre, or random), dependency of rate constants of formation on the position of the photochromic groups in the dendrimer (eg internal vs. external groups), size and shape change as a consequence of the photoreactions (hydrodynamic radii), kinetics of the photoreactions and the aggregation behaviour.Based on these investigations, synergistic effects of photochromic groups being in close spatial proximity in dendrons and the overall dendrimer effect will be determined. Conversely the effect of the inclusion of photochromic groups in the dendrimer structure will be understood and design strategies and rules for classes of new functional materials will be formulated.