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

目前，树枝状大分子的研究是化学中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/医学方面，这一研究主题在几年内从一个边缘兴趣转变为一个对许多研究学科至关重要的学科。这是因为树状结构的建筑之美及其潜在的应用令人着迷。最初的研究集中在初始树枝状大分子体系的设计和建造上，并确定基本结构性质的相关性。目前的研究关注于增加树枝状结构的功能，并准确地确定树枝状结构在哪里增强了特定的特性。虽然已经报道了大量的工作，但令人惊讶的是，很少有人关注树枝状大分子的光引导形状和性质调节，特别是据申请人所知，还没有关于在树枝状大分子的支化点定位光致变色基团的影响的系统和定量研究的报道。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，这也是本提案的主题。最终的目的是在光化学控制下实现对树枝状大分子的定量理解。这是为了将来的树枝状大分子的设计，通过使用光，a)树状大分子的整体形状可以改变，b)树枝状大分子的内部可以暴露(释放有效载荷或用于催化目的)。c)可以研究自组装行为，d)可以利用光学机械行为(例如人工肌肉)。为了进行系统的研究，将在第三代之前制备树枝和在分支点上含有光致变色基团的树枝状大分子和树枝状大分子。将使用光致变色基团、热可逆的色烯基团和双稳的二硫乙烯基团。需要定量研究的是消光系数、光转化的量子产率、光转化的量子产率和/或变化在这些研究的基础上，将确定光致变色基团在树枝上的空间接近的协同效应以及树枝状大分子的整体效应。相反，将了解树枝状大分子结构中光致变色基团的影响，并将制定针对各类新功能材料的设计策略和规则。

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.