Photoinduced Charge-Shifting and Self-Assembly of Photochromic Polyelectrolytes

有许多方法可以通过化学反应来控制物质的性质。光是引起化学反应的一种特别独特的工具，它有许多优点：它可以穿过玻璃等屏障，可以长距离传播，并且很容易打开和关闭。为了用光控制物质，一种叫做光色素的特殊类型的光敏化学物质特别有用。光致色素是一种分子，当受到光照射时，会改变结构和颜色。当光的颜色被改变或光被移除时，光致变色剂也会变回原来的形状。光致变色物质的例子是在户外变成太阳眼镜的眼镜，但在室内是普通的眼镜。塔夫茨大学塞缪尔·托马斯教授的研究小组研究了光致变色物质的新形式：塑料不仅会改变颜色，而且在受到光照射时还会溶解在水中。由于光导致光致变色分子在两种结构之间可逆地转换，当光被移除时，这些聚合物可以从水中分离出来，从而允许“按需”控制这些塑料的水溶性。这项基础研究有可能造福社会，因为它为一种新型的水纳米材料打开了大门，这种材料利用光的力量来远程控制其性能，最终用于生物医学应用中药物的封装和释放，具有可切换渗透性的膜和可逆粘合剂。除了为研究生和博士后提供实践训练外，该项目还为来自不同种族和民族背景的大学生提供了首次从事原创科学研究的经验。托马斯教授继续他从邦克希尔社区学院招募学生的成功计划，以及他在塔夫茨访问和早期研究学者经验（VERSE）计划中的合作伙伴关系，该计划从历史上的黑人学院和大学招募暑期研究学生。该项目由美国国家科学基金会化学部的大分子、超分子和纳米化学计划以及材料研究部的聚合物计划共同支持，开发了一种新的方法来控制聚电解质的性质，并使新的、自组装的纳米结构和凝聚体的形成能够经历可逆的、由电荷移动引起的光诱导相变。该项目的第一阶段是确定聚合物和亲水性共聚物上的螺吡喃悬垂物的结构如何影响光致变色单元的关键性质。这些性质包括质子化的merocyanine的酸度，merocyanine环闭合的量子产率，以及spiropyran打开的速率。光致溶液pH和zeta电位变化的测量有助于将光学性质的变化与聚电解质电荷的变化联系起来。该项目的第二阶段利用这些系统衍生的结构-性能关系，通过在接近中性ph下聚合物电荷的相应变化，开发可逆的光诱导形成的聚电解质纳米结构和凝聚。将这些原理进一步扩展到光致变色嵌段共聚物，旨在实现可逆的自络合和自凝聚。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

There are many ways to control the properties of matter through chemical reactions. Light is an especially unique tool for causing chemical reactions that has many advantages: it can pass through barriers such as glass, travel long distances, and be switched on and off easily. To control matter with light, a particular type of light-sensitive chemical called a photochrome is especially useful. Photochromes are molecules that change structure and color when irradiated with light. Photochromes also changes back to their original form when the color of light is changed or the light is removed. Examples of photochromic matter are glasses that become sunglasses outdoors, but are regular glasses indoors. The research group of Professor Samuel Thomas at Tufts University investigates new forms of photochromic matter: plastics that not only change color, but also dissolve in water when irradiated with light. Because light causes photochromic molecules to switch reversibly between two structures, these polymers can then be separated from water when the light is removed, allowing for "on-demand" control over the water solubility of these plastics. This fundamental research has the potential to benefit society by opening the door to a new class of aqueous nanomaterials that harnesses the power of light to remotely control their properties for eventual utility in encapsulation and release of drugs in biomedical applications, membranes with switchable permeability, and reversible adhesives. Beyond the hands-on training that this research provides to graduate students and postdoctoral scholars, this project also provides college students from a variety of racial and ethnic backgrounds with their first experiences in original scientific research. Professor Thomas continues his successful program of recruiting students from BunkerHill Community College, as well as his partnerships in the Tufts Visiting and Early Research Scholars Experiences (VERSE) program, which recruits summer research students from Historically Black Colleges and Universities. Jointly supported by the Macromolecular, Supramolecular, and Nanochemistry Program of the NSF Division of Chemistry and the Polymers Program of the Division of Materials Research, this project develops a novel approach to control the properties of polyelectrolytes and enable the formation of new, self-assembled nanostructures and coacervates that undergo reversible, photoinduced phase changes as a result of charge shifting. The first phase of this project is to establish how the structures of both spiropyran pendants on polymers and hydrophilic co-monomers influence the key properties of the photochromic units. These properties include the acidity of protonated merocyanines, the quantum yields of merocyanine ring closures, and the rates of spiropyran openings. Measurements of photoinduced changes in solution pH and zeta potentials serve to correlate changes in optical properties to changes in polyelectrolyte charge. The second phase of this project harnesses these systematically-derived structure-property relationships to develop reversible, photoinduced formation of polyelectrolyte nanostructures and coacervates through the corresponding changes in polymer charge at near-neutral pH. Further extension of these principles to photochromic block copolymers is aimed at reversible self-complexation and self-coacervation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.