CAREER: Monolayer Templated Growth of Organic Crystals

摘要CTS-9703102这个职业项目研究使用单层模板生长晶体的替代界面化学方法。在有机晶体的合成和加工中，形态控制是非常重要的，特别是那些具有各向异性的电子、光学、导电和电磁性质的有机晶体。分子水平的模型化合物在目前的晶体生长方法中很大程度上缺乏。有一类多亚胺染料非常适合于这种模型，因为它(1)二向色性光学性质，(2)对界面作用力的形态敏感性，(3)J-聚集体和内部的能量转移，(4)在彩色摄影和记录，以及在超快激光和强烈增强的非线性光学中的作用。有机晶体的结构和形态由范德华力、库仑力、氢键和电子轨道力的微妙平衡决定。初步结果表明：(1)聚亚甲基晶体表现出很强的光学各向异性；(2)它的结构对微小的空间变化很敏感；(3)它的形态对基质的疏水性很敏感。通过使用与不同晶面具有不同亲和力的单层模板改变核附着能来控制晶体形态是可能的。这项研究的目标是通过控制有机晶体在衬底表面的微结构和取向来生长具有均匀光学性能的有机晶体。单层模板晶体生长的原理是界面能和特定结合位促进了相似晶面的附着，抑制了其他晶面的附着。在下图中，可以通过使用与晶体表面具有不同亲和力的表面基团来实现晶体取向，并且晶体尺寸可以受到混合单层中的晶格失配和微区的限制。采用固体结合自组装和液体结合的朗缪尔单分子膜作为晶体生长模板。硅烷的自组装通过结合不同临界表面张力和亲电性程度的末端基团来修饰二氧化硅底物。电荷共振和电子给体/受体相互作用被认为是J聚集体光谱红移的根源。利用定制的含色团两亲分子和常规两亲分子在空气/水界面形成的单分子膜，比较研究了共轭染料官能团之间的特异性相互作用。采用光谱和显微相结合的方法，在分子水平和原位表征了单分子膜/晶体复合体。利用原子力显微镜(AFM)和傅立叶变换红外光谱(FTIR)对染料晶体在单层模板上的微观结构和界面键合发展进行实时研究。此外，单分子膜的结构和表面能由接触角、FTIR、AFM成像和拉力测量相结合来确定。通过原子力显微镜、偏光显微镜和紫外可见光谱，借助计算机生成的三维模型，确定了其晶体结构和光学性质。这项研究的实验结果有望在以下方面为该领域做出贡献：(1)将有机单分子膜功能化以用于晶体生长；(2)通过模板诱导的晶面取向获得均匀的光学性质；(3)基于微观结构研究了解染料晶体和J-聚集体的光电性质；(4)观察和了解结晶的早期阶段；以及(5)帮助开发具有多功能和新性质的感光染料。总之，本项目提出了一种基于分子水平相互作用和表征的合理设计和组装有机薄膜和晶体的替代化学方法。它追求有机薄膜的功能化，作为一类重要材料的晶体生长模板。它有可能理解有序材料自组织背后的分子原理。在教育计划中，PI计划通过先进与材料工程综合实验室将她在界面材料工程方面的研究与本科教育相结合，并通过将汽车相关研究的最新发展纳入聚合物工程研究生证书计划来弥合学术界和产业界之间的差距。

ABSTRACT CTS-9703102 This Career project investigates alternative interfacial chemical approach to crystal growth using monolayer templates. Morphological control is extremely important in the synthesis and processing of organic crystals, especially those with anisotropic electronic, optical, conductive, and electromagnetic properties. Molecular-level model compounds are largely lacking in current approaches to crystal growth. One class of polymethine dyes is well suited for such models, due to its (1) dichroic optical properties, (2) morphological sensitivity to interfacial forces, (3) J-aggregates and energy transfer within, and (4) functions in color photography and recording, as well as in ultra-fast lasers and strongly enhanced nonlinear optics. The structure and morphology of organic crystals are determined by a delicate force balance of van der Waals, Coulombic, hydrogen bonding, and electron orbital forces. Preliminary results have shown: (1) the polymethine crystal displays strong optical anisotropy; (2) its structure is sensitive to minor steric changes; and (3) its morphology is sensitive to substrate hydrophobicity. It is possible to control crystal morphology by changing the nuclei attachment energy using monolayer templates with varying affinities to different crystal faces. The goal of the proposed research is to grow organic crystals with uniform optical properties by controlling their microstructure and orientation at the substrate surface. The principle of monolayer templated crystal growth is that the interfacial energy and specific binding sites enhance the attachment of similar crystal faces and inhibit the others. In the following figure, crystal orientation can be achieved by using surface groups with different affinities to the faces of the crystal, and the crystal size can be limited by lattice mismatch and micro-domains in mixed monolayers. Solid-bound self-assembled and liquid-bound Langmuir monolayers are used as crystal growth templates. Self-assembly of silanes modifies the silica substrate by incorporating terminal groups of different critical surface tensions and degrees of electrophilicity. The charge resonance and electron donor/receptor interactions have been suggested to be the origin of the spectral red-shift in J-aggregates. Monolayers formed at the air/water interface of mixed custom-made chromophore-containing amphiphiles and regular amphiphiles are used to study comparatively the specific interaction between conjugated dye functional groups. A combined approach of spectroscopic and microscopic methods is used to characterize the monolayer/crystal complex at the molecular level and in situ. Real-time investigation of the microstructure and interfacial bonding development of dye crystals on monolayer templates will be carried out by atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR). In addition, the monolayer structure and surface energy are determined by a combination of contact angle, FTIR, AFM imaging and pull-off force measurements. The crystal structure and optical properties are determined by AFM, polarized optical microscopy, and UV-vis spectroscopy with the aid of computer-generated three dimensional models. Experimental results from the proposed research are expected to contribute to the field in the following ways: (1) functionalizing organic monolayers for crystal growth; (2) obtaining uniform optical properties by template-induced orientation of crystal faces; (3) understanding optoelectronic properties of dye crystals and J-aggregates based on microstructural study; (4) observing and understanding early stages of crystallization; and (5) assisting the development of photographic sensitizing dyes with versatile and new properties. In conclusion, this program proposes an alternative chemical approach to rational design and assembly of organic thin films and crystals based on molecular level interactions and characterization. It pursues the functionalization of organic thin films as templates for crystal growth of an important class of materials. It has the potential of understanding the molecular principles behind the self-organization of ordered materials. In the education plan, the PI plans to interface her research in interfacial materials engineering with the undergraduate education through the integrated Advanced and Materials Engineering Laboratory, and to bridge the gap between academia and industry by incorporating the latest development in automotive related research into the Graduate Certificate Program in Polymer Engineering.