The Series Of Alkyl Substituted Quinacridone Derivatives: Polymorphs And Properties

Organic optical and electronic materials have attracted more and more attentions because of their advantage compared with the inorganic materials, such as broader selecting of materials, easier processing and easier making large area films. For organic solid materials, the constituent molecules may form strong intermolecular interactions and assembly packing structures resulting in that the properties of these materials are governed by the whole collective rather than by individual molecules. Therefore, understanding and controlling molecular arrangement in solid state are fundamental issues for obtaining the desired chemical and physical properties of a material. The increasing demand for new molecular organic functional materials requires elucidating the relationship between molecular packing characteristics and optical and electronic properties, which enable the development of new strategies towards high performance organic materials.Organic single crystals as the thermodynamic stability, with the highly ordered molecule packing and low impurity content, often show high transfer rate. Simultaneously, organic single crystals could provide an ideal model for the study that the effect on the solid-state luminescent efficiency, carrier mobility and other physical properties as the clear molecule structure and molecule packing arrangement. Crystal engineering is one of the most intuitionistic and powerful method for analyzing the structure of the organic materials. The intuitionistic molecule structure and molecule packing structure are using for understanding the relationship between the material structure and the material properties deeply.Polymorphism is the existence of the same chemical composition in more than one crystalline modification. For organic solid materials, the properties are often governed by the whole collective rather than by individual molecules. As all properties of organic solids depend directly on the structural characteristics, different polymorphs may have different properties. Therefore, the study on the polymorphism in the solid state provides an approach to understand and control the relationship between the molecular packing characteristics and the resulting properties of organic materials at molecular level. During the past years, a great progress for the study of the polymorphism phenomena in the fields of organic luminescent materials has been made.Quinacridone (QA) and its derivatives are widely used as organic pigments and they display excellent fastness properties as well as pronounced photovoltaic and photoconductive activities. Different polymorphic forms of quinacridone differ significantly in terms of their coloristic properties. The solid powders of quinaridone display different colour, which change from orange to purple. This phenomenon owes to the quinacridone exhibit polymorphism properties. In order to deeply understand and control the relationship, we selected quinacridone as the research system. We have synthesized a series of alkyl substituted quinacridone derivatives. We have obtained two or more than two polymorphs of each compound. Detailed description and comparison of the structures and emission properties of the crystals are presented. Our studies mainly focus on the three aspects:(1) Six forms of n,n-di(n-butyl)quinacridone (DBQA) have been prepared by altering the choice of solvent and the crystallization conditions. The X-ray analysis reveals that the six crystals exhibit differences in the molecule structures: In form A, B or C, there is a crystallographically independent DBQA molecule, but in phase D, E or F, there are two kinds of crystallographically independent DBQA molecules; The molecular geometry of form A or C is non-centrosymmetric, but the molecular geometry of form B, D, E or F is non-centrosymmetric; In form A, two n-butyl chains favor the cis conformation. In form B or C, two n-butyl chains do not favor the all-trans conformation and the tails of n-butyl chains bent to the core. While in form D, E or F, two n-butyl chains favor all-trans conformation. Generally, there are only slight differences among the six forms. They display significant differences in molecular packing arrangements. The different molecular packing structures result in the different luminescence properties of the six solid-states: The forms B and C exhibit similar emission properties because of their similar intermolecular contact characteristic. In form B and C, molecules assemble into molecular columns based on - interactions. The overlap area of the - interaction of form B and C is about two and a half phenyl rings. While the overlap area of the - interaction of forms E and F is about three and a half phenyl rings. So the emission maximum of the crystals of form E or F undergo a red shift of about 35 nm compared with phase B or C. The emission peak of the polymorph A located between the emission peaks of B (or C) and E (F). The overlap area of the - interaction of form A is about three and a half phenyl rings, similar to form E (F). But in form A, the intermolecular stacking interactions only exist within the molecular dimmers and are disconnected. In contrast, the stacking contacts are infinity in the form E (F). On the other hand, although the stacking interactions in A are disconnected, they are obviously stronger than that in B or C. Non-covalent intermolecular interactions such as hydrogen bonding, interactions are all able to strongly influence the final packing structure. The comparison between the emission spectra and the molecular packing structure of the different polymorphs reveals that the stronger intermolecular S S stacking interactions between the quinacridone cores could result in the more remarkable red-shift of the emission spectra. (2) A polymorph A and a pseudopolymorph B of N,N-di(n-octyl)quinacridone (DOQA) have been prepared by altering the crystal growth conditions. The molecular structures of two forms are similar to each other. The non-covalent intermolecular interactions of two polymorphs are significantly different. Both crystals are characterized by the intermolecular interactions and C-H O hydrogen interactions. But the distances and the degrees of overlap of interactions are remarkably different. Simultaneously, molecules of two forms show different hydrogen bonding patterns. The different packing structures result in the different morphologies in crystals of polymorphs. The morphology of crystal A is block, while the morphology of crystal B is needle. The detailed analysis of structures and properties has demonstrated that the non-covalent intermolecular interactions are so complex.(3) The crystallization of N,N-di(n-cetyl)quinacridone (DCQA) gives two crystalline polymorphs A and B. The two crystalline phases show slight differences in the QA core structures and the orientations of the n-butyl chains, while they adopt remarkable different packing structures. Molecules assemble into one-dimensional molecular columns through strong interactions. In form B, every DCQA core is sandwiched by two n-cetyl chains of two adjacent DCQA molecules through C-H interactions. The DCQA cores are isolated from each other by the n-cetyl chains, so no stacking interaction between the DCQA cores has been observed. The luminescent spectrum of polymorph A shows a red shift of about 10 nm compared with polymorph B, which should be ascribed to the remarkable difference in their packing structures. The face-to-face intermolecular interaction can have a dramatic impact on the luminescence properties of organic materials and lead to red shifts of emission spectra. Therefore, the molecular packing properties have certain effect on their emission properties. (4) We prepared the co-crystallization of two molecules of DHQA and TCTDT by diffusing petroleum ether vapor into a chloroform solution containing DHQA and TCTDT with different molar ratio. Two co-crystal polymorphs have been obtained. They display both different and similar molecular structures and molecular packing structures. Two kinds of molecules of DHQA and TCTDT connected to each other through the hydrogen bonds and interactions in both of two forms. This demonstrated that the non-covalent intermolecular interactions are so important.In conclusions, we have designed and synthesized a series of alkyl substituted quinacridone derivatives and obtained polymorphs of these compounds. We also characterized the PL and EL preference based on compounds and elucidated the relationships between molecular packing structures and luminescent properties.