Design,Facile Synthesis,and Solid State Structures Of C₃-Symmetric Molecules With Novel Photophysical Properties

C3-symmetry is universal in nature and widely seen in the artificial work. In chemical context, the C3-symmetric molecules not only show the beautiful structures but also have potential applications in asymmetric catalysis, molecular recognition, molecular self-assembly and so on. Thus, the design and synthesis of molecules possessing C3-symmetry is of great interest for the development of functional materials, since the C3-symmetric molecules would result in novel packing motifs as well as unique physicochemical properties in solid state.Xanthone framework has rigid conjugated system and is observed in various natural products due to their excellent biological activities. While most of attentions have been paid to the pharmacologic properties of xanthone derivatives, the application of them on material science is less explored. Regarding the good thermo-oxidative and hydrolytic stability of xanthone nucleus, the exploration of xanthone-based functional materials is highly desirable. On the other hand, triphenylamine (TPA) is a typical C3-symmetric molecule with flexible skeleton. The excellent hole transport properties make the TPA derivatives receiving a great deal of attentions in organic functional materials, i.e., as hole transport layer in organic light-emitting diodes (OLEDs). However, only a handful of reports concern the star-shaped conjugated system containing multiple TPA units due to the synthetic obstacle.We herein designed and synthesized three new types of conjugated-systems, employing the unique feature of xanthone and TPA in both chemical and physical aspects, to explore functional materials with excellent optoelectronic properties. The first one is xanthone-based rigid system possessing C3-symmetry, particularly for molecules having multiple xanthone frameworks condensed onto a central benzene ring. The second one is the C3-symmetric and flexible system derived from TPA, which would result in various supramolecular architecture due to the rotational freedom of TPA derivatives. Finally, we make the hybrid of TPA and xanthone to construct new conjugated polyheterocycles. The contents of this thesis are as following. In Chapter1, we firstly made the brief introduction about the functional materials and the organic optoelectronic materials, and summarized the development of organic functional materials based on partial xanthone core and/or triphenylamine. Then, we outlined the application of C3-symmetry molecules in asymmetric catalysis, molecular recognition and supramolecular assembly. Finally, we introduced our strategies on molecular design and synthesis, as well as their structural and physicochemical features.In Chapter2, we reported a facile and highly efficient three-step synthetic approach toward condensed xanthones, which resulted in the creation of three dixanthones and an unprecedented C3-symmetric trixanthone in moderate yields. Crystallographic studies indicate that these condensed xanthones possess planar π-conjugated system and form tight face-to-face columnar stacks. The electronic structures of these condensed xanthones were elucidated by optical absorption spectra and theoretical studies, which indicate that the condensed xanthones show the narrower HOMO-LUMO gaps than the pristine xanthone caused by the extension of conjugation. Moreover, the condensed xanthones, especially the C3-symmetric trixanthone, show the distinctly red-shifted emissions with enhanced quantum yields. Further studies demonstrate that the fluorescence of the condensed xanthones can be significantly modulated by acidification without decomposition, which led to the red-shift of emission with enhanced intensity. Apart from the modulation of photophysical properties, the acidification of xanthones also shows the significant effect on their crystalline forms.In Chapter3, we synthesized a series of star-shaped fluorescent molecules through a three-step approach, employing1,3,5-triphenoxybenzene as core and triarylamine as terminal groups. The stability, solid state structure, and photophysical properties of these compounds were investigated. The total molecular geometry of the star-shaped molecule adopts propeller type conformation with quasi-C3-symmetry, as revealed by crystallographic study. In comparison with pristine triphenylamine, the present star-shaped compounds display strong fluorescence with quantum yields up to81.8%. Electrochemical study indicates that the half-wave redox potentials of these star-shaped compounds show the positive shifts. Besides, the influence of the aryls on the electronic properties of the resulting star-shaped molecules has been evaluated. The further investigation on their supramolecular assembly is ongoing.In Chapter4, we made the hybridation of xanthone and TPA. TAP is intrinsically electron rich, whereas the xanthone unit is electron deficient as compared with TPA. The combination of the TPA and xanthone would result in new conjugated system showing novel photophysical properties. Thus we have prepared several new compounds derived from xanthone and TPA. Preliminary investigation reveals the following facts:(1) the conjugated system of the molecule is distinctly twisted to show a screw-like chirality;(2) the electronic states of the compound are very sensitive to the environmental variations. The further study on this system is underway.