Design And Preparation Of Non-Centrosymmetric Organic Donor-Acceptor Cocrystals And Their Applications

Asymmetric synthesis of organic compounds has become a very successful field of chemistry in its own right,and it is of great interest to generate materials with unique physicochemical properties through the transfer,breaking and disappearance of symmetry centers in different hierarchical states of self-assembled structures.Organic cocrystals act as stoichiometric multi-component molecular crystals in which different components are assembled by heterogeneous molecular interactions（hydrogen bonding,halogen bonding,charge transfer,andπ-πinteractions）.The obtained cocrystals are a reliable mode of intermolecular interactions that can be used to generate supramolecular functional materials.As an important part of crystal engineering,cocrystal system can obtain materials with asymmetric centers through the co-assembly of heterogeneous molecules without the introduction of additional functional groups,which provides a new idea for the research of optical nonlinearity,chirality and polarity.The specific research work of this thesis is as follows:（1）Organic donor-acceptor complexes as new organic semiconductor class have attracted wide attention,due to their potential applications in functional optoelectronics.Herein,we present two new charge transfer cocrystals of dicyanodiazafluorene-perylene（DCPE）and di-cyanodiazaflfluorene-pyrene（DCPY）through a rational cocrystal-engineering strategy.Although they are both 1:1 mixed stacking cocrystals with similar chemical structures,the DCPE cocrystal possesses a non-centrosymmetric space group and narrower band gap compared to DCPY cocrystal,because of the noncovalent bonding variation.The electrostatic potential accumulated in the lateral facets leads to highly twisted DCPE nanobelts,and the small band gap causes near infrared fluorescence.Meanwhile,the DCPY crystals with centrosymmetric space groups and weaker intermolecular interactions exhibited an untwisted morphology and red emission.This study will be helpful for the design and understanding of functional cocrystal materials that can be used in flexible micro/nano-mechanics,mechanical energy,and optical devices.（2）Supramolecular chirality is essential for the development of functional materials.In this study,we report the synthesis of twisted nanobelts based on charge-transfer（CT）complexes using self-assembly cocrystallization starting from asymmetric components.An asymmetric donor,DBCz,and a typical acceptor,tetracyanoquinodimethane,were used to construct a chiral crystal architecture.An asymmetric alignment of the donor molecules induced polar±（102）facets that,accompanied with free-standing growth,resulted in a twisting along the b-axis due to the electrostatic repulsive interactions.Meanwhile,the alternately oriented±（001）side-facets were responsible for the propensity of the helixes to be righthanded.Addition of a dopant significantly enhanced the twisting probability by reducing the surface tension and adhesion influence,even switching the chirality preference of the helixes.In addition,we could further extend the synthetic route to other CT systems for formation of other chiral micro/nanostructures.Our study offers a novel design approach for chiral organic micro/nanostructures for applications in optically active systems,micro/nano-mechanical systems and biosensing.（3）In addition to polyvinylidifluoride（PVDF）polymers,efficient organic piezoelectric materials are widely in demand due to their great potential in cryogenic,solution handling,low-cost energy harvesting,mechanical sensors,and biomedical applications.In this paper,a novel piezoelectric charge-transfer complex is synthesized,which consists of a mixed stacked dibenzazole derivative（DBCz）and a tetracyanodinazafluorene derivative（TCAF）.The DBCz-TCAF complex can undergo reversible crystal-crystal transition at about 40℃or under mechanical force,and obtain multiple metastableα’phase eutectic with symmetry breaking.Transverse piezoelectric nanogenerators（PENGs）based on thermally evaporating cocrystal composite films exhibit significant energy conversion behavior under mechanical agitation.We obtained a piezoelectric coefficient of about 4.9 pC/N for d₃₃ by testing,and a piezoelectric coefficient of～37.1 pC/N for d₃₁by calculation.In addition,we have successfully constructed a piezo-triboelectric energy harvesting device using rod-coated DBCz-TCAF complex/multilayer graphene/polydimethylsiloxane（PDMS）composite film,which exhibits excellent output performance with an open-circuit voltage up to 60 V and a short-circuit current density of 0.67μA/cm².This single-electrode nanogenerator can work as a multifunctional electronic skin to achieve a good response to various mechanical external forces for information transmission.We believe that the discovery of this new family of organic piezoelectric materials,charge-transfer complexes,will have great potential in flexible,wearable and self-powered electronics.