Because of the low cost, light weight, flexibility property, π-conjugated polymers have been widely utilized for organic semiconductor devices, including organic light-emitting diodes (OLEDs) [1
], organic field-effect transistors (OFETs) [3
] and organic solar cells (OSCs) [6
]. In recent years, π-conjugated polymers based on the donor (D) and acceptor (A) alternative structures are of great importance and interest on account of their special optical, thermal and electrochemical properties and potential applications in organic semiconductor devices. Imide-based molecules are broadly used as electron deficient materials due to their high photochemical stability, ease of synthetic modification, and outstanding charge transmission ability [9
]. Many of the best-performing materials for OLEDs, OFETs and OSCs contain an electron-withdrawing carbonyl group in the form of an amide or imide functionality, such as diketopyrrolopyrrole (DPP) [12
]pyrrole-4,6-dione (TPD) [14
], perylene diimide (PDI) [9
], naphthalene diimide (NDI) [17
] and so on [19
]. The strong electron-withdrawing nature of imides also makes them excellent candidates as building blocks for photoactive materials. To date, a majority of materials use such moieties in the core of the small molecules [22
] or as acceptor units in D-A conjugated polymers [11
In the last five years, many reports have detailed the use of direct (hetero)arylation polymerization (DHAP) [25
], which is a novel and powerful method for the synthesis of π-conjugated small molecules and polymers with a large variety of small molecule building blocks. Compared with the traditional metal-catalyzed cross-coupling such as the Stille coupling and Suzuki coupling, DHAP allows the formation of carbon-carbon bonds between arenes and aryl halides directly and possesses many advantages such as the needless of prefunctionalization of monomers using hazardous reagents; reducing the reaction steps and cost; avoiding the employing of dangerous reagents as well as the formation of toxic byproducts, and decreasing environmental pollution [30
]. So far, there were a lot of articles and reviews reported about DHAP [32
], which could really give the reader essential information about the DHAP reaction and the advances contained in the paper with respect to state-of-the-art. And synthesizing organic semiconductor materials also included many different electron deficiency units such as DPP, TPD and so on. Punzi and coworkers successfully synthesized TEG-substituted DPP-based low band gap polymers via the DHAP method firstly [34
]. The same group also applied this synthetic methodology to the TPD unit and the reaction was successfully carried out in green solvents [35
], which really corresponded to an “environmentally benign” procedure, so the DHAP synthetic methodology could provide a green, convenient, and inexpensive shortcut to more complex materials, including small molecules and polymers for organic electronics, even for production on an industrial scale [36
Designs principle for new π-conjugated polymers for organic electronic applications include lowering the LUMO energy level for higher stability and improving the intermolecular and intramolecular interactions for efficient charge transport [37
]. A far less studied electron deficient functional group is the thiocarbonyl group, where that is carbonyl oxygen is replaced with sulfur [39
]. Previously, this functionality had been mainly studied in biological chemistry [41
]. However, the introduction of thionating materials for organic electronics has been gradually introduced [43
]. These thionation methods typically lead to a lowering of the LUMO level and increaed electron affinity. Furthermore, thionation gives rise to the possibility of S–S contacts, which can increase the intermolecular and intramolecular interactions to obtain high charge mobility.
Recent studies have demonstrated that the tailoring of molecular structures of π-conjugated polymers may result in new materials with unusual and exceptional properties. The LUMO levels of imide compounds can be stabilized by introducing strong electron-withdrawing groups into the molecules’ framework [38
]. The simple thionation of imide compounds not only increases the intermolecular interactions, but also reduces their LUMO levels compared with those of their parent compounds [49
]. Moreover, the electron mobility of thionated compounds is higher than those of parent compounds under a nitrogen atmosphere. The LUMO levels of thionated molecules were much lower, indicating the electron-withdrawing ability of the thioimide group is stronger than the imide group. Although the electronegativity of oxygen is stronger than that of sulfur, oxygen atoms are comparatively small and their electronic orbitals are very tight. The interelectronic repulsion induced by an additional electron is higher in oxygen than that in sulfur. Therefore, the electron-withdrawing ability of the imide group is lower than expected [45
]. The low lying LUMO level could improve the air-stability of the devices and on account of the intense S–S intermolecule and intramolecule interactions, the material could achieve higher charge mobility.
To date, several studies have illustrated the importance of thioimides in improving the properties of these small molecules by lowering the LUMO energy level and increasing the charge mobility even further [45
]. The stabilization of the LUMO level can improve the air-stability of the devices. Furthermore, the intense S–S contact increases the intermolecular electronic coupling in the solid state, thereby enhancing the charge mobility of the device. Therefore, thionation not only increases the electron mobility of the original imide molecules but also improves the air-stability of these compounds.
Phthalimide (PhI) is a famous building block for constructing conjugated polymers used in organic electronic devices [53
]. Over the past several years, the PhI unit has emerged as one of the most widely used acceptor units to construct high performance materials for organic semiconductor applications [55
]. The PhI molecule is electron-withdrawing, on account of the electrophilic imide group, which plays an important role in D-A conjugated polymers. It possesses a small aromatic core which is favorable for tuning its chemical structure and physical properties. Moreover, the original source of PhI, phthalic anhydride, is much cheaper and easier to prepare, which provides an advantage for large scale synthesis. More importantly, incorporation of PhI units between linking bridges could also increase the intermolecular and intramolecular interactions which would be beneficial for the charge transport. Bithiophene (BTh) has been widely employed as the donor unit for high mobility polymers in organic electronic devices due to its high degree of polymer backbone planarity and the ordering film morphology [56
Because of the promising potential of PhI-based conjugated polymers as practical organic electronic materials, efficient DHAP of PhI-based monomers are highly desirable. As we all know, the carbon-hydrogen bonds on the benzene ring possess lower reaction activity, so it is lightly difficult for them to be involved in the reaction [58
]. DHAP is a technology to develop sustainable, atom-efficient, and environmentally benign polymerization method for the large-scale synthesis of high performance copolymers with less branching defects, allowing the formation of carbon-carbon bonds between carbon-hydrogen bonds and carbon-halogen bonds directly. Thus, it is necessary to enlarge the scope of application of DHAP with the carbon-hydrogen bones based on any aromatic rings participating in this kind of reaction. In our study, we have systematically investigated the reaction conditions of DHAP for PhI units. The results from this study provide a generally applicable methodology for the broad application of DHAP to synthesize more useful π-conjugated materials.
Among the well-established chemical modification strategies for conjugated polymers, substitution of the imide oxygen atoms with sulfur, known as thionation, which renders the core more electron deficient, may provide an excellent approach to tuning the optoelectronic properties of materials [46
]. This study aims to investigate the effect of thionation of imide groups on the conjugated polymers. In contrast to the original imide, the thionation of PhI-based polymer has been barely studied, and fewer considering the limited understanding of the difference between the original and thionated polymers, studies addressing the design and synthesis of thionated material should be of great interest.
In this work, we described the synthesis of a series of novel polymers with or without thiocarbonyl by optimal DHAP method and characterization of the optical, electrochemical and thermal properties. We anticipated that by replacing one or two of the carbonyl groups with thiocarbonyl, we could tune the optoelectronic properties in this system and prepare materials with broad absorption region and low LUMO energy level, which is attractive for potential application in organic semiconductor devices.
In summary, we have investigated the DHAP with the carbon-hydrogen bond on the benzene ring and the effect of thionation on the optical, electrochemical and thermal properties in solution and thin film for the first time, and show that introducing thionation could lead to a red-shift of the absorption, reduce the LUMO energy level as well as improve the Td. DSC characterization demonstrated that liquid crystalline phases appeared in the thionated polymers. Furthermore, when compared to the parent imides, the thionated derivatives possess lower LUMO energy levels and narrower bandgaps that are attractive for potential applications in organic semiconductor devices such as OLEDs, OFETs and OSCs. These results show that thionation can be used to tune the optical, thermal properties of imide-containing polymers and the LUMO energy levels of imide compounds can be simply stabilized by thionation. When combined with the relative ease of the synthetic transformation, these findings suggest that thionation may be a promising method for the design of novel conjugated polymer materials for organic semiconductor devices.
We prove that this work represents a critical step for broadening the application scope of high performance conjugated polymers that can be synthesized by DHAP. Future studies will be concerned with the DHAP of PhI-like monomers to yield the corresponding polymers, and with thionation of polymers containing PhI type chromophores in the backbone. Moreover, the availability of relevant databases about the research on DHAP based on benzene ring and thionation of carbonyl is quite insufficient. Therefore, development of new polymerization processes and thionated molecules is necessary to prepare relevant materials and improve the quantitative understanding of structure–performance relationship, which can potentially optimize the performance of charge mobility as well as power conversion efficiency.