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Two charge transfer (CT) cocrystals (cocrystal-O and cocrystal-R) were prepared by solvent evaporation method with 9-(3-(pyren-1-yl)phenyl)-9H-carbazole (1PY-mCzB) as donor (D), 1,2,4,5-tetracyanobenzene (TCNB) as acceptor (A). The spectroscopic studies, single-crystal X-ray diffraction structure and theoretical calculations are carried out to explore the relationship between structures and properties of cocrystal system. The results indicate both cocrystals were assembled as the … D-A-D … alternated stacking way, implying strong CT interactions. Cocrystal-O with larger structural overlap and smaller intermolecular distance shows orange fluorescence with λmax=592nm, while cocrystal-R with smaller structural overlap and larger intermolecular distance shows red fluorescence with λmax=614nm. Actually, the decreasing structural overlap plays a key role in bathochromically shift emission for cocrystal-R, and it make the higher CT excited transition proportions in cocrystal-R (92.74%) than that in cocrystal-O (89.96%). This work not only provides a deep insight into CT cocrystals, but also supplies some guidance for the design and preparation of multicomponent luminescent materials.
In recent years, organic solid-state luminescent materials have attracted considerable research attention because of their promising applications in the fields of lighting , display [, , ], optical data storage  and sensors . Consequently, great efforts have been paid on the design and preparation of solid-state light-emitting materials. Among the methods, cocrystal engineering, as a strategy to prepare new materials, displays its particular advantages, such as can be prepared by simple, fast, convenient and low cost liquid-phase method , and the phase and morphology of materials could be changed by selecting different co-former component , and further they could exhibit novel and multifunctional applications that single component can not achieve [, , ]Scheme 1.
Cocrystals assemble by two or more components via noncovalent interactions, such as π-π stacking [12,13], charge-transfer (CT) interactions [9,, , , , ], halogen bonds [7,19,20] and hydrogen bonds [21,22]. Among them, CT interactions are most found in the cocrystals, and they can improve the stability and quality of cocrystals so as to form closed packing structures [23,24]. This kind of interaction formed by strong electron-donating donor (D) and easy electron-withdrawing acceptor (A) units. After the introduction of CT interactions, the optical band gap narrows, thus cocrystals exhibit red-shift spectra compared with their constituent components. At the same time, this kind of cocrystals also display excellent optical and electrical properties [8,25,26]. Up to now, organic cocrystals with CT interactions have been found that they have great application potential in photoelectric field, such as bipolar charge transfer [27,28], organic light-emitting transistor (OLET) , non-linear optics [30,31] and optical waveguide [23,, , ].
To our knowledge, researchers have made much effort to understand the insight into the single-component aggregations and have made some achievements [, , , , ]. For example, with decreasing intermolecular distance and increasing structural overlap (intermolecular interaction), emission color shows bathochromically shifts (Fig. 1) [36,37]. Though the researches in two-component D-A aggregations have been widely reported, there are still some questions need to be answered. For example, whether the distance and structural overlap between D and A have the similar effect on luminous color as single-component aggregations? In order to answer this problem, basic model need to be constructed. For previous works, a large number of single crystals with polymorph have been reported [, , ]. However, polymorph in cocrystals, especially in CT cocrystals, have rarely been reported [, , , ]. It severely restricts the deep understand into two-component aggregation with intermolecular CT interaction. Therefore, preparing polymorphs of CT cocrystals and constructing relevant model are really important.
Herein, we designed a new kind of CT cocrystal (1PY-mCzB:TCNB) by slowly solvent evaporation method, wherein 9-(3-(pyren-1-yl)phenyl)-9H-carbazole (1PY-mCzB) as electron donor, and 1,2,4,5-tetracyano-benzene (TCNB) as electron acceptor, shown in Fig. 1. Interestingly, two types of crystal phases (cocrystal-O and cocrystal-R) were obtained. By means of crystal structural analysis, we found both exhibit trimeric (D-A-D) stacking. Combined with photophysical properties surveys and quantum chemical calculations, it was found that the decreasing structural overlap induces bathochromically shift emission. This work not only constructs a model to study intermolecular CT aggregation, but also provides deeper understanding of organic CT cocrystals in fundamental research.
1H NMR and 13C NMR spectra were recorded on a Bruker AVANCE 500 spectrometer at room temperature, using tetramethylsilane (TMS) as the internal standard. The MALDI-TOF-MS mass spectra were recorded using an AXIMA-CFRTM plus instrument. UV–vis absorption spectra are recorded on a UV-2550 spectrophotometer (Shimadzu Company, Japan). Photoluminescence (PL) spectra, time-resolved PL spectra and PL quantum yields (PLQYs) were collected on an Edinburgh FLS980 Spectrometer. PLQYs were measured
Results and discussion
In order to verify whether the co-former components were suitable for growth cocrystal, the surface electrostatic potentials distribution were firstly calculated (Fig. 2(a)). The red areas represent negative potential, and the blue areas represent positive potential. From Fig. 2(a), we can find that the red regions are in the center of pyrene (PY) unit and carbazole (Cz) unit of 1PY-mCzB molecule, indicating electron-rich characteristics, and the blue regions are on the benzene ring of TCNB
In conclusion, we have successfully prepared two CT cocrystals polymorphs (cocrystal-O and cocrystal-R) based on 1PY-mCzB as donor and TCNB as acceptor. They exhibit a distinct emission color: orange for cocrystal-O and red for cocrystal-R. Crystal structures analysis indicate that both cocrystals show trimeric stacking, but there is larger intermolecular distance and small structural overlap between D and A in cocrystal-R. Combined with theoretical calculations and experimental measurements,
CRediT authorship contribution statement
Yue Shen: Project administration, Funding acquisition, Conceptualization, Methodology, Validation, Investigation, Data curation, Formal analysis, Writing – original draft. Jia-wang Hou: Investigation, Methodology, Data curation. Yun-ting Liu: Validation, Investigation, Data curation. Xiao-han Wan: Validation, Investigation. Meng-yu Bai: Methodology, Formal analysis. Xin-dong Jiang: Project administration, Funding acquisition, Supervision. Wen-long Duan: Resources, Writing – review & editing,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work was supported by the National Natural Science Foundation of China (22078201, U1908202), the Scientific Foundation of Liaoning Provincial Department of Education (LJKMZ20220791).
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