Locally twisted donor-π-acceptor fluorophore based on phenanthroimidazole-phenoxazine hybrid for electroluminescence
Introduction
Luminescent organic materials have received increasing attention and found their applications in chemical sensing [1,2], information security [3,4], optoelectronics [5], [6], [7], [8], and biotechnology [9], [10], [11], [12], etc. Display and lighting devices based on organic light-emitting diode (OLED) technology are the most prosperous products of luminescent organic materials in the current market [13], [14], [15]. Organic light-emitting materials showing high luminescent efficiency and tunable emission wavelength are essential to electroluminescence (EL) applications [16], [17], [18], [19], [20]. So far, many efficient organic luminophores, such as anthracene [21], [22], [23], tetraphenylethene, [24], [25], [26] and pyrene [27], [28], [29], quinacridone [30], [31], [32], quinoxaline [33], [34], [35] derivatives have been developed as light-emitting cores in EL devices due to their highly emissive π-π* transition states. Phenanthroimidazole (PI) is a recently emerging building block for highly efficient EL materials, by virtue of its π extended skeleton for high photoluminescence (PL) quantum yield, bipolar electronic characteristics, and facile synthesis [5,[36], [37], [38], [39], [40], [41], [42]]. Since the first report of PI-based EL materials in 2009 [43], PI has demonstrated its versatility in OLED materials showing remarkable performances [44], [45], [46], [47], [48], [49], [50]. However, because of its planar conformation, PI-based emitters often suffer from aggregation caused quenching (ACQ), which needs to be modified with bulky substituents or moieties of aggregation-induced emission (AIE) [46,51,52]. Furthermore, although PI shows bipolar electronic features suitable for acting as donor or acceptor, its electron push-pull effect is medium, which is hard to realize intense charge transfer (CT) for tuning electronic and excited state property [53].
This work presents the design, synthesis, characterization and application of a new PI-based donor-π-acceptor (D-π-A) fluorophore, 10-(4-(1-(4-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-10H-phenoxazine (PIPXZ, Scheme 1). In this design, the PI group serves as the A, while the electron-rich phenoxazine (PXZ) acts as the D [54]. A 4-(trifluoromethyl)phenyl (TFP) group was introduced to the PI skeleton to improve solubility in organic solvents and further modulate the electronic structure of PI due to its weak electron-withdrawing ability. The new emitter PIPXZ was systematically investigated by absorption/PL spectroscopy, thermoanalysis, density functional theory (DFT) calculation, etc. We found that PIPXZ showed a locally twisted D-π-A conformation, which can realize good emission efficiency and pronounced CT excited-state character. Finally, PIPXZ was successfully applied in OLEDs as the light-emitting layer.
Section snippets
Materials
Starting material 4-(10H-phenoxazin-10-yl)benzaldehyde was prepared according to the literature procedures [55]. Other chemicals and solvents were directly used as received from commercial suppliers.
Synthesis of PIPXZ: Phenanthrene-9,10-dione (5 mmol), 4-(10H-phenoxazin-10-yl)benzaldehyde (5 mmol) and 4-(trifluoromethyl)aniline (7 mmol), AcONH4 (50 mmol) were refluxed in AcOH (30 mL) for 8 h under N2 protection. After cooling down, 60 mL methanol was added with stirring to obtain a suspended
Results and discussion
The synthetic procedure of PIPXZ is shown in Scheme 1. First, the key intermediate 4-(10H-phenoxazin-10-yl)benzaldehyde was prepared via a nucleophilic aromatic substitution reaction with 10H-phenoxazine and 4-fluorobenzaldehyde [55]. Then PIPXZ was obtained by a “one-pot” procedure using phenanthrene-9,10-dione, 4-(trifluoromethyl)aniline, and AcONH4 in refluxed AcOH [36]. The chemical structure of the final product was characterized by NMR and mass spectroscopy.
To study the structure-property
Conclusion
We developed a new PI-based EL emitter consisting of a strong electron-donating PXZ. It was revealed that the introduction of PXZ can induce a high twisting angle with the phenylene π linker, while the PI-phenylene segment maintains a relatively planar configuration, leading to a locally twisted D-π-A conformation. This unique molecular structure endowed the new material with an intense CT excited state and decent fluorescence efficiency simultaneously. The PIPXZ-based nondoped device emitted
CRediT authorship contribution statement
Dongwei Sun: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Writing – original draft. Yudong Wen: Investigation, Data curation. Jia-Min Jin: Investigation, Data curation. Lu Zhou: Investigation, Formal analysis. Qiang Liu: Investigation, Formal analysis. Bo Liu: Software, Validation. Xiaohui Wu: Formal analysis. Nian Tang: Resources. Yongyan Zhou: Formal analysis. Shaomin Ji: Methodology. Yanping Huo: Data curation, Funding acquisition,
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Dongwei Sun, Nian Tang and Yongyan Zhou were employed by the Electric Power Research Institute of Guangdong Power Grid Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Acknowledgments
This work was financially supported by the Science and Technology Project of China Southern Power Grid (No. GDKJXM20200404), National Natural Science Foundation of China (Nos. U2001222, 21975055, 21975053), Guangdong Basic and Applied Basic Research Foundation (No. 2022B1515020041).
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