In recent years, bifluoreylidene and its derivatives have attracted great attention, since their unique conformational behavior [1, 2] and they are promising as new generation of electron acceptors in organic electronics [3-6]. Nowadays, the structures of related molecules are limited, which can mainly be divided into three types, namely, BF-1 [7-13], BF-2 [3, 14-15] and BF-3 [3, 14, 16] (Fig. 1). As for compounds BF-1 with functional groups in different positions, they present various conformations and physicochemical properties. For instance, the dihedral angle between two fluorene π-planes is range from 31° to 52° . For compounds BF-2 and BF-3 , due to the effect of the enhanced π-conjugation, their absorption onsets of UV–vis spectra can be shifted to long wavelengths of 560 nm and 603 nm, respectively, which is beneficial to harvest more sunlight. Even though BF-2 based organic photovoltaics devices realized a power conversion efficiency (PCE) of 1.7% , its narrow light absorption range and high lowest unoccupied molecular orbital (LUMO) level strongly restrict further device efficiency improvement.
|Fig. 1. Chemical structures of bisfluorenylidene derivatives (R represents functional groups) and 3D-imide|
To overcome the aforementioned issues and further enrich the bifluoreylidene structure library, the tetra-phthalimide endcapped bifluorenylidene (3D-imide) is designed based on the following considerations: (1) The light absorption onset can redshift since effective π-conjugation is elongated; (2) The electron-withdrawing dicarboximides units are introduced to decrease the LUMO energy level; (3) Alkyl chains can be easily attached at the nitrogen atoms of the end-capped dicarboximides to ensure the good solubility in common solvents. Herein, we report the synthesis, physical and electrochemical properties of a novel 3D acceptor. In the end, density functional theory (DFT) calculation method is employed, whose results are well in agree with the experimental discoveries.
The target compound 3D-imide could be synthesized via two routes as illustrated in Scheme 1. 2, 2', 3, 3', 6, 6', 7, 7'-Octamethyl-9, 9'- bifluorenylidene (2) was synthesized in 32% yield through Lawesson's reagent-assisted reductive coupling with 2, 3, 6, 7-tetramethyl-9H-fluoren-9-one (1) as starting material. Thereafter, radical bromination was done with bromine in CCl4 upon irradiation with a 250 W halogen lamp , which afforded 2, 2', 3, 3', 6, 6', 7, 7'-octakis(dibromomethyl)-9, 9'-bifluorenylidene (3) in 75% yield. Precursor 3 was directly used in the next step without further purification. Finally, the target compound 3D-imide was obtained as purple solid in 50% yield via a Diels-Alder cycloaddition. It is noted that using our reported precursor 4  as starting material can also yield 3D-imide. Its chemical structure was unambiguously confirmed by high-resolution mass spectrometry (HRMS). Fig. 2 displays a single peak at m/z 1704.1512 Da, which is consistent with its expected molecular mass of 1704.1529 Da. Also, the isotopic distribution observed perfectly matches with the simulated pattern. A well-resolved 1H NMR spectrum of 3D-imide could be recorded in CDCl3 at room temperature. As shown in 1H NMR spectrum (see the Supporting information), we can assign the protons near nitrogen atoms to the chemical shift of 3.6 ppm and the four single peaks (8.0–9.5 ppm) are assigned to the varying environmental aromatic protons of 3D-imide, whose FT-IR spectrum was also recorded (Fig. S1 in Supporting information). 3D-imide presents very good solubility in common organic solvents and excellent stability in the solid state, facilitating all further characterizations. Note that a resolved 13C NMR spectrum could not be obtained, even though we enhanced the concentration of the CDCl3 solution and extended the accumulation time of NMR experiments, which is similar to the reported results .
|Fig. 2. MALDI-TOF mass spectrum of 3D-imide. Inset: the corresponding experimental and simulated isotopic distributions|
The solution (chloroform) and thin-film UV–vis absorption spectra of 3D-imide are shown in Fig. 3. The first band peaking at 326 nm is corresponding to the π–π* transitions of the diphthalimide fused fluorene, while the band in the long-wave length region is ascribed to the π–π* transition of the tetraphthalimide end-capped bifluorenylidene conjugated structure. No obvious spectral shift is observed from solution to thin film, indicating that there is no appreciable intermolecular aggregation, which might be caused by the 3D geometry. Notably, the absorption edge of 3D-imide is 655 nm, which is redshifted with respect to that of compound BF-3, due to the extension of the effective conjugation length by the incorporation of the imide moieties . The optical gap of 3D-imide is determined to be 1.90 eV based on the equation Eopt = 1240/λonset.
|Fig. 3. UV–vis absorption spectra of 3D-imide in chloroform and in thin film|
To investigate the photoinduced charge transfer and charge separation between poly(3-hexylthiophene) (P3HT) and acceptor 3D-imide, the photoluminescence (PL) quenching experiments were carried out in the solution state. Fig. 4 displays the PL spectra of P3HT solution and the mixtures of P3HT/3D-imide solutions. With the increasing concentration of 3D-imide, the PL intensity of the P3HT is decreasing correspondingly. Notably, we observe that the intensity of 581 nm emission peak for P3HT decreases faster in comparison with its shoulder peak, which is different to the reported results . With the 1 ×10-4 mol/L 3D-imide addition, the 581 nm emission can be almost completely quenched. Therefore, based on the pioneering works [15, 19-20], we can conclude that 3D-imide represents a promising novel electron acceptor for organic electronics.
|Fig. 4. PL quenching spectra of P3HT: 3D-imide in CHCl3 with increasing concentration of 3D-imide (λEx = 540 nm)|
To investigate the electrochemical properties of 3D-imide, cyclic voltammetry was employed. As shown in Fig. 5, two reversible reduction waves are recorded, which are assigned to the formation of the corresponding radical anion and di-ion . By measuring the difference between the onset of reduction and the half-wave potential of the ferrocene standard, lowest unoccupied molecular orbital (LUMO) is calculated to be -3.76 eV, according to the equation: LUMO = -[Ered/onset-E(Fc+/Fc) + 4.8] eV. This low LUMO level is comparable with that of conventional electron acceptors, such as dicyanovinylene substituted tetraindenospirofluorene , perylene bis(dicarboximide) analogues  and B → N bridged aromatic units [23-25]. In comparison with the LUMO energy level (-3.35 eV) of BF-3, such deep LUMO of 3Dimide can be attributed to the strong electron-withdrawing character of end-capped imide units. The HOMO energy of 3Dimide is -5.66 eV, calculated from its optical gap according to the equation HOMO = LUMO-Eg.
|Fig. 5. Cyclic voltammetric profile of 3D-imide in dichloromethane with 0.1 mol/L Bu4NPF6 as supporting electrolyte|
To gain more insights into the electronic properties of 3D-imide compared with its carbon analogue BF-3, density functional theory (DFT) calculations were performed on the basis of B3LYP/6-311G (d, p). The optimized geometries of 3D-imide model and BF-3 reveal similar dihedral angles (~35°) between their planar backbones. Also, as shown in Fig. 6, the introduction of imide moieties presents almost no influence on the electron delocalization over the whole conjugated systems. Moreover, the frontier energy levels of 3D-imide are deeper than those of BF-3, which is well in agree with the experimental results. This can further emphasize the function of the imide units. According to timedependent DFT (TD-DFT) calculations, the lowest-energy absorption of 3D-imide and BF-3, can be assigned to the HOMO → LUMO transitions, which is corresponding to the absorption bands at ~602 nm and ~577 nm, respectively.
|Fig. 6. The HOMO–LUMO transition of 3D-imide (left) and BF-3 (right) using TDDFT at the B3LYP6-311G(d,p) level|
In conclusion, we have first time introduced the imide groups into the bifluorenylidene system. The yielding 3D-imide presents stronger electron accepting capability, with a LUMO energy as low as -3.76 eV. P3HT emission quenching experiments confirm its potential as acceptor in the field of photovoltaics. We expect that 3D-imide can be widely used in electronic device.Acknowledgment
This work is supported by National Natural Science Foundation of China (No. 51603055)Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2018.01.033.
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