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Cite this: Chem. Commun., 2013, 49, 11560 Received 24th August 2013, Accepted 16th October 2013 DOI: 10.1039/c3cc46472e

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Reaction of tetrachlorinated perylene bisimide in a strong base to form an asymmetric compound with charge transfer optical properties† Wenqiang Zhang,ab Xuehong Zhou,a Zengqi Xie,*a Bing Yang,b Linlin Liua and Yuguang Ma*a

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A novel perylene oxazine imide (POI) with donor–p-bridge–acceptor (D–p–A) asymmetric structure was synthesized from tetrachlorinated perylene bisimide (PBI) through transformation of one imide group to oxazine, which exhibits significant charge transfer optical properties in solutions. Scheme 1

Perylene bisimide is a kind of high grade organic color pigment, and is also among the archetype n-type semiconducting materials, which have been widely investigated in the past several decades.1 The chemistry of PBI, the groundwork for the development of PBIbased materials, was well developed by many chemists, such as solving solubility problems,2 expanding perylene core structures,3 developing electron donor–acceptor compounds,4 isolating helical enantiomers5 and so on.6 In most recent years, perylene bisimide was successfully applied as an electron accepting unit in low band gap conjugated polymers through Suzuki coupling and Stille coupling reactions under mild basic conditions.7 Perylene bisimide is commonly viewed as chemically very inert for the six-membered ring dicarboximides fused with aromatic substituents.2a However, saponification reaction of the imide groups is well known in strong hydrolyzing reagents like KOH in tert-butyl alcohol.8 More than 20 years ago, Langhals et al. reported that perylene–naphthalene bisimides may react with alkali metal hydroxides in an ethanol– methanol mixture to form ring-contracted lactamimides.9 In order to know the possibility of using perylene bisimide in some reactions like Wittig reaction10 in which strong base NaOCH3 is usually used, we tried to test the stability of a series of perylene bisimides under different base conditions. During the experiments, we found that tetrachlorinated perylene bisimide reacted with NaOCH3 in dimethylformamide (DMF) to form an asymmetric compound, a

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-20-87110606; Tel: +86-20-22236311 b State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China † Electronic supplementary information (ESI) available: Experimental details, synthesis, structure determination, optical properties, and electrochemical characterization. See DOI: 10.1039/c3cc46472e

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Synthesis of POI.

namely perylene oxazine imide (POI). POI possesses D–p–A structure and shows unique charge transfer optical properties. Herein, we report the results. Tetrachlorinated perylene bisimide (PBI, the chemical structure is shown in Scheme 1) was dissolved in DMF which gave orange color, and then the mixture of sodium methoxide in methanol was added to the above solution dropwise at room temperature. The system turned deep blue immediately, indicating that PBI reacted with sodium methoxide readily. After overnight stirring under ambient conditions, the reaction mixture was treated with glacial acid and the deep colored product was successfully isolated by column chromatography. The yield of the product was typically lower than 10% and was still unsatisfactory even if a larger amount of NaOCH3 was applied in the reaction system, and most of the unchanged starting material could be isolated at the same time. The reaction product was proved to be stable and the chemical structure was well verified as perylene oxazine imide (POI, Scheme 1) by 1H NMR, 13C NMR, HMQC, HMBC, NOESY, FT-IR and HRMS analyses (see ESI†). In contrast, when we used tetraphenoxy-substituted perylene bisimide to do the same reaction under the same conditions, only saponification reaction occurred and no asymmetric compound was formed, which indicates the important role of tetrachlorine substituents in the above reaction. Similar to the case of tetracarboxylic bisimide-lactam ring contraction reaction reported by Langhals et al.,9 the substituents at imide positions affect the reaction process significantly; that is when the aromatic substituents were replaced by aliphatic substituents the transformation of imide to oxazine becomes more difficult (see ESI†). We also investigated the effect of the base on the reaction and found that the reaction becomes more and more difficult with the increased size of the alkoxy anions (see ESI†). Besides, through changing the reaction This journal is

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Scheme 2

Reaction mechanism of PBI with two-fold of NaOCH3.

solvents, the asymmetric compound could be formed when treated with DMF and DMSO. In light of these experimental observations, the reaction mechanism is proposed as shown in Scheme 2. In an alkaline environment, nucleophilic attack of CH3O to one of the carbonyl groups occurs forming an intermediate species (IM1). Similar to the mechanism in the unsubstituted perylene–naphthalene bisimide systems,9 the rearrangement of the electronic skeleton may lead to the formation of a quinoid structure (IM1-q). However, herein the enhanced electron affinity due to the electronwithdrawing chlorine atoms enables the negative charge to be well dispersed on the perylene skeleton (Scheme 2, top) and the transformation between different conformers might be reversible. The electron-withdrawing effect of chlorine atoms was crucial, which was demonstrated by the unreacted trial via replacing the chlorine atoms with electron-donating phenoxy substituents (see ESI†). The increased stability of the intermediate structure allows one more nucleophilic attack by CH3O to form IM2. After the structural rearrangement from IM2 to IM2-q, an oxazine unit is formed; this intermediate is then protonated readily under acidic conditions during the subsequent treatment process and further oxidized by the oxygen in air. In the experiment, we did not observe the formation of ring concentrated perylene lactamimide, which may be attributed to the electron-withdrawing effect of chlorine atoms resulting in antioxidant molecular structure (IM1-q + H) by oxygen under ambient conditions during the acid treatment process and then back to the starting compound (Scheme 2). The reversible reaction from IM1 to the starting compound under acidic conditions corresponds to the low total reaction yield reported here, and further experiments are ongoing in our lab to increase the yield based on this reaction mechanism. As shown in Scheme 1, the transformation of one of the imide groups to the oxazine group endows the resulting compound POI an electron donor (oxazine, D)–p-bridge (tetrachlorinated perylene, p)–electron acceptor (imide, A) structure. This D–p–A structure predicts possible intramolecular charge transfer during the excitation process, which is well addressed by the absorption spectra as discussed below. Fig. 1 illustrates the absorption spectrum of POI in dichloromethane, and that of the PBI is also given as a reference. The absorption of POI is obviously This journal is

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Fig. 1 Absorption spectra of POI and PBI in dichloromethane solutions recorded at room temperature. The concentration for the experiments is 1.55  10 5 mol L 1. The inset shows a photograph of the solutions under natural light.

red-shifted, less structured and broader compared with that of PBI, resulting in jewelry blue color in dichloromethane solution as shown in the inset of Fig. 1. The maximum absorption peak and the extinction coefficient of POI in dichloromethane were determined to be 588 nm and 17 500 M 1 cm 1, respectively. We also observed that POI showed almost no fluorescence with quantum efficiency as low as 1% in dichloromethane. Both absorption and emission behaviors of POI clearly demonstrate intramolecular charge transfer optical properties. The solvatochromic effect of POI was measured in various solvents and the UV/Vis absorption spectra are depicted in Fig. S7(a) (ESI†). In low polar solvents like hexane the absorption spectrum showed a broad and structureless band in the region of 450–700 nm with a full width at half maximum of 121 nm. With the increased polarity of the solvent the absorption is red-shifted but the shifts are not large when the high polar solvent is used, e.g. the shift of the absorption maxima from hexane solution to dichloromethane solution is 17 nm (from 571 nm to 588 nm). The small red shift could be explained by the distribution of the frontier molecular orbitals of POI based on DFT analysis as shown in Fig. 3. Owing to the very large conjugated perylene core, the distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was not obviously separated, leading to the charge transfer process not being significantly affected by the polarity of solvents. However, one interesting phenomenon was observed that when very strong polar solvents such as DMF and dimethyl sulfoxide (DMSO) were applied a very special solvatochromic effect occurred as shown in Fig. 2, totally unlike in other common solvents. The maximum peak was red-shifted to 814 nm and obvious electronic and vibronic structure features were observed. Besides this, a new peak signal near 500 nm appeared. Based on the features of the absorption spectrum in DMF, we propose total charge separation at the ground state to form zwitterions11 as shown in the inset of Fig. 2. The absorption band at around 500 nm could be attributed to the absorption of quinone structure.12 Further investigation on the properties and applications of POI is ongoing in our lab currently. Chem. Commun., 2013, 49, 11560--11562

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Communication (2013ZZ0001), the Guangdong Natural Science Foundation (S2012030006232), and Introduced Innovative R&D Team of Guangdong (201101C0105067115).

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Notes and references

Fig. 2 Absorption spectra of POI in hexane and DMF recorded at room temperature. The concentration for the experiments is 1.55  10 5 mol L 1. Molecules in aromatic structure and zwitterion structure are given in the inset.

Fig. 3

Frontier molecular orbitals of POI.

In conclusion, a novel D–p–A asymmetric structured perylene oxazine imide (POI) was synthesized from tetrachlorinated perylene bisimide through transformation of one imide group to oxazine. The asymmetric POI shows special charge transfer optical characteristics, namely the absorption pronouncedly red-shifted to the region of near-IR light in strong polar solvents like DMF that correspond to the formation of zwitterions. The unique properties of perylene derivatives may expand the scope of this new reaction to more academic and industrial applications such as nonlinear optic devices and organic electronic devices. We are thankful for the support from the Natural Science Foundation of China (51373054), the National Basic Research Program of China (973 Program) (2013CB834705, 2014CB643504), Fundamental Research Funds for the Central Universities

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Reaction of tetrachlorinated perylene bisimide in a strong base to form an asymmetric compound with charge transfer optical properties.

A novel perylene oxazine imide (POI) with donor-π-bridge-acceptor (D-π-A) asymmetric structure was synthesized from tetrachlorinated perylene bisimide...
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