Download PDF
Synlett 2017; 28(16): 2121-2125
DOI: 10.1055/s-0036-1590808
letter
© Georg Thieme Verlag Stuttgart · New York

Tri-Petal Lilac-Like Perylene: Asymmetrical Substituted Platform for Regioselective Ether-Exchange Reaction

Manxi Zhou
a   State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. of China   Email: qflee@mail.buct.edu.cn
,
Lei Zhu
a   State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. of China   Email: qflee@mail.buct.edu.cn
,
Zhimin Sun
a   State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. of China   Email: qflee@mail.buct.edu.cn
,
Zhenqing Yang
b   State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. of China   Email: caodp@mail.buct.edu.cn
,
Dapeng Cao*
b   State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. of China   Email: caodp@mail.buct.edu.cn
,
Qifang Li*
a   State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. of China   Email: qflee@mail.buct.edu.cn
› Author Affiliations

This research was financially supported by the National Natural Science Foundation of China (No. 51273017, 20974013, 11374070).
Further Information

Publication History

Received: 30 April 2017

Accepted after revision: 28 May 2017

Publication Date:
06 July 2017 (online)

 
 PDF Download Permissions and Reprints All articles of this category


Abstract

An asymmetrical tri-petal lilac-like platform based on perylene bisimide (PBI) was designed and synthesized to further perform the ether-exchange reaction, while common tetraphenoxy PBI analogue cannot do it. We found that the tri-petal lilac-like platform strategy not only avoids the regioisomers of difunctionalized PBI, but also is a precise and facile way to achieve regioselective introduction of alkyloxy, alkylthio and C=C double bond ended substituents onto the 1-position of perylene bay without the use and removal of the protecting groups. Due to the tunable photoelectrical properties and functional groups at bay position, these n-type PBI derivatives are promising materials for photovoltaic and supramolecular application.


#

Key words

perylene - asymmetrical - regioselective substitution - acceptor - photovoltaic

Perylene-3,4,9,10-tetracarboxylic acid bisimides (PBIs) are n-type organic semiconductors with unique optoelectronic characteristics,[1] high molar absorptivity, high fluorescence quantum yields, ease of chemical functionalization, self-assembling and prominent photochemical and thermal stability. Their derivatives have shown wide range applications in organic solar cells,[2] [3] optical sensors,[4,5] supramolecular materials,[6,7] gel networks,[8] [9] catalyst,[10] [11] [12] photosensitizers,[13] and DNA architecture.[14] [15] [16]

However, the planar structure of the perylene core that facilitates the formation of π–π aggregates resulted in the poor solubility of these dyes. In addition, introducing substituent at imide position does not significantly affect the photophysical and electronic properties of PBI.[1] Therefore, the introduction of functional groups onto the perylene bay position not only helps to improve the solubility of PBI-based systems but also opens an efficient way to tune their electrochemical, optical and electronic properties.

In general, perylene dianhydride can be di- or tetrabrominated and subsequently after imidization to prepare di- or tetrafunctionalized PBIs at bay position with interesting properties. However, dibromination reaction is not regioselective and usually yields a regioisomeric mixture of 1,7- and 1,6-dibromo PBIs.[17] [18] In the case of tetrabrominated PBIs, the substituents are usually limited to same four symmetrical groups, such as phenoxy groups, aryls, pyrrolidinyl groups, and cyano groups.[19–22] But asymmetrical substitution and incorporation of C=C double bond were known to be challenging. Furthermore, four phenoxy groups at bay position are quite stable even under the strong base condition, such as KOH, resulting in low possibility of further exchanging of the functional groups, which reduces the diversity of photoelectrical properties.

Herein, we have designed and synthesized an asymmetrical tri-petal lilac-like platform to obtain regioselective substituted PBI derivatives by ether-exchange reaction, shown in Figure [1]. Interestingly, only triphenoxy PBI showed this reactivity at bay 1-position, but tetraphenoxy PBI counterpart cannot do it. We found a simple way to introduce only one further reactable group, such as C=C double bond, into the bay 1-position without the use and removal of the protecting groups, because the ether-exchange reaction temperature (66 °C) and time (2 h) are much lower/ shorter than those used in previous methods (100–180 °C, 15–69 h).[23] [24] [25] [26] [27]

Zoom Image
Figure 1 The ether-exchange reaction of tri-petal lilac-like perylene
Zoom Image
Scheme 1 Synthesis of PBI 3–6 and the model compound 7. Reagents and conditions: (i) Br2, I2, H2SO4, 95 °C, 68 h; (ii) 2-ethylhexylamine, propionic acid, reflux, 3 h; 4-tert-butylphenol, K2CO3, NMP, 120 °C, 4 h; (iii) NaH, THF, reflux, 2 h.

The synthesis of asymmetrical trifunctionalized PBIs follows the strategy outlined in Scheme [1]. The reaction of perylene bisanhydride with seven equivalents of Br2 in the mixture of sulfuric acid and fuming sulfuric acid at 95 °C for 68 hours afforded a mixture of 1,6,7,12-tetra- and 1,6,7-tribromoperylene bisanhydride. Owing to the poor solubility of brominated perylene bisanhydride in common organic solvents, the crude product was directly used for the subsequent imidization with 2-ethylhexylamine in propionic acid, followed by reacting with 4-tert-butylphenol/K2CO3 in NMP. Then the products were purified by column chromatography with CH2Cl2/petroleum ether to yield PBI-OAr3 3 and model compound PBI-OAr4 7. In the presence of excess sodium hydride, the desired products 4–6 were synthesized by treating PBI-OAr3 3 with two equivalents of respective alcohol or alkylthiol in refluxing THF for two hours, and its yield reached 71–78%.[28] Compared with perylene dianhydride and bromination product, compounds 3–7 exhibit great solubility in CH2Cl2, chloroform, THF, and ethyl acetate.

Similar to the amination of PBIs reported by Wasielewski, the mechanism of ether-exchange reaction is SNAr2,[29] and the nucleophilicity of alkanol and alkylthiol anions are stronger than that of phenol anions. Theoretically, both PBI-OAr3 3 and PBI-OAr4 7 should show the reactivity by this methodology, but in fact only asymmetric PBI-OAr3 3 is active, which might be probably associated with the effect of steric hindrance. In order to investigate the observation, density functional theory (DFT) with B3LYP level of theory and a basis set of 6-31G(d) were used to predict the ground state (GS) and excited state (ES) geometries of the tri- and tetrasubstituted PBIs.[30] The computational methodology is described in the Supporting Information. Compared to the PBI-OAr4 7, the calculations of PBI-OAr3 3 revealed an important decrease of the dihedral angle value (from 31° to 16°) on perylene bay 1,2,11,12-positions (Supporting Information, Table S1). Moreover, in all cases the frontier orbitals in PBI-OAr4 7 are homogeneously distributed all over the molecule. In contrast, a lower density of the HOMO orbitals on the bay 1-position was observed for PBI-OAr3 3 (Supporting Information, Table S2). Therefore, the highly twisted perylene backbone of PBI-OAr4 7 and the large size of four 4-tert-butylphenoxy groups sterically hinder the nucleophilic displacement whereas the lower barrier and electron densities of PBI-OAr3 3 at bay 1-position brings the reactivity of ether-exchange reaction.

Zoom Image
Figure 2 1H NMR spectra of PBI 3, 5, 6 and 7 in aromatic regions

The typical 1H NMR spectra of trifunctionalized PBIs appear to be complicated because of their asymmetrical structure (Figure [2]). For PBI-OAr3 3, two doublets and one singlet, corresponding to the protons of perylene backbone at positions 11, 12 and 8, appear between δ = 8.3–9.5 ppm. The protons at positions 2 and 5 show two singlets (around δ = 8.26 ppm) due to their similar chemical environment. Comparatively, in the simple spectrum of highly symmetric PBI-OAr4 7, only one singlet appears at δ = 8.25 ppm. Two doublets appear between δ = 6.80–7.50 ppm in compound 7, but two doublets and two quartets in compound 3 assigned to the protons of phenyloxy groups at positions 1 and 6 (or 7) appear in the same region. After ether-exchange reaction, alkoxy/alkylthio substituent at position 1 brings different chemical environment. Therefore, an obvious downfield shift singlet was observed, which belongs to the proton of PBI 4–6 at position 2.

To further identify the molecular structure, 2D ROESY NMR spectrum of PBI-OC3H7 5 was recorded in CDCl3(see Figure S12 in the Supporting Information). A signal corresponding to the correlation between the protons at perylene position 2 with those on propoxy group appears in the spectrum. In addition, four signals indicating the correlations of the protons at positions 8 and 5 of PBI ring and the protons of phenoxy group were also observed in the 2D ROESY spectrum of compound 5. These results give additional support to the structure of the 1-alkyl-6,7-phenoxy-substituted PBI.

Figure [3a] shows the UV–Vis absorption spectra of PBI 3–7 in CH2Cl2. The curves exhibit two distinct absorption bands, as many bay-functional PBI derivatives.[22] [31] [32] The maximum absorption peak of PBI-OAr4 7 is observed at λ = 579 nm, while that of PBI-OAr3 3 is blue-shifted to λ = 566 nm depending on the identity of electron-donating aromatic groups. Further introduction of alkoxy/alkylthio instead of phenoxy group onto the perylene core red shifts the λabs to 573, 576 and 574 nm, respectively. The gain of detailed vibronic structure (shoulder appears around λ = 450 nm) for the lower absorption bands in trifunctionalized PBI 3–6, assigned to the electronic S0–S2 transition, were attributed to the asymmetrical steric twisting of the perylene core and the electron-donating effect of the side groups.[20] [33]

Table 1 Summary of Photophysical and Electrochemical Properties of PBI 3–7

PBI

λabs [nm]a

λem [nm]b

E oxd [V]c

E red [V]c

LUMO [eV]d

HOMO [eV]d

3

566

603

1.70

–0.67

–3.94

–6.31

4

573

611

1.56

–0.78

–3.83

–6.17

5

576

614

1.58

–0.76

–3.85

–6.19

6

574

629

1.68

–0.71

–3.90

–6.29

7

579

614

1.61

–0.72

–3.89

–6.22

a Measured in CH2Cl2.

b Measured in CH2Cl2 with excitation at λ = 420 nm.

c Measured in a solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in MeCN versus Fc/Fc+, scan rate = 50 mV/s.

d LUMO and HOMO energy was calculated with reference to ferrocene; HOMO = − (E [onset, ox vs. Fc/Fc+] + 5.1), LUMO = − (E [onset, red vs. Fc/Fc+] + 5.1) from cyclic voltammetry.

The photoluminescence spectra of PBI 3–7 in CH2Cl2 solution excited at λ = 420 nm is shown in Figure [3b]. All these PBIs are red emitters with solution spectra maxima at λ = 603–629 nm. Notably, alkylthio-substituted PBI-SC8H17 6 was found to display larger Stokes shifts (55 nm) in comparison with that of alkoxy- and phenoxy-substituted analogues (35–38 nm). This observation is in agreement with earlier studies,[33] which indicates larger structure relaxation during the photoexcitation process.

The electrochemical behavior of PBIs was investigated by cyclic voltammetry (CV) in acetonitrile and all of the relevant optical and electrochemical parameters are summarized in Table [1]. Figure [3c] shows the CV curve of PBI-OC3H7 5 as a representative of these PBIs. The CV curves undergo one irreversible oxidation wave at E oxd values of 1.56–1.70 V, and one quasi-reversible reduction wave at E red values of –0.67 to –0.78 V. The replacement of phenoxy group in PBI-OAr3 3 with alkoxy/alkylthio group in 4–6 results in potential value shift to the negative direction for both oxidation and reduction processes, reflecting the larger electron-donating ability of the alkoxy/alkylthio group than that of phenoxy group. Therefore, the HOMO and LUMO energy levels of these PBIs were determined by CV, which were –6.17 to –6.31 eV and –3.83 to –3.94 eV under calibration to ferrocene. As expected, a useful estimation trend of band gaps and energy levels can be predicted by DFT calculations (Table S1), but small differences between the theoretical and experimental values still exist. This observation is in agreement with earlier studies,[30] probably because DFT calculations were performed on simplified model and limited by computational restrictions. Consequently, tuning of the absorption spectra, fluorescence spectra and energy levels of PBI derivatives has been achieved by introducing substituents with varying electron-donating ability onto the bay 1-position.

Zoom Image
Figure 3 Normalized UV–Vis absorption (a) and photoluminescence spectra (b) of PBI 3–7 in dichloromethane; cyclic voltammograms of PBI-OC3H7 5 (c) in acetonitrile

In conclusion, we have demonstrated that the tri-petal lilac-like asymmetrical platform is an effective strategy to achieve regioselective substitution on the perylene core by ether-exchange reaction. Compared with tetraphenoxy PBI, triphenoxy PBI showed lower sterical barrier and electron densities at bay 1-position, which leads to the reactivity of ether-exchange. Due to the low temperature and short time of this method, various functional groups can be precisely and easily incorporated onto PBI bay that might offer an entry into research on photoelectrical tunable n-type PBI derivatives for photovoltaic and supramolecular applications.


#

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/s-0036-1590808.
  • Supporting Information


   Permissions and Reprints All articles of this category
Zoom Image
Figure 1 The ether-exchange reaction of tri-petal lilac-like perylene
Zoom Image
Scheme 1 Synthesis of PBI 3–6 and the model compound 7. Reagents and conditions: (i) Br2, I2, H2SO4, 95 °C, 68 h; (ii) 2-ethylhexylamine, propionic acid, reflux, 3 h; 4-tert-butylphenol, K2CO3, NMP, 120 °C, 4 h; (iii) NaH, THF, reflux, 2 h.
Zoom Image
Figure 2 1H NMR spectra of PBI 3, 5, 6 and 7 in aromatic regions
Zoom Image
Figure 3 Normalized UV–Vis absorption (a) and photoluminescence spectra (b) of PBI 3–7 in dichloromethane; cyclic voltammograms of PBI-OC3H7 5 (c) in acetonitrile