Novel derivatives of 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylic acid: synthesis, electrochemical and optical properties

Rajeev K. Dubey *ab, Nick Westerveld a, Ernst J. R. Sudhölter a, Ferdinand C. Grozema b and Wolter F. Jager *a
aLaboratory of Organic Materials & Interfaces, Department of Chemical Engineering, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands. E-mail:;
bLaboratory of Optoelectronic Materials, Department of Chemical Engineering, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands

Received 20th July 2016 , Accepted 6th September 2016

First published on 6th September 2016

A family of novel unsymmetrical “peri”-substituted perylene-3,4,9,10-tetracarboxylic acid derivatives (5–10), with 1,6,7,12-tetrachloro-substituents at the bay-positions, has been synthesized. Subsequently, their redox and optical properties have been explored with the intent of unveiling opto-electronic characteristics of these newly synthesized compounds. To synthesize these new compounds, pure 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylic tetra-n-butylester (4) has been employed as the precursor and the structural modifications have been carried out exclusively at the “peri” positions in an efficient manner. The two synthons prepared in this work, 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylic di-n-butylester monoanhydride (5) and 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylic monoimide monoanhydride (8), are extremely valuable and versatile starting materials as they possess free anhydride functionality at the “peri” position in addition to the 1,6,7,12-tetrachloro-bay-substituents. Finally, the conventional methodology for the synthesis of 1,6,7,12-tetraphenoxy-bay-functionalized perylene bisimides and perylene bisbenzimidazoles has been modified to make it faster and more convenient.


The chemistry of perylene-based dyes has been thoroughly investigated and developed in both the academic and the industrial domains.1–3 Over the course of time, a number of important synthetic routes towards perylene-3,4,9,10-tetracarboxylic acid derivatives have been developed. Some representative classes of compounds that have been synthesized are presented in Fig. 1, such as perylene-3,4,9,10-tetracarboxy tetraesters (A1–A3),4–7 perylene-3,4,9,10-tetracarboxy monoimide diesters (B1–B2),5,8,9 perylene-3,4,9,10-tetracarboxy bisimides (C1–C3),1,3 and perylene-3,4,9,10-tetracarboxy bisbenzimidazoles (D1–D3).8,10 In addition, methods to obtain perylene-3,4-dicarboxylic acid derivatives have also been reported.11,12 These perylene derivatives have gained a lot of attention because of their inherently beneficial properties, e.g. chemical robustness, photo and thermal stability, strong absorption and emission in the visible region, as well as high electron affinities and charge carrier mobilities. These compounds have been extensively employed as the photo-functional materials for the construction of efficient and tunable electron donor–acceptor systems13–17 and light-harvesting arrays.18–21 These dyes have also been used in other photo-physical processes of current interest, such as photocatalysis,22,23 singlet exciton fission,24,25 triplet–triplet annihilation,26 organic photovoltaics,27–29 lasing,30 fluorescence probing,31–33 and bio-labelling.34
image file: c6qo00374e-f1.tif
Fig. 1 Previously prepared perylene-tetracarboxylic acid derivatives with tunable optical and electronic properties.

As compared to other organic dyes, perylene dyes provide a wealth of opportunities for the tuning of their properties via structural modification. These structural modifications can be achieved at three distinct positions around the perylene core; namely, the “peri” (3,4,9,10) positions, the “bay” (1,6,7,12) positions, and the “ortho” (2,5,8,11) positions (Fig. 1). Among these, the functionalization of peri and bay positions have been the preferred choices so far. At the peri positions, mostly imide, ester, and benzimidazole groups have been substituted (A1–D1; Fig. 1). The absorption and emission spectra shift bathochromically upon going from A1 to D1.5,20 These peri-substituents influence other properties as well. Generally, the ester functionalized derivatives are the least electron-deficient and exhibit the best solubility.9,32 On the other hand, the imide and benzimidazole groups significantly increase the electron deficiency of the perylene core but decrease the solubility.5,20 In particular, the bisbenzimidazole derivatives have extremely low solubility in common organic solvents.35

Once the peri positions are functionalized, the bay positions (1,6,7,12) provide an additional “handle” to control the opto-electronic properties of the dye. Simultaneously, the bay substituents significantly increase the solubility of the dye by twisting the otherwise planer perylene core.1,36,37 For the bay functionalization, 1,7-disubstitution and 1,6,7,12-tetrasubstitution have been used as the two major approaches, for which 1,7-dibromo- and 1,6,7,12-tetrachloro-perylene derivatives have been used as precursors, respectively.3

Since the halogen atoms can undergo a large variety of substitution and carbon–carbon coupling reactions, the bay-halogenated derivatives (A2–D2 and A3–D3) are highly valuable and versatile synthons. For attaching substituents to bay area, various methods, such as Suzuki21 and Sonogashira38 couplings, and nucleophilic aromatic substitution reactions, mostly with sulfides,16,39,40 cyclic amines,14,33,41 and phenols,5,15,20,41,42 have been reported.

The ortho substitution is an additional and recently developed method to prepare perylene derivatives with altered properties.43–45 Contrary to the bay-substitution, the ortho-substitution does not twist the perylene core. Therefore, this substitution is especially important for applications where planarity of the perylene core is an important requirement. This approach is still in the phase of development and has exclusively been applied on the perylene bisimide derivatives, so far.

We present, herein, the very first synthesis of unsymmetrical peri-substituted 1,6,7,12-tetrachloro-perylene-3,4,9,10-tetracarboxilic acid derivatives 5–10. These bay-chlorinated compounds are excellent starting materials for the synthesis of a large range of perylene derivatives due to the presence of four chlorine atoms that can be easily substituted. In addition, we have modified the conventional synthetic methodology for the tetraphenoxy-bay-substitution to achieve high chemical yield in significantly less reaction time as demonstrated with the synthesis of previously reported tetraphenoxy-bay-area-substituted compounds 12 and 14.

Results and discussion

Synthesis and characterization

The synthesis of the pure perylene precursor 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylic tetra-n-butylester 4 was carried out from commercially available perylene-3,4,9,10-tetracarboxylic bisanhydride (PBA, 1) and is outlined in Scheme 1.
image file: c6qo00374e-s1.tif
Scheme 1 Synthesis of pure 1,6,7,12-tetrachloroperylene tetra-n-butylester 4.

The first step involved the tetra-chlorination of PBA 1 using chlorosulfonic acid. In our recent studies, we showed that the conventional reaction conditions (70 °C, 20 h) to obtain 1,6,7,12-tetrachloro-PBA 2 produces ca. 20% of 1,2,6,7,12-pentachloro-PBA 3 as the side product.7 In this study, we observed that the amount of the side product 1,2,6,7,12-pentachloro-PBA 3 highly depends on the temperature of the reaction. By keeping the reaction mixture at 66 °C for 16 h, we could reduce the amount of pentachloro impurity to less than 10%. The reaction produced trichloro-derivative at reaction temperatures lower than 66 °C along with the pentachloro-derivative. Hence, a further decrease in the reaction temperature was not possible. In view of the sensitivity of this reaction towards temperature, it is recommended not to use an oil-bath, which often results in large temperature fluctuations. For this reaction, we specifically used an aluminum based dry heating block to attain a good temperature control.

In the second step, the crude mixture of 2 and 3 was converted into the corresponding perylene tetra-n-butylester derivatives, because these are highly soluble and crystalline compounds. The esterification reaction with n-butanol and n-bromobutane, in dimethylformamide46 at 60 °C, was highly efficient and gave a mixture of corresponding tetra- and pentachloroperylene tetra-n-butylesters in the expected ratio of ca. 9[thin space (1/6-em)]:[thin space (1/6-em)]1 as revealed by the 1H NMR spectroscopy.7 Eventually, the desired tetrachloro-derivative 4 was obtained in the pure form by recrystallization from acetonitrile/dichloromethane mixtures in overall 60% yield. It is important to emphasize that the pure compound 4 can also be obtained by silica gel column chromatography, using toluene as the eluent.

Scheme 2 depicts the route to the new peri-substituted perylene derivatives that we have synthesized from pure perylene tetra-n-butylester 4. In the first step, the pure perylene tetra-n-butylester 4 was converted to perylene monoanhydride di-n-butylester 5 by an acid catalyzed removal of two ester moieties in refluxing n-heptane.5,9 The synthesis of compound 5 was the most challenging step in the whole synthetic scheme because of the simultaneous formation of corresponding tetrachloro-bisanhydride 2. The yield of the product 5 was found to be highly sensitive to various parameters, such as temperature, amount and type of the solvent, and the amount of p-TsOH·H2O (Table S1, ESI). Noticeably, the selectivity of this reaction towards yielding 5 was significantly reduced at temperatures higher than 99 °C, in particular when using higher amount of p-TsOH·H2O, and when solvents that are more polar than n-heptane were used. This was because of an enhanced formation of the side product perylene bisanhydride 2. Herein, we maximized the yield of 5 by carrying out the reaction with ca. 1.1 equivalents of p-TsOH·H2O using a limited amount of n-heptane at its reflux temperature.

image file: c6qo00374e-s2.tif
Scheme 2 Synthesis of novel 1,6,7,12-tetrachloro substituted perylene derivatives 5–10.

The isolation of compound 5 in pure form was also challenging. Column chromatography could not be employed because the anhydride group of compound 5 strongly adheres to the silica. The further purification attempts based on solubility difference between 5 and 2 also failed. This was mainly because of the moderate solubility of side product 2 in common organic solvents, such as dichloromethane, chloroform, and toluene. The solubility of side product 2 can be explained by the presence of four chlorine atoms at the bay positions. We experienced that alcohols, in particular methanol and ethanol, could not be used for the isolation of compound 5 because alcohols react with compounds 2 and 5 at higher temperatures. Refluxing mixtures of mono anhydride 5 and bisanhydride 2 in alcohols, resulted in the formation of red and highly fluorescent water-soluble compounds. These compounds are most likely formed by the opening of anhydride functionalities by a nucleophilic attack of an alcohol, thus forming ester carboxylate functionalities on a peri-position. This reaction is known for perylene anhydrides, but only occurs when the strong base DBU is added.46 The reason why this reaction takes place for tetrachloro perylene anhydrides 2 and 5 without DBU activation, is the higher reactivity of these chlorinated compound. We ascribe this high reactivity to the higher electron deficiency of the perylene core induced by the strongly electron withdrawing bay-chlorines, that is clearly reflected by the lower reduction potential of these compounds. Finally, compound 5 was purified by recrystallization using refluxing acetonitrile in ca. 65% overall yield. The product has a purity of 97% as 3% contamination of bisanhydride still persisted.

Perylene monoanhydride di-n-butylester 5 is an important intermediate because of the presence of free monoanhydride group, which can be utilized to generate new functionalities at the peri-positions. Herein, compound 5 was first reacted with 1,2-diaminobenzene in propionic acid at 100 °C to get tetrachloroperylene monobenzimidazole di-n-butylester 6 in 91% yield.6 It has to be emphasized that the di-n-butylester moieties present on compound 6 can further be converted to monoanhydride group, which can be used as active site to prepare unsymmetrically substituted bisbenzimidazoles.

The imidization of the anhydride 5 with 2,6-diisopropylaniline was performed in refluxing propionic acid to achieve tetrachloroperylene monoimide di-n-butylester 7 in an extremely high yield (95%). Clear enough, the ester functionalities in 5, as well as the chlorine atoms at the bay area, were stable under these harsh reaction conditions. Subsequently, the di-n-butylester moieties of compound 7 were converted to monoanhydride functionality by the treatment with an excess of p-TsOH·H2O in refluxing n-heptane. This reaction resulted in an hitherto unknown compound 1,6,7,12-tetrachloroperylene monoimide monoanhydride 8 in 95% yield. Compound 8 is an important intermediate, which is capable of providing an easy access to unsymmetrical 1,6,7,12-tetrachloroperylene bisimides and 1,6,7,12-tetrachloroperylene monoimide monobenzimidazoles. To prove this point, compound 8 was converted to unsymmetrical perylene bisimide 9 by the reaction of 4-methoxyaniline in high yield. Furthermore, the synthesis of the unsymmetrical perylene monoimide monobenzimidazole derivative 10 was also carried out in 85% yield by treating 8 with 1,2-diaminobenzene in propionic acid at reflux.

The starting compound 1,6,7,12-tetrachloroperyle tetraester 4 can also be used for the synthesis of pure 1,6,7,12-perylene bisanhydride 2, which is a highly important intermediate to obtain perylene bisimides (like 11). This is done by using an excess of p-TsOH·H2O at higher dilution in n-heptane and reflux conditions for 24 h (Scheme S1, ESI). Subsequently, the imidization of bisanhydride 2 with 2,6-diisopropylaniline was performed in refluxing propionic acid and corresponding perylene bisimide 11 was obtained in 91% yield.

In view of the issues associated with low solubility of perylene bisimides, the substitution of the four bay-chlorine atoms by the phenoxy groups to give 1,6,7,12-tetraphenoxyperylene bisimides (like 12) is a highly desirable process (Scheme 3). The presence of four phenoxy groups at the bay-area generally results in dramatic increase in solubility. This nucleophilic aromatic substitution reaction has been extensively performed with various phenols in the presence of K2CO3 in NMP. The literature shows that a wide range of reaction times (8–48 h)47,48 and reaction temperatures (80–140 °C)47,49 have been employed to perform this reaction, for which low to extremely high yields (30–95%)42,49 have been reported.

image file: c6qo00374e-s3.tif
Scheme 3 Synthesis of 1,6,7,12-tetraphenoxyperylene bisimide 12.

In this study, we examined the efficacy of two other reactions to produce the tetraphenoxy derivative 12 (Scheme 3). In the first method, tetrachloroperylene bisimide 11 was treated with 4-tert-butylphenol in the presence of Cs2CO3 in DMF at 100 °C.5 Interestingly, the reaction was completed in only 3 hours and a high yield (81%) was achieved. Because of the short reaction time, this methodology provides a promising alternative to the exclusively used K2CO3/NMP reaction, which requires longer reaction times. In the second method, 4-tert-butylphenol was reacted with compound 11 in the presence of K2CO3 and 18-crown-6 was used as the phase transfer catalyst in toluene at 100 °C. This reaction has been previously employed on 1,7-dibromoperylene bisimides and was found to be quite fast and efficient.15,41 However, in this case, a maximum yield of only 35% could be obtained for compound 12, even after long reaction time (24 h). Interestingly, the reaction also yielded the triphenoxy-substituted product in a noticeable amount (ca. 30%).

As depicted in Scheme 2, the perylene derivatives with one free monoanhydride group (5 and 8) are important intermediates for the synthesis of corresponding derivatives with one benzimidazole group at the peri-position (6 and 10). The presence of benzimidazole group results in an extension of π-system of the perylene core.35 This effect significantly reduces the solubility of the dye. The solubility is still manageable for the monobenzimidazole derivatives (e.g.6 and 10) due to the presence of better soluble groups on the other side of perylene core.8 However, for the bisbenzimidazole perylene derivatives, tetraphenoxy-bay-substituents are essential to achieve moderate solubility in common organic solvents.10,35 Previously, the synthesis of tetraphenoxy-bay-substituted bisbenzimidazoles was performed in an indirect manner from 1,6,7,12-tetraphenoxyperylene bisanhydrides, which were obtained by the saponification of 1,6,7,12-tetraphenoxyperylene bisimides as described in Scheme S2 (ESI).10,35

In this study, we followed a direct route for an efficient synthesis of bisbenzimidazoles with solubilization improving tetraphenoxy groups at the bay area (Scheme 4). First, 1,6,7,12-tetrachloroperylene bisanhydride 2 was converted into the corresponding 1,6,7,12-tetrachloroperylene bisbenzimidazole 13 by treatment with 1,2-diamonobenzene in refluxing propionic acid, resulting in 85% yield. As expected, compound 13 exhibited poor solubility, so that further reaction for phenoxy-substitution in NMP could not be realized. Therefore, similar to perylene bisimides, the coupling reaction of compound 13 with 4-tert-butylphenol was performed in DMF and it proceeded smoothly at 105 °C to afford the desired product 14 in 74% yield. In the aromatic region of the 1H NMR spectrum of 14, the presence of syn- and anti-isomers was evident by the appearance of four singlets (between 8.4 to 8.6 ppm) originating from perylene core protons (Fig. 2).10 The integration of these singlets revealed the presence of these isomers in a ratio of ca. 3[thin space (1/6-em)]:[thin space (1/6-em)]2. The unambiguous assignment of these singlets to particular isomers could not be done due the extensive overlapping of NMR signals. Therefore, it could not be determined which is the prevailing isomer. Noticeably, the two isomers were reported in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio, when the tetraphenoxy-bisbenzimidazoles were prepared from 1,6,7,12-tetraphenoxy-perylene bisanhydrides in quinolone at 220 °C.10

image file: c6qo00374e-s4.tif
Scheme 4 New direct approach to synthesize 1,6,7,12-tetraphenoxyperylene bisbenzimidazole 14.

image file: c6qo00374e-f2.tif
Fig. 2 The 1H NMR spectrum (aromatic region) of compound 14.

Electrochemical properties

The electrochemical characterization has been carried out using cyclic voltammetry (CV) to understand the electron-accepting nature of the synthesized chromophores 4, 6, 7, and 10–14. The obtained redox potentials in dichloromethane versus Fc/Fc+, together with calculated HOMO and LUMO energy levels versus vacuum, are listed in Table 1.
Table 1 Redox potentials (vs. Fc/Fc+) and electronic energy levels (vs. vacuum) of perylene derivatives 4, 6, 7, and 10–14a
Compd E 1red E 2red E 1ox E g[thin space (1/6-em)]f (eV) E LUMO[thin space (1/6-em)]g (eV) E HOMO[thin space (1/6-em)]h (eV)
a The redox potentials (V vs. Fc/Fc+) measured by cyclic voltammetry in dichloromethane (scan rate = 0.10 V s−1). The potentials are reported as E1/2 (= (Eap + Ecp)/2) and quoted to the nearest 0.01 V. b Not observed. c Quasi-reversible. d Not visible very clearly. e Could not be measured because of poor solubility. f Optical band gap calculated from equation Eg = hc/λa.e. ≈ 1240/λa.e. (nm); where λa.e. denotes the absorption edge wavelength in nm, obtained from offset wavelength derived from the low energy absorption band.50 g Estimated vs. vacuum level from ELUMO = −(E1red + 4.8 eV). h Estimated from EHOMO = ELUMOEg.
4 −1.30 −1.52 b 2.53 −3.50 −6.03
6 −0.95 −1.17d b 2.16 −3.85 −6.01
7 −0.95 −1.31c b 2.35 −3.85 −6.20
10 −0.78 −0.98 b 2.06 −4.02 −6.08
11 −0.78 −1.00 b 2.25 −4.02 −6.27
12 −1.17 −1.39c +0.84 2.00 −3.63 −5.63
13 e e e 1.94 e e
14 −1.15 −1.31 +0.72 1.83 −3.65 −5.48

The perylene derivatives mostly used for applications are the archetypal perylene bisimides (e.g.11). In general, the perylene bisimides are good n-type semiconductors due to the presence of two electron-withdrawing imide groups, that exhibit two reversible reduction waves at low negative potentials corresponding to the formation of the radical anion and dianion.1

In this series of compounds, the perylene tetraester 4 exhibits first reduction potential at ca. −1.30 V, i.e. the most negative value in this series. This means, it is the least electron deficient compound. Upon moving to compounds 6 and 7, the electron deficiency increases because of the presence of either a benzimidazole or an imide group, respectively, that are more electron-withdrawing than the ester groups. Notably, both compounds 6 and 7 exhibit a first reduction at same potential. It means that the benzimidazole and imide groups exert electron-withdrawing effect of the same magnitude on the perylene core. Compound 10, with one imide and one benzimidazole groups, and perylene bisimide 11 exhibit the least negative values of first and second reduction potentials. Therefore, they are the most electron-deficient compounds in the series. It is worth noting that four electron-withdrawing bay-chlorine atoms make these derivatives comparatively more electron deficient than the corresponding derivatives with no bay-substituents.9 In the case of compounds 12 and 14, the presence of four phenoxy substituents at the bay-area clearly exerts a negative impact on the electron deficiency of the perylene core. For these compounds, the first reduction occurs at significantly more negative potential (ca. −1.15 V) compared to that (ca. −0.78 V) observed for similar perylene derivatives with four chlorine atoms (10 and 11).

Absorption and emission properties

The normalized absorption and emission spectra of the compounds 4, 6, 7, and 10–14 in chloroform are shown in Fig. 3, and the relevant optical data of these compounds are summarized in Table 2.
image file: c6qo00374e-f3.tif
Fig. 3 The normalized UV/Vis absorption (left) and emission (right) spectra of synthesized compounds in chloroform.
Table 2 Optical properties of the compounds 4, 6, 7, and 10–14 in chloroform
Compd λ abs (nm) ε (M−1 cm−1) λ em (nm) Stokes shift (cm−1) Φ f[thin space (1/6-em)]a τ f[thin space (1/6-em)]b (ns) k F[thin space (1/6-em)]c (108 s−1) k Q[thin space (1/6-em)]d (108 s−1)
a Fluorescence quantum yield. b Fluorescence life-time. c Rate of fluorescence. d Rate of fluorescence quenching. e Could not be measured because of poor solubility.
4 457 25[thin space (1/6-em)]400 503 2001 0.26 1.6 1.63 4.63
6 530 34[thin space (1/6-em)]200 584 1745 0.36 4.8 0.75 1.33
7 496 38[thin space (1/6-em)]800 533 1400 0.98 5.4 1.81 0.04
10 561 44[thin space (1/6-em)]000 611 1459 0.38 4.7 0.81 1.32
11 521 46[thin space (1/6-em)]900 550 1012 0.91 4.9 1.86 0.18
12 585 53[thin space (1/6-em)]600 618 913 0.72 6.3 1.14 0.44
13 589 e 644 1450 0.28 4.3 0.65 1.67
14 636 50[thin space (1/6-em)]700 672 842 0.20 5.0 0.40 1.60

All the compounds exhibit well-defined S0–S1 absorption and emission bands in the visible region, which is a characteristic feature of the aromatic perylene core. However, a systematic trend has been displayed by these compounds in their optical profile depending on the substituents either at “peri” or “bay” positions. The perylene tetraester 4, which carries four chlorine atoms at bay-positions, has the most blue-shifted absorption (λmax = 457 nm) and emission (λmax = 503 nm) spectra. Surprisingly, compound 4 shows a low fluorescence quantum yield (φf = 0.26) and life-time (τf = 1.6 ns). This is striking because the “bare” perylene tetraester, with no substituents at bay-area, is highly fluorescent with a fluorescence quantum yield of almost unity and a long life-time of 4.0 ns.51 The perylene monoimide diester 7, in which two ester groups are replaced by one imide moiety, gives bathochromically shifted absorption (λmax = 496 nm) and emission (λmax = 533 nm) spectra. In addition, an extremely high fluorescence quantum yield (ϕf = 0.98) and a long life-time (τf = 5.4 ns) is observed for this derivative. A high fluorescence (ϕf = 0.91) and a further red-shifted absorption (λmax = 521 nm) and emission (λmax = 550 nm) were observed for perylene bisimide 11.

For the compounds 6, 10, and 13, which contain either one or two benzimidazole groups at “peri” positions, a larger red-shift of the absorption and emission spectra is observed as compared to that of corresponding derivatives with imide groups (Table 2). This larger red-shift in benzimidazole based derivatives can be explained by an effective extension of π-system of the perylene core.35 Therefore, attaching a benzimidazole moiety as compared to imide group, works better to achieve bathochromic shifts in absorption and emission spectra. However, the presence of benzimidazole moiety clearly reduces the fluorescence quantum yield as compared to imide groups. Surprisingly, the benzimidazoles 6, 10, 13, and 14 possess long fluorescence life-times, which identifies low rates of fluorescence (kF) along with higher quenching rates (kQ), as the cause for the low fluorescence quantum yields of these compounds (Table 2).

Along with “peri” substitutions, the “bay” substitution also influences the optical properties of perylene dye. The tetraphenoxy-bay-substituents present in the derivatives 12 and 14 further shift the absorption band to longer wavelengths by 47–64 nm. This significant bathochromic shift was observed in emission spectra as well. These observations verify the importance of both “peri” and “bay”-functionalizations to fine-tune opto-electronic properties of perylene chromophores.

The HOMO and LUMO levels of synthesized perylene derivatives are shown in Fig. 4, which depicts the clear effect of peri and bay substitution on the electronic structure of perylene core. A noticeable feature is that the derivatives with benzimidazole group (e.g.6) and the corresponding derivatives with imide group (e.g.7) exhibit almost same LUMO energy levels. However, their HOMO levels are significantly different.

image file: c6qo00374e-f4.tif
Fig. 4 The HOMO and LUMO levels of synthesized perylene derivatives against vacuum.


We have devised an efficient and convenient synthetic methodology for the preparation of novel unsymmetrically “peri”-substituted perylene-3,4,9,10-tetracarboxylic acid derivatives 5–10 starting from pure tetrachloroperylene tetra-n-butylester 4. The efficient preparation of unsymmetric tetrachloroperylene monoanhydride di-n-butylester 5 and the inertness of the bay-chlorine atoms and ester functionalities under the applied reaction conditions are the major factors contributing to the success of our method. The newly synthesized derivatives have four chlorine atoms at the 1,6,7,12-bay-positions of perylene core and therefore they are excellent starting materials for the synthesis of a large variety of perylene derivatives bearing bay-area substituents. We have achieved tetraphenoxy-bay-substitution on 1,6,7,12-tetrachloro-perylene bisimide and perylene bisbenzimidazole compounds with high yields in remarkably shorter reaction times using modified reaction conditions. The synthesized compounds displayed systematic trends in optical and electrochemical properties as a function of structure modification at “peri” positions. In this way, the synthetic modifications described in this manuscript provide excellent control over the opto-electronic characteristics of the obtained materials. The studied derivatives altogether are capable of absorbing light in a broad range of visible region of solar spectrum (i.e. 350–700 nm), which makes them highly desirable compounds as photo-functional materials. Further studies focusing on the selective bay-functionalization of the perylene core to produce promising materials for optoelectronic and photovoltaic applications are currently underway and will be reported in due course.

Experimental section


All the reagents utilized in the synthesis were purchased from commercial suppliers and used as received unless otherwise stated. The DMF used in the synthesis was of anhydrous grade. Toluene was dried over sodium under an argon atmosphere prior to use. The purification of the products were performed by column chromatography. The TLC plates and the sorbent for the column chromatography (silica gel 40–63, mesh size 0.230–0.400 mm) were purchased from commercial suppliers.

Instrumentation and characterization

The NMR spectra were recorded with 400 MHz pulsed Fourier transform NMR spectrometer in CDCl3 at room temperature. The 1H NMR spectra of compounds 2 and 13 were measured in D2SO4 and CF3COOD, respectively. The chemical shifts are quoted relative to CDCl3 [δ = 7.26 ppm (1H, singlet); 77.00 (13C, triplet)]. The chemical shift values are given in ppm and J values in Hz. High-resolution mass spectra were collected on an AccuTOF GCv 4G, JMS-T100GCV, mass spectrometer (JEOL, Japan). The FD/FI probe (FD/FI) was equipped with an FD Emitter, Carbotec (Germany), FD 10 μm. Typical measurement conditions were as follow: current rate 51.2 mA min−1 over 1.2 min; counter electrode −10 kV; ion source 37 V. The samples were prepared in dichloromethane.

Electrochemical behavior of the compounds was studied using cyclic voltammetry (CHI 600D electrochemical analyzer) in a three-electrode single-compartment cell consisting of a platinum wire in glass as the working electrode, silver wire as the reference electrode, and a platinum sheet as the counter electrode. The cell was connected to the computer controlled potentiostat (CH Instruments Inc. 600D). Pre-dried CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate was used as solvent. The measurements were done under continuous flow of nitrogen. The concentration of the prepared samples was ca. 0.5 mM. Under these experimental conditions, the ferrocene oxidation was observed at 0.52 V. The potentials of all the reversible peaks are reported as E1/2 (= (Eap + Ecp)/2) in V vs. Fc/Fc+ and quoted to the nearest 0.01 V. The measurements were carried out at 0.10 V s−1 scan rate.

All the spectroscopic measurements were carried out at room temperature. The absorption spectra were recorded with a double beam UV/vis spectrophotometer. The emission spectra were corrected for the wavelength response of the detection system. Fluorescence quantum yields were determined by the comparative method using following compounds as reference: perylene-3,4,9,10-tetracarboxylic tetramethylester (Φf = 0.95 in CH2Cl2) and N,N′-bis(1-hexylheptyl)-perylene-3,4,9,10-tetracarboxy bisimide (Φf = 0.99 in CHCl3).51 Fluorescence lifetime measurements were performed on a Lifespec-ps Fluorescence spectrometer from Edinburgh Instruments. Samples were placed in 1 cm quartz cuvettes and degassed prior to the measurement. The time correlated fluorescence was analyzed by exponential tail fit with F900 Lifespec software. The rates of fluorescence (kF) and rates of fluorescence quenching (kQ) were obtained from steady state and time-resolved optical measurements using eqn (1a)–(d).

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image file: c6qo00374e-t3.tif(1c)
image file: c6qo00374e-t4.tif(1d)

Synthesis of a mixture of 1,6,7,12-tetrachloro- and 1,2,6,7,12-pentachloro-perylene-3,4,9,10-tetracarboxy bisanhydride (2 + 3) from perylene-3,4,9,10-tetracarboxy bisanhydride (1). A mixture of perylene-3,4,9,10-tetracarboxy bisanhydride 1 (10.00 g, 25.49 mmol) and iodine (1.66 g, 6.56 mmol), in chlorosulfonic acid (60 mL), was stirred for 18 h at 66 °C on aluminum based dry heating block. The reaction mixture was poured into an ice-water mixture, and the solid was collected by filtration using a glass filter. The solid residue was washed with several portions of water and dried under vacuum to afford the crude product (13.37 g, 99%). It was contaminated with 5–10% of corresponding pentachloro-derivative as evident by the small peak at 8.78 ppm in NMR spectrum.71H NMR (400 MHz, D2SO4): δ = 8.75 ppm (s, 4H). 13C NMR spectrum could not be measured because of its low solubility. MS (FD-TOF): [M]+ calculated for C24H4Cl4O6 (529.8736) and C24H3Cl5O6 (563.8345); found, 529.8848 and 563.8390.
Synthesis of 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxy tetrabutylester (4). In a 500 mL round-bottomed flask, crude 1,6,7,12-tetrachloroperylene bisanhydride (2 + 3) (10.0 g, 18.86 mmol) and DMF (125 mL) were taken. To this mixture, subsequently, butanol (13.8 mL, 150.8 mmol), and DBU (11.3 mL, 75.4 mmol) were added. The resultant mixture was stirred at 60 °C for 30 min. Thereafter, bromobutane (16.3 mL, 150.8 mmol) was taken in DMF (75) mL. The solution was added to the reaction mixture and the combined mixture was stirred for another 90 min. at 60 °C. The reaction mixture was poured into water and the precipitate was filtered off on a glass filter, washed with water, and dried. The crude product (14.6 g, 98%) was contaminated with ca. 5–10% of corresponding pentachloro derivative, as evident by the 1H NMR and HR mass analysis. The pure 1,6,7,12-tetrachloroperylene tetrabutylester 4 (8.9 g, 60%) was obtained by the repetitive recrystallization from dichloromethane/acetonitrile as reported previously.7 The pure product 4 can also be obtained by silica-gel column chromatography. For column chromatography separation, a portion of crude product 4 (1.0 g) was subjected to silica-gel and eluted with toluene to obtain desired compound 4 (0.85 g, 85%) in pure form. 1H NMR (400 MHz, CDCl3): δ = 8.09 (s, 4H), 4.44–4.32 (m, 8H), 1.87–1.75 (m, 8H), 1.55–1.45 (m, 8H), 1.05–0.96 ppm (m, 12H). 13C NMR (100 MHz, CDCl3): δ = 166.9, 133.9, 133.5, 131.9, 130.7, 127.2, 123.1, 66.0, 30.6, 19.2, 13.8 ppm. MS (FD-TOF): [M]+ calculated for C40H40Cl4O8, 790.1455; found, 790.1431.
Synthesis of 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxy monoanhydride dibutylester (5). Compound 4 (1.00 g 1.27 mmol) and p-toluenesulfonic acid monohydrate (p-TsOH·H2O) (0.26 g, 1.38 mmol) were taken in n-heptane (8 mL). The resulting suspension was stirred at reflux for 48 h. The product 5 starts to precipitate from the reaction mixture after a few hours. After 48 h, the reaction mixture was cooled to room temperature and heptane was evaporated under vacuum. The crude product was washed with a few portions of water and then with methanol to remove impurities based on p-toluenesulfonic acid. Subsequently, the dried orange precipitate was refluxed in hexane (100 mL) for ca. 2 h to remove the soluble starting compound 4. The crude product 5 (0.73 g, 87%) was collected by filtration, which was contaminated with ca. 10% of side product i.e. tetrachloroperylene bisanhydride 2. Finally, the crude product was purified by recrystallization in acetonitrile. In a typical procedure, the crude product (0.73 g) was stirred in refluxing acetonitrile (115 mL) for 2 h. The resultant solution was left at room temperature overnight. The crystals of the product were collected by filtration and dried to obtain the desired product 5 (545 mg, 65%). The product has the purity of ca. 97% as 3% contamination of bisanhydride still persisted. The recrystallization process can be repeated to achieve higher purity. Soxhlet extraction with hexane can also be used as an alternate purification procedure. But it is not a preferred choice because of the sluggishness of this procedure. Purification by column chromatography is not possible as the product sticks to silica. Mp = 213 °C. 1H NMR (400 MHz, CDCl3): δ = 8.65 (s, 2H), 8.13 (s, 2H), 4.41–4.31 (m, 4H), 1.85–1.75 (m, 4H), 1.54–1.45 (m, 4H), 0.99 ppm (t, J = 7.6 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ = 166.6, 158.6, 135.1, 134.7, 134.6, 133.9, 132.2, 132.1, 131.5, 130.9, 125.9, 125.1, 123.4, 118.5, 66.3, 30.5, 19.2, 13.7 ppm. MS (ESI-TOF): [M]+ calculated for C32H22Cl4O7, 658.0119; found, 658.0105.
Synthesis of 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxy monobenzimidazole dibutylester (6). 1,6,7,12-Tetrachloroperylene-3,4,9,10-tetracarboxy monoanhydride dibutylester 5 (50 mg, 0.08 mmol) and 1,2-diaminobenzene (16 mg, 0.15 mmol) were taken in propionic acid (2 ml). The mixture was stirred at 100 °C for 2 h. After cooling down to room temperature, the reaction mixture was poured into water. The resultant precipitate was collected by filtration and washed with several portions of water to remove all the propionic acid. The precipitate was dried and chromatographed on silica, eluting with DCM, to afford the pure product (50 mg, 91%). Mp = 172 °C. 1H NMR (400 MHz, CDCl3): δ = 8.69 (s, 1H), 8.61 (s, 1H), 8.32–8.27 (m, 1H), 8.14 (s, 1H), 8.13 (s, 1H), 7.71–7.66 (m, 1H), 7.34–7.27 (m, 2H), 4.44–4.32 (m, 4H), 1.88–1.76 (m, 4H), 1.57–1.46 (m, 4H), 1.01 ppm (m, 6H). 13C NMR (100 MHz, CDCl3): δ = 166.8, 158.6, 147.2, 143.8, 135.2, 134.6, 134.3, 133.9, 133.2, 132.9, 132.2, 131.9, 131.7, 131.1, 130.3, 128.4, 127.1, 126.8, 126.5, 126.4, 126.3, 123.3, 123.1, 122.3, 121.1, 120.5, 115.8, 66.1, 30.6, 19.2, 13.8 ppm. MS (ESI-TOF): [M]+ calculated for C38H26Cl4N2O5, 730.0596; found, 730.0612.
Synthesis of N-(2,6-diisopropylphenyl)-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxy monoimide dibutylester (7). A 50 mL round-bottomed flask was charged with tetrachloroperylene monoanhydride dibutylester 5 (0.95 g, 1.44 mmol), 2,6-diisopropylaniline (0.38 g, 2.14 mmol), and propionic acid (20 mL). The reaction mixture was refluxed for 16 h and then allowed to cool down to room temperature. The reaction mixture was poured into the water to precipitate the crude product. The precipitate was filtered off and washed with several portions of water to remove propionic acid and aniline. The precipitate was dried and chromatographed on silica, eluting with 2[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2–hexane, to afford the desired product 7 (1.12 g, 95%). Mp = 220 °C. 1H NMR (400 MHz, CDCl3): δ = 8.69 (s, 2H), 8.15 (s, 2H), 7.51 (t, J = 7.9 Hz, 1H), 7.36 (d, J = 7.9 Hz, 2H), 4.45–4.32 (m, 4H), 2.79–2.68 (m, 2H), 1.89–1.79 (m, 4H), 1.59–1.46 (m, 4H), 1.22–1.14 (m, 12H), 1.02 ppm (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ = 166.8, 162.5, 145.6, 134.7, 134.5, 133.8, 133.2, 132.1, 131.7, 131.6, 130.0, 129.9, 129.6, 126.5, 124.2, 123.6, 123.4, 122.4, 66.2, 30.5, 29.2, 24.0, 19.2, 13.8 ppm. MS (ESI-TOF): [M]+ calculated for C44H39Cl4NO6, 817.1532; found, 817.1518.
Synthesis of N-(2,6-diisopropylphenyl)-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxy monoimide monoanhydride (8). Compound 7 (700 mg, 0.85 mmol) and p-toluenesulfonic acid monohydrate (p-TsOH·H2O) (894 mg, 4.70 mmol) were taken in heptane (15 mL). The resulting mixture was refluxed for 20 h and then allowed to cool to room temperature. Thereafter, the reaction mixture was filtered and the solid residue was thoroughly washed with water and a small amount of cold methanol. The residue was dried to obtain the desired product 8 (560 mg, 95%). Mp > 350 °C. 1H NMR (400 MHz, CDCl3): δ = 8.78 (s, 2H), 8.75 (s, 2H), 7.53 (t, J = 7.8 Hz, 1H), 7.37 (d, J = 7.8 Hz, 2H), 2.79–2.65 (m, 2H), 1.19 ppm (t, J = 5.8 Hz, 12H). 13C NMR (100 MHz, CDCl3): δ = 161.8, 158.1, 145.2, 135.9, 135.2, 134.5, 133.2, 131.7, 131.3, 129.8, 129.7, 129.4, 128.0, 125.1, 123.9, 123.5, 123.3, 29.0, 23.7 ppm. MS (ESI-TOF): [M]+ calculated for C36H21Cl4NO5, 689.0144; found, 689.0142.
Synthesis of N-(2,6-diisopropylphenyl)-N′-(4-methoxyphenyl)-1,67,12-tetrachloroperylene-3,4,9,10-tetracarboxy bisimide (9). Synthesized from compound 8 (100 mg, 0.15 mmol), 4-methoxyaniline (36 mg, 0.29 mmol), and propionic acid (2 mL). The reaction mixture was refluxed for 8 h. After cooling down to room temperature, the reaction mixture was poured into the water to precipitate the crude product. The precipitate was filtered off and washed with several portions of water to remove all the propionic acid. The precipitate was dried and chromatographed on silica, eluting with CH2Cl2, to afford the desired product 9 (89 mg, 77%). Mp > 350 °C. 1H NMR (400 MHz, CDCl3): δ = 8.76 (s, 2H), 8.74 (s, 2H), 7.52 (t, J = 7.6 Hz, 1H), 7.36 (d, J = 7.6 Hz, 2H), 7.23 (d, J = 8.8 Hz, 2H), 7.09 (d, J = 8.8 Hz, 2H), 3.89 (s, 3H), 2.78–2.66 (m, 2H), 1.23–1.15 ppm (m, 12H). 13C NMR (100 MHz, CDCl3): δ = 162.6, 162.3, 159.9, 145.6, 135.6, 135.5, 133.4, 133.3, 131.7, 133.6, 130.0, 129.9, 129.4, 128.9, 128.8, 126.9, 124.2, 123.9, 123.7, 123.5, 123.2, 114.9, 55.5, 29.3, 24.0 ppm. MS (ESI-TOF): [M]+ calculated for C43H28Cl4N2O5, 792.0752; found, 792.0726.
Synthesis of N-(2,6-diisopropylphenyl)-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxy monoimide monobenzimidazole (10). A mixture of compound 8 (200 mg, 0.29 mmol) and 1,2-diaminobenzene (62 mg, 0.58 mmol) was refluxed in for 1 h in propionic acid (5 ml). After cooling down to room temperature, it was poured into water to precipitate the crude product. The precipitate was filtered off and washed with several portions of water to remove all the propionic acid. The precipitate was dried and chromatographed (silica/DCM) to afford the pure product (187 mg, 85%). Mp > 350 °C. 1H NMR (400 MHz, CDCl3): δ = 8.94 (s, 1H), 8.86 (s, 1H), 8.75 (s, 2H), 8.55–8.51 (m, 1H), 7.93–7.90 (m, 1H), 7.55–7.50 (m, 3H), 7.36 (d, J = 7.6 Hz, 2H), 2.78–2.68 (m, 2H), 1.20–1.16 ppm (m, 12H). 13C NMR (100 MHz, CDCl3): δ = 162.4, 158.4, 147.1, 145.6, 143.9, 136.1, 135.5, 135.4, 135.0, 133.5, 133.3, 133.1, 132.2, 131.8, 131.1, 130.0, 129.9, 129.6, 129.3, 128.9, 128.6, 126.8, 126.5, 126.5, 124.2, 124.0, 123.9, 123.3, 122.7, 122.6, 122.1, 120.7, 115.9, 29.3, 24.0 ppm. MS (ESI-TOF): [M]+ calculated for C42H25Cl4N3O3, 759.0650; found, 759.0652.
Synthesis of 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylic bisanhydride (2). Pure compound 4 (2.0 g, 2.53 mmol) and p-toluenesulfonic acid monohydrate (p-TsOH·H2O) (4.8 g, 25.5 mmol) were placed in a round-bottomed flask equipped with a reflux condenser. Subsequently, heptane (35 mL) was added and the reaction mixture was stirred at reflux (ca. 24 h) until the heptane solution became completely clear. The completion of reaction was confirmed by the TLC analysis. Afterwards, the reaction mixture was allowed to cool down to room temperature. The solid residue was collected by filtration, washed with methanol and water several times, and dried to obtain the desired product (1.3 g, 97%). 1H NMR (400 MHz, D2SO4): δ = 8.73 (s, 4H) ppm. 13C NMR spectrum could not be measured because of its low solubility. MS (FD-TOF): [M]+ calculated for C24H4Cl4O6, 529.8736; found, 529.8771.
Synthesis of N,N′-bis(2,6-diisopropylphenyl)-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxy bisimide (11). Prepared from compound 2 according to the previously described procedure.71H NMR (400 MHz, CDCl3): δ = 8.78 (s, 4H), 7.53 (t, J = 8.0 Hz, 2H), 7.37 (d, J = 8.0 Hz, 4H), 2.83–2.69 (m, 4H), 1.27–1.15 ppm (m, 24H). 13C NMR (100 MHz, CDCl3): δ = 162.3, 145.6, 135.6, 133.4, 131.8, 130.0, 129.9, 128.9, 124.3, 123.9, 123.3, 29.3, 24.0 ppm. MS (FD-TOF): [M]+ calculated for C48H38Cl4N2O4, 848.1566; found, 848.1593.
Synthesis of N,N′-bis(2,6-diisopropylphenyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4,9,10-tetracarboxy bisimide (12)52. The compound was synthesized by the following three methods:

(i) In a 25 mL round-bottomed flask, weighed amounts of 1,6,7,12-tetrachloroperylene bisimide 11 (100 mg, 0.12 mmol), 4-tert-butylphenol (124 mg, 0.83 mmol), and Cs2CO3 (308 mg, 0.95 mmol) were taken. Subsequently, anhydrous DMF (6 mL) was added and the reaction mixture was stirred at 100 °C for 3 h under argon atmosphere. Afterwards, the reaction mixture was poured in water and the precipitate was collected by filtration. The solid residue was washed with water and ethanol. The dried crude product was chromatographed (silica/DCM) to afford the desired product 12 (125 mg, 81%). Mp > 350 °C. 1H NMR (400 MHz, CDCl3): δ = 8.28 (s, 4H), 7.42 (t, J = 8.0 Hz, 2H), 7.26 (d, J = 8.0 Hz, 4H), 7.23 (d, J = 8.4 Hz, 8H), 6.87 (d, J = 8.4 Hz, 8H), 2.75–2.66 (m, 4H), 1.27 (s, 36H), 1.12 ppm (d, J = 6.4 Hz, 24H). 13C NMR (100 MHz, CDCl3): δ = 163.3, 155.9, 152.8, 147.3, 145.6, 133.2, 130.7, 129.4, 126.6, 123.8, 122.7, 120.7, 120.2, 120.1, 119.2, 34.3, 31.4, 29.1, 24.0 ppm.

(ii) 1,6,7,12-Tetrachloroperylene bisimide 11 (100 mg, 0.12 mmol), 4-tert-butylphenol (124 mg, 0.83 mmol), and K2CO3 (114 mg, 0.83 mmol) were stirred in NMP (5 mL) at 100 °C for 24 h under argon atmosphere. The work-up and purification was carried out according to above mentioned procedure to afford the desired product 12 (112 mg, 73%).

(iii) A mixture of 4-tert-butylphenol (248 mg, 1.65 mmol), K2CO3 (326 mg, 2.36 mmol) and 18-crown-6 (624 mg, 2.36 mmol), in dry toluene (30 mL), was stirred for 30 min. at room temperature under argon atmosphere. Subsequently, 1,6,7,12-tetrachloroperylene bisimide 11 (200 mg, 0.24 mmol) was added. The reaction mixture was stirred for 24 h at 100 °C and allowed to cool to room temperature. The solvent was removed by rotary evaporation and the residue was washed with water and dried. The crude product was subjected to silica gel column chromatography (DCM–hexane, 1[thin space (1/6-em)]:[thin space (1/6-em)]1) to afford the product 12 (108 mg, 35%).

Synthesis of 1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxy bisbenzimidazole (13). 1,6,7,12-Tetrachloroperylene bisanhydride 2 (300 mg, 0.57 mmol), 1,2-diaminobenzene (184 mg, 1.70 mmol), and propionic acid (10 ml) were subjected to a 25 mL round-bottomed flask. The reaction mixture was refluxed for 8 h and, subsequently, allowed to cool to room temperature. It was poured into water and the resultant precipitate was collected by filtration. The precipitate was washed with several portions of water and methanol and dried to afford the product (321 mg, 85%) as a mixture of syn and anti regioisomers. Mp > 350 °C. 1H NMR (400 MHz, CF3COOD): δ = 9.28 (s, 4H), 9.00 (d, J = 7.6 Hz, 2H), 8.14 (d, J = 7.6 Hz, 2H), 8.03 ppm (m, 4H). 13C NMR spectrum could not be measured because of its limited solubility.
Synthesis of 1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4,9,10-tetracarboxy bisbenzimidazole (14). In a 25 mL round-bottomed flask, weighed amounts of 1,6,7,12-tetrachloroperylene bisbenzimidazole 13 (60 mg, 0.09 mmol), 4-tert-butylphenol (110 mg, 0.73 mmol), and Cs2CO3 (232 mg, 0.71 mmol) were taken. Subsequently, anhydrous DMF (10 mL) was added and the reaction mixture was stirred at 105 °C for 5 h under argon atmosphere. Afterwards, the reaction mixture was poured in water and the precipitate was collected by filtration. The solid residue was washed with water and ethanol. The dried crude product was chromatographed (silica/CHCl3) to afford the desired product 14 (74 mg, 74%). 1H NMR (400 MHz, CDCl3): δ = 8.54 (s, 1.2H), 8.52 (s, 0.8H), 8.50–8.46 (m, 2H), 8.44 (s, 0.8H), 8.41 (s, 1.2H), 7.83–7.78 (m, 2H), 7.46–7.40 (m, 4H), 7.30–7.22 (m, 8H), 6.94–6.80 (m, 8H), 1.31 ppm (s, 36H). 13C NMR (100 MHz, CDCl3): δ = 159.95, 156.15, 156.03, 155.85, 155.77, 153.23, 153.19, 152.83, 148.61, 148.56, 147.44, 147.34, 147.07, 146.97, 143.84, 132.95, 132.91, 131.85, 126.73, 126.69, 125.92, 125.77, 125.71, 123.13, 122.52, 121.84, 121.44, 121.24, 120.60, 120.09, 119.99, 119.96, 119.45, 119.40, 119.21, 119.16, 119.12, 118.87, 118.70, 116.44, 116.26, 115.77, 34.37, 31.47 ppm.


The research leading to these results has received funding from the European Research Council Horizon 2020 ERC Grant Agreement no. 648433.

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Electronic supplementary information (ESI) available: Cyclic voltammograms of the all the compounds (Fig. S1 and S2); 1H and 13C NMR spectra of all synthesized compounds (Fig. S4–S15); mass spectra of all the new compounds (Fig. S16–S21). See DOI: 10.1039/c6qo00374e

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