J. Cent. South Univ. Technol. (2009) 16: 0206-0211

DOI: 10.1007/s11771-009-0035-1

Self-assembly constructed by perylene bisimide derivatives bearing complementary hydrogen-bonding moieties

YANG Xin-guo(杨新国)¹, YUAN Huan(袁 欢)¹, ZHAO Qiu-li(赵秋丽)¹,

YANG Qing(杨 青)², CHEN Xian-hong(陈宪宏)¹

 (1. College of Materials Science and Engineering, Hunan University, Changsha 410082, China;

2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China)

Abstract: An intermediate compound 2, 4-bis(laurylamino)-6-(1-(2-aminoethyl)-piperazine)-1, 3, 5-triazine was prepared by stepwise nucleophilic substitution on triazine ring by lauryl amine and subsequently 1-(2-aminoethyl)-piperazine. Then imidization of perylene-3, 4, 9, 10-tetracarboxylic acid dianhydride with 2,4-bis(laurylamino)-6-(1-(2-aminoethyl)-piperazine)-1, 3, 5-triazine was carried out to afford a novel perylene derivative bearing two melamine blocks (S₂) and 1, 6, 7, 12-tetra(4-tert-butyl phenoxy)-perylene-3, 4, 9, 10-tetracarboxylic acid bisimide (S₁). The hydrogen-bonding interactions between S₁ and S₂ were investigated by ¹H NMR spectrum, UV/Vis spectrum and fluorescence spectrum. The influences on the morphologies of S₁·S₂ aggregates were investigated. The results show that well-defined nanofibers with a diameter of about 100 nm can be obtained by self-assembly between S₁ and S₂ only in CH₂Cl₂ solution. Based on these results, guidelines for the molecular design and self-assembly of supramolecular polymer materials are presented.

Key words: perylene bisimide; self-assembly; hydrogen-bonding; synthesis

1 Introduction

The design and self-assembly of small functional molecule components into supramolecular aggregates have been one main branch of supramolecular chemistry [1-2]. An efficient control of highly mesoscopic order of molecule components by self-assembly is a subject of great importance because it is a promising avenue to developing novel functional materials and enabling the tuning of macroscopic properties of optoelectronic devices such as solar cells, light emitting diodes, and field effect transistors [3-6]. Because of the directionality and specificity of hydrogen bonding, much attention has been attracted in various supramolecular systems such as melamines with barbituric acids or imides [7-16]. The fascinating architecture and unique functionality motivate us to conceive well-defined functional nano- or mesoscopic structures from complementary molecular building blocks with the final goal of developing novel functional materials and optoelectronic devices.

Perylene derivatives are the best n-type organic semiconductors available to date. Based on their unique chemical, thermal and very high photochemical stability properties, perylene derivatives were chosen as functional building blocks for hydrogen-bonding assembly [2, 4-6, 9-18]. In this work, a novel perylene derivative containing two melamine blocks (S₂) and 1, 6, 7, 12-tetra(4-tert-butylphenoxy)-perylene-3, 4, 9, 10- tetracarboxylic acid bisimide (S₁) were prepared, then well-defined nanofibers were obtained by hydrogen- bonding self-assembly. The interaction of hydrogen bondings between imide in S₁ and the complementary melamine moiety in S₂ was investigated by ¹H NMR, optical spectroscopy and electron microscopy.

2.1 Reagents and measurements

Perylene-3, 4, 9, 10-tetracarboxylic acid dianhydride (PTCDA) was commercial product and was purified according to the following methods: PTCDA was dissolved in 5% KOH solution after being heated, then the precipitate was collected and hydrochloric acid was added in the filtrate. The precipitate was collected, washed to be neutral with water, and dried in vacuum at 100 ℃ for 24 h.

Cyanuric chloride and methylcyclohexane (MCH) were purchased from Alfa Aesar Corporation. Other chemical reagents were purchased from the markets and purified using standard procedures.

IR spectra were measured on a Nicolet-460 spectro- meter; ¹H NMR spectra were taken on an American INOVA-400 spectrometer; scanning electron micro- graphs were recorded on a JSM-6700F instrument; UV/Vis spectra were performed on a Japan UV-3010 and fluorescence spectra were measured on a Hitachi F-4500 spectrofluorometer.

2.2 Synthesis of compound S₁

Perylene bisimide (S₁) was synthesized according to Scheme 1 [13-15].

Scheme 1 Synthetic route for Compound S₁: a-n-Butylamine, H₂O/isopropanol, reflux, 20 h, yield of 91.0%; b-4-(Tert-butyl)phenol, K₂CO₃, 1-methyl-2-pyrrolidinone, 130 ℃, argon, 24 h, yield of 82.0%; c-KOH, isopropanol/H₂O, reflux, argon, 48 h, yield of 80.0%; d-Ammonium acetate, propionic acid, 140 ℃, argon, 40 h, yield of 31.0%

2.3 Synthesis of compound S₂

The new perylene derivative (S₂) containing bismelamine chromophores was prepared according to Scheme 2 [19].

Scheme 2 Synthetic route for Compound S₂

2.3.1 Synthesis of 2, 4-bis(laurylamino)-6-chloro-1, 3, 5- triazine (Compound 1)

Lauryl amine (5.0 g, 27 mmol) in CHCl₃ (10 mL) was added dropwise to a solution of cyanuric chloride (2.49 g, 13.5 mmol) and N, N′-diisopropylethylamine (DIPEA) (4.86 mL, 27 mmol) in CHCl₃ (50 mL). The solution was stirred at room temperature for 24 h, and 100 mL methanol was added. The resulting white precipitate was collected and redissolved in CHCl₃ and then reprecipitated in methanol to give Compound 1 as a white solid (5.6 g, 85.0%). MP: 159 ℃; ¹H NMR (CDCl₃), chemical shift: 0.864-0.897 (t, J=6.6 Hz, 6H, CH₃), 1.259-1.303 (m, 36H, CH₃(CH₂)₉), 1.553-1.587 (m, 4H, NCH₂CH₂), 3.387-3.42 (t, J=6.6 Hz , 4H, NCH₂), 5.407 (br s, 2H, NH); IR (KBr, cm^-1): 3 252,   3 120 (υ_N—H), 1 640 (υ_C=N), 1 656 (υ_C=O), 1 558 (δ_N—H).

2.3.2 Synthesis of 2, 4-bis(laurylamino)-6-(1-(2-amino- ethyl)-piperazine)-1, 3, 5-triazine (Compound 2)

Compound 1 (4.82 g, 10 mmol) was dissolved in CHCl₃ (50 mL), and 1-(2-aminoethyl)-piperazine (3.3 mL, 25 mmol) and DIPEA (0.9 mL, 5 mmol) were added. The solution was stirred at 35-40 ℃ for 24 h, and then water (100 mL) was added. The organic layer was separated and washed with water several times. The solvent was distilled off to give Compound 2 as a yellow solid that was not further purified for the following experiment.

2.3.3 Synthesis of Compound S₂

 Herylene derivativesMixture of Compound 2 (2.4 g, 4.2 mmol) and perylene-3, 4, 9, 10-tetracarboxylic acid dianhydride (0.494 g, 1.26 mmol) in quinoline (20 mL) were stirred at 100 ℃ for 12 h under nitrogen. After cooling to room temperature, the mixtures were poured into methanol (100 mL), and the resulting precipitate was collected and purified by column chromatography on silica eluting with chloroform-methanol (V(CHCl₃)?V(CH₃OH)=20?1) and reprecipitated from chloroform-methanol to obtain Compound S₂ as a red solid (0.55 g, 31.0%). MP: 223 ℃; ¹H NMR(CDCl₃), chemical shift: 0.855-0.888 (t, J=6.6 Hz, 12H, CH₃), 1.252-1.292 (m, 72H, CH₃(CH₂)₉), 1.511-1.544 (m, 8H, NHCH₂CH₂), 2.633 (s, 8H, H_Piper), 2.781 (s, 4H, NHCH₂), 3.322 (m, 8H, NCH₂), 3.758 (s, 8H, H_Piper), 4.397 (s, 4H, NCH₂CH₂), 4.773 (br s, 4H, NH), 8.456 (s, 4H, H_Pery), 8.565 (s, 4H, H_Pery); IR(KBr, cm^-1): 3 408 (υ_N—H), 1 691 (υ_C=O), 1 656 (υ_C=O).

3 Results and discussion

Fig.1 shows ¹H NMR spectra of S₁, S₂ and S₁·S₂ (the molar ratio of S₁ to S₂ is 1?1) in CDCl₃. The spectrum signals at 6.808-6.83, 7.227-7.249 of pure S₁ are assigned to benzene ring protons, and the signal at 8.214 is assigned to perylene ring protons. The chemical shift of the imide N-H protons of pure S₁ is at 8.407. In the spectrum of pure S₂, the signals at 8.456, 8.565 are assigned to perylene ring protons. The equimolar mixture of S₁ and S₂ in CDCl₃ exhibits a well-resolved spectrum with resonance of the hydrogen-bonded imide N-H protons at 9.157, benzene ring protons at 6.769-6.790, 7.219-7.241 in pure S₁, and perylene protons at 8.129, 8.542-8.563, and 8.600-8.620. The electronic densities of the protons involving hydrogen bondings decrease. Consequently their ¹H NMR signals shift to lower magnetic fields. These results indicate that hydrogen- bonding interactions take place between S₁ and S₂, which means that there exist S₁·S₂ complexes [14-15, 20].

Fig.1 Partial ¹H NMR spectra of S₁, S₂ and S₁·S₂ in CDCl₃ (Concentration of S₁ is 1 mmol/L)

The UV-Vis absorption spectra of pure S₁ in CH₂Cl₂ at concentration of 10 μmol/L show three maxima (Fig.2(a)), which correspond to the 0-0 (578 nm), 0-1 (528 nm) and 0-2 (452 nm) transitions and are typical of monomeric perylene bisimides. The absorption spectra of pure S₂ show three peaks at 524 (0-0), 488 (0-1) and 458 (0-2) nm. When the concentration of S₁ is kept constant upon increasing the content of S₂, the absorption intensity of S₁·S₂ at 578 nm, which belongs to S₁, distinctly increase and no linear dependence on the content of S₁ or S₂ is observed. On the other hand, the relative intensities of 0-0 and 0-1 transitions for S₁ and S₂ in CH₂Cl₂ essentially unchange, which indicates that the small molecules are not aggregated in a cofacial geometry. For methylcyclohexane (Fig.2(b)), the absorption spectra of pure S₁ or S₂ at 50 μmol/L show the characteristics of cofacial aggregates, but the spectra of S₁ and S₂ mixture exhibit fine structure features of monomeric perylene derivatives to some extent. This suggests that a few three-point hydrogen bondings form between the imide group of S₁ and the melamine group of S₂ in methylcyclohexane, and the solubility of S₁ and S₂ is endowed by their complexation with each other.

Fig.2 UV/Vis absorption spectra for stoichiometric mixture of S₁ (concentration of S₁ is kept at 10 μmol/L) and S₂ at different molar ratios (n(S₂)?n(S₁)=0?1, 1?0, 1?1, 2?1) in CH₂Cl₂ (a) and in MCH (b)

Fig.3 shows the fluorescence spectra of pure S₁, S₂ and S₁·S₂ in CH₂Cl₂. The spectrum of pure S₂ exhibits weaker fluorescence than that of pure S₁, indicating that there is intramolecular photoinduced electron transfer between perylene bisimide chromophores and bismelamine moieties in S₂. When the concentration of S₁ is kept constant, the fluorescence intensities of S₂ and S₁ in S₁·S₂ complexes further decrease with the addition of S₂. The formation of hydrogen bondings introduces the possibility of intermolecule electron transfer between S₂ and S₁ in those systems [21-23].

Fig.3 Fluorescence spectra (excited at 457 nm in CH₂Cl₂) for stoichiometric mixtures of S₁ (concentration of S₁ is kept constant at 10 μmol/L) and S₂ at different molar ratios (n(S₂)?n(S₁)= 0?1, 1?0, 1?1, 2?1 )

It has been reported that S₁ and melamine moieties in MCH tended to self-assembly more readily by hydrogen bondings because the high binding constants were observed in MCH [14]. As melamine moieties S₂ alone or its complexation with S₁ cannot disperse in MCH because of the limited solubility of S₂  in MCH, the well-defined superstructures are hardly observed with scanning electron microscope (SEM) after evaporation of a 50 μmol/L MCH solution of S₁?S₂ (Fig.4(a)). After evaporation of a 50 μmol/L solution of S₁·S₂ in CH₂Cl₂  at room temperature, a large quantity of fibrous nano- materials (Fig.4(b)) with diameters in the range of 90-150 nm and lengths of several micrometers can be observed with SEM. No well-defined superstructures (Figs.4(c) and 4(d)) are obtained in CH₂Cl₂ with high concentration of S₁·S₂. The hydrogen-bonding interactions between the imides group of S₁ and the melamines group of S₂ are possibly contributed to forming 1D supramolecular structure (Scheme 3). These results are of great importance for the future studies on 1D nanomaterials in the field of photoelectric devices, and the influences of solvents and other conditions on the morphologies of S₁·S₂ complexes will be further investigated.

Fig.4 FE-SEM images of mesoscopic structures obtained by evaporation of 50 μmol/L solution of S₁?S₂ (n(S₂)?n(S₁)=1?1) in MCH solution (a), 50 μmol/L (b), 5 mmol/L (c) and 50 mmol/L (d) solution of S₁·S₂ (n(S₂)?n(S₁)=1?1) in CH₂Cl₂ solution

Scheme 3 Superstructure formation of perylene derivatives S₁ with S₂ by hydrogen bonding self-assembly

(1) A new perylene bisimide derivative containing two melamine blocks (S₂) is synthesized starting from cyanuric chloride in 3-step simple reaction with overall yield of 26.4%. The intermediates and the target compounds are characterized by IR and ¹H NMR.

(2) The polarity of solvents plays an important role in self-assembly between S₁ and S₂. Well-defined long nanofibers with a diameter of about 100 nm are obtained by self-assembly S₁ with S₂ in CH₂Cl₂, but not in methylcyclohexane.

(3) ¹H NMR spectra show that the signals at chemical shift of 8.407 of the imide N—H protons of pure S₁ shift to 9.157 for S₁·S₂ complexes in CDCl₃, indicating that hydrogen-bonding interactions take place between S₁ and S₂. Optical spectroscopy results suggest hydrogen-bonding interactions make a contribution to intermolecular electron transfer between S₁ and S₂.

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Foundation item: Project(50573019) support by the National Natural Science Foundation of China

Received date: 2008-06-22; Accepted date: 2008-08-30

Corresponding author: YANG Xin-guo, Professor, PhD; Tel: +86-731-8821614; E-mail: xgyang@hnu.cn

 (Edited by CHEN Wei-ping)