Citation

Zhaozhong Li, Yang Li, Dong Wang, Qing Cui, Zhengqiang Li, Lei Wang, Huai Yang. Unique fluorescence properties of a self-assembling bis-pyrene molecule[J]. Chin. Chem. Lett., 2018, 29(11): 1645-1647  

Zhaozhong Li^a,b, Yang Li^a, Dong Wang^a, Qing Cui^a, Zhengqiang Li^a, Lei Wang^a,b, Huai Yang^a,c    

Pyrene and its derivatives are one sort of prototypical molecule with the long excited-state lifetime fluorescence and distinct solvatochromic shifts. They have been widely used in many research fields as fluorescent probes and fluorescent sensors, the key technology of which is the control of fluorescence by external stimuli including solvents, chemicals and biochemical additives [1]. For supramolecular systems, the fluorescence is responded to the changes of self-assembled superstructures determined by the intra and/or intermolecular interactions, such as π-π stacking, hydrogen-bonding, electrostatic interactions [2].

The self-assembling capability of pyrene derivatives can be designed and imparted according to the molecular characteristics of discotic liquid crystalline (LC) mesogens, which are typically composed of a central aromatic core substituted with flexible alkyl chains [3]. Generally, the LC mesogens have abundant intermolecular interactions and can form hierarchical self-assembled structures with the high degree of organization. Recently, we synthesized a series of asymmetric pyrene derivatives and there were no LC properties found, probably because the pyrene as a discotic core was too small to form discotic mesogen phase [4, 5]. Herein, we designed and prepared a large π-conjugated bis-pyrene (BP) molecule, which showed room-temperature discotic mesophase with highly-ordered structures. Furthermore, the BP formed nanofibers and showed bright green fluorescence against aggregation-caused quenching (ACQ) effect (Scheme 1).

Scheme 1

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Scheme 1. The molecular structure of bis-pyrene (BP) and its self-assembly and fluorescence properties.

The target molecule BP was synthesized in three steps including Friedel-Crafts acylation, bromination and Hagihara-Sonogashira cross-coupling reaction as shown in Scheme 2. Friedel-Crafts acylation procedure (ⅰ) for synthesis of 1, 4-phenylenebis(pyren-1- ylmethanone), pyrene (7.1 g, 34.8 mmol) and terephthaloyl dichloride (3.4 g 17.0mmol) were dissolved in carbon disulfide (50mL), the mixture was cooled to 0 ℃. After the portionwise addition of AlCl₃ (6.8 g, 51.0mmol), the mixture was heated under reflux overnight, then poured into ice-water and the resulting mixture was stirred until the color of the organic phase turned from black to yellow. The layers were then separated, the aqueous phase was extracted with dichloromethane, the combined organic phases were dried with MgSO₄, and the solvent was evaporated. The residue was purified by column chromatography, and the product was obtained (3.8 g, 42%), which was for the next step of bromination procedure (ⅱ) for synthesis of 1, 4-phenylenebis((3, 6, 8-tribromopyren-1-yl)methanone). 1, 4-Phenylenebis(pyren-1-ylmethanone) (5.3 g, 10.0mmol) was dissolved in nitrobenzene. Undervigorous stirring, bromine (2.0 mL, 40.0mmol) was added slowly. After complete addition, the temperature was increased to 160 ℃ and maintained for 8 h. The cooled reaction suspension was poured into acetone and the precipitate filtered off. Further drying of the precipitate in high vacuum gave the crude product (8.7 g, 87%), whichwas used without further purification because it was insoluble in common organic solvents. Finally, the BP was obtained from Hagihara–Sonogashira cross-coupling procedure (ⅲ), 1, 4-phenylenebis((3, 6, 8-tribromopyren-1-yl)methanone) (0.640 g, 0.640 mmol), Pd(PPh₃)₂Cl₂ (0.028 g, 0.039 mmol), CuI (0.008 g, 0.039 mmol), and PPh₃ (0.020 g, 0.078mmol) were added to degassed solution of triethylamine (10 mL) and THF (10 mL) under argon. While stirring, the reaction mixture was heated to 70 ℃ and 1-ethynyl-4-pentylbenzene (680mg, 4 mmol) was injected correspondingly. After 15 min of stirring at this temperature the reaction was heated to 80 ℃ and stirred overnight under argon atmosphere. The cooled reaction mixture was diluted with DCM and extracted with water.Theorganic phase was dried over MgSO₄, and the solvent was removed under reduced pressure. The crude product was purified by column chromatography to affordBP (0.866 g, 0.550mmol)withhighyieldof 87%. ¹H NMR (400 MHz, CDCl₃): δ 0.89 m, 18H), 1.33 (m, 24H), 1.63 (m, 12H), 2.64 (m, 24H), 7.24 (m, 12H), 7.60 (m, 12H), 8.05 (m, 4H), 8.30 (m, 2H), 8.43-8.47 (m, 2H), 8.69 (m, 2H), 8.81 (m, 4H). FT-IR (KBr): 3039, 2926, 2854, 2326, 2197, 1657, 1496, 1246, 1009, 823 cm^-1. MALDI-TOF-MS (dithranol): m/z calcd. for C₁₁₈H₁₀₆O₂: 1554.82, found: 1555.73 [MH]⁺. Elemental analysis calcd. (%) for C₁₁₈H₁₀₆O₂: C, 91.08; H, 6.87; O, 2.06; Found: C, 91.08; H, 6.87; O, 2.06. The ¹H NMR and UV–vis absorption spectra can be found in Supporting information (Figs. S1–S4 in Supporting information).

Scheme 2

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Scheme 2. Procedure for the synthesis of BP. (ⅰ) AlCl3, CS2, reflux; (ⅱ) Br2, nitrobenzene, 120 ℃; (ⅲ) CuI, PdCl2(PPh3)2, ethynyl, THF, NEt3, 80 ℃.

The large aromatic core and the alkyl chains imparted the selfassembly capabilityof BP. In the condensed state, the self-assembled structures were characterized by multiple-techniques, including differential scanning calorimetry (DSC), polarized optical microscope (POM), and 1D wide angle X-ray diffraction (WAXD). The DSC thermogram (Fig. 1a) showed isotropic phase above 227.6 ℃ during the second heating process, which was confirmed by black POM images under crossed polarizers (Fig. 1b) [6, 7]. When it was cooled from the isotropic state, there was only one phase transition at 137.8 ℃ for BP during the second cooling process down to room temperature. The POM images below the transition temperature indicated that it was a nematic columnar phase (Fig. 1c). In order to confirm the phase and the self-assembled structures of BP, 1DWAXD measurement was carried out after the second cooling process. As shown in Fig. 1d, the WAXD pattern of the discotic BP molecule showed one distinct reflection in the small-angle regime, and one halo in the wide-angle area. This phase was assigned to nematic columnar phase. The calculated d-spacing of 17.1 Å for the reflection in the small-angle region approximated the diameter of the discotic unit. The spacing of the wide-angle halo was calculated to be 4.1 Å, probably ascribed to the π-π stacking [8, 9].

Figure 1

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Fig. 1. (a) The second heating and subsequent cooling thermal diagrams for BP at scanning rate of 10 ℃/min. The POM observation of BP at 245.0 ℃ (b) and 25.0 ℃ (c) during the second heating and cooling process, respectively. (d) 1D WAXD powder patterns of BP at room temperature after the second cooling process.

The self-assembly in solution was performed by using interface diffusion of solvents. BP was firstly dissolved in dichloromethane (DCM, "good" solvent), followed by the careful addition of hexane ("poor" solvent) on top of DCM solution (Fig. 2a). The mixture was coated on the silicon substrate and observed by scanning electron microscopy (SEM). The BP self-assembled into long nanofibers with the width of ~800 nm and the length of ~100 mm Figs. 2b and c). The structure of the nanofibers was further studied by powder XRD measurement (Fig. 2d). The results indicated that the supramolecular arrangement of BP along the fiber corresponded to a columnar mesomorphic-like organization and d-spacing of 3.9 Å indicating the strong π-π interactions [8]. The BP was also characterized by the cyclic voltammogram as shown in Fig. S5 (Supporting information). The HOMO energy level was estimated according to the equation HOMO = -([E_onset]^ox + 4.8) eV, and the data were also listed in Table S1 (Supporting information). Subsequently, the LUMO energy level was estimated, by combining the HOMO energy levels together with the optical band gap.

Figure 2

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Fig. 2. (a) Schematic illustration of the self-assembly process via bisolvent phase transfer. (b) SEM images showing the fibers of BP and a zoomed-in image (c). (d) The XRD diagram of self-assembled nanofibers of BP.

Surprisingly, the solution of BP in DCM was non-emissive. Right after the addition of "poor" solvent hexane, bright light emitted from the interface of solvent under UV irradiation, indicating that the self-assembled structures displayed fluorescence (Fig. 3a). Subsequently, we prepared a series of BP solution in mixed solvent, with the increase of hexane ratio. As shown in Fig. 3b, the fluorescence rose up with the increase of hexane in mixed solvent. The fluorescence spectrum of BP solution in DCM was nearly a flat line parallel to the abscissa. Green fluorescence emitted and became brighter and brighter along with the increased percentage of hexane, which were taken with a digital camera under a laboratory UV lamp irradiation (Fig. 3b, inset). In other words, BP molecules were almost non-fluorescence when well dissolved in good solvent and became highly emissive in self-assembly state in the mixed solvent, which was different from aggregation-caused quenching (ACQ) effect. The unique fluorescence property of BP was agreement with termed aggregation-induced emission (AIE), firstly found by Tang [10]. The non-emissive materials become fluorescence when aggregation, which could be widely used for the monitoring the biological process with assembly.

Figure 3

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Fig. 3. (a) Photo taken under illumination of a UV lamp of BP in mixed solvent (up: hexane, down: DCM). (b) Fluorescence emission spectra of BP in solutions in DCMhexane mixtures containing different volume fractions of hexane upon excitation at 375 nm (Inset: Photographs taken under illumination of a UV lamp). HOMO (c) and LUMO (d) of BP molecule in excited state with charge transfer.

There was still another question, why the BP was quenched in DCM solution? In AIE system, there was no fluorescence for AIE molecules due to intramolecular rotation in solution, where the subunit of AIE active molecule is none fluorescence, such as phenyl biphenyl and silole [11, 12]. Meanwhile, BP molecule should be emissive due to the fluorescence feature of pyrene, which was validated by our previous mono-pyrene derivatives showing strong fluorescence [5]. Assisted by the quantum chemistry calculation Figs. 3c and d), the charge transfer occurred in excited state of BP, which could drive the intramolecular rotations of BP molecule, resulting in self-quenching of BP molecules fluorescence quenching [11-15]. When the "poor" solvent was added, BP molecules aggregated into nanostructures by intermolecular π-π interactions, inhibiting the molecular rotations. Furthermore, steric hindrance may prohibit the perfect packing of BP molecules so that no quenching happened in aggregation state, but an increasing in fluorescence through intermolecular π-π interaction [16].

In conclusion, a self-assembling pyrene derivative BP was designed and prepared. On the one hand, BP can self-assemble into highly-ordered columnar mesophase structure. On the other hand, BP can self-assemble into nanofibers in solution. More importantly, the BP showed interesting dually controllable quenching and emission fluorescence on solvent, which is an AIE phenomenon. The BP is a promising fluorescent probe and fluorescent sensor material for various applications.

This work was supported by the National Natural Science Foundation of China (Nos. 51573031 and 51473188).

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2018.01.056.