Diazocine as a Versatile Building Block Enables Excellent Photoswitching and Chromic Properties in Self-Assembled Organogels

Open AccessCCS ChemistryRESEARCH ARTICLE10 Apr 2021Diazocine as a Versatile Building Block Enables Excellent Photoswitching and Chromic Properties in Self-Assembled Organogels Yingying Wang†, Mengwei Li†, Chunmei Yan, Ning Ma and Yulan Chen Yingying Wang† Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Institute of Molecular Plus, Tianjin University, Tianjin 300354 , Mengwei Li† Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Institute of Molecular Plus, Tianjin University, Tianjin 300354 , Chunmei Yan Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Institute of Molecular Plus, Tianjin University, Tianjin 300354 , Ning Ma Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Institute of Molecular Plus, Tianjin University, Tianjin 300354 and Yulan Chen *Corresponding author: E-mail Address: [email protected] Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Institute of Molecular Plus, Tianjin University, Tianjin 300354 https://doi.org/10.31635/ccschem.021.202100798 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Novel types of photoresponsive organogelators containing a diazocine moiety have been synthesized, and their gelation behaviors have been characterized systematically. Subtle changes in the pendant structure of these bent-core molecules cause significant changes in the molecular packing mode in their self-assembled gels. By making use of the unconventional conformational and photochromic properties of the diazocine core, the obtained gels exhibited excellent photoswitching and -chromic properties. For the first time, the diazocine unit has been demonstrated as a versatile building block to construct photoresponsive supramolecular gels. The sensitive interconversion between multiple states of these chromic gels highlights their promising applications in fabricating logic gates with appropriate addressing mode (stimulated by visible light or heating). Download figure Download PowerPoint Introduction Photochromic switches, accessible by light of distinct wavelengths, are potentially applicable in optical data memory devices,1,2 molecular machines,3,4 catalysts,5,6 and logic devices.7 Particularly, the development of photoactive chromophores that can undergo reversible E/Z isomerization with remarkably different colors, structures, and functionalities is an important research topic focused on smart materials.8 By far, azobenzene and its derivatives are the most frequently used molecular photoswitches.9–11 They are thermodynamically more stable in the E-form and can be stimulated by light to isomerize to the Z-form. However, they are not actually suitable for photoactive switches. First, the overlapping absorption bands of the two isomers between 400 and 500 nm can lead to incomplete photoswitching under visible irradiation.12,13 Second, due to the planarity, E-azobenzenes usually exhibit a great tendency to aggregate, where they are very likely insensitive to or even silent upon light exposure.14,15 Furthermore, UV light required to drive the E/Z transition is not a friendly trigger for soft materials since other molecular moieties may suffer photodamage.16 Bridged azobenzenes,2,17,18 namely diazocines, show reversed switchability and favorable photochromic properties. The ethylene bridge stabilizes the bent and bulky Z-conformation, which can photoisomerize to the thermodynamically less stable, highly twisted E-isomer. Rather than the common color fading associated with the transition from the Z- to E-form of azobenzene, the inverse stability of diazocine enables a more visible coloration phenomenon. The bridge also leads to a large separation of the absorption bands (85 nm) of the two isomers and the red-shift of the Z to E wavelength, resulting in high switching efficiency triggered by visible light instead of potentially harmful UV light.15 Moreover, diazocine is not planar and therefore not prone to π–π stacking. Collectively, these superior photochemical and -physical properties offer great opportunities to construct photoresponsive materials with improved efficiency and sensitivity using diazocine as the molecular photoswitch. However, this unconventional type of azobenzene has only been recognized recently, and its applications in smart materials are still limited,15,19 For example, Zhang et al.2 reported a class of diazocine-containing photoresponsive polyureas, which could be used as rewritable photopatterning materials and visible-light driven actuators. Besides bulk polyureas, the activity of diazocine in other matrices, particularly the fundamentally and technologically important soft materials with distinct size and shape, has not been explored yet. Photoresponsive low molecular weight organogelators (LMOGs) formed by supramolecular self-assembly of photoswitchable building blocks are a kind of multifunctional soft material.20–22 They have aroused great interest because of their dynamically controllable features, well-defined architectures, and applications in diverse fields such as biomaterials,23,24 sensors,25 and molecular switches.26 Elaborating the intermolecular interactions between gelators at the molecular level is essential for gelation aggregation, as well as for photoswitching, since most photoswitchable chromophores are not active in the gelation state when they are bound together with synergistic, multiple intermolecular interactions. For example, we once reported a class of azobenzene-cored, coil–rod–coil molecules as the gelators.27 Although they could form supramolecular organogels in several solvents, the photoisomerization did not take place in these gels but only in solution or liquid crystal states. To the best of our knowledge, highly sensitive photoinduced Z/E isomerization of gelators alongside the sol–gel phase transition is greatly desirable but still a challenge.9,28–31 In this work, by replacing the azobenzene central core with a diazocine, new types of photochromic, bent-core molecules Azo-X-Cn, were synthesized and explored as supramolecular gelators (Scheme 1). With the cooperative interactions of π–π interactions, van der Waals forces, and H-bonding,32–34 they can self-assemble into nanofibers or spherical aggregates in the form of supramolecular gels. The different packing structures (lamellar or hexagonal columnar aggregations) in these organogels are dependent on the peripheral alkoxyl chain number of the gelators. Interestingly, unlike the azobenzene analogues, all the gels exhibit visible-light-triggered, prominent photochromic characteristics accompanied by the sol–gel phase transition, which is ascribed to the facile and fast Z/E isomerization of the steric diazocine core. Scheme 1 | Chemical structures of Azo-X-Cn and their reversible E/Z transition. Download figure Download PowerPoint Experimental Section Materials and equipment 1,2-Bis(4-aminophenyl)ethane [TCI (Shanghai) Development Co., Ltd., 97%] was used without further purification. Dichloromethane (DCM) was distilled over CaH2. All reactions were performed under an atmosphere of nitrogen and monitored by thin-layer chromatography (TLC) with silica gel 60 F254. Column chromatography was carried out on silica gel (100–200 mesh). The 1H and 13C NMR spectra were recorded on a BRUKER AVANCE III HD (400 or 600 MHz) (Bruker, Germany) spectrometer. The mass spectroscopy measurements were performed on a matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometer Bruker Autoflex Speed TOF/TOF (Bruker, Germany). UV–vis absorption spectra were obtained on a Lamda 750 (Perkin Elmer, United States) spectrophotometer with the standard procedure. Fourier transform infrared (FT-IR) spectra of the xerogels were recorded with a ALPHA (Bruker, Germany) spectrometer. Atomic force microscopy (AFM) measurements were performed using a Bruker Multimode 8 (Bruker Nano Inc., United States) atomic force microscope operating in tapping mode. The scanning electron microscopy (SEM) images were recorded with a field-emission scanning electron microscope SU8010 (Hitachi, Japan) and Apreo S LoVac (FEI, United States) operating at an accelerating voltage of 5 kV. The samples for SEM observation were prepared by inserting the silicon wafer into the gel and drying it at room temperature. The transmission electron microscopy (TEM) images were recorded with transmission electron microscopes JEM-2100F (JEOL, Japan) operated at 200 kV. The samples for TEM observation were prepared by smearing the corresponding organogel onto copper mesh and drying in the air at room temperature. X-ray diffraction (XRD) data were collected by using monochromated Cu Kα radiation (λ = 1.5406 Å) with an X Max-80T diffractometer (Oxford Instruments, United Kingdom). The Azo-2-C8/n-octane, Azo-2-C10/n-octane, and Azo-3-C8/n-decane organogels were dried on a silicon wafer and tested at small and wide angles. Rheological measurements of all gels were performed using a Discovery HR-2 Hybrid Rheometer (TA Instruments, United States). More experimental details and characterization are available in the Supporting Information. General procedure for the synthesis of Azo-X-Cn Compound 3 (0.30 g, 1.26 mmol) and triethylamine (0.42 g, 4.16 mmol) were dissolved in DCM (35 mL), then a solution of benzoyl chloride (8a–d) (3.78 mmol) in DCM (50 mL) was slowly added at 0 °C. After stirring at room temperature for 3.5 h, the reaction solution was extracted with DCM, washed with brine, and the organic phase was collected and dried over anhydrous Na2SO4. After concentrating under vacuum, the residual yellow viscous solid was purified by column chromatography (eluent:petroleum ether/ethyl acetate = 6:1 ∼ 3:1) to obtain a yellow solid. Azo-2-C8 (1.03 g, yield: 85.0%): 1H NMR (600 MHz, CDCl3, δ): 7.96 (s, 2H), 7.35 (dd, J = 8.3, 2.2 Hz, 2H), 7.08 (d, J = 2.1 Hz, 2H), 6.95 (d, J = 8.3 Hz, 2H), 6.86 (d, J = 2.2 Hz, 4H), 6.55 (t, J = 2.2 Hz, 2H), 3.90 (t, J = 6.6 Hz, 8H), 2.92 (dd, J = 14.9, 5.9 Hz, 2H), 2.72 (dd, J = 14.9, 6.0 Hz, 2H), 1.76–1.71 (m, 8H), 1.44–1.38 (m, 8H), 1.35–1.24 (m, 32H), 0.88 (t, J = 7.0 Hz, 12H). 13C NMR (151 MHz, CDCl3, δ): 165.9, 160.5, 155.6, 136.8, 136.7, 130.4, 124.2, 119.1, 110.4, 105.4, 104.9, 68.4, 31.9, 31.9, 31.9, 31.9, 31.9, 31.3, 29.4, 29.3, 29.3, 29.3, 29.2, 26.1, 22.7, 22.7, 14.2. MALDI-TOF (m/z): [M + Na]+ calcd for C60H86N4O6Na, 981.64; found: 981.88. Azo-2-C10 (1.07 g, yield: 79.3%): 1H NMR (400 MHz, CDCl3, δ): 7.84 (s, 2H), 7.36 (dd, J = 8.3, 2.2 Hz, 2H), 7.11 (d, J = 2.2 Hz, 2H), 6.96 (d, J = 8.3 Hz, 2H), 6.87 (d, J = 2.2 Hz, 4H), 6.56 (t, J = 2.2 Hz, 2H), 3.93 (t, J = 6.5 Hz, 8H), 2.94 (dd, J = 14.8, 5.8 Hz, 2H), 2.74 (dd, J = 14.9, 5.9 Hz, 2H), 1.79–1.70 (m, 8H), 1.41 (dd, J = 14.8, 6.9 Hz, 8H), 1.28 (d, J = 13.1 Hz, 48H), 0.88 (t, J = 6.9 Hz, 12H). 13C NMR (151 MHz, CDCl3, δ): 165.8, 160.5, 160.5, 155.6, 136.8, 136.7, 130.4, 124.2, 119.0, 110.4, 105.3, 104.9, 68.4, 32.0, 29.7, 29.7, 29.7, 29.6, 29.6, 29.5, 29.5, 29.4, 29.4, 29.4, 29.4, 29.3, 26.1, 22.8, 22.7, 14.2. MALDI-TOF (m/z): [M + H]+ calcd for C68H103N4O6, 1071.79; found: 1071.99. Azo-3-C8 (1.13 g, yield: 74.0%): 1H NMR (400 MHz, CDCl3, δ): 7.99 (s, 2H), 7.32 (dd, J = 8.3, 2.2 Hz, 2H), 7.10 (d, J = 2.1 Hz, 2H), 6.94 (d, J = 7.5 Hz, 6H), 3.95 (dt, J = 22.4, 6.6 Hz, 12H), 2.90 (dd, J = 14.9, 5.8 Hz, 2H), 2.71 (dd, J = 14.9, 5.9 Hz, 2H), 1.81–1.69 (m, 12H), 1.43 (t, J = 11.0 Hz, 12H), 1.36–1.22 (m, 48H), 0.92–0.83 (m, 18H). 13C NMR (151 MHz, CDCl3, δ): 166.2, 155.5, 153.1, 141.2, 136.9, 130.3, 129.6, 124.2, 119.3, 110.5, 105.6, 73.6, 69.2, 32.0, 31.9, 31.9, 31.9, 31.3, 30.4, 29.6, 29.5, 29.4, 29.4, 29.4, 26.2, 26.1, 22.8, 22.7, 14.2. MALDI-TOF (m/z): [M + Na]+ calcd for C76H118N4O8Na, 1237.88; found: 1238.19. Azo-3-C10 (1.45 g, yield: 83.0%): 1H NMR (600 MHz, CDCl3, δ) 8.21 (s, 2H), 7.31 (d, J = 7.7 Hz, 2H), 7.06 (s, 2H), 6.92 (s, 6H), 3.99–3.83 (m, 12H), 2.86 (d, J = 8.9 Hz, 2H), 2.69 (d, J = 8.9 Hz, 2H), 1.79–1.68 (m, 12H), 1.47–1.38 (m, 12H), 1.26 (s, 72H), 0.87 (t, J = 6.5 Hz, 18H). 13C NMR (151 MHz, CDCl3, δ): 165.9, 155.5, 153.1, 141.4, 136.8, 130.2, 129.5, 124.1, 119.1, 110.4, 105.7, 73.5, 69.3, 31.9, 31.9, 31.2, 30.4, 29.7, 29.7, 29.6, 29.6, 29.5, 29.4, 29.4, 29.3, 26.1, 26.1, 22.7, 14.1. MALDI-TOF (m/z): [M + Na]+ calcd for C88H142N4O8Na, 1406.07; found: 1406.01. Results and Discussion The target Azo-X-Cn are bent-core molecules consisting of a diazocine core with alkoxyl-chain-modified phenyl groups attached at the periphery and two amide groups as the linkers. The straightforward synthesis of Azo-X-Cn entailed acylation of bis(amine)-functionalized diazocine35 with different acyl chlorides in good yields (∼80%, Scheme 2). All the final products were unambiguously characterized with NMR (1H and 13C) and MALDI-TOF mass spectroscopy ( Supporting Information Figures S16–S31). Scheme 2 | The synthetic routes of the compounds Azo-X-Cn. Download figure Download PowerPoint The three parts of Azo-X-Cn —the diazocine core, alkoxyl chains, and amide groups—provide photoresponsive capability and multiple non-covalent interactions such as π–π stacking, van der Waals forces, and H-bonding interactions, respectively. The steric effect derived from the bent core and alkoxyl chains can adjust the spacing of aromatic nuclei. Therefore, except for Azo-3-C10 that was readily soluble in most organic solvents, Azo-2-C8, Azo-2-C10, and Azo-3-C8 could self-assemble into gels with solvents trapped inside ( Supporting Information Figure S1). Among the three, Azo-3-C8 exhibited the best gelation capability: stable, yellow, and transparent organogels could be obtained through the heating–cooling process in diverse solvents such as n-pentane, n-hexane, n-heptane, n-octane, and n-decane. Because of the low solubility of Azo-3-C8 in acetonitrile, the formed gel appeared opaque with critical gelation concentrations (CGC) determined to be 5.5 mM, manifesting that each gelator molecule can trap ca. 4300 solvent molecules. The gelation behaviors of all the molecules are summarized in Supporting Information Table S1. Rheological experiments of the Azo-2-C10 and Azo-3-C8 gels (formed in n-octane and n-decane, respectively) were conducted to study their viscoelastic properties. In the frequency sweep range of 0.1–100 rad/s, the elastic modulus G′ and the viscous modulus G″ values did not depend strongly on oscillation frequency, with G′ < G″ at all tested frequencies ( Supporting Information Figures S2a and S2c), thereby indicating that the gel was elastically resistant to external forces and dominated the viscous properties.9 Furthermore, the modulus value (G′ and G″) hardly changes over a long time ( Supporting Information Figures S2b–S2d), suggesting that it was stable once a gel network was formed. The supramolecular structures of the obtained xerogels were characterized systematically. First, SEM, TEM, and AFM studies revealed different morphologies formed by the three gelators. The xerogel of Azo-3-C8 exhibited a three-dimensional network composed of densely intertwined fibers of ca. 40–300 nm in width and several microns in length (Figures 1a–1c). The AFM images of the Azo-3-C8 xerogels formed in different solvents are consistent with their SEM and TEM measurements, showing network structures hierarchically formed by nanofibers with a high aspect ratio (Figure 1c and Supporting Information Figures S3a–S3d). In contrast, Azo-2-C8 and Azo-2-C10, with fewer alkoxyl chains, aggregated into nanospheres with diameters of ca. 20–200 nm or larger hollow nanospheres, which were evidenced by SEM and TEM images (Figures 1d–1f and Supporting Information Figure S3). Figure 1 | (a, d, and e) SEM, (b and f) TEM, and (c) AFM images of Azo-3-C8 xerogels from n-decane (a and b) and n-hexane (c), and Azo-2-C8 xerogels from n-octane (d and f) and acetonitrile (e). Download figure Download PowerPoint Variable temperature (VT) UV–vis, 1H NMR, FT-IR, and XRD were used to uncover the intermolecular interactions and driving forces of these supramolecular organogels. First, the VT UV–vis absorption spectra of Azo-3-C8 in n-octane (concentration: 20.6 mM, above CGC) showed that in the sol state (373 K), the n–π* transition of the diazocine peaked at 405 nm, corresponding to the existence of the stable Z-isomer.16 When the temperature decreased from 373 to 288 K, a transparent gel was formed together with the n–π* transition band red-shifting gradually by approximately 8 nm (λabs = 413 nm at 288 K), indicating the diazocine cores stacked into an aggregation mode during the gelation ( Supporting Information Figure S4c).36 The absorption spectra of Azo-2-C8 and Azo-2-C10 in n-octane exhibited a similar but slightly red-shift phenomenon ( Supporting Information Figures S4a and S4b), which may be due to the relatively poor gelation ability under the experimental conditions. Through the above analysis, it can be concluded that π–π interactions are the main forces that induce gel formation. The FT-IR spectra ( Supporting Information Figure S5) of the xerogels Azo-2-C8, Azo-2-C10, and Azo-3-C8 from n-octane showed their N–H stretch band at 3280 cm−1 and amide I band at 1653 cm−1, respectively. The relatively low wavenumbers suggested the presence of strong intermolecular H-bonding.37 All the gels showed CH2 stretching vibrations located at relatively low frequencies (ʋanti: 2923 cm−1; ʋsym: 2855 cm−1), which could be ascribable to an all-trans conformation due to the close packing of the alkoxyl chains.38 Concentration-dependent 1H NMR spectra of all the gelators in CDCl3 further confirmed these results ( Supporting Information Figure S6). In detail, when the concentration of Azo-3-C8 increased from 1.6 to 24.7 mM, the proton peak assigned to the amide group shifted successively from 7.74 to 8.21 ppm. This downfield shift implied that H-bond interactions became prominent at a concentrated state, and such interactions probably played an important role in mediating the assembly of the gelators. Meanwhile, signals of the aromatic protons shifted upfield. Such shifts most probably corresponded to the shielding effect from the ring current of neighboring aromatic molecules by π–π stacking. Moreover, in contrast to the symmetric conformation of the azobenzene, herein, the bridge-CH2 protons gave rise to two separate signals because of the twisted and asymmetric conformation of the diazocine core, which is critical to guide the packing structures of these gelators.39 Similar concentration-dependent spectral changes were observed for Azo-2-C8 and Azo-2-C10 ( Supporting Information Figures S6a and S6b). Importantly, the morphological differences among different gels suggested different packing modes for these gelators, which could be deduced from the results of XRD experiments. Well-resolved small-angle X-ray diffraction (SAXD) diagrams of the three xerogels ( Azo-2-C8, Azo-2-C10, and Azo-3-C8) demonstrated the long-range aggregation of the gelators. The Azo-2-C8 xerogel three with of and nm, to a structure (Figure as for Azo-2-C10, the presence of a stable its self-assembled gel was also observed ( Supporting Information Figure The values are consistent with the molecular of Azo-2-C8 and Azo-2-C10, and nm, by molecular force Supporting Information Figure for its xerogel three with the of (Figure Therefore, a hexagonal columnar with a value of nm was formed its Furthermore, the X-ray diffraction of Azo-2-C8, Azo-2-C10, and Azo-3-C8 xerogels ( Supporting Information Figures several strong and in the range of corresponding to the packing of the alkoxyl chains and the diazocine cores Figure 2 | of Azo-2-C8 xerogel from n-octane and Azo-3-C8 xerogel from n-decane. (c) self-assembled structures of Azo-2-C8 Azo-2-C10 and Azo-3-C8 Download figure Download PowerPoint Collectively, it was that π–π interactions, and van der Waals forces played cooperative in the The resulting gels could be into two to their molecular packing Azo-2-C8 and Azo-2-C10, with a fewer number of alkoxyl chains, structures with the azobenzene herein, the bent and steric diazocine core the of the the of stable hollow with in the of due to the large steric effect by the peripheral alkoxyl chains, a structure was in its This could be by the presence of stable hexagonal the further that were ca. two gelator molecules in each columnar unit ( Supporting Information Table which could together a can in this The two of packing modes are in Figure With the diazocine in the gelators, all the gels were not only but also The the properties of the gelators but not the properties of the resulting and upon a prominent chromic place together with the sol–gel transition. For example, upon 5 with light the Azo-3-C8 gels gradually and into transparent (Figure Such from yellow to was by the The color fading of the by the of organogels was by the of the sol by light nm) and the of and the chromic phase transition was reversible and could be in multiple good of Z- Azo-3-C8 (Figure Therefore, upon two distinct three states of our gelators with different and properties could be obtained which results the promising of these gelators in fabricating logic gates with an appropriate addressing mode heating). Figure 3 | sol–gel and chromic of Azo-3-C8 gels to formed in n-pentane, n-hexane, n-heptane, n-octane, and triggered by different external as or with UV–vis absorption spectra of Azo-3-C8 organogel under the of light and (c) sol under the of UV–vis absorption changes of at 413 and nm under the of and light in n-octane, 1 Download figure Download PowerPoint The photochromic properties of the gels were characterized in more by UV–vis and 1H NMR Similar spectral place in their gel state, as well as in solution (Figure and Supporting Information Figures and the Azo-3-C8 gel exhibited absorption band peak at nm, which was consistent with the absorption of the Z with light and of this n–π* absorption the and of a new and band at nm was corresponding to the n–π* transition of the E The state was the gel for approximately (Figure resulting in a sol morphologies were and an absorption with that of the E ( Supporting Information Figures This Z to E photoisomerization was also monitored by 1H NMR spectroscopy ( Supporting Information Figure which showed the of an of aromatic protons under light By the of the two aromatic for example, at and 7.35 for and Z- Supporting Information Figure the state was determined with a E/Z ratio of and approximately thereby with the UV–vis The of E to Z at different were in Supporting Information Figure the E to Z transition place very slowly at room temperature and became with temperature. For the obtained state to the Z state upon the sol state at for approximately ( Supporting Information Figure of this E to Z transition was also when the sol with light for (Figure After a yellow gel could be which showed and morphological as of the Z- Azo-3-C8 In a similar the photoresponsive properties of Azo-2-C8 and Azo-2-C10 were all of which prominent and reversible photochromic in different states ( Supporting Information Figures and The diazocine was demonstrated as a versatile building block to construct photoresponsive supramolecular gels. The effect from the packing of the diazocine core, the van der Waals forces between the peripheral alkoxyl chains, and the H-bonding interactions by the amide the self-assembly of all the gelators aggregation modes (lamellar or hexagonal columnar packing were dependent on the number of the peripheral In contrast to the azobenzene analogues, the unconventional conformational bulk and and photochromic properties Z to E of the diazocine excellent photoswitching and -chromic properties in their gel states. The fast and interconversion between multiple states of these chromic gels their promising applications as smart soft materials to molecular logic memory and Supporting Information Supporting Information is available and experimental details on the synthesis of all the and and Figures of is of interest to This was by the Key and Development of and the of and the and the of Tianjin The Zhang for and for kind in AFM Yan Zhang of Photoresponsive Bridged as and of by a Molecular