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Non-conventional low-molecular-weight organogelators with superhydrophobicity based on fluorescent β-diketone-boron difluorides
Journal Pre-proof Non-conventional low-molecular-weight organogelators with superhydrophobicity based on fluorescent β-diketone-boron difluorides Pengcheng Zhu, Xiaojing Yan, Yang Li, Haichuang Lan, Shuzhang Xiao PII:
S0143-7208(19)32342-3
DOI:
https://doi.org/10.1016/j.dyepig.2019.108176
Reference:
DYPI 108176
To appear in:
Dyes and Pigments
Received Date: 6 October 2019 Revised Date:
28 November 2019
Accepted Date: 27 December 2019
Please cite this article as: Zhu P, Yan X, Li Y, Lan H, Xiao S, Non-conventional low-molecular-weight organogelators with superhydrophobicity based on fluorescent β-diketone-boron difluorides, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2019.108176. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Non-conventional
low-molecular-weight
organogelators
with
superhydrophobicity based on fluorescent β-diketone-boron difluorides Pengcheng Zhu, Xiaojing Yan, Yang Li, Haichuang Lan, Shuzhang Xiao* College of Biological and Pharmaceutical Sciences, China Three Gorges University, Yichang 443002, Hubei, P. R. China. Corresponding author: Dr. Shuzhang Xiao Email: [email protected] Tel: 86-717-6397478 Fax: 86-717-6397478 Abstract: A series of β-diketone-boron difluoride derivatives bearing different methyl-substituted phenyl groups were designed and synthesized, and two of these boron-difluorides could form stable organogels in non-polar solvents, demonstrating that the methyl substituents have a significant effect on their gelation ability. Through temperature-dependent 1H NMR and photophysical properties analysis, we deemed that intermolecular non-covalent interactions facilitate the one-dimensional packing. Moreover, superhydrophobic surfaces, characterized by water contact angles around 150º could be obtained by means of the gelation of these boron-difluoride dyes. Thus, our work provided a unique example for designing non-conventional organogelators with superhydrophobic surfaces by introducing methyl-substituted phenyl groups into β-diketone-boron difluoride skeleton.
Key words: Boron-difluoride, non-conventional, organogel, fluorescence, superhydrophobicity
Low-molecular-weight gelator (LMWGs) is a class of soft material with large specific area and has received increasing attention due to their significance in fundamental scientific research and practical applications. Especially, LMWGs containing π-conjugated fluorescent chromophores are even more attractive because of their unique optoelectronic properties,
[1-6]
and potential applications in solar
cells,[7] field-effect transistors,[8-11] and chemosensors [12]. In order to promote the gelation of molecule in suitable solvents, functional groups bearing amide units and long-chain alkyl moieties were usually introduced into the gelator molecule to construct multiple non-covalent interactions, such as hydrogen bonding, van der Waals interactions, π–π stacking, and solvophobic forces. Although amide units and long-chain alkyl moieties could promote one-dimensional array to facilitate the gelation, they usually donot contribute to their photoelectric properties. Recently, a few examples of non-conventional LMWGs without alkyl chains have been reported, in which the balanced π-π interactions and hydrogen-bonding between various aromatic rings play important roles in the gel formation. These organogelators usually have a planar π system and a few short-chain alkyl substituents to balance the solubility and crystallinity.
[13-21]
Some gelators comprise
only rigid aromatic rings and can gelate organic solvents even without additional substituents.
[22-26]
Among all the non-conventional π gelators, we are interested in boron-difluoride skeleton, because boron difluoride-based dyes have been widely investigated due to their various advantages over other dyes, such as large molar absorption coefficients, controllable emission wavelength and high fluorescence quantum yields. Through introduction of suitable substitutes to the boron-difluoride chromophore, one-dimensional molecular packing might be obtained to make them excelletn non-conventional organogelators. Currently, it has been found that halogen atom, and tetraphenyl ethylene unit
[27-29]
and even t-butyl group
[30-33]
[19]
triphenylamine
play important role in the
self-assemblying of boron-difluoride dyes and might facilitate the one-dimensional molecular packing to promote gelation. However, rational design of non-conventional LMWGs based on boron-difluoride chromophore remains a significant challenge. Considering that halogen atoms and t-butyl unit could help to balance intermolecular interactions, we suppose that some other small substitute could also play important role in molecule’s self-assembly to facilitate the gelation. As the smallest alkyl group, methyl unit has never been reported to be important substitute for gelation. Herein, we designed a series of β-diketone-boron difluoride derivatives bearing different number of methyl groups and found that methyl group could significantly affect the self-assembly of β-diketone-based boron difluoride dyes (Scheme 1). Among the five dyes, only BF2-3 and BF2-4 could form stable organogels in non-polar solvents, indicating that both the number and the position of the methyl units affect the gelation. To the best of our knowledge, the gelators reported here are the boron-difluoride gelators with smallest molecular weight.
Scheme 1
2. Experimental
2.1 Materials and measurements
All starting materials were obtained from commercial supplies and used as received. 1H NMR spectrum was recorded on Bruker 400 NMR instruments, using CDCl3 or cyclohexane-d12 as solvent. HRMS data were recorded on Applied Biosystems Voyager-DE STR mass spectrometer. SEM images of the xerogels were obtained by using SSX-550 (Shimadzu). Uv-vis and fluorescent measurements
were carried out on Shimadzu UV-2600 and Hitachi F-4600, respectively. The fluorescence quantum yields in solutions are calculated using quinine sulfate as a reference.
The gelator and solvent were put in a septum-capped test tube and heated until the solid dissolved. The sample vial was then cooled to room temperature. Qualitatively, gelation was considered a success if no sample flow was observed upon inversion of the container at room temperature (the inverse flow method). Xerogel samples were obtained by evaporation of the solvent from the gel via freeze-drying.
Geometry optimization were performed by density functional theory (DFT) at B3LYP with the 6-31+G(d,p) basis sets.
[34]
The simulation of molecular packing was performed by Accelrys
Polymorph Predictor module of the Materials Studio software, using the simulated annealing algorithm of Karfunkel and Gdanitz. [35]
2.2 Synthesis
2.2.1 BF2-1 and BF2-2 were synthesized according the procedure reported previously (yield: 82% for BF2-1, 60% for BF2-2). [36-37]
BF2-1: 1H NMR (400 MHz, CDCl3): δ 8.16 (dd, J = 8.5, 1.2 Hz, 4H), 7.75-7.66 (m, 2H), 7.57 (dd, J = 11.1, 4.5 Hz, 4H), 7.21 (s, 1H). BF2-2:
1
H NMR (400 MHz, CDCl3): δ 8.05 (d, J = 8.3 Hz, 4H), 7.35 (d, J = 8.1 Hz, 4H), 7.13 (s, 1H),
2.47 (s, 6H). 2.2.2 BF2-3 and BF2-4 were synthesized according to the following procedure: In a three-neck flask with condenser, anhydrous AlCl3 (8.51g, 64.0 mmol) was stirred in CS2 (50 mL) under nitrogen at -5 ºC, followed by addition of m-xylene or p-xylene (60.0 mmol) in one portion. Then, malonyl dichloride (2.00 g, 15.0 mmol) was added by syringe drop by drop. After the addition of malonyl dichloride, the
reaction mixture was heated at 50 ºC for 5 h (the produced HCl gas was absorbed by NaOH solution). Then the reaction mixture was allowed to cool to room temperature and poured into cooled aqueous HCl solution slowly. After extracted by dichloromethane, the organic phase was washed by water and saturated NaCl solution, then treated by concentrated HCl (2 mL). The mixture was refluxed under 100 ºC for 12 h, and then neutralized by NaOH solution. After dried over anhydrous Na2SO4, the solvent was removed to provide a deep yellow solid. The powder was further dissolved in anhydrous chloroform (20 mL) under nitrogen, followed by addition of BF3.Et2O (3.78 mL, 30.0 mmol) drop by drop through a syringe. After the addition of BF3.Et2O, the reaction mixture was allowed to stir at room temperature overnight and then quenched by water. The mixture was extracted by dichloromethane and dried over anhydrous Na2SO4. The obtained crude product was then subjected to column chromatography to provide target products (yield: 26% for BF2-3, 30% for BF2-4). BF2-3: 1H NMR (400 MHz, CDCl3): δ: 7.65 (d, J = 7.6 Hz, 2H), 7.14 (t, J = 12.0 Hz, 4H), 6.68 (s, 1H), 2.63 (s, 6H), 2.40 (s, 6H).
13
C NMR (100 MHz, CDCl3) δ 186.63, 144.67, 140.18, 133.47, 130.38,
129.98, 128.24, 127.17, 100.13, 21.76. HRMS: calcd for C19H19BF2O2 [M+Na]+ 351.1342, found 351.1342. BF2-4: 1H NMR (400 MHz, CDCl3): δ 7.53 (s, 2H), 7.31 (dd, J = 7.8, 1.2 Hz, 2H), 7.22 (d, J = 7.8 Hz, 2H), 6.67 (s, 1H), 2.60 (s, 6H), 2.39 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 187.29, 136.62, 135.93, 134.24, 132.34, 130.31, 100.81, 21.01, 20.81. HRMS: calcd for C19H19BF2O2 [M+H]+ 329.1522, found 329.1522. 2.2.3 BF2-5 was synthesized according to the following procedure: NaH (0.40 g, 0.017mol) was stirred in DMSO (10 mL) at 0 ºC under nitrogen, followed by addition of methyl 3,4-dimethylbenzoate (1.50 g， 9.1 mmol) in 5 mL DMSO and 3,4-Dimethylacetophenone (1.48 g，10.0 mol) in 5 mL DMSO through
a syringe. The reaction mixture was stirred at room temperature for 1 h and then at 30 ºC for 1h. After the reaction was finished, the reaction mixture was poured into cooled aqueous HCl solution slowly and extracted by ethyl acetate. The organic phase was dried over anhydrous Na2SO4 and the solvent was removed by evaporation under vacuum to provide a pale yellow solid. The solid was then dissolved in chloroform (15 mL) under nitrogen, followed by addition of BF3.Et2O (2.29 mL, 18.2 mmol) drop by drop through a syringe. Then, the reaction mixture was allowed to stir at room temperature overnight and then quenched by water. The mixture was extracted by dichloromethane and dried over anhydrous Na2SO4. The obtained crude product was then subjected to column chromatography to provide a white solid (18%). 1
H NMR (400 MHz, CDCl3): δ 7.93 (s, 2H), 7.88 (dd, J = 8.0, 1.7 Hz, 2H), 7.30 (d, J = 8.0 Hz, 2H),
7.12 (s, 1H), 2.37 (d, J = 4.0 Hz, 12H). 13C NMR (100 MHz, CDCl3): δ 145.44, 137.75, 130.38, 129.85, 126.60, 92.65, 20.36, 19.80. HRMS: calcd for C19H19BF2O2 [M+H]+ 329.1522, found 329.1522.
Results and discussion The synthesis of BF2-1 and BF2-2 have been reported previously.
[36-38]
For BF2-3 and BF2-4,
Friedel-Craft reaction with methyl-substituted benzene and malonyl dichloride as start materials, using anhydrous AlCl3 as catalyst, provided the β-diketone ligands. The Claisen condensation reaction between methyl-substituted acetophenone and methyl-substituted ethyl acetate provided the ligand for BF2-5. Complexation of the ligands with boron trifluoride-diethyl etherate, afforded target products with moderate yields.
Figure 1
The UV/Vis absorption and fluorescence emission spectra of boron-difluorides in chloroform are shown in Figure 1. BF2-1 clearly shows absorption bands at 275, 364, 380 nm, and the absorption bands in the range of 316-408 nm should be due to π-π* transitions. Compounds BF2-3 and BF2-4 exhibit similar absorption bands as those of BF2-1, indicating that the two methyl groups substituted at o, p and o, m positions have little effect on the absorption propeties. However, the absorption bands of BF2-2 arising from π-π* transitions are located at 373 and 390 nm, which are slightly red-shifted compared with those of BF2-1. Moreover, the absorption bands of BF2-5 arising from π-π* transitions red-shifted to 380 and 394 nm. As a weak electron-donor, introducing methyl group on p position of phenyl ring should be able to facilitate the intramolecular charge transfer (ICT) to red-shift the absorption. However, multiple methyl groups on phenyl rings might affect distribution of electron clouds on HOMO and LUMO orbitals. Quantum calculations performed by density functional theory (DFT) at the B3LYP/6-31g+(d,p) level proved this hypothesis. As shown in Figure 2, the HOMOs of all the five boron-difluoride dyes were distributed along the π conjugated system (except the B, F, O atoms), and the LUMOs were located all over the whole molecules. Thus, all these boron-difluorides posses weak ICT property. Solvent-dependent absorption also supports this conclusion because their absorption bands are only slightly dependent on solvent polarity (Figure S1). It`s noteworthy that the calculated energy levels of HOMO and LUMO for BF2-3 and BF2-4 are quite close to that of BF2-1, but the energy gaps of BF2-2 and BF2-5 are comparatively smaller, which explains their red-shifted absorptions.
Figure 2
β-diketone-boron difluoride derivatives usually emit intense fluorescence both in dilute solutions and in the solid state. The emissions of all these synthesized boron-difluorides are strong, their fluorescent quantum yields in chloroform are above 0.20 (except BF2-4), using quinine sulfate as standard. Compare to BF2-1, the boron-difluorides bearing methyl groups exhibited slightly red-shifted fluorescence, indicating that methyl group acts as electron donor to facilitate the ICT in the excited state. Moreover, their fluorescences are also slightly dependent on solvent polarity, manifesting weak ICT characteristics.
Table 1
The gelation properties of the boron-difluorides were tested in various solvents by means of the “stable to inversion of a test tube” method. As summarized in Table 1, BF2-1 did not dissolve in non-polar solvents, such as hexane, cyclohexane and petroleum ether. In toluene, it tends to precipitate as cooled to room temperature, and it’s well soluble in all the other solvents. BF2-2 and BF2-5 did not dissolve in non-polar solvents just as BF2-1, and only precipitates were observed in the other solvents. However, BF2-3 and BF2-4 could form stable gels in hexane, cyclohexane, petroleum ether and toluene. BF2-3, BF2-4 and BF2-5 are isomers with same chemical formula but different chemical structures, but clearly they exhibit totally different gelation behaviors. These results suggested that methyl group (number, position) played vital role in the self-assembly of these boron-difluoride dyes in organic solvents, and then affects the gel formation. On the other hand, none of the ligands of all the boron-difluorides were able to form gel in studied organic solvents. The morphologies of the organogels were determined by Scanning Electron Microscopy (SEM). As
shown in Figure 3, flake-like lamellar structure was observed for the xerogels of both BF2-3 and BF2-4. These small flakes entangled with each other to form a porous network on mica, and these large volumes of solvents could be immobilized within the porous networks, resulting in gelation.
Figure 3
In
order
to
investigate
the
intermolecular
interactions
during
the
gel
formation,
temperature-dependent 1H NMR were measured for the gels of BF2-3 and BF2-4 in cyclohexane-d12 (Figure 4). The concentration of gels were 5 mg/mL for both samples, in order to balance the signal intensity and their solubility in hot cyclohexane-d12 (Figure S3). In the 1H NMR of BF2-3 gel, there appeared a doublet at 7.45 ppm, one singlet at 6.68 ppm and another doublet at 6.93 ppm, referenced to the protons on phenyl rings. The proton on vinyl unit in BF2 chromophore appeared at 6.40 ppm. With temperature increasing to destroy the gel, there is minimal signal change about their chemical shifts, including the signals of methyl groups. Based on this result, we can conclude that intermolecular π-π interactions between phenyl rings should be absent in the gel of BF2-3, because intermolecular π-π interactions between aromatic rings results in broadening of the signals and new resonance peaks. [24-26] Therefore, the gelation of BF2-3 should be ascribed to the one-dimensional packing facilitated by boron-difluoride skeleton.
Figure 4
For BF2-4 gel in cyclohexane-d12, the protons on the phenyl rings appeared at 7.33, 7.07 and 7.04 ppm, respectively. The signal ascribed to the proton on the vinyl unit was located at 6.38 ppm as a
singlet, and the protons on methyl groups appeared at 2.46 and 2.24 ppm. With temperature increasing to break the gel, slight chemical shift changes were observed for the protons ascribed to the phenyl rings, vinyl unit and methyl groups. Considering that the chemical shift change of the proton on phenyl rings are very small, the intermolecular π-π interactions between phenyl rings should be negligible. Moreover, methyl groups should participate in the interactions to facilitate the gelation because the protons on methyl groups clearly shift to up-field.
Figure 5
To further investigate the driving force for the gelation, the fluorescence spectral changes during the gelation process (from sol to gel) of BF2-3 and BF2-4 were tested. BF2-3 emits weak blue fluorescence at 429 nm in hot cyclohexane solution. Upon cooling naturally, this emission band was first intensified gradually, and then weakened with slight red-shift, and then intensified again. During the sol to gel process, the red-shift of the fluorescence is 15 nm. BF2-4 in hot cyclohexane solution also showed weak blue fluorescence centred at 426 nm, and the fluorescent intensity increased first upon cooling. But similar as BF2-3, the fluorescence turned weakened with obvious red-shift, and then intensified again. The red-shift of its fluorescence is 34 nm during the sol to gel process.
Figure 6
The XRD patterns of the dried gel of BF2-3 and BF2-4 obtained from cyclohexane are given in Figure S4. Two peaks at d-spacing of 1.47 nm and 1.02 nm emerged in small-angle region of BF2-3. Comparatively, three peaks at d-spacing of 1.22 nm, 1.02 nm and 0.92 nm appears in the XRD of
BF2-4 xerogel, which indicated that these two boron-difluoride dyes should exhibit different molecular packing modes in the gel state. According to the results of temperature-dependent 1H NMR and fluorescence of the gel in cyclohexane, we can conclude that there should be no intermolecular π-π interactions between neighbouring phenyl rings in BF2-3 gel. Considering that there is 15 nm red-shift of its fluorescence in the gel state compared to that in solution (Figure 5A), conformation change might happen to facilitate more efficient intramolecular charge transfer in the gel state. Thus, we propose that the dihedral angles between BF2 plane and phenyl rings turned larger, which would red shift its fluorescent emission. In addition, BF2-3 molecules should pack into a layered structure in the gel state, in which one BF2 chromophore remains far away from another one, and non-covalent bonding between fluorine atoms on neighbouring molecules should be the main driving force for the gelation. This hypothesis is in agreement with the result obtained by computational modeling.
[24, 26]
The dihedral angles between BF2
plane and phenyl rings are predicted as 58.76 and 53.81º, which induces much more efficient intramolecular charge transfer compared to that in gas state (Figure S5). In addition, the molecular length is calculated to be 1.40 nm, and the interlayer distance is 1.02 nm (Figure S6), which agrees to the XRD result. But in BF2-4 gel, only slight intermolecular π-π interactions between phenyl rings should exist, and methyl group also interacts with neighbouring molecule. Because much more significant fluorescence red-shift was observed in the gel state, intramolecular charge transfer in BF2-4 gel might be more efficient or overlapping between neighbouring BF2 chromophores might exist. Based on these results, the molecular packing is obtained by computational modeling, as shown in Figure 6B. For BF2-4, BF2 skeleton overlaps with neighbouring BF2 chromophore to form a molecular cluster, but the distance
between two BF2 chromophore is around 4.37 Å, which induces minimal red-shift of its fluorescence. Comparatively, conformation change is more significant. The dihedral angles between BF2 plane and phenyl rings are predicted as 66.01 and 55.45º, which facilitates efficient intramolecular charge transfer (Figure S5). In the predicted molecular cluster, one phenyl ring interacts with neighbouring phenyl ring through multiple non-covalent bondings to afford one-dimensional aggregate, and methyl groups help to establish the one-dimensional packing. This hypothesis agrees well with the XRD results. The molecular length of BF2-4 in the gel state is measured to be 1.21 nm, and the interlayer distances between different layers are 1.01 and 0.94 nm, respectively (Figure S6).
Figure 7
Superhydrophobic surfaces have attracted much attention for their promising applications in the field of self-cleaning and pollutant removal. [39] Normally, the surface with water contact angles greater than 150º could be considered as superhydrophobic surfaces. To effectively develop such functionality, it is essential to control both the chemical compositions of compounds and the geometrical roughness of surfaces. In consideration of the hydrophobic character of boron-difluoride dyes and the rough surfaces of BF2-3 and BF2-4 xerogels as evidenced by SEM (Figure 2), we suppose that the xerogel film of these two boron-difluoride dyes might exhibit superhydrophobic character. The xerogel films of BF2-3 and BF2-4 were prepared by spreading the cyclohexane organogel on the glass plate without any difficulty. The slow evaporation of the solvent in the organogel provided the xerogel coating on the glass plate. As shown in Figure 7, a water contact angle of 153º was obtained for the the xerogel film of BF2-3, which is enough to call the surface superhydrophobic. The xerogel film of BF2-4 is also highly
hydrophobic, with a contact angle 145 º. The superhydrophobicity could be attributed to self-assembled nanometer-scale porous structure with nano/micrometer-scale roughness.
Conclusions We synthesized a series of β-diketone-boron difluoride derivatives bearing different methyl-substituted phenyl groups, among which two boron-difluorides could form stable organogels in non-polar solvents. According to temperature-dependent 1H NMR and fluorescence measurements for the gels, the mechanisms of the organogel formation were proposed, and intermolecular π-π stacking and non-covalent bonding between methyl group and neighboring molecule were the main driving forces for the gelation of both compounds. Due to the hydrophobic character of boron-difluoride dyes and the rough surfaces of xerogel films, superhydrophobic surface was obtained. This work provided a unique example for designing non-conventional organogelators with superhydrophobic surfaces by introducing methyl-substituted phenyl groups into β-diketone-boron difluoride skeleton.
Acknowledgements We are grateful for the financial support from the National Natural Science Foundation of China (21472111) for financial support. The authors thank Dr. Y. Lu from East China University of Science and Technology for the quantum calculation and Material analysis and testing center of CTGU for the XRD and SEM measurements.
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Scheme 1. Chemical structure of synthesized boron-difluoride dyes Table 1. Gelation test of boron-difluoride dyes Figure 1. Absorption and fluorescent spectra of boron-difluoride dyes in chloroform (1.0×10-5 M) Figure 2. Energy levels and molecular orbital surfaces in the optimized ground-state structures of boron-difluorides by density functional theory calculations at the b3lyp/6-31g+(d,p) level Figure 3. SEM images of (A) BF2-3 and (B) BF2-4 from cyclohexane gel Figure 4. Temperature-dependent 1H NMR of cyclohexane-d12 gel (A) BF2-3; (B) BF2-4 Figure 5. Time-dependent fluorescence emission spectra and images of (A) BF2-3; (B) BF2-4 in hot cyclohexane upon cooling Figure 6. Proposed molecular packing model of (A) BF2-3; (B) BF2-4 in the gel phase Figure 7. Water contact angle experimental results of the xerogel films formed in cyclohexane: (A) BF2-3; (B) BF2-4
Table 1 BF2-3
BF2-4
(CGC)
(CGC)
In
G (4.0)
G (4.4)
In
In
In
G (2.2)
G (3.3)
In
Petroleum ether
In
In
G (2.6)
G (5.0)
In
Toluene
P
P
G (35.0)
G (36.7)
P
dichloromethane
S
P
S
S
P
THF
S
P
S
S
P
Solvents
BF2-1
BF2-2
Cyclohexane
In
Hexane
BF2-5
Ethyl acetate
S
P
S
S
P
acetonitrile
S
P
S
S
P
In: insoluble; P: precipitate; S: soluble; G: gel. CGC: critical gelation concentration (mg/mL)
A series of β-diketone-boron difluoride derivatives bearing different methyl-substituted phenyl groups were designed and synthesized. Two boron-difluorides could form stable organogels in non-polar solvents. The xerogel films of these boron-fluoride dyes exhibit superhydrophobic character.
Declaration of interests The authors declare no conflict of interest.
Pengcheng Zhu: Methodology, Investigation. Xiaojing Yan: Software, Investigation. Yang Li: Investigation, Visualization. Haichuang Lan: Writing- Original draft preparation. Shuzhang Xiao: Conceptualization, Writing- Reviewing and Editing, Supervision.
S0143-7208(19)32342-3
DOI:
https://doi.org/10.1016/j.dyepig.2019.108176
Reference:
DYPI 108176
To appear in:
Dyes and Pigments
Received Date: 6 October 2019 Revised Date:
28 November 2019
Accepted Date: 27 December 2019
Please cite this article as: Zhu P, Yan X, Li Y, Lan H, Xiao S, Non-conventional low-molecular-weight organogelators with superhydrophobicity based on fluorescent β-diketone-boron difluorides, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2019.108176. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Non-conventional
low-molecular-weight
organogelators
with
superhydrophobicity based on fluorescent β-diketone-boron difluorides Pengcheng Zhu, Xiaojing Yan, Yang Li, Haichuang Lan, Shuzhang Xiao* College of Biological and Pharmaceutical Sciences, China Three Gorges University, Yichang 443002, Hubei, P. R. China. Corresponding author: Dr. Shuzhang Xiao Email: [email protected] Tel: 86-717-6397478 Fax: 86-717-6397478 Abstract: A series of β-diketone-boron difluoride derivatives bearing different methyl-substituted phenyl groups were designed and synthesized, and two of these boron-difluorides could form stable organogels in non-polar solvents, demonstrating that the methyl substituents have a significant effect on their gelation ability. Through temperature-dependent 1H NMR and photophysical properties analysis, we deemed that intermolecular non-covalent interactions facilitate the one-dimensional packing. Moreover, superhydrophobic surfaces, characterized by water contact angles around 150º could be obtained by means of the gelation of these boron-difluoride dyes. Thus, our work provided a unique example for designing non-conventional organogelators with superhydrophobic surfaces by introducing methyl-substituted phenyl groups into β-diketone-boron difluoride skeleton.
Key words: Boron-difluoride, non-conventional, organogel, fluorescence, superhydrophobicity
Low-molecular-weight gelator (LMWGs) is a class of soft material with large specific area and has received increasing attention due to their significance in fundamental scientific research and practical applications. Especially, LMWGs containing π-conjugated fluorescent chromophores are even more attractive because of their unique optoelectronic properties,
[1-6]
and potential applications in solar
cells,[7] field-effect transistors,[8-11] and chemosensors [12]. In order to promote the gelation of molecule in suitable solvents, functional groups bearing amide units and long-chain alkyl moieties were usually introduced into the gelator molecule to construct multiple non-covalent interactions, such as hydrogen bonding, van der Waals interactions, π–π stacking, and solvophobic forces. Although amide units and long-chain alkyl moieties could promote one-dimensional array to facilitate the gelation, they usually donot contribute to their photoelectric properties. Recently, a few examples of non-conventional LMWGs without alkyl chains have been reported, in which the balanced π-π interactions and hydrogen-bonding between various aromatic rings play important roles in the gel formation. These organogelators usually have a planar π system and a few short-chain alkyl substituents to balance the solubility and crystallinity.
[13-21]
Some gelators comprise
only rigid aromatic rings and can gelate organic solvents even without additional substituents.
[22-26]
Among all the non-conventional π gelators, we are interested in boron-difluoride skeleton, because boron difluoride-based dyes have been widely investigated due to their various advantages over other dyes, such as large molar absorption coefficients, controllable emission wavelength and high fluorescence quantum yields. Through introduction of suitable substitutes to the boron-difluoride chromophore, one-dimensional molecular packing might be obtained to make them excelletn non-conventional organogelators. Currently, it has been found that halogen atom, and tetraphenyl ethylene unit
[27-29]
and even t-butyl group
[30-33]
[19]
triphenylamine
play important role in the
self-assemblying of boron-difluoride dyes and might facilitate the one-dimensional molecular packing to promote gelation. However, rational design of non-conventional LMWGs based on boron-difluoride chromophore remains a significant challenge. Considering that halogen atoms and t-butyl unit could help to balance intermolecular interactions, we suppose that some other small substitute could also play important role in molecule’s self-assembly to facilitate the gelation. As the smallest alkyl group, methyl unit has never been reported to be important substitute for gelation. Herein, we designed a series of β-diketone-boron difluoride derivatives bearing different number of methyl groups and found that methyl group could significantly affect the self-assembly of β-diketone-based boron difluoride dyes (Scheme 1). Among the five dyes, only BF2-3 and BF2-4 could form stable organogels in non-polar solvents, indicating that both the number and the position of the methyl units affect the gelation. To the best of our knowledge, the gelators reported here are the boron-difluoride gelators with smallest molecular weight.
Scheme 1
2. Experimental
2.1 Materials and measurements
All starting materials were obtained from commercial supplies and used as received. 1H NMR spectrum was recorded on Bruker 400 NMR instruments, using CDCl3 or cyclohexane-d12 as solvent. HRMS data were recorded on Applied Biosystems Voyager-DE STR mass spectrometer. SEM images of the xerogels were obtained by using SSX-550 (Shimadzu). Uv-vis and fluorescent measurements
were carried out on Shimadzu UV-2600 and Hitachi F-4600, respectively. The fluorescence quantum yields in solutions are calculated using quinine sulfate as a reference.
The gelator and solvent were put in a septum-capped test tube and heated until the solid dissolved. The sample vial was then cooled to room temperature. Qualitatively, gelation was considered a success if no sample flow was observed upon inversion of the container at room temperature (the inverse flow method). Xerogel samples were obtained by evaporation of the solvent from the gel via freeze-drying.
Geometry optimization were performed by density functional theory (DFT) at B3LYP with the 6-31+G(d,p) basis sets.
[34]
The simulation of molecular packing was performed by Accelrys
Polymorph Predictor module of the Materials Studio software, using the simulated annealing algorithm of Karfunkel and Gdanitz. [35]
2.2 Synthesis
2.2.1 BF2-1 and BF2-2 were synthesized according the procedure reported previously (yield: 82% for BF2-1, 60% for BF2-2). [36-37]
BF2-1: 1H NMR (400 MHz, CDCl3): δ 8.16 (dd, J = 8.5, 1.2 Hz, 4H), 7.75-7.66 (m, 2H), 7.57 (dd, J = 11.1, 4.5 Hz, 4H), 7.21 (s, 1H). BF2-2:
1
H NMR (400 MHz, CDCl3): δ 8.05 (d, J = 8.3 Hz, 4H), 7.35 (d, J = 8.1 Hz, 4H), 7.13 (s, 1H),
2.47 (s, 6H). 2.2.2 BF2-3 and BF2-4 were synthesized according to the following procedure: In a three-neck flask with condenser, anhydrous AlCl3 (8.51g, 64.0 mmol) was stirred in CS2 (50 mL) under nitrogen at -5 ºC, followed by addition of m-xylene or p-xylene (60.0 mmol) in one portion. Then, malonyl dichloride (2.00 g, 15.0 mmol) was added by syringe drop by drop. After the addition of malonyl dichloride, the
reaction mixture was heated at 50 ºC for 5 h (the produced HCl gas was absorbed by NaOH solution). Then the reaction mixture was allowed to cool to room temperature and poured into cooled aqueous HCl solution slowly. After extracted by dichloromethane, the organic phase was washed by water and saturated NaCl solution, then treated by concentrated HCl (2 mL). The mixture was refluxed under 100 ºC for 12 h, and then neutralized by NaOH solution. After dried over anhydrous Na2SO4, the solvent was removed to provide a deep yellow solid. The powder was further dissolved in anhydrous chloroform (20 mL) under nitrogen, followed by addition of BF3.Et2O (3.78 mL, 30.0 mmol) drop by drop through a syringe. After the addition of BF3.Et2O, the reaction mixture was allowed to stir at room temperature overnight and then quenched by water. The mixture was extracted by dichloromethane and dried over anhydrous Na2SO4. The obtained crude product was then subjected to column chromatography to provide target products (yield: 26% for BF2-3, 30% for BF2-4). BF2-3: 1H NMR (400 MHz, CDCl3): δ: 7.65 (d, J = 7.6 Hz, 2H), 7.14 (t, J = 12.0 Hz, 4H), 6.68 (s, 1H), 2.63 (s, 6H), 2.40 (s, 6H).
13
C NMR (100 MHz, CDCl3) δ 186.63, 144.67, 140.18, 133.47, 130.38,
129.98, 128.24, 127.17, 100.13, 21.76. HRMS: calcd for C19H19BF2O2 [M+Na]+ 351.1342, found 351.1342. BF2-4: 1H NMR (400 MHz, CDCl3): δ 7.53 (s, 2H), 7.31 (dd, J = 7.8, 1.2 Hz, 2H), 7.22 (d, J = 7.8 Hz, 2H), 6.67 (s, 1H), 2.60 (s, 6H), 2.39 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 187.29, 136.62, 135.93, 134.24, 132.34, 130.31, 100.81, 21.01, 20.81. HRMS: calcd for C19H19BF2O2 [M+H]+ 329.1522, found 329.1522. 2.2.3 BF2-5 was synthesized according to the following procedure: NaH (0.40 g, 0.017mol) was stirred in DMSO (10 mL) at 0 ºC under nitrogen, followed by addition of methyl 3,4-dimethylbenzoate (1.50 g， 9.1 mmol) in 5 mL DMSO and 3,4-Dimethylacetophenone (1.48 g，10.0 mol) in 5 mL DMSO through
a syringe. The reaction mixture was stirred at room temperature for 1 h and then at 30 ºC for 1h. After the reaction was finished, the reaction mixture was poured into cooled aqueous HCl solution slowly and extracted by ethyl acetate. The organic phase was dried over anhydrous Na2SO4 and the solvent was removed by evaporation under vacuum to provide a pale yellow solid. The solid was then dissolved in chloroform (15 mL) under nitrogen, followed by addition of BF3.Et2O (2.29 mL, 18.2 mmol) drop by drop through a syringe. Then, the reaction mixture was allowed to stir at room temperature overnight and then quenched by water. The mixture was extracted by dichloromethane and dried over anhydrous Na2SO4. The obtained crude product was then subjected to column chromatography to provide a white solid (18%). 1
H NMR (400 MHz, CDCl3): δ 7.93 (s, 2H), 7.88 (dd, J = 8.0, 1.7 Hz, 2H), 7.30 (d, J = 8.0 Hz, 2H),
7.12 (s, 1H), 2.37 (d, J = 4.0 Hz, 12H). 13C NMR (100 MHz, CDCl3): δ 145.44, 137.75, 130.38, 129.85, 126.60, 92.65, 20.36, 19.80. HRMS: calcd for C19H19BF2O2 [M+H]+ 329.1522, found 329.1522.
Results and discussion The synthesis of BF2-1 and BF2-2 have been reported previously.
[36-38]
For BF2-3 and BF2-4,
Friedel-Craft reaction with methyl-substituted benzene and malonyl dichloride as start materials, using anhydrous AlCl3 as catalyst, provided the β-diketone ligands. The Claisen condensation reaction between methyl-substituted acetophenone and methyl-substituted ethyl acetate provided the ligand for BF2-5. Complexation of the ligands with boron trifluoride-diethyl etherate, afforded target products with moderate yields.
Figure 1
The UV/Vis absorption and fluorescence emission spectra of boron-difluorides in chloroform are shown in Figure 1. BF2-1 clearly shows absorption bands at 275, 364, 380 nm, and the absorption bands in the range of 316-408 nm should be due to π-π* transitions. Compounds BF2-3 and BF2-4 exhibit similar absorption bands as those of BF2-1, indicating that the two methyl groups substituted at o, p and o, m positions have little effect on the absorption propeties. However, the absorption bands of BF2-2 arising from π-π* transitions are located at 373 and 390 nm, which are slightly red-shifted compared with those of BF2-1. Moreover, the absorption bands of BF2-5 arising from π-π* transitions red-shifted to 380 and 394 nm. As a weak electron-donor, introducing methyl group on p position of phenyl ring should be able to facilitate the intramolecular charge transfer (ICT) to red-shift the absorption. However, multiple methyl groups on phenyl rings might affect distribution of electron clouds on HOMO and LUMO orbitals. Quantum calculations performed by density functional theory (DFT) at the B3LYP/6-31g+(d,p) level proved this hypothesis. As shown in Figure 2, the HOMOs of all the five boron-difluoride dyes were distributed along the π conjugated system (except the B, F, O atoms), and the LUMOs were located all over the whole molecules. Thus, all these boron-difluorides posses weak ICT property. Solvent-dependent absorption also supports this conclusion because their absorption bands are only slightly dependent on solvent polarity (Figure S1). It`s noteworthy that the calculated energy levels of HOMO and LUMO for BF2-3 and BF2-4 are quite close to that of BF2-1, but the energy gaps of BF2-2 and BF2-5 are comparatively smaller, which explains their red-shifted absorptions.
Figure 2
β-diketone-boron difluoride derivatives usually emit intense fluorescence both in dilute solutions and in the solid state. The emissions of all these synthesized boron-difluorides are strong, their fluorescent quantum yields in chloroform are above 0.20 (except BF2-4), using quinine sulfate as standard. Compare to BF2-1, the boron-difluorides bearing methyl groups exhibited slightly red-shifted fluorescence, indicating that methyl group acts as electron donor to facilitate the ICT in the excited state. Moreover, their fluorescences are also slightly dependent on solvent polarity, manifesting weak ICT characteristics.
Table 1
The gelation properties of the boron-difluorides were tested in various solvents by means of the “stable to inversion of a test tube” method. As summarized in Table 1, BF2-1 did not dissolve in non-polar solvents, such as hexane, cyclohexane and petroleum ether. In toluene, it tends to precipitate as cooled to room temperature, and it’s well soluble in all the other solvents. BF2-2 and BF2-5 did not dissolve in non-polar solvents just as BF2-1, and only precipitates were observed in the other solvents. However, BF2-3 and BF2-4 could form stable gels in hexane, cyclohexane, petroleum ether and toluene. BF2-3, BF2-4 and BF2-5 are isomers with same chemical formula but different chemical structures, but clearly they exhibit totally different gelation behaviors. These results suggested that methyl group (number, position) played vital role in the self-assembly of these boron-difluoride dyes in organic solvents, and then affects the gel formation. On the other hand, none of the ligands of all the boron-difluorides were able to form gel in studied organic solvents. The morphologies of the organogels were determined by Scanning Electron Microscopy (SEM). As
shown in Figure 3, flake-like lamellar structure was observed for the xerogels of both BF2-3 and BF2-4. These small flakes entangled with each other to form a porous network on mica, and these large volumes of solvents could be immobilized within the porous networks, resulting in gelation.
Figure 3
In
order
to
investigate
the
intermolecular
interactions
during
the
gel
formation,
temperature-dependent 1H NMR were measured for the gels of BF2-3 and BF2-4 in cyclohexane-d12 (Figure 4). The concentration of gels were 5 mg/mL for both samples, in order to balance the signal intensity and their solubility in hot cyclohexane-d12 (Figure S3). In the 1H NMR of BF2-3 gel, there appeared a doublet at 7.45 ppm, one singlet at 6.68 ppm and another doublet at 6.93 ppm, referenced to the protons on phenyl rings. The proton on vinyl unit in BF2 chromophore appeared at 6.40 ppm. With temperature increasing to destroy the gel, there is minimal signal change about their chemical shifts, including the signals of methyl groups. Based on this result, we can conclude that intermolecular π-π interactions between phenyl rings should be absent in the gel of BF2-3, because intermolecular π-π interactions between aromatic rings results in broadening of the signals and new resonance peaks. [24-26] Therefore, the gelation of BF2-3 should be ascribed to the one-dimensional packing facilitated by boron-difluoride skeleton.
Figure 4
For BF2-4 gel in cyclohexane-d12, the protons on the phenyl rings appeared at 7.33, 7.07 and 7.04 ppm, respectively. The signal ascribed to the proton on the vinyl unit was located at 6.38 ppm as a
singlet, and the protons on methyl groups appeared at 2.46 and 2.24 ppm. With temperature increasing to break the gel, slight chemical shift changes were observed for the protons ascribed to the phenyl rings, vinyl unit and methyl groups. Considering that the chemical shift change of the proton on phenyl rings are very small, the intermolecular π-π interactions between phenyl rings should be negligible. Moreover, methyl groups should participate in the interactions to facilitate the gelation because the protons on methyl groups clearly shift to up-field.
Figure 5
To further investigate the driving force for the gelation, the fluorescence spectral changes during the gelation process (from sol to gel) of BF2-3 and BF2-4 were tested. BF2-3 emits weak blue fluorescence at 429 nm in hot cyclohexane solution. Upon cooling naturally, this emission band was first intensified gradually, and then weakened with slight red-shift, and then intensified again. During the sol to gel process, the red-shift of the fluorescence is 15 nm. BF2-4 in hot cyclohexane solution also showed weak blue fluorescence centred at 426 nm, and the fluorescent intensity increased first upon cooling. But similar as BF2-3, the fluorescence turned weakened with obvious red-shift, and then intensified again. The red-shift of its fluorescence is 34 nm during the sol to gel process.
Figure 6
The XRD patterns of the dried gel of BF2-3 and BF2-4 obtained from cyclohexane are given in Figure S4. Two peaks at d-spacing of 1.47 nm and 1.02 nm emerged in small-angle region of BF2-3. Comparatively, three peaks at d-spacing of 1.22 nm, 1.02 nm and 0.92 nm appears in the XRD of
BF2-4 xerogel, which indicated that these two boron-difluoride dyes should exhibit different molecular packing modes in the gel state. According to the results of temperature-dependent 1H NMR and fluorescence of the gel in cyclohexane, we can conclude that there should be no intermolecular π-π interactions between neighbouring phenyl rings in BF2-3 gel. Considering that there is 15 nm red-shift of its fluorescence in the gel state compared to that in solution (Figure 5A), conformation change might happen to facilitate more efficient intramolecular charge transfer in the gel state. Thus, we propose that the dihedral angles between BF2 plane and phenyl rings turned larger, which would red shift its fluorescent emission. In addition, BF2-3 molecules should pack into a layered structure in the gel state, in which one BF2 chromophore remains far away from another one, and non-covalent bonding between fluorine atoms on neighbouring molecules should be the main driving force for the gelation. This hypothesis is in agreement with the result obtained by computational modeling.
[24, 26]
The dihedral angles between BF2
plane and phenyl rings are predicted as 58.76 and 53.81º, which induces much more efficient intramolecular charge transfer compared to that in gas state (Figure S5). In addition, the molecular length is calculated to be 1.40 nm, and the interlayer distance is 1.02 nm (Figure S6), which agrees to the XRD result. But in BF2-4 gel, only slight intermolecular π-π interactions between phenyl rings should exist, and methyl group also interacts with neighbouring molecule. Because much more significant fluorescence red-shift was observed in the gel state, intramolecular charge transfer in BF2-4 gel might be more efficient or overlapping between neighbouring BF2 chromophores might exist. Based on these results, the molecular packing is obtained by computational modeling, as shown in Figure 6B. For BF2-4, BF2 skeleton overlaps with neighbouring BF2 chromophore to form a molecular cluster, but the distance
between two BF2 chromophore is around 4.37 Å, which induces minimal red-shift of its fluorescence. Comparatively, conformation change is more significant. The dihedral angles between BF2 plane and phenyl rings are predicted as 66.01 and 55.45º, which facilitates efficient intramolecular charge transfer (Figure S5). In the predicted molecular cluster, one phenyl ring interacts with neighbouring phenyl ring through multiple non-covalent bondings to afford one-dimensional aggregate, and methyl groups help to establish the one-dimensional packing. This hypothesis agrees well with the XRD results. The molecular length of BF2-4 in the gel state is measured to be 1.21 nm, and the interlayer distances between different layers are 1.01 and 0.94 nm, respectively (Figure S6).
Figure 7
Superhydrophobic surfaces have attracted much attention for their promising applications in the field of self-cleaning and pollutant removal. [39] Normally, the surface with water contact angles greater than 150º could be considered as superhydrophobic surfaces. To effectively develop such functionality, it is essential to control both the chemical compositions of compounds and the geometrical roughness of surfaces. In consideration of the hydrophobic character of boron-difluoride dyes and the rough surfaces of BF2-3 and BF2-4 xerogels as evidenced by SEM (Figure 2), we suppose that the xerogel film of these two boron-difluoride dyes might exhibit superhydrophobic character. The xerogel films of BF2-3 and BF2-4 were prepared by spreading the cyclohexane organogel on the glass plate without any difficulty. The slow evaporation of the solvent in the organogel provided the xerogel coating on the glass plate. As shown in Figure 7, a water contact angle of 153º was obtained for the the xerogel film of BF2-3, which is enough to call the surface superhydrophobic. The xerogel film of BF2-4 is also highly
hydrophobic, with a contact angle 145 º. The superhydrophobicity could be attributed to self-assembled nanometer-scale porous structure with nano/micrometer-scale roughness.
Conclusions We synthesized a series of β-diketone-boron difluoride derivatives bearing different methyl-substituted phenyl groups, among which two boron-difluorides could form stable organogels in non-polar solvents. According to temperature-dependent 1H NMR and fluorescence measurements for the gels, the mechanisms of the organogel formation were proposed, and intermolecular π-π stacking and non-covalent bonding between methyl group and neighboring molecule were the main driving forces for the gelation of both compounds. Due to the hydrophobic character of boron-difluoride dyes and the rough surfaces of xerogel films, superhydrophobic surface was obtained. This work provided a unique example for designing non-conventional organogelators with superhydrophobic surfaces by introducing methyl-substituted phenyl groups into β-diketone-boron difluoride skeleton.
Acknowledgements We are grateful for the financial support from the National Natural Science Foundation of China (21472111) for financial support. The authors thank Dr. Y. Lu from East China University of Science and Technology for the quantum calculation and Material analysis and testing center of CTGU for the XRD and SEM measurements.
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Scheme 1. Chemical structure of synthesized boron-difluoride dyes Table 1. Gelation test of boron-difluoride dyes Figure 1. Absorption and fluorescent spectra of boron-difluoride dyes in chloroform (1.0×10-5 M) Figure 2. Energy levels and molecular orbital surfaces in the optimized ground-state structures of boron-difluorides by density functional theory calculations at the b3lyp/6-31g+(d,p) level Figure 3. SEM images of (A) BF2-3 and (B) BF2-4 from cyclohexane gel Figure 4. Temperature-dependent 1H NMR of cyclohexane-d12 gel (A) BF2-3; (B) BF2-4 Figure 5. Time-dependent fluorescence emission spectra and images of (A) BF2-3; (B) BF2-4 in hot cyclohexane upon cooling Figure 6. Proposed molecular packing model of (A) BF2-3; (B) BF2-4 in the gel phase Figure 7. Water contact angle experimental results of the xerogel films formed in cyclohexane: (A) BF2-3; (B) BF2-4
Table 1 BF2-3
BF2-4
(CGC)
(CGC)
In
G (4.0)
G (4.4)
In
In
In
G (2.2)
G (3.3)
In
Petroleum ether
In
In
G (2.6)
G (5.0)
In
Toluene
P
P
G (35.0)
G (36.7)
P
dichloromethane
S
P
S
S
P
THF
S
P
S
S
P
Solvents
BF2-1
BF2-2
Cyclohexane
In
Hexane
BF2-5
Ethyl acetate
S
P
S
S
P
acetonitrile
S
P
S
S
P
In: insoluble; P: precipitate; S: soluble; G: gel. CGC: critical gelation concentration (mg/mL)
A series of β-diketone-boron difluoride derivatives bearing different methyl-substituted phenyl groups were designed and synthesized. Two boron-difluorides could form stable organogels in non-polar solvents. The xerogel films of these boron-fluoride dyes exhibit superhydrophobic character.
Declaration of interests The authors declare no conflict of interest.
Pengcheng Zhu: Methodology, Investigation. Xiaojing Yan: Software, Investigation. Yang Li: Investigation, Visualization. Haichuang Lan: Writing- Original draft preparation. Shuzhang Xiao: Conceptualization, Writing- Reviewing and Editing, Supervision.