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thiadiazole-based amphiphiles with glycerol groups
Accepted Manuscript Synthesis, liquid-crystalline, photophysical and chemosensor properties of oxadiazole/thiadiazole-based amphiphiles with glycerol groups
Yulong Xiao, Hongfei Gao, Tingyan Wang, Ruilin Zhang, Xiaohong Cheng PII: DOI: Reference:
S0167-7322(17)33156-2 doi: 10.1016/j.molliq.2017.08.110 MOLLIQ 7818
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
15 July 2017 27 August 2017 29 August 2017
Please cite this article as: Yulong Xiao, Hongfei Gao, Tingyan Wang, Ruilin Zhang, Xiaohong Cheng , Synthesis, liquid-crystalline, photophysical and chemosensor properties of oxadiazole/thiadiazole-based amphiphiles with glycerol groups, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.08.110
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Synthesis, liquid-crystalline, photophysical and chemosensor properties of oxadiazole/thiadiazole-based amphiphiles with glycerol groups
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Yulong Xiao, ‡ [a] Hongfei Gao, ‡ [a] Tingyan Wang, [b,a] Ruilin Zhang, [c,a] Xiaohong Cheng*[a]
Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry Department,
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Yunnan University Kunming,
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Yunnan 650091, P. R. China
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Fax: (+86) 871 65032905 E-mail: [email protected]
College of Science, Beijing University of Chemical Technology, Beijing 100029, P.R.China
[c]
Forensic Medicine of Kunming Medical University, Kunming 650500, P. R. China
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[b]
‡Both authors contributed equally to this work
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Abstract: Two series of heterocycle-based mesogens consisting of a long 2,5-diphenyl-1,3,4-oxadiazole / thiadiazole rigid core with three lipophilic and flexible alkyl chains at one end and a polar glycerol group at the opposite end have been synthesized via one pot cyclization reaction. These compounds were investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray scattering, cyclic voltammetry, UV-vis spectroscopy and photoluminescence measurements. Upon elongation of the alkyl chains, a transition from hexagonal columnar phase to Pm3n -type cubic phase was observed in both series. Oxadiazole series are blue luminescent LCs with binding selectivity to Li+, while thiadiazole series are blue-green luminescent LCs with binding selectivity to Fe3+. Keywords: 1,3,4-oxadiazole; 1,3,4-thiadiazole; self-assembly; luminescent liquid crystals; chemosensor 1 Introduction Liquid crystals possessing order and dynamics are unique self-organized functional soft materials, and they have been widely applied as organic electronic devices and sensors etc [1]. Usually molecules which can display mesogenic properties are anisometric molecules which incorporate either a large anisometric units such as rod or disclike units, or amphiphilic molecules which contain a polar groups with strong intermolecular attractive forces, such as hydrogen bonding and ionic groups (amphiphilic molecules) [2]. Recently by combining anisotropic molecular structure
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with amphiphilic units, novel mesogens such as block molecules [3] and bolaamphiphiles [4] have been designed, they can display a huge number of novel nanostructures and have great potentials as electrooptical materials. Heterocycles play an important role in the design of new advanced functional materials [5]. The incorporation of heterocyclic moieties as core units in liquid crystalline structures can result in variation of phase structure, bent angle, polarity, geometry, luminescence and other physical and chemical properties due to the presence of the heteroatoms (S, O and N) [6]. Luminescent liquid crystals are rarely reported, because intramolecular interactions could usually lead to a fluorescence quenching due to the π–π stacking [7]. Therefore selection of proper π-conjugated cores, linkages and terminal functional groups is essential to design new luminescent liquid crystals [8]. 1,3,4-Oxadiazole and 1,3,4-thiadiazole have been investigated as cores in different mesogenic structures to obtain luminescent liquid crystals [9,10]. Such five membered rings bring some favorable properties for applications in electro-optical devices, such as significant dipole moments, high fluorescence quantum yields, low viscosity and high birefringence. 1,3,4-Oxadiazole and 1,3,4-thiadiazole based liquid crystals are also good candidates for application in electron-transporting materials, emitting layers in electroluminescent diodes and non-linear optical materials with high photoluminescence quantum yield and good thermal and chemical stabilities [6e,11]. Although a number of 1,3,4-oxadiazole and 1,3,4-thiadiazole based liquid crystals [6c] including star shape [12,13,14,15,16] and polycatenars [17,18,19,20,21,22,23] have been reported. The relationship among the molecular structures and their properties are not totally understood, moreover 1,3,4-oxadiazole and 1,3,4-thiadiazole based amphiphilic liquid crystals with a diol terminal group have never been reported. In our previous work [3e], 1,2,3-triazole-based amphiphilic liquid crystals with a diol terminal group have been investigated. They can show interesting self-assemble behavior with formation of hexagonal columnar phase, micellar cubic phase (CubI) and spheroidic cubic phase (CubSph), however they are nonluminescent LCs [ 24 ], additionally the investigation on the practical application of the self-assembly superstructures has not begun. Therefore, in order to further understand the relationships between the structure of the molecular tectons and the details of the self-assembly superstructures, and to explore the potential applications of these soft matters, herein the 1,2,3-triazole unit was replaced with 1,3,4-oxadiazole and 1,3,4-thiadiazole, the photophysical property and chemosensor property of these compounds were investigated. Metal ion recognition is significant in supramolecular chemistry and has great potentials in environmental, chemical and biological fields. Considerable efforts have been made to the development of chemosensors for the selective detection of metal ions. Due to their excellent optical properties and the availability of additional ligating sites with metal ions, 1,3,4-oxadiazoles and 1,3,4-thiadiazoles have been employed as the molecular framework for constructing multiple fluorescent sensors. Till now 1,3,4-oxadiazole or 1,3,4-thiadiazole based chemical sensors containing simultaneously ionophores, such as amides [25], crown ether [26], schiff base [27], pyridyl [ 28 ], azobenzene [ 29 ], phenol [ 30 ], tris(2-aminoethyl)amine [ 31 ], (2-hydroxyethyl)piperazine [32], calix[4]arene [33] and 1,2,3-triazole [34], etc have been reported. Fe3+ as a physiologically important metal ion, plays a catalytic role in chemical and biological processes such as oxygen metabolism and electron transfer, and both its deficiency and excess in the human body can induce a variety of diseases [35,36]. On the other side, lithium-containing
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drug preparations are routinely applied for the treatment of manic-depressive psychosis. The concentration of lithium ions in blood serum after drug intake varies strongly from person to person and has to be monitored in the individual patient regularly. Thus, it is imperative to develop analytic and detective methods for sensitive sensing of these two ions, because of their potential applications in clinical biochemistry and in environmental research. Therefore herein we report the design, synthesis as well as thermotropic, photophysical and chemosensor behaviors of two novel series of heterocycle-based molecules consisting of a long 2,5-diphenyl-1,3,4-oxadiazole or 2,5-diphenyl-1,3,4-thiadiazole rigid core, with three lipophilic and flexible alkyl chains at one end and a polar glycerol group at the opposite end. The aim of this work was threefold: to design new 1,3,4-oxadiazole- and 1,3,4-thiadiazole-based amphiphiles, secondly to investigate their mesophase behavior, understanding the relationship between structure and mesomorphic properties, thirdly to investigate their photophysical properties.
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2 Results and discussion
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2.1 Synthesis
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The previously reported synthetic protocols for the 1,3,4-oxadiazole or 1,3,4-thiadiazole rings are multi-step in nature, and generally involve the cyclization of acid hydrazides with a variety of reagents, such as thionyl chloride, phosphorus oxychloride, sulfuric acid or phosphorus pentasulfide, usually under harsh reaction conditions [37,38]. Although the cyclization of acid hydrazides with Lawesson’s reagent can be carried under mild and efficient reaction condition, the overall yield of cyclization is about 31-55% [12b,17a,17c, 39 ]. Herein the 2,5-bis(4-methoxyphenyl)-1,3,4-oxadiazole or 2,5-bis(4-methoxyphenyl)-1,3,4-thiadiazole is efficiently obtained by one pot cyclization reaction of 4-methoxybenzoyl chloride 2 with 4-methoxybenzoyhydrazide 4 and phosphorus pentasulfide or phosphorus oxychloride, the yield of these cyclization is much higher than the Lawesson’s methods. Demethylation of 5 with BBr3 yielded the phenol 7, which was monobenzylated with alkoxysubstituted benzyl chlorides 6/n [40] to yield compounds 8. 8 was etherified with allybromide, followed by dihydroxylation of the double bonds with OsO4/N-methylmorpholine-N-oxide (NMMNO) [41 ] to yield the target compounds ON/n and TN/n. Detailed synthetic procedures and analytical data are given in the Supplementary data.
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Scheme 1. Synthesis of compounds ON/n and TN/n: Reagents and conditions: i) SOCl2, THF; ii) NH2NH2·H2O,
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2-methoxyethanol, 100 °C, 4 h, 91%; iii) pyridine, POCl3/ or P2S5, 100 °C, 12 h, 82-85%; iv) BBr3, CH2Cl2, 0 °C-RT, 8 h; v) K2CO3, DMF, 90 °C, 12 h, 70-75%; vi) allybromide, NaH, THF, 50 °C, 5 h, 87-92%; vii) OsO4,
2.2 Mesomorphic properties
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NMMNO, H2O, acetone, 45 °C, 10 h, 80-86%.
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The liquid crystalline self-assembly of compounds ON/n and TN/n was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The transitions and accompanying enthalpy changes of the synthesized heterocycle-based compounds are collected in Table 1. All compounds are enantiotropic LCs. In both series, compounds with short alkyl chains (ON/10 and TN/12) exhibit columnar phases, while the rest compounds all exhibit cubic phases. In most cases, the 1,3,4-thiadiazole series show wider mesophase temperature ranges, lower melting temperatures and higher clearing temperatures than the corresponding 1,3,4-oxadiazole analogues. Similar phenomenon has been also observed in 1,3,4-oxadiazole- / 1,3,4-thiadiazole-based star-shaped [12b], polycatenar [17a,c,22a,c] and dimer [39] LCs. This may be due to the stronger bend angle of 1,3,4-oxadiazole (ca. 134°), which disturbs the linear shape of the whole molecule, leading to the mesophases destabilization [42]. Table 1 Phase transitions, lattice parameters, and other data compound N/14 [3e].
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of compounds ON/n, TN/n and the reported
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Phase transition T/°C (ΔH/kJ·mol-1)
a/nm (T/oC)
μ
θ (o )
ON/10
Cr1 105[6.2] Cr2 112[11.5] Colhex/p6mm 153[0.7] Iso
5.70 (100)
8.83
40.8
ON/12
Cr 94[18.4] CubI/ Pm3n 127[0.3] Iso
11.83 (120)
129.2
2.8
ON/14
Cr 100[20.0] CubI/ Pm3n 139[0.2] Iso
11.76 (140)
114.9
3.1
ON/16
Cr 102[22.6] CubI/ Pm3n 159[0.2] Iso
12.48 (120)
125.4
2.9
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TN/12
Cr 98[49.5] Colhex/p6mm 166[0.5] Iso
6.14 (110)
9.05
39,8
TN/14
Cr1 45[2.0] Cr2 58[7.4] CubI/ Pm3n 155[0.3] Iso
13.13 (130)
157.9
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TN/16
Cr 87[16.1] CubI/ Pm3n 171[0.3] Iso
13.27 (140)
149.0
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N/14
Cr 67[23.2] CubI/ Pm3n 181[0.2] Iso
12.19 (150)
127.2
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Transition temperatures were determined by DSC (5 K
min -1)
and confirmed by POM, peak temperatures from
the second heating scan are given; values in brackets are transition on cooling; abbreviations: Cr = crystalline solid, Iso = isotropic liquid; Colhex/p6mm = hexagonal columnar phases; CubI/ Pm3n = micellar cubic mesophase with space group Pm3n ; For Colhex/p6mm phase: (a
2
2
) 3h( N A M ) , NA = Avogadro constant, M = molecular
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mass assuming a density of ρ = 1 g·cm-3; for CubI/ Pm3n phase: μ = ncell/8; θ = projection of the cone angle,
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2.2.1 Hexagonal columnar phases
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The columnar phases of compounds ON/10 and TN/12 were investigated by polarizing microscopy between crossed polarisers, birefringent spherulitic textures as for typical for columnar phases are observed (Fig. 1a and Fig. S1a), these columnar phases are optically negative as observed with an additional λ-retarder plate (Fig. 1b and Fig. S1b). This means that in the columns the preferred direction of the intramolecular π-conjugation pathway (i.e. the long axis of the rigid cores) is on average perpendicular to the column long axis. XRD patterns of the columnar phases of ON/10 and TN/12 show three small angle reflections with their reciprocal spacing in the ratio of 1 : 31/2 : 2, which can be indexed to the (10), (11), (20) indicating a hexagonal columnar arrangement with p6mm symmetry (see Fig. 1c, Fig. S5a, Table S1 and Table S5). The number of molecules organized in a slice of the columns with a height of h = 0.45 nm (maximum of the diffuse wide angle scattering) is about 9. Considering the strong intramolecular H-bonding interaction between the terminal glycerols, it is reasonable to assume that the polar glycerols self-assemble into the column centers, which are surrounded by the 2,5-diphenyl-1,3,4-oxadiazole or 2,5-diphenyl-1,3,4-thiadiazole rigid cores. These cylinders then self-assemble into a hexagonal lattice with the space of adjacent columns filled with the alkyl chains (Fig. 1e). This model is corresponding to the reconstructed electron density map of the Colhex/p6mm of TN/12 and ON/10 (Fig. 1d and Fig. S5b, for alternative phase combinations, see Fig. S5c and S8 in the Supporting Information). The glycerol groups assemble exclusively in the centers of the columns which correspond to the purple/blue high electron density regions in the electron-density map. The aromatic cores form distinct shells (green circles) separating the polar cores from the lipophilic outside continuum. The alkyl chains are completely disordered and interdigitated in the continuum of the low electron density (red region) around these columns.
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Fig. 1. Investigation of compound TN/12 a) texture of Colhex/p6mm phase at T = 135 °C cooling from isotropic state; b) the same domain with λ-retarder plate; c) XRD pattern (small-angle region, inset shows small-angle region) at T = 100 oC; d) reconstructed electron-density map of the Colhex/p6mm phase of compound TN/12; e)
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2.2.2 Micellar cubic phases
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model of Colhex/p6mm phase of compound TN/12.
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Compounds with long alkyl chains ON/n (n = 12, 14, 16) and TN/n (n = 14, 16) (see Table 1) all exhibit cubic phases. The cubic phases are optically isotropic (nonbirefringent) with high viscous. By increasing the temperature, a significant decrease in viscosity was observed at defined temperature (Fig. S1c-e and Fig. S2). It is clearly indicated from the XRD measurements of compounds ON/n (n = 12, 14, 16) and TN/n (n = 14, 16) that their distinct XRD patterns are for cubic phase with space group of Pm3n (Table S2-4 and S6-7 and Fig. 2a and Fig. S6-7 and S9-10). The calculated lattice parameters are acub = 11.83, 11.76, 12.48, 12.13, 12.27 nm for ON/n (n = 12, 14, 16) and TN/n (n = 14, 16) respectively. The Pm3 n lattice is the most commonly observed lattice for thermotropic micellar cubic phases (CubI) (Fig. 2b) formed by eight spheroidic aggregates with soft corona [44,45]. It was calculated [46] that for compounds ON/n (n = 12, 14, 16) and TN/n (n = 14, 16), each of the eight micelles is on average formed by approximately 129, 115, 125, 158, 149 molecules respectively (Table 1 and Table S8). The large distance between the alkyl substituted end and the polar glycerol group gives rise to a rather small projection cone angle (θ = 2.3º – 3.1º, see Table 1), which results in per aggregate with large number molecules (μ > 114). It is remarkable that such molecules with such a long aromatic rigid core, relatively small cone angles are still capable of forming columnar and micellar cubic phases. The formation of large
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dynamic hydrogen bonding networks between the diol groups, π-π stacking of conjugated rigid rod-like core as well as the microsegregation of the incompatible hydrophilic and lipophilic parts of the individual molecules into separate regions are the driving forces for the self-assembly of these molecules. The polar-apolar segregation along the core can favor parallel side-by-side packing of the cores. The bulky terminal alkyl chains are forced to organize side by side. Chain flexibility can be enhanced by the flexibility of the benzylether linkage between the 3,4,5-trialkoxybenzene units and the core units. The hydrogen bonding provided by the terminal diol groups leads to the dense packing of the cores. All this leads to a strong interfacial curvature, giving rise to the formation of hexagonal columnar and Pm3n cubic phases.
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Fig. 2. a) X-ray diffraction pattern of an aligned sample of compound ON/14 at T = 140 oC; b) possible model for CubI/ Pm3 n .
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It should be noted that 1,3,4-oxadiazole-based compound ON/12 display cubic phase while its corresponding 1,3,4-thiadiazole-based analogue TN/12 forms only hexagonal columnar phase, this is due to the larger bend angle, more linearity, less interface curvature of the thiadiazole molecules as compared with those of oxadiazoles (Fig. 3a). The influence of different heterocycles on the formation of cubic phases is done by comparison of the mesomorphic behavior among ON/14, TN/14 and the reported 1,2,3-triazole-based analogue N/14 [3e]. Their chemical structures differ only in the type of the central heterocycles. As shown in Fig. 3a, the dipole moment of the heterocycle unit is in the order triazole > thiadiazole > oxadiazole, which is consistent with mesophase stability and mesophase range. It is evident that the larger dipole moment of the heterocylce leads to the lower melting temperature, the higher clearing temperature and therefore the wider temperature range of mesophase (Fig. 3b and 3c). The reason is that dipole-dipole interactions can enhance core-core interactions, leading to the increasing stability of the mesophase in the case of compound with larger dipole moment. The bent angle of the heterocycle unit is in the order thiadiazole > triazole > oxadiazole as shown in Fig. 3a, which is in line with the number of molecules organized in per spherical aggregate for the cubic phases observed in compounds TN/14, N/14 and ON/14 (Table 1, Fig. 3d). Larger bent angle of molecule leads to smaller projection cone angle of molecule, and therefore more molecules are needed to fill per spherical aggregate (Fig. 3c). The large dipole moment and bent angle of the heterocycles could contribute to the formation of large size of micellar cubic phases for all these heterocycle-based compounds.
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Fig. 3. a) Values of bent angles in the rigid core induced by the heterocylces and dipole moments of the different heterocycles; b) the thermal behaviour of researched compounds ON/n, TN/n and the reported compound N/14; c) the effect of dipole moment on melting point, clearing point and range of mesophase of compounds ON/14, TN/14 and N/14; d) the effect of the bent angle on the number of molecules in per spheroidic aggregate for compounds
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2.3 Photophysical properties
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ON/14, TN/14 and N/14.
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The UV-vis absorption and photoluminescence emission spectroscopic data of ON/12 and TN/12 (c = 5×10-6 mol·L-1) in dichloromethane (CH2Cl2) solutions and in the LC states are shown in Fig. 4, and the corresponding photophysical properties are summarized in Table 2. The absorption spectra of ON/12 in CH2Cl2 solution showed a maxima at 310 nm (Fig. 4a), while the absorption maxima of compound TN/12 in CH2Cl2 solution exhibited a red shift with a maxima at 340 nm (Fig. 4a) The absorption band in CH2Cl2 solution may be attributed to the π-π* transition of the conjugated backbone. Compound ON/12 showed a maximum at 300 nm in LC state, which is blue shifted by about 10 nm (Fig. 4b), while the absorption spectra for compound TN/12 in LC state showed a maximum at 327 nm in LC state, which is blue shifted by about 13 nm (Fig. 4b). The blue shifts of both compounds ON/12 and TN/12 suggest the formation of π-stacked aggregates with a H-type parallel stacking mode in LC states [47]. The emission spectra of compound ON/12 exhibited a maxima at 371 nm in CH2Cl2 solution, with a maxima at 375 nm in the LC state, hence the Stokes shifts have large values of 61 nm and 75 nm in CH2Cl2 solution and in LC state respectively (Fig. 4). Compound TN/12 showed emission with a maxima at 417 nm in the CH2Cl2 solution, and a maxima at 425 nm in LC state, hence the Stokes shifts have large values of 77 nm and 98 nm in CH2Cl2 solution and in LC state respectively (Fig. 4). The large Stokes shifts of compounds ON/12 and TN/12 confirm the strong intramolecular charge transfer [48]. On irradiation with UV light of wavelength 365 nm, blue light
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Fig. 4. a) Normalized absorption (solid trace) and emission spectra (dotted trace) in CH2Cl2 solution obtained for ON/12 (blue trace) and TN/12 (green trace). Pictures of micromolar solutions of compounds ON/12 and TN/12 in
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CH2Cl2, as seen with the illumination of 365 nm UV light (inset); b) Normalized absorption (solid line) and emission spectra (dotted line) in LC state obtained for ON/12 at 120 oC (blue trace) and TN/12 at 120 oC (green trace).
state
Solutiona
TN/12
Solutiona
ON/12
LC state
TN/12
LC state
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Absorption (nm)
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Table 2 UV-vis absorption and fluorescence spectroscopy data of ON/12 and TN/12.
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Stokes shift nm
ФFLd
(cm-1)c
310
371
61(5303)
0.32
340
417
77(5431)
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300
375
75(6666)
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425
98(7052)
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2.4 Electrochemical properties The energy band gap of compounds ON/12 and TN/12, which was estimated from the onset of the absorption in thin film, is about 2.65 eV for compound ON/12 and 2.78 eV for compound TN/12 (Fig. S11) [52,53]. The EHOMO and ELUMO energy levels of representative compounds ON/12 and TN/12 in thin film were determined by cyclic voltammetry (CV) on a glassy carbon electrode (Fig. S12 and Table S9), from the onset of oxidative or reductive potentials and the band gap is calculated to be 2.52 eV for compound ON/12 and 2.75 eV for compound TN/12, which are in
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line with those obtained from their absorption spectra in thin film. In order to investigate the conformation and electron distributions of ON/12 and TN/12, calculations based on density functional theory (DFT) with the Gaussian 03W program package at B3LYP/(6-31G) level were performed with the model compounds ON/OCH3 and TN/OCH3, having methoxy groups instead of the alkoxy chains. The electron distributions of the HOMO and LUMO of ON/OCH3 and TN/OCH3 are shown in Fig. S13. As it can be seen in the HOMO orbital, the electrons are localized on 1,4-diphenyl-1,3,4-oxadiazole or 1,4-diphenyl-1,3,4-thiadiazole of the central-conjugated core, in the LUMO orbital, the electrons are mainly localized on the 1,3,4-oxadiazole or 1,3,4-thiadiazole unit, indicating that intramolecular charge transfer (ICT) exists in the ON/OCH3 and TN/OCH3 molecules. The photophysical and eletrochemical properties indiacate that these compounds have potential application in organic semiconductor materials.
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2.5 Chemosensor behavior
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In order to get insights into the binding properties of compounds ON/n and TN/n toward metal ions, the selectivities of compounds ON/12 and TN/12 for metal ions were examined by monitoring the change in fluorescence intensity upon the addition of the perchlorate salt of a wide range of cations including Ag+, Cd2+, Fe3+, Hg2+, Li+, Cu2+, K+, Pb2+, Zn2+, Mg2+, Ni2+ and Co2+. The fluorescence profiles of ON/12 and TN/12 in the presence of 10 equiv. of selected cations in CH3CN : CH2Cl2 = 2 : 1 solution were investigated. Upon the addition of Li+, the fluorescence of ON/12 was largely quenched. On the contrary, the presence of other tested cations induces slight fluorescence quenching (Fig. 5a and 5b). The fluorescence intensity of TN/12 changed little in the presence of most metal ions examined, except Fe3+ as shown in Fig. 6a and 6b. The selective response of ON/12 to Li+ and TN/12 to Fe3+ can be observed by naked eye under UV light (Fig. 5e and 6e). To evaluate the binding natures for ON/12 and TN/12, the fluorescence titration curves of ON/12 and TN/12 were performed in CH2Cl2 : CH3CN = 2 : 1 solution. With the addition of an increasing amount of Li+ (from 1 to 14 equiv), the emission intensity of ON/12 at 360 nm gradually decreased and finally reached its fluorescence quenching plateau at 12 equiv (Fig. 5c and 5d). The fluorescence intensity emerged at 405 nm gradually were quenched with the increasing concentration of Fe3+ from 1 equiv to 10 equiv. When the concentration of Fe3+ ran up to about 8.0 equiv, the quenching of the fluorescence intensity of TN/12 reached a plateau and almost contained a constant (Fig. 6c and 6d). The fluorescence quenching may be due to the chelation enhanced quenching (CEQ) sensing mechanism [54].
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Fig. 5. Fluorescence spectra of compound ON/12 (10-6 M) at room temperature; all anions are ClO4- at pH = 7.0. (a and b) CH2Cl2 : CH3CN = 2 : 1 (λex = 310 nm); (F0 - F)/F0 × 100 depicts the cation selective fluorescence quenching efficiency of compound ON/12, namely the fluorescence responses of different metal ions; abbreviation:
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F0 = the fluorescence emission maximum of a blank sample; F = the fluorescence emission maximum of samples with addition of different metal ions; c) Fluorescence spectra of compound ON/12 (10-6 M), upon addition of an increasing concentration of Li+ ions (0-14 equiv) measured in CH2Cl2 : CH3CN = 2 : 1 (λex = 310 nm) at room temperature. d) Titration curve of the integrated fluorescence as a function of Li + concentration; e) The samples
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Fig. 6. Fluorescence spectra of compound TN/12 (10-6 M) at room temperature; all anions are ClO4- at pH = 7.0. (a and b) CH2Cl2 : CH3CN = 2 : 1 (λex = 330 nm); (F0 - F)/F0 × 100 depicts the cation selective fluorescence quenching efficiency of compound TN/12, namely the fluorescence responses of different metal ions; abbreviation:
ACCEPTED MANUSCRIPT F0 = the fluorescence emission maximum of a blank sample; F = the fluorescence emission maximum of samples with addition of different metal ions; c) Fluorescence spectra of compound TN/12 (10-6 M), upon addition of an increasing concentration of Fe3+ ions (0-10 equiv) measured in CH2Cl2 : CH3CN = 2 : 1 (λex = 330 nm) at room temperature. d) Titration curve of the integrated fluorescence as a function of Fe3+ concentration; e) The samples with addition of different metal ions, under irradiation with 365 nm light.
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For better understanding the coordination of ON/12 with Li+ and TN/12 with Fe3+, the fluorescence Job’s plot analysis [55] was conducted (Fig. S14). The results showed that the maximum fluorescence quenching was observed when the mole fraction of Li+ ion was close to 0.48 (Fig. S14a), which confirmed that the chemosensor ON/12 and Li+ ion formed a 1 : 1 or 2 : 2 complex [29]. On the other side, the maximum fluorescence quenching was observed when the mole fraction of Fe3+ ion was close to 0.33 (Fig. S14b), which confirmed that the chemosensor TN/12 and Fe3+ ion formed a 2 : 1 complex. In order to investigate the binding site of ON/12 with Li+ and TN/12 with Fe3+, FTIR spectroscopies of solid ON/12, ON/12-Li+, TN/12 and TN/12-Fe3+ were performed (Fig. S15 and S16). The spectrum in Fig. S15 indicated that the O-H and C-O-C stretching vibrations of ON/12 are located at 3414 and 1129 cm-1 respectively [56]. After adding Li+, the peaks of O-H vibration and C-O-C vibration shift to lower wavenumbers of 3390 cm-1 and 1118 cm-1, respectively. Therefore, the binding site of ON/12 to Li+ is much likely to coordinate to the O atom of -OH and O atom of 1,3,4-oxadiazole moiety (Fig S17a). On the other side, comparing the FTIR spectrum before and after adding Fe3+, the peak of N-N vibration (996 cm-1) disappeared and the peak of C-S-C vibrational mode (632 cm-1) [57] shifts to lower wavenumber of 621 cm-1 after adding Fe3+, indicating that Fe3+ ion is binding with 1,3,4-thiadiazole, while the peak of O-H vibration (3387 cm-1) with no change from addition of Fe3+, indicating that Fe3+ is not binding with O-H groups. Therefore, the binding site of TN/12 to Fe3+ is much likely to coordinate to the S and N atoms of the 1,3,4-thiadiazole unit (Fig. S17b). 3 Conclusion
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Therefore heterocycle 1,3,4-oxadiazole or 1,3,4-thiadiazole has great impact on the molecular packing, mesophase stability and photophysical properties etc. Both 1,3,4-oxadiazole- and 1,3,4-thiadiazole-based derivatives reported here display enantiotropic mesophases. 1,3,4-Thiadiazole derivatives exhibit wider range of enantiotropic mesophase than their 1,3,4-oxadiazole analogues. With the elongation of the alkyl chains, a phase transition from Colhex/p6mm to CubI/ Pm3n was observed in 1,3,4-oxadiazole- and 1,3,4-thiadiazole-based compounds. Such compounds are luminescent LCs and can be used as fluorescence chemosensors for selective response to Li+ (ON/n) and Fe3+ (TN/n) in CH2Cl2/CH3CN solution. Our study should provide some fundamental knowledge in designing novel functional luminescent LCs. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 21664015, 21364017, 21274119 and 21602195) and the Yunnan Provincial Department of Education Foundation (No. ZD2015001, 2016FD008). We thank beamline 1W2A at Beijing Accelerator
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Laboratory and beamline BL16B1 at Shanghai Synchrotron Radiation Facility (SSRF), China. The calculations were performed with the support of the Yunnan University Supercomputer Center.
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights 1 Oxadiazol and thiadiazole based LCs with Colhex/p6mm and CubI/ Pm3n phases were synthesized via one pot cyclization. 2. Influences of heterocycles on the liquid crystalline and photophysical properties etc are discussed. 3. These luminescent LCs can act as chemosensors to recognize metal ions [1] (a) S. Sergeyev, W. Pisula, Y.H. Geerts, Chem. Soc. Rev. 36 (2007) 1902–1929;
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Yulong Xiao, Hongfei Gao, Tingyan Wang, Ruilin Zhang, Xiaohong Cheng PII: DOI: Reference:
S0167-7322(17)33156-2 doi: 10.1016/j.molliq.2017.08.110 MOLLIQ 7818
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
15 July 2017 27 August 2017 29 August 2017
Please cite this article as: Yulong Xiao, Hongfei Gao, Tingyan Wang, Ruilin Zhang, Xiaohong Cheng , Synthesis, liquid-crystalline, photophysical and chemosensor properties of oxadiazole/thiadiazole-based amphiphiles with glycerol groups, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.08.110
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Synthesis, liquid-crystalline, photophysical and chemosensor properties of oxadiazole/thiadiazole-based amphiphiles with glycerol groups
[a]
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Yulong Xiao, ‡ [a] Hongfei Gao, ‡ [a] Tingyan Wang, [b,a] Ruilin Zhang, [c,a] Xiaohong Cheng*[a]
Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry Department,
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Yunnan University Kunming,
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Yunnan 650091, P. R. China
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Fax: (+86) 871 65032905 E-mail: [email protected]
College of Science, Beijing University of Chemical Technology, Beijing 100029, P.R.China
[c]
Forensic Medicine of Kunming Medical University, Kunming 650500, P. R. China
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[b]
‡Both authors contributed equally to this work
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Abstract: Two series of heterocycle-based mesogens consisting of a long 2,5-diphenyl-1,3,4-oxadiazole / thiadiazole rigid core with three lipophilic and flexible alkyl chains at one end and a polar glycerol group at the opposite end have been synthesized via one pot cyclization reaction. These compounds were investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray scattering, cyclic voltammetry, UV-vis spectroscopy and photoluminescence measurements. Upon elongation of the alkyl chains, a transition from hexagonal columnar phase to Pm3n -type cubic phase was observed in both series. Oxadiazole series are blue luminescent LCs with binding selectivity to Li+, while thiadiazole series are blue-green luminescent LCs with binding selectivity to Fe3+. Keywords: 1,3,4-oxadiazole; 1,3,4-thiadiazole; self-assembly; luminescent liquid crystals; chemosensor 1 Introduction Liquid crystals possessing order and dynamics are unique self-organized functional soft materials, and they have been widely applied as organic electronic devices and sensors etc [1]. Usually molecules which can display mesogenic properties are anisometric molecules which incorporate either a large anisometric units such as rod or disclike units, or amphiphilic molecules which contain a polar groups with strong intermolecular attractive forces, such as hydrogen bonding and ionic groups (amphiphilic molecules) [2]. Recently by combining anisotropic molecular structure
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with amphiphilic units, novel mesogens such as block molecules [3] and bolaamphiphiles [4] have been designed, they can display a huge number of novel nanostructures and have great potentials as electrooptical materials. Heterocycles play an important role in the design of new advanced functional materials [5]. The incorporation of heterocyclic moieties as core units in liquid crystalline structures can result in variation of phase structure, bent angle, polarity, geometry, luminescence and other physical and chemical properties due to the presence of the heteroatoms (S, O and N) [6]. Luminescent liquid crystals are rarely reported, because intramolecular interactions could usually lead to a fluorescence quenching due to the π–π stacking [7]. Therefore selection of proper π-conjugated cores, linkages and terminal functional groups is essential to design new luminescent liquid crystals [8]. 1,3,4-Oxadiazole and 1,3,4-thiadiazole have been investigated as cores in different mesogenic structures to obtain luminescent liquid crystals [9,10]. Such five membered rings bring some favorable properties for applications in electro-optical devices, such as significant dipole moments, high fluorescence quantum yields, low viscosity and high birefringence. 1,3,4-Oxadiazole and 1,3,4-thiadiazole based liquid crystals are also good candidates for application in electron-transporting materials, emitting layers in electroluminescent diodes and non-linear optical materials with high photoluminescence quantum yield and good thermal and chemical stabilities [6e,11]. Although a number of 1,3,4-oxadiazole and 1,3,4-thiadiazole based liquid crystals [6c] including star shape [12,13,14,15,16] and polycatenars [17,18,19,20,21,22,23] have been reported. The relationship among the molecular structures and their properties are not totally understood, moreover 1,3,4-oxadiazole and 1,3,4-thiadiazole based amphiphilic liquid crystals with a diol terminal group have never been reported. In our previous work [3e], 1,2,3-triazole-based amphiphilic liquid crystals with a diol terminal group have been investigated. They can show interesting self-assemble behavior with formation of hexagonal columnar phase, micellar cubic phase (CubI) and spheroidic cubic phase (CubSph), however they are nonluminescent LCs [ 24 ], additionally the investigation on the practical application of the self-assembly superstructures has not begun. Therefore, in order to further understand the relationships between the structure of the molecular tectons and the details of the self-assembly superstructures, and to explore the potential applications of these soft matters, herein the 1,2,3-triazole unit was replaced with 1,3,4-oxadiazole and 1,3,4-thiadiazole, the photophysical property and chemosensor property of these compounds were investigated. Metal ion recognition is significant in supramolecular chemistry and has great potentials in environmental, chemical and biological fields. Considerable efforts have been made to the development of chemosensors for the selective detection of metal ions. Due to their excellent optical properties and the availability of additional ligating sites with metal ions, 1,3,4-oxadiazoles and 1,3,4-thiadiazoles have been employed as the molecular framework for constructing multiple fluorescent sensors. Till now 1,3,4-oxadiazole or 1,3,4-thiadiazole based chemical sensors containing simultaneously ionophores, such as amides [25], crown ether [26], schiff base [27], pyridyl [ 28 ], azobenzene [ 29 ], phenol [ 30 ], tris(2-aminoethyl)amine [ 31 ], (2-hydroxyethyl)piperazine [32], calix[4]arene [33] and 1,2,3-triazole [34], etc have been reported. Fe3+ as a physiologically important metal ion, plays a catalytic role in chemical and biological processes such as oxygen metabolism and electron transfer, and both its deficiency and excess in the human body can induce a variety of diseases [35,36]. On the other side, lithium-containing
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drug preparations are routinely applied for the treatment of manic-depressive psychosis. The concentration of lithium ions in blood serum after drug intake varies strongly from person to person and has to be monitored in the individual patient regularly. Thus, it is imperative to develop analytic and detective methods for sensitive sensing of these two ions, because of their potential applications in clinical biochemistry and in environmental research. Therefore herein we report the design, synthesis as well as thermotropic, photophysical and chemosensor behaviors of two novel series of heterocycle-based molecules consisting of a long 2,5-diphenyl-1,3,4-oxadiazole or 2,5-diphenyl-1,3,4-thiadiazole rigid core, with three lipophilic and flexible alkyl chains at one end and a polar glycerol group at the opposite end. The aim of this work was threefold: to design new 1,3,4-oxadiazole- and 1,3,4-thiadiazole-based amphiphiles, secondly to investigate their mesophase behavior, understanding the relationship between structure and mesomorphic properties, thirdly to investigate their photophysical properties.
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2 Results and discussion
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2.1 Synthesis
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The previously reported synthetic protocols for the 1,3,4-oxadiazole or 1,3,4-thiadiazole rings are multi-step in nature, and generally involve the cyclization of acid hydrazides with a variety of reagents, such as thionyl chloride, phosphorus oxychloride, sulfuric acid or phosphorus pentasulfide, usually under harsh reaction conditions [37,38]. Although the cyclization of acid hydrazides with Lawesson’s reagent can be carried under mild and efficient reaction condition, the overall yield of cyclization is about 31-55% [12b,17a,17c, 39 ]. Herein the 2,5-bis(4-methoxyphenyl)-1,3,4-oxadiazole or 2,5-bis(4-methoxyphenyl)-1,3,4-thiadiazole is efficiently obtained by one pot cyclization reaction of 4-methoxybenzoyl chloride 2 with 4-methoxybenzoyhydrazide 4 and phosphorus pentasulfide or phosphorus oxychloride, the yield of these cyclization is much higher than the Lawesson’s methods. Demethylation of 5 with BBr3 yielded the phenol 7, which was monobenzylated with alkoxysubstituted benzyl chlorides 6/n [40] to yield compounds 8. 8 was etherified with allybromide, followed by dihydroxylation of the double bonds with OsO4/N-methylmorpholine-N-oxide (NMMNO) [41 ] to yield the target compounds ON/n and TN/n. Detailed synthetic procedures and analytical data are given in the Supplementary data.
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Scheme 1. Synthesis of compounds ON/n and TN/n: Reagents and conditions: i) SOCl2, THF; ii) NH2NH2·H2O,
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2-methoxyethanol, 100 °C, 4 h, 91%; iii) pyridine, POCl3/ or P2S5, 100 °C, 12 h, 82-85%; iv) BBr3, CH2Cl2, 0 °C-RT, 8 h; v) K2CO3, DMF, 90 °C, 12 h, 70-75%; vi) allybromide, NaH, THF, 50 °C, 5 h, 87-92%; vii) OsO4,
2.2 Mesomorphic properties
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NMMNO, H2O, acetone, 45 °C, 10 h, 80-86%.
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The liquid crystalline self-assembly of compounds ON/n and TN/n was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The transitions and accompanying enthalpy changes of the synthesized heterocycle-based compounds are collected in Table 1. All compounds are enantiotropic LCs. In both series, compounds with short alkyl chains (ON/10 and TN/12) exhibit columnar phases, while the rest compounds all exhibit cubic phases. In most cases, the 1,3,4-thiadiazole series show wider mesophase temperature ranges, lower melting temperatures and higher clearing temperatures than the corresponding 1,3,4-oxadiazole analogues. Similar phenomenon has been also observed in 1,3,4-oxadiazole- / 1,3,4-thiadiazole-based star-shaped [12b], polycatenar [17a,c,22a,c] and dimer [39] LCs. This may be due to the stronger bend angle of 1,3,4-oxadiazole (ca. 134°), which disturbs the linear shape of the whole molecule, leading to the mesophases destabilization [42]. Table 1 Phase transitions, lattice parameters, and other data compound N/14 [3e].
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of compounds ON/n, TN/n and the reported
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Phase transition T/°C (ΔH/kJ·mol-1)
a/nm (T/oC)
μ
θ (o )
ON/10
Cr1 105[6.2] Cr2 112[11.5] Colhex/p6mm 153[0.7] Iso
5.70 (100)
8.83
40.8
ON/12
Cr 94[18.4] CubI/ Pm3n 127[0.3] Iso
11.83 (120)
129.2
2.8
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Cr 100[20.0] CubI/ Pm3n 139[0.2] Iso
11.76 (140)
114.9
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ON/16
Cr 102[22.6] CubI/ Pm3n 159[0.2] Iso
12.48 (120)
125.4
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Cr 98[49.5] Colhex/p6mm 166[0.5] Iso
6.14 (110)
9.05
39,8
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Cr1 45[2.0] Cr2 58[7.4] CubI/ Pm3n 155[0.3] Iso
13.13 (130)
157.9
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Cr 87[16.1] CubI/ Pm3n 171[0.3] Iso
13.27 (140)
149.0
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Cr 67[23.2] CubI/ Pm3n 181[0.2] Iso
12.19 (150)
127.2
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Transition temperatures were determined by DSC (5 K
min -1)
and confirmed by POM, peak temperatures from
the second heating scan are given; values in brackets are transition on cooling; abbreviations: Cr = crystalline solid, Iso = isotropic liquid; Colhex/p6mm = hexagonal columnar phases; CubI/ Pm3n = micellar cubic mesophase with space group Pm3n ; For Colhex/p6mm phase: (a
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2
) 3h( N A M ) , NA = Avogadro constant, M = molecular
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mass assuming a density of ρ = 1 g·cm-3; for CubI/ Pm3n phase: μ = ncell/8; θ = projection of the cone angle,
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2.2.1 Hexagonal columnar phases
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The columnar phases of compounds ON/10 and TN/12 were investigated by polarizing microscopy between crossed polarisers, birefringent spherulitic textures as for typical for columnar phases are observed (Fig. 1a and Fig. S1a), these columnar phases are optically negative as observed with an additional λ-retarder plate (Fig. 1b and Fig. S1b). This means that in the columns the preferred direction of the intramolecular π-conjugation pathway (i.e. the long axis of the rigid cores) is on average perpendicular to the column long axis. XRD patterns of the columnar phases of ON/10 and TN/12 show three small angle reflections with their reciprocal spacing in the ratio of 1 : 31/2 : 2, which can be indexed to the (10), (11), (20) indicating a hexagonal columnar arrangement with p6mm symmetry (see Fig. 1c, Fig. S5a, Table S1 and Table S5). The number of molecules organized in a slice of the columns with a height of h = 0.45 nm (maximum of the diffuse wide angle scattering) is about 9. Considering the strong intramolecular H-bonding interaction between the terminal glycerols, it is reasonable to assume that the polar glycerols self-assemble into the column centers, which are surrounded by the 2,5-diphenyl-1,3,4-oxadiazole or 2,5-diphenyl-1,3,4-thiadiazole rigid cores. These cylinders then self-assemble into a hexagonal lattice with the space of adjacent columns filled with the alkyl chains (Fig. 1e). This model is corresponding to the reconstructed electron density map of the Colhex/p6mm of TN/12 and ON/10 (Fig. 1d and Fig. S5b, for alternative phase combinations, see Fig. S5c and S8 in the Supporting Information). The glycerol groups assemble exclusively in the centers of the columns which correspond to the purple/blue high electron density regions in the electron-density map. The aromatic cores form distinct shells (green circles) separating the polar cores from the lipophilic outside continuum. The alkyl chains are completely disordered and interdigitated in the continuum of the low electron density (red region) around these columns.
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Fig. 1. Investigation of compound TN/12 a) texture of Colhex/p6mm phase at T = 135 °C cooling from isotropic state; b) the same domain with λ-retarder plate; c) XRD pattern (small-angle region, inset shows small-angle region) at T = 100 oC; d) reconstructed electron-density map of the Colhex/p6mm phase of compound TN/12; e)
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model of Colhex/p6mm phase of compound TN/12.
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Compounds with long alkyl chains ON/n (n = 12, 14, 16) and TN/n (n = 14, 16) (see Table 1) all exhibit cubic phases. The cubic phases are optically isotropic (nonbirefringent) with high viscous. By increasing the temperature, a significant decrease in viscosity was observed at defined temperature (Fig. S1c-e and Fig. S2). It is clearly indicated from the XRD measurements of compounds ON/n (n = 12, 14, 16) and TN/n (n = 14, 16) that their distinct XRD patterns are for cubic phase with space group of Pm3n (Table S2-4 and S6-7 and Fig. 2a and Fig. S6-7 and S9-10). The calculated lattice parameters are acub = 11.83, 11.76, 12.48, 12.13, 12.27 nm for ON/n (n = 12, 14, 16) and TN/n (n = 14, 16) respectively. The Pm3 n lattice is the most commonly observed lattice for thermotropic micellar cubic phases (CubI) (Fig. 2b) formed by eight spheroidic aggregates with soft corona [44,45]. It was calculated [46] that for compounds ON/n (n = 12, 14, 16) and TN/n (n = 14, 16), each of the eight micelles is on average formed by approximately 129, 115, 125, 158, 149 molecules respectively (Table 1 and Table S8). The large distance between the alkyl substituted end and the polar glycerol group gives rise to a rather small projection cone angle (θ = 2.3º – 3.1º, see Table 1), which results in per aggregate with large number molecules (μ > 114). It is remarkable that such molecules with such a long aromatic rigid core, relatively small cone angles are still capable of forming columnar and micellar cubic phases. The formation of large
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dynamic hydrogen bonding networks between the diol groups, π-π stacking of conjugated rigid rod-like core as well as the microsegregation of the incompatible hydrophilic and lipophilic parts of the individual molecules into separate regions are the driving forces for the self-assembly of these molecules. The polar-apolar segregation along the core can favor parallel side-by-side packing of the cores. The bulky terminal alkyl chains are forced to organize side by side. Chain flexibility can be enhanced by the flexibility of the benzylether linkage between the 3,4,5-trialkoxybenzene units and the core units. The hydrogen bonding provided by the terminal diol groups leads to the dense packing of the cores. All this leads to a strong interfacial curvature, giving rise to the formation of hexagonal columnar and Pm3n cubic phases.
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Fig. 2. a) X-ray diffraction pattern of an aligned sample of compound ON/14 at T = 140 oC; b) possible model for CubI/ Pm3 n .
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It should be noted that 1,3,4-oxadiazole-based compound ON/12 display cubic phase while its corresponding 1,3,4-thiadiazole-based analogue TN/12 forms only hexagonal columnar phase, this is due to the larger bend angle, more linearity, less interface curvature of the thiadiazole molecules as compared with those of oxadiazoles (Fig. 3a). The influence of different heterocycles on the formation of cubic phases is done by comparison of the mesomorphic behavior among ON/14, TN/14 and the reported 1,2,3-triazole-based analogue N/14 [3e]. Their chemical structures differ only in the type of the central heterocycles. As shown in Fig. 3a, the dipole moment of the heterocycle unit is in the order triazole > thiadiazole > oxadiazole, which is consistent with mesophase stability and mesophase range. It is evident that the larger dipole moment of the heterocylce leads to the lower melting temperature, the higher clearing temperature and therefore the wider temperature range of mesophase (Fig. 3b and 3c). The reason is that dipole-dipole interactions can enhance core-core interactions, leading to the increasing stability of the mesophase in the case of compound with larger dipole moment. The bent angle of the heterocycle unit is in the order thiadiazole > triazole > oxadiazole as shown in Fig. 3a, which is in line with the number of molecules organized in per spherical aggregate for the cubic phases observed in compounds TN/14, N/14 and ON/14 (Table 1, Fig. 3d). Larger bent angle of molecule leads to smaller projection cone angle of molecule, and therefore more molecules are needed to fill per spherical aggregate (Fig. 3c). The large dipole moment and bent angle of the heterocycles could contribute to the formation of large size of micellar cubic phases for all these heterocycle-based compounds.
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Fig. 3. a) Values of bent angles in the rigid core induced by the heterocylces and dipole moments of the different heterocycles; b) the thermal behaviour of researched compounds ON/n, TN/n and the reported compound N/14; c) the effect of dipole moment on melting point, clearing point and range of mesophase of compounds ON/14, TN/14 and N/14; d) the effect of the bent angle on the number of molecules in per spheroidic aggregate for compounds
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2.3 Photophysical properties
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The UV-vis absorption and photoluminescence emission spectroscopic data of ON/12 and TN/12 (c = 5×10-6 mol·L-1) in dichloromethane (CH2Cl2) solutions and in the LC states are shown in Fig. 4, and the corresponding photophysical properties are summarized in Table 2. The absorption spectra of ON/12 in CH2Cl2 solution showed a maxima at 310 nm (Fig. 4a), while the absorption maxima of compound TN/12 in CH2Cl2 solution exhibited a red shift with a maxima at 340 nm (Fig. 4a) The absorption band in CH2Cl2 solution may be attributed to the π-π* transition of the conjugated backbone. Compound ON/12 showed a maximum at 300 nm in LC state, which is blue shifted by about 10 nm (Fig. 4b), while the absorption spectra for compound TN/12 in LC state showed a maximum at 327 nm in LC state, which is blue shifted by about 13 nm (Fig. 4b). The blue shifts of both compounds ON/12 and TN/12 suggest the formation of π-stacked aggregates with a H-type parallel stacking mode in LC states [47]. The emission spectra of compound ON/12 exhibited a maxima at 371 nm in CH2Cl2 solution, with a maxima at 375 nm in the LC state, hence the Stokes shifts have large values of 61 nm and 75 nm in CH2Cl2 solution and in LC state respectively (Fig. 4). Compound TN/12 showed emission with a maxima at 417 nm in the CH2Cl2 solution, and a maxima at 425 nm in LC state, hence the Stokes shifts have large values of 77 nm and 98 nm in CH2Cl2 solution and in LC state respectively (Fig. 4). The large Stokes shifts of compounds ON/12 and TN/12 confirm the strong intramolecular charge transfer [48]. On irradiation with UV light of wavelength 365 nm, blue light
ACCEPTED MANUSCRIPT was observed in the emissive state for compound ON/12, while blue-green light was observed for compound TN/12 (Fig. 4a). Compounds ON/12 and TN/12 have good quantum yields of 0.31 and 0.37 respectively in solution of CH2Cl2 relative to the quinine sulfate in 0.1 M H2SO4 (ФFL = 0.54) as the standard [49]. The large Stokes shifts and high quantum yields indicate that these compounds have significant potentials for applications in fluorescence sensors [ 50 ] and photoactive functional assemblies [51]. b)
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Fig. 4. a) Normalized absorption (solid trace) and emission spectra (dotted trace) in CH2Cl2 solution obtained for ON/12 (blue trace) and TN/12 (green trace). Pictures of micromolar solutions of compounds ON/12 and TN/12 in
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CH2Cl2, as seen with the illumination of 365 nm UV light (inset); b) Normalized absorption (solid line) and emission spectra (dotted line) in LC state obtained for ON/12 at 120 oC (blue trace) and TN/12 at 120 oC (green trace).
state
Solutiona
TN/12
Solutiona
ON/12
LC state
TN/12
LC state
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Absorption (nm)
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Table 2 UV-vis absorption and fluorescence spectroscopy data of ON/12 and TN/12.
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Stokes shift nm
ФFLd
(cm-1)c
310
371
61(5303)
0.32
340
417
77(5431)
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300
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75(6666)
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425
98(7052)
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2.4 Electrochemical properties The energy band gap of compounds ON/12 and TN/12, which was estimated from the onset of the absorption in thin film, is about 2.65 eV for compound ON/12 and 2.78 eV for compound TN/12 (Fig. S11) [52,53]. The EHOMO and ELUMO energy levels of representative compounds ON/12 and TN/12 in thin film were determined by cyclic voltammetry (CV) on a glassy carbon electrode (Fig. S12 and Table S9), from the onset of oxidative or reductive potentials and the band gap is calculated to be 2.52 eV for compound ON/12 and 2.75 eV for compound TN/12, which are in
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line with those obtained from their absorption spectra in thin film. In order to investigate the conformation and electron distributions of ON/12 and TN/12, calculations based on density functional theory (DFT) with the Gaussian 03W program package at B3LYP/(6-31G) level were performed with the model compounds ON/OCH3 and TN/OCH3, having methoxy groups instead of the alkoxy chains. The electron distributions of the HOMO and LUMO of ON/OCH3 and TN/OCH3 are shown in Fig. S13. As it can be seen in the HOMO orbital, the electrons are localized on 1,4-diphenyl-1,3,4-oxadiazole or 1,4-diphenyl-1,3,4-thiadiazole of the central-conjugated core, in the LUMO orbital, the electrons are mainly localized on the 1,3,4-oxadiazole or 1,3,4-thiadiazole unit, indicating that intramolecular charge transfer (ICT) exists in the ON/OCH3 and TN/OCH3 molecules. The photophysical and eletrochemical properties indiacate that these compounds have potential application in organic semiconductor materials.
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2.5 Chemosensor behavior
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In order to get insights into the binding properties of compounds ON/n and TN/n toward metal ions, the selectivities of compounds ON/12 and TN/12 for metal ions were examined by monitoring the change in fluorescence intensity upon the addition of the perchlorate salt of a wide range of cations including Ag+, Cd2+, Fe3+, Hg2+, Li+, Cu2+, K+, Pb2+, Zn2+, Mg2+, Ni2+ and Co2+. The fluorescence profiles of ON/12 and TN/12 in the presence of 10 equiv. of selected cations in CH3CN : CH2Cl2 = 2 : 1 solution were investigated. Upon the addition of Li+, the fluorescence of ON/12 was largely quenched. On the contrary, the presence of other tested cations induces slight fluorescence quenching (Fig. 5a and 5b). The fluorescence intensity of TN/12 changed little in the presence of most metal ions examined, except Fe3+ as shown in Fig. 6a and 6b. The selective response of ON/12 to Li+ and TN/12 to Fe3+ can be observed by naked eye under UV light (Fig. 5e and 6e). To evaluate the binding natures for ON/12 and TN/12, the fluorescence titration curves of ON/12 and TN/12 were performed in CH2Cl2 : CH3CN = 2 : 1 solution. With the addition of an increasing amount of Li+ (from 1 to 14 equiv), the emission intensity of ON/12 at 360 nm gradually decreased and finally reached its fluorescence quenching plateau at 12 equiv (Fig. 5c and 5d). The fluorescence intensity emerged at 405 nm gradually were quenched with the increasing concentration of Fe3+ from 1 equiv to 10 equiv. When the concentration of Fe3+ ran up to about 8.0 equiv, the quenching of the fluorescence intensity of TN/12 reached a plateau and almost contained a constant (Fig. 6c and 6d). The fluorescence quenching may be due to the chelation enhanced quenching (CEQ) sensing mechanism [54].
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Fig. 5. Fluorescence spectra of compound ON/12 (10-6 M) at room temperature; all anions are ClO4- at pH = 7.0. (a and b) CH2Cl2 : CH3CN = 2 : 1 (λex = 310 nm); (F0 - F)/F0 × 100 depicts the cation selective fluorescence quenching efficiency of compound ON/12, namely the fluorescence responses of different metal ions; abbreviation:
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F0 = the fluorescence emission maximum of a blank sample; F = the fluorescence emission maximum of samples with addition of different metal ions; c) Fluorescence spectra of compound ON/12 (10-6 M), upon addition of an increasing concentration of Li+ ions (0-14 equiv) measured in CH2Cl2 : CH3CN = 2 : 1 (λex = 310 nm) at room temperature. d) Titration curve of the integrated fluorescence as a function of Li + concentration; e) The samples
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Fig. 6. Fluorescence spectra of compound TN/12 (10-6 M) at room temperature; all anions are ClO4- at pH = 7.0. (a and b) CH2Cl2 : CH3CN = 2 : 1 (λex = 330 nm); (F0 - F)/F0 × 100 depicts the cation selective fluorescence quenching efficiency of compound TN/12, namely the fluorescence responses of different metal ions; abbreviation:
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For better understanding the coordination of ON/12 with Li+ and TN/12 with Fe3+, the fluorescence Job’s plot analysis [55] was conducted (Fig. S14). The results showed that the maximum fluorescence quenching was observed when the mole fraction of Li+ ion was close to 0.48 (Fig. S14a), which confirmed that the chemosensor ON/12 and Li+ ion formed a 1 : 1 or 2 : 2 complex [29]. On the other side, the maximum fluorescence quenching was observed when the mole fraction of Fe3+ ion was close to 0.33 (Fig. S14b), which confirmed that the chemosensor TN/12 and Fe3+ ion formed a 2 : 1 complex. In order to investigate the binding site of ON/12 with Li+ and TN/12 with Fe3+, FTIR spectroscopies of solid ON/12, ON/12-Li+, TN/12 and TN/12-Fe3+ were performed (Fig. S15 and S16). The spectrum in Fig. S15 indicated that the O-H and C-O-C stretching vibrations of ON/12 are located at 3414 and 1129 cm-1 respectively [56]. After adding Li+, the peaks of O-H vibration and C-O-C vibration shift to lower wavenumbers of 3390 cm-1 and 1118 cm-1, respectively. Therefore, the binding site of ON/12 to Li+ is much likely to coordinate to the O atom of -OH and O atom of 1,3,4-oxadiazole moiety (Fig S17a). On the other side, comparing the FTIR spectrum before and after adding Fe3+, the peak of N-N vibration (996 cm-1) disappeared and the peak of C-S-C vibrational mode (632 cm-1) [57] shifts to lower wavenumber of 621 cm-1 after adding Fe3+, indicating that Fe3+ ion is binding with 1,3,4-thiadiazole, while the peak of O-H vibration (3387 cm-1) with no change from addition of Fe3+, indicating that Fe3+ is not binding with O-H groups. Therefore, the binding site of TN/12 to Fe3+ is much likely to coordinate to the S and N atoms of the 1,3,4-thiadiazole unit (Fig. S17b). 3 Conclusion
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Therefore heterocycle 1,3,4-oxadiazole or 1,3,4-thiadiazole has great impact on the molecular packing, mesophase stability and photophysical properties etc. Both 1,3,4-oxadiazole- and 1,3,4-thiadiazole-based derivatives reported here display enantiotropic mesophases. 1,3,4-Thiadiazole derivatives exhibit wider range of enantiotropic mesophase than their 1,3,4-oxadiazole analogues. With the elongation of the alkyl chains, a phase transition from Colhex/p6mm to CubI/ Pm3n was observed in 1,3,4-oxadiazole- and 1,3,4-thiadiazole-based compounds. Such compounds are luminescent LCs and can be used as fluorescence chemosensors for selective response to Li+ (ON/n) and Fe3+ (TN/n) in CH2Cl2/CH3CN solution. Our study should provide some fundamental knowledge in designing novel functional luminescent LCs. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 21664015, 21364017, 21274119 and 21602195) and the Yunnan Provincial Department of Education Foundation (No. ZD2015001, 2016FD008). We thank beamline 1W2A at Beijing Accelerator
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Laboratory and beamline BL16B1 at Shanghai Synchrotron Radiation Facility (SSRF), China. The calculations were performed with the support of the Yunnan University Supercomputer Center.
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights 1 Oxadiazol and thiadiazole based LCs with Colhex/p6mm and CubI/ Pm3n phases were synthesized via one pot cyclization. 2. Influences of heterocycles on the liquid crystalline and photophysical properties etc are discussed. 3. These luminescent LCs can act as chemosensors to recognize metal ions [1] (a) S. Sergeyev, W. Pisula, Y.H. Geerts, Chem. Soc. Rev. 36 (2007) 1902–1929;
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