Fluorescent Hydrogel Generated Conveniently from a Perylene

Jan 22, 2018 - CCDC 1573319–1573321 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via ww...
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Fluorescent Hydrogel Generated Conveniently from a Perylene Tetracarboxylate Derivative of Titanium(IV) Alkoxide Dan-Hong Zou, Peng Wang, Wen Luo, Jin-Le Hou, Qin-Yu Zhu,* and Jie Dai* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People’s Republic of China S Supporting Information *

ABSTRACT: Organic gelators and metal-coordination frameworks based on perylene derivatives as functional materials have attracted great attention because of their intense fluorescence emission as well as unique electronic and photonic properties. We report here the structures and properties of a luminescent titanium(IV) coordination compound of a perylene tetracarboxylate (PTC) derivative, [Ti2(OiPr)6(L1)(phen)2] (1), along with its two naphthalene analogues, [Ti2(OiPr)6(L2)(phen)2] (2) and [Ti2(OiPr)6(L2)(bpy) 2 ] (3), where L 1 = 3,9-dicarboxylate-(4,10diisopropanolcarboxylate)perylene, phen = 1,10-phenanthroline, L2 = 1,5-dicarboxylate-(2,6-diisopropanolcarboxylate)naphthalene, and bpy = 2,2′-bipyridine. Compound 1 is a rare early-transition-metal PTC coordination compound that can be simply prepared in one pot as crystals by a low-heat synthesis. Unlike those of paramagnetic late-transition-metal PTC compounds, compound 1 showed intense fluorescence emission. More remarkably, the crystals of 1 can be turned immediately to a fluorescent hydrogel just through a simple procedure, putting the crystals in water and then treating with ultrasound. No acid catalyst or pH adjustment is needed. Hydrolysis of the titanium isopropanol group in water and π−π interaction of the perylene and phen play important roles in the gelation process. The film prepared from the gel can be used as a visual fluorescence sensor for aromatic amines and phenols, which are hazards for the human and environment.

1. INTRODUCTION Perylene derivatives, such as perylene bisimides (PBIs) and perylene tetracarboxylates (PTCs), have attracted great attention because of their properties including intense fluorescence emission as well as electronic and photonic properties.1,2 PBIs and PTCs are able to construct fascinating supramolecular architectures through π−π interaction3,4 and even sols or gels.5−7 For the sols or gels of PBIs or PTCs, some organic gelator moieties must be introduced into the perylene core in order to increase the physicochemical interactions. Perylene derivatives with long hydrocarbon and polyether chains can be used to form effective low-molecular-weight gelators, and the gelation can be achieved through a physical entanglement of the long chains.5 Amino acid functionalized PBIs or PTCs can form hydrogels through hydrogen bonds and electrostatic interactions.6 The luminescent hydrogels have widespread applications in various fields such as biomedicine and soft electronics. The gelation can also be achieved through covalent chemical cross-linking, such as the polymerization of alkoxysilane derivatives of perylene.7 On the other hand, metal-coordination-directed selfassemblies based on PBI and PTC derivatives have been investigated as a means to obtain metallosupramolecular structured materials,8 metal-coordinated cages, and functional metal−organic frameworks.9 Nanocrystalline metal-coordination polymers of PTCs have also been used to gain metal oxide © XXXX American Chemical Society

nanostructures with desired morphology that could be elaborately controlled by the synthesis conditions in coordination chemistry.10 In coordination assembly, noble metals and late transition metals were used as metal-ion sources, while early-transition-metal coordination compounds of PDI and PTC derivatives have rarely been isolated as far as we know. Although strong fluorescence is the characteristic of PTC dyes, it is usually partially or completely quenched in some of the late transition metals because of their paramagnetic electron structure, which is known to promote intercrossing and nonradiative relaxation.9b,11 We report here a luminescent titanium(IV) coordination compound of a PTC derivative, [Ti2(OiPr)6(L1)(phen)2] (1), along with its naphthalene analogues, [Ti2(OiPr)6(L2)(phen)2] (2) and [Ti2(OiPr)6(L2)(bpy)2] (3). Herein L1, L2, phen, and bpy are 3,9-dicarboxylate-(4,10-diisopropanolcarboxylate)perylene, 1,5-dicarboxylate-(2,6-diisopropanolcarboxylate)naphthalene, 1,10-phenanthroline, and 2,2′-bipyridine, respectively. The compounds are small molecules and can be easily prepared in one pot by a low-heat synthesis. Compound 1 is an unusual early-transition-metal PTC coordination compound that shows high fluorescent quantum yield in solutions. More remarkably, in most cases, the synthesis of organic gelator Received: November 23, 2017

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DOI: 10.1021/acs.inorgchem.7b02985 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

atoms were positioned with idealized geometry and refined with fixed isotropic displacement parameters. Relevant crystal data, collection parameters, and refinement results can be found in Table 1. CCDC 1573319−1573321 contain the supplementary crystallographic data for 1−3, respectively.

derivatives of perylene is somewhat difficult and multistep reactions and catalysts are inevitable, but 1 can be obtained easily and turned immediately into a fluorescent hydrogel just through a treatment with water. A film prepared from the gel can be further used as a visual fluorescence sensor for aromatic amines and phenols, which are hazards for humans and the environment.

Table 1. Crystal Data and Structural Refinement Parameters for Compounds 1−3

2. EXPERIMENTAL SECTION

1

2.1. General Remarks. 3,4,9,10-Perylenetetracarboxylic acid (PTCA) was prepared by the dissolution of 3,4,9,10-perylenetetracarboxyl dianhydride (PTCDA) into a dilute solution of KOH and then the addition of dilute HCl.12 Other analytically pure reagents were purchased commercially and used without further purification. Elemental analyses of carbon, hydrogen, and nitrogen were performed using a VARIDEL III elemental analyzer. The Fourier transform infrared (FT-IR) spectra were recorded as KBr pellets on a Nicolet Magna 550 FT-IR spectrometer. Solid-state room-temperature optical diffuse-reflectance spectra of the microcrystal samples were obtained with a Shimadzu UV-2600 spectrometer. The emission spectra and quantum yield were recorded using Hitachi F-2500 and Edinburgh FLS-920 fluorescence spectrometers with a quartz cell. Roomtemperature X-ray diffraction (XRD) data were collected on a D/ MAX-3C diffractometer using a copper tube source (Cu Kα; λ = 1.5406 Å). The morphologies of the electrodes were recorded with a JSM-5600LV scanning electron microscope. Thermoanalytical measurements were performed using a TGA-DCS 6300 microanalyzer, and the sample was heated under a nitrogen stream of 100 mL min−1 at a heating rate of 20 °C min−1. 2.2. Synthesis of Compounds. [Ti2(OiPr)6(L1)(phen)2]·HOiPr (1). Analytically pure Ti(OiPr)4 (0.1 mL, 0.26 mmol), PTCA (20 mg, 0.05 mmol), phen (20 mg, 0.11 mmol), and 0.3 mL of toluene were mixed and degassed rapidly using argon. The mixture was sealed in a thick glass tube, and the sealed tube was heated at 40 °C for 6 days and then cooled to room temperature to yield orange crystals (38% yield based on PTCA). The crystals were rinsed with toluene, dried in vacuum, and preserved in a desiccator. Anal. Calcd for C75H88N4O15Ti2 (MW 1381.30): C, 65.22; H, 6.42; N, 4.06. Found: C, 64.94; H, 6.59; N, 4.17. Important IR data (KBr, cm−1): 2968 (m), 1713 (s), 1628 (s), 1590 (s), 1425 (m), 1312 (m), 1281 (s), 1102 (s), 848 (s), 725 (s), 591 (m). [Ti2(OiPr)6(L2)(phen)2] (2). Compound 2 was prepared in a manner similar to that used for 1 except that PCTA was replaced by naphthalenetetracarboxyl dianhydride (NTCDA; 10 mg, 0.05 mmol). The mixture was heated at 40 °C for 6 days to yield pale-yellow crystals (41% yield based on NTCDA). Anal. Calcd for C62H76N4O14Ti2 (MW 1197.06): C, 62.21; H, 6.40; N, 4.68. Found: C, 62.44; H, 6.68; N, 4.41. Important IR data (KBr, cm−1): 2973 (m), 1720 (s), 1624 (s), 1518 (m), 1428 (w), 13226 (m), 1276 (m), 1105 (s), 988 (m), 848 (s), 731 (s), 610 (m). [Ti2(OiPr)6(L2)(bpy)2] (3). Compound 3 was prepared in a manner similar to that used for 2 except that phen was replaced by bpy (20 mg, 0.15 mmol). Analytically pure Ti(OiPr)4 (0.1 mL, 0.26 mmol), alizarin (6.0 mg, 0.025 mmol), 4-fluorobenzoic acid (4.5 mg, 0.032 mmol), and 0.3 mL of anhydrous isopropanol were mixed. The mixture was heated at 40 °C for 6 days to yield pale-yellow crystals (36% yield based on NTCDA). Anal. Calcd for C58H76N4O14Ti2 (MW 1149.02): C, 60.63; H, 6.67; N, 4.88. Found: C, 60.72; H, 6.87; N, 4.53. Important IR data (KBr, cm−1): 2970 (m), 1714 (s), 1602 (s), 1444 (m), 1326 (w), 1275 (m), 1188 (s), 1100 (s), 988 (w), 846 (m), 768 (s), 597 (m). 2.3. X-ray Crystallographic Study. The measurement of 1 was carried out on a Agilent Gemini Atlas CCD diffractometer at room temperature. The measurements of 2 and 3 were carried out on a Rigaku Mercury CCD diffractometer at room temperature with graphite-monochromated Mo Kα (λ = 0.71075 Å) radiation. The structures were solved by direct methods using SHELXS-2014, and the refinement was performed against F2 using SHELXL-2014. All of the non-hydrogen atoms were refined anisotropically. The hydrogen

formula fw cryst size (mm3) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) F(000) μ (mm−1) T (K) Rint reflns collected unique reflns obsd reflns no. of param GOF on F2 R1 R1 [I > 2σ(I)] wR2 wR2 [I > 2σ(I)]

C75H88N4O15Ti2 1381.30 0.23 × 0.33 × 0.35 triclinic P1̅ 9.6701(19) 11.876(2) 16.176(3) 86.93(3) 86.12(3) 76.09(3) 1797.7(7) 1 1.297 720 0.291 293(2) 0.0395 14365 6312 4541 423 1.034 0.0898 0.0618 0.1205 0.1085

2 C62H76N4O14Ti2 1197.06 0.2 × 0.3 × 0.4 triclinic P1̅ 9.1388(18) 9.5095(19) 19.052(4) 98.90(3) 91.47(3) 102.39(3) 1594.7(5) 1 1.232 618 0.314 293(2) 0.0245 15439 8657 5446 790 1.049 0.1054 0.0684 0.1472 0.1319

3 C58H76N4O14Ti2 1149.02 0.21 × 0.23 × 0.30 triclinic P1̅ 9.1440(18) 12.255(3) 13.919(3) 87.62(3) 84.48(3) 82.71(3) 1539.3(5) 1 1.240 608 0.323 293(2) 0.0268 12043 5313 3239 342 1.028 0.1196 0.0793 0.1597 0.1438

3. RESULTS AND DISCUSSION Synthesis and Characterization. Compounds 1−3 were prepared directly by a one-pot synthesis in toluene at moderate temperature, 40 °C (see the Experimental Section). Sample 1 was obtained as orange crystals, while samples 2 and 3 were obtained as colorless or pale-yellow crystals (Figure S1). The identity of the collected bulky crystal sample with the single crystals used in structure analysis was checked by comparing the experimental XRD pattern with the calculated pattern from the crystal data (Figure S2). Their main peaks are in agreement with each other, except for some differences in their peak intensities. For each compound, the IR stretches around 1700 and 1600 cm−1 indicate the coordinated carboxyl group of L1 and L2. Isopropoxy groups are detected by the νC−H (between 2980 and 2850 cm−1) and νTi−O−C (about 1110 and 1100 cm−1) vibrations (Figure S3). Compounds 1−3 show multistep thermal decomposition in the range of 50−800 °C (Figure S4). The isopropoxy groups are easily lost below 200 °C, and then the organic ligands are removed until near 600 °C. For 1, the first-step weight loss is the cocrystallized HOiPr (5%). The weight at 800 °C for 1 and 2 is more than that of two TiO2, which should be due to carbon deposition. The crystals are stable under a dry atmosphere, but they easily lose isopropanol B

DOI: 10.1021/acs.inorgchem.7b02985 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Ball-and-stick plot of 1. (b) Parallel arrangement of the perylene and phen planes in 1. (c) Ball-and-stick plot of 2 (d) Ball-and-stick plot of 3. Hydrogen atoms are omitted for clarity.

Figure 2. (a) Solid-state absorption spectra of 1−3 in a representation of the electronvolt coordinate. (b) Solid-state absorption spectra of 1−3 in a representation of the wavelength coordinate. (c) Fluorescent spectra of 1 in different solvents (1.0 × 10−4 mmol L−1). (d) Fluorescence spectra of 1 and 2 in methanol and dichloromethane for comparison.

groups in ambient conditions. Compound 3 is the most unstable one that lost HOiPr even at room temperature. Although perylene derivatives as efficient photosensitive dyes

have been used in dye-sensitized TiO2 electrodes, the crystals of titanium coordination compounds of perylene were isolated for the first time. C

DOI: 10.1021/acs.inorgchem.7b02985 Inorg. Chem. XXXX, XXX, XXX−XXX

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compound, it can also be used as a gelator to generate organic− inorganic hybrid hydrogels in water. Figure 3 gives the

The collected high-quality single crystals allow X-ray determination of the structures of the molecules in the solid state. All three molecules crystallize in the triclinic P1̅ space group with a centrosymmetrical center (Figure 1a−d). The titanium(IV) atom has a distorted octahedral geometry coordinated by one carboxylate oxygen atom from the ligand L1 or L2, two nitrogen atoms from the chelated phen or bpy, and three isopropanol oxygen atoms. The two titanium centers are bridged by ligand L1 or L2 via two carboxyl groups of the perylene/naphthalenetetracarboxylates, and the other two carboxyl groups of the ligands are esterified with isopropanol groups. The perylene core in 1 is a conjugated plane with 0.0415 Å average deviation from the plane of the 20 carbon atoms and arranged parallel to the two phen molecules with a dihedral angle of 2.76(5)° (Figure 1b). The average distance between the planes is 3.40 Å, and hence there is π−π interaction between the planes. Every titanium(IV) atom is coordinated by three isopropanol groups, which are arranged to direct two opposite sides. The molecular packing of 1 showing the contacting columns along the b axis is given in Figure S5. The ligand arrangement in 3 is similar to that in 1 (the dihedral angle between naphthalene and bpy is 2.17(8)°; Figure 1d), but that in 2 is quite different. The dihedral angle between naphthalene and phen is 68.1(3)° (Figure 1c). Solid-state UV−vis absorption spectra of 1, calculated from the diffuse-reflectance spectra, show a broad absorption band around 450 nm with an onset wavelength of 530 nm (orange), covering part of the main range of the visible spectra, while the absorption spectra of 2 and 3 are below 400 nm (Figure 2a,b). The highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) energy gap estimated from the onset energy of 1 is 2.3 eV, while those of 2 and 3 are 3.2 and 3.3 eV, respectively. The low-energy absorption of 1 is assigned to the representative band of the perylene13 because of its extended π system in comparison with those of 2 and 3. Fluorescence spectra of 1 in different solventsdichloromethane, toluene, water, and methanolare shown in Figure 2c. A solvent-induced sharp fluorescence change was observed. The fluorescence spectra can be divided into two different groups (water and toluene and also dichloromethane and methanol) because of their different solvation effects. Figure 2d gives the fluorescence spectra of 1 and 2 in dichloromethane and methanol solvents. The emission wavelength of 2 is 400 nm lower than the 550 nm of 1. The results show that (1) the perylene complex of titanium(IV) (d0 electron state) is a fluorescent compound, while most of the late-transition-metal PTC compounds do not have or only have weak fluorescence for the paramagnetic quenching9b,11 and (2) increasing the conjugated system of the ligand narrows the energy gap between the ground and excited states, and, consequently, the emission band shifts to the visible range and the fluorescence emission increases. Gelation and Characterization of the Gel. In the past decades, the supramolecular gels and gelation properties of a series of structurally related PBIs or PTCs have been studied. Although π−π interaction of the perylene core of PBIs or PTCs can facilitated by the formation of supramolecular architectures, some organic gelator moieties, such as long hydrocarbon, polyether, or polyamine chains, must be introduced into the perylene core to obtain sols or gels.5 In most cases, the synthesis of organic gelator derivatives of perylene is somewhat difficult, and multistep reactions and catalysts are inevitable. It is attractive that, although compound 1 is a simple coordination

Figure 3. Schematic view of the procedures from crystals of compound 1 to TiO−perylene gel.

schematic view of the procedure. Two or three small pieces of crystals ( diethylamine > ethyldiamine. Similarly, aromatic phenols such as phenol and catechol can completely quench the emission of the film, while the aliphatic alcohols of ethylene alcohol, ethylene glycol, and butylene alcohol showed little influence on the emission. The selectivity mainly results from the preferable matching of the HOMO and LUMO energy levels between PTCA and the aromatic compounds.16 Fluorescence films of perylene for aniline vapor detection have been reported;16 however, the films used for phenol detection have rarely been discussed, especially those in which the sensing could be performed in a visualized manner. A phenol sensor based on a cholesterol-functionalized calix[4]perylene bisimide diacid was studied,17 but our TiO− PTC system is more economic in the synthesis and easier to prepare.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the NSF of China (Grant 21771130), the Program of the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the project of scientific and technologic infrastructure of Suzhou (SZS201708).

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4. CONCLUSION A luminescent dinuclear titanium(IV) alkoxyl compound of the PTC derivative (1) along with two naphthalene analogues (2 and 3) were prepared. The compounds are small molecules and can be easily prepared in one pot by a low-heat synthesis. Compound 1, constructed with perylene, phen, and alkoxyl groups, is an unusual early-transition-metal-coordination compound. Unlike paramagnetic transition-metal PTC compounds, compound 1 shows high fluorescence emission. More F

DOI: 10.1021/acs.inorgchem.7b02985 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02985 Inorg. Chem. XXXX, XXX, XXX−XXX