Synthesis and Photochromic Studies of Dithienylethene-Containing

May 20, 2016 - ... University Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong. Inorg...
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Synthesis and Photochromic Studies of Dithienylethene-Containing Cyclometalated Alkynylplatinum(II) 1,3-Bis(N‑alkylbenzimidazol-2′yl)benzene Complexes Michael Ho-Yeung Chan, Hok-Lai Wong, and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S Supporting Information *

ABSTRACT: Several photochromic cyclometalated alkynylplatinum(II) complexes with tridentate 1,3-bis(Nalkylbenzimidazol-2′-yl)benzene (bzimb) ligands have been synthesized by the reaction of the corresponding chloroplatinum(II) bzimb precursor complexes with the photochromic ligand TMS-CC-Th-DTE in the presence of sodium hydroxide. They have been characterized by 1H NMR spectroscopy and positive-ion FAB or ESI mass spectrometry and confirmed by elemental analysis. One of the complexes has also been characterized by X-ray crystallography. Their photophysical, photochromic, and electrochemical properties have been studied. Upon photoexcitation, the yellow solutions in benzene display green phosphorescence originating from the triplet intraligand (3IL) excited state. All the cyclometalated alkynylplatinum(II) bzimb complexes exhibit reversible photochromism with solution colors changing between yellow and purple upon photoirradiation. The thermal bleaching kinetics of complex 2 has been studied in toluene at various temperatures with the activation barrier for the thermal cycloreversion reaction determined.

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destabilize the ligand field (LF) excited state via the raising of the energy of the dx2−y2 orbital on the platinum(II) center, the cyclometalated platinum(II) bzimb system has been demonstrated to exhibit rich photophysical properties with inefficient nonradiative decay from the high-lying LF excited state.7a,g−i By combining the cyclometalated platinum(II) bzimb system with the photochromic diarylethene unit, it is envisaged that efficient utilization of the excited states would become possible, which would lead to interesting photochromic behavior and photophysical properties. Herein we report the synthesis, characterization, photophysical, photochromic and electrochemical studies of a new class of photochromic cyclometalated alkynylplatinum(II) bzimb complexes.

INTRODUCTION Photochromic molecular functional materials have aroused tremendous research interest owing to their potential applications ranging from sunglass lenses, photochromic inks, and optoelectronic devices to optical memories.1−6 Among the variety of photochromic building blocks, the photochromic diarylethene system has received the most attention due to its excellent fatigue resistance and thermal irreversibility.1−3 In order to improve the robustness of the materials, strategies like linking the photochromic moiety to metal complexes have been employed since a relatively nondestructive lower-energy excitation source could be utilized to sensitize the photocyclization reaction from the metal-to-ligand charge transfer (MLCT) excited states.1−6 Recently, with the incorporation of metal centers to photochromic groups such as azobenzenes,4 stilbenes,5 spirooxazines,6 and diarylethenes,1−3 intriguing photochromic and photophysical behaviors have been demonstrated. On the other hand, cyclometalated platinum(II) complexes often display strong phosphorescence and electroluminescence such that they have been utilized as the emissive materials for the phosphorescent organic light-emitting diodes (PHOLEDs).7 Our group has reported the utilization of a new class of cyclometalating ligand, namely, 1,3-bis(N-alkylbenzimidazol-2′-yl)benzene (bzimb) ligands for the fabrication of platinum-based PHOLEDs.7a,g−i By utilizing the advantage of the anionic and strongly σ-donating bzimb ligand which would © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials. Copper(I) iodide and 1-bromooctane were purchased from Aldrich Chemical Co. N,N-Diisopropylamine was purchased from Lancaster Synthesis Ltd. (Trimethylsilyl)acetylene was obtained from GFS Chemical, Inc. Triphenylphosphine was obtained from Acros Organics, Inc. Potassium tetrachloroplatinate(II) and palladium(II) chloride were purchased from Strem Chemicals, Inc. Tetrakis(triphenylphosphine)palladium(0)8 as catalyst for Suzuki crosscoupling, dichlorobis(tiphenylphosphine)palladium(II)9 as catalyst for Sonogashira coupling, 2,3-bis(2,5-dimethylthiophen-3-yl)-5-iodoReceived: March 11, 2016

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DOI: 10.1021/acs.inorgchem.6b00619 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

MHz, CDCl3, 298 K): δ 0.88 (t, J = 6.4 Hz, 6H, −CH3), 1.26 (m, 16H, −CH2−), 1.46 (m, 4H, −CH2−), 1.94 (m, 4H, −CH2−), 4.50 (t, J = 7.4 Hz, 4H, −N−CH2−), 7.15 (m, 2H, benzimidazolyl), 7.31 (m, 2H, benzimidazolyl), 7.54 (m, 2H, −C6H2−), 8.14 (m, 2H, benzimidazolyl), 9.05 (m, 2H, benzimidazolyl). Positive FAB-MS: m/z 747 [M − Cl]+. [Pt(L1)(CC-Th-DTE)] (1). Complex 1 was prepared by a modification of a literature procedure for [Pt(bzimb)(CCPh)].7g−i To a stirred solution of TMS-CC-Th-DTE (65 mg, 0.16 mmol) in methanol (150 mL) was added NaOH powder (6.2 mg, 0.16 mmol). The reaction mixture was stirred at room temperature for 30 min. A solution of [Pt(L1)Cl] (58.6 mg, 0.08 mmol) in dichloromethane (20 mL) was added to the reaction mixture. It was stirred and heated at 120 °C overnight. After removal of solvent under reduced pressure, the yellow solid was washed with deionized water, methanol, and diethyl ether. The crude product was subjected to further purification by layering diethyl ether over its concentrated dichloromethane solution to afford the pure product as a yellow crystal. Yield: 48.9 mg, 0.046 mmol; 60%. 1H NMR (400 MHz, CD2Cl2, 298 K): δ 0.87 (t, J = 7.1 Hz, 6H, −CH3), 1.28 (m, 16H, −CH2−), 1.48 (m, 4H, −CH2−), 2.00 (q, J = 7.7 Hz, 4H, −CH2−), 2.12 (s, 3H, −CH3), 2.14 (s, 3H, −CH3), 2.40 (s, 6H, −CH3), 4.60 (t, J = 7.7 Hz, 4H, −N−CH2−), 6.52 (s, 1H, dimethylthienyl), 6.57 (s, 1H, dimethylthienyl), 7.08 (s, 1H, thienyl), 7.32 (t, J = 8.0 Hz, 1H, −C6H3−), 7.38 (m, 4H, benzimidazolyl), 7.44 (m, 2H, benzimidazolyl), 7.64 (m, 2H, benzimidazolyl), 8.89 (d, J = 8.0 Hz, 2H, −C6H3−). Positive FABMS: m/z 1055 [M]+. Anal. Found (%): C 60.92, H 5.76, N 5.51. Calcd (%) for C54H60N4PtS3·0.5H2O: C 60.88, H 5.77, N 5.26. [Pt(L2)(CC-Th-DTE)] (2). Complex 2 was prepared according to a procedure similar to that of [Pt(L1)(CC-Th-DTE)], in which [Pt(L2)Cl] (116 mg, 0.14 mmol) was used instead of [Pt(L1)Cl]. Yield: 125 mg, 0.11 mmol; 79%. 1H NMR (500 MHz, CDCl3, 298 K): δ 0.87 (t, J = 7.1 Hz, 6H, −CH3), 1.26 (m, 16H, −CH2−), 1.44 (s, 9H, −CH3), 1.52 (m, 4H, −CH2−), 2.00 (q, J = 7.6 Hz, 4H, −CH2−), 2.09 (s, 3H, −CH3), 2.12 (s, 3H, −CH3), 2.39 (s, 6H, −CH3), 4.57 (t, J = 7.6 Hz, 4H, −N−CH2−), 6.50 (s, 1H, dimethylthienyl), 6.55 (s, 1H, dimethylthienyl), 7.10 (s, 1H, thienyl), 7.24 (m, 2H, benzimidazolyl), 7.29 (m, 2H, benzimidazolyl), 7.40 (m, 2H, benzimidazolyl), 7.64 (s, 2H, −C6H2−), 8.94 (m, 2H, benzimidazolyl). Positive ESI-MS: m/z 1111 [M + H]+. Anal. Found (%): C 62.19, H 6.23, N 5.20. Calcd (%) for C58H68N4PtS3·0.5H2O: C 62.12, H 6.20, N 5.00. [Pt(L3)(CC-Th-DTE)] (3). Complex 3 was prepared according to a procedure similar to that of [Pt(L1)(CC-Th-DTE)], in which [Pt(L3)Cl] (62.5 mg, 0.08 mmol) was used instead of [Pt(L1)Cl]. Yield: 66.2 mg, 0.06 mmol; 73%. 1H NMR (400 MHz, CDCl3, 298 K): δ 0.86 (t, J = 6.6 Hz, 6H, −CH3), 1.23 (m, 16H, −CH2−), 1.40 (m, 4H, −CH2−), 1.87 (q, J = 7.4 Hz, 4H, −CH2−), 2.13 (s, 3H, −CH3), 2.16 (s, 3H, −CH3), 2.42 (s, 6H, −CH3), 4.48 (t, J = 7.4 Hz, 4H, −N− CH2−), 6.54 (s, 1H, dimethylthienyl), 6.59 (s, 1H, dimethylthienyl), 7.10 (m, 2H, benzimidazolyl), 7.16 (s, 1H, thienyl), 7.18 (d, J = 10.1 Hz, 2H, −C6H2−), 7.28 (m, 2H, benzimidazolyl), 7.38 (m, 2H, benzimidazolyl), 8.86 (m, 2H, benzimidazolyl). 19F{1H} NMR (376.4 MHz, CDCl3, 298 K): δ −117.9 (t, J = 10.1 Hz, 1F, −C6H2F−). Positive ESI-MS: m/z 1072 [M + H]+. Anal. Found (%): C 59.45, H 5.59, N 5.09. Calcd (%) for C54H59FN4PtS3·H2O: C 59.37, H 5.63, N 5.13. [Pt(L4)(CC-Th-DTE)] (4). Complex 4 was prepared according to a procedure similar to that of [Pt(L1)(CC-Th-DTE)], in which [Pt(L4)Cl] (60 mg, 0.09 mmol) was used instead of [Pt(L1)Cl]. Yield: 56 mg, 0.06 mmol; 64%. 1H NMR (400 MHz, CDCl3, 298 K): δ 0.98 (t, J = 7.3 Hz, 6H, −CH3), 1.48 (m, 4H, −CH2−), 1.94 (m, 4H, −CH2−), 2.12 (s, 3H, −CH3), 2.14 (s, 3H, −CH3), 2.41 (s, 6H, −CH3), 4.56 (t, J = 7.4 Hz, 4H, −N−CH2−), 6.52 (s, 1H, dimethylthienyl), 6.57 (s, 1H, dimethylthienyl), 7.13 (s, 1H, thienyl), 7.22 (m, 2H, benzimidazolyl), 7.29 (m, 3H, −C6H3−), 7.40 (t, J = 7.6 Hz, 2H, benzimidazolyl), 7.54 (d, J = 7.6 Hz, 2H, benzimidazolyl), 8.95 (d, J = 8.1 Hz, 2H, benzimidazolyl). Positive FAB-MS: m/z 944 [M]+. Anal. Found (%): C 58.05, H 4.49, N 5.90. Calcd (%) for C46H44N4PtS3·0.5H2O: C 57.97, H 4.76, N 5.88.

thiophene, 3i ((5,6-bis(2,5-dimethylthiophen-3-yl)thiophen-2-yl)ethynyl)trimethylsilane (TMS-CC-Th-DTE), 3 i 1,3-bis(benzimidazol-2′-yl)benzene (bzimb),7a,g−i and [Pt(L4)Cl]7a,g−i (where L4 = 1,3-bis(N-butyl-benzimidazol-2′-yl)benzene) were synthesized according to literature procedures. All amines were distilled over potassium hydroxide and stored over potassium hydroxide before use. Tetrahydrofuran and diethyl ether were purified using Innovative Technology, Inc., model PureSolv MD 5 Solvent Purification System before use. All other solvents and reagents were of analytical grade and were used as received. 1,3-Bis(N-octyl-benzimidazol-2′-yl)benzene (L1). L1 was prepared by modification of a procedure for the synthesis of 1,3bis(N-butyl-benzimidazol-2′-yl)benzene7a,g−i using 1-bromooctane instead of 1-bromobutane. Yield 1.3 g, 2.43 mmol; 75%. 1H NMR (400 MHz, CDCl3, 298 K): δ 0.82 (t, J = 6.8 Hz, 6H, −CH3), 1.21 (m, 20H, −CH2−), 1.81 (q, J = 7.5 Hz, 4H, −CH2−), 4.27 (t, J = 7.5 Hz, 4H, −N−CH2−), 7.32 (m, 4H, benzimidazolyl), 7.43 (m, 2H, benzimidazolyl), 7.56 (t, J = 7.5 Hz, 1H, −C6H4−), 7.82 (m, 2H, benzimidazolyl), 7.88 (d, J = 7.5 Hz, 2H, −C6H4−), 8.04 (s, 1H, −C6H4−). Positive FAB-MS: m/z 535 [M + H]+. 1,3-Bis(N-octyl-benzimidazol-2′-yl)-5-tert-butylbenzene (L2). L2 was prepared according to a procedure similar to that of 1,3bis(N-octyl-benzimidazol-2′-yl)benzene (L1), in which 1,3-bis(benzimidazol-2′-yl)-5-tert-butylbenzene (1.0 g, 2.73 mmol) was used instead of 1,3-bis(benzimidazol-2′-yl)benzene to afford the product as a pale yellow solid. Yield: 1.1 g, 1.86 mmol; 68%. 1H NMR (500 MHz, CDCl3, 298 K): δ 0.82 (t, J = 6.9 Hz, 6H, −CH3), 1.19 (m, 20H, −CH2−), 1.44 (s, 9H, −tBu), 1.81 (q, J = 7.5 Hz, 4H, −CH2−), 4.27 (t, J = 7.5 Hz, 4H, −N−CH2−), 7.31 (m, 4H, benzimidazolyl), 7.43 (m, 2H, benzimidazolyl), 7.78 (t, J = 1.5 Hz, 1H, −C6H3−), 7.83 (m, 2H, benzimidazolyl), 7.89 (d, J = 1.5 Hz, 2H, −C6H3−). Positive FAB-MS: m/z 592 [M + H]+. 1,3-Bis(N-octyl-benzimidazol-2′-yl)-5-fluorobenzene (L3). L3 was prepared according to a procedure similar to that of 1,3-bis(Noctyl-benzimidazol-2′-yl)benzene (L1), in which 1,3-bis(benzimidazol2′-yl)-5-fluorobenzene (5.3 g, 16.1 mmol) was used instead of 1,3bis(benzimidazol-2′-yl)benzene to afford the product as an oil. Yield: 4.8 g, 8.68 mmol; 54%. 1H NMR (500 MHz, CDCl3, 298 K): δ 0.82 (t, J = 7.5 Hz, 6H, −CH3), 1.21 (m, 20H, −CH2−), 1.81 (q, J = 7.4 Hz, 4H, −CH2−), 4.28 (t, J = 7.4 Hz, 4H, −N−CH2−), 7.24 (m, 4H, benzimidazolyl), 7.32 (m, 2H, benzimidazolyl), 7.61 (m, 2H, −C6H3−), 7.82 (m, 2H, benzimidazolyl), 7.86 (s, 1H, −C6H3−). Positive FAB-MS: m/z 554 [M + H]+. [Pt(L1)Cl]. [Pt(L1)Cl] was prepared by a modification of a literature procedure for the preparation of the related chloroplatinum(II) bzimb complexes.7a,g−i The reaction was performed under inert atmosphere by using standard Schlenk techniques. To a stirred solution of L1 (160 mg, 0.30 mmol) in glacial acetic acid (5 mL) was added K2[PtCl4] (100 mg, 0.24 mmol) in deionized water (0.5 mL). The reaction mixture was heated under reflux until completion, during which a yellow solid with green emission precipitated. The solid was filtered and washed with deionized water, methanol, and diethyl ether to afford the product as a yellow solid. Yield: 58.6 mg, 0.077 mmol; 32%. 1H NMR (300 MHz, CDCl3, 298 K): δ 0.86 (t, J = 6.3 Hz, 6H, −CH3), 1.22 (m, 20H, −CH2−), 1.80 (m, 4H, −CH2−), 4.37 (t, J = 7.4 Hz, 4H, −N−CH2−), 7.02 (m, 4H, benzimidazolyl), 7.19 (m, 3H, benzimidazolyl, −C6H3−), 7.29 (m, 2H, benzimidazolyl), 8.86 (d, J = 8.1 Hz, 2H, −C6H3−). Positive FAB-MS: m/z 729 [M − Cl]+. [Pt(L2)Cl]. [Pt(L2)Cl] was prepared according to a procedure similar to that of [Pt(L1)Cl] in which L2 (635 mg, 1.07 mmol) was used instead of L1. Yield: 223 mg, 0.27 mmol; 52%. 1H NMR (300 MHz, CDCl3, 298 K): δ 0.87 (t, J = 6.6 Hz, 6H, −CH3), 1.26 (m, 16H, −CH2−), 1.44 (m, 4H, −CH2−), 1.51 (s, 9H, −tBu), 1.87 (m, 4H, −CH2−), 4.47 (t, J = 7.6 Hz, 4H, −N−CH2−), 7.01 (m, 2H, benzimidazolyl), 7.21 (m, 2H, benzimidazolyl), 7.30 (m, 2H, benzimidazolyl), 7.45 (s, 2H, −C6H2−), 8.84 (d, J = 7.8 Hz, 2H, benzimidazolyl). Positive FAB-MS: m/z 784 [M − Cl]+. [Pt(L3)Cl]. [Pt(L3)Cl] was prepared according to a procedure similar to that of [Pt(L1)Cl] in which L3 (149 mg, 0.27 mmol) was used instead of L1. Yield: 62.5 mg, 0.08 mmol; 35%. 1H NMR (300 B

DOI: 10.1021/acs.inorgchem.6b00619 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthetic Route and Molecular Structures of Complexes 1−4

Physical Measurements and Instrumentation. 1H NMR spectra were recorded on a Bruker DPX-300 (300 MHz), a Bruker AVANCE 400 (400 MHz), or a Bruker DRX 500 (500 MHz) Fourier transform NMR spectrometer with chemical shifts relative to that of tetramethylsilane (Me4Si). 19F{1H} NMR spectra were recorded on a Bruker AVANCE 400 (400 MHz) Fourier transform NMR spectrometer with chemical shifts relative to trichlorofluoromethane (CFCl3). All measurements were performed at 298 K unless specified otherwise. Electron impact (EI) and positive-ion fast atom bombardment (FAB) mass spectra were recorded on a Thermo Scientific DFS high-resolution Magnetic Sector mass spectrometer while positive-ion electrospray ionization (ESI) mass spectra were recorded on a Finnigan LCQ mass spectrometer. Elemental analyses were performed on a Flash EA 1112 elemental analyzer by the Institute of Chemistry at the Chinese Academy of Sciences in Beijing. UV−vis absorption spectra were recorded on a Hewlett-Packard 8452A diode array spectrophotometer. For photochromic studies, photoirradiation was carried out using a 300 W Oriel Corporation model 66011 Xe (ozone free) lamp with Applied Photophysics F 3.4 monochromator for wavelength selection. All measurements were performed at room temperature. Steady-state emission spectra at room temperature and 77 K were recorded on a Spex Fluorolog-3 model FL3-211 spectrofluorometer equipped with a R2658P photomultiplier tube (PMT) detector. For solution emission spectra, samples were degassed on a high-vacuum line in a degassing cell with a 10 cm3 Pyrex round-bottom flask connected by a side arm to a 1 cm quartz fluorescence cuvette which was sealed from atmosphere by a Rotaflo HP6/6 quick-release Teflon stopper. The samples were degassed with no fewer than four freeze−pump−thaw cycles before conducting the measurements. Solid-state emission studies at room temperature were recorded in a quartz tube loaded with solid samples inside a quartzwalled Dewar flask. Solid-state emissions at low temperature (77 K) and in butyronitrile glass were recorded similarly, with liquid nitrogen inside the optical Dewar flask. Excited-state lifetimes of solid, glass, and solution samples were measured using a conventional laser system. The excitation source was the 355 nm output (third harmonic, 8 ns) of a Spectra-Physics Quanta-Ray Q-switched GCR-150 pulsed Nd:YAG laser (10 Hz). Luminescence decay traces at a selected wavelength were detected by a Hamamatsu R928 photomultiplier tube connected to a 50 Ω load resistor, and the voltage signal was recorded on a Tektronix Model TDS 620A digital oscilloscope (500 MHz, 2 GS/s). The lifetime (τ) determination was achieved by the exponential fittings of the luminescence decay traces with the model equation, I(t) = I0 exp(−t/τ), where I(t) and I0 refer to the luminescence intensity at time = t and time = 0, respectively. Luminescence quantum yield was measured by optical dilute method which was developed by Demas and Crosby.10a A degassed aqueous solution of [Ru(bpy)3]Cl2 was used as standard10b,c at 298 K. Cyclic voltammetric measurements were performed by using a CH Instruments, Inc., model CHI 620E electrochemical analyzer interfaced to a personal computer. The

electrolytic cell used was a conventional two-compartment cell. The salt bridge of the reference electrode was separated from the working electrode compartment by a vycor glass. Electrochemical measurements were performed in dichloromethane (oxidation) and tetrahydrofuran (reduction) solutions with 0.1 mol dm−3 nBu4NPF6 as supporting electrolyte at room temperature. The reference electrode was a Ag/AgNO3 (0.1 M in acetonitrile) electrode, and the working electrode was a glassy carbon (CH Instrument) electrode with a platinum wire as a counter electrode in a compartment separated from the working electrode by a sintered-glass frit. The ferrocenium/ ferrocene couple (FeCp2+/0) was used as the internal reference.11 All solutions for electrochemical studies were deaerated with prepurified argon gas before measurement. All the cyclic voltammetric measurements were performed with scan rate at 100 mV s−1. Chemical actinometry was employed for the photochemical quantum yield determination.12 Incident light intensities were taken from the average values measured just before and after each photolysis experiment using ferrioxalate actinometry.12 In the determination of the photochemical quantum yield, the sample solutions were prepared at concentrations with absorbance slightly greater than 2.0 at the excitation wavelength. The quantum yield was determined at a small percentage of conversion by monitoring the initial rate of change of absorbance (ΔA/Δt) in the absorption maximum of the closed forms in the visible region. Crystal Structure Determination. All the experimental details are given in Table S1 in the Supporting Information. Complex 4 has been synthesized for X-ray crystal structure analysis, in which the octyl chain on the bzimb in complex 1 is replaced by a butyl chain. The single crystal was obtained by slow diffusion of diethyl ether into a concentrated solution of the complex. A crystal of dimensions 0.50 mm × 0.20 mm × 0.01 mm mounted in a glass capillary was used for data collection at 28 °C on a Bruker Smart CCD 1000 using graphite monochromatized Mo-Kα radiation (λ = 0.710 73 Å). The structure was solved by direct methods employing the SHELXS-97 program13 on PC and expanded using Fourier techniques. All non-H atoms were refined anisotropically by full-matrix least-squares using program SHELXL-9713 on PC. Detailed experimental procedures and other results including atomic coordinates, equivalent isotropic displacement parameters, bond lengths, bond angles, anisotropic displacement parameters, hydrogen coordinates, and isotropic displacement parameters for the complex were included in Tables S2−S4 in the Supporting Information.

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RESULTS AND DISCUSSION Synthesis. Complexes 1−3 were prepared by incorporation of the alkyne, TMS-CC-Th-DTE, into different chloroplatinum(II) bzimb precursor complexes in the presence of sodium hydroxide in dichloromethane and methanol under reflux conditions to give the desire complexes (Scheme 1), C

DOI: 10.1021/acs.inorgchem.6b00619 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

conformation of the complex. The interplanar angles between the dimethylthiophene rings and the thiophene core are 44.61(8)° and 51.24(11)°, respectively. Similar to the previously reported cyclometalated N^C^N platinum(II) complexes,7,14 the platinum(II) center is found to adopt a distorted square planar geometry, in which the C13Pt1N1 and C29Pt1N1 bond angles are 79.4(18)° and 100.2(17)°, respectively. The bond length of Pt1C29 is 2.06(5) Å, which is comparatively longer than that of the alkynylplatinum(II) bzimpy complexes, attributed to the strong trans effect by the strong Pt−C(bzimb) σ-bond.7g−i The Pt−Pt distance of the closest two complexes in the crystal packing is 8.10(12) Å, indicating the absence of Pt···Pt interactions in the crystalline form. Electronic Absorption and Emission Properties. The cyclometalated alkynylplatinum(II) bzimb complexes 1−3 give yellow solutions in benzene. According to previous reports on related cyclometalated alkynylplatinum(II) bzimb complexes,7g−i the high-energy intense absorption bands in the range 306−316 nm, with molar extinction coefficients in the order of 104 dm3 mol−1 cm−1, are attributed to the intraligand [π → π*] transitions of the alkynyl and cyclometalated bzimb ligands, while the lower-energy absorption bands at about 378−388 nm, with molar extinction coefficients in the order of 104 dm3 mol−1 cm−1, are ascribed as intraligand [π → π*] transitions of the cyclometalated bzimb ligand mixed with metal-to-ligand charge transfer (MLCT) [dπ(Pt) → π*(bzimb)] transitions, probably with some mixing of ligand-to-ligand charge transfer (LLCT) [π(alkynyl) → π*(bzimb)] character. It is observed that the lower-energy absorption band is red-shifted when the platinum

similar to the previously reported alkynylplatinum(II) bzimb complexes. 7g−i The identities of complexes 1−3 were confirmed by 1H NMR spectroscopy, FAB or ESI mass spectrometry, and elemental analysis. Complex 4, an analogue of complex 1, has also been synthesized and was structurally characterized by X-ray crystallography. X-ray Crystallography. The perspective drawing of complex 4 is depicted in Figure 1 showing the antiparallel

Figure 1. Perspective drawing of complex 4 with atomic numbering scheme. Hydrogen atoms are omitted for clarity. Thermal ellipsoids were shown at the 30% probability level.

Table 1. Photophysical Data for Complexes 1−3 in Both Open and Closed Forms emission

complex

configuration

absorption λabs/nm (ε/dm3 mol−1 cm−1)

medium (T/K)

1

open

306 sh (37 460), 316 (39 700), 382 (19 980), 432 sh (6870)

benzene (298)

a

solid (298) solid (77) glassd (77) closed 2

open

304 sh (37 930), 314 (39 120), 349 (39 240), 368 (51 340), 390 sh (28 110), 405 sh (25 090), 558 (11 360) 307 sh (33 960), 316 (35 850), 378 (17 670), 432 sh (7060)

benzene (298) solid (298) solid (77) glassd (77)

closed 3

open

305 sh (33 650), 315 (34 400), 350 (34 740), 370 (44 820), 390 sh (26 030), 410 (21 020), 559 (10 500) 312 sh (32 980), 320 (34 980), 388 (17 400), 440 sh (6660)

benzene (298) solid (298) solid (77) glassd (77)

closed

vibrational progressional spacing in cm−1

ϕlumc

518, 556, 602 (4.3) e e 495 (8.8), 560 (136.5)

1320, 1370

0.02

519, 558, 604 (4.2) 516, 561, 680 (0.1) 568, 605 (13.7) 497 (7.7), 570 (156.4)

1350, 1370

6.2 × 10−3

532, 572, 620 (3.8) 528, 571, 635 (0.1) 635 (8.7) 507 (7.0), 549 (129.4)

1310, 1350

λemb/nm (τo/μs)

1550 1080

0.01

1420

310 sh (32 530), 320 (34 160), 348 (38 430), 370 (41 310), 402 (21 670), 426 (19 330), 558 (10 130)

a Data obtained in benzene at 298 K. bCorrected emission maxima. cDegassed aqueous solution of [Ru(bpy)3]Cl2 at 298 K was used as standard for reporting the relative luminescence quantum yields. dIn butyronitrile glass. eNot measurable due to rapid solid-state photochromism.

D

DOI: 10.1021/acs.inorgchem.6b00619 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry center is coordinated to a more π-accepting bzimb ligand with the absorption energies in the order 2 (378 nm) > 1 (382 nm) > 3 (388 nm). The large molar extinction coefficients on the order of 104 dm3 mol−1 cm−1 also suggest the involvement of intraligand absorption character. The absorption shoulders at about 432−440 nm are lower in energy as compared to that of the previously reported chloroplatinum(II) precursor (402 nm),7g−i suggestive of the involvement of the alkynyl ligands which would lower the absorption energy. The absorption data are summarized in Table 1. Figure 2a shows the UV−vis absorption spectra of complexes 1−3.

Scheme 2. Photochemical Reaction of Complexes 1−3

Figure 2. (a) Electronic absorption spectra and (b) normalized emission spectra of the open form of complexes 1−3 in benzene solutions at 298 K. Figure 3. Electronic absorption spectral change of complex 1 in degassed benzene solutions at 298 K upon photoirradiation at λ = 390 nm.

Upon excitation at λ = 390 nm, the open forms of complexes 1−3 exhibit green emission at about 518−531 nm in degassed benzene solution (Figure 2b), with lifetimes in the microsecond regime. Together with the large Stokes shifts and the vibronicstructured band, with vibrational progressional spacing of ca. 1350 cm−1, which is typical of the aromatic vibrational modes of the bzimb ligand, the emission origin is ascribed to originate from triplet intraligand 3IL excited states of the bzimb ligand, probably mixed with triplet metal-to-ligand charge transfer 3 MLCT [dπ(Pt) → π*(bzimb)] excited-state characters, which are commonly observed from the previously reported cyclometalated alkynylplatinum(II) bzimb complexes.7g−i When a t Bu group is introduced to the 5-position of the bzimb ligand, only a slight perturbation in the emission maximum is observed while a red shift is observed when an electron-withdrawing fluoro group is introduced. The red shift is ascribed to the strong negative inductive effect of the fluoro group,15 which stabilizes the LUMO more than the HOMO of the complex, resulting in a lower emissive energy. The emission data with the corresponding luminescence quantum yields of complexes 1−3 are summarized in Table 1. Photochromic Properties. Upon photoexcitation at λ = 390 nm, the degassed benzene solutions of 1−3 are found to turn from yellow to purple with the emergence of new lowerenergy absorption bands located at ca. 560 nm, which are attributed to the increase in π-conjugation in the closed form (Scheme 2). The UV−vis spectral changes of complex 1 are depicted in Figure 3. In a comparison of the absorption spectra of their respective closed forms, as shown in Figure S1, the perturbation resulting from the variation of the substituents on the cyclometalated bzimb ligands is observed in the new absorption bands that emerge at ca. 300−450 nm. Hence, these

bands are sensitive toward electronic effects, suggestive of the involvement of the bzimb ligand. However, the new lowerenergy absorption bands at ca. 560 nm show insignificant differences among complexes 1−3, which reveals the predominant involvement of the diarylethene moiety. It is worthwhile to note that the dimethylthienyl rings are freely rotating as revealed from the 1H NMR spectra of the complexes, such that the parallel and antiparallel forms are interconvertible at room temperature. According to the Woodward−Hoffmann rule for pericyclic reactions, only the antiparallel forms of the complexes are photochromic-active and would undergo photocyclization under photoirradiation to account for the spectroscopic changes. In addition, the isosbestic points observed in the UV−vis spectra indicate the clean conversion between the open and closed forms. Upon excitation into the absorption band of the closed form in degassed benzene solutions at ca. 560 nm at 298 K, the UV−vis spectral changes are reversed, and are attributed to the regeneration of the open form. The cycloreversibility of representative complex 1 in degassed benzene solution at 298 K has been studied, demonstrating good fatigue resistance over at least five cycles of alternating excitation at 390 and 560 nm as revealed in the photocycloreversion study shown in Figure 4. Furthermore, the emission change of the representative complex 1 has been studied. Upon photoexcitation at the isosbestic wavelength, a decrease in the luminescence intensity is observed. This has been attributed to the quenching of the emissive excited state via energy transfer processes to the newly E

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Figure 4. UV−vis absorbance changes of complex 1 at 560 nm on alternate excitation at 390 and 560 nm over five cycles in degassed benzene solution at 298 K.

Figure 5. Plot of ln(A/A0) versus time for the absorbance decay of complex 2 at 560 nm at various temperatures in nitrogen-flushed toluene solution. A denotes absorbance at time t, and Ao denotes the initial absorbance; solid lines represent the linear least-squares fit.

generated closed form. The luminescence intensity change of complex 1 is depicted in Figure S2. The photochemical quantum yields of the photochemical reactions for 1−3 have been determined using an excitation wavelength of 406 nm for the forward reaction and of 500 nm for the backward reaction. Generally, it is commonly observed for photochromic diarylethene systems that the photocyclization quantum yields are larger than that of photocycloreversion.3a,c−e When comparing complexes 1−3 to the photochromic ligand TMS-CC-Th-DTE, higher photocyclization quantum yields are observed for the complexes, which could probably be ascribed to the enhanced rigidity in the complexes such that the vibrational relaxation decay process becomes less favorable. In addition, the percentage conversion at the photostationary state (PSS) has been determined, in which a higher conversion is also observed for complexes 1−3. The photochemical quantum yields and the percentage conversion at PSS are summarized in Table 2. A study on the thermal stability of the closed form has been carried out on complex 2 in which the half-life is found to be about 150 h at 288 K while it is about 34 h at 318 K. The activation energy of the thermal cycloreversion of complex 2 is found to be 30.4 kJ mol−1. The plots of ln(A/A0) versus time for the absorbance decay and ln(k) versus T−1 for complex 2 are shown in Figure 5 and Figure S3, respectively. Electrochemical Studies. Cyclic voltammetry has been performed to study the electrochemical behavior of complexes 1−3 in dichloromethane solutions (0.1 mol dm−3 nBu4NPF6). However, due to the limitation of the solvent window of dichloromethane in the reduction of the complexes, tetrahydrofuran solutions (0.1 mol dm−3 nBu4NPF6) have been used for the reductive scans. The electrochemical data are

summarized in Table 3. Complexes 1−3 showed irreversible oxidation waves (Epa) at ca. +0.68 to +1.38 V versus SCE, and a Table 3. Electrochemical Data for Complexes 1−3 in Dichloromethane (Oxidation) and Tetrahydrofuran (Reduction) Solutions (0.1 mol dm−3 nBu4NPF6) at 298 Ka complex

oxidation [Epac /V vs SCE]

reduction E1/2b V vs SCE [Epcd /V vs SCE]

1 2 3

[0.69], [1.36] [0.68], [1.38] [0.73], [1.38]

−1.77 [−1.78] −1.61

Working electrode, glassy carbon; scan rate, 100 mV s−1. bE1/2 = (Epa + Epc)/2; Epa and Epc are anodic and cathodic peak potentials, repectively. cEpa is reported for irreversible oxidation wave. dEpc is reported for irreversible reduction wave. a

quasireversible (E1/2) or irreversible reduction couple (Epc) at ca. −1.61 to −1.78 V versus SCE. With different substituent groups on the 5-position of the bzimb ligand, the first irreversible oxidation wave is found to be sensitive toward the nature of the substituent groups, in which the electronwithdrawing group would lead to a more positive potential (2 (+0.68 V) < 1 (+0.69 V) < 3 (+0.73 V)). This can be attributed to the inductive effect of the substituent group in that the electron-withdrawing group could render the metal center less electron-rich, stabilizing the HOMO making it more difficult to be oxidized. This is in line with the previously reported cyclometalated alkynylplatinum(II) bzimb complexes, in which the first irreversible oxidation wave is tentatively assigned as the

Table 2. Photochemical Quantum Yields for 1−3 and TMS-CC-Th-DTE3i in Degassed Benzene Solution at 298 K photochemical quantum yield/ϕb compd 1 2 3 TMS-CC-Th-DTEf

closed-form absorption λabs/nm a

photocyclization c

558 559 558 572

0.57 0.55c 0.58c 0.23e

photocycloreversion d

0.05 0.06d 0.05d 0.009d

conversion at PSS (%) 62.0g 62.3g 69.7g 47e

62.2h 59.1h 62.0h

a Only the lowest energy absorption peak maxima shown. bData obtained with ±10% estimated error. cData obtained by using 406 nm as the excitation wavelength. dData obtained by using 500 nm as the excitation wavelength. eData obtained by using 320 nm as the excitation wavelength. f Data adapted from ref 3i. gDetermined by UV−vis spectroscopy at PSS with 390 nm as the excitation wavelength. hDetermined by NMR spectroscopy at PSS with 390 nm as the excitation wavelength.

F

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alkynyl oxidation with the mixing of metal-centered oxidation.7g−i The second irreversible oxidation wave is not sensitive toward the nature of the substituent group and is assigned as the ethynylthiophene-centered oxidation, which is also observed in TMS-CC-Th-DTE.3i Moreover, complexes 1− 3 displayed a quasireversible (1 and 3) or irreversible (2) reduction couple which varies with the electronic properties of the bzimb ligand. The more electron-withdrawing the substituent is, the less negative the reduction potential is (2 (−1.78 V) < 1 (−1.77 V) < 3 (−1.61 V)) since it stabilizes the LUMO by inductive effect. Hence, it is assigned as the bzimb ligand-centered reduction, which is commonly observed in the related alkynylplatinum(II) bzimb complexes.7g−i Such electrochemical behavior is in good agreement with the observation in the electronic absorption studies where the lower-energy absorption bands for complexes 1−3 are assigned to intraligand [π → π*] transitions of the cyclometalated bzimb ligand mixed with metal-to-ligand charge transfer (MLCT) [dπ(Pt) → π*(bzimb)] and ligand-to-ligand charge transfer (LLCT) [π(alkynyl) → π*(bzimb)] character.

CONCLUSION A new class of photochromic cyclometalated alkynylplatinum(II) bzimb complexes has been successfully synthesized and characterized. The photophysical, photochromic and electrochemical studies have been performed. They exhibit green phosphorescence, reversible photochromism with solution color change from yellow to purple and enhanced photocyclization quantum yields relative to that of their ligand counterparts. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00619.

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REFERENCES

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Crystallographic data (CIF) Additional characterization data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.W.-W.Y. acknowledges the support from The University of Hong Kong and the URC Strategic Research Theme on New Materials. This work has been supported by the University Grants Committee Areas of Excellence Scheme (AoE/P-03/ 08) and a General Research Fund (GRF) grant from the Research Grants Council of Hong Kong Special Administrative Region, P.R. China (HKU 17305614). M.H.-Y.C. acknowledges the receipt of a postgraduate studentship and a University Postgraduate Fellowship, both administered by The University of Hong Kong. H.-L.W. acknowledges the receipt of a University Postdoctoral Fellowship. Dr. Anthony Yiu-Yan Tam is gratefully acknowledged for his helpful discussions. Technical assistance in solving the crystallographic data by Dr. Lap Szeto is gratefully acknowledged. G

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