Article pubs.acs.org/IC
1‑D “Platinum Wire” Stacking Structure Built of Platinum(II) Diimine Bis(σ-acetylide) Units with Luminescence in the NIR Region Jiajia Kang,† Xiaoxin Zhang,† Huajun Zhou,‡ Xuqiao Gai,† Ting Jia,† Liang Xu,† Jianjun Zhang,† Yanqin Li,† and Jun Ni*,† †
College of Chemistry, Dalian University of Technology, Linggong Road No. 2, Dalian 116024, P. R. China High Density Electronics Center, University of Arkansas, Fayetteville, Arkansas 72701, United States
‡
S Supporting Information *
ABSTRACT: A square-planar platinum(II) complex, Pt(DiBrbpy)(CCC6H4Et-4)2 (1) (DiBrbpy = 4,4-dibromo2,2′-bipyridine), and crystals of its three solvated forms, namely, 1·DMSO, 1·1/2(CH3CN), and 1·1/8(CH2Cl2), were developed and characterized. 1·DMSO and 1·1/2(CH3CN) contain quasi-dimeric and dimeric structures with luminescence in the visible range, whereas 1·1/8(CH2Cl2) exhibits NIR luminescence at 1022 nm due to its intrinsic 1-D “platinum wire” stacking structure with strong Pt−Pt interactions. 1·1/8(CH2Cl2) represents the first compound based on platinum(II) diimine bis(σ-acetylide) molecular units with the NIR luminescence beyond 1000 nm. 1 selectively responds to DMSO and CH3CN by changing its color and luminescence property and the three solvated forms can be reversibly converted to each other upon exposure to corresponding solvent vapors. Their desolvated forms, namely 1a, 1b, and 1c, obtained after heating 1·DMSO, 1·1/2(CH3CN), and 1·1/ 8(CH2Cl2), respectively, can also be restored to the original solvated forms upon exposure to corresponding solvent vapors. 1a and 1b emit NIR luminescence peaked at 998 and 1018 nm respectively, suggesting indirect synthetic methods as powerful alternatives to achieve NIR luminescence with long wavelength. In contrast, 1c exhibits a red luminescence with a broad unstructured emission band centered at 667 nm. All the responses to organic solvent vapors and heating are due to the structural transformations which result in the conversion of the lowest energy excited states between 3MLCT/3LLCT and 3MMLCT in solid-state as supported by time-dependent density functional theory (TD-DFT) calculations.
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INTRODUCTION Near-infrared (NIR) refers to the electromagnetic spectrum in the range of 700−2526 nm according to the American Society for Testing and Materials (ASTM). Due to its versatile advantages such as strong penetrating ability, biological safety, and visibility, NIR has found wide applications in many fields including medicine, agriculture, military, science, energy resource and industry.1 Many kinds of materials with NIR emission have been reported including lanthanide compounds,2 organic molecules,3 inorganic hybrid materials,4 quantum dots,5 nanoparticles,6 and transition metal complexes.7−17 Among them transition-metal complexes with phosphorescence in NIR range have received significant attention during the past decade due to their tunable emissions. Though a series of such transition-metal complexes including those based on Cu(I),7 Cu(II),8 Cr(III),9 Re(I),10 Re(III),11 Ru(II),12 Os(II),13 Ir(III),14 Pt(II),15 Pd(II),16 and Au(I)17 have been developed, only a few8c,12,13,15e−g,16a demonstrate NIR emission beyond 1000 nm. Since the optical transparency window for biological tissues is 1060−1150 nm,18 development of transition-metal complexes with emission in that range is of great importance. Square-planar platinum(II) complex can form different aggregates via intermolecular metal−metal interactions, leading © XXXX American Chemical Society
to decreased energy gaps between ground and excited states, decreased emission energy, and thus increased emission wavelengths.19,20 We have found that some molecules based on platinum(II) diimine bis(σ-acetylide) complexes can dimerize through Pt−Pt interactions and exhibit the low energy 3MMLCT (metal−metal-to-ligand charge transfer) emission in NIR region with the wavelength up to 766 nm.20e Recently, the 1-D “platinum wire” structure via continuous intermolecular Pt−Pt interactions in a platinum(II) cyanoximate complex15d was found to be responsible for the luminescence beyond 1000 nm. Inspired by this success, we propose the emission energy of molecules based on platinum(II) diimine bis(σ-acetylide) complexes can be further reduced by forming 1-D “platinum wire” structure via continuous intermolecular Pt−Pt interactions. In this paper, we report the use of 4,4′-dibromo-2,2′bipyridine (DiBrbpy) and 4-ethylphenylacetylene to build a new bis(σ-acetylide) platinum(II) complex, namely, [Pt(DiBrbpy)(CCC6H4-Et-4)2] (1). The two ligands were chosen due to their potential capabilities to enable the Received: June 13, 2016
A
DOI: 10.1021/acs.inorgchem.6b01426 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry formation of the 1-D “platinum wire” structure via a close packing of the platinum(II) molecule. First, both ligands are molecularly planar; second, the strong electron-withdrawing ability of Br group in DiBrbpy can decrease the electron density on the Pt(II) effectively, thereby reducing electronic repulsions between Pt(II) centers and favoring shorter Pt···Pt distances and stronger Pt−Pt interactions; third, the ethyl group on the ancillary ligand can effectively weaken intermolecular, intramolecular, and solvent-molecule hydrogen bonds. Thus, Pt···Pt and π−π interactions play a more significant role in Pt(II) molecule packing. Besides, the electron-donating nature of the ethyl group can further effectively induce the red-shift of the emission. Single-crystals of three solvated forms [1·DMSO, 1· 1/2(CH3CN), and 1·1/8(CH2Cl2)] were successfully developed and were converted to their corresponding desolvated forms (1a, 1b, and 1c) upon heating. Among them, there are rich reversible conversions including heat-induced desolvation and vapor-induced solvation and solvent exchanges. Interestingly 1·1/8(CH2Cl2), 1a, and 1b demonstrate NIR emissions with longer wavelength at about 1000 nm. All the in situ photoluminescence and PXRD analyses as well as theoretical simulations were investigated to understand the origins of these NIR emissions.
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solutions, respectively. The data of crystals was collected on a Bruker SMART APEX II CCD area detector system. Structures were solved using direct methods and refined by a full-matrix least-squares methods procedure on F2 with anisotropic thermal parameters for all non-hydrogen atoms using the SHELXTL−97 program package.22 Table S1 gives the detailed crystallographic data and structure refinement parameters. Theoretical Calculation Methodology. Time-dependent density functional theory (DFT/TD-DFT)23 with the gradient-corrected correlation functional PBE1PBE24 were used to explore the electronic and spectroscopic properties of the complex 1. The ground-state structural geometry of 1 in solution was first optimized without symmetry constrain at the DFT level of theory. And then the TD-DFT method were performed to calculate the 60 singlet and 10 triplet excited states for the determination of the vertical excitation energies based on the optimized ground-state structures. The polarized continuum model method (PCM)25 was used to mimic the influence of CH2Cl2 solvent in calculating the exicted-state properties of 1 in liquid state. In all calculations, the Stuttgart-Dresden (SDD)26 basis set consisting of the effective core potentials (ECP) and the 6-31G (p, d) polarized double-ζ basis set were used for the platinum(II) atom and the remaining nonmetal atoms, respectively. One extra f-type polarization function was used for the platinum(II) atom (αf = 0.18)27 to accurately describe the electronic structures. The TD-DFT calculations were also performed on the crystalline species 1·DMSO, 1·1/2(CH3CN), and 1·1/8(CH2Cl2) using a pair of symmetry-related platinum(II) moieties (dimeric model) with the shortest intermolecular Pt···Pt distance based on their crystal structures. Furthermore, three-, four-, and five-molecule models of 1· 1/8(CH2Cl2) mimicking the 1-D “platinum wire” structure of the complex were also employed for the TD-DFT calculations. Since the solvate molecules were found to play an insignificant role in the calculated results, all the calculations on 1·DMSO, 1·1/2(CH3CN), and 1·1/8(CH2Cl2) were performed without the solvent molecules. The Wiberg bond indices (WBIs)28 with the Natural Bond Orbital (NBO 3.1) program were used to estimate the Pt−Pt interactions.29 All calculations were done using Gaussian 09.30
EXPERIMENTAL SECTION
General Procedures and Materials. Unless noted otherwise, all reactions were performed using standard Schlenk techniques and vacuum-line systems protected by dry argon. The solvents with spectroscopic grade purity were used for spectroscopic measurements. Other solvents were purified by standard methods and degassed before use. 4,4′-Dibromo-2,2′-bipyridine (DiBrbpy) and intermediate Pt(DiBrbpy)Cl2 were prepared by following the reported procedures.21 Other commercial reagents were used without any pretreatment unless stated otherwise. Pt(DiBrbpy)(CCC6H4Et-4)2 (1). A mixture containing Pt(DiBrbpy)Cl2 (116 mg, 0.20 mmol), 4-ethylphenylacetylene (65 mg, 0.50 mmol), CuI (1 mg), and diisopropylamine (2 mL) in 50 mL of CH2Cl2 was stirred overnight at room temperature. The mixture was dried under vacuum and then subjected to column chromatography over silica gel (300−400 mesh) and eluted with dichloromethane to afford the pure product. Yield: 79%. Anal. Calcd for C30H24Br2N2Pt: C, 46.95; H, 3.15; N, 3.65. Found: C, 46.98; H, 3.19; N, 3.62. ESI-MS (m/z): 768 [M + H] +. 1H NMR (d6-DMSO, ppm): 9.37 (d, 2H, J = 8.0 Hz, bpy), 9.12 (s, 2H, bpy), 8.20 (d, 2H, J = 8.0 Hz, bpy), 7.26 (d, 4H, J = 8.0 Hz, C6H4), 7.12 (d, 4H, J = 8.0 Hz, C6H4), 2.56(q, 4H, J = 8.0 Hz, -CH2CH3), 1.15(t, 6H, J = 8.0 Hz, −CH2CH3). IR (KBr disk, cm−1): 2114s (CCPt). Physical Measurements. Infrared (IR) spectrum of KBr pellet of complex was obtained from a Magna 750 FT-IR spectrophotometer. A Bruker Advance III (400 MHz) spectrometer was used to acquire 1H NMR spectra with SiMe4 as the internal reference, while a PerkinElmer Lambda 25 UV−vis spectrophotomet was used to acquire UV−vis absorption spectra. A Finnigan LCQ mass spectrometer was used to acquire electrospray ionization mass spectra (ESI-MS) and dichloromethane-methanol was used as the mobile phases. Elemental analysis of C, H and N were obtained from a PerkinElmer model 240 C elemental analyzer. Luminescent properties were measured on the PerkinElmer LS55 luminescence spectrometer and Edinburgh analytical instrument (F910 fluorescence spectrometer). The emission quantum yield (Φem) in degassed dichloromethane solution was measured and calculated using a degassed acetonitrile solution of [Ru(bpy)3](PF6)2 as the standard (Φem = 0.062). Crystal Structural Determination. A crystal of 1·1/8(CH2Cl2) was obtained from diffusing petroleum ether onto its dichloromethane solution. Crystals of 1·DMSO or 1·1/2(CH3CN) were obtained by cooling their hot saturated corresponding DMSO and CH3CN
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RESULTS AND DISCUSSION Preparation and Characterization. Complex 1 was synthesized from the reaction between Pt(DiBrbpy)Cl2 and 4-ethylphenylacetylene in the solvent media of CH2Cl2 in the presence of i-Pr2NH and CuI according to Sonogashira’s method. The complex was fully characterized by 1HNMR, elemental analyses, ESI-MS, IR spectroscopy, and by X-ray crystallography for 1·VOCs (VOCs = DMSO, CH3CN and CH2Cl2). Complex 1 can get dissolved in most common organic solvents and have excellent stability in both solution and solid states. The crystal structures of 1·DMSO, 1·1/2(CH3CN), and 1·1/ 8(CH2Cl2) were determined by X-ray crystallography. In all of them, each platinum(II) center is coordinated by two N atoms from a chelating bipyridyl ligand and two C atoms from 4ethylphenylacetylene ligands in a distorted square-planar coordination geometry (Figure 1), and the bond distances of Pt−N [2.039(7)−2.074(5) Å] and Pt−C [1.9055(6)− 1.9797(7) Å] match those reported for diimine platinumalkynyl complexes (Table S2, Supporting Information). In 1· DMSO, two neighboring platinum(II) moieties with Pt···Pt distance of 4.845 Å display an antiparallel packing pattern and form a quasi-dimeric structure via intermolecular π−π stacking interactions between the pyridine group and alkynyl group from two neighboring ligands (Figure 1a, Figure S1a and Table S3 in the Supporting Information). Neighboring quasi-dimeric structures are well separated by DMSO solvent molecules and no direct interaction exists between them. In 1·1/2(CH3CN), B
DOI: 10.1021/acs.inorgchem.6b01426 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
with no weak interactions such as hydrogen bonds and π−π stacking interactions. The formation of 1-D “platinum wire” structure has been reportedly affected by many competing factors including Pt···Pt interactions, ligand-field strength, π-donor/acceptor characteristics of ligands, steric effects, and intermolecular hydrogen bonds.31 Herein we increased the strength of Pt···Pt interactions through introducing a strong electron-withdrawing Br group in the ligand, and decreased the strength of hydrogen bond through introducing an electron-donating ethyl group on the ancillary ligand. Furthermore, hydrogen bond strength was further adjusted by choosing different guest solvent molecules. In 1·DMSO and 1·1/2(CH3CN), strong multihydrogen bond was formed between the solvent molecules and the Pt(II) moieties (Figure S3 and Table S3 in the Supporting Information) so that the Pt(II) moieties were well separated by the solvent molecules and could not form 1 D “platinum wire” structure (Figure S4 in the Supporting Information). In contrast, in the absence of such interaction between CH2Cl2 and Pt(II) moiety in 1·1/8(CH2Cl2) (Figure S5, Supporting Information), the Pt(II) units can be packed closely to afford the 1 D structure. The π−π stacking interaction also plays a key role in the molecular packing of the platinum(II) complexes. 1· DMSO and 1·1/8(CH2Cl2) only have pyridine-alkynyl π−π interaction, while 1·1/2(CH3CN) has both pyridine-alkynyl and pyridine-benzene π−π interactions. However, the packing of the Pt(II) moieties of 1·1/8(CH2Cl2) is closest among the three phases, as evidenced by the moiety separation and the interplanar distance (Figure S1−S2, and Figure S6, in the Supporting Information). All above factors result in the 1D structure of 1·1/8(CH2Cl2). Photophysical Properties. The solution of 1 in CH2Cl2 exhibited high-energy absorption peaks at about 241, 265, 275, and 291 nm, together with broad low-energy absorption peaks at ca. 429 and 469 nm (Figure 2a). The high-energy bands mainly arise from intraligand (1IL) and metal-centered (1MC) transitions as well as some ligand-to-ligand charge transfer (1LLCT) and metal-to-ligand charge transfer (1MLCT) transitions. The broad low-energy absorption mostly likely results from a mixture of 1MLCT [dπ(Pt) → π*(DiBrbpy)] and 1LLCT [π(CCPhEt-4) → π*(DiBrbpy)] transitions as revealed from TD-DFT studies (vide infra) and further confirmed by the negative solvatochromism of the low-energy absorption band (Figure S7, Supporting Information).32 In degassed CH2Cl2 solution at ambient temperature, 1 displays a bright and featureless red luminescence peak at 640 nm with
Figure 1. Crystal packing diagrams of adjacent platinum moieties in 1· VOCs. (a) 1·DMSO, (b) 1·1/2(CH3CN), and (c) 1·1/8(CH2Cl2). Solvate molecules and hydrogen atoms are omitted for clarity. Symmetry code: a. 1-x, -y, -z; b, 1-x, 1-y, 1-z.
adjacent Pt(II) moieties are also arranged in an antiparallel pattern and form dimers through π−π interactions with the shortest Pt···Pt distance being 4.882 Å (Figure 1b). Neighboring dimers are perpendicularly packed (dihedral angle is 89.47°) and connected via their hydrogen bonds with CH3CN solvent molecules and π−π stacking interactions among dimers (Figure S1b, S2 and Table S3, Supporting Information). The large Pt···Pt separations in both 1·DMSO and 1·1/2(CH3CN) reveal the absence of metallophilic interaction in both structures. In 1·1/8(CH2Cl2), platinum(II) moieties are paralleled to each other and display the alternate arrangement of antiparallel and staggered packing patterns through intermolecular Pt−Pt contacts and π−π stacking interactions between bipyridyl ligand and alkynyl group to afford the 1-D “platinum wire” (Figure 1c, Figure S1c and Table S3 in the Supporting Information). The much shorter Pt···Pt distances (3.341 Å for antiparallel packing pattern and 3.438 Å for staggered packing pattern) indicate the presence of strong metal−metal contacts in 1·1/8(CH2Cl2). Furthermore, the 1-D “platinum wires” are well separated from each other
Figure 2. Absorption (a) and emission (b) spectra of solid sample 1·DMSO (green), 1·1/2(CH3CN) (red), 1·1/8(CH2Cl2) (black), and 1 in CH2Cl2 solution (blue) at ambient temperature. C
DOI: 10.1021/acs.inorgchem.6b01426 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry the lifetime and quantum yield being 0.35 μs and 2.14% (Figure 2b and Table 1). The somehow solvent-dependent emission
8(CH2Cl2) has a smaller HOMO−LUMO energy gap than that of 3MLCT state in the other two compounds and normal 3 MMLCT with dimeric structure.19 Although complexes containing 1D platinum-wire structure always exhibit lowenergy 3MMLCT luminescence, only a few of them have their luminescence beyond 1000 nm. Compared to those complexes, 1·1/8(CH2Cl2) additionally employs a proper ligand to successfully realize the long wavelength emission. The strong electron-withdrawing ability of Br group and the electrondonating nature of ethyl group were introduced into bipyridine ligand and ancillary phenylacetylene ligand, respectively. On one hand, these two substituents will not influence the planarity of the platinum molecule and the formation of 1D “platinum wire” stacking structure; on the other hand, they can further reduce the energy of 3MMLCT exited states. Both factors jointly lead to the long wavelength NIR luminescence of 1·1/ 8(CH2Cl2). Vapor- and Heat-Induced Structural and Spectral Changes. Figure 3a shows the photographic images when 1
Table 1. Luminescence Data of 1 in Different States at Ambient Temperature sample
medium
λem (nm)
τem (μs)
Φem (%)
1 1·DMSO 1a 1·1/2(CH3CN) 1b 1·1/8(CH2Cl2) 1c
CH2Cl2 solution crystalline solid state crystalline solid state crystalline solid state
640 540, 570sh 998 625 1018 1022 667
0.35 2.47 4.91 2.28 1.71 2.09 2.27
2.14a 2.54
