Influence of the Diphosphine Coordination Mode on the Structural and

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Influence of the Diphosphine Coordination Mode on the Structural and Optical Properties of Cyclometalated Platinum(II) Complexes: An Experimental and Theoretical Study on Intramolecular Pt···Pt and π···π Interactions S. Reza Barzegar-Kiadehi,† Mohsen Golbon Haghighi,*,† Mahboubeh Jamshidi,‡ and Behrouz Notash† †

Department of Chemistry, Shahid Beheshti University, Evin, Tehran 19839-69411, Iran Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany

‡

S Supporting Information *

ABSTRACT: The reaction of [Pt(C^N)(CF3CO2)(SMe2)] (1), in which C^N is either benzo[h]quinolinate (bhq), 1a, or 2-phenylpyridinate (ppy), 1b, with 1 equiv of bis(diphenylphosphino)methane (dppm) gave the bischelate complexes [Pt(C^N)dppm]CF3CO2 (2). The binuclear complexes [Pt2(C^N)2(CF3CO2)2(μ-dppm)] (3) were prepared, using an unusual reaction pathway, by the addition of equimolar amount of complexes 1 and 2, through the ring opening of the chelating dppm ligand and coordination of the CF3CO2 anion to the platinum center. The proposed reaction pathway and effect of the solvent polarity were investigated by density functional theory (DFT) calculations. The crystal structure of 3a shows considerable intramolecular Pt···Pt and π···π interactions. The crystal structure and formation pathway toward 3 were compared with the similar analogue [Pt2(bhq)2(Cl)2(μ-dppm)] (5). All complexes were fully characterized using multinuclear NMR spectroscopy and elemental analysis. Furthermore, the crystal structures of some complexes including 1b, 2a, 2b, 3a, and 5 were confirmed by X-ray crystallography. The effect of dimerization via a change in the coordination mode of dppm, from a chelate mode in complex 2 to a bridge mode in complexes 3 and 5, upon the excited states of the studied compounds was investigated in their distinguished absorption and emission profiles. The appearance of a remarkably low energy band in the absorption spectra of 3, which was assigned to a metal−metal to ligand charge transfer [MMLCT; dσ*(Pt2) → π*(C^N)] transition showing negative solvatochromism, is important evidence for the Pt···Pt intramolecular interaction. The vibronically resolved and long-lifetime emission of 2a in poly(methyl methacrylate) media and powder states at 77 and 300 K, along with time-dependent DFT calculations, suggested that the triplet ligand-centered (3LC) emission was mixed with some 3MLCT character. Unstructured and short-lifetime emission in 3 refers to the phosphorescence 3MMLCT [dσ*(Pt2) → π*(C^N)] transition. Although complex 5 is a binuclear compound, the long distance of the Pt···Pt interaction caused the occurrence of the 3MMLCT transition to fade and act as a mononuclear unit, and the emission originated mostly from the 3MLCT transition. As a result, more metal participation leads to more red-shifted absorption and emission spectra of the studied complexes upon going from LC to MLCT to MMLCT transitions (λabs and λem: 3a > 3b > 5 > 2a > 2b).

1. INTRODUCTION Cyclometalated organometallic compounds have been extensively investigated by many research groups because of their catalytic reactivity in organic synthesis and diverse medical and industrial applications.1−6 In particular, cyclometalated platinum(II) complexes have been widely developed for their applications in organic light-emitting diodes (OLEDs),7−9 vapor sensing,10 oxygen sensing11,12 and DNA applications.13,14 However, most applications are restricted to mononuclear cyclometalated organoplatinum(II) complexes,15 in which different kinds of electronic transitions, such as metal to ligand charge transfer (MLCT), ligand-centered (LC), and ligand to ligand charge transfer (L′LCT), are involved in any related emission operation. Also, the molecular interactions in platinum(II) complexes lead to significantly red-shifted long-wavelength © XXXX American Chemical Society

emission, which is of considerable interest in near-infrared (NIR) absorbing and emitting organic materials and potentially applied in optical telecommunication platforms, sensing, and bioimaging16 in recent decades. Although it is still difficult to design extended π-conjugated compounds, one way to create a red-shifted spectrum is expansion of the π-conjugation length in a cyclometalating ligand.17,18 Furthermore, binuclear platinum complexes have drawn attention because of their particular photophysical properties.19−25 Metal−metal to ligand charge transfer (MMLCT) transitions exist in binuclear platinum(II) complexes when the Pt···Pt distance is less than 3.5 Å (the sum of two platinum van der Waals Received: January 15, 2018

A

DOI: 10.1021/acs.inorgchem.8b00137 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

in which R is either methyl or p-MeC6H4 and C^N is either benzo[h]quinolinate (bhq) or 2-phenylpyridinate (ppy),38 [Pt(C^N)(CF3CO2)(SMe2)] (1a or 1b, respectively),39 [Pt(bhq)Cl(DMSO)] (4; DMSO = dimethyl sulfoxide),40 and [Pt2(bhq)2(Cl)2(μ-dppm)] (5),41 were synthesized similar to the literature methods. 2.2. Synthesis of the Complexes. [Pt(bhq)(dppm)]CF3CO2 (2a). To a solution of 1a (54.8 mg, 0.1 mmol) in dichloromethane (10 mL) was added dppm (38.4 mg, 0.1 mmol). The mixture was stirred at room temperature for 1 h. The solvent was removed, and the residue was triturated with cold diethyl ether (Et2O; 2 × 2 mL). The product as a yellow solid was dried under vacuum. Yield: 74 mg, 86%. Anal. Calcd for C40H30F3NO2P2Pt (870.7): C, 55.2; H, 3.47; N, 1.61. Found: C, 55.6; H, 3.56; N, 1.46. 1H NMR: δ 4.95 (t, 2JP−H = 10.3 Hz, 2H, CH2 fragment of the dppm ligand); aromatic protons, δ 8.67 (br t, 1H, H2), 8.53 (d, 3JH−H = 8.1 Hz, 1H, H4); other aromatic protons of the bhq and dppm ligands, δ 7.2−8.0. 31P NMR: δ −27.6 (d, 1JPt−P = 1450 Hz, 2JP−P = 43 Hz, P trans to C of the phenyl ring of bhq), −32.1 (d, 1JPt−P = 3360 Hz, 2JP−P = 42 Hz, P trans to N of the pyridine ring of bhq). The suitable crystal for X-ray diffraction structural determination was obtained by trichloromethane (CHCl3)/Et2O diffusion. [Pt(ppy)(dppm)]CF3CO2 (2b). This was prepared similarly to complex 2a using 1b. Yield: 72 mg, 85%. Calcd for C38H30F3NO2P2Pt (846.7): C, 53.9; H, 3.57; N, 1.65. Found: C, 54.5; H, 3.55; N, 1.26. 1 H NMR: δ 4.85 (t, 2JP−H = 10.5 Hz, 2H, CH2 fragment of the dppm ligand); aromatic protons, δ 8.37 (br t, 1H, H2); other aromatic protons of the ppy and dppm ligands, δ 6.9−8.1. 31P NMR: δ −25.7 (d, 1JPt−P = 1407 Hz, 2JP−P = 37 Hz, P trans to C of the phenyl ring of ppy), −33.6 (d, 1JPt−P = 3389 Hz, 2JP−P = 37 Hz, P trans to N of the pyridine ring of ppy). The suitable crystal for X-ray diffraction structural determination was obtained by CHCl3/n-hexane diffusion. [Pt2(bhq)2(CF3CO2)2(μ-dppm)] (3a). To a solution of 1a (27.4 mg, 0.05 mmol) in benzene (5 mL) was added 2a (43.5 mg, 0.05 mmol). The mixture was stirred at room temperature for 2 days. The solvent was removed, and the residue was triturated with Et2O (2 × 2 mL). The product as a green solid was dried under vacuum. Yield: 55 mg, 81%. Calcd for C55H38F6N2O4P2Pt2 (1357.02): C, 48.6; H, 2.82; N, 2.06. Found: C, 48.8; H, 3.05; N, 1.89. 1H NMR: δ 4.40 (t, 2JP−H = 13.6 Hz, 2H, CH2 fragment of the dppm ligand); aromatic protons, δ 7.14 (d, 3 JH−H = 7.2 Hz, 3JPt−H = 70 Hz, 2H, H9), 8.05 (d, 3JH−H = 8.0 Hz, 2H, H4), 8.40 (br t, 2H, H2); other aromatic protons of the bhq and dppm ligands, δ 6.4−8.0. 31P NMR: δ 6.64 (s, 1JPt−P = 4490 Hz, 3JPt−P ≈ 30 Hz, Pt−P). The suitable crystal for X-ray diffraction structural determination was obtained by the slow evaporation of its benzene solution. [Pt2(ppy)2(CF3CO2)2(μ-dppm)] (3b). This was prepared similarly to complex 3a using the appropriate precursors. Yield: 48 mg, 75%. Calcd for C51H38F6N2O4P2Pt2 (1309.0): C, 46.8; H, 2.93; N, 2.14. Found: C, 46.5; H, 3.22; N, 2.06. 1H NMR: δ 4.17 (t, 2JP−H = 12.6 Hz, 2H, CH2 fragment of the dppm ligand); aromatic protons, δ 8.30 (br t, 2H, H2); other aromatic region protons of the ppy and dppm ligands, δ 6.4−8.0. 31 P NMR: δ 8.48 (s, 1JPt−P = 4470 Hz, 3JPt−P = 32 Hz, Pt−P). 2.3. Computational Details. Density functional theory (DFT) calculations were performed with the program suite Gaussian 0942 using the B3LYP level of theory. The LANL2DZ basis set43 was chosen to describe the platinum atom. The 6-31G(d) basis set was used for other atoms. The geometries of complexes for elucidation of the reaction mechanism were fully optimized by employing the DFT without imposing any symmetry constraints. To evaluate and ensure the optimized structures of the molecules, frequency calculations were carried out using analytical second derivatives. In all cases, only real frequencies were obtained for the optimized structures. Solvent effects have been taken into account using the polarizable continuum model.44,45 The computations of the electronic absorption spectra using time-dependent DFT (TD-DFT) were carried out at the same level. Percentage compositions of molecular orbitals and theoretical absorption spectra were plotted using the Chemissian program. 2.4. X-ray Structure Determination. The X-ray diffraction measurements were carried out on a STOE IPDS-II or IPDS-2T diffractometer with graphite-monochromated Mo Kα radiation. All single crystals were mounted on a glass fiber and used for data collection. Cell constants and an orientation matrix for data collection were

radius), leading to marked red shifts compared to the corresponding emission peaks of mononuclear platinum complexes.26 Although mononuclear platinum(II) complex units tend to interact with each other in the solid state and in concentrated solutions, Pt···Pt interactions are generally not strong enough to emerge in a diluted solution at room temperature.27 Moreover, even in the solid state, fine control of the Pt···Pt interactions is rather difficult for self-assembled mononuclear platinum(II) complexes. From this viewpoint, double-decker platinum(II) complexes with well-defined Pt···Pt distances, built by bridging ligands, are very attractive. Furthermore, Pt···Pt distances can be precisely controlled using the appropriate bridging ligands. The formation of MMLCT transitions is due to charge transfer between a full Pt−Pt dσ* orbital and a partially empty ligand π* orbital on the cyclometalated ligand to accomplish a significantly red-shifted spectrum.28−30 In most cases, to design these systems, double bridging ligands with N^N, N^S, and NÔ coordinating atoms were used, but the complexes with single P^P bridging ligands, such as bis(diphenylphosphino)methane (dppm), are still rare.31−33 Phosphine ligands as nonconjugated ancillary ligands have π-acceptor character and so increase the MLCT transition energy in contrast with the most oxygen- or nitrogen-donor ligands.34 In this manner, bisphosphine ligands could have dual structural reactivity, as chelating or bridging ligands. In this regard, complexes with chelating dppm could be used as a precursor to construct various types of binuclear complexes by the ring opening of a four-membered chelate ring and the formation of a monodentate dppm ligand with a free phosphine moiety in the intermediate of these reactions.35−37 The aim of the present work is to introduce several emissive mononuclear cyclometalated platinum(II) complexes with a chelating dppm ligand and a new class of binuclear complexes with a bridging dppm ligand. The luminescence properties of mononuclear complexes with chelating dppm and binuclear complexes with bridging dppm were studied; the influence of bridging dppm on its metal−metal interactions and their emission were explored.

2. EXPERIMENTAL SECTION 2.1. General Remarks. Microanalyses were performed using an Elementar CHN elemental analyzer. The NMR spectra were recorded on a Bruker Avance DPX 300 MHz spectrometer. The operating frequencies and references, respectively, are shown in parentheses as follows: 1H (300 MHz, TMS) and 31P (121 MHz, 85% H3PO4). The chemical shifts and coupling constants are given in parts per million and hertz, respectively. NMR data are in CDCl3. The UV−vis absorption spectra were recorded on a Shimadzu UV-2100 spectrophotometer in a cuvette with a 1 cm and/or 1 mm path length. Emission and excitation spectra were recorded with a Horiba Jobin-Yvon Fluorolog 3 steadystate fluorescence spectrometer at 300 and 77 K. This spectrometer was modified to allow for measurements of emission decay times. A PicoQuant FB-375 pulsed-diode laser (λexc = 378 nm; pulse width 100 ps) was applied as the excitation source. The emission signal was detected with a cooled photomultiplier attached to a FAST ComTec multichannel scalar card with a time resolution of 250 ps. Photoluminescence (PL) quantum yields were determined using a Hamamatsu system for absolute PL quantum yield measurements (type C9920-02) equipped with an integrating sphere with a Spectralon inner surface coating. Polymer films containing about 1 wt % of the platinum complexes were obtained by dissolving the emitter and poly(methyl methacrylate) (PMMA) in dichloromethane and spin-coating the solutions onto quartz glass substrates. All measurements were performed under a continuous flow of nitrogen gas in order to minimize emission quenching by oxygen. The monomeric precursors [Pt(R)(C^N)(SMe2)], B


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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement 1b formula Mt [g mol−1] T [K] λ [Å] cryst syst space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z ρ [Mg m −3] μ [mm−1] F(000) cryst size [mm] θ range [deg] limiting indices collected data unique data R(int) abs corr R1, wR2 [I > 2δ (I)] R1, wR2 (all data)

2a.CHCl3·H2O

3a·2C6D6

2b·CHCl3

5

C15H14F3NO2PtS 524.41 120(2) 0.71073 monoclinic P21/c 11.420(2) 8.4832(17) 16.659(3) 90 100.04(3) 90 1589.2(5) 4 2.192 8.999 992 0.20 × 0.20 × 0.05

C41H33Cl3F3NO3P2Pt 1008.04 120(2) 0.71073 monoclinic P21/c 13.529(3) 21.548(4) 14.226(3) 90 115.21(3) 90 3752.2(16) 4 1.781 4.096 1976 0.20 × 0.18 × 0.10

C39H31Cl3F3NO2P2Pt 966.03 298(2) 0.71073 triclinic P1̅ 12.377(3) 13.795(3) 14.286(3) 108.61(3) 101.12(3) 115.79(3) 1917.7(13) 2 1.673 4.001 948 0.50 × 0.40 × 0.30

C67H38D12F6N2O4P2Pt2 1525.26 120(2) 0.71073 triclinic P1̅ 13.466(3) 14.377(3) 14.7 88(3) 86.65(3) 79.18(3) 85.36(3) 2800.1(11) 2 1.795 5.121 1476 0.38 × 0.18 × 0.15

C51H38Cl2N2P2Pt2 1201.84 120(2) 0.71073 triclinic P1̅ 9.3847(19) 14.942(3) 16.820(3) 63.85(3) 79.87(3) 83.86(3) 2083.0(9) 2 1.916 6.953 1156 0.20 × 0.20 × 0.20

1.81−25.00

2.47−25.00

2.42−25.00

2.46−25.00

2.42−25.00

−13 ≤ h ≤ 13, −10 ≤ k ≤ 9, −19 ≤ l ≤ 19 7186

−13 ≤ h ≤ 16, −22 ≤ k ≤ 25, −16 ≤ l ≤ 16 17532

−14 ≤ h ≤ 14, −16 ≤ k ≤ 16, −16 ≤ l ≤ 16 13843

−16 ≤ h ≤ 16, −16 ≤ k ≤ 17, −17 ≤ l ≤ 17 20974

−10 ≤ h ≤ 11, −17 ≤ k ≤ 17, −20 ≤ l ≤ 20 15555

2775 0.1315 numerical 0.0574, 0.1255

6548 0.0942 numerical 0.0511, 0.0876

6716 0.0659 numerical 0.0435, 0.0819

9826 0.0637 numerical 0.0350, 0.0684

7295 0.1398 numerical 0.0671, 0.0919

0.0922, 0.1353

0.0923, 0.0993

0.0712, 0.0863

0.0532, 0.0739

0.1459, 0.1070

Scheme 1. Synthetic Route for the Preparation of Bischelate Complexes 2a and 2b

obtained by a least-squares refinement of the diffraction data. Diffraction data were collected in a series of ω scans in 1° oscillations and integrated using the STOE X-AREA46 software package. A numerical absorption correction was applied using X-RED47 and X-SHAAPE48 software. The data were corrected for Lorentz and polarization effects. The structures were solved by direct methods49 and subsequent difference Fourier maps and then refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters.50 Atomic factors are from the International Tables for X-ray Crystallography.51 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. All refinements were performed using the XSTEP32 crystallographic software package.52 Atomic coordinates and displacement parameters are deposited with the Cambridge Crystallographic Data Centre. CCDC 1813692 (5), 1813693 (3a), 1813694 (2b), 1813695 (2a), and 1813696 (1b) contain the supplementary crystallographic data for this paper (Table 1).

(2) is depicted in Scheme 1. The known starting cyclometalated organoplatinum(II) complexes [Pt(R)(C^N)(SMe2)],38,53 in which R is methyl or p-MeC6H4 and C^N is either deprotonated benzo[h]quinoline (bhq) or deprotonated 2-phenylpyridine (ppy), were reacted with 1 equiv of trifluoroacetic acid (CF3CO2H), yielding cleavage of the Pt−C bond of the Pt−R moiety and preparing cycloplatinated(II) complexes [Pt(C^N)CF3CO2(SMe2)] (1).39 The structure of the typical cycloplatinated(II) complex 1b has been confirmed by X-ray structure determination (Figure 1i). The suitable crystals were grown through CH2Cl2/n-hexane diffusion. The complex exhibits a distorted square-planar geometry around the platinum center in which the chelating ppy bite angle (N1−Pt1−C11) is 81.9°. This angle is significantly smaller than 90°, which implies that the chelates are probably under strain. Also, through metal-π interaction, complexes were assembled to each other, as shown in Figure 1ii (the mean interplanar separation is 3.4 Å). The cycloplatinated complexes 1 were reacted with 1 equiv of dppm to give the bischelate complexes 2, in which first the

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. The synthetic route to prepare the bischelate complexes [Pt(C^N)dppm]CF3CO2 C


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Inorganic Chemistry

Figure 1. (i) ORTEP representation and (ii) intermolecular interaction for 1b. Ellipsoids are drawn at the 30% probability level, and hydrogen atoms are shown as spheres of arbitrary radius. Selected geometrical parameters (Å and deg): Pt1−C11 1.972(15), Pt1−N1 2.043(12), Pt1−O1 2.118(13), Pt1−S1 2.260(4); C11−Pt1−N1 81.9(5), C11−Pt1−O1 171.4(6), N1−Pt1−O1 89.6(5), C11−Pt1−S1 92.5(4), N1−Pt1−S1 174.3(4), O1−Pt1−S1 96.1(4).

π−π stacking. The geometrical parameters of complexes 2 have good coincidence with similarly reported structures with a PF6 counteranion.54,55 For preparing the binuclear complexes, as a usual procedure, the cycloplatinated complexes 1 were reacted with 0.5 equiv of dppm, but it was surprising that 0.5 equiv of bischelate complexes 2 was formed and 0.5 equiv of unreacted precursors 1 remained. However, after several days of keeping the reaction mixture in solvent, the formation of binuclear complexes 3 occurred (Scheme 2). For a further study on this reaction, 1 equiv of bischelate complexes 2 was added to 1 equiv of cycloplatinated complexes 1 in various solvents, followed by NMR spectroscopy. In a nonpolar solvent, such as benzene, all precursor peaks completely disappeared after 2 days and the peaks of the binuclear complex were completely obvious; meanwhile, no intermediates were detectable. Typically, a triplet signal with broad platinum satellites at δ 4.40, 2JPH = 13.6 Hz, due to CH2 protons of bridging dppm was observed in the 1H NMR spectrum (Figure S5) of 3a. Its 31P NMR spectrum contained a single resonance [δ 6.6, 1JPtP = 4490 Hz, 3JPtP ≈ 30 Hz], illustrated in Figures 3iii and S6. The satellite spectra arise from coupling to 195Pt, with both 1JPtP and 3 JPtP couplings resolved in these spectra. A comparison of the 1JPtP value of this complex by similar previously reported structures with bridging dppm38,41 demonstrates that this great value of 1 JPtP corresponds to its trans ligand with lower trans influence, such as a nitrogen atom of the bhq ligand. Also, in polar solvents, such as dichloromethane or acetonitrile, the reaction progress is much slower than that in benzene, and after a week, large amounts of precursors were observed in the NMR spectra of the reaction mixture. Also, the single-crystal structure of 3a was confirmed by the X-ray crystallography method (Figure 4i). In comparison with similar previously reported crystal structures, the bridging dppm ligand in the aryl-cycloplainated analogues38 adopts an anti conformation in which the two square-planar platinum centers are well separated (Pt1···Pt2 = 6.73 Å) and allow intermolecular π···π interaction. The structure of 3a shows a bent conformation with the Pt(bhq)(CF3CO2)(PPh2) moieties situated face-toface. On the other hand, the methyl-cycloplatinated analogues56

diphosphine ligand displaces SMe2 and then the CF3CO2 ligand is disposed from the coordination sphere to form the cationic complexes 2. The complexes 2 were characterized in solution using 1H and 31 P NMR spectroscopies and in the solid state by single-crystal X-ray determination. Typically, in the 1H NMR spectrum of complex 2a, a signal due to CH2 protons of the chelating dppm was observed as a triplet at δ 4.95, which was accompanied by broad platinum satellites. The other assignable signal in the aromatic region was due to the CH proton adjacent to the ligating nitrogen atom of bhq, appearing as a broad triplet at δ 8.67 with broad platinum satellites (Figure S1). The 31P NMR spectrum of this complex (2a) shows two different phosphorus atoms appearing as two doublet signals flanked by platinum satellites. The coordinated phosphorus atom of dppm trans to the coordinated nitrogen atom of bhq appears at δ −32.1 with 1 JPtP = 3360 Hz and 2JPP = 42 Hz. At δ −27.6, the coordinated phosphorus atom of dppm trans to the coordinated carbon atom of bhq with a much lower value of 1JPtP = 1450 Hz is seen (Figure S2). The smaller value of 1JPtP is due to the significantly greater trans influence of the coordinated carbon atom compared to the nitrogen atom. Also, the geometry of complex 2a in the crystal structure (Figure 2i) is best described as distorted square-planar because the chelating bhq and dppm bite angles; i.e., N1−Pt1− C12 and P1−Pt1−P2 are respectively 81.12° and 71.74°, being significantly smaller than 90° and implying that the chelates are probably under strain. The Pt1−P1 bond length (2.33 Å, trans to the carbon atom of the phenyl ring) is longer than the Pt1−P2 bond length (2.25 Å, trans to the nitrogen atom of the pyridyl ring), complying with the carbon atom having a higher trans influence than that of the nitrogen atom and consistent with the Pt−P coupling constant values in 31P NMR spectroscopy. In addition, different types of intermolecular and intramolecular π stacking of the aromatic groups to form aggregations are also illustrated in Figure 2ii. Also, similar parameters were observed in the crystal structure of 2b, as shown in Figure 2iii,iv. One of the major differences between these two crystal structures is the interplanar separation of their cyclometalating ligands; in 2a, this separation is ≈0.4 Å less than that in 2b, which could be due to the larger π-conjugated system in bhq and stronger D


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Figure 2. (i and iii) ORTEP representations and (ii and iv) π−π-stacking interactions for 2a and 2b, respectively. Ellipsoids are drawn at the 30% probability level, hydrogen atoms are shown as spheres of arbitrary radius, and cocrystallized solvents are omitted for clarity. Selected geometrical parameters for 2a (Å and deg): Pt1−C12 2.056(9), Pt1−N1 2.101(6), Pt1−P2 2.252(2), Pt1−P1 2.325(2); C12−Pt1−N1 81.1(3), C12−Pt1−P2 102.5(2), N1−Pt1−P2 171.0(2), C12−Pt1−P1 174.1(2), N1−Pt1−P1 104.8(2), P2−Pt1−P1 71.75(8). Selected geometrical parameters for 2b (Å and deg): Pt1−C1 2.050(6), Pt1−N1 2.085(6), Pt1−P1 2.2596(19), Pt1−P2 2.3059(19); C1−Pt1−N1 80.1(3), C1−Pt1−P1 102.5(2), N1−Pt1−P1 177.2(2), C1−Pt1−P2 174.3(2), N1−Pt1−P2 104.4(2), P1−Pt1−P2 72.90(7).

Scheme 2. Synthetic Route for the Preparation of Binuclear Complexes 3a and 3b

dppm ligand and coordination of the CF3CO2 anion to the platinum center. Then the free phosphine moiety of the monodentate dppm ligand could react with the cycloplatinated complex 1 and substitutes with dimethyl sulfide, as a good leaving group. The intermediate may have two possible structures, as depicted in Figure 5i; however, as was mentioned earlier, the Pt−P bond in bischelate complexes 1 has different strengths and depends on the trans influence of the atoms. So, the phosphine trans to carbon atom of the cyclometalated ligand could have a longer bond length and it could be broken more easier than the other one. Also, the DFT calculations confirm that IM-1 is around 35 kJ mol−1 more stable than IM-2 in various solvents. Furthermore, the reactions for the preparation of binuclear complexes 3 have different rates in several solvents. For example, in benzene, all precursors disappeared after 2 days, but the NMR yield in dichloromethane was less than 50%, even after more than 10 days,

have a bent conformation similar to that for dppm but with staggered [(bhq)Pt] units, although the same units in the crystal structure of 3a have a eclipsed conformation. The torsion angle about the Pt1−Pt2 axis (defined by the angle between the N1−Pt1−Pt2 and N2−Pt2−Pt1 planes) is 19°, which shows a nearly parallel face-to-face conformation (Figure 4iii), supported by intramolecular π···π interactions. The Pt···Pt distance is 3.28 Å, which falls within the range of intermetal distances (3.09−3.50 Å) observed in monomeric platinum(II) linear-chain structures.57 In addition, through intermolecular π stacking between parallel bhq groups, the binuclear complex 3a could assemble to give a supramolecular polymer structure (Figure 4ii). The reaction pathway in different solvents was investigated by DFT calculations (Figure 5). The intermediate [Pt(C^N)(CF3CO2)(η1-dppm)] (IM) would be considered for this reaction, which could be formed by ring opening of the chelating E


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Inorganic Chemistry

Figure 3. 31P NMR spectra of the reaction of 1a with 0.5 equiv of dppm in benzene (i) after the addition, (ii) after 1 day, and (iii) after 2 days. The signal assignments are shown. The asterisk is shown for the impurity.

Figure 4. (i) ORTEP representation, (ii) intramolecular and intermolecular π−π-stacking interaction, and (iii) view through the Pt1−Pt2 axis for 3a. Ellipsoids are drawn at the 30% probability level. Hydrogen atoms and cocrystallized benzene molecules are omitted for clarity. Selected geometrical parameters for 3a (Å and deg): Pt1−C10 2.016(6), Pt1−N1 2.080(5), Pt1−O1 2.133(4), Pt1−P1 2.2262(16), Pt2−C23 2.017(6), Pt2−N2 2.094(5), Pt2−O3 2.113(4), Pt2−P2 2.2265(16); C10−Pt1−N1 81.7(2), C10−Pt1−O1 169.18(19), N1−Pt1−O1 88.09(18), C10−Pt1−P1 96.51(17), N1−Pt1−P1 177.94(14), O1−Pt1−P1 93.73(12), C23−Pt2−N2 82.2(2), C23−Pt2−O3 168.5(2), N2−Pt2−O3 86.32(18), C23−Pt2−P2 94.33(17), N2−Pt2−P2 176.09(14), O3−Pt2−P2 97.14(12).

was formed as about 50% yield. In addition, other species such as the symmetrical binuclear complexes 3a and 3b were detected in the reaction mixture (Figures S9 and S10). All attempts for the purification of 3c from this mixture were not successful. The 31 P NMR spectrum of 3c illustrates two singlet peaks for two different phosphorus atoms at δ 6.5 with 1JPtP = 4462 Hz for the coordinated phosphorus atom of dppm trans to the coordinated nitrogen atom of bhq and at δ 8.8 with 1JPtP = 4477 Hz for the coordinated phosphorus atom of dppm trans to the coordinated nitrogen atom of ppy. Increasing the time of reaction was unsuccessful in gaining more yield of unsymmetrical binuclear complex 3c. To explain the formation of this mixture, it could be assumed that, by the dissociation of each dppm−Pt bond in unsymmetrical binuclear species, different types of bischelate

and the reaction progress in acetonitrile was even lower than those in the aforementioned solvents. This could be due to the precursors 2, which are ionic. The intermediates IM are neutral complexes, so the IM should be considerably more stable in nonpolar solvents, and their formation is faster. Also, DFT calculations demonstrate that IM-1 in benzene is around 13.5 and 18 kJ mol−1 more stable than those in dichloromethane and acetonitrile, respectively. For the synthesis of unsymmetrical binuclear complexes, a strategy similar to that of Scheme 2 was used. A total of 1 equiv of the cyclometalated complex 1b was added to 1 equiv of the bischelate complex 2a in benzene. The 31P NMR spectrum of the reaction mixture showed that the unsymmetrical binuclear complex [Pt(ppy)(CF3CO2)(μ-dppm)Pt(bhq)(CF3CO2)] (3c) F


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Inorganic Chemistry

Figure 5. (i) Proposed reaction pathway and (ii) energy profile in different solvents for the preparation of binuclear complex 3b.

Figure 6. (i) ORTEP representation and (ii) π−π-stacking interactions for 5. Ellipsoids are drawn at the 30% probability level, and hydrogen atoms are shown as spheres of arbitrary radius. Selected geometrical parameters for 5 (Å and deg): Pt1−C10 2.015(19), Pt1−N1 2.107(16), Pt1−P1 2.234(4), Pt1−Cl1 2.404(4), Pt2−C23 2.017(14), Pt2−N2 2.077(14), Pt2−P2 2.244(5), Pt2−Cl2 2.374(3); C10−Pt1−N1 80.5(7), C10−Pt1−P1 95.6(5), N1−Pt1−P1 176.0(5), C10−Pt1−Cl1 170.2(5), N1−Pt1−Cl1 91.4(5), P1−Pt1−Cl1 92.55(15), C23−Pt2−N2 81.9(6), C23−Pt2−P2 93.6(5), N2−Pt2−P2 175.5(4), C23−Pt2−Cl2 171.7(5), N2−Pt2−Cl2 90.2(3), P2−Pt2−Cl2 94.32(14).

the monodentate dppm [Pt(bhq)(Cl)(η1-dppm)] in NMR spectra at room temperature. This difference in the reactivity could be interpreted as a stronger coordination ability of the Cl− counteranion relative to CF3CO2−,58 so it has a higher tendency for coordination and formation of a binuclear compound. Also, suitable single crystals of 5 for X-ray crystallography determination were grown through two different solvent systems, benzene/ n-hexane and CH2Cl2/n-hexane diffusion, the resulting crystals of which both show almost similar structures (Figure 6), and the bridging dppm ligands in both crystal structures have anti conformation, similar to arylcycloplainated analogues.53 The distance

complexes, 2a and 2b, and cyclometalated complexes, 1a and 1b, could be formed. In this situation, the reaction between them leads to the formation of different types of binuclear complexes, 3a−3c, based on the probability distribution. It is interesting to note that the reactivity and crystal structure of similar analogues, from the reaction of 4 with 0.5 equiv of dppm, are completely different with complex 1a. By the addition of 0.5 equiv of dppm to 4 in several solvents such as dichloromethane, acetone, benzene, and acetonitrile, the binuclear complex 541 was formed, in the usual pathway, without any detectable intermediates such as the chelating dppm [Pt(bhq)dppm]Cl or G


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Inorganic Chemistry

the solvent polarity and the lowest-energy bands of binuclear complexes in benzene are not easily distinguishable as peak maxima in the spectra (see Table S1 and Figure S13). These transitions are noticeably red-shifted compared with the typical transition because the combined LC π−π* and MLCT excited states are attributed to 1MMLCT, the intramolecular π···π stacking in binuclear complexes 3 of which promoted their formations.66,67 The dependency of the low-energy absorption bands on the cyclometalated ligand is clear in all complexes. These bands are red-shifted upon going from complexes with less electrondelocalized systems (3b, 2b, and 1b) to higher ones (3a, 2a, and 1a) (λmax: 3b < 3a; 2b < 2a; 1b < 1a), which have more extended π-conjugated systems.68,69 Interestingly, when the coordinated CF3CO2 ligand in 3a is replaced with the chloride ligand, the 1MMLCT transition (see Figure 7) in complex 5 is eliminated. Although, the more electron-donation ability of chloride compared to that of the CF3CO2 ligand leads to more electron-rich platinum centering, which reduces the 1MLCT transition energy (λ = 402 nm for 5 and 394 nm for 3a), but the presence of the chloride ligand causes a totally different orientation of the bhq ligands compared to that of 3a. The intramolecular π···π stacking disappeared and the Pt···Pt distance enhanced to 6.61 Å in 5 (Figure 6). This value is as big as the interaction of d orbitals from two centers can possibly be. As a result, binuclear compound 5 behaves as a mononuclear platinum(II) complex. 3.2.2. PL Investigations. For most technological applications, such as OLEDs and sensors, luminescent molecules must be embedded in a solid matrix,9 we investigated the PL properties of complexes 2 and 3 investigated in pure solid and polymer states. The data of the emission peak positions, lifetimes, and quantum yields are summarized in Table 2. As shown in Figure 8, the mononuclear bischelate compound 2a, as well as its binuclear neutral compound, 3a, exhibits luminescence in powder and PMMA film. Although the mononuclear complex 2a shows weak emission in a CH2Cl2 solution, the binuclear complex 3 does not show emission in a CH2Cl2 solution even after oxygen was carefully removed by at least five freeze−pump−thaw cycles. The effect of dimerization on the excited states of studied compounds is reflected undoubtedly in their distinguished emission profiles. The broad and low-energy emission of the binuclear compound 3a with relatively short lifetimes (1.0 μs in powder and 3.7 μs in PMMA) is placed against the green structured emission of the mononuclear bischelate compound 2a with long lifetimes (60 μs in powder and 45.4 μs in PMMA). Moreover, the presence of a new low-energy absorption band even in a low concentrated CH2Cl2 solution (Figure 7) suggests the presence of intramolecular π···π (bhq) and Pt···Pt interactions, which are also observable in its crystal structure (Figure 4). Therefore, on the basis of the experimental evidence in conjunction with theoretical calculations (Table 3), it is manifested that the emission of 3a originates from the phosphorescence 3[dσ*(Pt2) → π*(bhq)] (3MMLCT) excited state, as in other stacked binuclear platinum systems.32,33,56,70−73 At 77 K, this unstructured emission band narrows and shifts 6 nm to lower energies (λmax = 604 nm) in the powder state (Figure S14) and the lifetime increases to 4.2 μs (Table 2). Upon cooling at 77 K (Figure S15), the emission of complex 2a becomes more vibronically resolved and intense and shifts 3 nm to higher energy (λmax = 502 nm) with very long lifetime (170 μs) indicative of an emission having a predominant 3LC parentage

between two square-planar platinum centers, Pt1···Pt2, is 6.6 Å; therefore, intramolecular π−π interactions similar to those of 3a were inhibited, and only intermolecular π−π interactions are detectable. 3.2. Optical Properties. 3.2.1. Absorption Investigations. Absorption spectra of complexes 1−3 are recorded in CH2Cl2, benzene, and acetonitrile solvents, and the data are summarized in Table S1. As illustrated in Figure 7, for all complexes in a CH2Cl2 solution, intense bands (ε > 25000 M−1 cm−1) in the region of

Figure 7. Normalized absorption spectra of complexes 1 and 2 (top) and 3 and 5 (bottom) in CH2Cl2 at 298 K (the insets show the expansion of the region of the low-energy bands, just increased in the x axis).

250−340 nm appeared, which are attributed to metal-perturbed π−π* ligand-centered transitions (1LC) located on the C^N and dppm ligands.59−61 In the range between 350 and 415 nm, the lowest-energy absorption bands of complexes 2 and the secondlower-energy bands for complexes 3 are observable. These absorption bands in 2 are slightly blue-shifted and have approximately half of the ε values compared to the related bands in binuclear complexes 3 (Table S1). Resolving the vibronic structures of these bands in benzene as well as TD-DFT calculations manifest that these bands have mostly contributions from 1LC (π−π* of the C^N ligand) and 1MLCT transitions. The blueshifted spectra of neutral binuclear complexes 3 in this region relative to the mononuclear bischelate complexes 2 is ascribed to an increased contribution of the 1MLCT transition together with a reduced 1LC [π ··· π*(C^N)] transition in these low-energy bands, which are supported by TD-DFT studies and can be seen in the natural transition orbital (NTO) plots in Table 3 (vide infra). In the absorption spectra of complexes 3, at wavelengths above 430 nm, broad bands that were absent in the monomer’s absorption spectra are observed. The presence of this band, which is independent from the concentration, caused the occurrence of intermolecular transition to fade from interactions between two adjacent binuclear complexes.62,63 The lowest-energy absorption bands are blue-shifted upon going from the least to most polar solvent [ε (M−1 cm−1): benzene (2.27) < CH2Cl2 (8.93) < MeCN (37.5)] [3a: 463 (benzene), 456 nm (CH2Cl2), 443 nm (MeCN), 3b; 459 nm (benzene), 441 nm (CH2Cl2), 419 nm (MeCN)], indicative of a negative solvatochromic effect, proposing a certain degree of charge transfer for this transition.64,65 The ε values increase with H


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Inorganic Chemistry Table 2. Emission Data for Complexes 2, 3, and 5 T, K

media 2a

2b 3a

3b

3cd

5e

solid solid PMMA PMMA CH2Cl2 solid solid solid solid PMMA PMMA solid solid PMMA PMMA solid solid PMMA PMMA solid

300 77 300 77 300 300 77 300 77 300 77 300 77 300 77 300 77 300 77 300

λem/λexc, nm

ΦPL, %

kr,b s−1

60 170 45.4 270 a

1.1

τem, μs

505max, 536, 584sh 502max, 538, 579, 624sh 473max, 506, 540, 583sh 473max, 505, 542, 587sh 396, 419max, 444sh 488, 517max, 558sh 485, 514max, 550sh 598 604 592 601 561 570 580 577 593 572 586 584 531max, 600

knr,c s−1

68

4

1.1 × 10

5.3 × 103

25

5.5 × 103

1.7 × 104

41 1.0 4.2 3.7 5.7 1.8 4.8 2.9 4.9 1.9

9

9.0 × 104

8.9 × 105

30

8.1 × 104

1.9 × 105

14

7.8 × 104

4.7 × 105

25

8.6 × 104

2.6 × 105

5

2.6 × 104

5.0 × 105

3.4 9.9 8.8

27

7.9 × 104

2.1 × 105

4.5 × 103

1.1 × 105

4

Too weak to be measured. Radiative rate constant kr = Φ/τ. Nonradiative rate constant knr = (1 − Φ)/τ. Measured as mixtures of products. From ref 39.

a

b

c

d

e

quantum yield of 3a is enhanced from ΦPL(powder) = 9.0% to ΦPL(PMMA) = 30%. Complex 3b displays a similar but blue-shifted low-energy broad band compared to 3a (Figure S16a), which is positioned at 580 nm with a smaller lifetime of τ = 2.9 μs in PMMA. Upon cooling at 77 K, a very similar and more intense emission can be seen at 577 nm for 3b. The lifetime increases (3b, 4.9 μs), while access to the low-energy emission is still strong, which proves that the emission comes from the dimer itself and is not the reason for aggregation of the complexes at room temperature.59 It was observed that complexes 3 in PMMA have the same trend of λem that originated from 3MMLCT (likely affected by the π−π interactions) as the trend of λmax in the absorption (1MMLCT) spectra. Therefore, the more π-expansion systems resulted in more red-shifted emission and absorption bands [λem: 3a (592 nm) > 3b (580 nm].77,68 It is noteworthy that the quantum yields of all binuclear complexes observed in PMMA films (3a, 30%; 3b, 25%) are higher than those in the solid state (3a, 9%; 3b, 14%), which can be explained with an increase of the probable occurrence of aggregation-induced self-quenching in the solid state than in the diluted doped PMMA film, which leads to enhanced nonradiative decays (see knr in Table 2).78−80 Additionally, shorter lifetimes in powder form at 300 K besides the red-shifted emission compared to the PMMA film, proposing easier access to deactivation of the excited states in powder form.81 Finally, it is expected that reducing the intramolecular Pt···Pt distances that caused stronger overlap between the dz2 orbitals of the platinum centers leads to destabilization of the highest occupied molecular orbital that is localized on the dz2 orbital of the platinum centers and concludes in red-shifted emission spectra (Figure 9). By evidence of the blue-shifted emission spectra of complex 3b compared to 3a, we propose longer Pt···Pt distances in 3b (3.28 Å < Pt···Pt < 3.5 Å). The considerable long Pt···Pt distance (6.61 Å) in complex 5 (λem= 531 nm) prevents the chance of interaction of two d orbitals from two platinum centers and the formation of dσ*(Pt2).

Figure 8. Ambient-temperature luminescence spectra of 2a (green) and 3a (red), recorded for powder (solid lines), PMMA (dashed lines), and CH2Cl2 (c ≈ 10−5 M) (dotted lines) samples (λexc = 400 nm). The PMMA film was doped with a ≈1 wt % complex.

mixed with some 3MLCT character, which is coincident with TD-DFT calculations (Table 3). The emission of 2a doped in PMMA (λmax = 473 nm) is blueshifted by 32 nm compared to the emission measured in powder and even more blue-shifted in CH2Cl2 (λmax = 419 nm), which is due to the absence of interactions between the platinum complexes with a greater degree of intermolecular π···π interactions in solution compared to those in PMMA and powder.74,75 Furthermore, the quantum yield of 2a decreases distinctly (under degassed conditions) from ΦPL(powder) = 68% to ΦPL(PMMA) = 25% to ΦPL(CH2Cl2) = 1.1% with a decrease in the radiative rate constant and an increase in the nonradiative rate constant (Table 2). This is related to increasing distortions of monomeric platinum(II) complexes that may occur upon excitation with decreasing matrix rigidity.76 However, the I


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Inorganic Chemistry

Table 3. NTOsa Representing Transitions That Correspond to the First and Second Lower-Energy “Bands” in CH2Cl2 and the Lowest Triplet Excited State in the Gas Phase for 2a, 3a, and 5

a

NTOs shown in this figure were produced with the isovalue of 0.02 and visualized with Chemissian software.

Besides, the shorter lifetime of 5 (8.8 μs) compared to 2a (60 μs) and the more resolved vibronic structured emission of 2a prove the more metal participation in the T1 state of 5. In conclusion, more metal participitation leads to more red-shifted spectra of studied complexes (λem: 3a > 3b > 5 > 2a), which leads to shorter lifetimes, according to the energy-gap law82 [τ = 2a (60 μs) > 5 (8.8 μs) > 3b (1.8 μs) > 3a (1.0 μs)]. 3.2.3. TD-DFT Calculations. To gain insight, TD-DFT calculations on singlet geometry (S0) have been carried out for complexes in CH2Cl2, and the data are summarized in Table 3. We have used the crystal structures of the investigated complexes as inputs for TD-DFT calculations to get the most accurate and

precise results. For complexes 3b and 3c (with no crystal structure), we restricted the Pt···Pt distances to values of less than 3.5 Å. Calculated absorption spectra for the complexes are blue-shifted compared to the experimental data. Actually, this deviation comes from the fact that the calculated spectra correspond to vertical absorption. Vertical or idealized absorption is the energy difference between electronically excited states and ground states determined at the ground-state geometry (S0).83 Nevertheless, they show a considerable agreement with the experimental data and reproduce the same experimental trends (Figure 10 and Table S1). Herein we focused on complexes 2a, 3a, and 5 in CH2Cl2. The lowest-energy transition of complexes 2a and 5 appeared at 368 nm, J


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Inorganic Chemistry

Figure 9. Emission spectra for 2a (green), 2b (blue), 3a (red), and 3b (black) by λexc = 400 nm in powder form at room temperature.

while the first and second low-energy transitions of complex 3a were located at 435 and 387 nm, respectively. For analysis of the nature of these singlet excited states, natural transition orbitals (NTOs)84 were obtained from TD-DFT calculations. NTOs are linear combinations of ground-state molecular orbitals that contribute to a given excited state. Each electronic transition Sn is presented as a single electron−hole pair excitation from the occupied “hole” NTO to the unoccupied “electron” NTO. As shown in Table 3, the lowest-lying singlet excited state (S1) of complex 2a stems from the 1LC (1π → π*) transition in the cyclometalated bhq ligand mixed with 1MLCT from the dxz and dyz orbitals of the platinum center to π* of the bhq ligand. Interestingly, the character of the NTOs contributing to the second lower-energy absorption band (see the NTOs for the fourth excited state, S4, in Table 3) of complex 3a has a nature very similar to that of the S1 transition of complex 2a. With a closer look at the corresponding NTOs, increased contribution of the 1MLCT transition together with a reduced [π···π*(C^N)] 1 LC transition in complex 3a compared to 2a is seen. Complex 5 shows a similar but red-shifted S1 state compared to 2a, referring to more participation of metal orbitals in this transition. No Pt···Pt interaction is seen, and the high ε value compared to that of 2a suggests that 5 acts as two monomer platinum(II) units. The lowest-energy absorption band (NTOs for S1) in complex 3a displays an interaction between the dz2 orbitals of two platinum centers in the hole besides a small contribution of the π orbital of the phenyl group of bhq coordinated to the platinum center and the π* orbital of bhq in the particle. As a result, this transition could be assign predominantly to the 1MMLCT [dσ*(Pt2) → π*(bhq)] excited state. Phosphorescence energies were calculated as vertical triplet excitations calculated via TD-DFT in the gas phase (see Table 3). The emission of complex 3a shows a red shift compared to 5 and 2a and originates from a phosphorescence 3[dσ*(Pt2) → π*(bhq)] (3MMLCT) excited state that is similar to the S1 transition in the absorption spectrum. The emissions of complexes 2a and 5 are assigned to 3LC/3MLCT excited states in addition to the higher contribution of the 3MLCT transition together with chloride participation in the T1 state.

Figure 10. Experimental UV−vis spectra of (i) complexes 2a, (ii) 3a and (iii) 5 in CH2Cl2 (10−5 M) at 300 K and calculated absorption spectra shown by bars in CH2Cl2.

By the reaction of complexes 1 with 1 equiv of dppm, the bischelate complexes 2 were prepared. Their crystal structures show strained angles of C−Pt−N in cyclometalated ligands and of P−Pt−P in dppm ligands. The absorption spectra for bischelate complexes 2 have 1π → π* and 1MLCT bands between 250 and 415 nm, which were supported by TD-DFT calculations. The luminescence properties of 2a were studied in CH2Cl2, solid, and PMMA media that has the highest intensity in powder media at λmax = 505 nm with 68% quantum yield

4. CONCLUSION The present study extends the information on the structures and electronic transitions of double-decker cyclometalated platinum(II) complexes containing a bridging dppm ligand. K


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Inorganic Chemistry and a 60 μs lifetime predominant 3LC parentage mixed with 3 MLCT. By an equimolar reaction of 1 and 2, through ring opening of chelating dppm, the symmetrical binuclear complexes 3 were produced in an unusual pathway. This transformation was studied in different solvents and by DFT calculations; it was shown that, by reducing the solvent polarity, the proposed intermediate [Pt(C^N)(CF3CO2)(η1-dppm)] (IM-1) could be more stable. Also, the crystal structure of 3a possesses a bent conformation for bridging dppm, and face-to-face platinum moieties have an eclipsed conformation with a 19° torsion angle. This structure has remarkable Pt···Pt intramolecular interaction with 3.28 Å distance, which demonstrates an interaction between dz2 platinum orbitals. The result of this interaction was observed by significant 1MMLCT bands above 430 nm in UV−vis spectra, in addition to 1π−π* and 1MLCT transitions in the higher-energy region. Also, a negative solvatochromic effect was observed as a result of a certain degree of charge transfer for this transition. The luminescence properties were acquired for binuclear complexes 3, in which 3a has the highest intensity in PMMA media at λmax = 592 nm with 30% quantum yield and 3.7 μs lifetime. The redshifted band (≈100 nm), compared with its monomeric analogue 2a, is due to the phosphorescence 3[dσ*(Pt2) → π*(bhq)] (3MMLCT) excited state, which was supported by TD-DFT analysis. The considerably long Pt···Pt distance (6.61 Å) in complex 5 prevents the chance interaction of two d orbitals from the platinum centers and the formation of dσ*(Pt2). Thus, it acts as two monomer platinum(II) units in the T1 state. In conclusion, more metal participation leads to more redshifted absorption (S1 transition) and emission (T1 transition) spectra of studied complexes upon going from LC to MLCT to MMLCT transitions (λabs and λem: 3a > 3b > 5 > 2a > 2b). So, tuning the Pt···Pt distances in binuclear cyclomatelated platinum(II) complexes, which is an applicable method for red emitters, depends on three main factors: (a) the bite angle of the bridged ligand, (b) the extent of π···π interactions in π-conjugated ligands, and (c) the effect of the monocoordinated ligand on the electronic and steric parameters of the complex.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.G.H. is thankful for financial support from the Iran National Science Foundation (Grant 94010886) and Shahid Beheshti University Research Councils. Technical support of the Chemistry Computational Center at Shahid Beheshti is gratefully acknowledged. M.J. gratefully acknowledges Professor Hartmut Yersin and A. Schinabeck for providing facilities during her visit at Regensburg University.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00137. 1 H and 31P NMR spectra, experimental absorption parameters, absorption spectra, and normalized excitation and emission spectra (PDF) Accession Codes

CCDC 1813692−1813696 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mohsen Golbon Haghighi: 0000-0002-2422-9075 L


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Inorganic Chemistry

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