Highly Stable and Strongly Emitting N-Heterocyclic Carbene Platinum

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

Highly Stable and Strongly Emitting N‑Heterocyclic Carbene Platinum(II) Biaryl Complexes Dominik Suter,† Luuk T. C. G. van Summeren,†,‡ Olivier Blacque,† and Koushik Venkatesan*,†,§ †

Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057, Zurich, Switzerland Department of Molecular Materials, Radboud University, Nijmegen, 6525 AJ, Netherlands § Department of Molecular Sciences, Macquarie University, North Ryde, NSW 2109, Australia ‡

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S Supporting Information *

ABSTRACT: C^C cyclometalated platinum(II) triplet emitters bearing electronically different N-heterocyclic carbenes(1,3diisopropyl-4-(trifluoromethyl)-imidazol-2-ylidene (d), 1,3-diisopropyl-benzimidazol-2-ylidene (e), and 1,3-diisopropyl-imidazol-2ylidene (f))as neutral ligands and biphenyl (bph) as well as its fluorinated derivative octafluorobiphenyl (oFbph) as dianionic cyclometalating ancillary ligand were synthesized and structurally characterized by 1H, 13C, 19F, and 195Pt NMR, single crystal X-ray diffraction, and HR-ESI-MS studies. Detailed photophysical investigations carried out reveal a strong influence on the excitedstate properties exerted by the electronic nature of the N-heterocyclic carbenes as well as the fluorine functional groups on the ancillary biphenyl moiety. The solid-state structures of all complexes reveal a nearly planar and slightly distorted square planar geometry around the platinum center. Introduction of fluorine groups into the ligand framework leads to a less structured emission centered at 513 nm in poly(methyl methacrylate) (PMMA) thin films, compared to the highly structured emission profile of the bph analogues. Additionally, a hypsochromic shift of approximately 10−12 nm was found in the absorption as well as in the emission profiles and is attributed to the electron deficient nature of the oFbph ligand. Three wt % of the compounds doped in PMMA exhibit photoluminescence efficiencies as high as 92% in thin films. DFT and TD-DFT calculations on selected molecules revealed the charge transfer to be an admixture of intraligand (3ILCT) and metal-to-ligand charge transfer (3MLCT) and the frontier orbitals corresponding to the emission to be mainly localized on the bph and oFbph ligands, which is consistent with the observations from the photophysical investigations. The thermal stability of the complexes evaluated by thermogravimetric analysis (TGA) shows an enhanced thermal stability for the complexes bearing fluorine functional groups.

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INTRODUCTION Molecules exhibiting phosphorescence have attracted considerable attention due to their intrinsic efficient triplet harvesting property, and consequently, they have been evaluated for use in a number of applications such as light harvesting systems, catalysis, sensors, and phosphorescent light emitting diodes (PhOLEDs).1−4 In particular, transition metal complexes have been intensively studied during the last decades because of their light emitting behavior. Platinum and iridium based complexes have been the most investigated metals among the transition metals.3−5 While most of the complexes display high emission efficiencies in the red and green part of the electromagnetic spectrum, there are very few complexes that display strong emission as well as high thermal stability at higher emission energies particularly in the blue and the deep blue region.6−10 Recent work from our group based on Pt(II) bis-alkyne-bis-NHC complexes and Pt(II) cyclometalated pyridine-NHC-bis-alkyne complexes showed deep blue and tunable white emission with moderate to good quantum yields (ϕem).11,12 However, these complexes were thermally unstable due to the tendency of the alkyne ligands that are bound in the © XXXX American Chemical Society

cis configuration to undergo reductive elimination. Furthermore, the rotational freedom of the two monodentate alkyne ligands rendered the complexes nonemissive in solution and additionally contributed to a decrease in ϕem in thin films and solid state. In order to develop thermally stable and highly efficient Pt(II) based emitters, we hypothesized that bidentate chelating ligands such as biphenyl (bph) would be a suitable ligand that was expected to impart more rigidity as well as stability to the complex and therefore reduce the probability for the nonradiative decay of the excited state. The further use of the perfluorinated derivative, octafluorobiphenyl (oFbph) was expected to lead to lowering of the energy of the HOMO, thereby increasing the HOMO−LUMO gap and thus shifting the emission to shorter wavelengths. These electronic features were expected to render complexes with emission wavelengths at higher energies of the electromagnetic spectrum with high photoluminescent efficiencies and increased thermal stability.13 While the chelating bidentate bph derivatives were chosen to Received: March 6, 2018

A

DOI: 10.1021/acs.inorgchem.8b00564 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthetic Pathway for Complexes 4−9

base KOtBu were used to substitute COD in 3.43 Since the lithiated oFbph species is unstable at the ambient room temperature conditions necessary for its addition to the metal complex in contrast to the bph compound, a different strategy comprising of transmetalation by an organotin reagent was utilized. Me2Sn(oFbph) was treated with Pt(COD)Cl2 using a modified literature procedure to form 3.42 The complexes 4−9 were isolated in moderate to good yields (36−90%) and were extensively characterized by 1H, 13C, 195Pt, and 19F NMR studies. 2D NMR experiments were further performed for selected complexes to assign all proton resonances. All complexes 4−9 showed 195Pt coupling in the 1H and 19F NMR spectra, respectively. The bph and oFbph ligands displayed four distinct signals in the 1H and 19F NMR spectra with 195Pt coupling due to the H or F nucleus on the 2 and 2′ position (Figure 1a). The coupling constants were in the range of 3JH−Pt = 48.3−49.5 Hz and 3JF−Pt = 234.0−252.9 Hz, respectively. A clear trend was observed in the case of bph compounds with the aromatic protons shifted more downfield with increasing σ-donor ability of the ancillary carbene,

serve as the chromophore, two additional L-type ligands are needed to complete the coordination sphere of the platinum(II) metal center. Previous work from our group and others has shown that N-heterocyclic carbenes (NHCs) are suitable ligands, since NHCs are strong field ligands, and hence the metal-centered (MC) d-d levels are destabilized, which results in lowering the probability for the radiationless deactivation and therefore is anticipated to result in an increased emission efficiency.11,12,14−34 In addition, the introduction of the Nheterocyclic carbenes was expected to result in emission energies blue-shifted in comparison to the bipyridine and phosphine analogues due to the increased σ-donation ability resulting in the destabilization of the π*-orbitals.35,36 On the basis of the study on the σ-donor properties of a variety of NHCs by Gusev and co-workers, a number of carbenes with different σ-donor properties was further investigated in order to elucidate the impact of raising the low lying d-d levels on the quantum yields.37 A trifluoromethyl-substituted carbene was chosen because the incorporation of a CF3 group was expected to have an added beneficial impact by minimizing intermolecular interactions, thereby suppressing quenching of the luminescence in solid-state and thin films. In addition, they would allow for easy sublimation that could simplify the vacuum deposition and purification process. Last but not least, the electron mobility in the PhOLED could be improved as well.38

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RESULTS AND DISCUSSION Synthesis and Characterization. The imidazolium and benzimidazolium salts (a−c) and biphenyl ligands were synthesized according to previously reported or slightly modified procedures (Scheme 1).39,40 Complexes 1−3 were synthesized based on modified reported procedures.41,42 While silver(I)−NHC complexes were used for substitution of the neutral diethyl sulfide ligand and 1,5-cyclooctadiene (COD) ligand in 1 and 2, respectively, imidazolium salts after subsequent in situ deprotonation with the noncoordinating

Figure 1. (a) Depiction of the 3J couplings between Pt and H in complexes 4−6 and between Pt and F in 7−9; (b) Illustration of the diastereotopic isopropyl groups highlighted in green/blue and the HOESY signal indicated with the arrows, exemplified on compound 9. B

DOI: 10.1021/acs.inorgchem.8b00564 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Molecular structures of 3−9. Thermal ellipsoids are drawn at the 30% probability level. Disordered atoms, solvent molecules, and H atoms are omitted for clarity.

whereas deshielding with respect to the NHC in the perfluorinated bph was found in the trend e, d, and f. Remarkably, 1H NMR studies revealed resonances for the isopropyl-CH3 protons as two doublets both with an integral of 12 H instead of the expected one doublet with an integral of 24 H. Heteronuclear Overhauser effect spectroscopy (HOESY) reveal a NOE signal for only one of the CH3 groups with the fluorine atom on position-2 of the octafluorobiphenyl ligand (Figure 1b). The presence of these two signals can be attributed to arise from the inequivalent magnetic environments encountered by the two diastereotopic isopropyl groups of the NHC due to the restricted rotation of the NHC ligands around the carbon−platinum bond caused by the steric hindrance. HR-ESI-MS and single crystal X-ray diffraction studies were carried out to further corroborate the chemical identity of the final complexes. Crystal Structure Determination. X-ray diffraction studies were carried out on single crystals of 3−9. Crystals of 4 and 7 were obtained by slow evaporation of DMSO, 5 and

8 by slow diffusion of pentane into a saturated solution of dichloromethane and crystals of 3, 6, and 9 by slow evaporation of CDCl3. Plots of the molecular structures are shown in Figure 2. The crystallographic details are summarized in the Supporting Information (Table S1−S2). All complexes exhibit slightly distorted square planar coordination geometry with the bidentate bph or oFbph ligand and two NHC ligands disposed cis to each other (4−9). The parent bph ligand in the complexes 4−6 has the two six-membered rings almost coplanar, with the dihedral angles between the mean planes of the rings of 2.6(1), 7.1(2), and 1.83(6)°, respectively. That coplanarity is broken in the case of oFbph ligand because of the apparent steric reasons. In the molecular structures of 7−9 the distortion is clearly shown by the corresponding dihedral angles of 21.99(9), 25.91(7), and 20.5(2)°, respectively. The twist of the substituted biphenyl ligand has no influence on the central C−C bond which remains in the narrow range of 1.468(5)−1.486(5) Å but induces a greater distortion of the square planar coordination geometry around the Pt center. C

DOI: 10.1021/acs.inorgchem.8b00564 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Selected Bond Lengths and Bite Angles of Complexes 4−9 Distance/Å Compound 4 5 6

7 8 9

Pt−CNHC 2.058(7) 2.045(6) 2.056(2) 2.0512(17) Distance/Å

Angle/deg Pt−Cbph

CNHC−Pt−CNHC

2.057(6) 2.055(6) 2.059(2) 2.0411(16)

99.4(2)

Cbph−Pt−Cbph 80.6(3)

101.54(11) 93.01(9)

80.60(12) 80.92(9) Angle/deg

Pt−CNHC

Pt−CoFbph

CNHC−Pt−CNHC

CoFbph−Pt−CoFbph

2.030(2) 2.030(2) 2.034(4) 2.035(4)

2.052(2) 2.057(2) 2.067(3) 2.056(4)

93.30(12) 94.65(12) 91.52(14)

80.48(12) 80.52(13) 80.48(14)

Indeed, the planes defined by the three atoms CNHC/Pt/CNHC and Cbph/Pt/Cbph are almost coplanar in 4−6 as indicated by the dihedral angles of 4.05(5), 5.8(2), and 8.64(5)° while larger values of 22.7(2), 19.5(2), and 18.0(1)° characterize the distortion in 7−9. The Cbph−Pt−Cbph (or CoFbph−Pt−CoFbph) bond angles are in the short-range of 80.48(14)−80.92(9)°, while the CNHC−Pt−CNHC bite angles fall in the wider range 91.52(14)−101.54(11)° (Table 1). As a general trend, the bulkier the NHC (e > d > f), the smaller is the CNHC−Pt− CNHC bite angle. The bond length of the Pt−CNHC was found to be slightly shorter in the oFbph derivatives than those in the bph complexes.13 It is presumed that the electronegative character of the fluorine atoms results in a better overlap of the bonding orbitals; hence, a stronger and shorter bond is formed. Comparing the NHC bound Pt(II) complexes with the bipyridine (bpy) and the triphenylphosphine Pt(II) analogues bearing the bph ligand, neither the bph bite angle (bpy: 80.2(4)°; PPh3: 79.7(2)°) nor the bond length of the Pt−Cbph bond (bpy: 1.993(9) Å and 2.021(10) Å; PPh3: 2.068(5) Å and 2.092(5) Å) was found to differ significantly with respect to the NHC bound complexes.41,44 A careful analysis of the intermolecular interactions in the crystal structures showed that there are clearly no metal−metal interactions, the smallest Pt···Pt distance is as large as 7.4655 Å in 5 (no standard deviation since the metal centers are located on special positions). Despite the presence of conjugated rings, there are also no π···π interactions; only weak C−H···π and C−H···F hydrogen interactions are observed in the crystal packings. Photophysical Properties. The UV−vis absorption spectra of complexes 4−9 display two absorption bands in the range 230−270 and one at 320−340 nm with molar extinction coefficients in the range 104−105 M−1 cm−1 in dichloromethane (Figure 3). The higher energy absorption band at 230−270 nm can be attributed to an intraligand charge transfer (1ILCT) [π−π*] transition which corresponds to the absorption bands of the slightly hypsochromically shifted free biphenyl (λmax 207 and 247 nm in MeOH).45 This red shift is in good agreement with the higher rigidity in the complex due to the less torsion of the two phenyl rings. The ancillary Nheterocyclic carbene and the fluorine groups on the biphenyl ligand, respectively, do not have a strong impact on this transition. The absorption bands in the region of 280−301 nm which are only observed in complexes 5 and 8 are assigned to an 1ILCT[π−π*] of the benzimidazole-NHC. The lowest energy transition at 320−340 nm can be described by an admixture of 1ILCT[π−π*] and metal-to-ligand charge transfer (1MLCT) [d−π*]. Although, the bph and the corresponding

Figure 3. UV−vis profiles of complexes 4−9 recorded in CH2Cl2 at room temperature.

oFbph analogues displayed similar absorption profiles, the absorption bands of the fluorinated complexes were hypsochromically shifted by approximately 10 nm as a consequence of a stabilization of the HOMO, thus resulting in an enhanced HOMO−LUMO bandgap. Since, the transitions are presumed to be predominantly derived from an 1ILCT[π−π*] and 1MLCT[d−π*] involving only the platinum center and the bph moiety, electronic changes in the carbene ligand do not significantly influence the low-energy transition. A solvent dependent UV−vis absorption investigation was carried out exemplarily for complexes 6 and 9. The lowest energy transition revealed a positive solvatochromic behavior (Figures S1−S2 in the Supporting Information). While in complex 6 only a minor shift of 1−2 nm was observed, this behavior is much more distinct in the case of 9 with a shift of up to 8 nm, suggestive of the excited state of the fluorinated complexes being more polar than the ground state. A summary of the relevant photophysical data is provided in Table 2 and in the Supporting Information for complexes 2 and 3 (Table S3 and Figures S3−S4). The emission properties of the complexes were investigated in solution at room temperature, doped in amorphous PMMA thin films (3 wt % in PMMA) and as neat powder. In solution, the compounds showed no observable emission, which can be attributed to the radiationless deactivation resulting from thermal quenching caused by the solvent molecules as well as the too high rotational freedom of the monodentate carbene ligands. While the emission profile of the bph complexes doped in amorphous PMMA matrix and as neat powder showed two peaks at 490 and 525 nm with a shoulder at 560 nm, the D

DOI: 10.1021/acs.inorgchem.8b00564 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 2. Photophysical Properties of Complexes 4−9 Recorded at 298 K in Solution and in Amorphous PMMA Thin Films Absorption (CH2Cl2) 4 5 6 7 8 9

Emission (3 wt % in PMMA)

λmax/nm (ε/M−1cm−1)

complex 233 235 232 229 230 230

(32085), (45531), (32319), (25313), (41833), (28019),

263 265 267 248 245 251

(25295), (28032), (20866), (23278), (30620), (22602),

331 285 334 321 280 326

(6672) (29667), 301 (28487), 334 (11045) (7221) (6058) (27105), 293 (27577), 325 (10759) (7133)

λmax/nm (τ/μs)

ϕem/%

kr/×104 s−1

knr/×104 s−1

489, 525, 559 (15.4) 489, 525, 558 (14.5) 490, 526, 560 (13.8) 486 sh, 513 (5.5) 486 sh, 511 (6.8) 488 sh, 513 (9.2)

91.7 73.9 69.9 30.6 45.6 46.1

5.95 5.10 5.06 5.58 8.68 5.01

0.54 1.80 2.18 12.65 5.98 5.86

Thermogravimetric analysis (TGA) measurements carried out on complexes 6 and 9 to evaluate the thermal stability of the two complexes revealed the onset of thermal degradation (Td) at 309 °C for the bph complex 6 and at 374 °C for the oFbph analogue 9. As envisaged, the oFbph derivative showed an increased thermal stability in comparison to the parent biphenyl analogue. Both these complexes exhibit high thermal stability in comparison the cis-bisalkyne complexes. DFT and TD-DFT Calculations. The emission properties of compounds 6 and 9 were exemplarily investigated by means of DFT and TD-DFT calculations (Supporting Information). No significant differences could be identified from the calculations between the former complex bearing the biphenyl ligand and the latter that bears the octafluorobiphenyl ligand. The T1 excited states arise from HOMO−LUMO excitations, with the two frontier orbitals mainly located on the biphenyl ligands and the metal center leading to an admixture of intraligand (3ILCT) and metal-to-ligand (3MLCT) charge transfers. The spin density surfaces calculated from the optimized triplet states (unrestricted DFT) further confirm the significant involvement of the biphenyl ligands (Figure 5). Furthermore, the S0−T1 transition is lower in energy for 6 than for 9 by 0.12 eV (21 nm), which is in agreement with the experimental data.

emissions of the oFbph complexes were broad and structureless with a global emission maximum at 512 nm, except in the case of 9, which has an emission maximum at 550 nm in neat solid (Figure 4 and Figure S5 in the Supporting

Figure 4. Normalized emission spectra of complexes 4−9 doped at 3 wt % in PMMA and recorded at 298 K.

Information). The absence of distinct structural features in the emission profile might arise from a greater horizontal displacement of the two potential energy surfaces, more specifically the singlet ground state (S0) and the first excited triplet state (T1), compared to the bph complexes as well as to closer vibrational levels in S0 due to the fluorine functional groups. This would lead to less discrete bands, since the probability for much more transitions is increased. Both thin film and neat solid spectra showed a hypsochromic shift of the emission maxima of the fluorinated biphenyl compounds in comparison to nonfluorinated complexes (Table S3 and Figure S5 in the Supporting Information). This shift is consistent with the increasing electron deficient character of the oFbph ligand, which effects a lowering of the HOMO and overall resulting in a larger HOMO−LUMO gap in comparison to the bph. Since the emission spectra of the compounds in neat powder were found to be very similar to the highly diluted samples in PMMA. it can be presumed that most intermolecular interactions in the solid state can be excluded (Figure S5 in the Supporting Information). The ϕem of samples, which were 3 wt % doped in PMMA, were found to be strongly dependent on the specific electronic character combination of the carbene and the ancillary bph derivative. Enhancement of ϕem was observed with increasing electron density on one side and an increasing electron withdrawing character on the other side. The ϕem was found to be in a range 31−92% (Table 2). In the solid state, no specific trend was observed and ϕem values were found in the range 8− 54% (Table S3 in the Supporting Information).

Figure 5. Spin density surface for the optimized triplet states of (a) 6 and (b) 9.

Electrochemical Properties. Cyclic voltammetry studies were carried out in CH2Cl2 at room temperature for the final complexes. All six compounds showed a reversible oxidation peak in the region of 0.279−0.432 V (vs Fc+/Fc0 couple) for the bph derivatives and in the region of 0.908−1.087 V for the oFbph analogues, respectively, which were attributed to a predominantly ligand centered event, as the removal of an electron takes place from an orbital composed mainly of the πsystem of the biphenyl moiety and only to a small extent of the metal (Table 3 and Figures S6−S11 in the Supporting Information). Additionally, an irreversible oxidation peak was observed in the region of 0.768−0.958 V for the bph E

DOI: 10.1021/acs.inorgchem.8b00564 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Cyclic Voltammetry Data of Complexes 4−9a complex

Eox,rev/V

Eox,irrev/V

4 5 6 7 8 9

0.432 0.376 0.279 1.087 1.028 0.908

0.958b 0.949b 0.768b -c -c 1.176b

1,3-Diisopropyl-4-(trifluoromethyl)-imidazolium Iodide (a). 4(Trifluoromethyl)-1H-imidazole (95%, 1.053 g, 7.35 mmol) was added together with 1.1 equiv of K2CO3 (1.117 g, 8.08 mmol) into a Young Schlenk apparatus under nitrogen atmosphere. Dry CH3CN (20 mL) was added and the mixture was heated to 90 °C for 1 h. Three equiv of 2-iodopropane (3.748 g, 22.04 mmol) were added and the suspension was heated to 90 °C. After 24 h an additional portion of 2-iodopropane (3.748 g, 22.04 mmol) was added and the mixture was stirred at 90 °C for another 3 days before cooling to room temperature again. The solvent was removed under reduced pressure followed by dissolving the residues in CH2Cl2, and subsequent filtration through a glass filter frit gave a yellowish filtrate of which the solvent was evaporated under reduced pressure to yield in an orangereddish oil. After adding a small amount of EtOAc and sonication of the mixture, a slightly yellowish precipitate started to form. Washing with EtOAc (2 × 10 mL) and hexane (2 × 10 mL) yielded a colorless solid (1.931 g, 5.55 mmol, 75%). Yield: 75% (1.931 g, 5.55 mmol). 1 H NMR (500.25 MHz, CDCl3, 300 K): δ (ppm) = 11.20 (s, 1H, NCHN), 7.87 (s, 1H, CH), 5.40 (sept, 1H, 3JHH = 6.7, CH(CH3)2), 5.40 (sept, 1H, 3JHH = 6.8, CH(CH3)2), 1.87 (d, 6H, 3JHH = 6.7, CH(CH3)2), 1.74 (d, 6H, 3JHH = 6.8, CH(CH3)2). 13C{1H}-NMR (125.78 MHz, CDCl3, 300 K): δ (ppm) = 139.2 (d, 1C, NCHN), 123.6 (q, 1C, 2JCF = 42.6, CCF3), 121.0 (q, 1C, 3JCF = 3.8, CH), 118.5 (q, 1C, 1JCF = 269.3, CF3), 55.09 (d, 2C, CH(CH3)2), 23.8 (q, 1C, CH(CH3)2), 23.3 (q, 1C, CH(CH3)2). 19F-NMR (470.61 MHz, CDCl3, 300 K): δ (ppm) = −60.0 (s, 3F, CF3). (+)-HR-ESI-MS (MeOH): calcd for C7H10F3N2+ [M − C3H7I + H]+: m/z 179.0791, found: 179.0792; calcd for C10H16F3N2+ [M − I]+: m/z 221.1260, found: 221.1260. Pt(COD)(oFbph), 3. One equiv of 2,2′-dibromo-octafluorobiphenyl (0.750 g, 1.68 mmol) was dissolved in dry Et2O (40 mL) under nitrogen atmosphere. The solution was cooled to −78 °C using a bath of acetone and liquid N2. Under stirring, 2 equiv of nBuLi (2.5 M, 1.35 mL, 3.36 mmol) were added dropwise by syringe and lithiated over a period of 30 min. One equiv of Me2SnCl2 (0.361 g, 1.68 mmol) was added at −78 °C to the lithiated octafluorobiphenyl. The mixture was allowed to slowly warm to room temperature and was then dried under a N2 flow. The residues were dissolved in hexane, and the suspension was sonicated and filtered through 0.5 cm of Celite in a glass filter frit. The solvent was removed under reduced pressure. The crude Me2Sn(oFbph) was used without further purification and dissolved together with 1.1 equiv of Pt(COD)Cl2 (0.506 g, 1.85 mmol) in chloroform (20 mL) in a microwave tube. The reaction mixture was irradiated in a sealed tube for 1 h at 130 °C. The solvent was removed under reduced pressure and the crude was purified by column chromatography on silica gel after washing with hexane. A yellow solid was obtained. Eluent: CH2Cl2:hexane 2:1. Yield: 67%, (0.658 g, 1.10 mmol). 1H NMR (500.25 MHz, CD2Cl2, 300 K): δ (ppm) = 6.27 (m, 4H, CHCH), 2.51 (m, 8H, aliph. CH2). 13C{1H} NMR (125.78 MHz, CD2Cl2, 300 K): δ (ppm) = 152.4, 150.6, 144.0, 142.0, 141.4, 140.6, 139.5, 138.6, 135.0, 130.7, 106.1, 30.8. 19F-NMR (470.61 MHz, CD2Cl2, 300 K): δ (ppm) = −128.4 (m, 2F, 3JFPt = 103.6, oFbph), −130.0 (m, 2F, oFbph), 157.4 (m, 2F, oFbph), 159.1 (m, 2F, oFbph). 195Pt-NMR (107.12 MHz, CD2Cl2, 298 K) δ (ppm) = −3862. (+)-HR-ESI-MS (MeOH/CHCl 3 3:2): calcd for C20H13F8Pt+ [M + H]+: m/z 600.0532, found: 600.0534. General Procedure for the Synthesis of 4−6. Five equivalents of the imidazolium salt (a−c) were added together with 2.5 equiv of Ag2O (0.066 g, 0.29 mmol) into a round-bottom flask under nitrogen atmosphere. Dry CH2Cl2 (20 mL) was added under exclusion of light and stirred for 1 h at room temperature. 1 (0.100 g, 0.11 mmol) was then added to the light protected mixture and stirring was continued for 15 h. The reaction progress was monitored by TLC (CH2Cl2/ hexane 2:1). CH2Cl2 (20 mL) was added, and after sonication the mixture was filtered over a pad of approximately 1 cm of silica gel and the filtrate was evaporated to dryness. The residues were washed with pentane and dried in vacuo yielding the desired product. Pt(C10H15F3N2)2(bph) (4). A pale yellow solid was obtained. Yield: 80% (0.144 g, 0.18 mmol). 1H NMR (400.13 MHz, CDCl3, 298 K): δ (ppm) = 7.46 (s, 1H, Im), 7.43 (s, 1H, Im), 7.42 (dt, 2H, 3JHH = 7.6,

a Scan rate = 100 mV s−1 in 0.1 M [nBu4N][PF6] (glassy carbon electrode; E0 vs Fc+/Fc; 298 K; CH2Cl2). bAnodic peak potential. c No irreversible oxidation observed in the electrochemical window of the solvent.

complexes. The electrochemical window of the solvent CH2Cl2 was too narrow for complexes 7 and 8. Therefore, only an irreversible oxidation peak was observed for 9 at 1.176 V. A clear trend was observed with respect to the increasing σdonating nature of the N-heterocyclic carbenes leading to a decrease in the oxidation potential regardless of whether it is a reversible or an irreversible event. A shift of approximately 600−700 mV to a more positive potential was observed in comparing the bph and the perfluorinated analogue oFbph, which is consistent with the more electron deficient nature of the latter, which makes it difficult to oxidize the complexes. The trend of the oxidation potential is also consistent with the shift of the lowest energy absorption bands of complexes 4−9.

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CONCLUSION Six novel platinum(II) complexes bearing two N-heterocylcic cabenes in cis configuration and a bph derivative as ancillary ligand were prepared in order to study the influence of the electronic properties of the ligands on the photophysical behavior. Structural and photophysical investigations of the new complexes were carried out. The chosen synthetic route was shown to be effective toward the preparation of the desired complexes, which was reflected in yields of up to 90%. The complexes show bright emission in solid and in PMMA thin films upon irradiation under UV light with the highest ϕem of 92%. A hypsochromic shift of approximately 10 nm was observed in the absorption profile as well as in the emission wavelength when the bph ligand was replaced with its perfluorinated analogue, oFbph. The results from the studies suggest a significant influence of the electronic structure of the bph ligand on the emission wavelength as well as the emission quantum yield of the complex. Thermogravimetric analysis revealed an enhanced thermal stability for the complex bearing the perfluorinated ligand. Although deep blue emission could not be obtained from these specific complexes, the high photoluminescent quantum efficiencies and improved thermal stability of the complexes in comparison to their dialkyne analogues provide a greater impetus for the further development of deep blue emitters based on the Pt(II) NHC biaryl structural motif.

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EXPERIMENTAL SECTION

General Procedure. All manipulations were carried out under standard Schlenk techniques with commercially available anhydrous solvents. Diethyl ether (Et2O) and tetrahydrofuran (THF) were freshly distilled over sodium with benzoquinone. Further details on instrumentation are provided in the Supporting Information. Preparation. The imidazolium salts 1,3-diisopropyl-benzimidazolium bromide39 (b) and 1,3-diisopropyl-imidazolium iodide39 (c) were synthesized according to an already reported procedure. F

DOI: 10.1021/acs.inorgchem.8b00564 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry bph CH-5), 6.95 (td, 2H, 3JHH = 7.3, 4JHH = 1.1, bph CH-4), 6.71 (tt, 2H, 3JHH = 7.2, 4JHH = 1.3, bph CH-3), 6.31 (m, 2H, 3JHPt = 49.5, bph CH-2), 6.00 (sept, 1H, 3JHH = 6.7, CH(CH3)2), 5.75 (sept, 2H, 3JHH = 6.7, CH(CH3)2), 5.55 (sept, 1H, 3JHH = 6.7, CH(CH3)2), 1.38 (dd, 6H, 3JHH = 7.0, 4JHH = 2.5, CH(CH3)2), 1.31 (d, 6H, 3JHH = 6.7, CH(CH3)2), 1.29 (dd, 6H, 3JHH = 6.7, 4JHH = 1.5, CH(CH3)2), 1.19 (d, 6H, 3JHH = 6.9, CH(CH3)2). 13C{1H}-NMR (125.78 MHz, CDCl3, 300 K): δ (ppm) = 190.4, 159.8, 158.3, 138.5, 125.5, 123.5, 122.9 (q, 2JCF = 40.9, CCF3), 122.5 (q, 2JCF = 40.9, CCF3), 121.8 (q, 3 JCF = 5.4, CH), 121.4 (q, 3JCF = 5.4, CH), 120.4 (q, 1JCF = 267.6, CF3), 119.6, 54.9, 52.3, 23.3, 23.2, 21.4, 21.3, 21.0. 19F-NMR (376.46 MHz, CDCl3, 300 K): δ (ppm) = −55.9 (s, 3F, CF3), −56.0 (s, 3F, CF3). 195Pt-NMR (107.54 MHz, CDCl3, 295 K) δ (ppm) = −4000. (+)-HR-ESI-MS (MeOH): calcd for C30H41N4Pt+ [M − C12H10 + H]+: m/z 634.1944, found: 634.1937; calcd for C30H41N4Pt+ [M + H]+: m/z 652.2973, found: 652.2977. Pt(C13H18N2)2(bph) (5). A pale yellow solid was obtained. Yield: 65% (0.112 g, 0.15 mmol). 1H NMR (400.13 MHz, CD2Cl2, 300 K): δ (ppm) = 7.65 (m, 4H, ArH-3,4), 7.34 (m, 2H, bph CH-5), 7.24 (m, 4H, ArH-2,5), 6.87 (td, 2H, 3JHH = 7.4, 1.4, bph CH-4), 6.58 (td, 2H, 3 JHH = 7.3, 1.4, bph CH-3), 6.34 (m, 2H, 3JHPt = 48.7, bph CH-2), 6.06 (sept, 4H, 3JHH = 7.1, CH(CH3)2), 1.54 (d, 12H, 3JHH = 7.4, CH(CH3)2), 1.40 (d, 12H, 3JHH = 7.1, CH(CH3)2). 13C{1H}-NMR (125.81 MHz, CD2Cl2, 300 K): δ (ppm) = 193.5, 160.7, 158.2, 138.6, 133.8, 125.2, 122.9, 121.7, 119.2, 112.7, 52.7, 20.5, 20.2. 195Pt-NMR (107.54 MHz, CDCl3, 295 K) δ (ppm) = −4055. (+)-HR-ESI-MS (MeOH): calcd for C38H45N4Pt+ [M + H]+: m/z 752.3286, found: 752.3296. Pt(C9H16N2)2(bph) (6). A pale yellow solid was obtained. Yield: 44% (0.066 g, 0.10 mmol). 1H NMR (400.13 MHz, CDCl3, 300 K): δ (ppm) = 7.40 (m, 2H, bph CH-5), 7.00 (m, 4H, Im), 6.91 (td, 2H, bph CH-4), 6.68 (td, 2H, bph CH-3), 6.37 (m, 2H, 3JHPt = 48.3, bph CH-2), 5.46 (sept, 4H, 3JHH = 6.7, CH(CH3)2), 1.32 (d, 12H, 3JHH = 6.7, CH(CH3)2), 1.16 (d, 12H, 3JHH = 6.8, CH(CH3)2). 13C{1H}NMR (125.78 MHz, CDCl3, 300 K): δ (ppm) = 183.3, 161.7, 158.5, 138.8, 125.3, 122.8, 119.4, 116.3, 51.3, 23.6, 23.3. 195Pt-NMR (107.54 MHz, CDCl3, 295 K) δ (ppm) = −4006. (+)-HR-ESI-MS (MeOH): calcd for C18H31N4Pt+ [M − C12H10 + H]+: m/z 498.2196, found: 498.2195; calcd for C30H41N4Pt+ [M + H]+: m/z 652.2973, found: 652.2977. General Procedure for the Synthesis of 7−9. Three equiv of the imidazolium salt (a−c) were added together with 3 equiv of KOtBu (0.056 g, 0.50 mmol) and 3 (0.100 g, 0.17 mmol) into a Young Schlenk apparatus under nitrogen atmosphere. Dry THF (15 mL) was added and the mixture was stirred for 24 h at 80 °C. The reaction progress was monitored by TLC (CH2Cl2/hexane 2:1). The solvent was removed under reduced pressure. CH2Cl2 (50 mL) was added, and after sonication the mixture was filtered over a pad of approximately 1 cm of silica gel and the filtrate was evaporated to dryness. The residues were washed with pentane and dried in vacuo yielding the desired product. Pt(C10H15F3N2)2(oFbph) (7). A pale yellow solid was obtained. Yield: 36% (0.056 g, 0.06 mmol). 1H NMR (400.13 MHz, CDCl3, 300 K): δ (ppm) = 7.33 (m, 2H, Im), 5.61 (sept, 1H, 3JHH = 7.0, CH(CH3)2), 5.42 (sept, 1H, 3JHH = 6.9, CH(CH3)2), 5.32 (sept, 1H, 3 JHH = 6.8, CH(CH3)2), 5.16 (sept, 1H, 3JHH = 6.7, CH(CH3)2), 1.28 (d, 6H, 3JHH = 7.0, CH(CH3)2), 1.21 (m, 6H, CH(CH3)2), 1.15 (d, 6H, 3JHH = 6.8, CH(CH3)2), 1.05 (m, 6H, CH(CH3)2). 13C{1H}NMR (125.78 MHz, CDCl3, 300 K): δ (ppm) = 179.0, 149.8, 148.0, 144.7, 142.7, 140.0, 139.2, 138.0, 137.3, 136.3−135.6, 122.9 (q, 2JCF = 41.3, CCF3), 122.6 (q, 2JCF = 41.3, CCF3), 121.4 (q, 3JCF = 5.4, CH), 121.1 (q, 3JCF = 5.5, CH), 120.2 (q, 1JCF = 267.7, CF3), 55.3, 55.3, 52.7, 52.6, 22.9, 22.9, 22.8, 22.7, 21.0, 21.0, 20.5, 20.4. 19F-NMR (376.46 MHz, CDCl3, 300 K): δ (ppm) = −56.0 (CF3), −56.1 (CF3), −120.6 (3JFPt = 234.0, oFbph), −131.1 (oFbph), −158.7 (oFbph), −162.4 (oFbph). 195Pt-NMR (107.54 MHz, CDCl3, 295 K) δ (ppm) = −4082. (+)-HR-ESI-MS (MeOH): calcd for C20H29F6N4Pt+ [M − C12H2F8 + H]+: m/z 634.1944, found: 634.1942; calcd for C32H31F14N4Pt+ [M + H]+: m/z 932.1967, found: 932.1977; calcd for C32H30F14N4NaPt+ [M + Na]+: m/z 954.1787, found: 954.1790.

Pt(C13H18N2)2(oFbph) (8). A pale yellow solid was obtained. Yield: 80% (0.066 g, 0.07 mmol). 1H NMR (400.13 MHz, CDCl3, 300 K): δ (ppm) = 7.55 (m, 4H, ArH-2,5), 7.21 (m, 4H, ArH-3,4), 5.77 (sept, 4H, 3JHH = 6.8, CH(CH3)2), 1.49 (d, 12H, 3JHH = 7.0, CH(CH3)2), 1.24 (d, 12H, 3JHH = 7.0, CH(CH3)2). 13C{1H}-NMR (125.81 MHz, CD2Cl2, 300 K) δ (ppm) = 182.5, 157.6−136.3, 133.3, 122.5, 113.3, 53.3, 20.2, 20.1. 19F-NMR (376.46 MHz, CD2Cl2, 300 K) δ (ppm) = −118.5 (3JFPt = 252.9, oFbph), −131.3 (oFbph), −158.9 (oFbph), −162.8 (oFbph). 195Pt-NMR (107.54 MHz, CDCl3, 295 K) δ (ppm) = −4055. (+)-HR-ESI-MS (MeOH): calcd for C26H35N4Pt+ [M − C12H2F8 + H]+: m/z 598.2504, found: 598.2507; calcd for C38H37F8N4Pt+ [M + H]+: m/z 896.2533, found: 896.2543; calcd for C38H36F8N4NaPt+ [M + Na]+: m/z 918.2352, found: 918.2358. Pt(C9H16N2)2(oFbph) (9). A pale yellow solid was obtained. Yield: 90% (0.119 g, 0.15 mmol). 1H NMR (400.13 MHz, CDCl3, 300 K): δ (ppm) = 6.95 (s, 4H, Im), 5.14 (sept, 4H, 3JHH = 6.7, CH(CH3)2), 1.27 (d, 12H, 3JHH = 6.7, CH(CH3)2), 1.74 (d, 12H, 3JHH = 6.8, CH(CH3)2). 13C{1H}-NMR (125.78 MHz, CDCl3, 300 K): δ (ppm) = 171.4, 144.7−134.6, 116.3, 51.6, 23.1, 22.8. 19F-NMR (376.50 MHz, CDCl3, 300 K): δ (ppm) = −120.7 (3JFPt = 246.6, oFbph), −131.7 (oFbph), −159.3 (oFbph), −163.6 (oFbph). 195Pt-NMR (107.54 MHz, CDCl3, 295 K) δ (ppm) = −4084. (+)-HR-ESI-MS (MeOH): calcd for C18H31N4Pt+ [M − C12H2F8 + H]+: m/z 498.2196, found: 498.2193; calcd for C30H33F8N4Pt+ [M + H]+: m/z 796.2220, found: 796.2218; calcd for C30H32F8N4NaPt+ [M + Na]+: m/z 818.2039, found: 818.2034.

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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.8b00564. Crystal data and refinement details of complexes 3−9, photophysical data, cyclic voltammetry data (PDF) Accession Codes

CCDC 1585222−1585228 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Koushik Venkatesan: 0000-0002-3046-2017 Funding

Financial support from the Swiss National Science Foundation (Grant no. 200020_156967), the University of Zurich, and Macquarie University is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Prof. Roger Alberto for generous support and Dr. Thomas Fox for his support in NMR structure elucidation.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b00564 Inorg. Chem. XXXX, XXX, XXX−XXX