Enhanced quantum yields by sterically demanding aryl-substituted β-diketonate ancillary ligands

Luminescent organometallic platinum(II) compounds are of interest as phosphors for organic light emitting devices. Their emissive properties can be tuned by variation of the ligands or by specific electron-withdrawing or electron-donating substituents. Different ancillary ligands can have a profound impact on the emission color and emission efficiency of these complexes. We studied the influence of sterically hindered, aryl-substituted β-diketonates on the emission properties of C^C* cyclometalated complexes, employing the unsubstituted methyl-phenyl-imidazolium ligand. The quantum yield was significantly enhanced by changing the auxiliary ligand from acetylacetonate, where the corresponding platinum(II) complex shows only a very weak emission, to mesityl (mes) or duryl (dur) substituted acetylacetonates. The new complexes show very efficient emission with quantum yields >70% in the sky-blue spectral region (480 nm) and short decay times (<3 μs).

We herein present the synthesis and photophysical properties of two new C^C* cyclometalated platinum complexes. Both are based on the original 3-methyl-1-phenylimidazolium (MPIM) ligand system which together with the acac auxiliary ligand showed only a very low quantum yield of 7%. We introduced sterically demanding aryl substituted β-diketonate auxiliary ligands to further examine their influence on the emission properties of the resulting platinum(II) complexes.

Results
The mesityl-and duryl-substituted 3-methyl-1-phenylimidazole complexes 2, Pt(MPIM)(mes) and 3, Pt(MPIM)(dur), were synthesized from 3-methyl-1-phenylimidazolium iodide (1) according to a modified literature procedure (Scheme 1) [41,42]. The starting imidazolium salt 1 was prepared from phenylimidazole by addition of methyl iodide as previously described [43]. Complexes 2 and 3 were obtained as yellow solids in isolated yields of 5% and 18%, respectively (Scheme 1). They were characterized by standard methods, NMR techniques ( 1 H, 13 C, and 195 Pt) as well as mass spectrometry (ESIMS). The purity of all compounds was verified by elemental analyses. Additionally we could unequivocally determine the structural parameters of 3 by a solid-state structure ( Figure 1). Details of the structure determination are given in Supporting Information File 1, Table S1.
The absorption spectra ( Figure 2) were measured in dichloromethane solution at ambient temperature. The complexes show almost identical absorption behavior with only minor deviations in the absorption intensity. Both complexes exhibit a strong absorption in the ultraviolet spectral region with an intense shoulder at 241 nm. Two weak and one more intense absorption bands are additionally located at 280 nm, 293 m, and 313 nm, respectively.
Photoluminescence spectra ( Figure 3) were measured at ambient temperature in a PMMA matrix (2 wt % complex) and at 77 K in 2-MeTHF (0.5 mM). The room-temperature emission spectra of both complexes exhibit one broad, structurally unresolved band in the sky-blue spectral region.
The low-temperature emission maxima of both complexes display only a minor hypsochromic shift compared to the emission at ambient temperature: 5 nm for complex 2 and 8 nm for     (2) and 73% (3) at ambient temperatures as well as short decay times around 3 μs (Table 1) were measured. The complexes show no aggregation behavior at higher concentrations (10 wt % in PMMA and 100% amorphous film measurements, see Figures S1, S2 and Tables S2, S3 in Supporting Information File 1), which can be assigned to the steric demand of the aryl-substituted diketonate counter ligand.
Cyclic voltammograms of complexes 2 and 3 were measured in DMF with ferrocene as an internal reference. For both compounds, one irreversible oxidation wave was measured ( Figure 4), which is commonly found for platinum(II) complexes [16,44]. Irreversibility of the measured signals was confirmed by variation of the scan rate (30 mV/s to 1 V/s). The peak potential of the oxidation is located at 0.69 V vs ferrocene for both complexes. No reduction was observed for both complexes in the electrochemical window of the solvent. Thus, the electrochemical behavior of the newly synthesized substances is comparable.
The higher emission efficiency is accompanied by a red shift in emission color of about 40 nm ( Figure 6). An improved quantum yield of Φ = 30% (5 wt % in PMMA) has already been reported for a 3-methyl-1-phenylimidazolium cyclometallated platinum(II) complex by the introduction of a sterically demanding ancillary ligand (α-duryl substituted acac) in the central position of the acetylacetonate between the two C=O groups [35]. Besides an increased quantum yield, the complex displayed a small red shift (λ exc = 467 nm) compared to Pt(MPIM)(acac) and a decay time of 8.7 μs. When mesityl or duryl groups replace both methyl groups of the acetylacetonate, the quantum yield is further enhanced. Such a severe influence of the mesityl-and duryl-substituted auxiliary ligands on the quantum yield is unprecedented, although enhanced quantum yields have been reported for both ligands [18,[39][40][41]. Additionally, the decay times of Pt(MPIM)(mes) and Pt(MPIM)(dur) are shorter compared to the phosphorescence decay of the α-durylsubstituted complex (8.7 µs at 77 K in 2-MeTHF).
The observed red shift in emission color is also in agreement with the results of the DFT calculations (Supporting Information File 1, Table S4) of the predicted emission wavelength, ac- cording to a previously published procedure [60]. The bathochromic shift in emission color of complexes 2 and 3 can be assigned to the delocalization of electron density on the arylsubstituted auxiliary ligands.

Conclusion
As shown above, we observed an unprecedented enhancement of the quantum yield for platinum(II) complexes with 3-methyl-1-phenylimidazole as C^C* cyclometalating ligand by changing the ancillary ligand from acetylacetonate (R = CH 3 ) to sterically demanding aryl-substituted β-diketones (R = 2,4,6-trimethylphenyl, 2,3,5,6-tetramethylphenyl). The drastically increased quantum yield was accompanied by a shift in the emission color from the deep-blue to the sky-blue spectral region. Besides a very efficient phosphorescent emission, the two newly synthesized complexes also exhibit very short decay times of less than 3 μs. The profound impact of the counter ligand on the complexes' emission properties originates from the diketonate ligand, which was also confirmed by DFT calculations.

Experimental
Both complexes were characterized by 1 H, 13  Chemical shifts are given in ppm downfield from TMS, coupling constants J in Hz (the signal splitting is abbreviated as followed: s = singlet, d = doublet, t = triplet, m = multiplet). Elemental analyses were performed by the analytical laboratory of the department using a Eurovektor Hekatech EA-3000 elemental analyzer. Melting points were measured on a Wagner and Munz Poly Therm A system and are not corrected.

X-ray crystallography
Crystallographic data for compound 3 were collected on Bruker D8 VENTURE Kappa Duo PHOTON200 by IμS micro-focus sealed tube Mo Kα 0.71073 Å at a temperature of 100(2) K. The absorption corrections were carried out using numerical methods. The structure was solved by direct methods (XP) and refined by full matrix least squares based on F 2 (SHELXL2014).

Photophysical characterization
Absorption spectra of all complexes were measured on a Perkin Elmer lambda 25 spectrophotometer in dichloromethane solution. Photoluminescence measurements were performed in amorphous PMMA thin films doped with the emitter. Films were prepared by doctor blading a solution of 2 wt % emitter in a 10 wt % PMMA solution in dichloromethane on a quartz substrate with a 60 μm doctor blade. Film emission was measured under nitrogen flux. Excitation was carried out at different wavelengths (Xe-lamp with monochromator) and the emission was detected with a calibrated quantum-yield detection system (Hamamatsu, model C11347). The phosphorescence decay of all complexes was measured with an Edinburgh Instruments mini-τ by excitation with a pulsed EPLED (360 nm, 20 kHz) and time-resolved photon counting (TCSPC). Frozen 2-MeTHF glass emission samples at 77 K were prepared by inserting a sealed quartz tube, containing the solution under argon atmosphere, into liquid nitrogen. Spectroscopic grade 2-methyltetrahydrofuran (2-MeTHF) was purchased from ABCR and used as received.

Cyclic voltammetry
Electrochemical measurements were performed with a BioLogic SP-150 potentiostat in degassed, dry N,N-dimethylformamide using a Pt counter electrode, a glassy carbon working electrode, and a Ag/Ag + pseudo reference electrode.  The product was obtained following the general procedure reported for 2 using 1-methyl-3-phenyl-1H-imidazol-3-ium iodide (