Electroluminescence of Tetradentate Pt(II) Complexes: O^N^N^O versus C^N^N^O Coordination

Alkylation of one of the phenolic hydroxyl groups in a salen-type tetradentate ligand changes the coordination mode from O^N^N^O to the cyclometallating C^N^N^O type. The ligand was used to synthesize a new cyclometalated luminescent Pt(II) complex 2. While in solution the complex is poorly luminescent, in the solid state the emission is reinstated, which allowed one to evaluate complex 2 as a phosphorescent emitter in organic light-emitting diodes. 2 displays external quantum efficiency (EQE) = 9.1% and a maximum luminance of 9000 cd m–2 in a vacuum-deposited device. We carried out comparative analysis of photo- and electroluminescence of complex 2 with O^N^N^O complex 1 and demonstrated that the similar luminescent properties of the O^N^N^O and C^N^N^O complexes are rather coincidental because they display different excited-state landscapes. Surprisingly, the two complexes display a dramatically different electrochemical behavior, with O^N^N^O coordination leading to the formation of a stable electropolymer but C^N^N^O coordination fully preventing electropolymerization.


Materials and Methods
All solvents and reagents were purchased from Sigma-Aldrich, Acros Organics or Alfa-Aesar and used without further purification unless otherwise specified. Reactions were monitored by TLC using silica gel with UV254 fluorescent indicator. NMR spectra were recorded on a JEOL ECS400FT Delta spectrometer (399.78 MHz for 1 H NMR, 100.53 MHz for 13 C NMR). Chemical shifts are reported in parts per million (ppm) relative to tetramethylsilane as internal standard. Coupling constants (J) are measured in hertz. Multiplets are reported as follows: b = broad, s = singlet, d = doublet, dd = double doublet, t = triplet, q = quartet, qu = quintet, m = multiplet, app d = apparent doublet, app t = apparent triplet.

Photophysics
Absorption spectra of 10 -5 M solutions were recorded with UV-3600 double beam spectrophotometer (Shimadzu). Photoluminescence (PL) spectra of solutions and films were recorded using a QePro compact spectrometer (Ocean Optics) or FluoroLog fluorescence spectrometer (Jobin Yvon). Photoluminescence decays in solution and film were recorded using nanosecond gated luminescence and lifetime measurements (from 400 ps to 1 s) using the third harmonic of a high-energy pulsed Nd:YAG laser emitting at 355 nm (EKSPLA) or with a Horiba DeltaFlex TCSPC system using a 330 nm SpectraLED or a 405 nm DeltaDiode light source. Further details are available in reference 1 . Temperature-dependent experiments were conducted using a liquid nitrogen cryostat VNF-100 (sample in flowing vapour, Janis Research) under nitrogen atmosphere, while measurements at room temperature were recorded under vacuum in the same cryostat. Solutions were degassed using five freeze-pumpthaw cycles. Thin films for photophysics were deposited from toluene solutions. The films were fabricated through spin-coating and dried under vacuum at room temperature. Solid state photoluminescence quantum yield was obtained using an integrating sphere (Labsphere) coupled with a 365 nm LED light source and QePro (Ocean Optics) detector.

Determination of photoluminescence quantum yields in solution 2
Photoluminescence quantum yields in solution were obtained using a gradient method in which we study relation (gradient) between total photoluminescence intensity and absorbance at the excitation wavelength (same for both standard and analyte) in a range of concentrations for both analyte and standardsee equation below. We only consider data points with a constant gradient, so that the relation between photoluminescence intensity and absorbance is linearindication of the photoluminescence yield being independent of concentration in this region.
Where: , photoluminescence quantum yield of analyte and standard, respectively; , -gradient (slope) of the linear relation between photoluminescence intensity and solution absorbance at the excitation wavelength, for analyte and standard, respectively; , -refractive index of solvent used for analyte and standard, respectively.

Calculations
To assist the interpretation of the experimental results, we have performed density functional theory (DFT) and time-dependent density functional theory (TDDFT) simulations with Tamm-Dancoff approximation (TDA) using ORCA 4.2.1 quantum chemistry software [3][4][5] . All molecular orbital (MO) iso surfaces were visualised using Gabedit 2.5.0. 6 Geometry optimisations were performed at the B3LYP 7,8 /def2-TZVP 9 level of theory with RIJCOSX 10,11 approximation to accelerate calculations and def2/J 12 auxiliary basis set. Atom-pairwise dispersion correction with the Becke-Johnson damping scheme (D3BJ) 13,14 was included in the calculation. Single point energy calculations were performed using ZORA-corrected variants of the def2-TZVP basis set. All geometries were verified to be true energy minima by a frequency calculation. All optimisations were performed with tight SCF and geometry convergence criteria. Excited state energy of TDDFT states was calculated using the resulting S0 or T1 geometry. In this case relativistically corrected triplezeta basis sets with the zeroth-order regular approximation (ZORA) 15,16 were used: ZORA-def2-TZVP 9 with the SARC/J 17 auxiliary basis for all atoms except Pt for which a segmented all-electron relativistically contracted (SARC) SARC-ZORA-TZVP 17 basis set was used. Spin-orbit coupling (SOC) calculations were performed as implemented in the ORCA software. SOC matrix elements (SOCME) and SOC-corrected excitations (SOC-TDDFT states) were computed using the same settings as for the TDDFT states. In order to accelerate the calculations RIJCOSX 10,11 approximation was used in all cases and the RI-SOMF(1X) setting was used to accelerate SOC calculations. All computations were performed using a dense grid (Grid6, GridX6).

Electrochemistry
Cyclic voltammetry was conducted in a three-electrode, one-compartment cell. All measurements were performed using 0.1 M Bu4NBF4 (99%, Sigma Aldrich, dried) solution in dichloromethane (ExtraDry AcroSeal®, Acros Organics). All solutions were purged with nitrogen prior to measurement and the measurement was conducted in a nitrogen atmosphere. Electrodes used in the experiment were: working (Pt disc d = 1 mm), counter (Pt wire), and reference (Ag/AgCl calibrated against ferrocene). All cyclic voltammetry measurements were performed at room temperature with a scan rate of 50 mV s -1 .
The ionization potential (IP) and electron affinity (EA) are obtained from onset redox potentials; these figures correspond to HOMO and LUMO values, respectively. The ionization potential is calculated from onset oxidation potential IP = Eox CV + 5.1 and the electron affinity is calculated from onset reduction potential EA = Ered CV + 5.1. 18,19,20,21 An uncertainty of ±0.02 V is assumed for the electrochemical onset potentials.

OLED devices
OLEDs were fabricated by thermal evaporation or by spin-coating / evaporation hybrid method. We used pre-cleaned indium-tin-oxide (ITO) coated glass substrates with a sheet resistance of 20 Ω cm -2 and ITO thickness of 100 nm. The substrates were first washed with acetone and then sonicated in acetone and isopropanol, for 15 min each time. Substrates were dried with compressed air and transferred into an ozone-plasma generator for 6 min at full power. Thermally deposited layers were obtained using Kurt J. Lesker Spectros II deposition system at 10 -6 mbar base pressure. All organic materials and aluminium were deposited at a rate of 1 Å s -1 . The LiF layer was deposited at a rate of 0.1-0.2 Å s -1 . Characterisation of OLED devices was conducted in a 10 inch integrating sphere (Labsphere) connected to a Source Measure Unit (SMU, Keithley) and coupled with a spectrometer USB4000 (Ocean Optics). Further details are available in reference. 13 Devices of 4  2mm pixel size were fabricated. Solution processing. Hole injection layer (Heraeus Clevios HIL 1.3) was spin-coated and annealed on a hotplate at 200 ˚C for 3 min to give a 60 nm film. Emitting layer was prepared from toluene solution of PVK:PBD (60:40 w/w) with a total concentration of host 20 mg mL -1 . The dopants were dissolved in the solution of blend host in order to obtain final 5% (w/w) concentration in the emitting layer. The solution containing the host and the platinum complex was applied onto a substrate with HIL 1.3 layer and spun at 2500 RPM for 60 s; the substrate was then annealed at 120 ˚C for 15 min. All solutions were filtrated directly before use with a PVDF (organic solvents) syringe filter with a 0.45 µm pore size. The electron transport (TPBi) and electron injection (LiF) layers as well as cathode (Al) were thermally evaporated. 5

5
A mixture of 4 (1.1 g, 2.62 mmol), 1-bromo-2-ethylhexane (505 mg, 2.62 mmol) and potassium carbonate (1084 mg, 7.85 mmol) in DMF (20 mL) was stirred at 115°C for 2 days. The DMF was evaporated. The residue was treated with DCM and filtered through a short pad of silica gel using a mixture of DCM/PE/EA (5/5/1 v/v) to elute the product. By this procedure, the dark impurity on the baseline is removed. The filtrate was evaporated to dryness. The residue was treated with acetone (20 mL) and filtered. The solid on the filter was washed with acetone and dried to give unreacted diphenol 4 (201 mg). The filtrate was again evaporated to dryness and the mixture was then purified by column chromatography using silica gel as stationary phase and a mixture of PE/EA (8/1 v/v) as mobile phase. The first product to elute with the highest Rf is the symmetrical dialkylated product. Yield: 68 mg (4%). The major product is the desired ligand 5 which was the second product to elute and was isolated as an oil. Yield: 520 mg (37%). 1 Figure S9. HOMO and LUMO iso-surfaces at T1 geometry at the B3LYP/def2-TZVP/CPCM(toluene) level for complexes 1 and 2. Figure S10. Cyclic voltammograms of 2 (c = 10 -3 M) recorded at the first and second oxidation peak. Figure S11. Current efficiency-current density characteristics of devices 1-3. Figure S12. EQE-current density characteristics of devices 1-3.