Monochromatic red electroluminescence from the homodinuclear europium(III) complex of a β-diketone tethered by 2,2ʹ-bipyrimidine

A new bright red emitting complex [Eu(btfa)3bpm]2 has been synthesized. The photophysical properties were analyzed and it was utilized as an emitter to fabricate red OLEDs.


Spectroscopic measurements
Optical absorption spectrum of 1 was obtained in 1×10 -5 M dichloromethane (DCM) solution at room temperature (RT) using Varian Cary 50 spectrophotometer. Excitation and emission spectra of 1 (1×10 -3 M DCM solution) at RT were recorded on an Edinburgh FS5 fluorimeter.
The same instrument was used to determine the decay profile of 1 and the data obtained were fitted by in-built software. The goodness of the fit (GOF) was judged by the value of the reduced chi-squared (χ 2 ) and is shown in their respective figures in Supporting Information.
Absolute PLQY was determined by using a calibrated integrating sphere on a C-9920-02 instrument and details are reported in our very recent article. 1

Ground state geometry and singlet (S) and triplet (T) energy level
Structural modeling of [Eu(btfa) 3 ] 2 bpm was performed using as reference the crystal structure  semiempirical model 4 has also been used, replacing the Eu 3+ ions by a 3e+ point charge. For   the INDO configuration interaction single excitation approach, 20 occupied molecular orbitals and 20 virtual molecular orbitals were considered. The absorption spectra were obtained from fitting to a Lorentzian function with a half width at half maximum (HWHM) of 25 nm for all calculations.

2.2.
Theoretical J-O intensity parameters (Ω λ ) (λ = 2, 4 and 6) The theoretical intensity parameters derived from the Judd-Ofelt theory 5 are important parameters for the spectroscopic characterization of systems containing lanthanide, and the B3LYP/SVP/MWB52 geometry were considered in the calculation. The J-O parameters are theoretically calculated by: Eq. S2 contains the contribution from the forced electric dipole and dynamic coupling mechanisms, which depend on the so-called odd-rank ligand field parameter ( p t ) and Γ p t , respectively. These quantities are calculated by the following expressions: The j index corresponds to each atom coordinated directly to the lanthanide ion, R j is the distance between a given j atom and the lanthanide ion,  j and  j are the corresponding angular coordinates. As the complex studied has two Eu 3+ centers, one of the centers was considered in the calculation. All quantities present in Eq. S2 can be consulted in reference 6 as well as the complete procedure used to calculate Ω λ . The ρ j (2β j ) t+1 term from Eq. S3 represents the correction introduced by the Simple Overlap Model (SOM) advanced by Malta 7 for the crystal field parameters of the point charge electrostatic model (PCEM).
The procedure commonly used to calculate the theoretical Ω λ consists of adjusting the charge factors (g j ) and polarizabilities (α j ), from Eq. S3 and S4, respectively, to reproduce the experimental values of Ω λ . Instead of treating the g j and α j quantities as adjustable parameters, the QDC model 8 which is implemented into LUMPAC 9 postulates that the g j are obtained from the product between adjustable parameters (Q) and each ZDO (Zero Differential Overlap) electronic density (q j ) of each atom j. In addition, the model postulates that the α j are calculated by using the D and C adjustable parameters From the energy of the occupied molecular orbital and the corresponding linear coefficients calculated with any Sparkle model or with RM1 10 , the ZDO electronic density and the electrophilic superdelocalizability (SE) can be obtained using LUMPAC, as shown in the reference 8 .
From theoretical intensity parameters, the decay rate (A rad ) was calculated, which for the europium ion is given by: where e is the elementary charge; 2J+1 is the degeneracy of the initial state, for Eu 3+  ( 2) / 9 n n    the refractive index n equal to 1.424 was considered in this work, corresponding to the DCM solvent. The magnetic dipole strength of the 5 D 0  7 F 1 transition is theoretically evaluated as being S md = 9,6 × 10 -42 esu 2 cm 2 . 11

Theoretical Quantum Yield
The PLQY (q) for [Eu(btfa) 3 ] 2 bpm was calculated with LUMPAC 1.4.1. The PLQY is defined as the ratio of the number of emitted photons by the lanthanide ion to the number of absorbed photons by the ligand, and for the Eu(III) ion is calculated by the Eq. S8.
where W ij is the transfer rate from level i to level j, if i = j, W ij = 0.