Efficient Red Organic Light Emitting Diodes of Nona Coordinate Europium Tris(β‐Diketonato) Complexes Bearing 4′‐Phenyl‐2,2′:6′,2′′‐Terpyridine

Abstract

Two novel nona-coordinated Eu(III) complexes [Eu-(btfa) 3 (Ph-TerPyr)] (Eu-1) and [Eu(NTA) 3 (Ph-TerPyr)] (Eu-2) have been synthesized and characterized. The structure of the complexes was elucidated by density functional theory (DFT) methods. The experimental photophysical properties of the complexes were investigated and complemented with theoretical calculations. Effective energy transfer (ET) pathways for the sensitized red luminescence is discussed. The complexes were tested as emitting layers (EML) in organic light emitting diodes (OLEDs). At the optimum doping concentration of 4 wt.%, the double-EML OLEDs of Eu-1 exhibited red electro-luminescence (EL) with an EQE of 4.0 % and maximum brightness (B) = 1179 cd/m 2 , maximum current efficiency (η c ) = 5.64 cd/A, and maximum power efficiency (η p ) = 4.78 lm/W at the current density (J) of 10 mA/cm 2 . Interestingly, the double-EML OLEDs of Eu-2 at the optimum concentration of 3 wt.%, displayed an outstanding EL performance with EQE of 7.32 % and B = 838 cd/m 2 , η c = 10.19 cd/A and η p = 10.33 lm/W at J = 10 mA/cm 2 . The EL performance of this device is among the best reported for devices incorporating a europium complex as a red emitter.

Highly Efficient Red-Emitting OLEDs Prepared from Nona-Coordinated Europium(III) Complexes

“…To underpin the results of absorption spectroscopy and to navigate the role of btfa and TerPy ligands in the light absorption phenomenon of the present OEuCs, we calculated the absorption spectra by the TD-DFT method (Figure b), where the solvent effect DCM was implicitly considered in the calculations. It is possible to note that the λ abs max value for all calculated spectra was slightly underestimated, and this behavior has previously been documented in research conducted by our group, even by applying different density functionals in the TD-DFT approach. ,, In this context, the results attest that the accurate determination of λ abs max is one of the main shortcomings of the TD-DFT method. Nevertheless, an examination of the theoretical spectra reveals that the positions of the absorption bands most shifted to longer wavelengths are consistent with the experimental observations.…”

“…The magnitudes of the rates are very similar to those found in a recent study conducted by us, in which the theoretical modeling of ET for the complexes [Eu(btfa) 3 (Ph-terPy)] and [Eu(NTA) 3 (Ph-terPy)] was performed. 30 Figure 5 shows a postulated energy level diagram that illustrates the most important channels involved in the ligandto-Eu(III) ET processes for all three complexes. Furthermore, as the MO analysis revealed the contribution of the βdiketonate and neutral ancillary TerPy ligands to the composition of the S 1 and T 1 states, only these two states were considered in the Jablonski diagram.…”

“…However, this is not the case for the complexes under consideration. To reproduce the experimental η sen and Q Eu L of the complexes, the ligand decay rates related to the S 1 → S 0 , S 1 → T 1 , and T 1 → S 0 transitions were adjusted, following a method already applied in our previous works. ,, …”

“…The terpyridine ligands were synthesized by following the synthetic protocol reported previously . A general synthetic approach for 4′-(furan-2-yl)-2,2′:6′,2″-terpyridine (FurTerPy) is described here as a representative of other TerPy ligands.…”

Deep-Red Organic Light-Emitting Diodes with Increased External Quantum Efficiency and Extended Operational Lifetime by Managing the Composition of Mixed Cohosts

“…When the emission wavelength is extended to the near-infrared (NIR) region (700–1400 nm), light can easily transmit in the human body, and thus, the strong demand for invisible sensing in phototherapy or healthcare equipment has stimulated the development of NIR materials. − Organic light-emitting diodes (OLEDs) possess many intrinsic advantages of ultralight weight, flexibility, and low cost; they are considered potential alternative of LEDs . Despite having obtained considerable values for both light sources and displays in the visible range through the continuous efforts of researchers, − OLEDs in the DR/NIR region suffer from lower emission efficiencies caused by aggregation-induced quenching, unbalanced charge transport and trapping, and accelerated nonradiative deactivation caused by the energy gap law. − Recently, much progress has been made in developing DR/NIR thermally activated delayed fluorescence (TADF) materials, such as the work of Xu et al demonstrating an external quantum efficiency (EQE) higher than 30% at about 690 nm and very recently Fan et al who developed a multiple subacceptor strategy for constructing NIR-TADF materials and obtained a peak EQE of 21.9% at 712 nm . However, the efficiency roll-off of DR/NIR-OLEDs is still serious at the high-current region. , Although operational stability is one of the key factors in practical application, few reports have given enough attention to the durability of DR/NIR-OLEDs, implying that it is still a great challenge to achieve compatibility of both high performance and excellent operational durability …”