Polypyridyl Ligands: Challenges and possibilities in SS-LEDs

Polypyridyl Ligands: Challenges and possibilities in SS-LEDs - Part 2

This article content is the continuation of our earlier Scientific News Blog (SNB).

Prof. Hashem Shahroosvand and his research team, from the University of Zanjan, Iran, Group for Molecular Engineering of Advanced Functional Materials (GMA) have surveyed about on Polypyridyl ligands that can be used for solid-state light-emitting devices(SS-LEDs). Among all of the emitters reported to-date, cyclometalate complexes have attracted significant and considerable interest in the novel photophysical properties which are well suited for a broad range of applications [1,2]. Hundreds of organometallic compounds have been reported, since the first discovery of highly efficient organic light-emitting diodes (OLEDs) prepared from cyclometalate Ir(III) complexes [3]. Cyclometalate complexes have the general formula [M(X)_3-n(Y)_n]^x+which are comprised of a metal cation (M^x+), a bidentate chelating ligand involved in at-least one organometallic carbon-to-metal bond, as well as an ancillary ligand. The metal center is generally a cation with a full tⁿ_2g configuration e.g. Ir(III), Ru(II), Os(II), or Pt(II). The redox properties of the metal cations facilitate electron transport between the layers of the device in these complexes.

Figure 1. Polypyridyl ligands as a versatile platform for solid-state light-emitting devices (SS-LEDs) [1].

Volz, et al., have reported that, an NHetPHOS complex containing a butterfly-shaped Cu₂I₂ unit coordinated by two ancillary phosphine ligands and bridged by an N,P-ligand, and complex (1), to modify thermally activated delayed fluorescence (TADF) in Cu(I) complexes[4]. This compound has a lower triplet energies ∆E(T₁–S₀) of up to 200 meV and air stable, when compared to other green-emitting Cu(I) compounds [5]. Another advantage of binuclear Cu-complex is its high thermal stability (T_decomp = 290°C) due to the four-fold bridging of Cu(I) centers. Because of its well-defined structure, good processability, high PL quantum yields (PLQY) in PL quantum yield doped pyridine (PYD2) films and low triplet energy, due to the singlet harvesting mechanism, this multi-bridged binuclear Cu(I) complex lends well itself to OLED applications [4]. The fact is that, for OLEDs containing complex (1), a high EQE value of 23% was obtained, the highest value reported to date for OLEDs fabricated from either vacuum or solution processed emissive layers of Cu(I) complexes. Moreover, this value is comparable to best efficiencies reported for state-of-the-art vacuum sublimed devices based on Ir(III) emitters,which are very important. The former device shows a pronounced roll-off the current efficiency with increasing luminance due to a loss of charge balance, most likely since most of the hole transport occurs via the emitter itself and it does not occurs via the host material. Nevertheless, this multi-bridged binuclear Cu(I) complex paves the way for the future development of new families of very efficient OLEDs.

Shafikov and Yersin (2017) a team of researchers have studied the effects of substituent in two important studies [6] on the photophysical properties as well as improving the thermally activated delayed fluorescence (TADF) efficiency of a series of new Ag(I) complexes containing a phenanthroline core and negatively charged electron donor (P2-nCB) ligands namely, Ag(phen)(P2-nCB) (2), Ag(idmp)- (P2-nCB) (3), Ag(dmp)(P2-nCB) (4) and Ag(dbp)(P2-nCB) (5), in which P2-nCB = bis(diphenylphosphine)-nido-carborane, phen= 1,10-phenanthroline, idmp = 4,7-dimethyl-1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-nbutyl-1,10-phenanthroline shown in (Figure 2). Ag(I) complexes generally exhibit long lived phosphorescence from ligand-centered (LC) excited states of ³π–π* character [7].

Figure 2. From left to right, molecular structures of the Ag(I) complexes (2), (4) and (5) [6].

In order to gain significant contributions from HOMO, HOMO-1, and HOMO-2 frontier orbitals, the d orbitals of these complexes need to be destabilized for the TADF. In this aspect, the use of a strong electron donating ligand can address this challenge, especially if the ligand is negatively charged. 1,10-phenanthroline was coupled with a phosphine ligand to stabilize the orbitals (LUMO, LUMO+1) of the complexes whose FPL values follow the order Ag(phen)(P2-nCB) (2) ˂ Ag(idmp)(P2-nCB) (3) ˂ Ag(dmp)(P2-nCB) (4) ˂ Ag(dbp)(P2-nCB) (5) have been studied. This trend reflects the increasing rigidity of the complexes that in turn increases the radiative decay and decreases the non-radiative decay rates.

One of the main important parameters for TADF efficiency is proportional to the radiative rate of the oscillator strength of the S₁↔S₀ transition f (S₁↔S₀), which also follows the above trend due to its dependence on the coordination geometry of the complex in the emitting state. Furthermore, the oscillator strength for the S₀→S₁ transition in these complexes is an order of magnitude higher when compared to other organometallic TADF materials with similar ∆E(S₁-T₁) splitting, In fact, the large, allowed S₀↔ S₁ transition for Ag(dbp) (P2-nCB) (5), results in extraordinary emission properties because it exhibits ground breaking TADF efficiency, including a high TADF rate with a significantly short radiative TADF decay time of only τ_r = 1.4 µs. The Φ_PL reported for a powder sample of complex (5) at ambient temperature reached to 100% [6]. As a result, these compounds are very exciting for applications in lighting technology. In fact, Ag(dbp)(P2-nCB) (5) is the first compound ever reported with a TADF decay time that is shorter than the phosphorescence decay time τ_r of 1.5 µs reported for the benchmark Ir(ppy)₃[7].

Our SNB Team recommended this research article to enrich our viewer’s knowledge about the research studies on the development of solid-state light-emitting devices using polypyridyl ligands. They believed that future studies on high performance OLEDs will not only continue to advance their commercial applications but will also accelerate the discovery of new fundamental science. Many new exciting advances in this field are possible, if the chemists continue to work, successfully alongside scientists from the neighboring fields in the near future.

References

B.Pashaei et al., Chem. Soc. Rev., (2019), DOI: 10.1039/c8cs00075a.
Y. You and W. Nam, Chem. Soc. Rev., 41, 7061 (2012).
C. Ulbricht, et al., Adv. Mater., 21, 4418 (2009).
D. Volz, et al., Adv. Mater., 27, 2538 (2015).
M. Wallesch, et al.,Chem. Eur. J., 20, 6578 (2014).
M. Z. Shafikov, et al.,Inorg. Chem., 56, 13274 (2017).
T. Hofbeck and H. Yersin, Inorg. Chem., 49, 9290 (2010).

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Dr. S. Thirumurugan

Assistant Professor

National College, Tiruchirappalli

Tamil Nadu, India

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