Investigation of a sterically hindered Pt(II) complex to avoid aggregation-induced quenching: Applications in deep red electroluminescent and electrical switching devices
Graphical abstract
Introduction
Phosphorescent metal complexes have attracted considerable attention in the development of high-performance electroluminescent devices [1], [2], [3], [4], [5]. Indeed coordination of heavy metals increases the rate of radiative decay from triplet states by enhancing intersystem crossing from singlet to triplet states thanks to spin–orbit coupling [6]. In particular, organic light emitting diodes (OLEDs) with 100% internal quantum yield could be theoretically achieved by using phosphorescent dyes in which both singlet and triplet excited states participate [7]. Due to the broad variety of available ligands, metal–organic complexes are considered as promising materials for industrial and practical applications [3] such as OLEDs [8], [9], [10], [11]. Increasing the rigidity of the ligands leads to complexes with less participation of non-radiative decay and therefore to more efficient devices [12], [13]. Among the various ligands used in the metal complexes, Schiff base ligands were widely developed because they are easily obtained, and can be modified in different ways [14], [15], [16]. As a consequence, the physical and photophysical properties, such as color tunability of the corresponding complexes can be easily adjusted [15], [17], [18]. Moreover, increasing the rigidity of the ligands with tetradentate Schiff bases, provides Pt(II) complexes with high thermal stability [17], [18]. In particular, Pt(II) Salophen-type complexes have shown interesting photophysical features: they absorb and emit in the red region, with high emissive quantum yields [19], which make them efficient materials in light emitting devices. In fact, Pt(II) complexes emitting in the deep red/NIR wavelength region have stimulated particular interest for practical applications such as biomedical uses, security or telecommunications [20]. Due to their simple integration, possible cost-effective and potential miniaturization, deep red OLEDs are considered as a promising laptop sources for biosensors [21], [22], [23] and could be useful in phototherapy [24]. However, as compared to the other colors, the design of deep red emitters is more difficult in accordance with the energy gap law [25], [26], [27], since a loss in quantum efficiency in the saturated-red region by self-quenching and triplet-triplet annihilation (TTA) is often observed [28], [29], [30], [31], [32], [33]. Nevertheless, the trade-off issue between efficiency and color purity of OLEDs especially within deep red/NIR region should still be solved [20].
Due to their high stability under ambient conditions (i.e. room temperature and atmospheric pressure), Pt(II) Schiff base complexes were used as dopants in electrophosphorescent materials for robust OLEDs emitting from yellow to red [17], [34], [35], [36], [37].
By promoting intermolecular ligand–ligand and/or metal–metal interactions [38] the planar geometry around the platinum atom favors aggregation and leads to quenching of emission. Thereby, the performance of Pt(II)-based OLEDs exhibits strong dependence on dopant concentration, limiting the optimized doping concentration below 5 wt.% [34], [37], [39]. We also have to keep in mind that some of reported Pt(II) complexes exhibit relatively long emission lifetimes, which might cause saturation of emissive sites, and be involved in TTA, particularly at high current density [40].
In this study, investigations based on materials design, photophysical and electroluminescence properties of symmetric Pt(II) Schiff base complexes as deep red emitters, are reported. Herein, the impact of tert-butyl substitution on chemical and photophysical properties of Pt(II) Salophen complexes is explored. Tert-butyl substituents are chosen in order to reduce excimer formation by precluding aggregation as already proposed by Che [34] and Wong [36]. In this work, [Pt(II) Salophen] 2 is also studied as a reference for comparison. Light emitting devices with an emission wavelength extended into the near IR spectral region have been fabricated by using Pt(II) complexes as dopants in Tris(8-hydroxyquinolinato)aluminium(III) (Alq3) which emits a green light with a maximum of emission centered between 520 nm and 550 nm [41]. As the emission band of Alq3 (host) overlaps the visible absorption band of the platinum(II) Salophen type complexes (guest) an energy transfer from host to guest (Förster transfer [42]) is potentially allowed.
The paper is organized as follows: we first describe the experimental and theoretical frameworks used to characterize and to evaluate the performance of the two Pt(II) complexes 1 and 2 (Scheme 1) from a molecular level to a single emitting layer based device. Then, we discuss the absorption and emission spectra, assign the main electronic transitions by TD-DFT and characterize the lowest triplet state by DFT. Electrochemical data complete the energy level diagram of the complexes. A comparative investigation on opto-electronical properties is carried out on electro-phosphorescent OLEDs obtained by doping Alq3 host matrix with Pt(II) complexes 1 and 2. Finally, the electrical features of various devices of Pt(II) complexes are discussed in relation within the photo/electro-chemical analyses of the two complexes.
Section snippets
Materials and measurements
Salicylaldehyde, 3,5-di-tert-butylsalicylaldehyde and 1,2-phenylenediamine were used as received from Aldrich or TCI. K2PtCl4 was provided by Strem Chemicals. Tetra-tert-butyl-salophen (N,N″-bis-(3,5-di-tert-butylsalicylidene)-1,2-phenylenediamine) was synthesized as previously reported [43a]. All reactions were performed under Ar atmosphere. Chromatographies were carried out on silica gel 70-230 mesh from Aldrich. Solvents were used as received from commercial sources, unless otherwise stated.
Synthesis and thermal properties
Complexes 1 [34] and 2 [37] were obtained by reacting the platinum salt with the ligand in the presence of a base in DMSO under heating at 80 °C overnight. Incorporation of tert-butyl groups enhanced solubility of Pt(II) complexes in most common organic solvents [34], [36].
Therefore, synthetic yield of the more soluble complex 1 decreases because it does not precipitate as soon as it is obtained and some ligand remains unreacted as confirmed by 1H NMR of the crude product. Increasing the
Conclusion
[Pt(II)(tetra-tert-butylSalophen)] 1 and [Pt(II) Salophen] 2, were used as dopants in Tris-(8-hydroxyquinoline)aluminium (Alq3) for the fabrication of organic light-emitting diodes (OLEDs). The crystal structure [Pt(II)(tetra-tert-butylSalophen)] shows that tert-butyl substituents limited complex stacking by introducing steric hindrance between adjacent molecules. Photo/electrochemical analysis supporting by TD-DFT calculations showed that although the tert-butyl substituents had a weak effect
Supplementary data
Figures S1–S19 are available in the Supplementary Information. The structure of the [Pt(II)(tetra-tert-butylSalophen)] was deposited in Cambridge under the number CCDC 1447941.
Acknowledgements
Region Midi-Pyrénées and University Paul Sabatier are acknowledged for B.B's fellowship. The authors thank Dr. SOURNIA-SAQUET Alix and M. MOREAU Alain for the electrochemistry measurements, and Mrs. SEYRAC Stéphanie for the ATD/ATG measurements. M. SCHLEGEL Benoît is acknowledged for his contribution in the setting up of the cleaning part in the OLEDs making process. This work was performed using HPC resources from CALMIP (Grant n° 2011-P1133).
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