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Letter
In Search of Deeper Blues: trans-N-Heterocyclic Carbene Platinum Phenylacetylide as a Dopant for Phosphorescent OLEDs James D. Bullock, Amin Salehi, Charles J. Zeman, Khalil A. Abboud, Franky So, and Kirk S. Schanze ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12107 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017
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ACS Applied Materials & Interfaces
In Search of Deeper Blues: trans-N-Heterocyclic Carbene Platinum Phenylacetylide as a Dopant for Phosphorescent OLEDs James D. Bullocka, Amin Salehid, Charles. J. Zeman IVa, Khalil A. Abbouda, Franky Soc and Kirk S. Schanzeb* a
Department of Chemistry, University of Florida, Gainesville, FL, 32611 Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249 c Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695 d Department of Physics, North Carolina State University, Raleigh, NC 27695 Keywords: OLED, platinum carbene, phosphorescence, electrophosphorescence, blue emission b
ABSTRACT: A trans N-heterocyclic carbene (NHC) platinum(II) acetylide complex bearing phenyl acetylene ligands (NPtPE1) has been synthesized via the Hagihara reaction in 64% yield. The complex features spectrally narrow deep blue emission with a phosphorescence quantum yield (0.30) and lifetime (~10 µs) in the solid state. The modest quantum yield and lifetime make NPtPE1 a candidate for incorporation into an organic light emitting diode (OLED). Prototype devices exhibited a maximum EQE of 8% with CIE (0.20,0.20). To our knowledge this is the first example of a platinum(II) acetylide bearing NHC ligands to be incorporated into an OLED.
Organic light emitting diodes (OLEDs) remain a promising solution for flat panel displays and lighting sources with some commercial acceptance already realized. Much of this success can be attributed to advances in phosphorescent metal complexes which are able to harvest and emit 100% of the generated excitons across much of the visible spectrum.1-3 One area that remains of interest is in deep blue phosphorescent metal complexes and while much progress has been made challenges remain in terms of color purity and stability. Much of the current research into deep blue phosphorescent emitters has focused on iridium(III) complexes with Nheterocyclic carbene (NHC) ligands4-5 which generally show deeper blue emission but lower efficiency when compared to fluorinated iridium(III) complexes such as FIrpic which shows sky blue emission and was incorporated into an OLED more than a decade ago.6 Recently devices incorporating FIrpic have been optimized to afford efficiencies as high as 30% EQE and CIE coordinates of about (0.18, 0.33),2 which does not meet the standard blue requirement of (0.14, 0.08) set by the National Television System Committee (NTSC). More recently the strategy of incorporating NHC ligands complexed to iridium has seen significant improvements in emitter and device design, resulting in a device with CIE coordinates of (0.16, 0.15) with a maximum EQE of 14.4% for the mer-isomer while the fac-isomer exhibited CIE coordinates of (0.16, 0.09) with a maximum external quantum efficiency of 10% while exhibiting high luminance.7 This represents devices closer to the color standard but fall short on efficiency when compared to devices that used FIrpic.
While less common there are also examples of advances with platinum(II) complexes. One of which, platinum(II) [6-(1,3-dihydro-3-methyl-2H-imidazol-2-ylidene-κC2)-4tert-butyl-1,2-phenylene-κC1]oxy[9-(4-tert-butyltpyridin-2yl-κN)-9H-carbazole-1,2-diyl-κC1] PtON7-dtb reported by Li et. al. was both efficient and pure blue with an external quantum efficiency (EQE) of 24.8% and international commission on illumination (CIE) values (0.15, 0.08).8 Unfortunately the device showed a significant efficiency roll-off dropping below 12% at 1000 cd/m2, attributed to degradation of the co-host materials. N-Heterocyclic carbene ligands have gained interest with platinum complexes just as with the iridium complexes because of their strong σ-donating ability. This strong σdonating character of NHCs results in stronger Pt(II) – ligand bonds to reduce the likelihood of non-radiative deactivation of excited states through dissociation of the ligand upon excitation.9 Additionally, the strong σ-donation limits thermal activation of metal-centered non-emissive d-d states by pushing these states to higher energies.10 Pt(II) acetylides bearing NHC ligands have been reported previously, though most are cis complexes with a bidentate ligand.11-13 Though examples of trans-isomers have been reported, they were obtained in yields of 4-27% through isomerization from the corresponding cis complex. These complexes are of interest as they offer reasonable photophysical properties as OLED dopants exhibiting higher quantum yields in the solid state than they do in solution.11
In this work a trans N-heterocyclic platinum (II) acetylide platinum(II) bis(1,3-dicyclohexyl-1H-3l4-imidazol-2yl)bis(phenylethynyl) NPtPE1 (Figure 1) was synthesized directly through a strategy developed in our labs by Winkel ACS Paragon Plus Environment
ACS Applied Materials & Interfaces et. al.14 which showed favorable photophysical properties for OLED device fabrication with a solid state phosphorescence yield of 0.30 doped in a PMMA film with a corresponding average lifetime of 9.4 µs and a narrow deep blue photoluminescence with CIE coordinates of (0.14, 0.12).
phine complex, PPtPE1, are included for comparison in Table 1. The absorption onset of 350 nm, emission onset of 425 nm, and steady state spectroscopy structural features are similar between PPtPE1 and NPtPE1 suggesting the HOMO and LUMO orbitals are located primarily on the acetylide ligands. The most striking difference is seen in NPtPE1 having a much longer triplet lifetime which results in a 2800-fold increase in lifetime and greater than 50-fold increase in quantum yield in THF. This suggests that non-radiative decay pathways, such as thermal population of a metal centered d-d dark state, have been suppressed with the change from the phosphine ligand to the stronger σ-donating carbene ligand. This is due to destabilization of the d(x2-y2) orbital by the stronger field carbene ligands. The quantum yield increases another 10-fold when NPtPE1 is doped into a PMMA matrix along with a longer triplet excited state lifetime. The UV-visible absorption and emission spectra
Figure 1. Structures of NPtPE1 and PPtPE1. PPtPE1 and was characterized previously but is included here for comparison purposes. Upon incorporation into an OLED NPtPE1 achieved a maximum external quantum efficiency of 7.8% with CIE color coordinates of (0.2, 0.2). This work suggests that the possible development of a new approach to high efficiency blue emitters based on the trans-Pt-acetylide motif. NPtPE1 was characterized by 1H NMR, 13C NMR and the structure was confirmed by single crystal X-ray crystallography (Figure 2). The unit cell in the X-ray crystal struc-
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Figure 3. Absorption and emission of NPtPE1 in degassed THF solution. of NPtPE1 in THF are shown in Figure 3. The emission in a solid PMMA matrix is qualitatively the same (Figure S5). There are two strong absorption bands with maxima at 278 nm and 312 nm. The representation of the HOMO and LUMO from DFT calculations (Figure S4) supports the conclusion that these absorption bands are primarily ligand centered (π π*), although there does appear to be some contribution from the metal in the HOMO indicating the transition features a minor contribution from metal-toligand charge transfer (MLCT).
Figure 2. X-Ray crystal structure of NPtPE1 with thermal displacement ellipsoids at 40%. ture contains two unique molecules that differ subtly in geometry. The Pt1A-C1A (carbene) distance is 2.027 Å while the alkyne bond distances were different with Pt1AC16A at 2.051 and Pt1A-C46A at 1.906 Å. One of the molecular structures is shown in Figure 2 and it clearly shows the trans-configuration of the Pt(II)(ICy)2C2 unit. A summary of the photophysical properties of NPtPE1 along with the properties of the analogous trialkyl phos-
The emission spectrum in THF has a maximum at 444 nm and a second maximum at 463 nm but no emission is obTable 1. Photophysical Properties of Selected Platinum Acetylide Complexes Compound
λmaxabs nm
NPtPE1
PPtPE115
ε / M-1cm-1
λmaxem / nm
Φph / THF
τsoln
Φph PMMA
τsolid
278
33,000 (278 nm) 28,200 (312 nm)
444
0.029
1.7 µs
0.30
9.4 µs
324
24,700
435
