Phosphorescence Properties of Ir(ppy)₃Films

Abstract

The pioneering investigations by Thompson and Forrest introduced fac-tris(2-phenyl pyridine)iridium (Ir(ppy) 3 ) as a highly efficient phosphorescent material because the green emission of Ir(ppy) 3 takes advantage of both electrically generated singlet and triplet excitons.1,2 The luminescence efficiency can be increased to a quantum yield of up tõ 100%, 1-6 which suggests emission-related applications. In general, emission from triplet to singlet states is forbidden but the electrons of Ir(III) can induce a quantum mechanical effect, such as spin-orbit coupling, and allow a phosphorescent transition.1-6 In this regard, most studies have focused on monomeric Ir(ppy) 3 in dilute solution (1 × 10 −5 M) or thin films with a low doping ratio (< 1 wt%), whereas there is little information on the intermolecular interactions of Ir(ppy) 3 molecules.2,3 This Note reports the luminescent properties of neat Ir(ppy) 3 films investigated by temperatureand time-dependent spectroscopy. Three triplet substates were found to be responsible for the phosphorescence. Moreover, energy transfer between Ir(ppy) 3 molecules was observed due to the physical proximity in neat films.The photoluminescence (PL) spectrum of a typical neat Ir(ppy) 3 film at 300 K is shown in Figure 1(a). The emission peak was observed at 509 nm, which is similar to the phosphorescence of Ir(ppy) 3 in solution, [4][5][6] suggesting that the triplet states are the emitting states, even in the neat film. The quantum yield is related to the strength of spin-orbit coupling because the phosphorescence is allowed due to the influence of spin-orbit coupling induced by the heavy center ion, Ir(III). In other words, the triplet state with a large extent of metal-to-ligand charge-transfer (MLCT) state, which involves the 5d orbitals of Ir(III) (the highest occupied molecular orbitals, HOMOs) and the π* orbitals of 2-phenylpyridine (the lowest unoccupied molecular orbitals, LUMOs), 4 would show efficient phosphorescence. The contribution of the MLCT character can be estimated by several ways. One of them is the magnitude of zero-field splitting because it is determined by the strength of spinorbit coupling. The triplet states of Ir(ppy) 3 have a zero-field splitting value of 170 cm −1 in the CH 2 Cl 2 solution, 4 suggesting a large MLCT character. The rigidochromism of the emitting states is another explanation. A blue-shift of emission was observed for a transition of the MLCT state in a rigid condition 5,7 because a charge-transfer transition state was less relaxed by dipolar interactions of the solvents in the rigid condition. An emission peak in the neat film was observed at 509 nm, which was blue-shifted by ~10 nm compared to that in the CH 2 Cl 2 solution, 4 indicating the MLCT character in the emitting triplet states of the neat Ir(ppy) 3 film. The MLCT character was also distinguished by a broad and unstructured spectral shape of the phosphorescence spectrum, 7-9 whereas the ligand-centered (LC) transitions exhibited vibronic fine structures.7-10 The PL spectrum...

TCTA:Ir(ppy)3 Green Emissive Blends in Organic Light-Emitting Transistors (OLETs)

“…The photoluminescence spectrum for a neat Ir­(ppy) 3 film (30 nm) is also included as reference (dash line). The overall shape of the photoluminescence spectrum is consistent with previously reported emission for Ir­(ppy) 3 and Ir­(ppy) 3 blends . All spectra show a broad emission due to strong charge-transfer (CT) characteristics of Ir-based cyclometalated complexes, with the main peak at around 515 nm, consistent with phosphorescence signal in Ir­(ppy) 3 solution .…”

“…The overall shape of the photoluminescence spectrum is consistent with previously reported emission for Ir(ppy) 3 and Ir(ppy) 3 blends. 21 All spectra show a broad emission due to strong charge-transfer (CT) characteristics of Ir-based cyclometalated complexes, with the main peak at around 515 nm, consistent with phosphorescence signal in Ir(ppy) 3 solution. 22 Further, we also observed two clear contributions originating from the guest at 545 and 585 nm.…”

Electron-Rich 4-Substituted Spirobifluorenes: Toward a New Family of High Triplet Energy Host Materials for High-Efficiency Green and Sky Blue Phosphorescent OLEDs

“…One can note that the turn-on voltage V on (defined as the driving voltage to achieve the brightness of 1 cd/m 2 ) of 4-Ph­(OMe) 3 -SBF (4.3 V) is higher than that of 4-PhCz-SBF (3.7 V), translating to better charge injection in the former in accordance with the different energy gap of the molecules (Table ). Finally, the electroluminescent spectra of both devices exclusively display the emission of the phosphor Ir­(ppy) 3 in accordance with the emission of the pure Ir­(ppy) 3 …”

“…Finally, the electroluminescent spectra of both devices exclusively display the emission of the phosphor Ir(ppy) 3 in accordance with the emission of the pure Ir(ppy) 3 . 57 Sky blue PhOLEDs were also fabricated and characterized (Figure 11 and Supporting Information). With 4-PhCz-SBF as the host, the maximum EQE reaches 6.7% with corresponding CE and PE of 18.0 cd/A and 11 lm/W.…”

4-Pyridyl-9,9′-spirobifluorenes as Host Materials for Green and Sky-Blue Phosphorescent OLEDs

“…The electroluminescence (EL) spectra of the 4-4Py-SBF green device presented Figure (right) and those of the devices based on the different host materials are identical, exclusively showing the emission of the green dopant at 516/540 nm close to the photoluminescence of pure Ir(ppy) 3 film (509/540 nm) with no parasite emission in the range of the nondoped device (see EL spectra of the devices using 4-3Py-SBF , 4-2Py-SBF , and 2-4Py-SBF in Figures S50, S52, S54, and S56, Supporting Information). This result demonstrates an efficient energy transfer from the four SBF- derivatives to Ir(ppy) 3 due to their high triplet energy levels.…”