A Detailed Evaluation for the Nonradiative Processes in Highly

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 4029−4036

A Detailed Evaluation for the Nonradiative Processes in Highly Phosphorescent Iridium(III) Complexes Jin-Hyoung Kim,† So-Yoen Kim,† Yang-Jin Cho,† Ho-Jin Son,*,† Dae Won Cho,*,†,‡ and Sang Ook Kang*,† †

Department of Advanced Materials Chemistry and ‡Center for Photovoltaic Materials, Korea University, Sejong, 30019, Korea S Supporting Information *

ABSTRACT: To understand the intrinsic nature of nonradiative processes in heteroleptic cyclometalated Ir(III) complexes, highly phosphorescent Ir3+ complexes containing 2-(3-sulfonylfluorophenyl)pyridine (ppySO2F) as the cyclometalated ligand were newly synthesized. Three ancillary ligands, acetylacetonate (acac), picolinate (pic), and tetrakis-pyrazolyl borate (bor), were employed for the preparation of the Ir(III) complexes [Ir(ppySO2F)2(acac)] (Ir-acac), [Ir(ppySO2F)2(pic)] (Ir-pic), and [Ir(ppySO2F)2(bor)] (Ir-bor). The molecular structures were characterized by X-ray crystallography. Blue phosphorescence maxima were observed at 458, 467, and 478 nm for Ir-bor, Ir-pic, and Ir-acac, respectively, at 77 K, and the corresponding emission quantum yields were determined to be 0.79, 0.80, and 0.98 in anaerobic CH2Cl2 at 300 K. Additionally, the phosphorescence decay times were measured to be 3.58, 1.94, and 1.44μs for Irbor, Ir-pic, and Ir-acac, respectively. No temperature dependence was observed for the emission lifetimes in 298−338 K. These results indicate that there is no activation barrier to crossing to a nonradiative state like metal-centered (MC, d−d) state. The radiative rate constants (kr) are within a narrow range of 3.0−5.5 × 10−5 s−1. However, the nonradiative rate constants (knr) are within a wide range of 14.2−0.52 × 10−4 s−1. The knr values showed exponetial correlation with the energy gap. We carried out ab initio calculations to evaluate the energy states and their corresponding orbitals. The nonemissive MC states lie at higher energies than the emissive metal-to-ligand charge transfer (MLCT) state, and hence, the MC states can be excluded from the nonradiative pathway. Scheme 1. Schematic Energy Diagram of Ir(III)-Complexa

1. INTRODUCTION Iridium complexes have been considered for use as phosphorescent dopants in organic light-emitting diodes (OLEDs) because of their high quantum efficiency and short lifetime of triplet excited states.1−5 Ir(III) complexes used in OLEDs have a six-coordinate octahedral structure, which can be further classified as homoleptic6,7 or heteroleptic.8−12 In most cases, identical cyclometalating ligands are used to form homoleptic Ir-complexes through covalent and coordinate bonding. Two types of geometric isomers (i.e., facial and meridional) are found in homoleptic complexes.13 Although there are exceptions, it is accepted that facial Ir-complexes have better photophysical properties, including high-emission quantum yields and suitable lifetimes, than meridional Ircomplexes. Heteroleptic Ir-complexes have two identical cyclometalating ligands and one ancillary ligand. On the basis of the geometry of the cyclometalating ligands, heteroleptic complexes can also be classified as one of two isomers (i.e., cis or trans).14,15 To prepare highly phosphorescent Ir-complexes, which are essential for practical applications in OLEDs, nonradiative processes should be suppressed. Therefore, much effort has been devoted to understanding the nature of nonradiative processes and how they may be inhibited. As shown in Scheme 1, the molecular energy levels of metal complexes are assigned to electronic transitions that can be © 2018 American Chemical Society

①, ②, and ③ denote the MC (d−d), MLCT, and LC (π−π*) transitions, respectively. a

regarded as (1) ligand-centered (LC, π−π*), (2) metalcentered (MC, d−d), and (3) charge-transfer (CT) transitions. The latter transition may be either metal-to-ligand CT (MLCT) or ligand-to-metal CT (LMCT) and involves radical Received: December 20, 2017 Revised: January 30, 2018 Published: January 31, 2018 4029

DOI: 10.1021/acs.jpcc.7b12449 J. Phys. Chem. C 2018, 122, 4029−4036

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The Journal of Physical Chemistry C Scheme 2. Molecular Structures of Ir-acac, Ir-pic, and Ir-bor

Figure 1. (a) Absorption spectra for Ir-acac, Ir-pic, and Ir-bor in CH2Cl2, (b) the emission spectra in CH2Cl2 measured at 298 K (λex = 300 nm), and (c) the emission spectra in MTHF at 77 K (λex = 355 nm).

the ligands. It has been established that electron-withdrawing groups incorporated into the cyclometalated ligand tend to stabilize the HOMO by reducing electron density on the metal.22−25 Moreover, we have examined ancillary ligand effects on the control of the HOMO−LUMO electronic energy gap, the stabilization of the HOMO level, and the strengthening of the metal−ligand bonding. To achieve these goals, we have prepared a series of heteroleptic Ir-complexes composed of 2(3-sulfonylfluorophenyl)pyridine (ppySO2F) as the cyclometalated ligand. Ancillary ligands, acetylacetonate (acac), picolinate (pic), and tetrakis-pyrazolyl borate (bor), were employed for the preparation of [Ir(ppySO2F)2(acac)] (Iracac), [Ir(ppySO2F)2(pic)] (Ir-pic), and [Ir(ppySO2F)2(bor)] (Ir-bor), respectively (Scheme 2). All Ir-complexes showed highly phosphorescent emissions. Synthesized complexes were well characterized by NMR and Xray crystallographic analysis. Theoretical study was performed to understand the orbital shapes and relative energy levels. As a result, we found that the MC state of Ir-complexes is much higher than the emissive MLCT state. We also found that the ancillary ligand influenced the band gaps and HOMO levels, which in turn affect the nonradiative rate constant. We explored the nonradiative behaviors of Ir-complexes applying the “energy gap law”. Actually, the radiationless transitions such as intramolecular internal conversion and intersystem crossing is governed by the energy gap law.

formation by redistribution of electron density on both metal and ligand orbitals. The Ir(III) ion has a d6 electron configuration; therefore, low-spin Ir(III) complexes have empty metal−ligand (M−L) antibonding σM* (dz2* or dx2−y2*) orbitals (Scheme 1). An electron can be populated in the antibonding σM* state by an electronic d−d (MC) transition or by crossing from a phosphorescent state. The weakened M−L bond that results is associated with increased nonradiative deactivation because of structural distortion.16,17 However, it is well-known that cyclometalated heteroleptic Ir(III) complexes have large d-orbital splitting because of strong ligand-field stabilization energy with strong-field ligands.18 Consequently, the MC state is not thermally accessible at room temperature. Despite this, many researchers consider the MC state to be a major nonradiative pathway. One aim of this work is to investigate the location of the MC energy state relative to the emissive state. To develop a deep-blue phosphorescent Ir-complex with a large highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) band gap, two strategies have been considered: (1) the LUMO can be shifted up to higher energy or (2) the HOMO can be shifted down to lower energy. These alternate strategies are depicted by the direction of the yellow arrows in Scheme 1. The former approach may lead to an increase of the MLCT (or LC) states toward the region of the higher lying, antibonding MC (eg*) state. In the MC state, nonradiative processes take place efficiently through bond-breaking or weakening of metal−ligand bonding. Crossing from the emissive MLCT state to the MC state through thermal deactivation can suppress the phosphorescence quantum yield, especially in deep-blue colored emission.19−21 Because of these complications associated with increasing the LUMO energy, we have examined the strategy of decreasing the HOMO energy by utilizing electron-withdrawing groups on

2. RESULTS AND DISCUSSION 2.1. Steady-State Absorption and Emission Properties. Figure 1a shows the absorption spectra of Ir-bor, Ir-pic, and Ir-acac in CH2Cl2. The absorption spectral bands can be assigned to the following: (1) strong spin-allowed, ligandcentered (LC, π → π*) transitions at wavelengths shorter than ∼320 nm, (2) spin-allowed singlet metal-to-ligand charge 4030


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The Journal of Physical Chemistry C

Figure 2. Emission decay profiles of Ir-acac, Ir-pic, and Ir-bor measured (a) in MTHF at 77 K, (b) in anaerobic CH2Cl2 at 300 K, and (c) in aerobic CH2Cl2 at 300 K. The excitation wavelength was 355 nm, and the monitoring wavelength was 480 nm.

Table 1. Photophysical and Electrochemical Parameters of the Heteroleptic Complexes in Ar-Saturated CH2Cl2 at 300 K and in MTHF at 77 K at 300 K

Ir-acac Ir-pic Ir-bor

at 77 K

λp (nm)

τP (μs)

ϕP

kr (105 s−1)

knr (104 s−1)

λp (nm)

τp (μs)

Eox/Ered (eV)a

490.2, 520.3 473.7, 503.5 463.7, 496.2

1.44 1.94 3.58

0.79(5) 0.80(4) 0.98(3)

5.52 4.14 3.01

14.2 10.1 0.52

478.3, 513.3 466.5, 500.4 458.0, 491.9

3.9 3.2 4.3

0.81/−2.04 1.02/nd 1.17/−2.04

ELUMO/3ELUMOc/EHOMOb (eV) (exp)

ELUMO/EHOMO (eV) (theo)

−1.96/−2.50/−5.61 −2.17/−2.60/−5.82 −2.27/−2.56/−5.97

−2.13/−5.89 −2.32/−6.08 −2.31/−6.31

1

a

Redox potential values were measured in CH2Cl2 using a saturated calomel electrode (SCE), and their values are converted on the basis of Fc/Fc+ couple. bThe EHOMO values were determined using eq 1. cThe 1ELUMO and 3ELUMO values were determined using eq 2.

shown in Figure 2a. The lifetimes of 3.2−4.3 μs are similar between the three complexes because of the structural restriction of the rigid ligand environment. However, in fluid CH2Cl2 at 300 K, the lifetimes of Ir-acac and Ir-pic become substantially shortened compared to those at 77 K (Figure 2b). The emission lifetime of Ir-bor is remarkably longer than the other complexes at 300 K. We will discuss in detail in later sections the correlation between the change in lifetime and the energy gap. The emission lifetimes of Ir-acac, Ir-pic, and Ir-bor in aerobic CH2Cl2 were measured as 0.28, 0.48, and 1.84 μs, respectively. The short length of these lifetimes is attributed to quenching by oxygen (Figure 2c). Furthermore, the ϕP of Iracac, Ir-pic, and Ir-bor in aerobic conditions were very low at 0.06, 0.08, and 0.17, respectively. 2.3. Electrochemistry. To determine the energy gap between the HOMO and LUMO, cyclic voltammetry (CV) measurements were carried out. The oxidation waves of Iracac, Ir-pic, and Ir-bor are shown in Figure 3, and their anodic

transfer (1MLCT, Ir-dπ or Ir-dxy → (ppySO2F)-π*) transitions in the range of 320−425 nm, and (3) spin-forbidden triplet MLCT (3MLCT) transitions showing a very weak absorption band at wavelengths longer than 425 nm. The relatively low absorption coefficient strongly suggests that this band can be assigned as a transition to a spin-coupled triplet state. The spinforbidden 3MLCT transitions acquire intensity by effective mixing with higher energy spin-allowed transitions because of the strong spin−orbit coupling of iridium atoms. As shown in Figure 1b, although Ir-bor, Ir-pic, and Ir-acac have identical cyclometalated ligands, their phosphorescence emissions were observed at different wavelengths (464, 474, and 490 nm, respectively) in accordance with their differing ancillary ligands. Broad vibronic bands were observed in the emission spectra at 298 K. Relative intensity and peak position of satellite vibronic bands also depend on the ancillary ligands. At 77 K, sharp emission spectra were observed with a rigidochromic shift to shorter wavelengths. For Ir-bor, an intense emission band was observed at 458 nm with a distinctive satellite vibronic band at 492 nm. Ir-pic and Iracac also showed similarly intense bands and satellite emission bands at 467 and 500 nm for Ir-pic, and at 478 and 513 nm for Ir-acac, as shown in Figure 1c. 2.2. Photodynamic Properties. The phosphorescence lifetimes (τp) were measured in CH2Cl2 at 300 K as shown in Figure 2b and Table 1. Ir-bor showed the longest lifetime of 3.58 μs relative to 1.44 μs for Ir-acac and 1.94 μs for Ir-pic. The phosphorescence quantum yields (ϕP) of Ir-complexes were in the range of 0.79−0.98 in CH2Cl2. Using the τp and ϕP values, the radiative (kr) and nonradiative (knr) decay rate constants were determined by the following equations: kr = ϕP/ τP and knr = (1/τp) − kr. The kr values for these complexes were markedly larger than their knr values. This result indicates that the nonradiative deactivation pathway is negligible in the emissive excited state. The emission lifetimes of Ir-complexes were measured in a glassy 2-methyl tetrahydrofuran (MTHF) matrix at 77 K, as

Figure 3. Oxidation waves for 1 mM CH2Cl2 solution of Ir-acac, Irpic, and Ir-bor containing 0.1 M TBAP electrolyte obtained with a scan rate of 0.1 V/s by cyclic voltammetry. Inset: reduction waves of Ir-acac and Ir-bor acquired under identical experimental conditions. 4031


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Figure 4. Single crystal structures of Ir-complexes. Hydrogen atoms are omitted for clarity. 1

Egopt value of 3.65 eV (340 nm), the LUMO energies can be calculated to be −1.96, −2.17, and −2.27 eV for Ir-acac, Ir-pic, and Ir-bor, respectively. While the LUMO and HSOMO levels of Ir-complexes shift slightly to lower energy in order of Ir-acac, Ir-pic, and Ir-bor, the HOMO levels undergo a larger shift to lower energy, in the same order. As a result, the energy gaps between the HOMO and LUMO (or HSOMO) increase from Ir-acac through Ir-pic to Ir-bor. These results indicate that the ancillary ligand plays an important role in energy gap control. 2.4. X-ray Crystallographic Analysis. The molecular structures of Ir-acac, Ir-pic, and Ir-bor were determined by Xray crystallographic techniques (Figure 4). Ir-acac, Ir-pic, and Ir-bor crystallized in monoclinic C2, orthorhombic Pccn, and trigonal R3̅ space groups, respectively. These Ir-complexes feature octahedral geometry with cis-metalated carbons and trans-pyridine nitrogen atoms from the cyclometalated ligands. The N−Ir−N angles for the two trans-N,N atoms were 174.21°, 174.91°, and 172.96° for Ir-acac, Ir-pic, and Ir-bor, respectively. This small distortion may be the result of repulsive interactions with the ancillary ligand. The cyclometalated ligands have phenyl (ph) and pyridine (py) rings which are nearly coplanar with dihedral angles between the ph and py moieties of 0.22° for Ir-pic and 1.46° for Ir-acac. In Ir-bor, a larger degree of twisting between ph and py results in a dihedral angle of 5.49°, as a result of the bulky bor ligand. As listed in Table 2, the Ir−CPh bond lengths are 1.979 Å for Ir-acac, 1.977

waves were observed at oxidation potentials of 0.81, 1.02, and 1.17 V versus Fc/Fc+, respectively. The oxidation waves shifted gradually to higher potentials in order of Ir-acac, Ir-pic, and Irbor. In Ir-complexes containing identical cyclometalated ligands, the increasing of ligand field strength owing to the electron-accepting properties of the ancillary ligand leads to a higher oxidation potential, which is indicative of stabilization of the HOMO energy level. Therefore, the blue-shift in the absorption spectra is due to a stabilization of HOMO as a result of decreased electron density on the Ir ion extracted by the electron-accepting ancillary ligand. Reduction peaks of Ir-acac and Ir-bor were observed at −2.04 V, as shown in the inset of Figure 3; however, the reduction potential of Ir-pic was not obtained. The redox potentials for the Ir-complexes are summarized in Table 1. The electrochemical energy gap (Eg EC) was calculated as the difference between the onsets of the oxidation and reduction potentials (Eg EC = Eox − Ered). The Eg EC values of Ir-acac and Ir-bor were 2.85 and 3.21 eV, respectively, which are close to the energies of 1MLCT transition. After calibrating the Eox value against Fc/Fc+, the HOMO energy (EHOMO) value was evaluated according to the following eq 1: E HOMO (eV) = −(Eox − E Fc/Fc+ + 4.8)

(1)

Experimentally obtained HOMO energy values (EHOMO) are listed in Table 1. While the value of Eox is accepted as the HOMO energy, Ered cannot strictly be regarded as the LUMO energy. The electrochemically reduced species is an anionic radical, which is different from the excited neutral species. Therefore, the LUMO energy of anionic species is different to that of excited species. The LUMO energy of excited species can be determined using the following relation eq 2: E LUMO (eV) = −(E HOMO + Eg opt)

Table 2. Bond Distances (Å) between Ir and Donor Atoms of the Ligands for Ir-acac, Ir-pic, and Ir-bor Ir-acac Ir−Cph Ir−Cph Ir−Npy Ir−Npy Ir−Oacac Ir−Oacac

(2)

opt

where the Eg indicates the optical energy gap. To evaluate the Egopt, the longest absorption wavelength λonset (nm) is used usually and is converted to eV according to the equation (Egopt = 1242/λonset). The 3Egopt values for the triplet excited state of Ir-acac, Irpic, and Ir-bor were determined using the intersection point of the 3MLCT absorption bands and the phosphorescence spectra to be 3.11, 3.22, and 3.41 eV, respectively. The highest singly occupied molecular orbitals (HSOMO), corresponding to the triplet LUMO, are −2.50, −2.60, and −2.56 eV for Ir-acac, Irpic, and Ir-bor, respectively, as listed in Table 1. However, it is difficult to determine the 1Egopt values because the lowest-energy 1MLCT band merges with other absorption bands such as those corresponding to the π → π* or 3MLCT transitions and no fluorescence emission. Assuming that the cyclometalated ligands of all Ir complexes have an identical

Ir-pic 1.979 2.042 2.144

Ir−Cph Ir−Cph Ir−Npy Ir−Npy Ir−Opic Ir−Npic

Ir-bor 1.977 1.990 2.042 2.043 2.146 2.117

Ir−Cph Ir−Cph Ir−Npy Ir−Npy Ir−Nbor Ir−Nbor

2.000 2.012 2.052 2.054 2.142 2.153

and 1.990 Å for Ir-pic, and 2.000 and 2.012 Å for Ir-bor, which are similar to the values reported for Ir-complexes having phenylpyridine ligands.13,26 The Ir−O and Ir−N bond lengths of the ancillary ligands are longer than those of Ir−Cph and Ir− Cpy of the cyclometalated ligands, as listed in Table 2. In brief, the bond lengths Ir−Cph have covalent bond character and are shorter than coordination bonds such as Ir−Oacac, Ir−Npic, Ir− Opic, and Ir−Nbor of the ancillary ligands. The coordination bonds of Ir−Npy of the cyclometalated ligands are shorter than those of the ancillary ligands. These results imply strong bonding between Ir and the coordinated ligands, resulting in an increase in the d-orbital splitting; specifically, there is an 4032


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Figure 5. Frontier molecular orbitals and an energy-level diagram of singlet Ir-acac, Ir-pic, and Ir-bor from the DFT calculations.

Figure 6. (a) Arrhenius plots of ln(kp) versus the reciprocal temperatures. (b) The relationship between ln(knr) and energy gap: 1: Ir-acac; 2: Ir-pic; and 3: Ir-bor.

increase in energy of the antibonding dz2* and dx2−y2* orbitals. The lifting of these orbitals means that the MC state is located at much higher energy than the emissive 3MLCT state. 2.5. Density Functional Theory (DFT) Calculations. On the basis of the geometric parameters obtained by X-ray crystallography, we performed DFT calculations to evaluate the energy levels and orbital distribution. The B3LYP (UB3LYP for triplet state) hybrid functional and 6-31G(d,p) basis set were employed. The HOMO and HOMO−1 (H-1) of the Ircomplexes correspond to the dxy orbital and ligands, as shown in Figure 5. The dxy orbital is significantly mixed with the ligands. The HOMO is especially localized on the Ir ion (dxy) and is mixed with the phenyl moiety of the cyclometalating ligand. The H-1 is also localized on the Ir ion (dxy) but is mixed with the ancillary ligand. The LUMO of all the Ir-complexes is localized on the pyridine moiety of the cyclometalating ligand. Therefore, the transition between the HOMO and the LUMO is a characteristic MLCT transition. The LUMO+9 (L+9) (Iracac) and L+10 (Ir-pic and Ir-bor) show the characteristic antibonding dz2* orbital. Antibonding dx2−y2* orbitals are found at L+10 (Ir-acac), L+11 (Ir-pic), and L+13 (Ir-bor), respectively. The dz2* orbital lies lower in energy than the dx2−y2* orbital. Both d-orbitals lie in much higher energy states compared to the LUMO.27,18 Therefore, both states are hard to access by thermal activation near ambient temperatures. The dz2* and dx2−y2* orbitals are classified as antibonding orbitals; thus, the Ir-ligand bonding is weakened or broken by the d−d

transition from the dxy and dπ orbitals. Therefore, it is noteworthy that such a large d-orbital splitting is attainable for a cyclometalated Ir-complex, which makes it possible to limit nonradiative processes in these complexes. We also calculated the HSOMO (HS) and LSOMO (LS) energies for the triplet state (Figure S1) after the geometry optimization with the triplet spin multiplicity. The dz2* orbitals are found at HS+8, HS+8, and HS+10 of Ir-acac, Ir-pic, and Irbor, respectively. Moreover, the dx2−y2* orbitals are found at HS +10, HS+10, and HS+12 for the same Ir-complexes. Therefore, in the excited triplet state, the dz2* and dx2−y2* orbitals have higher energies than the emissive 3MLCT state. 2.6. Temperature Effect on Emission Decay. No change in phosphorescence lifetimes was observed in the temperature range of 298−338 K. For the Arrhenius plots shown in Figure 6a, the natural log of the phosphorescence rate constant (ln(kp)) values were constant with changes in temperature. This indicates that any activation energy barriers did not involve progress to deactivation pathways such as the MC state or drastic structural changes. Many reports have considered the MC state to be the nonradiative state without conclusive evidence.28 However, for the Ir-complexes studied here, any small activation energy for crossing to the MC state was not observed. Therefore, the nonradiative MC state can be excluded from the nonradiative pathway because the MC state (eg* state in Scheme 1) lies at much higher energy than the emissive MLCT state in cyclometalated Ir-complexes. 4033


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poured in water. The resulting precipitate was collected by filtration and was washed with water and n-hexane. The yellow power was purified by flash column chromatography (yield: 66%). 1H NMR (300.1 MHz, CDCl3): δ 8.827 (d, J = 5.4 Hz, 1H), 8.373 (d, J = 7.5 Hz, 1H), (dd, J = 11.7 Hz, 12.3 Hz, 2H), 8.111−7.891 (m, 5H), 7.688 (d, J = 4.8 Hz, 1H), 7.506−7.447 (m, 2H), 7.401−7.283 (m, 3H), 7.163 (t, J = 6.9 Hz, 1H), 6.657 (d, 8.7 Hz, 1H), 6.393 (d, 8.7 Hz, 1H). ESI-MS: m/z 788.4526 for [M + H]+ (calcd 787.0234 for C28H18F2IrN3O6S2). Ir-bor. The mixture of Ir-dimer complex (0.5 g, 0.36 mmol) and silver trifluoromethanesulfonate (AgSO3CF3, 0.19 g, 0.72 mmol) was dissolved in 30 mL solution of CH2Cl2/CH3OH (v/v = 1:1) and was stirred at room temperature for 2 h. The solution was filtered using cannular for remove salts; the filtrate was concentrated using the reduced pressure. The residue was dissolved in 10 mL CH3CN, and tetrakis(1-pyrazolyl)borate potassium (0.34 g, 1.08 mmol) was added. The solution was refluxed at 150 °C for 12 h under nitrogen condition and was cooled to room temperature. After cooling to room temperature, the crude product was extracted with CH2Cl2 and was dried over anhydrous MgSO4. After removal of the solvent under reduced pressure, flash chromatography gave the product (yield: 31%). 1H NMR (300.1 MHz, CDCl3): δ 8.148 (d, J = 1.2 Hz, 1H), 7.946 (d, J = 7.8 Hz, 1H), 7.772 (t, J = 7.5 Hz, 1H), 7.726 (s, 1H), 7.385−7.320 (m, 2H), 7.187 (d, J = 2.1 Hz, 1H), 6.942 (t, J = 6.3 Hz, 1H), 6.817 (d, J = 1.5 Hz, 1H), 6.401 (d, J = 7.8 Hz, 1H), 6.261 (t, J = 2.1 Hz, 1H), 6.186 (d, J = 2.1 Hz, 1H), 6.054 (d, J = 1.2 Hz, 1H). ESI-MS: m/z 945.7231 for [M + H]+ (calcd 944.1270 for C34H26BF2IrN10O4S2). 3.2. Spectroscopic Measurements. The steady-state absorption and photoluminescence spectra were recorded on an Agilent Technologies Cary 5000 and a Varian Cary Eclipse, respectively. The phosphorescence spectra at various low temperatures were measured using an intensified chargecoupled device (ICCD, Andor, iStar). The samples were excited by nanosecond laser pulses of the third harmonic generation (355 nm, full width at half-maximum (fwhm) of 4.5 ns) from a Q-switched Nd:YAG laser (Continuum, Surelite II10). Phosphorescence lifetimes were measured using a digital oscilloscope (Tektronix, TDS-784D) equipped with a fast photomultiplier tube (Zolix Instruments Co., CR 131). 3.3. CV Measurement. The cyclic voltammetry experiments were performed using an electrochemical analyzer (CH Instruments, CHI660e). The three-electrode cell system used comprised a platinum disk (diameter 1.6 mm) electrode as the working electrode and a platinum wire and standard calomel electrode (SCE) as the counter and reference electrodes, respectively. Freshly distilled, degassed CH2Cl2 was used as the solvent with 0.1 M tetrabutylammonium perchlorate (TBAP) electrolyte as the supporting electrolyte. To check the number of electrons transferred per molecule, a linear sweep voltammetry experiment using a carbon microdisk electrode was performed. The electrochemical data were calibrated against a ferrocenium/ferrocene redox couple (Fc+/Fc). 3.4. DFT Calculation. The DFT calculations were performed using the Gaussian 09 program package.30 The ground-state geometry was fully optimized at the DFT level using the B3LYP31,32 (UB3LTYP for triplet calculations) method. The 6-31G(d,p) basis set was applied for nonmetal atoms, and the LANL2DZ (Los Alamos National Laboratory 2 double-ζ) basis set was used to model the iridium atom.33 The molecular coordination obtained by X-ray crystallography was

Because the nonradiative pathway does not include the MC state, the Ir-complexes prepared here have relatively high ϕP values, as listed in Table 1. The knr values increase in the order of Ir-acac to Ir-pic to Ir-bor as listed in Table 1. The knr value is usually governed by the “energy gap law” (eq 3):29 ln(k nr) ∝ ( − γ /ℏωM) × (ΔE)

(3)

where ΔE is the energy gap between the excited triplet and ground states involved, γ is a term that can be expressed in terms of molecular parameters, and ωM is the maximum and dominant vibrational frequency available in the system. The experimental values for the energy gap were evaluated from the 0−0 vibronic peaks of the corresponding phosphorescence. A linear relationship between the ln(knr) and ΔE values was confirmed, as shown in Figure 6b, confirming that the nonradiative rate decreases exponentially for large energy gaps (ΔE). The nonradiative decay of the 3MLCT state is linear dependent on the energy gap. This means that the specific deactivation channel such as the crossing to MC state can be excluded. From the slope value, the γ/h̵ωM value for these Ircomplexes was estimated to be −22 eV−1, which is close to a low vibrational energy of a few hundred wavenumbers.

3. EXPERIMENTAL SECTION 3.1. Synthesis. Cyclometalated Ligand. Under a N2 atmosphere, 3-bromobenzene-1-sulfonyl fluoride (2 g, 8.37 mmol) and 2-(tri-butylstannyl)pyridine (3.08 g, 8.37 mmol) were mixed in toluene. Then, Pd(pph3)4 (0.48 g, 5 mol%) was added as a catalyst for the Stille cross coupling reaction. The reaction was allowed to proceed at 110 °C for 16 h. After cooling to room temperature, the solvent was removed under reduced pressure. The dark residue was purified by flash column chromatography (yield: 72%) 1H NMR (300.1 MHz, CDCl3): δ 8.721 (d, J = 4.8 Hz, 1H), 8.654 (s, 1H), 8.408 (d, J = 7.8 Hz, 1H), 8.027 (d, J = 7.5 Hz, 1H), 7.852−7.698 (m, 3H), 7.327 (t, J = 6.3 Hz, 1H). Electrospray ionization mass spectrometry (ESI-MS): m/z 238.2524 for [M + H]+ (calcd 237.0260 for C11H8FNO2S). Ir-Dimer. IrCl3·3H2O (0.5 g, 1.42 mmol) was added to a stirred solution of the cyclometalating ligand (0.84 g, 3.55 mmol) in 2-ethoxymethanol. The mixture was refluxed at 130 °C with stirring under N2 overnight. After cooling to room temperature, the solvent was removed in vacuo. The yellow solid, presumed to be the intermediate bis(μ-Cl) dimer complex, was washed with cold n-hexane to remove any free ligand and was dried. Ir-acac. The mixture of Ir-dimer complex (0.5 g, 0.36 mmol), acetylacetone (0.074 mL, 0.72 mmol), and Na2CO3 (0.38 g, 3.6 mmol) was dissolved in 10 mL of 2-ethoxyethanol and was refluxed for 12 h. After cooling to room temperature, the reaction mixture was poured into water. The resulting precipitate was collected by filtration and was washed with water and n-hexane. The yellow powder was purified by flash column chromatography (yield: 71%). 1H NMR (300.1 MHz, CDCl3): δ 8.487 (d, J = 5.4 Hz, 2H), 8.105 (s, 2H), 8.038 (d, J = 8.4 Hz, 2H), 7.944 (t, J = 7.8 Hz, 2H), 7.376 (t, J = 6.0 Hz, 2H), 7.224 (d, J = 8.1 Hz, 2H), 6.482 (d, J = 8.4 Hz, 2H), 5.29 (s, 1H), 1.822 (s, 6H). ESI-MS: m/z 765.3724 for [M + H]+ (calcd 764.0438 for C27H21F2IrN2O6S2). Ir-pic. The mixture of Ir-dimer complex (0.5 g, 0.36 mmol) and picolinic acid (0.09 g, 0.72 mmol) was reacted with Na2CO3 (0.13 g, 1.26 mmol) in 2-ethoxyethanol (10 mL). After cooling to room temperature, the reaction mixture was 4034


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The Journal of Physical Chemistry C Notes

applied as the initial geometries. The frontier molecular orbitals were visualized by a GaussView program with isodensity plots (contour = 0.035 au). Quantum Yield. The ϕP values of Ir-complexes in anaerobic CH2Cl2 solution were determined by relative method using well-established quantum yield standard, 9,10-diphenylanthracene (ϕF = 1).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2014R1A6A1030732 and NRF-2017R1D1A3B03033085). The research was supported by the International Science and Business Belt Program through the Ministry of Science and ICT (2017K000494). This research was supported also by the MOTIE (Ministry of Trade, Industry & Energy (10051379) and Korea Display Research Corporation (KDRC) support programs for the development of future device technology for the display industry.

4. CONCLUSIONS We have investigated the nonradiative decay pathway in highly phosphorescent heteroleptic Ir(III) complexes containing ppySO2F ligands. Three kinds of ancillary ligands were introduced to control the emission band gap. All the Ircomplexes showed high emission quantum yields of 0.79−0.98 in anaerobic CH2Cl2 at 300 K. Similar kr values were determined for all the Ir-complexes. However, the Ir-complexes have quite different knr values, which showed exponential correlation with the energy gap. The molecular energy levels and their orbitals were determined through ab initio calculations. The calculations indicate that the nonemissive MC (d−d) states are much higher in energy than the emissive state; hence, the emission lifetimes were not influenced by temperature change. We concluded that the MC states can be excluded from various nonradiative pathways because the covalent bonding character increases the MC state away from the emissive state, at least for the cyclometalated Ir-complexes studied here.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12449. Frontier molecular orbitals and an energy-level diagram for triplet state (PDF) Cambridge Crystallographic Data (PDF) Cambridge Crystallographic Data (PDF) Cambridge Crystallographic Data (PDF) Cambridge Crystallographic Data number CCDC 1576264 (CIF) Cambridge Crystallographic Data number CCDC 1576265 (CIF) Cambridge Crystallographic Data number CCDC 1576266 (CIF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jin-Hyoung Kim: 0000-0002-5709-0399 Ho-Jin Son: 0000-0003-2069-1235 Dae Won Cho: 0000-0002-4785-069X Sang Ook Kang: 0000-0002-3911-7818 4035


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A Detailed Evaluation for the Nonradiative Processes in Highly

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