Tuning State Energies for Narrow Blue Emission in Tetradentate

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Tuning State Energies for Narrow Blue Emission in Tetradentate Pyridyl-Carbazole Platinum Complexes Tyler Fleetham,†,§ Jessica H. Golden,‡,§ Muazzam Idris, Han-Ming Hau, Daniel Sylvinson Muthiah Ravinson, Peter I. Djurovich, and Mark E. Thompson* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

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ABSTRACT: Narrow, deep blue emitters are highly desired in the field of organic light emitting diodes for high quality full color display and solid-state lighting applications. PtNON is reported as a deep blue emitting phosphor but is limited by its broad emission spectrum, making it unsuitable for high quality full color display applications. In this work, we report a strategy to fine-tune the color and the emission line shape of PtNON derivatives by incorporating electron donating (methyl or methoxy) or withdrawing (trifluoromethyl) substituent groups at the positions para to the nitrogen of the pyridines in PtNON. These substitutions resulted in destabilization or stabilization of the charge transfer state (CT) relative to the ligand centered (LC) state, resulting in complexes with narrow or broad emission spectra in various media. PtNONOMe emits predominantly from the LC state, giving a narrow emission spectrum with fwhm = 48 nm in any media. PtNON-Me emits largely from the LC state in nonpolar media (fwhm = 54 nm) and predominantly from the CT state in polar media (fwhm = 83 nm). Last, PtNON-CF3 emits solely from the CT state in any media, giving it a broad emission spectrum (fwhm = 98 nm). The photoluminescence quantum yields of PtNON-OMe, PtNON-Me, and PtNON-CF3 in 1% doped PMMA films are 89, 95 and 20% with emission lifetimes of 27.1, 7.17, and 0.96 μs, respectively.

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thermally activated delayed fluorescent (TADF) materials, as well as materials based on other metals such as Cu,4 Au,4c,5 Pd,6 and Pt,6d,7 have emerged over the past several years, some with device efficiencies matching or exceeding those of Ir(III) derivatives, though many of these new emitter designs have yet to demonstrate comparable operational stabilities.6c,8 Recently, the success of Ir(III) complexes has been challenged by highly stable Pt(II) complexes utilizing rigid tetradentate ligands.9 Pt(II) complexes with tetradentate ligands, particularly those containing pyridyl-carbazole moieties, have shown remarkable operational lifetimes, highly saturated colors, and very high efficiencies.10 However, the simultaneous achievement of saturated blue emission, high efficiency, and long operational lifetime remains elusive for this class of materials just as for any other blue phosphors. The recent report of a tetradentate Pt complex PtNON (Scheme 1)11 with all six-membered-ring chelates, however, demonstrated an operational lifetime over 600 h at 1000 cd/m2 for an emission with a triplet energy in excess of 2.8 eV.11d The absence of 5-membered azole rings or fluorinated ligands, each thought to be a source of device degradation,12 as well as the complex’s rigid molecular design may be partially responsible for this extended operational lifetime. Unfortunately, the broad charge transfer (CT) character of PtNON at room temper-

INTRODUCTION Organic light emitting diodes hold promise as the premier technology for the next generation of display and lighting technologies. They are highly energy efficient and colortunable and provide the option of novel form factors such as curved displays and flat-panel lighting. The compatibility of amorphous organic materials with a wide range of substrates and deposition techniques has enabled their adoption in stateof-the-art displays and lighting for televisions, mobile devices, wearables, and automotive and aerospace applications.1 Rapid research progress and commercial adoption in these fields has been achieved over the past several years, but major deficiencies remain. In particular, the absence of a stable and efficient blue OLED has stunted operational lifetimes, energy efficiencies, and the reduction of costs for both display and lighting applications.2 It is clear that further progress in the OLED field as a whole hinges on the development of blue emissive phosphorescent materials and device architectures which stabilize the high energy charge carriers and excitons inherent to the emission of blue photons. Highly efficient blue OLEDs require phosphorescent dopants with high energy triplets (2.8 eV), high luminescent quantum yield, and high stability under operating conditions. Among the possible classes of emissive materials, phosphorescent organometallic materials, primarily those based on octahedral Ir(III) complexes, have shown the most promise due to their ability to harvest 100% of electrogenerated excitons.3 Pure organic © XXXX American Chemical Society

Received: June 25, 2019

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DOI: 10.1021/acs.inorgchem.9b01888 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

However, no general paradigm for blue-shifting and narrowing phosphorescent emission in these materials has been put forth. Li et al. have shown that functionalization of phenyl-pyrazole (ppz) based Pt complexes (PtON1, Scheme 1) is a method which can be used to tune the lineshapes of room temperature emission spectra.13c The proposed explanation was that narrow emission can be achieved by suppressing the contribution of metal to ligand charge transfer (MLCT) from the pyridylcarbazole relative to the ligand centered (LC) triplet of the ppz fragment. They demonstrated the utility of this strategy by substituting the pyridine with various electron donating (stabilized MLCT) or electron withdrawing (destabilized MLCT) moieties to make the phosphorescent emission broader (CT) or sharper (LC), respectively. Although this work served to illuminate the utility of state energy tuning to achieve narrowed emission features, the model reported therein proved insufficient to explain the 77 K emission properties of other previously reported ppz emitters, such as PtOO1, which has large first and second vibronic progressions. Moreover, the model does not explain the similarity in the 77 K emission spectra of PtNON to that of PtON1.14 In fact, many of the tetradentate complexes containing carbazolepyridine ligands, for example PtON1, PtON7-dtb (Scheme 1), and PtNON, showed similar sharp features in their cryogenic emission spectra (e.g., see Figure 1).11d,13a,14b Thus, it appears that the narrow emission in the PtON1 derivatives is not from the ppz portion of the ligand but rather from the pyridylcarbazole moiety that is common to all of these emitters. As shown in Figure 1, phosphorescence from PtNON shows positive solvatochromic behavior at room temperature, indicating emission from a charge transfer state that is stabilized in polar media. In 2-methyltetrahydrofuran (2MeTHF) at 77 K, however, phosphorescence in Pt(NON) is characterized by a narrow, vibronically structured emission profile. At room temperature, PtNON doped into thin films of poly(methyl methacrylate) (PMMA) displays an emission spectrum that is red-shifted from spectra recorded at 77 K, with a high energy shoulder that suggests a contribution from the higher energy state seen at lower temperatures. Broad, structureless electroluminescence (EL) is observed (λmax = 476 nm, fwhm = 90 nm) from OLEDs prepared using PtNON, indicating the charge transfer character dominates in the devices.11d Ideally, a narrow EL spectrum similar to emission observed at 77 K is highly desired in order to blue-shift the emission color without increasing the triplet energy. Thus, in this work we have sought to tune the relative contributions of the competing triplet states in order to achieve efficient and narrow blue emission at room temperature. To elucidate the origin of the narrow emission in pyridylcarbazole-based tetradentate Pt complexes, four PtNON derivatives with different substitutents at the 4 position of the pyridine were modeled (i.e., PtNON, PtNON-CF3, PtNON-Me, and PtNON-OMe in Scheme 1) via DFT at the B3LYP/LACVP** level of theory (Figure 2). The calculations show lowest unoccupied molecular orbitals (LUMO) localized on the pyridine rings in all cases, and highest occupied molecular orbitals (HOMO) localized predominantly on the platinum and the platinum-adjacent rings of the carbazole moieties. Consequently, the HOMO energies are minimally influenced by the pyridine substitutions, whereas the LUMO energies vary dramatically from −2.0 eV for PtNON-CF3 to −1.11 eV for PtNON-OMe upon this single site substitution.

Scheme 1. Structures of PtNON Derivatives

ature (Figure 1) leads to phosphorescence with an emission maximum at 492 nm in 2-MeTHF at room temperature,

Figure 1. Room temperature emission spectrum of PtNON in various media and 77 K spectrum in 2-MeTHF.

rendering it unsuitable for display-blue pixels or high colorrendering index (CRI) white lighting.11d It is highly desirable to develop emitters with similar structures to PtNON, but with narrower emission characteristics that can be tuned for display and lighting applications. This work makes an effort to establish design criteria for the synthesis of platinum-based phosphorescent materials capable of meeting the challenges inherent to highly efficient and stable blue OLEDs. In this report, the design, synthesis, and characterization of a series of functionalized PtNON derivatives (Scheme 1) with variable lineshapes and emission maxima ranging from 446 to 590 nm are presented, and materials design considerations for narrow, high efficiency emission from platinum phosphors are considered.

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RESULTS AND DISCUSSION Recently, a few reports have demonstrated narrow emission through a variety of ligand designs and substitutions.13 B

DOI: 10.1021/acs.inorgchem.9b01888 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Top row: LUMO densities and energies. Bottom row: HOMO densities and energies. DFT calculations were performed at the B3LYP/ LACVP** level.

Figure 3. Natural transition orbitals (NTOs) for the CT (bottom) and LE (top) states in PtNON derivatives. Blue and red plots refer to hole and electron densities, respectively, for each transition.

trifluoromethyl group. Thus, the T1 state is 3CT in character, wherein the hole is delocalized across the platinum carbazole and extended onto the pyridine, and the electron is confined to the pyridine. The T2 state in PtNON-CF3 has 3LE character, similar to that observed for the T1 state in the other PtNON complexes. The difference in energy between the T1 (3LE) and T2 (3CT) energies in PtNON-OMe is calculated to be 90 meV, which is nearly 4-fold greater than kT at room temperature, whereas the difference between the two states in PtNON-Me

The lowest energy triplet state (T1) in PtNON-OMe, PtNON-Me, and PtNON is locally excited (3LE, see NTOs in Figure 3 and Table 1), where the hole and the electron are localized predominately on the carbazole. The higher lying triplet, T2, involves a charge transfer transition, 3CT, see NTOs in Figure 3. In contrast, PtNON-CF3 shows a pyridinecentered LUMO that is significantly lower in energy than the comparable pyridine based MO in the other three derivatives, due to the strong electron withdrawing effect of the C

DOI: 10.1021/acs.inorgchem.9b01888 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Excited Triplet State Energies and Dipole Moments of the Complexes Computed Using TDDFTa equilibrium solvation

μ (D)

nonequilibrium solvation

T1

T2

T1

T2

S0

T1

T2

PtNON-CF3

2.26/CT

2.57/CT

2.38/CT

2.64/CT

PtNON

2.76/LE

2.81/CT

2.77/LE

2.84/CT

PtNON-Me

2.80/LE

2.84/CT

2.80/LE

2.87/CT

PtNON-OMe

2.83/LE

2.91/CT

2.83/LE

2.93/CT

4.44 (1.15, −4.27, -0.39) 8.39 (−0.84, −8.34, 0.47) 9.46 (−0.72, −9.41, 0.63) 11.58 (−0.84, −11.54, 0.36)

6.58 (4.49, 3.59, 3.20) 6.47 (−1.25, −6.32, −0.53) 8.36 (−2.17, −8.07, 0.11) 10.95 (−1.47, −10.84, 0.34)

7.54 (5.22, 4.26, 3.37) 4.42 (−2.80, −1.93, −2.82) 5.31 (−3.45, −2.96, −2.75) 7.13 (−3.50, −5.67, −2.51)

a

T1 and T2 energies were obtained from TD-DFT (CAM-B3LYP/LACVP**) using the IEFPCM solvation model for THF in the equilibrium and nonequilibrium limit. States are assigned as being either charge transfer (CT) or locally excited (LE). Dipole moment vectors are reported for a coordinate system with Pt as the origin. The y axis intercepts the oxygen atom connecting the two carbazoles, and the z axis goes through the plane of the ligands.

Scheme 2. Synthetic Scheme of PtNON Derivatives

response to the excited state of interest and can be used to model the effect of solvation in a fluid environment. On the basis of the TD-DFT results, the T1 and T2 states in the three more electron-rich derivatives (PtNON, PtNON-Me, and PtNON-OMe) are found to have predominantly 3LE and 3CT character, respectively, whereas in the electron-poor PtNONCF3 derivative, both the T1 and T2 states are found to be 3CT in character. In all cases, the 3CT states are destabilized in the nonequilibrium limit relative to the equilibrium, whereas the 3 LE states remain largely unperturbed. This difference in response is due to the orientation of 3CT transitions opposing the inherent ground state dipole moment in each of the complexes as shown in Table 1. For molecules such as these where the excited state dipole moment is oriented opposite the ground state moment, the 3 CT state is destabilized in the nonequilibrium limit due to the inability of the frozen medium to effectively stabilize the new electronic distribution. Meanwhile, in the equilibrium limit, both electronic and nuclear degrees of freedom of the solvent can respond to the electronic configuration of the 3CT state and can stabilize it, as is the case in a fluid medium. The 3LE states, in contrast, do not undergo any significant change in dipole moment upon photoexcitation relative to the ground state and thus remain largely unperturbed by the solvent. In the equilibrium solvation limit, the stabilization of the 3CT

and PtNON is calculated to be smaller (40 and 50 meV, respectively). The decrease in energy separation suggests that temperature and solvation effects may have a greater effect on the emitting state in these species than in PtNON-OMe (electron rich) and PtNON-CF3 (electron poor). Together, these calculations indicate that destabilizing the carbazolepyridine 3CT state relative to the ligand-centered carbazole 3 LE state could be accomplished by substitution at a single site on pyridine. The substituent will alter the energy of the pyridine-centered LUMO, leading to a relative reordering of the two lowest energy triplet states, thereby narrowing and blue-shifting the emitting spectra. To study the effects of solvation on the energies of the excited states, TD-DFT calculations were performed on each of the derivatives using the IEF-PCM solvation model15 both in the equilibrium and nonequilibrium limit as implemented in Gaussian 09.16 The nonequilibrium scheme accounts for the relaxation of the solvent’s electronic cloud in response to the excited state charge distribution, whereas the slower (nuclear) degrees of freedom are considered to be in equilibrium with the ground state charge distribution. Therefore, the scheme can be used to model rigidochromic effects in frozen polar matrices (77 K) that arise due to the inability of the solvent molecules to reorient in response to the excited state charge distribution. The equilibrium solvation calculation accounts for both electronic and nuclear relaxation of the solvent in D

DOI: 10.1021/acs.inorgchem.9b01888 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

pyridine charge transfer. The CT absorption in PtNON is blue-shifted relative to PtNON-CF3, and that of PtNON-Me is even further blue-shifted. In PtNON-OMe, there is no discernible CT absorption, although it is possible that it is blue-shifted and obscured by the π−π* absorption feature. These absorption characteristics indicate the expected tuning of the charge transfer energies by pyridine substitution, with CT energy for PtNON-OMe > PtNON-Me > PtNON > PtNON-CF3. The emission spectra of the four complexes in 2-MeTHF at room temperature, in frozen 2-MeTHF solution at 77 K, and in 1% doped PMMA thin films are shown in Figure 5, and their corresponding photophysical data are tabulated in Table 3. PtNON-CF3 shows broad, featureless emission spectra under all conditions (fluid solution, dilute PMMA film, and at 77 K) in accordance with the charge transfer character observed in the absorption spectra. The spectrum at room temperature displays a weak, broad orange-red emission band with a maximum at 590 nm that tailed past 700 nm. The low quantum yield (Φ = 0.02) and the short excited state decay time (τ = 74 ns) indicate a high nonradiative rate. The nonradiative decay rate is suppressed in the rigid PMMA (Φ = 0.20, τ = 1 μs) film and is further suppressed in frozen solution at 77 K (τ = 7.9 μs). These data suggest that the highly stabilized CT state, which delocalizes triplet spin density across the chromophore, is more sensitive to structural distortions than the more carbazole-localized triplet state observed in the parent structure PtNON. PtNON, which shows a much higher energy CT absorption than the CF3 derivative, nevertheless displays CT emission at room temperature (λmax = 492 nm in 2-MeTHF solution and 469 nm in PMMA). Unlike the CF3 derivative, PtNON exhibits highly efficient emission in both solution (Φ = 0.60) and PMMA (Φ = 0.99) with relatively short excited state decay times of 3.1 and 4.2 μs, respectively. Moreover, in frozen solution, PtNON forms a narrow and highly structured emission at 77 K, accompanied by a much longer emission lifetime of 11 μs. The sharp, structured emission and longer emission lifetime indicates a change in the nature of the emissive state from mostly charge transfer in character to a ligand centered state in frozen solution. PtNON-Me, which has an even higher energy CT absorption band (λmax = 480 nm), shows a blend of vibronically structured and CT emission depending on the matrix. The emission of PtNON-Me in PMMA is narrow and vibronically structured (λmax = 446 nm, Φ = 0.95), and the excited state decay is not monoexponential. A two-component fit with 3.2 μs (38%) and 9.6 μs (62%) components gives a χ2 of 0.9. In solution, however, the emission is characterized by an efficient (Φ = 0.79), mostly structureless emission band (λmax = 478 nm, τ = 4.5 μs). Cooling the dilute solution of PtNONMe to 77 K sharpens the emission relative to room temperature and results in a significantly lengthened emission lifetime (25.9 μs). The increased lifetime indicates that PtNON-Me phosphorescence has more ligand-centered character than the CF3 or unsubstituted derivatives. Fits of the excited state decay time of PtNON-OMe at room temperature in the polymer matrix to a biexponential are consistent with two different emission processes; i.e., both the 3 LE (T1) and CT (T2) states emit efficiently at room temperature. In the fourth derivative, a strong electron donating group (methoxy) further destabilizes the LUMO energy of the

state in PtNON and PtNON-Me brings it close in energy to the 3LE state (to within about 50 meV). Therefore, the 3CT and the 3LE states for these derivatives are predicted to be in thermal equilibrium at room temperature. However, thermal equilibration to the 3CT state is unfavorable at a frozen 77 K owing to the widening of the 3CT−3LE gap brought about by the relative destabilization of the 3CT state, along with the decreased temperature. In the extreme, a large 3CT−3LE separation in PtNON-OMe, both in the equilibrium and nonequilibrium limit, leads to the expectation that narrow 3LE emission will dominate at both cryogenic and room temperatures. In the case of PtNON-CF3, where both lowest energy triplet states are 3CT in nature, broad 3CT emission is expected in all media. Each of the four modeled complexes, PtNON-CF3, PtNON, PtNON-Me, and PtNON-OMe, were synthesized according to Scheme 2, and their electronic and photophysical properties were compared with calculated values to probe the effects of tuning the relative energies of the 3LE and CT states. In each of the four compounds, the oxidation potentials, which can be used to approximate the HOMO energy, remained relatively constant (Table 2).17 Two distinct reduction potentials were Table 2. Redox Potentialsa and HOMO and LUMO Levelsb

PtNONCF3 PtNON PtNONMe PtNONOMe

Eox. (V)

Ered. (V)

Ered. (V)

HOMO (eV)

LUMO (eV)

LUMO (eV)

0.51

−1.96

−2.76

−5.38

−2.52

−1.57

0.45 0.41

−2.41 −2.56

−2.65 −2.84

−5.31 −5.26

−1.99 −1.81

−1.70 −1.49

0.36

−2.76

c

−5.20

−1.57

c

a Acquired using differential pulse voltammetry in acetonitrile versus Fc+/0. bHOMO and LUMO levels were derived from redox potentials according to ref 17. cNot observed.

observed in all of the complexes except PtNON-OMe. The variation in reduction potential between the derivatives is consistent with the trend observed in the calculated LUMO values. Absorption spectra for the PtNON derivatives in 2-MeTHF solution are shown in Figure 4. Each of the four compounds is characterized by a strong bands between 300 and 350 nm that are attributed to π−π* transitions on the ligands. There is a broad absorption band evident between 350 and 500 nm in the absorption of PtNON-CF3, which is assigned to a carbazole-to-

Figure 4. Absorption spectra of PtNON derivatives in 2-MeTHF. E

DOI: 10.1021/acs.inorgchem.9b01888 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Photoluminescent spectra of substituted PtNON derivatives at 77 K (black line) and room temperature in a dilute PMMA film (blue, squares) and in fluid 2-MeTHF solution (red, circles). PtNON-OMe: excitation at 350 nm in all media. PtNON, PtNON-Me, and PtNON-CF3: excitation at 380 nm in all media.

Table 3. Summary of Photophysical Properties for PtNON-X Derivatives 2-MeTHFa,b

PMMAa,b −1

−1

77 Ka,c

X

λmax (nm)

Φ

τ (μs)

kr (× 10 s )

knr (× 10 s )

λmax (nm)

Φ

τ (μs)

λmax (nm)

τ (μs)

-OMe -Me -H -CF3

446 478 492 590

0.26 0.79 0.60 0.02

13 4.5 3.1 0.074

2.0 17 19 27

5.6 4.7 13 1300

446 446 469 539

0.89 0.95 0.99 0.20

27.1 3.2 (38%), 9.6 (62%) 4.2 0.45 (38%), 1.3 (62%)

438 438 440 514

40 26 11 7.9

4

4

a

PtNON-OMe: excitation at 350 nm. PtNON, PtNON-Me, and PtNON-CF3: excitation at 380 nm. bData collected at room temperature. cIn 2MeTHF.

closer in energy. A similar shift from 3CT to 3LE emission character on cooling solution samples to 77 K was observed for (carbene)M(N-carbazolyl) compounds (M = Cu, Ag, Au).4a,5c,18 In fluid solution, the polar 2-MeTHF solvent molecules organize around the large dipole moment of the metal complex, stabilizing the 3CT state. At 77 K, the solvent molecules are held rigidly in this ordered structure, which is destabilizing with respect to the CT excited state, shifting it to energies higher than that of the 3LE (3Cz) state. The deleterious effect of longer emission lifetime and lower PLQY of PtNON-OMe compared to PtNON-Me indicates that localizing the spin density extensively onto the carbazole (Figure 3) reduces the degree of spin−orbit coupling and thus limits the efficiency of phosphorescence relative to nonradiative decay. Thus, it appears that mediating the degree of CT character in the T1 is of paramount importance when attempting to both narrow and blue-shift phosphorescence emission while maintaining the high luminescent efficiencies and fast radiative rates which are desirable for OLED and lighting applications. The tunability of the CT state by simple para-substitution of the pyridine ring with electron-donating or withdrawing groups is a promising paradigm for applicationspecific derivatization of related complexes. Electroluminescence Properties. The high PLQY and deep blue color of PtNON-OMe in both dilute solutions and

pyridine ring. This substituent raises the energy of the CT state such that it is no longer observed in the emission spectrum in any media. The resultant emission is defined by a narrow, vibronically featured spectrum in all matrices with λmax = 446 nm for the room temperature solution and film spectra and λmax = 438 nm in the frozen solution at 77 K. This ligandcentered emission results in much longer excited state decay times of 13.1 μs in 2-MeTHF solution (Φ = 0.26) and 27.1 μs in PMMA (Φ = 0.89). In this case, the highly destabilized CT state is not involved in the emission, and a long-lived ligand centered emission dominates. The observation of identical emission λmax and line shape for PtNON-OMe, PtNON-Me, and PtNON in frozen solution suggests that phosphorescence in these species under rigid, cryogenic conditions is dominated by a common state (Table 3). This is strong support for the hypothesis that narrow, structured emission in carbazole-containing tetradentate platinum(II) complexes is a result of emission from a metalperturbed carbazole-centered (3LE) triplet state. In rigid films at room temperature, both PtNON-OMe and PtNON-Me again appear to share emission from states of identical origin (Table 3). It is clear that there is some rigidochromic effect at play in these systems, which modulates the degree of CT and 3 LE character in the emitting state, as can be seen especially in PtNON-Me and PtNON, where the CT and 3LE states are F

DOI: 10.1021/acs.inorgchem.9b01888 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

nm and a quantum efficiency of Φ = 0.89 in dilute PMMA film. This deep blue emission, which is typically achieved through the use of known unstable fluorine or azole moieties, provides a new avenue for stable deep blue OLEDs. Efforts are ongoing to make efficient OLEDs using these emitters, in particular, improving their thermal stability, as well as the development and implementation of higher energy host and transport materials.

as 1% doped PMMA films make this emitter a good candidate for deep blue OLEDs. However, to prepare a phosphorescent OLED, a suitable host matrix is required as well as much higher doping levels in the host matrix, typically 5−15% in molecular and polymeric hosts. Severe quenching occurred in common host materials such as mCP and mCBP at a 10% doping concentration due to the high triplet energy of the PtNON-OMe emitter (Table S1). Fortunately, in the phosphine oxide host DPEPO (T1 = 3.0 eV),19 PtNONOMe is able to retain a moderately high PLQY of 0.45 at a doping concentration of 10%. As a proof of concept, devices of PtNON-OMe were made in the structure: ITO/70 nm PEDOT:PSS/30 nm DPEPO:10% PtNON-OMe/50 nm TPBI/1 nm LiF/100 nm Al. Both the photoluminescent (PL) spectrum of the films and the electroluminescent (EL) emission spectrum from the device retain the ligand centered emission character, peaking at 446 nm for the PL and 448 nm for the EL (Figure 6). Unfortunately, PtNON-OMe decom-

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EXPERIMENTAL SECTION

Methods and Instrumentation. UV−visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer. Photoluminescence spectra were measured using a QuantaMaster Photon Technology International phosphorescence/fluorescence spectrofluorimeter. Quantum yield measurements were carried out using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere, and model C10027 photonic multichannel analyzer (PMA). Photoluminescence lifetimes were measured by time-correlated single-photon counting using an IBH Fluorocube instrument equipped with an LED excitation source. Molar extinction coefficients were obtained by plotting solutions at four concentrations between 0.1 and 0.9 on a Beer’s Law plot, with the y intercept set to zero. Line fitting for all samples provided R2 values greater than 0.98. Excitation, emission, photoluminescence quantum yield, and lifetime measurements were acquired from solutions at maximum optical densities between 0.1 and 0.2 to minimize the effects of solute−solute interactions and inner filter effects. Room temperature photophysical measurements were recorded in indicated solvents, and cryogenic photophysical measurements were carried out in 2-methyltetrahydrofuran at 77 K. NMR spectra were recorded on a Varian 400 NMR spectrometer and referenced to the residual proton resonance of chloroform (CDCl3), acetone-d, or benzene (C6D6) solvent, as indicated. High resolution mass spectra were recorded using a Bruker Autoflex Speed MALDI-TOF spectrometer. Computational Methods. All ground state DFT calculations reported here were performed at the B3LYP/LACVP** level using the Jaguar (version 9.5, release 11) program on the Schrödinger Materials Science Suite (2017−3).20 T1 optimized geometries for all the complexes were isolated via Unrestricted DFT (UDFT) at the CAM-B3LYP/LACVP** level using the integral equation formalism variant polarizable continuum model (IEF-PCM) solvation model for tetrahydrofuran as implemented in Gaussian 09.16 TD-DFT calculations were performed at the CAM-B3LYP/LACVP** level on the T1 optimized geometries with the IEF-PCM solvation model15 in the nonequilibrium and equilibrium limit using the state-specific approach. The CAM-B3LYP functional was chosen in order to accurately model CT states. All solvation calculations were performed using Gaussian 09. Synthesis. 2a−2d. A 0.25 M solution of pyridine 1 (1.1 equiv) in 20 mL of dry toluene was added to a dry 100 mL Schlenk flask equipped with a condenser and containing 2-bromocarbazole (1.0 equiv), lithium t-butoxide (1.5 equiv), copper(I) chloride (0.05 equiv), and 1-methylimidazole (0.10 equiv). The solution was sparged with nitrogen, then refluxed overnight with stirring. The reaction was cooled, quenched with 100 mL of water, and extracted into ethyl acetate (3 × 100 mL). The organic layers were combined, dried over sodium sulfate, and condensed by rotary evaporation. The product was purified by silica gel flash chromatography with an eluent of 30% ethyl acetate in hexanes, followed by recrystallization from a saturated dichloromethane solution layered with hexanes. 2a. Yield: 5.82 g (90%). Off-white solid. Spectra match those reported in the literature.21 2b. Yield: 8.91 g (86%). White solid. Spectra match those reported in the literature.21 2c. Yield: 1.61 g (94%). Colorless powdery solid. 1H NMR (400 MHz, CDCl3): δ 8.54 (dd, J = 5.8, 0.5 Hz, 1H), 8.07 (ddd, J = 7.8, 1.3, 0.7 Hz, 1H), 8.04−8.00 (m, 1H), 7.97−7.92 (m, 1H), 7.77 (dt, J = 8.3, 0.8 Hz, 1H), 7.45 (ddd, J = 8.4, 7.2, 1.3 Hz, 1H), 7.43−7.39 (m, 1H), 7.34−7.28 (m, 1H), 7.10 (dd, J = 2.3, 0.5 Hz, 1H), 6.87 (dd,

Figure 6. PL and EL of PtNON-OMe. Device architecture: ITO/70 nm ca. PEDOT:PSS/30 nm DPEPO:10 wt % PtNON-OMe/50 nm TPBI/1 nm LiF/100 nm Al.

poses upon sublimation so only solution processed OLEDs, whose efficiency was limited, could be fabricated (see SI). The low efficiencies, likely due to poor charge imbalance and quenching by either of the transport layers, may be improved with higher triplet energy host or transport materials. The fabrication of such a device is currently being explored and will be reported elsewhere.

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CONCLUSION A series of tetradentate Pt complexes were developed to tune the degree of charge-transfer and ligand-centered character in the emission spectra through a simple substitution of the pyridine ring in PtNON. By stabilizing the LUMO level with an electron withdrawing CF3 substitution, the charge transfer emission state is stabilized, resulting in a broad, structureless emission at both 77 K and room temperature. Conversely, the addition of electron donating methyl or methoxy substitutions destabilized the LUMO level and the corresponding charge transfer state, resulting in ligand centered transition with a narrow deep blue emission. Through computational modeling, the ligand centered transition originating from the carbazole moiety was identified as the origin of the narrow emission spectrum that has been characteristic of many recently reported efficient Pt complexes. Furthermore, methoxy substituents on PtNON-OMe afforded a tetradentate Pt complex with six-memberered chelates that retains efficient and narrow deep blue emission in all matrices at room temperature, characterized by a dominant emission peak at 446 G

DOI: 10.1021/acs.inorgchem.9b01888 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry J = 5.8, 2.3 Hz, 1H), 3.94 (s, 3H). 13C (1H decoupled) NMR (101 MHz, CDCl3): δ 167.71, 152.66, 150.70, 140.38, 139.64, 126.55, 123.98, 123.62, 123.09, 121.21, 121.20, 120.20, 119.77, 114.53, 111.19, 108.26, 105.13, 55.64. HRMS calculated for C18H13BrN2O [M+H]+: 355.0446. Found: 355.1368. 2d. Yield: 2.79 g (88%). White powder. Spectra match those reported in the literature.22 3a−3d. To a 50 mL Schlenk flask containing a 0.25 M solution of 2 in a 4:1 DMSO/H2O mixture was added lithium hydroxide (2.1 equiv), copper(I) chloride (0.05 equiv), and N1,N2-bis(4-hydroxy-2,6dimethylphenyl)ethanediamide (0.05 equiv). The slurry was sparged with nitrogen for 30 min, then heated to 110 °C and stirred overnight. The flask was then cooled to room temperature, quenched with 100 mL of water, and extracted into ethyl acetate (3 × 100 mL). The organic layers were combined, dried over sodium sulfate, and condensed to a light brown solid which was purified by silica gel flash chromatography with an eluent of 30% ethyl acetate in hexanes. The product eluted as the second major fraction; the first major fraction contained compound 4. 3a. Yield: 3.85 g (70%). Pale yellow powder. Spectra match those reported in the literature.22 3b. Yield: 1.85 g (45%). White solid. Spectra match those reported in the literature.22 3c. Yield: 0.50 g (76%). Colorless powdery solid. 1H NMR (400 MHz, CDCl3): δ 8.50 (d, J = 5.8 Hz, 1H), 8.00−7.96 (m, 1H), 7.90 (d, J = 8.4 Hz, 1H), 7.72 (dt, J = 8.3, 0.8 Hz, 1H), 7.36−7.32 (m, 2H), 7.30−7.26 (m, 1H), 7.11 (d, J = 2.3 Hz, 1H), 6.84 (dd, J = 5.8, 2.3 Hz, 1H), 6.82−6.77 (m, 1H), 5.83 (s, 1H), 3.93 (s, 3H). 13C (1H decoupled) NMR (101 MHz, CDCl3): δ 167.68, 155.12, 153.12, 150.37, 140.92, 139.55, 124.87, 124.51, 120.99, 120.94, 119.34, 118.02, 110.78, 109.90, 108.12, 104.91, 97.85, 55.61. HRMS calculated for C18H14N2O2 [M+3H]+: 293.1290. Found: 293.4116. 3d. Yield: 0.81 g (75%). White solid. Spectra match those reported in the literature.22 4a−4d. To a 50 mL Schlenk flask containing a 0.15 M solution of 3 in DMSO was added 2 (1.1 equiv), potassium phosphate (2.0 equiv), copper(I) iodide (0.10 equiv), and picolinic acid (0.20 equiv). The solution was sparged with nitrogen for 30 min, then heated to 110 °C and stirred for 3 days. The reaction was then cooled to room temperature, quenched with 100 mL water, and extracted into ethyl acetate (3 × 100 mL). The organic layers were combined, dried over sodium sulfate, and condensed to a powdery solid, which was purified by silica gel flash chromatography with an eluent of 30% ethyl acetate in hexanes. The product was further purified by recrystallization from a concentrated dichloromethane solution layered with hexanes. 4a. Yield: 2.56 g (65%). Pale yellow solid. Spectra match those reported in the literature.22 4b. Yield: 1.50 g (70%). Eggshell white powder. 1H NMR (400 MHz, CDCl3): δ 8.50 (dd, J = 5.2, 2.0 Hz, 2H), 8.05 (td, J = 8.3, 7.8, 2.3 Hz, 4H), 7.80 (dd, J = 8.3, 2.4 Hz, 2H), 7.57 (t, J = 2.5 Hz, 2H), 7.45−7.36 (m, 4H), 7.31 (td, J = 7.5, 2.1 Hz, 2H), 7.06 (ddt, J = 9.0, 6.3, 3.1 Hz, 4H), 2.39 (d, J = 3.4 Hz, 6H). 13C (1H decoupled) NMR (101 MHz, CDCl3): δ 156.78, 151.55, 150.05, 149.23, 140.69, 140.04, 125.46, 124.09, 122.61, 120.96, 120.91, 120.03, 119.67, 119.53, 112.64, 111.10, 101.85, 21.14. HRMS calculated for C36H26N4O [M +H]+: 531.2185. Found: 531.7696. 4c. Yield: 0.50 g (43%). Light brown powder. 1H NMR (400 MHz, CDCl3): δ 8.44 (d, J = 5.8 Hz, 2H), 8.03 (t, J = 8.3 Hz, 4H), 7.81 (d, J = 8.2 Hz, 2H), 7.53 (d, J = 2.2 Hz, 2H), 7.44−7.36 (m, 2H), 7.30 (t, J = 7.5 Hz, 2H), 7.10−7.02 (m, 4H), 6.78 (dd, J = 5.8, 2.3 Hz, 2H), 3.81 (s, 6H). 13C (1H decoupled) NMR (101 MHz, CDCl3): δ 167.57, 156.89, 152.94, 150.45, 140.58, 139.96, 125.51, 124.08, 121.02, 120.93, 120.04, 119.64, 112.81, 111.23, 108.34, 104.58, 101.95, 55.48. HRMS calculated for C36H26N4O3 [M+H]+: 563.2083. Found: 563.6362. 4d. Yield: 0.95 g (53%). Yellow powder/pale yellow to colorless crystals. 1H NMR (400 MHz, CDCl3): δ 8.79 (d, J = 5.0 Hz, 2H), 8.04 (m, 4H), 7.83 (m, 4H), 7.61 (d, J = 2.1 Hz, 2H), 7.46−7.40 (m, 4H), 7.34 (dt, J = 7.5, 0.94 Hz, 2H), 7.09 (dd, J = 8.5, 2.1 Hz, 2H). 13 C (1H decoupled) NMR (101 MHz, CDCl3): δ 156.88, 152.60,

150.73, 140.83 (q, J = 34.3 Hz), 140.21, 139.41, 125.86, 124.58, 123.78, 121.88, 121.12, 120.48, 119.93, 116.43 (q, J = 3.3 Hz), 114.17 (q, J = 3.7 Hz), 113.46, 110.85, 102.09. HRMS calculated for C36H20F6N4O [M+H]+: 639.1620. Found: 639.2459. 5a−5d. To a 100 mL Schlenk flask containing a 0.02 M solution of 4 in glacial acetic acid was added tetrabutylammonium bromide (0.10 equiv) and potassium tetrachloroplatinate(II) (1.1 equiv). The solution was sparged with nitrogen for 30 min, then stirred overnight at room temperature. After a night, the slurry, containing copious amounts of precipitated solids, was heated to 110 °C, redissolving the solids. It was then stirred at 110 °C for 3 days. The reaction was then cooled to room temperature and slowly poured into cold water. The precipitated solids were collected by filtration and rinsed with water. The filter cake was dissolved into dichloromethane, dried over sodium sulfate, and condensed by rotary evaporation to form a powdery solid which was purified by silica gel flash chromatography with an eluent of 25% ethyl acetate in hexanes. The product was further purified by recrystallization from a concentrated dichloromethane solution layered with methanol. PtNON, 5a. Yield: 1.82 g (70%). Pale yellow crystals. Spectra match those reported in the literature.22 PtNON-Me, 5b. Yield: 0.90 g (66%). Pale yellow crystals. 1H NMR (400 MHz, acetone-d): δ 8.55 (d, J = 6.1 Hz, 2H), 7.98 (ddd, J = 7.6, 1.4, 0.6 Hz, 2H), 7.87 (d, J = 8.3 Hz, 2H), 7.80 (d, J = 8.3 Hz, 2H), 7.69 (dt, J = 8.0, 0.9 Hz, 2H), 7.31−7.25 (m, 4H), 7.24−7.18 (m, 2H), 5.96 (ddd, J = 6.2, 1.8, 0.7 Hz, 2H), 1.47 (s, 6H). 13C (1H decoupled) NMR (101 MHz, acetone-d): δ 153.10, 150.70, 149.25, 148.53, 143.75, 138.13, 129.51, 123.12, 122.52, 119.87, 119.83, 116.06, 115.85, 113.38, 113.20, 94.82, 20.34. HRMS calculated for C36H24N4OPt [M]+: 723.1598. Found: 723.8128. PtNON-OMe, 5c. Yield: 0.36 g (60%). Light beige powder/pale yellow needle-like crystals. 1H NMR (400 MHz, CDCl3): δ 8.82 (d, J = 6.9 Hz, 2H), 8.08−8.01 (m, 2H), 8.00−7.93 (m, 2H), 7.82 (d, J = 8.3 Hz, 2H), 7.57 (d, J = 2.6 Hz, 2H), 7.43−7.34 (m, 4H), 7.34−7.30 (m, 2H), 6.63 (dd, J = 6.9, 2.6 Hz, 2H), 3.91 (s, 6H). 13C (1H decoupled) NMR (101 MHz, CDCl3): δ 166.84, 152.49, 152.30, 150.37, 143.24, 138.09, 129.22, 123.51, 122.62, 119.86, 115.89, 115.73, 113.55, 112.63, 107.98, 99.99, 93.34, 55.95. HRMS calculated for C36H24N4O3Pt [M]+: 755.1497. Found: 755.6792. PtNON-CF3, 5d. Yield: 0.54 g (62%). Bright orange powder. 1H NMR (400 MHz, C6D6): δ 8.37 (d, J = 6.3 Hz, 2H), 7.91 (dd, J = 7.7, 0.7 Hz, 2H), 7.87−7.77 (m, 6H), 7.61 (d, J = 8.1 Hz, 2H), 7.25 (td, J = 7.5, 0.9 Hz, 2H), 7.14 (t, J = 1.1 Hz, 2H), 6.26 (dd, J = 6.4, 1.8 Hz, 2H). 13C (1H decoupled)/UDC NMR (101 MHz, C6D6): δ 152.98, 152.58, 148.80, 142.95, 138.36 (q, J = 34.6 Hz), 137.60, 129.76, 124.00, 123.74, 121.08, 120.21, 116.66, 116.23, 113.82, 113.55 (d, J = 3.0 Hz), 112.79, 112.55 (d, J = 4.0 Hz), 93.60. HRMS calculated for C36H18F6N4OPt [M]+: 831.1033. Found: 831.2355.

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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.inorgchem.9b01888. Photophysical properties of PtNON-Me in different host materials; OLED device fabrication procedure and device characteristics; 1H and 13C NMR of 2c, 3c, 4b, 4c, 4d, 5b, 5c, and 5d (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peter I. Djurovich: 0000-0001-6716-389X Mark E. Thompson: 0000-0002-7764-4096 H

DOI: 10.1021/acs.inorgchem.9b01888 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Present Addresses

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†

Universal Display Corporation, Ewing, New Jersey 08618, United States ‡ Sepion Technologies, Inc., Emeryville, California 94608, United States Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare the following competing financial interest(s): M.E.T. has a financial interest in the Universal Display Corporation. This is indicated in the acknowledgements of the paper.

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ACKNOWLEDGMENTS This work was supported by Universal Display Corporation. One of the authors (Thompson) has a financial interest in the Universal Display Corporation. The authors would like to thank Professor Surya Prakash at the Loker Hydrocarbon Research Institute for help with the Gaussian 09 calculations reported here.

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DOI: 10.1021/acs.inorgchem.9b01888 Inorg. Chem. XXXX, XXX, XXX−XXX