Can Remote N-Heterocyclic Carbenes Be Used for ... - ACS Publications

May 22, 2019 - An Answer from Quantum Chemical Investigation ..... orbital characteristics of the involved states are listed in Table 3 and also shown...
1 downloads 0 Views 775KB Size
Download PDF
Article Cite This: J. Phys. Chem. C 2019, 123, 14216−14222

pubs.acs.org/JPCC

Can Remote N‑Heterocyclic Carbenes Be Used for Designing Efficient Blue Triplet Emitters? An Answer from Quantum Chemical Investigation Sharmistha Urinda,† Goutam Das,‡ Anup Pramanik,† and Pranab Sarkar*,† †

Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India GSSST, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

‡

Downloaded via UNIV OF SOUTHERN INDIANA on July 22, 2019 at 10:08:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: For the first time, we have tried to show the effect of remote carbene ligands on the emission and quantum efficiency of Irbased triplet emitters useable for organic light-emitting diodes. It has already been shown that N-heterocyclic carbenes (NHCs) are comparatively stronger than mesoionic carbene (MICs) and hence provide a blue emissive color, but the latter ones are superior in terms of quantum efficiency. Similar arguments prevail for remote NHCs (rNHCs), which are supposed to be even less stronger than MICs. Therefore, any triplet emitter designed with rNHC might experience a bathochromic shift and might not therefore achieve the blue emission. However, we demonstrate that the efficiency of rNHC-based triplet emitters is much higher than that of NHCs and MICs. Hence, our target is toward designing efficient blue emitters having rNHCs. We have used both pyrazolium-pyridine- and phenyl-pyridine-based rNHCs, and we find that the combination of NHC as a cyclometallating ligand and phenyl-pyridine-based rNHC as an ancillary ligand is the best for designing efficient blue emitters. Density functional theory (DFT) and time-dependent DFT have been employed to elucidate the effect of different substitutions on the emission color tuning and quantum efficiency.

1. INTRODUCTION During the last few decades, organic light-emitting diode (OLED) materials have acquired much attention because of their potential applications in display technology and other light sources.1−4 As introduced by Tang and VanSlyke,1 an efficient OLED comprising a p−n heterostructure of organic thin films should act even at low external voltage. Besides, an efficient OLED should possess self-emitting property, high luminescence efficiency with a wide range of color display capabilities, high contrast, low weight, and high flexibility.3 A conventional fluorescent material may produce only 25% singlet excitons, while remaining 75% triplet excitons are lost through nonradiative deactivation processes. Contrarily, a phosphorescent material may provide 100% internal quantum efficiency through intersystem crossing from an excited singlet to triplet state.5,6 Therefore, the phosphorescence properties of heavier transition metal complexes (such as Rh, Ir, Pt, and so forth) with suitable organic ligands have gained significant attention in this regard, especially with the aim of high energy (blue) emission. In particular, the organometallic complexes of N-heterocyclic carbenes (NHCs) have experienced explosive development during the last few years.7−13 The presence of a high field strength ligand such as an NHC in the complexes gives rise to high energy emissions and consequently to the desired blue color needed for OLED applications. Further© 2019 American Chemical Society

more, the great variability of NHC ligands for structural modifications, together with the use of other ancillary ligands in the complex, provides numerous possibilities for the synthesis of phosphorescent materials, with emission colors over the entire visible spectra.12 Over the last decade, many luminescent transition metal complexes have been synthesized and characterized to be used as OLED materials for applications such as flat panel displays and solid-state lighting sources.14,15 Among all the complexes, blue phosphore has great challenges because of its lower stability and lower quantum efficiency. To overcome these shortcomings, a logical approach for the synthesis of complexes with blue and near-UV phosphorescence was reported by Thompson and co-workers, in which NHC ligands were used to form tris(cyclometallated) iridium complexes.16 It was originated from the fact that tris(cyclometallated) complexes with (C∧N) ligands are excellent phosphors and substitution by a high field strength ligand such as the NHC could give rise to high energy emission from the perturbed high energetic 3LC state. Efficient, saturated blue electrophosphorescence was achieved with the facial and meridonial complexes doped into Received: March 10, 2019 Revised: May 20, 2019 Published: May 22, 2019 14216

DOI: 10.1021/acs.jpcc.9b02251 J. Phys. Chem. C 2019, 123, 14216−14222

Article

The Journal of Physical Chemistry C

2.2. Emission Energy. Now, from the optimized S0 and T1 state energies, we calculated the adiabatic emission energy (ΔE(T1 − S0)) which is very close to the experimental emission value at room temperature, 298.15 K. We, however, calculated vertical emission energy (ΔE(T1 − S0′ )) for comparison where S0′ designates some upper vibrational level of the singlet ground state having the same geometry as the optimized T1 state. For obtaining the S0′ state, we performed single point energy calculation at the T1 geometry with a singlet spin multiplicity. The emission characteristics of the complexes were determined by performing time-dependent DFT (TDDFT) calculations on the T1 states of the complexes with a spin multiplicity specified as 1. The TDDFT calculations provide emission wavelength in one hand and on the other hand the nature of the involved molecular orbitals in the excitation process.8,33 However, as TDDFT does not account the spin−orbit coupling (SOC) in cyclometallated Ir complexes, which is expected to be quite string, it therefore underestimates the emission wavelength. To remove this inconsistency, we have further carried out SOC-induced phosphorescence calculations where SOC is treated perturbatically.34 Nevertheless, we would still like to emphasize that the computed adiabatic emission energy values, ΔE(T1 − S0), provide a better comparison with the experimentally determined emission wavelength as compared to other more sophisticated theoretical calculations such as vertical transition including SOC as also evidenced in numerous previous literature studies.8,15,35 Moreover, for similar type of complexes, the qualitative trend in results is expected to be retained. 2.3. Radiative and Nonradiative Rate Constants and Quantum Efficiency. Now, the quantum efficiency (ϕ) of a phosphore could be expressed in terms of the rate constants of the radiative (kr) and nonradiative (knr) emission processes as below

the gap of a host. The emitted color was pure deep blue. These complexes are the suitable luminescent dopants in blue OLEDs.16 From these studies, it has already been established that by the combination of different NHCs as cyclometallating or ancillary ligands, deep blue emission can be achieved.10,17,18 However, the efficiency and the lifetime of such blue emitters are still far away from the practical requirements. There exist enormous challenges until date to synthesize good deep triplet emitters with higher quantum efficiency. In the recent past, some experimental as well as theoretical works have been performed toward the design of new complexes to enhance the efficiency of the triplet blue emitters.16,17,19−22 Monti et al.17 have demonstrated that Ir complexes with a mesoionic carbene (MIC) ligand could be used as better efficient triplet emitters. Further studies have also been shown that because of the presence of MIC, 3MC states are comparatively inaccessible from the triplet excited state (3MLCT), and consequently, nonradiative rate constants are reported to be lower, resulting in higher quantum yields.20,23 Now, in some of the recent findings,24 organometallic complexes containing pyrazole-derived remote NHC (rNHC) ligands have been synthesized. Apart from normal and abnormal NHCs, “rNHCs” have gained importance within organometallic chemistry because of their higher σ-donor capability.25 In our previous work, while comparing NHC and MIC, we have already demonstrated that increase in the σdonor and decrease in π-acceptance ability enhance the inaccessibility of the 3MC states, which results in higher efficient triplet emitters with MIC compared to its NHC analogue.26 Along the same direction, we conjecture that the rNHC might produce even higher efficient triplet emitters compared to MIC. In this paper, we therefore set out our goal toward comparing Ir-based complexes of MIC and rNHC as the ancillary ligand. However, our previous observation has also proved that NHC is a comparatively stronger ligand than MIC and hence provides a blue emission color. Similar arguments prevail for rNHC ligands, which are supposed to be even comparatively weaker than MIC. Therefore, any triplet emitter designed with rNHC might experience a bathochromic shift and might not therefore achieve the blue emission. Hence, our target would be toward designing efficient blue emitters having rNHCs as well.

ϕ=

kr (k r + k nr + k nr(T ))

(1)

where kr is the radiative rate constant, knr is the temperatureindependent nonradiative rate constant, and knr(T) is the temperature-dependent nonradiative rate constant. From the above equation, it is clear that the larger value of kr and smaller values of knr and knr(T) would improve the phosphorescence quantum efficiency ϕ. Thus, from the above equation, it can be stated that the larger value of kr or smaller values of knr or knr(T) would improve the value of ϕ. Here, kr values have been calculated with the help of perturbatically corrected dipole moment and the spin−orbit operators, as implemented in Dalton.36 It is noteworthy that an efficient phosphore should experience a large amount of spin forbidden transition (T1 → S0) resulting from effective SOC. This in turns depends on the extent of metal-to-ligand charge-transfer (MLCT) character in the emissive state. Thus, the larger the MLCT character in the T1 state, the more efficient the radiative process is. However, kr is the average of the individual components arrived from the three spin sublevels (indexed by i) of the involved emissive state (T1), which can be expressed as37,38

2. METHOD OF COMPUTATION 2.1. Ground Singlet (S0) and Excited Triplet (T1) State Geometries. The ground-state singlet (S0) and excited-state triplet (T1) geometries of the complexes were obtained by means of density functional theory (DFT)27 calculations using Gaussian 09 software package28 without any symmetry constraint. We adopted B3LYP29,30 exchange−correlation functional (xc) along with double-ζ 6-31G** basis sets for the lighter elements and the LANL2DZ31 basis set for the heavy metal atom, Ir. In the latter case, a relativistic effective core potential32 was used to replace the inner core electrons, leaving the outer core [5s25p6] and the valence [5d6] electrons under consideration. It has already been demonstrated that the B3LYP xc functional is the best choice for characterizing the ground and excited states of different heteroleptic cyclometallating Ir(III) complexes.13 However, normal mode analyses were done for confirming the structures in their respective local minima with all positive vibrational frequencies. 14217

DOI: 10.1021/acs.jpcc.9b02251 J. Phys. Chem. C 2019, 123, 14216−14222

Article

The Journal of Physical Chemistry C 3

3

complexes of similar structure. For that, we have chosen some Ir(III) complexes with pyrazole-based NHCs as an ancillary ligand in the presence of a phenylpyridine (ppy) as the cyclometallating ligand. Figure 1 shows the schematic

1 1 k r = ∑ k ri = ∑ kr(S0 , T1i ) 3 i=1 3 i=1 =

3 i yz j α3 1 ∑ jjjjj4 0 ΔES−T ∑ |M ji|2 zzzzz 3 i = 1 j 3t0 z j∈x ,y,z k {

(2)

where ΔES0−T1 is the transition energy, α0 is the fine structure constant, t0 = (4πϵ0)2/mee4, and Mij is the jth component of the electric dipole transition moment between S0 and the ith sublevel of the T1 state. It is noteworthy that at room temperature, only weighted phosphorescence rates can be measured using the above equation. On the other hand, the temperature-independent knr values have been calculated by using the energy gap law39 as follows ln(k nr) = k TIΔE(T1 − S0)

Figure 1. Schematic representations of some complexes studied here. L2 and L3 are unsubstituted/substituted phenylpyridine ligands, while L1 is either MIC (A−D) or remote carbene (rNHC, E and F) ligand. For complex E: R = R′ = H; complex E′: R = R′ = F; and complex E″: R = SO2CH2Ph, R′ = F.

(3)

where kTI is a proportionality constant. However, it should be pointed out that at room temperature, the overall knr value has a strong temperature dependence,37 and therefore, knr(T) must have some influential role on determining ϕ. Furthermore, as OLEDs work at ambient temperatures, it is indispensable to discuss over their temperature-dependent decaying process. Even in many cases, it has been found that at 300 K, knr(T) dominates over knr. Keeping in mind that there exists excited 3 MC state near the radiative 3MLCT state and the former cannot decay radiatively, we consider the possibility of thermal population of the 3MC state to calculate the temperaturedependent nonradiative rate constant (knr(T)). Now, the relative population of the 3MC state with respect to the 3 MLCT state can easily be determined by scanning the potential energy surface (PES) along a suitable reaction coordinate. Thus, knr(T) can be expressed in terms of the Arrhenius type of equation40 k nr(T ) = ka e−ΔEa / kBT

representations of the complexes, while optimized geometries of some representative structures are shown in Figure S1 in the Supporting Information. It has been well established in the literature that pyrazole-based NHCs are considered to be effective for preparing efficient blue OLED materials.9,12 Now, from Figure 2, it is clear that pyrazole-based carbenes can form both the MIC (A, B, C, D) and remote carbene (rNHC: E, F) depending on the position of the nitrogen atom in the pyrazole ring. We have computed the radiative and nonradiative rates and emission wavelengths of the complexes as listed in Table 1. From this table, it is clear that complexes containing MIC (A− D) emit at a shorter wavelength in comparison to that of the complexes containing rNHC (E, F). Complex A shows an emission wavelength of 467 nm. Similarly, complexes B, C, and D show emission wavelengths of 553, 590, and 610 nm, respectively. If we concentrate on the radiative rate constant, we find that complexes E and F show comparatively higher kr values compared to complexes A−D. We have calculated the kr values by using eq 2 with the help of similar methodology as mentioned earlier.36 As the SOC in the transition metal allows a singlet to triplet transition, the radiative rate constant (kr) becomes strongly dependent on the metal percentage involved in the emission process. This is due to the fact that the values of the SOC matrix elements increase as the percentage of the metal character increases in the transition from T1 to S0. While evaluating the nonradiative rate constant (knr(T)), we first evaluate the activation barrier (ΔEa) between the 3MLCT and 3 MC states. The activation barrier between these two excited states, ΔEa, plays an important role in the thermal deactivation process. To calculate the activation energy, we perform a constraint optimization while elongating the weakest bond between the ligand and the central metal atom, which is a notable convenient method.45 We then calculate the transition state over the PES along this reaction coordinate. We further prove that after crossing the barrier, the PES moves to the 3 MC state. This is proven by evaluating the corresponding charge-transfer characteristics of the concerned geometries. If this activation barrier is high enough, the excited 3MC state becomes poorly accessible, and thus, the triplet emitter works very efficiently. Using this method, we calculate ΔEa for all complexes A, B, C, D, E, and F. It is observed that complexes A, B, and C have higher knr(T) values, resulting in very low quantum efficiency. On the other hand, complexes E and F have very low knr(T) values, which result in high quantum

(4)

where ka is the proportionality constant, ΔEa is the activation energy barrier for the transformation of 3MLCT state to 3MC state, and kB is the Boltzmann constant. Considering that for similar type of complexes the value of ka is same, we can calculate the values of knr(T) for different complexes by using eq 4 provided the values of ka and ΔEa are known. We calculated the value of ka from the experimental data of the synthesized reference complex41 and finally computed the ϕ values by using eq 1 through eq 4.

3. RESULTS AND DISCUSSION The effective utilization of carbenes in OLED technology has evolved recently.10−12,16−18,20,26 It has been shown that MICs are more effective than the normal NHCs in terms of both color tuning and quantum efficiency.18,26 It has been demonstrated that the stronger σ-donation along with moderate π-acceptance property of MIC is the main guiding factor for its privileged role in OLED property in comparison to that of NHC.26 Herein, we extend this idea toward remote carbene (rNHC) on the basis of the fact that the σ-donor capability42 of different carbene ligands increases from NHC to MIC to rNHC progressively.25,43,44 Therefore, we postulate that rNHC might show better quantum efficiency. 3.1. Comparative Study between MIC and rNHC in Terms of Emission Wavelength and ϕ. We provide a comparative study between MIC and rNHC containing 14218

DOI: 10.1021/acs.jpcc.9b02251 J. Phys. Chem. C 2019, 123, 14216−14222

Article

The Journal of Physical Chemistry C

Figure 2. Structures of different MIC and remote (rNHC) carbenes. Gray, white, and blue colored balls indicate C, H, and N atoms, respectively.

terms of emission wavelength, reverse trend is observed. Further, from our previous work,26 we have found that NHC is better than MIC in terms of blue shifting but has reversed nature in terms of quantum efficiency. Therefore, we can conclude that (a) in terms of ϕ: rNHC > MIC > NHC while (b) in terms of emission wavelength: NHC > MIC > rNHC. 3.2. Strategic Substitution for Blue Emission. As already mentioned, in spite of having higher ϕ, the Ir(III) complexes with rNHC as the cyclometallating ligand emit relatively higher wavelength. Now, we explore the possibility to improve the blue emission properties of the rNHC complexes without compromising the efficiency of the triplet emitters. For doing this, we adopt two different methods of substitution either to the ligand or to the entire complex as discussed below. 3.2.1. Functional Group Substitution to the Ligand. In accordance with the previous endeavor,8,9,13,20 electronwithdrawing substituents shift the emission to the deep blue region. Thus, we have performed the substitution of the electron-withdrawing fluorine atom to the ppy ligand as shown in Figure 1. The computed emission wavelength and other associated properties of the complex along with some other designed complexes are listed in Table 2. The corresponding orbital characteristics of the involved states are listed in Table 3 and also shown in Figure S2 in the Supporting Information

Table 1. Computed Emission Energy, Radiative and Nonradiative Rates, Percentage of Metal (M) in the Involved Transition, and the Highest Coefficient of the Involved Pair of Orbitals Therein for Some Studied Complexes complex

% of M

CI

complex A complex B complex C complex D complex E complex F

31.21 29.6 29.94 29.65 32.11 31.03

0.59 0.70 0.69 0.69 0.70 0.70

kr (105 s−1) 3.71 4.02 3.99 3.90 4.10 4.20

knr(T) (105 s−1) 5.50 × 107 8.06 × 103 5.07 × 107 5.46 × 10−4 1.06 × 10−1 5.46 × 10−4

T1 − S0 (nm) 467 553 590 610 774 789

efficiency. For example, Figure 3a shows the PESs for complex E against the longest (weakest) bond around the central metal

Table 3. Calculated Transition and Orbital Characteristics for Some Studied Complexes

Figure 3. Representative PES, showing the variation of total energy with respect to the bond length of the weakest Ir−N bond for complex E (a) and complex H (b).

orbital contribution (%)

complex

atom, Ir, which gives the estimated value of ΔEa = 0.65 eV. Finally, knr(T) was calculated by using eq 4. The proportionality constant ka was evaluated from the experimental data given in ref 41. From the above discussion, it can be concluded that in terms of ϕ, rNHC is better than MIC; however, in

complex complex complex complex

E E′ E″ H

H H H H

→ → → →

L L L L

(61) (70) (70) (67)

transition characteristics L2/L3/Ir Ir/L2/L3 Ir/L2/L3 Ir/L2/L3

→ → → →

L1 L1 L1 L1

(MLCT/LLCT) (MLCT/LLCT) (MLCT/LLCT) (MLCT/LLCT)

Table 2. Computed Emission Energies, Radiative and Nonradiative Rates, Percentage of Metal (M) in the Involved Transition, and the Highest Coefficient of the Involved Pair of Orbitals Therein and ϕ for Some Selective Complexes complex complex complex complex complex

E E′ E″ H

% of M 32.11 29.61 30.16 33.12

CI 0.70 0.71 0.65 0.67

kr (105 s−1) 4.10 4.05 4.01 4.7 14219

knr(T) (105 s−1) 1.06 8.70 4.63 3.95

× × × ×

−1

10 10−1 10−1 10−3

T1 − S0 (nm)

ϕ (%)

774 613 652 476

97 82 89 100

DOI: 10.1021/acs.jpcc.9b02251 J. Phys. Chem. C 2019, 123, 14216−14222

Article

The Journal of Physical Chemistry C

Figure 4. Pictorial representations of some designed complexes with different remote carbenes.

for more clarity. From Table 2, it is clear that the electronwithdrawing substituent, fluorine exhibits a blue shift in the emission wavelength (complex E′, emission wavelength 613 nm). However, the main problem is that the emission color is still not in the deep blue region. The main orbitals involved in the transition here are the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), resulting in MLCT and ligand-to-ligand charge-transfer (LLCT) characters. HOMO is situated on Ir, L2, and L3, whereas LUMO is mainly concentrated on L1. Therefore, we have found that complex E′ has more blue-shifted emission compared to complex E with comparable ϕ value. Further, we have substituted one fluorine atom in the ppy moiety of complex E′ by more electron-withdrawing functional groups, −SO2CH2Ph, to produce complex E″ as shown in Figure 1. However, this substitution has a negligible effect on shifting the emission toward deep blue regions. Similarly, we have tried insertion of other combinations of different functional groups without having much success. Hence, we took the following alternative routes for achieving further hypsochromic shifts. 3.2.2. Entire Ligand Substitution. In our previous work, it has been established that NHC with proper substitution is suitable for the blue color emission.20 However, the efficiency of such triplet emitters is not high. It has already been demonstrated that the increase of nitrogen richness in cyclometallating/ancillary ligands of some Ir-based heteroleptic complexes increases the blue shifting in emission as well as quantum efficiency (ϕ).19,20 Taking these two strategies into consideration, we aim to design triplet emitters with a combination of NHC (as the cyclometallating ligand) and rNHC (as the ancillary ligand) toward efficient blue emitters. In this effort, we have designed complex E1 as shown in Figure 4, where we replace the entire cyclometallating ligand of complex E with nitrogen-rich triazine-substituted pyridine. On the other side, in complex E′1, we replace the cyclometallating ligand with nitrogen-rich normal NHC (1,2,3 triazine-based imidazolium carbene). However, from Table 4, it is observed that adaptation of both the strategies does not provide enough hypsochromic shift to incur blue emission.

Table 4. Computed Emission Wavelength for Some Designed Complexes (Changing Entire Ligand) complex complex complex complex complex complex

E1 E1′ G1 G2 H

T1 − S0 (nm) 626 610 688 843 476

From the above observations, it is clear that if we use MIC as the cyclometallating ligand, we find that it shows blue emission, but the efficiency is not much higher. However, in the case of rNHC (as the ancillary ligand), it shows higher quantum efficiency, but the emission wavelength is not shifted in the blue region. When we introduce more electronwithdrawing substituents in the ppy ligand, the emission wavelength is comparatively blue-shifted at the expense of compromising the quantum efficiency. However, from Table 4, it is observed that adaptation of both strategies [(a) introduction of more electronegative substituents and (b) substitution of the entire ligand] does not provide enough hypsochromic shift to incur blue emission. 3.2.3. Introduction of Six-Membered ppy-Based rNHC. Therefore, as an alternative strategy, we introduce a new ligand (ppy-based rNHC) as an ancillary ligand. As a result for this substitution, complexes G1 and G2 have been produced (Figure 4), which, however, demonstrate the emission wavelengths of 688 and 843 nm, respectively, as shown in Table 4. From Table 4, it is clear that in complex G1, introduction of an electron-withdrawing F atom in the cyclometallating ligand and the use of ppy-based remote carbene (rNHC) as the ancillary ligand are not very much helpful to achieve efficient blue emission. On the other hand, the emission wavelength of complex G2 (843 nm) demonstrates that the introduction of an electron-donating alkyl group in the six-membered remote carbene ligand produces further red shifting in emission. It can therefore be concluded that introduction of six-membered rNHC is also not very helpful to achieve blue emission. 14220

DOI: 10.1021/acs.jpcc.9b02251 J. Phys. Chem. C 2019, 123, 14216−14222

Article

The Journal of Physical Chemistry C

(2) Wen, S.-W.; Lee, M.-T.; Chen, C. H. Recent development of blue fluorescent OLED materials and devices. J. Disp. Technol. 2005, 1, 90−99. (3) Geffroy, B.; Le Roy, P.; Prat, C. Organic light-emitting diode (OLED) technology: materials, devices and display technologies. Polym. Int. 2006, 55, 572−582. (4) Chi, Y.; Chou, P.-T. Transition-metal phosphors with cyclometalating ligands: fundamentals and applications. Chem. Soc. Rev. 2010, 39, 638−655. (5) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. J. Appl. Phys. 2001, 90, 5048−5051. (6) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Highefficiency organic electrophosphorescent devices with tris(2phenylpyridine)iridium doped into electron-transporting materials. Appl. Phys. Lett. 2000, 77, 904−906. (7) Avilov, I.; Minoofar, P.; Cornil, J.; De Cola, L. Influence of substituents on the energy and nature of the lowest excited states of heteroleptic phosphorescent Ir(III) complexes: a joint theoretical and experimental study. J. Am. Chem. Soc. 2007, 129, 8247−8258. (8) De Angelis, F.; Fantacci, S.; Evans, N.; Klein, C.; Zakeeruddin, S. M.; Moser, J. E.; Kalyanasundaram, K.; Bolink, H. J.; Grätzel, M.; Nazeeruddin, M. K. Controlling phosphorescence color and quantum yields in cationic iridium complexes: a combined experimental and theoretical study. Inorg. Chem. 2007, 46, 5989−6001. (9) Bai, F.-Q.; Wang, J.; Xia, B.-H.; Pan, Q.-J.; Zhang, H.-X. DFT and TD-DFT study on the electronic structures and phosphorescent properties of 6-phenyl-2,2′-bipyridine tridentate iridium(III) complexes and their isomer. Dalton Trans. 2012, 41, 8441−8446. (10) Monti, F.; et al. Charged bis-cyclometalated iridium(III) complexes with carbene-based ancillary ligands. Inorg. Chem. 2013, 52, 10292−10305. (11) Li, T.-Y.; Liang, X.; Zhou, L.; Wu, C.; Zhang, S.; Liu, X.; Lu, G.Z.; Xue, L.-S.; Zheng, Y.-X.; Zuo, J.-L. N-heterocyclic carbenes: versatile second cyclometalated ligands for neutral iridium(III) heteroleptic complexes. Inorg. Chem. 2015, 54, 161−173. (12) Visbal, R.; Gimeno, M. C. N-heterocyclic carbene metal complexes: photoluminescence and applications. Chem. Soc. Rev. 2014, 43, 3551−3574. (13) Urinda, S.; Das, G.; Pramanik, A.; Sarkar, P. Tuning the phosphorescence and quantum efficiency of heteroleptic Ir(III) complexes based on pyridine-tetrazole as an ancillary ligand: An overview from quantum chemical investigations. Comput. Theor. Chem. 2016, 1092, 32−40. (14) Fu, H.; Cheng, Y.-M.; Chou, P.-T.; Chi, Y. Feeling blue? Blue phosphors for OLEDs. Mater. Today 2011, 14, 472−479. (15) Yersin, H. Highly Efficient OLEDs with Phosphorescent Materials; John Wiley & Sons, 2008. (16) Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Blue and near-UV phosphorescence from iridium complexes with cyclometalated pyrazolyl or N-heterocyclic carbene ligands. Inorg. Chem. 2005, 44, 7992−8003. (17) Baschieri, A.; Monti, F.; Matteucci, E.; Mazzanti, A.; Barbieri, A.; Armaroli, N.; Sambri, L. A mesoionic carbene as neutral ligand for phosphorescent cationic Ir(III) complexes. Inorg. Chem. 2016, 55, 7912−7919. (18) Monti, F.; La Placa, M. G. I.; Armaroli, N.; Scopelliti, R.; Grätzel, M.; Nazeeruddin, M. K.; Kessler, F. Cationic iridium(III) complexes with two carbene-based cyclometalating ligands: cis versus trans isomers. Inorg. Chem. 2015, 54, 3031−3042. (19) Urinda, S.; Das, G.; Pramanik, A.; Sarkar, P. Theoretical studies on the photophysical properties of some Iridium(III) complexes used for OLED. J. Phys. Chem. Solids 2016, 96−97, 100−106. (20) Urinda, S.; Das, G.; Pramanik, A.; Sarkar, P. Quantum chemical investigation on the Ir(III) complexes with an isomeric triazine-based imidazolium carbene ligand for efficient blue OLEDs. Phys. Chem. Chem. Phys. 2017, 19, 29629−29640.

3.2.4. Combination of Nitrogen-Rich NHC as the Cyclometallating Ligand and Six-Membered ppy rNHC. Finally, when we simultaneously use all the strategies together to produce complex H, as shown in Figure 4 (see Figure S3 in the Supporting Information for the optimized geometries), we observe that complex H shows an emission wavelength of 476 nm with appreciably higher ϕ. It shows mainly HOMO to LUMO transition having MLCT and LLCT characteristics. Higher MLCT character is responsible for higher k r value.26,37,38 We have performed PES scan along the weakest bond attached to the central metal atom to evaluate the activation barrier between the 3MLCT and 3MC states. Figure 3b shows the PESs for the complex H. The value of knr(T) was calculated by using eq 4. From this PES, we find the higher activation energy (ΔEa = 1.41 eV), resulting in lower access to the 3MC state. Therefore, the nonradiative rate constant (knr(T)) is observed to be significantly lowered, and as a consequence, the overall quantum efficiency is enhanced.

4. CONCLUSIONS In this work, we demonstrate the role of the rNHC ligand in color tuning and quantum efficiency (ϕ) of Ir(III) complexes. Using DFT and TDDFT calculations, we find that the rNHC ligand might be a better ligand in designing triplet emitters. The stronger σ-donation and moderate π-acceptance of rNHC are very helpful to lower the access of the nonemissive 3MC state, which ultimately results in the lowering of nonradiative rate constant (knr) and increasing the corresponding ϕ. Our computational analyses unravel that the combination of NHCs as cyclometallating ligands and phenyl-pyridine-based rNHC as the ancillary ligand is the most suitable one for designing efficient deeper blue emitting Ir(III) complexes, which might have potential application in OLEDs.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b02251.

■

Singlet state optimized geometries of some complexes and pictorial representation of transition and orbital characteristics (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pranab Sarkar: 0000-0003-0109-6748 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from DST, Govt. of India, through sponsored research grant (SR/ NM/NS-1005/2016). S.U. is thankful to DST for providing her with women scientist (scheme WOS-A) research grant (sanction order no. SR/WOS-A/CS-116/2012(G)).

■

REFERENCES

(1) Tang, C. W.; VanSlyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51, 913−915. 14221

DOI: 10.1021/acs.jpcc.9b02251 J. Phys. Chem. C 2019, 123, 14216−14222

Article

The Journal of Physical Chemistry C

(43) Schneider, S. K.; Julius, G. R.; Loschen, C.; Raubenheimer, H. G.; Frenking, G.; Herrmann, W. A. A first structural and theoretical comparison of pyridinylidene-type rNHC (remote N-heterocyclic carbene) and NHC complexes of Ni(II) obtained by oxidative substitution. Dalton Trans. 2006, 1226−1233. (44) Raubenheimer, H. G.; Cronje, S. One-N, six-membered heterocyclic carbene complexes and the remote heteroatom concept. Dalton Trans. 2008, 1265−1272. (45) Gonzalez-Vazquez, J. P.; Burn, P. L.; Powell, B. J. Interplay of zero-field splitting and excited state geometry relaxation in facIr(ppy)3. Inorg. Chem. 2015, 54, 10457−10461.

(21) Yang, C.-H.; Beltran, J.; Lemaur, V.; Cornil, J.; Hartmann, D.; Sarfert, W.; Fröhlich, R.; Bizzarri, C.; De Cola, L. Iridium metal complexes containing N-heterocyclic carbene ligands for blue-lightemitting electrochemical cells. Inorg. Chem. 2010, 49, 9891−9901. (22) Yang, L.; Okuda, F.; Kobayashi, K.; Nozaki, K.; Tanabe, Y.; Ishii, Y.; Haga, M.-A. Syntheses and phosphorescent properties of blue emissive iridium complexes with tridentate pyrazolyl ligands. Inorg. Chem. 2008, 47, 7154−7165. (23) Sarkar, B.; Suntrup, L. Illuminating iron: mesoionic carbenes as privileged ligands in photochemistry. Angew. Chem., Int. Ed. 2017, 56, 8938−8940. (24) Arduengo, A. J.; Bertrand, G. Carbenes Introduction. Chem. Rev. 2009, 109, 3209−3210. (25) Schuster, O.; Raubenheimer, H. G. Synthesis of the first rNHC (remote N-heterocyclic carbene) complexes with no heteroatom in the carbene carbon-containing ring. Inorg. Chem. 2006, 45, 7997− 7999. (26) Urinda, S.; Das, G.; Pramanik, A.; Sarkar, P. Essential Role of Ancillary Ligand in Color Tuning and Quantum Efficiency of Ir(III) Complexes with N-Heterocyclic or Mesoionic Carbene Ligand: A Comparative Quantum Chemical Study. J. Phys. Chem. A 2018, 122, 7532−7539. (27) Hohenberg, P.; Kohn, W. Inhomogeneous electron gas. Phys. Rev. 1964, 136, B864. (28) Frisch, M. J.; et al. Gaussian 09, Revision D.01; Gaussian Inc.: Wallingford CT, 2009. (29) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (30) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785. (31) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270−283. (32) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional Theory; John Wiley & Sons, 2015. (33) Hay, P. J. Theoretical studies of the ground and excited electronic states in cyclometalated phenylpyridine Ir(III) complexes using density functional theory. J. Phys. Chem. A 2002, 106, 1634− 1641. (34) Sałek, P.; Vahtras, O.; Helgaker, T.; Ågren, H. Densityfunctional theory of linear and nonlinear time-dependent molecular properties. J. Chem. Phys. 2002, 117, 9630−9645. (35) Censo, D. D.; Fantacci, S.; Angelis, F. D.; Klein, C.; Evans, N.; Kalyanasundaram, K.; Bolink, H. J.; Grätzel, M.; Nazeeruddin, M. K. Synthesis, Characterization, and DFT/TD-DFT Calculations of Highly Phosphorescent Blue Light-Emitting Anionic Iridium Complexes. Inorg. Chem. 2008, 47, 980−989. (36) Aidas, K.; et al. The Dalton quantum chemistry program system. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2014, 4, 269−284. (37) Escudero, D. Quantitative prediction of photoluminescence quantum yields of phosphors from first principles. Chem. Sci. 2016, 7, 1262−1267. (38) Escudero, D.; Jacquemin, D. Computational insights into the photodeactivation dynamics of phosphors for OLEDs: a perspective. Dalton Trans. 2015, 44, 8346−8355. (39) Caspar, J. V.; Meyer, T. J. Application of the energy gap law to nonradiative, excited-state decay. J. Phys. Chem. 1983, 87, 952−957. (40) Meyer, T. J. Photochemistry of metal coordination complexes: metal to ligand charge transfer excited states. Pure Appl. Chem. 1986, 58, 1193−1206. (41) Stringer, B. D.; Quan, L. M.; Barnard, P. J.; Wilson, D. J. D.; Hogan, C. F. Iridium complexes of N-heterocyclic carbene ligands: investigation into the energetic requirements for efficient electrogenerated chemiluminescence. Organometallics 2014, 33, 4860−4872. (42) Mayer, U. F. J.; Murphy, E.; Haddow, M. F.; Green, M.; Alder, R. W.; Wass, D. F. A New class of remote N-heterocyclic carbenes with exceptionally strong σ-donor properties: introducing benzo[c]quinolin-6-ylidene. Chem.Eur. J. 2013, 19, 4287−4299. 14222

DOI: 10.1021/acs.jpcc.9b02251 J. Phys. Chem. C 2019, 123, 14216−14222