Can Remote N-Heterocyclic Carbenes Be Used for Designing Efficient

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Can Remote N-Heterocyclic Carbenes (rNHCs) be Used for Designing Efficient Blue Triplet Emitters? An Answer from Quantum Chemical Investigation Sharmistha Urinda, Goutam Das, Anup Pramanik, and Pranab Sarkar J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 22, 2019

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The Journal of Physical Chemistry

Can Remote N-Heterocyclic Carbenes (rNHCs) 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 ‡G.S.S.S.T, Indian Institute of Technology, Kharagpur 721302, India E-mail: [email protected]

Abstract For the first time, we have tried to show the effect of remote carbene ligand (rNHC) on the emission and quantum efficiency of Ir-based triplet emitters usable for organic light emitting diode (OLED). It has already been shown that N-heterocyclic carbenes (NHC) are comparatively stronger than mesoionic carbene (MIC) and hence provide bluer emissive color but the latter ones are superior in terms of quantum efficiency. Similar arguments prevail for rNHCs which are supposed to be even less stronger than MIC. 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 NHCs and MICs. Hence, our target is towards designing efficient blue emitters having rNHCs. We have used both pyrazolium-pyridine and phenyl-pyridine based rNHC and we find that the combination of NHC as cyclometallating ligand and phenyl-pyridine

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based rNHC as ancillary ligand is the best for designing efficient blue emitter. Density functional theory (DFT) and time-dependent DFT have been employed to elucidate the effect of different substitution on the emission color tuning and quantum efficiency.

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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 et al., 1 an efficient OLED comprising p-n heterostructure of organic thin films should act even a low external voltage. Besides, an efficient OLED should possess self-emitting property, high luminescence efficiency with wide range of color display capability, high contrast, low weight and high flexibility. 3 A conventional fluorescent material may produce only 25% singlet exciton 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 (like Rh, Ir, Pt, etc.) 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. Furthermore, 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 like flat panel displays and solid state lighting sources. 14,15 Among all the complexes, blue phosphore has great challenges due to 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 coworkers, which is to use of NHC ligands to form tris(cyclometallated) iridium complexes. 16 It was sprouted from the basis that 3



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 3 LC state. Efficient, saturated blue electrophosphorescence was achieved with the facial and meridonial complexes doped into the gap of 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 till date to synthesize a good deep triplet emitters with higher quantum efficiency. In the recent past, some experimental as well as theoretical works have been performed towards 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 mesoionic carbene (MIC) ligand could be used as better efficient triplet emitters. Further studies has also been shown that due to the presence of MIC, 3 MC states are comparatively inaccessible from triplet excited state (3 MLCT) and consequently nonradiative rate constant are reported to be lower, resulting in higher quantum yield. 20,23 Now, in some of the recent findings, 24 organometallic complexes containing pyrazolederived remote N-heterocyclic carbene ligands have been synthesized. Apart from normal and abnormal NHCs, “remote NHCs” (rNHC) have gained importance within organometallic chemistry due to their higher σ-donor capability. 25 In our previous work, while comparing NHC and MIC, we have already demonstrated that increase in the σ-donor and decrease in πacceptance ability enhance the inaccessibility of the 3 MC states which results 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 towards comparing Ir based complexes of MIC and rNHC as ancillary ligand.

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However, our previous observation has also proved that NHC is a comparatively stronger ligand than MIC and hence provide bluer emission color. Similar arguments prevails for rNHC ligands which is 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 towards designing efficient blue emitters having rNHCs as well.

Method of Computation 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 Gaussian09 software package 28 without any symmetry constrain. We adopted B3LYP 29,30 exchangecorrelation functional (xc) along with double-ζ 6-31G∗∗ basis sets for the lighter elements and LANL2DZ 31 basis set for the heavy metal atom, Ir. In the latter case, relativistic effective core potential (ECP) 32 was used to replace the inner core electrons, leaving the outer core [5s2 5p6 ] and the valance [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.

Emission Energy Now, from the optimized S0 and T1 state energies, we calculated the adiabatic emission energy (∆E(T1 − S0 )) which is very close to the experimental emission value at room temperature, 298.15 K. We, however, calculated vertical emission energy (∆E(T1 − S00 )) for comparison where S00 designates some upper vibrational level of the singlet ground state hav5



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ing the same geometry as the optimized T1 state. For obtaining the S00 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, on the other hand the nature of the involving 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, ∆E(T1 -S0 ) provide a better comparison with the experimentally determined emission wavelength as compared to other more sophisticated theoretical calculations like vertical transition including SOC as also evidenced in numerous previous literature. 8,15,35 Moreover, for similar type of complexes, the qualitative trend in results is expected to be retained.

Radiative and Nonradiative Rate Constants and Quantum Efficiency Now, quantum efficiency (φ) of a phosphore could be expressed in terms of the rate constants of the radiative (kr ) and nonradiaive (knr ) emission processes as below

φ=

kr (kr + knr + knr (T ))

(1)

where kr is the radiative rate constant, knr is the temperature independent non-radiative rate constant, and knr (T) is the temperature dependent non-radiative rate constant. From the above equation it is clear that 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 6


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can be stated that 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 Noteworthy, 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 MLCT character in the emissive state. Thus, larger the MLCT character in the T1 state, more efficient the radiative process is. However, kr is the average of the individual components arrived from the three spin sub-levels (indexed by i) of the involved emissive state (T1 ) which can be expressed as 37,38 kr =

3 3 3 X 1 X α0 3 1X i 1X kr = kr (S0 , T1 i ) = (4 ∆E S−T |Mj i |2 ) 3 i=1 3 i=1 3 i=1 3t0 j∈x,y,z

(2)

where ∆ES0 −T1 is the transition energy, α0 is the fine-structure constant, t0 = (4π0 )2 /me e4 and Mji is the j th component of the electric dipole transition moment between S0 and the ith sub-level of the T1 state. Noteworthy, 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 energy gap law 39 as follows

ln(knr ) = kT I ∆E(T1 − S0 )

(3)

Where kT I 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 the mind that there exists excited 3 MC state near the radiative 3 MLCT state and the former cannot decay radiatively, we consider the possibility of thermal population of the 3 MC state to calculate the temperature dependent nonradiative rate constant (knr (T )). 7



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Now, the relative population of the 3 MC 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 equation: 40 knr (T ) = ka e−∆Ea /kB T

(4)

where ka is the proportionality constant, ∆Ea is the activation energy barrier for the transformation of 3 MLCT state to 3 MC 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 Equation 4 provided the values of ka and ∆Ea are known. We calculated the value of ka from the experimental data of the synthesized reference complex 41 and finally computed the φ values by using Equation 1 through Equation 4.

Results and Discussion The effective utilization of carbenes in OLED technology is being evolved recently. 10–12,16–18,20,26 It has been shown that mesoionic carbenes (MICs) are more effective than the normal Nheterocyclic carbenes (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 towards remote carbene (rNHC) on the basis that the σ-donor capability 42 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.

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Comparative Study Between MIC and rNHC in Terms of Emission Wavelength and φ Let us provide a comparative study between MIC and rNHC containing complexes of similar structure. For that, we have chosen some Ir(III) complexes with pyrazole based Nheterocyclic carbenes as a ancillary ligand in the presence of a phenylpyridine (ppy) as cyclometallating ligand. Figure 1 shows the schematic representations of the complexes, while optimized geometries of some representative structures are shown in Figure S1 in supporting information (SI). 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 masoionic (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, nonradiative rates and emission wave lengths of the complexes as listed in Table 1. From this table it 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 emission wavelength 467 nm. Similarly complexes B, C, D show emission wavelength 553 nm, 590 nm, 610 nm respectively. If we concentate on the radiative rate constant we find that complexes E and F show comparatively higher kr values compared to the complexes A–D. We have calculated the kr values by using Equation 2, with the help of similar methodology as mention earlier. 36 Since the spin-orbit coupling (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 metal character increases in the transition from T1 to S0 . While evaluating nonradiative rate constant (knr (T)), we first evaluate the activation barrier (∆Ea ) between the 3 MLCT and 3 MC states. The activation barrier between these two excited states, ∆Ea plays an important role in 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 9



atom, which is a convenient method, noteworthy. 45 We then calculate the transition state (TS) 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 3 MC state becomes poorly accessible and thus the triplet emitter works very efficiently. Using this method, we calculate ∆Ea for all the complexes A, B, C, D, E and F. It is observed that the complexes A, B, C have higher knr (T) values, resulting very low quantum efficiency. On the other hand, Complex E and F have very low knr (T) values which result in high quantum efficiency. As for example, Figure 3(a) shows the PESs for the complex E, against the longest (weakest) bond around the central metal atom, Ir which gives the estimated value of ∆Ea = 0.65 eV. Finally, knr (T) was calculated by using Equation 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 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 reversed nature in terms of quantum efficiency. Therefore, we can conclude that (a) in terms of φ: rNHC > MIC > NHC but (b) in terms of emission wavelength: NHC > MIC > rNHC.

Strategic Substitution for Blue Emission As we already mentioned, in spite of higher φ, the Ir(III) complexes with rNHC as cyclometalating 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.

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(a) Functional group substitution to the ligand: In accordance with the previous endeavor, 8,9,13,20 electron withdrawing substituents shifts the emission to the more blue region. Thus, we have performed substitution of electron withdrawing fluorine atom to the ppy ligand as shown 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 SI for more clarity. From the Table 2 it is clear that electron withdrawing substitution fluorine is influential in blue shift in the emission wavelength (Complex E0 , emission wavelength 613 nm). However, main problem is that the emission color is still not in the deep blue region. The main orbitals involved in the transition here are HOMO and LUMO, resulting in MLCT and LLCT characters. HOMO is situated on Ir, L2 and L3, while LUMO is mainly concentrated on L1. Therefore, we have found that Complex E0 have more blue shifted emission compared to Complex E with comparable φ value. Further, we have substituted one fluorine atom in the ppy moiety of the Complex E0 by more electron withdrawing functional group, – SO2 CH2 Ph to produce Complex E00 as shown in Figure 1. However, this substitution has negligible effect on shifting the emission towards more blue region. Similarly, we have tried insertion of others combination of different functional groups without having much success. Hence, we took the following alternative routes for achieving further hypsochromic shift.

(b) Entire ligand substitution: In our previous work, it has been established that NHC with proper substitution is suitable for the blue color emission. 20 But the efficiency of such triplet emitters are not high. It has already been demonstrated that the increment of nitrogen richness in cyclometalating/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 combination of NHC (as cyclometallating 11



ligand) and rNHC (as ancillary ligand) towards 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 E01 , we replace the cyclometallating ligand with nitrogen rich normal NHC (1,2,3 triazinebased 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. From these above observation it is clear that if we concentrate on MIC as cyclometallating ligand, we find that it shows blue emission but the efficiency is not much higher. But in case of rNHC (as ancillary ligand), it shows higher quantum efficiency but emission wavelength is not in the blue region. When we introduce more electron withdrawing substituents in the ppy ligand, 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 the strategies ((a) introduction of more electronegative substituents and (b) substitution of entire ligand) do not provide enough hypsochromic shift to incur blue emission.

(c) Introduction of six membered ppy based rNHC: Therefore, as an alternative strategy we introduce a new ligand (ppy based rNHC) as ancillary ligand. As a result for this substitution, Complex G1 and G2 have been produced (Figure 4) which, however, demonstrate the emission wavelength at 688 and 843 nm, respectively, as shown in Table 4. From Table 4 it is clear that in Complex G1 , introduction of electron withdrawing F atom in cyclometallating ligand and the use of ppy based remote carbene (rNHC) as 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 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 not also very helpful in blue emission.

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(d) Combination of nitrogen rich NHC as 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 (please see Figure S3 in SI for the optimized geometries), we observe that Complex H shows emission wave length at 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 kr value. 26,37,38 We have performed potential energy surface scan along the weakest bond attached to the central metal atom to evaluate the activation barrier between 3 MLCT and 3 MC state. Figure 3(b) shows the PESs for the complex H. The value of knr (T) was calculated by using Equation 4. From this PES we find the higher activation energy (∆Ea = 1.41 eV), resulting lower access to the 3 MC state. Therefore, the nonradiative rate constant (knr (T )) is observed to be significantly lowered and as a consequence the overall quantum efficiency is enhanced.

Conclusion In this work we demonstrate the role of 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 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 3 MC state which ultimately result 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 ancillary ligand is the most suitable one for designing efficient deeper blue emitting Ir(III) complexes which might have potential application in OLED.

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Supporting Information Available Singlet state optimized geometries of some complexes, pictorial representation of transition and orbital characteristics.

Acknowledgement The authors gratefully acknowledge the financial support from DST, Govt. of India, through sponsored research grant (SR/NM/NS-1005/2016). SU is thankful to DST for providing her with women scientist (scheme WOS-A) research grant (Sanction Order No. SR/WOS-A/CS116/2012(G)).

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References (1) Tang, C. W.; VanSlyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51, 913–915. (2) Wen, S.-W.; Lee, M.-T.; Chen, C. H. Recent development of blue fluorescent OLED materials and devices. J. Display Technol. 2005, 1, 90–99. (3) Geffroy, B.; Le Roy, P.; Prat, C. Organic light-emitting diode (OLED) technology: materials, devices and display technologies. Polymer International 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. High-efficiency organic electrophosphorescent devices with tris (2-phenylpyridine) iridium doped into electrontransporting 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) Angelis, F. D.; Fantacci, S.; Evans, N.; Klein, C.; Zakeeruddin, S. M.; Moser, J. E.; Kalyanasundaram, K.; Bolink, H. J.; M.Grätzel,; 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.; Xia, J. W. B.-H.; Pan, Q.-J.; Zhang, H.-X. DFT and TD-DFT study on the

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electronic structures and phosphorescent properties of 6-phenyl-2,20 -bipyridine tridentate iridium(III) complexes and their isomer. Dalton Tans. 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. 16


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(18) Monti, F.; Placa, M. G. I. L.; 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, 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. (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 Nheterocyclic carbene ligands for blue-light-emitting 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.

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(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 122, 7532– 7539. (27) Hohenberg, P.; Kohn, W. Inhomogeneous electron gas. Phys. Rev. B. 1964, 136, 864. (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 Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 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) Salek, P.; Vahtras, O.; Helgaker, T.; ˚ Agren, H. Density-functional 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. Inorg. Chem. 2008, 47, 980–989. (36) Aidas, K. et al. The Dalton quantum chemistry program system. Wiley Interdisciplinary Rev.: Comput. Mol. Sci. 2014, 4, 269–284. 18


Page 18 of 23

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(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, excitedstate 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 cew class of remote N-heterocyclic carbenes with exceptionally strong σ-donor properties: introducing benzo[c]quinolin-6-ylidene. Chem. Eur. J. 19, 4287–4299. (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 fac-Ir(ppy)3 . Inorg. Chem. 2015, 54, 10457–10461.

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Table 1: Computed emission energy, radiative, nonradiaive 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 Complex Complex Complex Complex Complex Complex

A B C D E F

% of M 31.21 29.6 29.94 29.65 32.11 31.03

CI 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

Table 2: Computed emission energies, radiative, nonradiaive rates percentage of metal (M) in the involved transition, the highest coefficient of the involved pair of orbitals therein and φ for some selective complexes. Complex Complex E Complex E0 Complex E00 Complex H

% of M 32.11 29.61 30.16 33.12

CI kr 0.70 0.71 0.65 0.67

(105 s−1 ) 4.10 4.05 4.01 4.7

knr (T ) (105 s−1 ) T1 – 1.06 × 10−1 8.70 × 10−1 4.63 × 10−1 3.95 ×10−3

S0 (nm) φ(%) 774 97 613 82 652 89 476 100

Table 3: Calculated transition and orbital characteristics for some studied complexes. Complex Complex Complex Complex Complex

E E0 E00 H

Orbital contribution (%) H→L (61) H→L (70) H→L (70) H→L (67)

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Transition characteristics L2/L3/Ir→ L1 (MLCT/LLCT) Ir/L2/L3→L1 (MLCT/LLCT) Ir/L2/L3→L1 (MLCT/LLCT) Ir/L2/L3→ L1 (MLCT/LLCT)


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Table 4: Computed emission wavelength for some designed complexes (changing entire ligand). Complex Complex E1 Complex E01 Complex G1 Complex G2 Complex H

T1 – S0 (nm) 626 610 688 843 476

Figure 1: Schematic representations of some complexes studied here. L2, L3 are unsubstituted/substituted phenylpyridine ligands, while L1 is either mesoionic (MIC, A–D) or remote carbene (rNHC, E and F)) ligand. For Complex E: R = R0 = H; Complex E0 : R = R0 = F; and Complex E00 : R = SO2 CH2 Ph, R0 = F.

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

Figure 3: Representative potential energy surface (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).

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

Graphical TOC Entry

Combination of NHC as cyclometallating ligand and phenyl-pyridine based rNHC as ancillary ligand is very effective for designing efficient blue emitter.

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Can Remote N-Heterocyclic Carbenes Be Used for Designing Efficient

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