Essential Role of Ancillary Ligand in Color Tuning and Quantum

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A: Kinetics, Dynamics, Photochemistry, and Excited States

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 Sharmistha Urinda, Goutam Das, Anup Pramanik, and Pranab Sarkar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05376 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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The Journal of Physical Chemistry

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 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, India E-mail: [email protected]

Abstract Tuning photoluminescence properties is of prime importance for designing efficient light emitting diode (LED) materials. Here, we perform a computational study on the effect of normal N-heterocyclic carbene (NHC) and abnormal mesoionic carbene (MIC) ligands on the photoluminescence properties of some Ir(III) complexes which are very promising LED materials. We find MIC as the privileged ligand in designing triplet emitters. The strong σ-donating and moderate π-accepting properties of MIC render a lower access to the non-emissive triplet metal-centered state (3 MC), resulting in lowering the non-radiative rate constant (knr ) and correspondingly achieves higher quantum efficiency. We also demonstrate that the judicial choice of ancillary ligand can improve the efficiency of these materials even further. This quantum chemical

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investigation focuses on the importance of MIC as cyclometallating ligand and the substantial effects of ancillary ligands in controlling the color tuning and quantum efficiency for optoelectronic applications.

Introduction Since its advent in 1987, 1,2 organic light emitting diode (OLED) has earned it’s prominence compared to other available display technology due to its energy efficiency and better aesthetics. OLEDs mainly rely on phosphorescence properties of organometallic complexes of heavier transition metal (Rh, Ir, Pt, etc.) atoms. In the recent past, the transition metal complexes have gained significant attraction within the photophysical research community due to their versatility in terms of the different excited states properties. 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. 3,4 In particular, the organometallic complexes of N-heterocyclic carbene (NHC) have become important in recent few years for their extraordinary activities. The synthesis and characterization of luminescent transition metal complexes containing NHC ligand, both by experimental and computational methods, have been demonstrated through multiple significant research studies over the last few years. 5–12 Due to the presence of NHC, having very high field strength, the corresponding complexes manifest high energy emissions. As a consequence, these complexes can be used for designing deep blue OLEDs. Furthermore, as NHC ligands have capability of exhibiting various structural modifications, they together with some other ancillary ligands, provide numerous possibilities of synthesizing phosphorescent materials having tunability of emission colors over the entire visible spectra. Thompson and coworkers have reported a logical approach for the synthesis of Ir(III) complexes with excellent blue and near-UV phosphorescence where they demonstrate the use of NHC ligands. 13 Plethora of works have been followed thereafter with normal NHC as cyclometallating ligands 2


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for synthesizing phosphoroscent OLED materials. 9,12,14–17 In the recent past, we have also demonstrated, using extensive computational studies, that the Ir-NHC carbene complexes are really useful in preparing OLEDs due to their deep blue emission with high quantum yield. 18–20 Besides the concept of normal heterocyclic carbenes (NHC) which are stabilized by the two adjacent heteroatoms, an abnormal class of carbenes, known as mesoionic carbenes (MIC) have also been considered which are produced by the replacement or displacement of one of the heteroatom (carbene derived from pyrazole, imidazole, isothiazole) and stabilized by remote positioned heteroatom. 16 Mesoionic/abnormal carbenes such as 1,2,3-triazol5-ylidenes 21 have become attractive ligands in transition-metal chemistry as they impart stronger donor ability than Arduengo-type 2-imidazolylidenes. 22,23 For example, it has been shown that the Fe/Ru-MIC complexes are very promising in the field of photochemical and photophysical research. 24–27 The Ru-MIC complexes are shown to have 3 MLCT (metal to ligand charge transfer) excited-state lifetimes in nanosecond or even microsecond order. 24 These results demonstrate the potential usefulness of NHC/MIC ligands for generating favorable excited-state properties in d6 metal complexes like Fe/Ru(II) or Ir(III) which have been proven to be potential triplet emitters in OLED industry. However, the effects have been attributed to the strong σ-donating and moderate π-accepting capabilities of the NHC/MIC ligands, both of which practically destabilize the metal-centered, 3 MC states of the complexes resulting an increase in the lifetimes of the 3 MLCT excited states. 25,28 Very recently, Chábera et al. 29 have reported a room temperature photoluminescent Fe(III)-MIC complex which displays an excited-state lifetime of 100 ps. The experimental and back to back theoretical studies reveal that the luminescence involves a spin-allowed transition between two doublet states while the 3 MC state is situated far above the doublet one. This renders a peculiar σ-donor and π-acceptor characters of the MIC ligands, which certainly necessities further studies. 30 At the same time, there is also increasing evidence that bidentate MIC ligands remarkably enhance the redox-stability of their metal complexes. 15,27,30–32

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Recently, Monti and coworkers 15 have synthesized two cationic Ir-MIC complexes with neutral MIC and anionic ancillary ligands. Although the photoluminescence quantum yields of the prepared complexes are comparable to those of the archetypal complexes containing bypyridine (bpy) or phenylpyridine (ppy) ligands, further improvements may be carried out by changing the MIC and/or the ancillary ligands. Inspired by this work of Monti and coworkers 15 we set our goal to perform a comparative density functional theory (DFT) study of the effects of NHC and MIC ligands on the color tuning and quantum efficiency of the complexes (please see Figure 1, Complex I–III). Initially, we observe that MICs are potentially capable of producing efficient triplet emitters. Thereafter, we demonstrate that the judicial choice of ancillary ligand can improve the efficiency even further.

Methodology and computational details: We adopt the following computational methodologies for characterizing the ground and excited state properties, and evaluating the quantum efficiency of the complexes.

Characterization of the S0 and T1 states of the Ir(III) complexes: To evaluate the singlet ground state (S0 ) geometries, DFT 33 calculation have been performed using B3LYP 34,35 exchange-correlation (xc) functional as implemented in Gaussian09 software package. 36 No symmetry constrain has been imposed as all the considered complexes are non-symmetric in nature. Basis set, (double-ζ) 6-31G∗∗ is used for the light atoms and LANL2DZ 37 is used for the heavy metal atom, Ir. Relativistic effective core potential (ECP) 38 is used for the Ir to replacement of the inner core electrons while leaving the outer core [5s2 5p6 ] and the valance [5d6 ] electrons for consideration. In our previous work we have demonstrated that for the characterizing such heteroleptic cyclometallating Ir(III) complexes, B3LYP is the best choice among the available functionals. 19 We further characterize the geometries of the first triplet excited (T1 ) state by using unrestricted B3LYP

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(uB3LYP) xc functional with triplet spin multiplicity. Same basis sets and ECP are used for the excited state optimization.

Characterization of emission: We next calculate the phosphorescence emission energy by using the optimized T1 state energy. Two methods are mostly effective for this calculation. (1) Adiabatic emission energy (∆E(T1 − S0 )) which is calculated as the energy difference between the optimized T1 and S0 states in their respective minima and (2) vertical emission energy (∆E(T1 − S00 )) which is the energy difference between T1 and S00 , where S00 is an upper vibrational level of S0 having the same geometry as the optimized T1 state. Note that, the ∆E(T1 − S0 ) is very close to the experimental value at room temperature, 298.15 K. The energy of the state S00 is calculated by performing a single point (SP) calculation at the triplet geometry with singlet spin multiplicity. Next, time-dependent DFT (TDDFT) calculation is performed on the triplet optimized geometry having a spin multiplicity specified as singlet. The TDDFT calculation provides emission orbital characteristics and other emission specific information. 6,39 However, TDDFT does not account the presence of strong spin-orbit coupling (SOC) in cyclometalated Ir complexes and therefore underestimate the emission wavelength. To overcome this we have further performed SOC induced phosphorescence calculation where SOC is treated perturbatically. 40 However, we would still like to emphasize that in numerous previous observation the experimental evidence has confirmed that the adiabatic separation ∆E(T1 -S0 ) provides a better estimated emission wavelength compared to theoretically accurate calculations like vertical transition including spin orbit coupling (SOC). 4,6,41

Prediction of quantum efficiency: Theoretically, quantum efficiency can be expressed as

φ=

kr (kr + knr + knr (T )) 5


(1)


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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. Here, we have calculated the kr values using the method as implemented in Dalton, 42 where the dipole moment and the spin-orbit operators are treated perturbatically on the unperturbed Hamiltonian at the ground state geometry. In order to ensure efficient phosphorescence, a large value of spin forbidden transition (T1 → S0 ) value is required as a result of strong SOC. In practice, the larger the MLCT character of the emissive state, the more efficient the radiative process. The phosphorescence radiative decay rate constants (kr ) from one of the three spin sub-levels (indexed by i) of the involved emissive state (T1 ) can be expressed as 43,44 3 3 3 X 1 X α0 3 1X i 1X i kr = kr (S0 , T1 ) = (4 ∆E S−T |Mj i |2 ) kr = 3 i=1 3 i=1 3 i=1 3t0 j∈x,y,z

(2)

where ∆ES−T is the transition energy, α0 is the fine-structure constant, t0 = (4π0 )2 /me e4 and Mji is the j axis projection of the electric dipole transition moment between the ground state and the ith sub-level of the emissive triplet state, T1 . At room temperature, only the weighted phosphorescence rate constants can be calculated using the above equation. On the other hand, to evaluate the temperature independent knr we use the energy gap law 45 as given below ln(knr ) = kT I ∆E(T1 − S0 )

(3)

where kT I is a proportionality constant. However, it is well known that the overall knr value has a strong temperature dependence 43 and therefore it provides mostly inaccurate nonradiative rate constant at room temperature. To calculate the temperature dependent nonradiative rate constant (knr (T )), i.e., the rate of thermal population to an upper level (e.g., the 3 MC state) that cannot decay radiatively, we consider the possibility of populating a 3 MC 6


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state situated nearby the excited 3 MLCT state along the potential energy surface (PES). As OLEDs should work at ambient temperatures, controlling the temperature-dependent behavior is vital for designing more efficient phosphors. It has been found that at 300 K, knr (T ) dominates over knr . Now, knr (T ) can simply be expressed in terms of the Arrhenius equation: 46 knr (T ) = ka e−∆Ea /kB T

(4)

where ka is the proportionality constant, ∆Ea is the activation energy between 3 MLCT and 3

MC states and kB is the Boltzmann constant. The value of ka and hence the exponential

decrease in knr (T ) as ∆Ea increases may be expected to be of similar magnitude for all the complexes. We can calculate the non-radiative rate constant (knr (T )) using Eq. 4 provided the value of ka is known. ka can be calculated once the experimental value of φ is known for a synthesized complex. For the same complex we can compute the value of ∆Ea and then substitute it in Eq. 1 through Eq. 4 while ignoring the term knr . Here we use the kr value obtained by experiment. If the experimental results provide phosphorescence lifetime (τ ), then kr is evaluated as the inverse of τ .

Results and discussion: In this section we demonstrate the comparative effects of NHC versus MIC ligands on color tuning and quantum efficiencies of the studied Ir(III) complexes. In Figure 2, we have shown schematically the structures of the N-heterocyclic and mesoionic carbenes that are chosen as the cyclometallating ligands for our purpose. From Figure 2, it is clear that normal carbene has three resonating structures in free condition but mesoionic carbene has two such structures. As a consequence, normal carbene is more stable compared to the MIC. MIC has strong σ donor character compared to NHC which destabilizes the 3 MC state more, resulting an increment of the lifetime of the 3 MLCT excited state.

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Frontier molecular orbitals and energy gap: We have listed the corresponding energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) and the corresponding HOMO-LUMO gaps for the considered complexes, Complex I–III in Table 1. Some of these complexes (Complex I and II in Figure 1) have recently been synthesized and characterized as phosphorescent emitters by Monti et al. 15 For Complex I and II, the cyclometallating ligands attached to Ir are MIC in nature (L2 = L3 = B), whereas for Complex III, these are NHC (L2 = L3 = A). The MIC precursor is able to behave as a bis-chelating ligand, as it contains a 2pyridyl-1,2,3-triazolylidene unit and a benzyl group capable of N-substituent coordination. Therefore, facilitates cyclometalation. As already told, our primary aim is to put forward a comprehensive comparison of MICs and the corresponding NHCs in terms of both color tuning and quantum efficiency. Here, the Complex I shows a HOMO-LUMO gap of 4.24 eV, whereas Complex II and III have HOMO-LUMO gaps 3.20 and 3.54 eV, respectively. On the next section, we further compare the emission characteristics.

Emission wavelength and characteristics of emission: We list the calculated adiabatic emission wavelengths for different complexes (Complex I– III) in Table 1. The corresponding transition characteristics of the first emissive states (T1 ) have been listed in Table 2. Complex I and II are formed through the chelation of the cyclometalated ligand (B, Figure 1) with Ir(III) and they demonstrate emission wavelengths of 527 and 470 nm, respectively (please see Table 1). Noteworthy, in both the complexes, the cyclometallating ligands are MIC. For Complex II, the main transition is characterized to be 3 LLCT. However, we observe a significant presence of 3 ILCT with a negligible admixture of 3 MLCT. On the other hand, Complex III is structured through binding of NHC (A) cyclometallating ligand and it shows lower emission wavelength of 379 nm with a predominant 3

LLCT emission characteristics. Therefore, from the perspective of emission wavelength, it

is clear that normal carbene is a stronger ligand, while the mesoionic (abnormal) carbene 8


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is comparatively weaker. Here, we have found that all the transition from T1 to S0 , for Complex II and III, are of mostly 3 LLCT characteristics. Therefore, they are more likely to have a rather well defined emission spectra 3 with comparatively sharper peak as in 3 LLCT, the PES is comparatively flatter due to the delocalization of π electron. 4 On the other hand, the presence of a strong 3 ILCT characteristic provides a relatively weaker color tuning property in Complex I. Therefore, it demonstrates multiple peaks around it’s main emission wavelength (527 nm) at room temperature. These facts about Complex I and II are already observed through experiment by Monti et al. ?

Radiative (kr ) and non-radiative (knr ) rate constants and quantum efficiencies (φ): In this section, we compare the radiative and non radiative constants for the complexes with MIC and NHC. First of all, we have calculated the kr values by using Eq. 2, as implemented in Dalton Software 42 and are listed in Table 3. From the table, it is clear that Complex I and II show higher kr values compared to the Complexes III. Since the spin orbit coupling (SOC) in transition metal allows 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 spin-orbit coupling matrix elements increase as the percentage of metal character increases in the transition from T1 to S0 . Complex III has such metal character of 1.65%, resulting in a small soc and thus, it has very low kr value and subsequently it offers low φ. Complex I and II has relatively higher metal contribution (4.29% and 13.85% respectively) in the transition from T1 to S0 . This can be attributed to the relatively moderate π back-bonding in MIC compared to that of NHC. From Figure 3, it is clear that for Complex II (containing MIC as cyclometallating ligand), the transition from T1 to S0 involves a major orbital contributions from (H-3) to L and H to L. (H-3) to L transition shows mainly 3 LLCT character along with slight 3 MLCT and 3

ILCT character. Similarly, other major orbital involvement occurs from H to L which 9



has 3 LLCT character along with slight 3 MLCT. Involvement of 3 LLCT is responsible for higher geometric deformation 47 and therefore reduces the vertical emission energy (T1 –S00 ) as listed in Table 1. The charge transfer occurs from L1 to L2, L3 to L2 and metal to L2. Note that these calculations of vertical emissions already include SOC. 42 In numerous previous publications it has already been demonstrated that the adiabatic emission often reproduces the emission energy most accurately as compared to the experimentally observed vertical emission energy at room temperature. 4,6,18–20,41 From the energy gap law (EGL), we can calculate the temperature independent nonradiative rate constant and quantum efficiency. As shown in some of the previous investigations, 48 at 77 K, this may lead to much higher quantum efficiency due to a very small value of knr (T ). In our previous investigation, 18,19 we have demonstrated that the presence of 3 LLCT introduces (in contrary to 3 ILCT) geometry deformation and hence reduces the T1 to S00 energy gap. At low temperature, EGL shows that the value of knr is directly proportional to (T1 –S00 ) energy difference. Therefore, design of high photoluminescence quantum yield (PLQY) at low temperature requires to ensure the presence of 3 ILCT with the proper admixture of 3 MLCT for adequate SOC. However, OLEDs generally work at ambient room temperature. It has also been experimentally shown that the knr (T ) > knr at room temperature. 49,50 Therefore, we have calculated the temperature dependent non-radiative rate constant (knr (T )). It has also been experimentally shown that knr (T ) has more significant role on the evaluation of φ compared to knr . Therefore, without loss of generality we can ignore the effect of knr for evaluating φ. To calculate the 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 atom, which is a convenient method, noteworthy. 51 We then calculate the transition state (TS) over the PES along this reaction coordinate. We further prove

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that, after crossing the barrier the PES moves to 3 MC state. This is proven by evaluating the corresponding charge transfer characteristics of the concerned geometries (given in the supporting information, SI). If this activation barrier is high enough, the excited 3 MC state becomes poorly accessible and thus the triplet emitter works very efficiently. The upper and lower left panels of Figure 4 show the PESs for the Complex II and III, respectively, against the longest (weakest) bond (Ir–C or Ir–N) around the central metal atom, Ir. In particular, Powell et al. 48 have demonstrated that the activation energy between 3 MLCT and 3 MC states have clear correlation with the experimentally observed knr (T ) values for different cyclometalated Ir complexes. We observe that, for Complex II, there is an initial nonlinear increase in the energy of the T1 state along the reaction coordinate followed by a smooth drop. As it is well established that a 3 MC state will enforce mostly non-radiative decay, we have tested the charge transfer characteristics of the T1 geometry obtained at the point in the PES after the energy drop which shows a strong 3 MC characteristic (Figure S1 in SI). We identify the corresponding TS and evaluate the activation energy (∆Ea = 1.09 eV). Similarly, Figure 4 also shows the PES for Complex III. There is sudden change in the geometry of the triplet state and consequently a sudden drop at the fourth point of the PES scan. We characterize this geometry and find that it shows a strong 3 MC characteristics. All the knr (T ) values are listed in Table 3. Among these complexes, Complex I has a presentable radiative rate constant. However, as it shows very high knr (T ) and as a result it has very low φ. Complex II (MIC as cyclometallating ligand) has a higher value of radiative rate constant and has moderately low value of knr (T ) and therefore, has a relatively higher value of φ. Complex III (containing NHC as cyclometallating ligand) has the least kr value with very high knr (T ), resulting the poorest value of φ.

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Enhancement of φ by judicial choice of ancillary ligand: We look for the further improvement of φ by judicial choice of ancillary ligand, a common strategy that has already been established. 10,18–20,52 The target has been three fold (i) to increase the 3 MLCT characteristics of the T1 excited state to enhance the SOC and hence kr value (ii) to keep T1 –S0 separation high enough to get blue emission (iii) to maintain a good admixture of 3 LLCT and 3 ILCT to have a higher activation energy to 3 MC and to fortify the sharp emission peak. In view of this, we have substituted ligand L1 of Complex II by phenylpyridine (ppy) ligand to produce Complex IIA and IIIA respectively. All the optimized bond lengths of the four complexes (in the first excited triplet state) around the central metal atom (Ir) are listed in Table 4. Ir-C and Ir-N bond length of L1 of Complex II are 2.090 and 2.106 Å, respectively. Similarly, the corresponding bond lengths for Ir-L2 and Ir-L3 are listed in Table 4. Among all the six bonds, Ir-N bond of Ir-L3 is the longest (2.116 Å) one. On the other hand, Ir-C and Ir-N bond lengths of L1 of Complex IIA are 2.095 Å and 2.168 Å, respectively. Comparing all six bonds we have found that Ir-N (of ppy ligand, L1) bond is the longest (2.168Å) one. The longest bond of the Complex II, Ir-L3 (Ir-carbene) is now shifted to Ir-L1 (Ir-ppy). The octahedral structure of the complex IIA is less strenuous due to the six membered ring chelation of ppy ligand. As a consequence, the overall stability of Complex IIA is higher than that of Complex II. Further, from Table 1, it is observed that HOMO-LUMO gap of Complex IIA and Complex IIIA are 2.98 eV and 3.61 eV, respectively. This is consistent with our earlier observation, however, the difference in HOMO-LUMO gap between the Complex IIA and IIIA enhances significantly as compared to that between Complex II and III. This indicates that the overall HOMO-LUMO splitting is not only dependent upon the field strength of the cyclometallating ligand but also dependent on the nature of the ancillary ligand. Here, we have found that the emission wavelengths (Complex IIA, 469 nm and Complex IIIA, 450 nm as shown in Table 1) correspond to the transition from T1 to S0 , having an admixture of MLCT, ILCT and LLCT (please see Table 2). Specially, for Complex IIA, we observe that T1 state has almost equal percentage 12


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of MLCT, ILCT and LLCT characteristics (3). This was in fact one of our major goal to achieve which enhances the overall PLQY with required blue shift and color purity. We further investigate the effect of ppy on the emission wavelength and quantum efficiency. Figure 3 shows the orbital transition characteristics of Complex II (MIC as cyclometallating ligand) and Complex IIA (normal NHC as cyclometallating ligand). As mentioned earlier, Complex II have major contribution of 3 LLCT along with moderate 3 MLCT and 3 ILCT in it’s T1 state, whereas, Complex IIA shows two main orbital contributions of HOMO-2 to LUMO (52%) and HOMO-5 to LUMO (27%) for T1 to S0 transition. It shows major orbital contribution of 3 ILCT and 3 MLCT due to the substitution by a ppy ligand. This substantially increases the metal percentage and corresponding kr value. Similarly, we have listed the orbital contributions of Complex III and IIIA for T1 to S0 transition in Table 2. All the emission related values for the Complex II, IIA, III and IIIA are listed in Table 3. Figure 4 shows the PES scan for all four complexes. For example, as shown earlier, for the complex II the activation energy is 1.09 eV and the corresponding knr (T ) is very high. By the substitution of ppy in complex IIA, the activation barrier between 3 MLCT and 3 MC is recorded to be much higher (1.9 eV), which results in a very low knr (T ) value. As the activation energy is higher, 3 MC state is not easily accessible for non-radiative decay for Complex IIA. As a consequence, complex IIA shows very high quantum efficiency (nearly 100 %). Similarly, for Complex IIIA, activation energy barrier (1.25 eV) is comparatively higher than that of Complex III which results in a lower knr (T ) value and correspondingly higher φ (nearly 96%). However, the knr (T ) value calculated for Complex IIIA is found to be significantly lower compared to that of Complex IIA. This is because of the presence of MIC in Complex IIA as explained earlier. Now, from Table 1, it is clear that complexes formed with NHC (III and IIIA) show higher blue emission compared the complexes with MIC (II and IIA). But from Table 3 it is clear that Complex IIA and Complex IIIA shows higher kr value and lower knr (T )

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resulting in higher quantum efficiency (φ). The calculated knr (T ) are dependent on the proportionality constant (ka ), which is calculated from the experimental data 15 reported by Monti et al. There exist only two different experimental values of phosphorescence life time for two different complexes. 15 Hence the proportionality constant evaluated might not be the exact one due to lack of availability of sufficient experimental data. In fact, this might vary depending on the system under study. However, the relative values of φ must be indicative and shall therefore predict correct trends. So, from these above calculated values (Table 3), we can conclude that Complexes IIA and IIIA are better triplet emitter compared to Complexes II and III. Therefore, from the above observation, it can easily be concluded that ancillary ligands have an important role on designing an efficient triplet emitter that could be used as OLED.

Conclusion In this work we demonstrate the role of ancillary ligand in color tuning and quantum efficiencies (φ) of Ir(III) complexes with N-heterocyclic or mesoionic carbene ligands. Using the density functional theory (DFT) and time-dependent DFT calculations we find that the mesoionic carbene ligand (MIC) is a privileged ligand in designing the triplet emitters used in OLED. The strong σ-donation and moderate π-acceptance of MIC are very helpful to lower the access of the non-emissive 3 MC state resulting in lowering non radiative rate constant (knr ) and the corresponding φ. We also demonstrate that the ancillary ligand has an important role in the enhancement of the quantum efficiency. Our computational analyses unravel the fact why the recently synthesized Ir(III) complexes show very low φ at room temperature. 15 We have substituted the existing ancillary ligand by phenylpyridine which results significant changes in the color tuning and quantum efficiencies of the existing complexes.

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Acknowledgments The authors gratefully acknowledge the UGC and CSIR, Govt. of India for the financial supports through research grants. The DST, Govt. of India is acknowledged for the women scientist (scheme WOS-A) research grant (Sanction Order No. SR/WOS-A/CS-116/2012(G)) to S.U.

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Table 1: Calculated HOMO-LUMO energies, their corresponding gaps, and emission energies for the studied complexes. Complex Complex Complex Complex Complex Complex

I II III IIA IIIA

HOMO (eV) -6.62 -8.75 -8.93 -10.68 -10.69

LUMO (eV) -2.38 -5.55 -5.39 -7.70 -7.08

H-L gap (eV) 4.24 3.20 3.54 2.98 3.61

T1 -S0 (nm) 527 470 379 469 450

T1 -S00 (nm) 688 729 523 610 571

Table 2: Calculated transition and orbital characteristics for the studied complexes. Complex Complex I Complex II Complex III Complex IIA

Complex IIIA

Orbital contribution (%) H→L (61) H→L (38) H-3→L (45) H-10→L (33) H-13→L (34) H-2→L (52) H→L (14) H-5→L (27) H →L+1 (16) H →L+2 (65)

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

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Table 3: Computed emission energies, radiative, nonradiaive rates and φ for the complexes. Complex Complex I Complex II Complex III Complex IIA Complex IIIA

% of M CI kr (105 s−1 ) ∆Ea (eV) knr (T ) (105 s−1 ) 4.29 0.61 0.73∗ 0.81 7.40 × 105 13.85 0.38 1.53 1.09 19.90 7.21 0.45 1.65 0.34 0.08 0.81 7.40 × 105 0.05 0.33 32.60 0.52 3.26 1.9 4.40 × 10−13 12.76 0.14 5.73 0.65 1.16 1.25 0.04 5.73 0.16 ∗ Expt. value: τ = 5.90 µs; kr = 1.69×105 s−1 (Ref. 15 ).

φ(%) 9.86 × 10−5 7 1.22 × 10−5 100 96

Table 4: Different optimized bond lengths (in Å in the excited triplet state geometries (T1 ) of Complex IIA and IIIA. Complex II Ligand Bond Bond length L1 Ir-C 2.090 Ir-N 2.106 L2 Ir-C 1.980 Ir-N 2.094 L3 Ir-C 2.032 Ir-N 2.116 Complex IIA Ligand Bond Bond length L1 Ir-C 2.095 Ir-N 2.168 L2 Ir-C 2.014 Ir-N 2.119 L3 Ir-C 2.059 Ir-N 2.105

Complex III Ligand Bond Bond length L1 Ir-C 2.095 Ir-N 2.168 L2 Ir-C 2.054 Ir-N 2.119 L3 Ir-C 2.059 Ir-N 2.105 Complex IIIA Ligand Bond Bond length L1 Ir-C 2.065 Ir-N 2.133 L2 Ir-C 2.127 Ir-N 2.099 L3 Ir-C 2.015 Ir-N 2.117

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Figure 1: Schematic representations of different complexes studied here.

Figure 2: Resonating structures of normal (NHC) and abnormal/mesoionic (MIC) carbenes.

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Figure 3: Pictorial representations of orbital transition characteristics for Complex II and Complex IIA.

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Figure 4: Potential energy surface scan along the weakest bond of some selective complexes.

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Essential Role of Ancillary Ligand in Color Tuning and Quantum

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