Intrinsic quantum efficiency enhancement in well-known Ir(III) complexes by virtue of a simple and controllable deuteriation strategy

Abstract

A series of well-known iridium(III) complex molecular materials are selected for a comprehensive theoretical investigation into their complete or partial ligand deuteriation products. These compounds, respectively, are two homoleptic compounds fac-Ir(ppy)₃ [ppy = 2-phenylpyridinate] (1) and fac-Ir(dfppy)₃ [dfppy = 2-(2′,4′-difluorophenyl)pyridyl] (2), and two heteroleptic compounds Ir(ppy)₂acac [acac = acetylacetonate] (3) and Ir(dfpypy)₂pic [dfpypy = 2′,4′-difluoro-2,3′-bipyridinato-N,C^4′, pic = picolinate] (4). Unlike traditional strategies for enhancing the radiative decay process or reducing the non-radiative decay process through cumbersome ligand decorations, our simple and controllable deuteriation strategy, which is insensitive to the radiative decay process, enables us to suppress the non-radiative decay process by almost 50%. It is consistent with previous experimental findings, under our theoretical evaluation, that deuterium has a significant influence on non-radiative deactivation processes due to the reduction in the amplitude and frequency of vibrational modes caused by the increased deuterium mass. The systematic computational approach for the evaluation of the non-radiative decay processes provides us with alterations of every single vibrational mode, from the primitive complexes to the completely or partially deuterated ligand complexes, in detail. The two-averaged modes assessment informs us that deuteriation exerts its main influence on the high-frequency modes, rather than the low-frequency modes. Additionally, partial ligand deuteriation has a correlation with the nature of the electronically excited state decay pathways.

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1 Introduction

Phosphorescent organic light-emitting diodes (phOLEDs), as second generation OLEDs, have been attractive in recent years, owing to their ability to attain nearly 100% internal electroluminescence quantum efficiencies.¹ Among the numerous phosphorescent materials, cyclometalated iridium(III) complexes have advantages over other heavy metal materials, due to their chemical stability and full-color emission tunability.^2–5 Watts and co-workers reported the well-known green emitting complex fac-Ir(ppy)₃ in 1991,⁶ and efficient fac-Ir(ppy)₃ OLED devices have since been fabricated.⁷ In order to improve the synthesis approaches for tris-cyclometalated homoleptic iridium compounds, heteroleptic compounds with ancillary ligands have become prevalent,⁸ such as the representative Ir(ppy)₂acac, which displays high quantum efficiency.⁹ The synthesis of blue-emitting Ir(III) complexes with considerable phosphorescence efficiency is still a genuine challenge because of a significant increase in the non-radiative deactivation processes.^10,11 Through introducing fluorine atoms onto the ppy ligand, a blue-shifted emission could be triggered, and hence, homoleptic fac-Ir(dfppy)₃, with dfppy representing 2-(4,6-difluorophenyl)pyridyl, was prepared for blue-emitting phosphors in 2003.¹² In searching for a suitable host for phOLED applications, heteroleptic complexes with appropriate ancillary ligands were then prepared to address energy limitation problems.^13–15 One of the typical heteroleptic blue-emitting Ir(III) compounds is Ir(dfpypy)₂pic, which has a considerable phosphorescence efficiency of 90% in solution.¹⁶

Nevertheless, phosphorescence emitting Ir(III) complexes still have potential in quantum efficiency enhancement. Combined with the thriving theoretical foundations and computational industries, molecular photophysical processes have been explored and assessed gradually from qualitative to quantitative levels.¹⁷ To obtain efficient phosphorescence emitting Ir(III) complexes, raising k_r and reducing k_nr are of great concern, and in both the radiative and non-radiative processes for phosphorescent complexes, the “heavy-atom effect”, which gives rise to large spin–orbit coupling (SOC),¹⁸ contributes to the electronic transition channels between the singlet and triplet states.¹⁹ The decoration of ligands for larger SOC integrals has been implemented to increase k_r, including enlarging the π-extension or adding substitute groups.^20,21 However, an increased SOC constant can also facilitate non-radiative deactivation processes.²²

As for the non-radiative deactivation process, factors involving structural distortions between the electronic transition states play a significant role in the determination of k_nr through its expressions.²³ The improvement of structural rigidity is an efficient approach to suppress k_nr.²⁴ Moreover, strongly σ-donating ligands enable the lifting of the detrimental d–d excited state, thereby contributing to the inhibition of k_nr to a significant extent.²⁵ Notably, deuteriation is a succinct method to suppress k_nr and maintain k_r at the same time.

The deuterium isotope effect on the photophysical properties of inorganic and organic systems is of significance due to the improvement it causes in the quantum efficiency, which was recognized as early as 1960.²⁶ The applications of deuteriation were first investigated by Kropp and Windsor,^27,28 who primarily focused on the fluorescence of rare earth ions. Nowadays, the deuterium isotope effect is still attractive, especially in the pursuit of photoelectric materials with high efficiency, and in the pursuit of mechanistic interpretations. For example, the working mechanism of aggregation-induced emission can be understood from the deuterated isotope effect in depth.²⁹ The photophysical exploration of the deuterated Ir(III) complex IrCl₂(bpy-d₈)₂⁺ (bpy = 2,2′-bipyridine) was first reported by Watts et al.³⁰ It was found that the emission lifetime was prolonged due to the deuterium effects on the ligands. The complete deuterium effect on the well-known Ir(ppy)₃ (ppy = 2-phenylpyridine) complex was also evaluated in different solvents.³¹ The quantum yield of the fully deuterated Ir(ppy)₃ was slightly higher than that of the primitive complex, independent of the solvents. Experimental results showed that the ligand deuterium isotope effect mainly acted upon the non-radiative decay process. Additionally, observations of such a deuterium isotope effect are often noted as a weak coupling mechanism for non-radiative decay occurrences,³² which means that the temperature-dependent non-radiative decay process that is prominent in strongly coupled ground and excited states cannot exert a significant influence.

Although numerous experimental studies have discussed the influence exerted by complete or partial deuteriation on certain molecules,^33,34 the mechanism of such a deuterium isotope effect on heavy-metal complexes has not been interpreted in depth from a theoretical perspective. Herein, to verify the pervasiveness of such a deuterium isotope effect, a series of well-known Ir(III) complexes, as mentioned above, are selected for a deuterium isotope effect investigation at a molecular electronic-structure level, as shown in Fig. 1. As widespread materials in phOLED applications, these four representative complexes are cogent and valuable for achieving higher quantum efficiency through such an intelligible approach. With the development of theoretical systems and the computational industry, simulations of the photophysical processes of heavy metal complexes on the basis of progressive theory can be conducted precisely and extensively.^35–37 Under the integrated theoretical scheme, these four complexes, as well as their completely and partially deuterated equivalents, are comprehensively evaluated through k_r and k_nr calculations. The computational results show that the completely deuterated complexes have smaller k_nr values compared with the primitive complexes, and decomposed vibrational modes that correspond to large reorganization energies are picked out for further deuterium effect research. For partially deuterated complexes, in each ligand of the complexes, only the complexes with a deuterated ligand that is involved in excited electronic transitions can result in a smaller k_nr. Variations in the molecular normal modes caused by partial deuteriation are also probed by resolving the typical vibrational modes into internal coordinations.

Fig. 1 Chemical structures of compounds 1–4.

2 Theoretical background and computational details

The radiative decay rate is evaluated under the Einstein coefficient of spontaneous emission:³⁸where ε₀ is the permittivity of vacuum, ħ is the Planck constant, η represents the refractive index of the medium (η = 1.424 for dichloromethane), E(T₁) is the transition energy from T₁ to S₀, and μ^α_T represents the T^α₁ → S₀ transition dipole moment, where α indicates three spin sublevels of the triplet state (α = x, y, or z).

With consideration of the zero-field splitting (ZFS), the energy difference between the sublevels of the triplet state is taken into account for the averaged radiative decay rate, which can be expressed as:

where k_I, k_II, and k_III represent the individual decay rates of the triplet sublevels I, II, and III, as ordered by energy. ΔE_I,II and ΔE_I,III are the energy intervals between the substates, and k_B is the Boltzmann constant. At ambient temperature, the triplet radiative decay rate can be further simplified by an approximated equation:

The temperature-independent non-radiative decay rate k_nr is decided by the SOC integral and the energy gap, as well as by the degree of structural distortion between the T₁ and S₀ states, and can be calculated under the adoption of Fermi's Golden Rule with the Condon approximation. A detailed formula can be expressed as:³⁹

where ν_T and ν_S indicate the collective vibrational quantum numbers of the T₁ and S₀ states, respectively, and f(t) is the thermally averaged Frank–Condon factor. In this study, the convolution method^38,40 is employed for the evaluation of k_nr on the basis of eqn (4). This convolution approach is more suitable for temperature-dependent calculations of the non-radiative decay process, owing to additional considerations of the frequency shift factor in the low-frequency region in electron transitions from the excited states to the ground state, and thus more temperature-dependent factors are taken into account. Such a convolution method can be formulated as:where S_j is the Huang–Rhys factor of the j-th normal mode, ω^S/T_j is the frequency of the j-th normal mode of the S₀/T₁ state, E₀₀ is the zero point energy difference between the T₁ and S₀ states, and n_M is the number of vibrational quanta of ħω_M, with the value corrected to the smaller integer. The high-frequency (hf) region is in the range of 1000 cm⁻¹ to 1700 cm⁻¹, which correlates mainly to intraligand vibrations. The low-frequency (lf) region is less than 1000 cm⁻¹, which correlates mainly to metal–ligand stretching vibrations.⁴¹ Other items in the equation are referred to in ref. 38.

For geometry optimization, the hybrid functional PBE0^42,43 is employed throughout this work, and a basis set combined with Stuttgart Dresden ECP (SDD)⁴⁴ is chosen for the heavy atom Ir(III), whilst a 6-311G* basis set is adopted for light atoms. The ground state (S₀) geometry optimization is obtained with restricted density functional theory (RDFT), and the T₁ state is obtained with unrestricted density functional theory (UDFT). Moreover, vibrational frequency calculations at the same level of theory are conducted to affirm the nature of the stationary points of the geometry. The properties of the electronic transitions are simulated with the B3LYP^45,46 method, which has been applied to various Ir(III) complexes^47,48 by means of time-dependent DFT (TDDFT). The geometry optimization and electronic transition properties are obtained with the Gaussian 09 program,⁴⁹ along with a consideration of the solvent effect of dichloromethane (CH₂Cl₂) using the polarized continuum model (PCM).⁵⁰

The T₁ → S₀ transition dipole moment, which serves as a crucial factor in k_r, and the SOC integral between them, which is indispensable for both k_r and k_nr, are obtained using the Amsterdam Density Functional (ADF) 2016 program.^51,52 For these property calculations, the B3LYP functional with a Slater type all-electron triple-ζ basis with polarizations (TZP)^53,54 is employed based on the optimized geometries. With consideration of the computational cost and accuracy, one-component zeroth order regular approximation (ZORA)^55,56 TDDFT calculations are performed on the 10 lowest scalar relativistic singlet and triplet excitations, with perturbation involvement of the SOC.^57,58 Also, the solvent effect of CH₂Cl₂ is simulated by the conductor-like screening model (COSMO).⁵⁹ The reorganization energy and Huang–Rhys factors, which contain information about structural distortions, are calculated via a normal mode analysis approach using the DUSHIN program.⁶⁰

3 Results and discussion

3.1 Electronic structure properties

All four iridium complexes are optimized with the PBE0 method. The deuterated complexes share the same electron configurations as their corresponding protonated complexes, and the main geometry parameters both at their S₀ and T₁ states are listed in Table S1 (ESI†). For homoleptic 1 and 2, the ground state geometries are constrained in C₃ symmetry.⁶¹ The calculated bond lengths of Ir–N and Ir–C for complex 1 are 2.155 and 2.018 Å, respectively, which are close to the experimental values of 2.130 and 2.016 Å.⁶¹ Moreover, the mean error of the calculated geometry parameters for complex 4 is less than 1% when compared with the available experimental data. The precision of the calculated geometries confirms the credibility and reliability of our chosen method. To clarify the major transition characteristics, natural transition orbital (NTO) analysis is employed, as depicted in Fig. 2. Combining the geometry parameters with the NTO representations, the ligand involved in the charge transfer transition for both holes and electrons is the one that shortens the distance of the metal ion from the S₀ and T₁ states for each complex. The holes and electrons are mainly centered on the metal ion and its cyclometalating ligands for complexes 1–3, while for complex 4, the holes and electrons are mainly centered on the ancillary pic ligand, besides the metal ion.

Fig. 2 The hole and electron pairs of complexes 1–4 obtained by natural transition orbital analysis at their optimized excited state geometries.

The uniform electronic structures of the deuterated and protonated complexes are also suitable for the evaluation of k_r. Herein, the Einstein coefficient of spontaneous emission is applied in the assessment of the radiative decay process, as shown in eqn (1). The formula of k_r employed in this study, which expresses primarily the electronic behaviour, is insensitive to deuteriation, in that the results for the deuterated and primitive complexes are the same. The calculated and experimental results are listed in Table 1. It can be seen that the computational results are fairly identical to the experimental findings, indicating the accuracy of this theoretical scheme, and the k_r values for all four complexes have the same order of magnitude.

Table 1 Calculated radiative decay rates k_r (×10⁵ s⁻¹) and the available experimental values for complexes 1–4. The available experimental k_r value for completely deuterated complex 1 is listed in parentheses

Complex	k ^xr	k ^yr	k ^zr	k r,avr	k r,exp.
a From ref. 31. b From ref. 12. c From ref. 9. d From ref. 16.
1	1.40	9.66	1.22	4.09	6.40 (5.50)^a
2	4.56	3.67	0.99	3.07	2.70^b
3	1.88	6.89	2.72	3.83	2.13^c
4	2.03	9.41	0.50	3.98	3.90^d

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3.2 Non-radiative decay

The increase in the atomic mass via deuteriation undoubtedly enables the trigger of vibrational frequency alterations in both the ground and excited states, and such a variation can be remarkably reflected in the calculation of the non-radiative decay rate constants. As has been proven in several previous studies, spectra of heavy metal complexes can be concisely and specifically simulated by mode averaging approaches.^36,37,62 Therefore, a two-averaged modes simulation method is employed in this theoretical evaluation. Computational results show that the deuteriation of these iridium complexes is bound to exert some influence on the non-radiative decay rate, and changes can be expected according to the nature of the electronically excited states. By virtue of further comparative analysis on the relationship of normal mode frequencies and reorganization energies, the alterations of the vibrational modes through deuteriation are evaluated.

3.2.1 Complete deuteriation. Firstly, the non-radiative decay processes are assessed for all four complexes and their completely deuterated equivalents. It has been reported that complete deuteriation of some heavy metal complexes could prolong their phosphorescence decay time, which is also verified by our theoretical calculations, in which the non-radiative decay rates of the completely deuterated complexes are decreased. Several vital parameters contained in the non-radiative decay rate formula are listed in Table 2. The reorganization energies in the low-frequency region (LF) (denoted λ₁) of the completely deuterated complexes are larger than those of their primitive equivalents, and in the high-frequency region (HF), the completely deuterated complexes have smaller averaged Huang–Rhys factors (S_M). The trends in the final non-radiative decay rates, as shown in Table 2, mainly hinge on the HF part, and a smaller value of S_M leads to a slower non-radiative decay process, which demonstrates the involvement of C–H vibrational modes in these complexes primarily as the acceptor modes in the theoretical assessment of k_nr.

Table 2 Values of ΔE₀₀, 〈T₁|H_SOC|S₀〉, λ₁ (reorganization energy in the low-frequency region) and μ with the units of (cm⁻¹), ħ²D₁² and ħ²P² with the units of (×10⁵ cm⁻²), and λ_M (cm⁻¹, reorganization energy in the high-frequency region), combined with S_M, and k_nr (×10⁵ s⁻¹) calculated viaeqn (5), as well as their available experimental values, are listed for complexes 1–4

	ΔE00	〈T1|HSOC|S0〉	λ 1	μ	ħ ^2 D 1 ^2	ħ ^2 P ^2	λ M	S M	k nr(theo.)	k nr(exp.)
a From ref. 31. b From ref. 12. c From ref. 9. d From ref. 16.
1	20 088	347	489	1489	3.04	1.30	1823	1.34	0.24	0.80^a
Dall	20 142		582	1520	3.89	1.14	1733	1.25	0.11	0.50^a
DppyCT	20 144		612	1538	4.22	1.11	1713	1.22	0.08
Dppy1	20 086		490	1486	3.08	0.90	1832	1.34	0.26
Dppy2	20 087		488	1482	3.04	0.98	1831	1.34	0.26
2	21 548	329	455	1651	3.15	1.75	2210	1.57	0.90	3.60^b
Dall	21 542		555	1680	4.06	1.56	2109	1.47	0.53
DdfppyCT	21 579		560	1722	4.18	1.73	2109	1.47	0.52
Ddfppy1	21 539		458	1608	3.22	1.18	2208	1.57	0.86
Ddfppy2	21 537		458	1601	3.21	1.18	2208	1.57	0.86
3	19 490	505	487	1389	2.92	1.17	1485	1.07	0.34	4.13^c
Dall	19 534		570	1495	3.68	1.13	1406	0.99	0.18
DppyCT	19 544		566	1510	3.63	1.08	1409	1.00	0.19
Dppy1	19 478		489	1464	2.93	1.07	1483	1.07	0.36
Dacac	19 488		492	1493	2.97	1.22	1484	1.07	0.36
4	23 711	155	2396	1250	14.11	0.85	3099	2.22	1.26	0.40^d
Dall	23 746		2685	1438	20.66	6.27	3004	2.13	0.59
DpicCT	23 751		2671	1447	20.27	5.94	3007	2.13	0.60
Ddfpypy1	23 712		2487	1405	16.96	6.14	3099	2.22	1.04
Ddfpypy2	23 746		2466	1428	16.39	6.71	3096	2.22	1.04

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The relationships between the vibrational modes and reorganization energies of these complexes and their corresponding deuterated equivalents are depicted in Fig. 3. The LF part of complex 1 has generally the same reorganization energy distributions as its deuterated equivalent (D_all-1). There is a comparative suppression in the reorganization energy in the LF region of complex D_all-2, especially in the normal mode range of 700–800 cm⁻¹. As for complexes 3 and 4, there are merely minor discrepancies between the reorganization energies in the LF regions for their primitive and deuterated complexes, suggesting a negligible effect of complete deuteriation on the LF promoting modes.

Fig. 3 Calculated reorganization energies versus the normal mode frequencies for compounds 1–4, along with their completely deuterated equivalents. The vibrational mode corresponding to the largest reorganization energy is labeled.

As for the HF regions, relatively large distinctions exist between the primitive and completely deuterated compounds. In the primitive complex 1, vibrational mode 141 corresponds to the largest reorganization energy contribution, and C–H-involved vibrations form up to 52.24% of this normal mode, as discovered through statistical analysis by decomposing the modes into internal coordinations. The vibrational composition of C–D in D_all-1 is subdued to 34.67% in mode 139, which corresponds to the largest reorganization energy contribution. Although the largest reorganization energy in D_all-1 is greater than that in its primitive complex, the overall reorganization energy in the high-frequency part is smaller, indicating fewer structure distortions in the process of electronic transition. Moreover, mode 139 of D_all-1 is mainly composed of C–C ring-breathing vibrations, as shown in Fig. S1 (ESI†), while mode 141 of the primitive complex 1 is mainly made up of C–H rocking vibrations. Vibrational mode 146 points to the largest reorganization energy contribution in both complexes 2 and D_all-2, and the 42.27% C–H vibrational ratio in primitive 2 is suppressed to a 24.75% C–D vibrational ratio in D_all-2. The heteroleptic complex 3 also has a decreased C–D-involved vibration constituent (decreased from 46.12% to 34.81%) when completely deuterated, on account of the resolved normal modes contributing the most to the reorganization energy. There is a special case in complex 4, in which the nature of the electronically exited state is centered on the pic ligand, and the C–H vibrations are not obviously inhibited in D_all-4. However, the overall reorganization energy in the HF region has been inhibited visibly.

In short, complete deuteriation of these four iridium complexes is in favor of retarding their non-radiative decay processes, thereby enhancing the probability of higher quantum efficiency.

3.2.2 Partial deuteriation. To figure out the deuterating effects of every single ligand in these complexes, partial deuteriation is conducted. It is noteworthy that the ligand alone, which is dominant in the excited charge transfer, could prolong the non-radiative decay process, and the rate suppression to a similar extent, when compared with the completely deuterated complexes. Therefore, one can deduce the nature of the electronically excited states of these complexes only through a partial deuteriation approach.

The relationships between the normal modes and reorganization energies of three partially deuterated ligand complexes of each bidentate ligand, with the addition of the primitive complexes, are presented in Fig. 4. Obviously, the partially deuterated complex of the ligand involved in the excited charge transfer (ligand_CT) has a most distinctive relationship. The other two partially deuterated complexes exert no influence on the vibrational modes, in comparison with the primitive complexes. Similar to the completely deuterated complexes, the partially deuterated complex containing the ligand_CT has a comparative suppression effect on the reorganization energy in the HF region, namely a smaller λ_M, as shown in Fig. 5.

Fig. 4 Calculated reorganization energies versus the normal mode frequencies for compounds 1–4, along with their partially deuterated ligand equivalents.

Fig. 5 Calculated reorganization energies versus the normal mode frequencies for compounds 1–4, along with the partially deuterated complexes containing the ligands that are dominant in charge transfer. The vibrational mode that corresponds to the largest reorganization energy is labeled.

Vibrational mode 133 corresponds to the largest reorganization energy contribution of partially deuterated complex 1 on ppy_CT (D_ppyCT-1). The decomposition of this mode into internal coordinates reveals that the proportion of C–D-involved vibrations is 29.31%, which is even smaller than that in D_all-1 (34.67%). By further resolving mode 142, which corresponds to the second largest reorganization energy, it is revealed that the proportion of C–D-involved vibrations is 28.84%. It can be concluded that the partial deuteriation of the ligand_CT may result in a more remarkable effect on the reduction of k_nr. As for D_dfppyCT-2, mode 143, for which the reorganization energy is 535 cm⁻¹, is picked out. The proportion of its C–D-involved vibrations is 33.66%, whilst the second largest reorganization energy contribution is from mode 148, and the proportion of its C–D-involved vibrations is 29.03%. Both of these are smaller than the C–H-involved vibration contributions in primitive complex 2.

Heteroleptic iridium complex 3 has a little discrepancy in the relationship between its normal modes and reorganization energies. When deuterating its acac ligand, there is no obvious variation in the reorganization energies when compared with primitive 3, but it is undoubtable that measurable differences appear for partially deuterated 3 on its ppy_CT ligand. A smaller C–D-involved vibrational proportion of 31.78% is calculated for the largest reorganization energy contribution from mode 128 in D_ppyCT-3. However, the reorganization energies also show an increase in the HF region when deuterating the other ppy ligand, as can be seen in Fig. 4. Nonetheless, the vibrational mode corresponding to the largest reorganization energy has a relatively large ratio of C–D-involved vibrations (42.20%). The nature of the excited electronic transition in complex 4 is centered on the ancillary pic ligand, which results in a distinct suppression of the reorganization energies when deuterating the pic ligand. The vibrational compositions are similar to those of the completely deuterated complex.

Traditional perspectives have interpreted that a decreased k_nr can be explained by the increased energy gap between two charge-transfer related states, as formulated by the energy gap law (EGL).⁶³ However, the variations in k_nr for completely and partially deuterated complexes do not abide by the EGL absolutely. It is evident in Fig. 6 that the relationship between k_nr and ΔE₀₀ does not always conform to a negative correlation. For example, the k_nr values are largely decreased from complex 2 to D_all-2, while the ΔE₀₀ values show the same trends, which violates the rule of the EGL. Additionally, as shown in Fig. 6, complexes D_dfpypy1-4 and D_dfpypy2-4 have nearly the same k_nr values, but the ΔE₀₀ values of these two complexes have a large discrepancy. Hence, it is inadequate to determine the value of k_nr under the consideration of the EGL. Alternatively, the convolution method, which takes strong coupling limits as well as more temperature-dependent factors into consideration, could serve as an improvement to the EGL approach. As shown in Fig. 7, the trend in k_nr has an absolute positive correlation with λ_M, which represents the reorganization energy in the high-frequency region. Consequently, the calculation of k_nr employed in this study, which serves as an improvement to the EGL approach, enables the production of more valuable results, in that the deuterium isotope effect mainly gives rise to HF accepting modes.

Fig. 6 Plots of the calculated k_nr and ΔE₀₀ values for the primitive and deuterated complexes of 1–4.

Fig. 7 Plots of the calculated k_nr and λ_M values for the primitive and deuterated complexes of 1–4.

4 Conclusions

On the basis of an in-depth theoretical evaluation, deuterium isotope effects on several well-known Ir(III) complexes are explored. Traditional perspectives on the reduction of k_nr in deuterated complexes, namely the traditional EGL, cannot serve as the fundamental standard. With respect to radiative deactivation processes, the calculated k_r values for the protonated complexes conform to experimental results. With such credible k_r calculations, neither complete deuteriation or partial ligand deuteriation compounds are altered when compared with corresponding primitive complexes. Subsequently, a two-averaged modes theoretical approximation is employed to evaluate k_nr. The computational decreased k_nr (decreased almost by half) of the completely deuterated complexes is consistent with experimental results, and the suppression of k_nr primarily results from reduced C–D vibrations in the high-frequency mode. Partial deuteriation enables us to distinguish the ligand involved in the excited charge transfer, due to the fact that the C–D vibrations exert the same influence as the completely deuterated complex, while other ligand deuteriations cannot make any changes to the non-radiative decay processes. Therefore, through our comprehensive theoretical approach, the impact of the deuterium isotope effects on the non-radiative deactivation processes, as well as the nature of the excited state decay processes, can be precisely evaluated. Notably, the crucial deuteriation location for smaller values of k_nr can also be expected from our theoretical investigation. Moreover, such a convincing theoretical evaluation system provides more possibilities to improve the quantum efficiency of Ir(III) complexes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the State Key Development Program for Basic Research of China (Grant No. 2013CB834801), the Natural Science Foundation of China (Grant No. 21573088), the Young Scholar Training Program of Jilin University, and the Open Project Funding of Beijing National Laboratory for Molecular Sciences (BNLMS).

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Footnote

† Electronic supplementary information (ESI) available: A detailed description of k_r and k_nr, the geometry optimization results, and detailed calculations of the displacement vectors of the typical vibrational modes for all complexes. See DOI: 10.1039/c8qm00061a

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