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Theoretical Studies of Photodeactivation Pathways of NHCChelate Pt(II) Compounds with Different Numbers of Triarylboron Units: Radiative and Non-Radiative Decay Processes Fengying Zhang, Yanyan Xu, Wenting Zhang, Wei Shen, Ming Li, and Rongxing He J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b10754 • Publication Date (Web): 01 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017

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The Journal of Physical Chemistry

Theoretical

Studies

of

Photodeactivation

Pathways

of

NHC-Chelate Pt(II) Compounds with Different Numbers of Triarylboron Units: Radiative and Non-Radiative Decay Processes Fengying Zhang, Yanyan Xu, Wenting Zhang, Wei Shen, Ming Li, Rongxing He

∗

Key Laboratory of Luminescence and Real-Time Analytical chemistry (Southwest University), Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China

∗

To whom correspondence should be addressed. E-mail: [email protected]. 1

ACS Paragon Plus Environment


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ABSTRACT: The radiative and non-radiative decay processes of four platinum(II) complexes chelated with triarylboron (TAB)-functionalized N-heterocyclic carbenes (NHC) are investigated by using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculation, for probing into the influence of different numbers of TAB on the phosphorescent emission properties. For the radiative decay processes, zero-field splitting energies, radiative rates and lifetimes are explored, and corresponding factors including transition dipole moments, singlet-triplet splitting energies as well as spin-orbit coupling matrix elements are also analyzed in detail. Additionally, energy-gap law is considered in the temperature-independent non-radiative decay processes; meanwhile, potential energy profiles are obtained to elaborate the temperature-dependent non-radiative decay processes. As a result, radiative rates declined slightly with the increased numbers of TAB. The minimum temperature-independent non-radiative decay may occur in BC-3 due to its smallest structural distortion between S0 and T1 states. According to the potential energy profiles of the deactivation pathways, four investigated phosphors have the similar temperature-dependent non-radiative decay processes because of the incredibly analogous energy barriers. We speculate that it does not mean the greater phosphorescent emission and the higher phosphorescent quantum yield with more TAB units, which would provide extraordinary assistance for further research in potential phosphors of OLEDs.

2


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1. INTRODUCTION Extensive efforts have been devoted to the potential N-heterocyclic carbenes (NHC)-based transition metal complexes owing to their strong σ-bonding, readily tunable steric and extraordinary electronic properties, since the X-ray crystal structure of NHC was revealed by Arduengo in 1991.1 A great number of functional materials, including liquid crystals,2,3 antimicrobial agents,4,5 supramolecular structures6-8 as well as luminescent compounds,9,10 are constructed on the basis of this NHC structure. Currently, transition metal compounds containing NHC ligands are taken as one kind of promising blue phosphorescent material in organic light-emitting diodes (OLEDs),11-13 settling some defects including the low emission quantum yield and poor stability. The energy of the metal-centered d-d excited state can be improved with the help of the strong ligand field from carbene, contributing to the increase of energy gap in the emissive excited state and the improvement of phosphorescence quantum yield.10 On the other hand, phosphors can realize extended operational lifetime on account of the great stability of metal-carbene bonds. To date, phosphorescent Ir(III) and Pt(II) complexes containing NHC structures have be explored extensively in OLEDs.14-17 In addition to singlet excitons, triplet excitons can also be extracted to light through reverse intersystem crossing (RISC) from the lowest triplet excited state (T1) to the lowest singlet excited state (S1), realizing nearly 100% electron-generated excitons utilized for electroluminescence.18-21 Similarly, numerous studies have uncovered that it is an efficient way to promote the phosphorescence efficiency by introducing one triarylboron (TAB) unit,22-25 3



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whose vacant p orbitals on the boron atoms can cause attractive electron-accepting properties, facilitating to both electron transport and charge-transfer luminescence. As one kind of powerful π-electron acceptors and optoelectronic building units, phosphors constructed with the TAB group have be reported in Pt(II)-based OLED widely.22,24-27 With the purpose of integrating both superiorities including electron-transporting and strong ligand field embodied in the TAB group and NHC structure respectively, TAB-functionalized NHCs compounds are constructed and received considerable attention due to their properties of strong intramolecular charge transfer (ICT). Wang and coworkers22 synthesized and reported the first TAB-functionalized NHC chelate ligand as well as their Pt(II) complexes with blue and blue-green phosphorescent. Subsequently, Su et al.26 revealed the effect of several different functional ligands on the phosphorescence efficiency of emitters. In addition, electronic structures and optical

properties

of

Pt(II)

complexes

with

different

substituents,

triarylboron-functionalized and triarylnitrogen-functionalized, were investigated by Zhang and coworkers.28 There is no doubt that TAB-functionalized metal-NHCs complexes induce efficient high-energy blue phosphorescence, and the presence of TAB moiety greatly improves the phosphorescent quantum yield (ΦPL). In addition, the substitution position of the TAB group on the phenyl ring chelated with Pt(II) center proved valuable in phosphors according to the work of Wang and coworkers.22 Nevertheless, the optimal number of TAB unit is not mentioned and stated. Recently, Cheng et al.29 synthesized 4


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two kinds of promising blue emitting materials with multiple TAB units. Kaji and coworkers30 explored host materials with three amine-based electron-donating units, realizing external quantum efficiencies exceeding 20%. The question might be raised that what’s the relationship between the number of TAB unit and the phosphorescent quantum yield? Whether there is a higher phosphorescent quantum yield with the numbers of TAB units increasing? How does the triarylboron moiety affect the radiative decay process and non-radiative decay process in the case of increased numbers of TAB units? Up to now, considerable researches are focused on the different substituents, conjugation degree31-34 as well as substitution position,22 there is no research mentioning and interpreting these issues above, letting alone the in-depth explanation for their radiative and non-radiative decay processes. Thus, it is meaningful to make a comprehensive inquire about the influence of different numbers of TAB groups on radiative and non-radiative decay processes of the NHC-chelate Pt(II) compounds. Based on the reported Pt(II) complexes BC1 (also named BC-1) bearing one TAB group in the favorable position of the phenyl ring,22 four complexes with different numbers of TAB units are devised and investigated, their structures and corresponding names are displayed in Figure 1. Here, both radiative and non-radiative decay processes of target phosphorescent transition metal complexes are analyzed roundly more than the simplified energy gap law. Importantly, their thermally activated non-radiative photo-deactivation processes, emission state 3ES → transition state TS[3ES/3MC] → metal-centered excited state 3MC → minimum energy crossing point MECP, are illustrated detailedly. We hope this theoretical 5



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investigation could provide great assistance for deep research on phosphorescent transition metal complexes with higher efficiencies, contributing to the potential OLEDs.

Figure 1. Structures and corresponding names of four target phosphors.

2. METHODS 2.1. Theoretical background As the benchmark of OLEDs performance, the phosphorescence quantum yield ΦPL is closely correlated with two factors: radiative rate constants κr and non-radiative rate constants κnr, of which κnr contains the temperature-independent non-radiative decay rate constant κnr(1) and temperature-dependent non-radiative decay rate constant κnr(2). Their relationship can be expressed as equation (1): Φ PL =

κr

(1)

κ r + κ nr (1) + κ nr (2) 6


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The larger κr and smaller κnr, including κr(1) and κr(2), would contribute to the higher ΦPL. Both the radiative rate constant κir and corresponding radiative decay lifetime τir from triplet sublevels (i=1, 2, 3) caused by the emissive triplet manifolds to ground states can be calculated as following equation.

κir =

4α03 1 r i = κ S , T = ∆ES −T i 3 ∑ M ij ( ) 0 em em 0 τir 3t0 j∈{ x , y , z}

2

(2)

where t0 = ( 4πε 0 ) h3 / me e 4 , α 0 denotes the fine-structure constant, ∆ES 2

i 0 −Tem

is the

energy difference of transition from Tem to S0, M ij represents the dipole moment with spin-orbit coupled Tem→S0 transition, which can be obtained from the following equation: ∞

M =∑ i j

〈 S0 |µˆ j |S n 〉〈 S n |Hˆ SO |Temi 〉

n=0

E ( S n ) − E (Tem )

∞

+∑

m =1

〈 S0 |Hˆ SO |Tm 〉 〈Tm |µˆ j |Temi 〉 E (Tm ) − E ( S0 )

(3)

where Hˆ SO and µˆ j refer to the spin-orbital Hamiltonian and electric dipole, respectively. To the best of our knowledge, there are larger energy differences between S0 and Tm than that of Sn and T1. But for transition dipole moments, there are greater values between S0 and Sn than that of Tm and T1. Namely, M ij depends mainly on the former. Considering the thermal population distribution controlled by Boltzmann statistics of three sublevels, the total radiative rate constant can be obtained via the following expression:

κr =

κ1r + κ2r exp ( − ZFS1,2 / κ BT ) + κ3r exp ( − ZFS1,3 / κ BT ) 1 + exp ( − ZFS1,2 / κ BT ) + exp ( − ZFS1,3 / κ BT )

(4)

where κB is the Bolzman constant, the zero-point splitting energy ZFS is defined as 7



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the energy differences between each of the three sublevels (1, 2, 3). For phosphorescence emitters, ZFS1,2 and ZFS1,3 are typically less than 200 cm-1.35,36 If kBT ≫ ZFS1,3, the total radiative rate constant can be simplified as follows:

κr =

1 3 r ∑κi 3 i =1

(5)

For the non-radiative decay rate constant κnr, it can be estimated roughly according to the energy-gap law:

{

κ nr (Tm → S 0 ) ∝ exp − β  E (Tm ) − E ( S 0 ) 

}

(6)

where β and E (Tm ) − E ( S0 ) correspond to the geometrical distortion and energy gap between S0 and Tm states, respectively. Generally, the greater geometrical deformation corresponding to the larger β would lead to the increase of electron-vibrational coupling constant between T1 and S0 states, so as to facilitating the nonradiative decay. Here, the structural deformation is estimated by Huang-Rhys factors as following: 1 S j = ω j K j2 2

(7)

where Sj, Kj represent the Hung-Rhys factor and the shift vector, respectively.

2.2. Computational details Optimized geometries of four investigated Pt(II) complexes in their ground-states and triplet-states are obtained based on the restricted and unrestricted density functional theory (RDFT and UDFT). Four representative functionals (B3LYP,37 CAM-B3LYP,38 PBE039 and M062X40) are performed to obtain the appropriate geometries parameters and energy levels being accordance with reported experiments. It’s not difficult to find from Table S1 (ESI†) that the more accurate molecular orbital energy is gained by B3LYP although its geometry parameters slightly inferior to that of PBE0. 8


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Considering comprehensively, the hybrid functional B3LYP is used for the optimization of all Pt(II) complexes including their ground states and lowest triplet excited states, and 6-31G(d) basis set41 is employed for C, H, O, N, P atoms, basis set LanL2DZ42 with corresponding pseudopotential is employed for heavy atom Pt. Similarly, absorption and emission properties of investigated complexes are also explored on the basis of optimized geometries at S0 and T1 states using the time-dependent density functional theory (TD-DFT). B3LYP and M062X are selected as the most appropriate functionals for the simulation of absorption and emission respectively, and results are collected in Tables S2 and S3 (ESI†). Additionally, during the investigation of temperature-dependent non-radiative decay processes, several key points in the potential energy surface curves, including metal centered d-d excited states 3MC, transition states TS between T1 and 3MC states as well as the minimal energy crossing points MECP, are computed at B3LYP/6-31G(d) level. Among them, 3MC states are obtained in terms of Persson’s work,43,44 and sobMECP program, the modified version of Harvey’s MECP,45,46 is used for the calculation of MECPs. Furthermore, intrinsic reaction coordinates (IRC) are also executed to illustrate the reliability of TS. Throughout the calculation, the polarized continuum model (PCM)47,48 in CH2Cl2 is adopted, and frequency calculations are implemented for every stationary point for ensuring all calculated structures are reliability. All the calculations above are realized in the Gaussian 09 program package.49 Moreover, based on optimized T1 geometries, the zero-point spiriting (ZFS) parameters and radiative decay rate constants (κr) are obtained in the ADF2014.04 9



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software50,51 using TD-DFT/PBE0 including SOC calculations together with zero order regular approximation (ZORA)52,53 Hamiltonian. An all-electron TZP basis set is employed for Pt atom; correspondingly, the all-electron DZP basis set is used for the other non-metal atoms.

3. RESULTS AND DISCUSSION 3.1 Geometries of phosphors at their S0 and T1 states As is widely known that structures of complexes play important roles in their photophysical properties, it is necessary to discuss their geometries in detail, especially for them at their S0 and T1 states. Based on the proper functional B3LYP, the main geometric parameters corresponding to optimized geometries are listed in

Table 1, matching atomic labels are plotted in Figure S1 (ESI†). Besides, other important bond lengths and bond angles surrounding at the boron (B) center are presented in Table S4 (ESI†). Table 1. Main structure parameters of complexes BC-1, BC-2, BC-3 and BC-4 at S0 and T1 states, respectively, including bond lengths (Å), bond angles and dihedral angle (°). All calculations are obtained at B3LYP/6-31G(d) level in CH2Cl2. BC-1

BC-2

BC-3

BC-4

S0

T1

S0

T1

S0

T1

S0

T1

Pt1-C1

1.969

1.972

1.969

1.972

1.970

1.971

1.970

1.972

Pt1-C2

2.008

1.960

2.007

1.959

2.008

1.962

2.009

1.962

Pt1-O1

2.156

2.149

2.159

2.142

2.157

2.145

2.158

2.138

Pt1-O2

2.099

2.106

2.100

2.105

2.096

2.107

2.096

2.102

C1-Pt1-O2

172.8

173.5

172.7

173.6

172.8

173.6

173.1

174.0

C2-Pt1-O1

178.8

179.0

178.6

178.5

178.6

178.7

178.4

178.3

10


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N1-C1-Pt1-O1

0.320

-0.570

1.044

-0.827

-0.202

0.805

0.742

0.743

From Table 1, there are negligible changes in the main bond lengths and bond angles connecting to Pt atom for four phosphors. However, for the dihedral angle N1-C1-Pt1-O1, BC-4 exhibits the smallest changes (0.001) between S0 and T1 states when comparing with BC-1 (0.890), BC-2 (1.871) and BC-3 (1.007). Exactly as mentioned in theoretical background, the geometrical distortion between S0 and T1 states is closely related with the non-radiative rate constant, especially for the temperature-independent

non-radiative

rate

constant.

The

larger

molecular

deformation contributes to the faster non-radiation transition process. Thus, the calculation of Huang−Rhys factor (Smax) is implemented to evaluate the degree of structural deformation between S0 and T1 states, and detailed discussion is presented in the following analyses of temperature-independent non-radiative decay process.

3.2 Electronic properties of phosphors at their S0 and T1 states To probe into the luminescence properties of four complexes, their electronic properties at the S0 and T1 states are investigated and discussed precisely. For S0 states, frontier molecular orbitals (FMOs) of all phosphors, including the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), are displayed in Figure 2. It is easy to obtain from the energy gap (Eg) defined as the difference between HOMO and LUMO that there are the smaller Eg with the increasing numbers of TAB group, only small discrepancy (0.03 eV) exists between BC-2 and BC-3. For the electron-withdrawing TAB unit, it brings enhanced electrophilicity with increased numbers, leading to the localized densities of 11



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molecular orbitals so as to reduce the band gap. What’s more, there are the similar FMOs distribution for four target compounds, HOMOs distribute on the general ∧

∧

formula [Pt(C C)(O O)], as for LUMOs, they are delocalized over the TAB groups and their surroundings. As a result, charges would be transferred from NCHs to TABs groups.

Figure 2. Schematic energy levels and orbital distributions of studied complexes at their S0 optimized geometries. All calculations are obtained at the B3LYP/6-31G(d) level in CH2Cl2.

Spin densities of four complexes in their T1 states are presented in Figure 3. There is no significant difference for the distribution of spin densities, and they are mainly ∧

distributed on the cyclometalated Pt(C C) ligand and adjacent B atom. These similar spin distributions of four compounds imply they have prominent 3MLCT character, meaning the number of TAB has no significant effect on the distribution of spin densities.

12


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Figure 3. Spin densities computed on optimized T1 geometries for investigated complexes (isovalue = 0.002).

From another perspective, transition characters of four phosphors are discussed through their natural transition orbitals (NTOs) on optimized T1 states. As depicted in

Figure 4, the unoccupied NTOs represent the “electron” transition orbitals, while the “hole” transition orbitals are denoted by the occupied NTOs. The homologous distributions of “electron” and “hole” delocalized on the metal Pt and adjacent ligand for the studied compounds reveal their transitions of T1 emission are mainly assigned to the intra-ligand charge transfer (ILCT) and metal-to-ligand charge transfer (MLCT). The number of TAB has little influence on the transition nature of T1 states.

13



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Figure 4. The NTO plots of four complexes at their lowest-lying triplet excited states (isovalue=0.02).

3.3 Absorption and emission For simulating absorption properties of phosphors appropriately, a series of functionals are tested and investigated, and B3LYP is selected as the best functional to reproduce experimental data of absorption. Calculated maximum absorption wavelengths (λmax) of objective compounds are 372, 388, 385 and 398 nm for BC-1 to BC-4, respectively, their simulated absorption curves are displayed in Figure 5. Furthermore, excitation energies (E), oscillator strength (f) and transition character corresponding to main spectral absorption are collected in Table S5 (ESI†). Overall, there are some slight red-shifts of absorption spectra following increased TAB groups, which mainly attributed to the gradually decreased Eg caused by faster reduced LUMO than HOMO. In addition, benefitted from the increased overlap integral between the beginning state and final state of electron transition, absorption intensities are significantly strengthened with the TAB unit increasing gradually, making the process of intersystem crossing easily accessible.

14


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Figure 5. Simulated absorption spectra of four complexes obtained at the B3LYP/6-31G(d) level in CH2Cl2 with their optimized S0 geometries.

As for the emission properties, the lowest-lying triplet energies of all phosphors are calculated by B3LYP, PBE0, CAM-B3LYP and M062X as listed in ESI†. It is not difficult to observe that results from M062X match better with the experimental value, and they are 490, 491, 489 and 490 nm for BC-1, BC-2, BC-3 and BC-4, respectively. These small differences uncover that it is improbable to tune the emission wavelength via appending TAB groups into phosphors.

3.4 Properties of phosphorescence As mentioned in theoretical background, both the radiative decay rate constants and non-radiative decay rate constants are key factors closely correlated with the phosphorescence quantum yield. With the attempt to improve ΦPL, it is indispensable to investigate κr and κnr for increasing κr as well as reducing κnr. In detail, the complicated

non-radiative

decay

process,

including

temperature-independent

non-radiative decay process and temperature-dependent non-radiative decay process are discussed respectively.

15



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3.4.1 Radiative decay process For the sake of discerning the radiative decay process, SOC-TD-DFT calculations with the ZORA Hamiltonian are conducted to explore ZFS parameters, radiative rate constants and phosphorescent lifetimes of four organo-metallic complexes. It is powerful evidence from Table 2 that these calculations are reliable, they agree well with corresponding experiments. Table 2. ZFS parameters (cm-1), radiative rate constants κr (s-1) and radiative decay lifetimes τ (s) for all studied complexes together with the available experimental values.

a

∆E1-2(ZFS)

∆E1-3(ZFS)

κav r

κr (Exp.) a

τav r

τp (Exp.) a

BC-1

2.10

5.40

1.37×105

1.26×105

7.32×10-6

6.90×10-6

BC-2

3.31

11.86

8.53×104

-

1.17×10-5

-

BC-3

3.63

11.05

7.42×104

-

1.35×10-5

-

BC-4

5.48

9.84

9.72×104

-

1.03×10-5

-

The available experimental values from Ref. 22.

Zero-point splitting energy ZFS, one of the vital parameters for assessing the magnitude of κr, can reflect the suitability of phosphorescent emitters well in OLEDs. Generally, the spin-orbit coupling would induce the triplet state splits into three sub-levels; meanwhile, it also leads to the spin-forbidden transition through the homogeneous mixing of singlet states and triplet states. In this view, the larger ZFS is more feasible for the spin-forbidden transition from Tm to S0, which is more beneficial to the phosphorescence emission. As results in Table 2 revealed, there is no significant difference in ZFS for four phosphors though the value of BC-2, BC-3 and BC-4 larger slightly than that of BC-1 regardless of ∆E1-2 and ∆E1-3. Importantly, BC-1 owns the largest κr (1.26×105 s-1) and the shortest τ (6.90×10-6 s). As for BC-2, 16


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BC-3 and BC-4, their κr are 8.53×104 s-1, 7.42×104 s-1 and 9.72×104 s-1, corresponding to lifetimes 1.17×10-5 s, 1.35×10-5 s and 1.03×10-5 s, respectively. It seems that κr is inconsistent with ZFS, that’s because ZFS just is a rough estimation for phosphorescence emission. In consequence, it is not a wise choice to improve radiative rate by introducing more TAB units, which usually fades the κr to some extent. With the aim of discovering what are the main factors affecting the raidiative decay rate constant and explaining why occur these small discrepancies, three parameters including transition dipole moments µ(Sn), singlet-triplet splitting energies ∆E(Sn–T1) and SOC matrix elements 〈T1| HSOC| Sn〉 are computed and listed in Table 3. According to equations (2) and (3), it would be favorable to the radiative decay process with larger µ(Sn) and 〈T1| HSOC| Sn〉, but smaller ∆E(Sn–T1). From the statistical results, phosphors BC-1, BC-2 and BC-3 show the similar µ(Sn) and ∆E(Sn–T1) , it is the discrepancy in SOC matrix elements that makes the small differences of κr exist between them. Furthermore, compound BC-1 exhibits greater SOC matrix elements than that of phosphors with two TAB groups (BC-2 and BC-3) in S4, S5 and S6 states obviously, but the abnormal 〈T1| HSOC| Sn〉 in S8 narrows their gaps. Although 〈T1| HSOC| Sn〉 reaches 708.47 cm-1, it can be explain reasonably according to equation (3): SOC matrix element just the one of the factors influencing the radiation rate constant, µ(Sn) and ∆E(Sn-T1) are also important indicators. Here, transition dipole moments µ(S6) is only 0.94, and ∆E(S6-T1) is 3.71, so the effect of

〈T1| HSOC| S6〉 on the radiation rate constant is small. In addition, with regard to 17



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BC-4, it has smaller SOC matrix elements than that of BC-1, BC-2 and BC-3 overall, which is adverse to its radiative decay process. However, on the other hand, BC-4 displays smaller ∆E(Sn–T1) from S1 to S8 in the statistics, contributing to its phosphorescence emission and narrowing its gap in κr compared with BC-1, BC-2 and BC-3. In summary, it is the SOC matrix element that is taken as the key factor for determining the radiative decay process and radiative decay rate constant. Table 3. Transition dipole moments µ(Sn) (Debye) for S0–Sn transitions, singlet–triplet splitting energies ∆E(Sn–T1) (eV) and the SOC matrix elements〈T1| HSOC| Sn 〉(cm-1) of objective complexes at their T1 optimized geometries. BC-1

BC-2

Sn

µ(Sn)

∆E(Sn-T1)

〈T1|HSOC|Sn〉

µ(Sn)

∆E(Sn-T1)

〈T1|HSOC|Sn〉

S1

2.56

0.67

46.76

2.53

0.59

34.88

S2

1.92

3.29

278.44

1.81

3.18

271.27

S3

2.01

3.36

125.64

2.30

3.26

135.14

S4

1.10

3.50

102.71

1.71

3.38

22.43

S5

1.48

3.58

311.77

1.73

3.39

56.33

S6

0.94

3.61

708.47

1.37

3.42

48.57

S7

0.76

3.71

38.03

1.88

3.46

68.65

S8

1.30

3.79

44.89

0.48

3.52

331.76

BC-3

BC-4

Sn

µ(Sn)

∆E(Sn-T1)

〈T1|HSOC|Sn〉

µ(Sn)

∆E(Sn-T1)

〈T1|HSOC|Sn〉

S1

2.73

0.60

33.31

2.42

0.54

24.92

S2

1.95

3.21

285.34

0.80

0.92

283.29

S3

2.11

3.33

82.73

1.75

1.09

43.23

S4

0.71

3.40

26.95

0.29

1.13

12.54

S5

1.33

3.42

31.52

0.69

1.14

23.61

S6

1.36

3.43

74.32

1.31

1.16

10.45

18


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S7

1.88

3.48

98.70

1.20

1.17

3.11

S8

0.65

3.56

743.36

1.62

1.21

80.54

3.4.2 Temperature-independent non-radiative decay process Based on the energy-gap law as equation (6) mentioned in the theoretical background, two major factors, the structural distortion parameter β and the energy gap

E (Tm ) − E ( S0 ) between the ground state and emission triplet state, are closely related to the non-radiative decay rate constants κnr. For the structural distortion parameter β, it can be signified by the parameters of Huang-Rhys factor Smax, which are 15.10, 24.20, 6.11 and 28.00 for BC-1, BC-2, BC-3 and BC-4, respectively. The minimum Smax is obtained in complex BC-3. On the other hand, the root mean square deviation (RMSD), an important parameter which evaluates the structural distortion on the whole, is also implemented in VMD 1.954 for excluding the inaccuracy of large Smax. As schematic diagrams of RMSD collected in Figure S2 (ESI†), they are 0.322, 0.342, 0.181, 0.332 for BC-1, BC-2, BC-3 and BC-4, respectively. It is easily found from RMSD results that they are consistent with Smax, and BC-3 manifests the minimal distortion and the smallest non-radiative decay. Besides geometrical distortion, it is also important for phosphors to investigate their energy gaps between S0 and Tm states for predicting the temperature-independent non-radiative decay rate constant. Intuitively, the energy gaps of four phosphors between their S0 and T1 states are computed and summarized, results are as the order of BC-1 (2.58 eV) > BC-3 (2.56 eV) > BC-2 (2.53 eV) > BC-4 (2.51 eV). To a great extent, these minor differences indicate the influence of energy gap for 19



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temperature-independent non-radiative rate constant can be neglected in our exploration. Namely, phosphor BC-3 may have the smallest temperature-independent non-radiative constant and the slowest temperature-independent non-radiation transition process due to its smallest deformation in structure, contributing to the improvement of phosphorescence quantum yield ΦPL.

3.4.3 Temperature-dependent non-radiative decay process In general, the strong temperature-dependent emissive behavior exists in many luminescent transition metal complexes, further exploration for these thermally activated radiationless processes helps to comprehend adequately these pathways from Tm returning to S0 without emission occurring. During the reported photo-deactivation process, the triplet metal-centered d–d excited states 3MC play a crucial role in the luminescence behavior among the thermal population of higher lying non-emissive excited states. For the sake of obtaining the geometries of 3MC states, phosphors are optimized via elongating the metal-ligand bonds and distorting the optimized S0 geometries. As illustrated in

Figure 6, the most obvious characteristic of 3MC states is that their spin densities are localized primarily on the Pt atom and its surroundings. Besides, transition states of all complexes between 3ES to 3MC are calculated and their vibration forms are depicted in Figures S3-6 (ESI†). Sole imaginary mode of -203.77, -207.84, -247.66 and -206.95 occur in phosphors BC-1, BC-2, BC-3 and BC-4, respectively. All these TS states are confirmed by the intrinsic reaction coordinate (IRC) calculations as shown in Figure S7 (ESI†). Once reaching the 3MC state, it is more likely to refer to 20


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the minimal energy crossing point, especially for the similar energies in 3MC state and MECP, which would induce a fast radiationless process to reach the ground state. In order to verify their reaction paths, the minimal energy crossing points of four phosphors are calculated, and their structures at the MECP including S0 and 3MC states are depicted in Figures S8-11 (ESI†). Geometries of S0 and 3MC at MECP imply their similar structural parameters and energies, meaning these calculation results are reliable. Additionally, MECP energies for all investigated phosphors are 67.8, 67.6, 67.5 and 67.5 kcal/mol, respectively, only the tiny differences are embodied among them.

Figure 6. Spin densities computed on optimized 3MC geometries for investigated complexes (isovalue = 0.002).

To discern the deactivation pathway thoroughly, four potential energy profiles are depicted 1

and

listed

in

7,

Figure

following

the

circulation

of

GS→3ES→TS[3ES-3MC]→3MC→MECP[3MC/1GS]→1GS. In the circulation, there

are two energy barriers needed to be surmounted. The higher energy barrier is 21



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unbeneficial for the temperature-dependent non-radiative decay process, which contributes to the luminescence behavior. The most prominent energy barrier is the transition state between the 3ES and 3MC. It is not difficult to find that there are the similar energy barriers with the numbers of TBA groups increasing, they are BC-1(10.8 kcal/mol), BC-2(11.2 kcal/mol), BC-3(11.0 kcal/mol) and BC-4(11.4 kcal/mol). These minor energy differences are insufficient to distinguish their abilities attaining the transition states. In other words, four phosphors have the almost equal ability to surmount the TS states. Once it is thermally accessible to the 3MC state, the other energy barrier MECP is reached. Nevertheless, according to the results calculated, the energy barriers of four compounds are not only small but almost identical, corresponding to BC-1 (3.5 kcal/mol), BC-3(3.3 kcal/mol), BC-2(3.5 kcal/mol) and BC-4(3.5 kcal/mol), respectively. That is to say there is little impact of MECP energies on the temperature-dependent nonradiative decay processes for investigated phosphors.

22


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Figure 7. Schematic potential energy profiles of the deactivation pathway for four studied complexes.

Consequently, there is no significant effect on temperature-dependent non-radiative decay processes with the increased numbers of TAB units, four phosphors we investigated go with the analogical deactivation pathways.

4. CONCLUSIONS In summary, the influence of different numbers of triarylboron groups on NHC-chelate Pt(II) compounds is explored roundly, including both radiative and non-radiative decay processes. Some significant conclusions are summarized as follows based on above detailed analyses: (1) According to the simulated absorption spectra, there are slightly wavelength bathochromic-shifts and enhanced oscillator strength from BC-1 to BC-4. Nevertheless, the number of TAB has little effect on the emission characteristic in terms of emission spectra. (2) The radiative rate constant of BC-1 is larger than the other three phosphors, implying there is no inevitable connection between the number of TAB and κr. It is not a wise choice to improve radiative rate by introducing more TAB units, which usually fading the κr to some extent. (3) From the analyses of influence factors for the radiative rate constant, the larger κr of BC-4 than that of BC-3 and BC-2 is ascribed to its smaller singlet-triplet splitting energies. (4) Based on the energy-gap law, phosphor BC-3 probably has the minimum temperature-independent non-radiative transition constant among four complexes due to the smallest structural distortion. 23



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Page 24 of 29

(5) The tiny difference embodied on four complexes in potential energy profiles of the deactivation pathway indicates that all phosphors go with the analogical deactivation pathways, and the number of TAB units has little effect on temperature-dependent non-radiative decay process. Comprehensive consideration, phosphors with one TAB group can induce efficient high-energy blue phosphorescence, while, the increased TAB moiety may not bring remarkable promotion in emission of phosphorescence.

ASSOCIATED CONTENT Supporting Information Selected bond lengths, bond angles, HOMO, λmax and ET1 of all studied complexes calculated by different functionals. Schematic diagrams of RMSD results. The vibration form of 3TS[3ES/3MC] and corresponding intrinsic reaction coordinate for four investigated phosphors. Structures of S0 and 3MC at MECP for BC-2, BC-2, BC-3 and BC-4, respectively. (PDF)

ACKNOWLEDGMENTS We acknowledge the generous financial support from Natural Science Foundation of China

(21173169),

Chongqing

Municipal

Natural

Science

Foundation

(cstc2013jcyjA90015), Fundamental Research Funds for the Central Universities (XDJK2016E060) and Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011).

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Theoretical Studies of Photodeactivation Pathways ... - ACS Publications

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