Yanyan Xu

Exploring the Photodeactivation Pathways of Pt[O^N^C^N] Complexes: A Theoretical Perspective

In this article, the influence of the tert-butyl unit on the photodeactivation pathways of Pt[O^N^C^N] (O^N^C^N=2-(4-(3,5-di-tert-butylphenyl)-6-(3-(pyridin-2-l)phenyl) pyridin-2-yl)phenolate) is investigated by DFT/TDDFT calculations. To further explore the factors that determine the radiative processes, the transition dipole moments of the singlet excited states, spin-orbit coupling (SOC) matrix elements, and energy gaps between the lowest triplet excited states and singlet excited states are calculated. As demonstrated by the results, compared with Pt-3, Pt-1 and Pt-2 have larger SOC matrix elements between the lowest triplet excited states and singlet excited states, an indicator that they have faster radiative decay processes. In addition, the SOC matrix elements between the lowest triplet excited states and ground states are also computed to elucidate the temperature-independent non-radiative decay processes. Moreover, the temperature-dependent non-radiative decay mechanisms are also explored via the potential energy profiles.

Theoretical Insights into the Photo-Deactivation of Emitting Triplet Excited State of (C^N)Pt(O^O) Complexes: Radiative and Nonradiative Decay Processes

In this study, density functional theory (DFT) and time-dependent DFT were employed to elucidate the photo-deactivation mechanisms of (C^N)Pt(O^O) complexes 1-4 (where C^N = 2-phenylpyridine derivatives, O^O = dipivolylmethanoate). To make thorough understanding of the radiative decay, the singlet-triplet splitting energies ΔE(Sn-T1) (n = 1, 2, 3, 4, ...), transition dipole moment μ(Sn) for S0-Sn transitions and the spin-orbit coupling (SOC) matrix elements ⟨T1|HSOC|Sn⟩ were all calculated. Moreover, the spin-orbit coupling between T1 and S0 ⟨T1|HSOC|S0⟩ and Huang-Rhys factors were calculated to estimate the temperature-independent nonradiative decay processes. Meanwhile, the thermal deactivation via metal-centered (3)MC was described to analyze the temperature-dependent nonradiative decay processes. As a result, the effective SOC interaction between the lowest triplet and singlet excited states successfully rationalize why complexes 1 and 3 have higher radiative decay rate constant than that of complex 2, while the larger ⟨T1|HSOC|S0⟩ and lower energy barrier for thermal deactivation in 3 reasonably explains why 3 has larger nonradiative rate than that of 1 and 2. Consequently, it can be concluded that it is the ⟨T1|HSOC|S0⟩ and thermal population of (3)MC that account for the nonemissive behavior of (C^N)Pt(O^O) complexes, and controlling π-conjugation is an efficient method for tuning phosphorescence properties of transition-metal complexes.

Theoretical study and design of cyclometalated platinum complexes bearing innovatively a highly-rigid terdentate ligand with carboranyl as a chelating unit

Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were employed to explore the electronic structures and phosphorescence properties of synthesized terdentate Pt(II) complexes bearing highly-rigid 3,6-bis(p-anizolyl)-2-carboranyl-pyridine as a cyclometalated ligand and triphenylphosphine (1) or t-butylisonitrile (2) as ancillary ligand. To understand the marked difference in phosphorescence quantum efficiency between 1 and 2, the relaxation dynamics of excited states were elucidated in detail. Aiming to formulate the radiative relaxation, the zero-field splitting (ZFS) and the radiative decay rate constant (kr) were calculated by SOC-perturbed TDDFT (pSOC-TDDFT). Meanwhile, the temperature-independent non-radiative relaxation was analyzed by calculating the Huang–Rhys factor (S), the SOC interaction between the emitting state and the ground state. While the temperature-dependent non-radiative decay mechanism was studied by depicting the thermal deactivation process via a metal-centered excited 3MC state. Based on the results, 1 and 2 show a few differences in their temperature-independent non-radiative rates. However, the activation barrier for the population of non-emissive 3MC is greatly raised for complex 2. Therefore, the temperature-dependent non-radiative decay behavior of 2 is considerably suppressed, which ultimately leads to a substantially enhanced phosphorescence quantum efficiency for 2. To further tune the emission wavelength towards blue, four new complexes 3–6 were theoretically designed by modifying the terdentate ligand with azole groups based on the parent complex 2. As a result, pyrazole modified complex 4 stands out with enhanced deep-blue phosphorescence located at 434 nm.

Theoretical Investigation and Design of Highly Efficient Blue Phosphorescent Iridium(III) Complexes Bearing Fluorinated Aromatic Sulfonyl Groups

Aromatic sulfonyl groups have attracted increasing interest due to their unique electronic features. In this article, a series of IrIII complexes bearing fluorinated phenylsulfonyl groups were evaluated by density functional theory and time-dependent density functional theory methods. To explore their phosphorescence efficiencies, factors that determine the radiative decay rate constant, kr , and the nonradiative decay rate constant, knr , were computed. As demonstrated by the results, complex 4, which has fluorinated phenylsulfonyl groups at the 5-positions of the phenyl rings for all three C^N ligands, was found to have the highest phosphorescence efficiencies with the largest kr and smallest knr values among these complexes. Moreover, it was found to exhibit significantly blueshifted behavior relative to complex 1 and emits in the blue region, and thus, it can serve as a highly efficient blue emitter for application in organic light-emitting diodes. These findings successfully illustrated the structure-properties relationship and provided valuable information for the development of future highly efficient blue-emitting phosphors.

DFT study on the Au(I)-catalyzed cyclization of indole-allenoate: counterion and solvent effects

A computational study using the B3LYP density functional was carried out to explore the effects of counterions and solvents on the Au(I)-catalyzed cyclization reaction of indole-allenoate to form dihydrocyclopenta[b]indole derivatives. The optimal reaction path includes intramolecular cyclization and proton transfer steps. In the first process, the counterions Cl−, BF4− and OTf− act as hydrogen-bond acceptors to promote the intramolecular cyclization between the C1 and C5 atoms. In the proton transfer step, the anions greatly reduce the energy barrier of proton migration, in the form of a proton-transfer shuttle. More importantly, the Bronsted/Lewis basicity of the counterions (Cl− > OTf− > BF4−) turns out to be the primary reason for the difference in the counterion catalytic activity in the proton-transfer process. During the protonation of the counterion, the catalytic capacities of the counterions show significant differences according to the series Cl− > OTf− > BF4−, and the order of the catalytic ability of the counterions was found to be Cl− < OTf− < BF4− in the deprotonation of the counterion-H. Interestingly, the strong coordinating capability of the solvents (DMF and DMSO vs. PhCH3) was found to be another important factor that critically affects the reaction yield (0%, 0% and 95% yield, respectively). Overall, our calculations not only explain the experimental phenomena well, but also put forward some guidance and advice for the selection of counterions and solvents for transition metal-catalyzed reactions, including proton-transfer processes.

Theoretical Studies of Photodeactivation Pathways of NHC–Chelate Pt(II) Compounds with Different Numbers of Triarylboron Units: Radiative and Nonradiative Decay Processes

The radiative and nonradiative 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 nonradiative decay processes; meanwhile, potential energy profiles are obtained to elaborate the temperature-dependent nonradiative decay processes. As a result, radiative rates declined slightly with the increased numbers of TAB. The minimum temperature-independent nonradiative 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 nonradiative decay processes because of the incredibly analogous energy barriers. We speculate that it does not mean greater phosphorescent emission and higher phosphorescent quantum yield with more TAB units, which would provide extraordinary assistance for further research in potential phosphors of organic light-emitting diodes.