Wei Shen

•Explored the root cause of the PQE difference between two Ir complexes.•Non-radiative dynamics was used to explain energy dissipation.•Several new complexes were designed.The photo-deactivation mechanism of heteroleptic Ir(III)(C^N)2 (LX) complexes [(dfpypy)2Ir(pic)](1) and [(dfppy)2Ir(pic)](2) [where dfpypy = 2′,6′-difluoro-2,3′-bipyridine, dfppy = 2-(2,4-difluorophenyl)pyridine, pic = 2-picolinate] were elucidated by density functional theory/time-dependent density functional theory (DFT/TD-DFT) to rationalize the nature of the significant differences in luminescence efficiency between 1 and 2. Specifically, the radiative deactivation was formulated through the calculation of zero-field splitting (ZFS) and the radiative decay rate constant (kr) based on spin-orbit coupling (SOC). Meanwhile, the non-radiative deactivation was estimated by considering the structural distortion, the SOC between the emitting state and the ground state, as well as the thermal deactivation process via metal-centered excited 3MC state. Based on the results, 1 has much smaller SOC matrix elements between T1 and S0 than 2, which determines its small non-radiative decay rate constant, thus one may understand why 1 has higher phosphorescence quantum efficiency than 2. To further explore the structure-property relationship of Ir(III) complexes, four other new complexes 3–6 were designed by incorporating trimethylphenyl(R1), phenyl(R2), ter-butyl (R3), diphenylamine(R4) to the pyridine rings of dfpypy ligand of 1, respectively. Through analyzing, complex 4 with larger radiative decay rate and smaller non-radiative decay rate may be considered as a potential candidate as robust blue-emitting material applied in organic light emitting diodes (OLEDs).Excited state intersystem crossing and the relaxation dynamics of phosphorescent Ir(III) complexes bearing bipyridine-based C^N ligand.Download high-res image (120KB)Download full-size image

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

Co-reporter:Yanyan Xu, Yafei Luo, Wenting Zhang, Wenqian Li, Ming Li and Wei Shen

RSC Advances 2016 vol. 6(Issue 82) pp:78241-78251

Publication Date(Web):12 Aug 2016

DOI:10.1039/C6RA14653H

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.

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

Co-reporter:Yafei Luo;Yanyan Xu;Wenting Zhang; Ming Li; Rongxing He ; Wei Shen

ChemPhysChem 2016 Volume 17( Issue 1) pp:69-77

Publication Date(Web):

DOI:10.1002/cphc.201500957

Abstract

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 Phosphorescence Quantum Yields of Cyclometalated (C∧C*) Platinum(II) NHC Complexes: π-Conjugation Controls the Radiative and Nonradiative Decay Processes

Co-reporter:Yafei Luo

The Journal of Physical Chemistry C 2016 Volume 120(Issue 6) pp:3462-3471

Publication Date(Web):January 25, 2016

DOI:10.1021/acs.jpcc.5b12214

In this article, the radiative and nonradiative decay processes of four cyclometalated (C∧C*) platinum(II) N-heterocyclic carbene (NHC) complexes were unveiled via density functional theory and time-dependent density functional theory. In order to explore the influence of π-conjugation on quantum yields of (NHC)Pt(acac) (NHC═N-heterocyclic carbene, acac = acetylacetonate) complexes, the factors that determine the radiative process, including singlet–triplet splitting energies, transition dipole moments, and spin–orbit coupling (SOC) matrix elements between the lowest triplet states and singlet excited states were calculated. In addition, the SOC matrix elements between the lowest triplet state and the ground state as well as Huang–Rhys factors were also computed to describe the temperature-independent nonradiative decay processes. Also, the triplet potential energy surfaces were investigated to elucidate the temperature-dependent nonradiative decay processes. The results indicate that complex Pt-1 has higher radiative decay rate than complexes Pt-2–4 due to the larger SOC matrix elements between the lowest triplet states and singlet excited states. However, complexes Pt-2–4 have smaller Huang–Rhys factors, smaller SOC matrix elements between the lowest triplet and the ground states, and higher active energy barriers than complex Pt-1, indicating that complexes Pt-2–4 have smaller nonradiative decay rate constants. According to these results, one may discern why complex Pt-2 has higher phosphorescence quantum efficiency than complex Pt-1; meanwhile, it can be inferred that the nonradiative decay process plays an important role in the whole photodeactivation process. In addition, on the basis of complex Pt-2, Pt-5 was designed to investigate the influence of substitution group on the photodeactivation process of rigid (NHC)Pt(acac) complex.

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

Co-reporter:Yanyan Xu, Yafei Luo, Ming Li, Rongxing He, and Wei Shen

The Journal of Physical Chemistry A 2016 Volume 120(Issue 34) pp:6813-6821

Publication Date(Web):August 12, 2016

DOI:10.1021/acs.jpca.6b03575

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 3MC 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 3MC 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.

The electronic structures and photophysical properties of platinum complexes with C^N^N ligands: the influence of the carborane substituent

Co-reporter:Wenting Zhang, Yafei Luo, Yanyan Xu, Li Tian, Ming Li, Rongxing He and Wei Shen

Dalton Transactions 2015 vol. 44(Issue 41) pp:18130-18137

Publication Date(Web):17 Sep 2015

DOI:10.1039/C5DT02110C

Carboranes have attracted increasing interest in the scientific community due to their remarkable structures and strong electron-withdrawing abilities. In this article, four platinum complexes [(C^N^N)PtCCPh](1), [(C^N^N)PtCC-TPA](2), [(C^N^N)PtCC-TAB](3), [(C^N^N)PtCC-CB](4) (where TPA = triphenylamine, TAB = triarylboryl, CB = o-carborane) have been calculated via density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods to mainly explore the influence of carborane substituents on electronic structures, photophysical properties and radiative decay processes. The calculated results reveal that 2 with electron-donating triphenylamine has a low radiative decay rate constant and a red-shifted emission band, but 3 and 4 containing electron-withdrawing triarylboryl and o-carborane exhibit the opposite properties, especially 4 is supposed to have the highest phosphorescence quantum yield with the smallest nonradiative decay rate constant. These findings successfully illustrated the structure–property relationship and the designed complex 4 with carborane can serve as a highly efficient phosphorescent material in the future.

Theoretical Insight Into the Role of Triarylboron Substituents in Tetradentate Dianionic Bis(N-heterocyclic carbene) Platinum(II) Chelates Improving the Performance of Blue Light Emission

Co-reporter:Li Tian;Yafei Luo;Luqiong Zhang;Ming Li;Rongxing He

European Journal of Inorganic Chemistry 2015 Volume 2015( Issue 11) pp:1902-1911

Publication Date(Web):

DOI:10.1002/ejic.201403092

Abstract

The electronic structures and photophysical properties of eleven Pt^II complexes divided into three series by their degree of π conjugation were studied through density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. To investigate the effect of triarylboron substituents and the changes caused by the extension of the π conjugation, the nonradiative and radiative decay efficiencies, the geometric relaxations, d orbital splitting, and spin–orbit couplings at the optimized S₀ and T₁ geometries were computed. The results show that complexes with triarylboron substituents may have higher phosphorescence efficiencies than those with cyano groups. Furthermore, complexes with larger π conjugation (anthracene groups) may weaken the effects caused by the introduction of triarylboron substituents and are less likely to possess enhanced phosphorescence efficiency. Predictions of the absorption spectra and emission colors indicated that complexes with triarylboron substituents would emit blue colors, whereas the emission colors of the complexes with larger π conjugation would be located in the near-infrared region. This work highlights that the introduction of the triarylboron substituents and appropriate π conjugation (naphthalene groups) can result in highly efficient phosphorescence in complexes containing donor N-heterocyclic carbene (NHC) ligands.

Exploration of phosphorescent platinum(II) complexes functionalized by distinct main-group units to search for highly efficient blue emitters applied in organic light-emitting diodes: A theoretical study

Co-reporter:Yafei Luo, Yanyan Xu, Li Tian, Wenting Zhang, Ming Li, Wei Shen

Inorganica Chimica Acta 2015 Volume 435() pp:109-116

Publication Date(Web):24 August 2015

DOI:10.1016/j.ica.2015.06.012

•According to the DFT calculations, the complex 4 is promising as deep-blue OLED material.•Emission wavelengths are in the range from 434 to 562 nm.•Radiative rate constants dramatically change from 1.00E+04 to 1.22E+05 s−1.•Relationship between zero-field splitting and radiative rates were discussed.In this study, five cyclometalated Pt(II) complexes were chosen as research subjects to investigate the effects of main-group moieties on the electronic structure, photophysical properties and radiative deactivation processes of the phosphorescent metal complexes. Density functional theory (DFT)/time-dependent DFT investigation was conducted to gain a better understanding of the properties of these Pt(II) complexes, including the ground and triplet state geometries, absorption spectra and emission wavelength. Moreover, the self-consistent spin–orbit coupling TDDFT (SOC-TDDFT) was used to calculate zero-field splitting (ZFS), radiative rate and radiative lifetime to unveil the radiative deactivation processes for these complexes. The results reveal that the different main-group moieties added on the 4′-position of the phenyl ring in [Pt(ppy)(acac)] could not only dramatically affect molecular and electronic structure, absorption and luminescence properties, but also radiative deactivation processes. And the emission wavelengths of five complexes are in the range from 434 to 562 nm. Furthermore, among the studied complexes, the designed complex 4 shows great potential to serve as an efficient deep-blue-light emitter in OLED.Incorporate substituents 1–5 displayed as the above graph into the 4′-position of the phenyl ring of the ppy ligand to study the structure–property relationship of cyclometalated Pt(II) complexes and provide valuable hints for the design of robust blue emitters doped in OLEDs.

A theoretical study on tuning the electronic structures and photophysical properties of newly designed platinum(II) complexes by adding substituents on functionalized ligands as highly efficient OLED emitters

Co-reporter:Luqiong Zhang, Li Tian, Ming Li, Rongxing He and Wei Shen

Dalton Transactions 2014 vol. 43(Issue 17) pp:6500-6512

Publication Date(Web):29 Jan 2014

DOI:10.1039/C3DT53209G

By imitating FIrpic, seven new platinum(II) complexes with pic (pic = picolinate) ligand have been designed to be guest materials by means of adding different substituents to functionalized ligands (ppy and fpy, ppy = phenylpyridyl-N,C and fpy = 2-(9′,9′-diethyl-9H-fluorenyl)pyridyl-N,C). In order to reveal their molecular structures, photophysical properties and structure–property relationships with typical host materials, an in-depth theoretical investigation was performed via quantum chemical calculations. The electronic structures and photophysical properties of these complexes were investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) using the B3LYP functional with LANL2DZ and 6-31G* basis sets. It turns out that electronic structures and photophysical properties can be tuned by substituent modifications on functionalized ligands. This work highlights that the match between guest materials and host materials in typical OLED structures can be weighed by the energy levels of the HOMO and LUMO and the adiabatic triplet energy of each complex. Also, a combined analysis of electronic structures, host–guest match, reorganization energies (λ) and triplet exciton generation fraction (χT) is helpful in exploring triplet emitters with high phosphorescence efficiency in OLEDs, which is an interesting and creative aspect of this work. Thereinto, λ reveals the capability of carrier transport and the balance between holes and electrons, whilst structural parameters and d-orbital splittings show that those complexes that have strong electron-withdrawing and electron-donating groups are nonemissive. Consequently, complexes 3–7 can be better triplet emitters than FIrpic. Moreover, the emission colors could be predicted by the 0–0 transition energy (E0–0) instead of the triplet vertical transition energy (Evert). Accordingly, complexes 3, 4 and 6 would be efficient phosphorescent materials with different predicted emission colors.

Theoretical investigations on fluorinated and cyano copolymers for improvements of photovoltaic performances

Co-reporter:Xiaorui Liu, Ming Li, Rongxing He and Wei Shen

Physical Chemistry Chemical Physics 2014 vol. 16(Issue 1) pp:311-323

Publication Date(Web):17 Oct 2013

DOI:10.1039/C3CP53268B

An effective way to improve the efficiency of organic solar cells is to adjust the electron-withdrawing strength in donor–acceptor (D–A) copolymers. To achieve this goal, starting from previously reported polymers (PCPDT-BT and PDTPr-FBT) which are based on benzothiadiazole (BT) electron-deficient unit connected to each of two electron-rich units (cyclopentadithiophene (CPDT) and dithienopyrrole (DTPr)), we introduced two strong electron-withdrawing fluorine atoms or a cyano group on the BT unit to replace BT with fluorinated BT (FBT) and cyano BT (CNBT) in PCPDT-BT and PDTPr-FBT, respectively, and designed two types of D–A copolymers with different electron-withdrawing strengths. From the calculated results, the introduction of strong electron-withdrawing groups onto the copolymer can not only obviously reduce the HOMO and LUMO level of molecules, which results in increasing the open circuit voltage (Voc) in solar cells, but can also enhance light-absorbing efficiency and charge transport ability of polymers. In the meantime, the cyano copolymers of PCPDT-1CNBT and PDTPr-1CNBT show the best performances with the smallest band gaps, lowest HOMO energy levels, the highest Voc, and the largest hole mobility (3.67 × 10−3 cm2 V−1 s−1 and 8.05 × 10−4 cm2 V−1 s−1, respectively) among all the considered systems. The power conversion efficiencies (PCEs) of ∼7.2% and ∼6.8% for organic solar cell made of designed polymers (PCPDT-1CNBT and PDTPr-1CNBT) are predicted by Scharber models. We presented several polymer donors for comparison of how the strong electron-withdrawing group influences the electronic properties and optical absorption of the polymers and the performances of organic solar cells made of the polymers, thereby obtaining promising organic solar cells with high power conversion efficiencies.

Molecular designing and DFT investigation of novel alternating donor–acceptor dibenzo[b,d]thiophen-based systems: from monomer to polymer

Co-reporter:Lidan Deng;Xiaohua Xie;Liusheng Jiang;Binyao Liu

Structural Chemistry 2012 Volume 23( Issue 1) pp:97-106

Publication Date(Web):2012 February

DOI:10.1007/s11224-011-9838-4

The structures and properties of dibenzo[b,d]thiophene (DBT) based alternating donor–acceptor conjugated oligomers, in which thieno[3,4-b]pyrazine (TP), thieno[3,4-b]thiadiazole (TD), and [1,2,5]thiadiazolo[3,4-e]thieno[3,4-b]pyrazine (TTP) as acceptors, and their periodic polymers were investigated by the density function theory (DFT) at the B3LYP/6-31G(d) level. The bond length, electron density at bond critical points (BCPs) and nucleus-independent chemical shift (NICS) are analyzed and correlated with the conductive properties. NICS shows that the conjugation degree is increased with main chain extension. Research results show the conductive ability of compounds with 1:2 D–A ratio is better than that with 1:1 D–A ratio. The reorganization energies and energy bands also are considered. The results suggest that (BTDDBT)n and (BTPDDBT)n have small reorganization energy (0.163 and 0.152 eV, respectively) and quite low energy gap (0.73 and 0.56 eV, respectively), which indicate that they may be potential organic conductive materials.

Fine tuning phosphorescent properties of platinum complexes via different N-heterocyclic-based CˆNˆN ligands

Co-reporter:Wenting Zhang, Jian Wang, Yanyan Xu, Wenqian Li, Wei Shen

Journal of Organometallic Chemistry (15 May 2017) Volumes 836–837() pp:26-33

Publication Date(Web):15 May 2017

DOI:10.1016/j.jorganchem.2017.02.029

The electronic structures and photophysical properties of platinum complexes with C^N^N ligands: the influence of the carborane substituent

Co-reporter:Wenting Zhang, Yafei Luo, Yanyan Xu, Li Tian, Ming Li, Rongxing He and Wei Shen

Dalton Transactions 2015 - vol. 44(Issue 41) pp:NaN18137-18137

Publication Date(Web):2015/09/17

DOI:10.1039/C5DT02110C

Co-reporter:Luqiong Zhang, Li Tian, Ming Li, Rongxing He and Wei Shen

Dalton Transactions 2014 - vol. 43(Issue 17) pp:NaN6512-6512