Hole transport in triphenylamine based OLED devices: from theoretical modeling to properties prediction

For the series of para-substituted triphenylamines, optimized geometries, HOMO and LUMO energy levels, ionization potentials Ip, reorganization energies for hole transport λ(+), and frontier orbital contours have been calculated by means of ab initio computations. Relationships between them and the Hammett parameter are presented. According to calculations, electron releasing substituents increase the HOMO and LUMO energies of TPA, while electron withdrawing ones decrease it. This behavior is reflected in subsequent decreasing and increasing of ionization potentials of substituted TPAs. Calculations show that there exists also a strong substituent effect on the reorganization energy λ(+), which is a dominating factor of hole mobility. It is concluded that proper tuning of the HOMO and LUMO levels (and, as a result, ionization potential, Ip) and reorganization energy λ(+) (consequently, hole mobility) of the triphenylamine can be done by alteration of the TPA electronic structure by an appropriate substitution. It is demonstrated that the proper adjustment of the HOMO levels of HTM facilitates the reduction of an energy barrier at the interface of ITO/HTL and HTL/EL and ensure the high hole injection and hole transport rate. On the other hand, appropriate adjustment of the LUMO level prevents an electron leak from the EL into the HTM layer. Results of these calculations can be useful in the process of designing new HTM materials of desired properties (high efficiency, stability, and durability).     

吲哚并咔唑类衍生物载流子传输性质的理论研究

以密度泛函理论结合跳跃模型, 重点研究了氯原子和烷基链的引入对吲哚并咔唑类衍生物传输性质的影响. 计算结果表明, 与吲哚并[3,2-b]咔唑(1)相比, 氯原子的引入增大了2,8-二氯吲哚并[3,2-b]咔唑(2)和2,8-二氯-5,11-二己基吲哚并[3,2-b]咔唑(3)的最高占据分子轨道(HOMO)的离域程度, 而对最低未被占据分子轨道(LUMO)则无显著贡献, 但明显降低了二者的能级. 上述结果表明, 对于LUMO, 氯原子体现了吸电子效应, 而对于HOMO, 氯原子体现了共轭效应. 烷基链的引入使化合物3的空穴迁移率明显高于化合物1和2, 这主要归因于化合物3具有更加紧密的分子堆积, 尤其在跳跃路径A中, 具有更大的分子间电子耦合和轨道重叠. 同时能带结构的计算结果进一步证明, 氯原子和烷基链的同时引入大大改善了吲哚并咔唑类衍生物的电荷传输性能.     

Synthesis and characterization of green-emitting phosphorescent Ir(III) complexes based on phenyl benzimidazole ligand

Several new Ir(III) complexes with 2-(4-bromophenyl)-1H-benzo[d]imidazole or 2-(4-bromophenyl)- 1-methyl-benzo[d]imidazole ligands as cylcometalated ligand and acetylacetonate or picolinate as the ancillary ligand were synthesized and their structures and photophysical properties were characterized. HOMO and LUMO energy levels and the molecular structures of Ir(III) complexes were scrutinized by DFT calculations. The complexes exhibited green luminescence at the maximum emission peaks at ca 495–522 nm. The methyl group substituent and replacing of acetylacetonate with picolinate complex can enhance the complex thermal stability. HOMO energy levels of the complexes vary from –4.99 to –5.44 eV, the LUMO energy levels are between –1.52 and –1.97 eV.     

Impact of replacement of the central benzene ring in anthracene by a heterocyclic ring on electronic excitations and reorganization energies in anthratetrathiophene molecules

The impact of changing the central benzene ring on the electronic excitations and reorganization energies (λ) of the anthratetrathiophene (ATT) molecules is studied by density functional theory (DFT) and time‐dependent DFT (TD‐DFT) quantum chemical calculations. The effect of changing the position of the sulfur atom at the periphery of anthracene on the optical and charge transfer properties is also studied. The calculated results suggest that the HOMO, LUMO, HOMO–LUMO energy gap, ionization potential (IP), electron affinity (EA), hole extraction potential (HEP), electron extraction potential (EEP), and reorganization energies (λ) are affected by replacing the central ring with different heterocyclic rings and the position of the sulfur atom. In addition, all molecules show good hole‐ and electron‐transport properties. This work may be helpful for future design and preparation of high‐performance charge‐transport materials.     

Theoretical Studies on the Electronic Structures and Optical Properties of Tri-aryl End-capped Terthiophene Derivatives

The relationships between electronic structures and spectra properties are investigated by DFT/TDDFT for terthiophene derivatives, BMA‐3T (tri‐aryl amine end‐capped terthiophene), BBA‐3T (tri‐aryl amine and tri‐aryl boron end‐capped terthiophene) and BPB‐3T (tri‐aryl boron end‐capped terthiophene). The calculated results show that BMA‐3T, BBA‐3T and BPB‐3T have higher HOMO energy level and lower ionization potentials (IPs) than 3T. BMA‐3T has good hole injection ability and hole‐transport property as reported in experiment. The designed molecule of BBA‐3T and BPB‐3T own lower LUMO level and higher electron affinities (EAs) than BMA‐3T, which facilitate electron injection and improve their electron‐transport properties. Surprisingly, BPB‐3T has preferable charge equilibrium property since its hole reorganization energy (λ_h) is close to electron reorganization energy (λ_e). The ΔE (HOMO−LUMO) and E_g of these three derivatives are narrower compared to 3T, and the absorption as well as emission spectrum exhibited red‐shifts.     

Relative stability of 1-(4-R-2,6-dinitrophenyl)-2,2-diphenylhydrazides

Substitution of the nitro group for methyl in the position 4 of the picrylic fragment of 1-(2,4,6-trinitrophenyl)-2,2-diphenylhydrazyl increases the relative stability of hydrazyl radical in reaction with CH-acids. The reaction rate of 1-(2,4,6-trinitrophenyl)-2,2-diphenylhydrazyl with acetylacetone and ethyl acetoacetate is described with equations of first order with respect to the radical and the СН-acid and is determined by the energy of electron transfer from HOMO of СН-acid on LUMO of the radical, which is its acceptor.     

The Pi‐Nature of Methyl Obtained by the Natural Bond Orbital Method: Orbital‐Based Rationalizations of Site‐Dependent Substitution Effects on Fine Color‐Tuning of Luminescence

Both C‐H bonding and antibonding (σ_CH and σ*_CH) of a methyl group would contribute to the highest occupied or lowest unoccupied molecular orbitals (HOMO or LUMO) in methylated derivatives of Ir(ppz)₂ 3 iq (ppz = 1‐phenylpyrazole and 3iq = isoquinoline‐3‐carboxylate). This is found by analysis of HOMO (or LUMO) formed by linear combination of bond orbitals using the natural bond orbital (NBO) method. The elevated level of HOMO (or LUMO) uniformly found for each methylated derivative, indicating the σ_CH‐destabilization outweighs the σ*_CH‐stabilization. To broaden the HOMO‐LUMO gap, methylation at a carbon having smaller contribution to HOMO and/or larger contribution to LUMO is suggested.     

Computational studies of the interactions of I<Superscript>−</Superscript> and I<Subscript>3</Subscript><Superscript>−</Superscript> with TiO<Subscript>2</Subscript> clusters: implications for dye-sensitized solar cells

Abstract  

First principle density-functional theory calculations have been carried out on the interaction of I⁻ and I₃ ⁻ with TiO₂ anatase surfaces, modeled by finite clusters that range in size from 48 to 180 atoms. The total energy per TiO₂ unit and the HOMO-LUMO gaps decrease with increasing the size of the clusters. Both redox species (I⁻ and I₃ ⁻) are strongly adsorbed on the TiO₂ surface with the adsorbtion of I⁻ being stronger. Adsorption of triiodide leads to its dissociation. The positions of the HOMO and LUMO of the adsorbed systems shift negatively from their respective cluster values. Solvation effects have been modeled using the CPCM model. Introducing solvent reduces the shifting of HOMO and LUMO. Implications for dye-sensitized solar cells (DSSC) are discussed. Both the HOMO-LUMO shifting and the strong adsorption might affect the performance of the cell.     

卤素、 氰基及N原子修饰对四硫富瓦烯衍生物载流子传输性质影响的理论研究

基于密度泛函理论结合跳跃模型和能带理论研究了氟、 氯、 氰基和N原子的引入对四硫富瓦烯(TTF)衍生物载流子传输性质的影响. 计算结果表明, 嵌N修饰会降低分子重组能, 特别是当N原子靠近TTF主体环时作用更明显. 与引入卤素修饰相比, 引入氰基修饰的分子具有更小的电子和空穴重组能及更低的前线分子轨道(FMO)能级. 同时迁移率的计算结果显示, 分子6具有1.15 cm²·V^-1·s^-1的高电子迁移率, 考虑其较低的LUMO能级, 推测其有望成为潜在的优异电子传输材料, 而相似的电子和空穴迁移率使分子2有望成为潜在的双极性传输材料. 同时还考察了S和N原子之间的弱相互作用, 当S或N原子对分子HOMO(或LUMO)有贡献时, 其相应的空穴(或电子)传输能力会有所提高.     

Developing Conjugated Polymers with High Electron Affinity by Replacing a CC Unit with a B←N Unit

The key parameters of conjugated polymers are lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Few approaches can simultaneously lower LUMO and HOMO energy levels of conjugated polymers to a large extent (>0.5 eV). Disclosed herein is a novel strategy to decrease both LUMO and HOMO energy levels of conjugated polymers by about 0.6 eV through replacement of a C? C unit by a B←N unit. The replacement makes the resulting polymer transform from an electron donor into an electron acceptor, and is proven by fluorescence quenching experiments and the photovoltaic response. This work not only provides an effective approach to tune the LUMO/HOMO energy levels of conjugated polymers, but also uses organic boron chemistry as a new toolbox to develop conjugated polymers with high electron affinity for polymer optoelectronic devices.     

Low-bandgap poly(thiophene-phenylene-thiophene) derivatives with broaden absorption spectra for use in high-performance bulk-heterojunction polymer solar cells

Two low-bandgap (LGB) conjugated polymers ( P1 and P2) based on thiophene-phenylene-thiophene (TPT) with adequate energy levels have been designed and synthesized for application in bulk-heterojunction polymer solar cells (PSCs). The absorption spectral, electrochemical, field effect hole mobility and photovoltaic properties of LGB TPT derivatives are investigated and compared with poly(3-hexylthiophene) (P3HT). Photophysical studies reveal bandgaps of 1.76 eV for P1 and 1.70 eV for P2, which could effectively harvest broader solar spectrum. In addition, the thin film absorption coefficients of P1 and P2 are 1.6 x 10 (5) cm (-1) (lambda approximately 520 nm) and 1.4 x 10 (5) cm (-1) (lambda approximately 590 nm), respectively. Electrochemical studies indicate desirable HOMO/LUMO levels that enable a high open circuit voltage while blending them with fullerene derivatives as electron acceptors. Furthermore, both materials show sufficient hole mobility (3.4 x 10 (-3) cm (2)/Vs for P2) allowing efficient charge extraction and a good fill-factor for PSC application. High-performance power conversion efficiency (PCE) of 4.4% is obtained under simulated solar light AM 1.5 G (100 mW/cm (2)) from PSC device with an active layer containing 25 wt% P2 and 75 wt% [6,6]-phenyl-C71-butyric acid methyl ester (PC 71BM), which is superior to that of the analogous P3HT cell (3.9%) under the same experimental condition.     

Synthesis and Properties of Thieno[3,2‐b]thiophene Derivatives for Application of OFET Active Layer

A series of thieno[3,2‐b]thiophene derivatives having styryl groups were synthesized via short steps and characterized by UV–vis absorption spectra and cyclic voltammetry. Based on these results, we found that the introduction of long chain alkyl groups to the terminal styryl groups leads to narrower HOMO–LUMO gaps and higher HOMO energy levels than the unalkylated styryl substituted molecules. Organic field–effect transistor (OFET) devices using these derivatives as the active layer were fabricated by a vacuum deposition process. It was demonstrated that these devices showed a relatively high hole mobility up to 3.5 × 10⁻² cm²/Vs. These devices also showed a good stability, namely their mobilities did not decrease over 100 days in air. Therefore, these facts suggested that the introduction of long‐chain alkylated styryl groups is an effective way to improve the hole mobilities in OFETs. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 24:25–35, 2013; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21059     

Square-planar mixed ligand nickel dithiolenes as second-order non-linear chromophores: synthesis and characterisation of [Ni(Me2pipdt)(dddt)]

Abstract  

The novel [Ni(Me₂pipdt)(dddt)] complex based on the Me₂pipdt (1,4-dimethylpiperazine-3,2-dithione) and dddt (5,6-dihydro-1,4-dithine-2,3-dithiolate) ligands has been synthesised and characterised. Structural data, vibrational marker, solvatochromic behaviour of the typical absorption in the near infrared region, and approximate theoretical calculations suggest that an unbalanced electron distribution at the dithiolene core occurs in the ground and excited states. In particular, the dddt ligand gives a prevailing contribution to the HOMO, and Me₂pipdt to the LUMO. The charge-transfer character of the HOMO–LUMO transition makes this complex a potential second-order non-linear optic chromophore.     

PHOTOCONDUCTIVE PROPERTIES OF PVK:Alq3 BLEND FILMS STUDIED BY STEADY-STATE AND TIME-RESOLVED TRANSIENT PHOTOCURRENT SPECTRA

The carrier transport properties of the blends of the hole transport material poly(N-vinylcarbazole) (PVK) and the electron transport material tris (8-hydroxyquinolinolato) aluminumⅢ(Alq_3) are investigated at room temperature using steady-state and time-resolved transient photocurrent measurements as a function of doping concentration of Alq_3.Due to lower LUMO and higher HOMO energy level of Alq_3 than those of PVK,Alq_3 molecules may act as carrier trap states in PVK films at low concentration.However...     

8-甲硫基喹啉及其两种衍生物UV-Vis光谱的理论研究

The ground state structures of 8-methylthioquinoline and its two absorption spectra were calculated using TD-DFT method.Calculation results agree with experimental values.Calculation results indicate that the position of lowest-energy absorption bands will be red-shifted and blue-shifted,when one hydrogen atom of methyl group is substituted by -Si(CH3)3 and -Si(OCH3)3 groups,respectively.And the energies of HOMO and LUMO of substitutents increase to a different extent,and the position of lowest-energy absorption bands and the energy gap vary slightly.     

Synthesis and characterization of alternating fluorene–thiophene copolymers bearing ethylene glycol side-chains

Abstract  

New alternating fluorene–thiophene copolymers are introduced bearing polar ethylene glycol-carboxylate functionalities on the thiophene ring to achieve enhanced solubility in polar solvents. Suzuki polycondensation was applied to synthesize a set of three polymers with differing lengths of the ethylene glycol side-chains. The polymers are thermally stable up to temperatures of 300 °C. Solutions of the polymers in CHCl₃ show an absorption maximum at approximately 397 nm and a luminescence maximum of 472 nm in solutions with quantum yield of 30%. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels have been determined to be at −5.7 and −2.6 eV, respectively.     

Bay‐annulated indigo based near‐infrared sensitive polymer for organic solar cells

A new conjugated polymer (PBAIIDTT) based on bay‐annulated indigo and indacenodithieno[3,2‐b]thiophene was designed, synthesized, and characterized. PBAIIDTT shows strong absorption in 400–500 and 600–800 nm, and its HOMO and LUMO energy levels are −5.45 eV and −3.65 eV, respectively. In organic field‐effect transistors, the polymer exhibits a relatively high hole mobility of 0.39 cm² V⁻¹ s⁻¹. PBAIIDTT was added to poly(3‐hexylthiophene) (P3HT) and phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) based organic solar cells. Ternary blend solar cells with 10% PBAIIDTT show an increased short circuit current density due to the broadened photocurrent generated in the near‐infrared region, and a power conversion efficiency of 3.78%, which is higher than that of the P3HT:PC₆₁BM binary control devices (3.33%). © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 213–220     

Theoretical Investigation into Electronic Structures and Charge Transfer Properties of π‐Conjugated System with Different Combinations of Thiophene and Vinyl/Butadiene

A series of combinations of thiophene and vinyl/butadiene were investigated by ab initio and DFT methods to explore their electronic structures and charge transfer properties. The results show that increasing thiophene ring and vinyl number is a rational strategy to raise the HOMO energy levels and lower the LUMO energy levels. Moving the vinyl from the periphery to the core has the slight effect on the HOMO and LUMO energy levels. Furthermore, replacing the middle vinyl and end‐capped vinyl of 3b (T5V4) with the butadiene can lower LUMO energy levels and then facilitate the electron injection. Above all, the close hole and electron reorganization energies (λ_h and λ_e) are observed from these compounds. However, the λ_es are smaller than their respective λ_hs in some compounds, which is relatively rare in organic materials. Especially, the promising ambipolar material 3c (T5B4) is recommended theoretically for possessing the equivalent minimum λ_h (0.24 eV) and λ_e (0.24 eV). The absorption wavelengths exhibit red shifts with the increasing of the thiophene ring and the vinyl number under the same configuration, which correspond to the reverse order of ΔE_H‐L and E_g. The linear relationships are found between experimental lowest singlet excited energies (E_exp) with theoretical values ΔE_H‐L and E_g.     

Ab initio investigation of 2,2′‐bis(4‐trifluoromethylphenyl)‐5,5′‐bithiazole for the design of efficient organic field‐effect transistors

The initial molecular structure of 2,2′‐bis(4‐trifluoromethylphenyl)‐ 5,5′‐bithiazole has been optimized in the ground state using density functional theory (DFT). The distribution patterns of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have also been evaluated. To shed light on the charge transfer properties, we have calculated the reorganization energy of electron λ_e, the reorganization energy of hole λ_h, adiabatic electron affinity (EA_a), vertical electron affinity (EA_v), adiabatic ionization potential (IP_a), and vertical ionization potential (IP_v) using DFT. Based on the evaluation of hole reorganization energy, λ_h, and electron reorganization energy, λ_e, it has been predicted that 2,2′‐bis(4‐trifluoromethylphenyl)‐5,5′‐bithiazole would be a better electron transport material. Finally, the effect of electric field on the HOMO, LUMO, and HOMO–LUMO gap were observed to check its suitability for the use as a conducting channel in organic field‐effect transistors. © 2015 Wiley Periodicals, Inc.     

螺噁嗪光致变色反应机理研究进展

20世纪80年代中期以来, 关于螺噁嗪在光致变色过程中的光化学和光物理现象的研究非常活跃. 综述了近20年来对溶液中的螺噁嗪、晶体螺噁嗪以及固体介质中的螺噁嗪的光致变色机理研究结果, 同时介绍了实验研究方法和理论研究方法在螺噁嗪光致变色过程研究中的应用, 分析了取代基对光致变色性质的影响. 阐述了机理研究对新型螺噁嗪设计合成的重要指导意义.