Lowest Unoccupied Molecular Orbital Distributions Research Articles

Substituent effect on the electronic and optical properties of newly designed pyrrole derivatives using density functional theory

Abstract This work explores six newly designed compounds obtained by several substitutions in 2,5-di(2-thienyl) pyrrole molecule. For this series of compounds, the electronic and optical properties were investigated using density functional theory and time-dependent density functional theory (TD-DFT). The new compounds were characterized by calculating the chemical parameters that correlated with their optical and electrical properties. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energies are calculated using the B3LYP functional with the 6-311G (d, p) basis set. The most low-lying energy level of the LUMO was found for Perr-NO2, indicating its effective electron injection capabilities and oxidation resistance. The HOMO and LUMO distributions of Perr-Cl and Perr-NO2 displayed a remarkable complementarity throughout each component of the two compounds, indicating an effective intermolecular charge transfer. The molecular electrostatic potential analysis demonstrated that the proposed compounds have a broad distribution of electrophilic and nucleophilic sites, which predict a high degree of chemical reactivity. The electron density analysis at the bonding and anti-bonding sites of the title compounds was performed using the electron localization function and local orbital locator techniques. Non-covalent interaction analysis using the reduced density gradient approach classified all types of interaction: repulsive, weak, and attractive interactions within compound fragments. All compounds exhibited a robust repulsive interaction, as proved by the red spikes at 0.038 a.u. The ultraviolet/visible (UV/vis) spectrum was obtained by TD-DFT using CAM-B3LYP models in conjunction with 6-311G (d, p) basis set and methanol as a solvent, the absorption bands were found within the UV range, and the maximum wavelength showed red-shifted increases. These compounds could serve as a base material for developing selective gas sensors with considerable UV/vis absorption (180–400 nm). According to the research results, the designed compounds are good candidates for use as precursors in polymer designs for optoelectronic and sensor applications due to their high electrical conductivity and photochemical properties.

Effective Approach by Computational Chemical Prediction and Experimental Verification to Elucidate SEI Formation Mechanism in LiPF6-, LiFSI-, and LiBF4-Containing Electrolyte Solutions

Solvation structures of ethylene carbonate-based electrolyte solutions containing 1 mol/L LiPF6, LiN(SO2F)2 (LiFSI), or LiBF4 are experimentally determined, and their electron affinity and lowest unoccupied molecular orbital (LUMO) distribution are calculated by density functional theory (DFT). The stability of the electrolyte solutions against reduction can be evaluated using electron affinity of the solvates, and the formation mechanism of the solid electrolyte interphase (SEI) can be predicted by their LUMO distributions. The calculation results suggest that 1 mol/L LiPF6 and LiFSI electrolyte solutions should form SEI consisting of carbonate and carboxylate which derives from solvent decomposition and LiF originating from anion decomposition. These predictions are verified experimentally by scanning electron microscopy–energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy measurements. On the other hand, intriguingly, the reductive decomposition of 1 mol/L LiBF4 electrolyte solution does not simply proceed as theoretically predicted, and the subsequent reactions of SEI need to be considered: further decomposition of the organic SEI and formation of inorganic particles containing lithium, fluoride, oxygen, and boron. These results are consistent with an increase in the resistance of SEI and interfacial lithium ion transfer at the graphite negative electrode. Thus, a combination of DFT prediction and experimental analysis is an effective approach to reveal SEI formation mechanisms and can be utilized as a versatile approach to develop new solvents, electrolyte salts, and additives and to design electrolyte solutions with an appropriate concentration.

Sonosensitized Aggregation-Induced Emission Dots with Capacities of Immunogenic Cell Death Induction and Multivalent Blocking of Programmed Cell Death-Ligand 1 for Amplified Antitumor Immunotherapy

Sonosensitized Aggregation-Induced Emission Dots with Capacities of Immunogenic Cell Death Induction and Multivalent Blocking of Programmed Cell Death-Ligand 1 for Amplified Antitumor Immunotherapy

1,8-Naphthalimide-based hybrids for efficient red thermally activated delayed fluorescence organic light-emitting diodes

Abstract The connection of an electron deficient 1,8-naphthalimide acceptor moiety with a phenoxazine or phenothiazine donor moiety afforded 6-(10H-phenoxazin-10-yl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (PXZ-NAI) and 6-(10H-phenothiazin-10-yl)-2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (PTZ-NAI) as two novel red thermally activated delayed fluorescence (TADF) emitters. The two emitters exhibited distinct TADF characteristics with small energy gaps between the lowest singlet and triplet excited states, which originated from the well-separated highest occupied molecular orbital and lowest unoccupied molecular orbital distributions. The optimized TADF organic light-emitting diodes based on PXZ-NAI and PTZ-NAI offered maximum external quantum efficiencies (EQEs) of 13.0% and 11.4% with maximum power efficiencies of 14.8 and 9.8 lm/W, respectively. Both devices emitted red electroluminescence spectra with peaks at 624 and 632 nm, and Commission Internationale de L'Eclairage coordinates of (0.610, 0.388) and (0.630, 0.368), respectively. These findings demonstrate that the combination of 1,8-naphthalimide with phenoxazine or phenothiazine could be an effective pathway for developing red TADF emitters.

Manipulating the LUMO distribution of quinoxaline-containing architectures to design electron transport materials: Efficient blue phosphorescent organic light-emitting diodes

A series of new quinoxaline-containing compounds, namely, 2,3,6,7-tetrakis(3-(pyridin-3-yl)phenyl)quinoxaline (Tm3PyQ), 2,3,6,7-tetrakis(3-(pyridin-4-yl)phenyl)quinoxaline (Tm4PyQ), 1,4-bis(2,3-dimethyl-7-(pyridin-3-yl)quinoxalin-6-yl)benzene (3PyDQB), and 1,4-bis(2,3-dimethyl-7-(pyridin-4-yl)quinoxalin-6-yl)benzene (4PyDQB) were designed and synthesized as electronic transporting materials. The lowest unoccupied molecular orbital (LUMO) distributions of these compounds vary with the locations of quinoxaline moieties, which result in adjustable intermolecular charge-transfer integrals. All the compounds exhibit favorable electron affinity (2.73–2.88 eV) and good thermostability (glass transition temperatures in the range of 112–148 °C). Using these compounds as electron transport layers, the bis(4,6-(difluorophenyl)pyridinato-N,C2′)picolinate iridium(Ⅲ) (Firpic)-based blue phosphorescent organic light emitting diodes (PhOLEDs) achieve good performances with a maximum current efficiency (ηc,max) of 30.2 cd A−1 and a maximum external quantum efficiency (ηext,max) of 14.2%. Moreover, these efficiencies reveal small roll-offs at high luminance.

Isomeric Thermally Activated Delayed Fluorescence Emitters for Color Purity-Improved Emission in Organic Light-Emitting Devices.

To improve the color purity of thermally activated delayed fluorescence (TADF) emitters, two isomeric compounds, oPTC (5'-(phenoxazin-10-yl)-[1,1':3',1″-terphenyl]-2'-carbonitrile) and mPTC (2'-(phenoxazin-10-yl)-[1,1':3',1″-terphenyl]-5'-carbonitrile), were designed and synthesized with same skeleton but different molecular restrictions. Both compounds exhibit similar highest occupied molecular orbital and lowest unoccupied molecular orbital distributions and energy levels, photophysical properties in nonpolar cyclohexane solution, and high external quantum efficiencies (19.9% for oPTC and 17.4% for mPTC) in the devices. With the increased molecular space restriction induced by the additional phenyl substitutions at meta-position of the cyano group from mPTC to oPTC, much weaker positive solvatochromic effect is observed for mPTC. And the color purity of emission from mPTC (full width at half-maximum (fwhm) of 86 nm) is also improved contrasted with that of oPTC (fwhm of 97 nm) in the devices. These results prove that increased restriction of the molecular structure is a simple and effective method to improve the color purity of the TADF emitters.

Purely organic electroluminescent material realizing 100% conversion from electricity to light.

Efficient organic light-emitting diodes have been developed using emitters containing rare metals, such as platinum and iridium complexes. However, there is an urgent need to develop emitters composed of more abundant materials. Here we show a thermally activated delayed fluorescence material for organic light-emitting diodes, which realizes both approximately 100% photoluminescence quantum yield and approximately 100% up-conversion of the triplet to singlet excited state. The material contains electron-donating diphenylaminocarbazole and electron-accepting triphenyltriazine moieties. The typical trade-off between effective emission and triplet-to-singlet up-conversion is overcome by fine-tuning the highest occupied molecular orbital and lowest unoccupied molecular orbital distributions. The nearly zero singlet–triplet energy gap, smaller than the thermal energy at room temperature, results in an organic light-emitting diode with external quantum efficiency of 29.6%. An external quantum efficiency of 41.5% is obtained when using an out-coupling sheet. The external quantum efficiency is 30.7% even at a high luminance of 3,000 cd m−2.

MoO3 as p-type dopant for Alq3-based p–i–n homojunction organic light-emitting diodes

Using high-work-function material MoO3 as a p-type dopant, efficient single-layer hybrid organic light-emitting diodes (OLEDs) with the p–i–n homojunction structure are investigated. When MoO3 and Cs2CO3 are doped into the conventional emitting/electron-transport material tris-(8-hydroxyquinoline) aluminum (Alq3), respectively, a significant increase in p- and n-type conductivities is observed compared to that of intrinsic Alq3 thin films. With optimal doping, the hole and electron mobilities in Alq3:MoO3 and Alq3:Cs2CO3 films was estimated to be 9.76×10−6 and 1.26×10−4cm2/Vs, respectively, which is about one order of magnitude higher than that of the undoped device. The p–i–n OLEDs outperform undoped (i–i–i) and single-dopant (p–i–i and i–i–n) OLEDs; they have the lowest turn-on voltage (4.3V at 1cd/m2), highest maximum luminance (5860cd/m2 at 11.4V), and highest luminous efficiency (2.53cd/A at 100mA/cm2). These values are better than those for bilayer heterojunction OLEDs using the same emitting layer. The increase in conductivity can be attributed to the charge transfer process between the Alq3 host and the dopant. Due to the change of carrier concentration in the Alq3 films, the Fermi level of Alq3 is close to the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels upon p- and n-type doping, respectively, and the carrier injection efficiency can thus be enhanced because of the lower carrier injection barrier. The carriers move closer to the center energy levels of the HOMO or LUMO distributions, which increases the hopping rate for charge transport and results in an increase of charge carrier mobility. The electrons are the majority charge carriers, and both the holes and electrons can be dramatically injected in high numbers and then efficiently recombined in the p–i–n OLEDs. As a result, the improved conductivity characteristics as well as the appropriate energy levels of the doped layers result in improved electroluminescent performance of the p–i–n homojunction OLEDs.

Spectral studies of multi-branched fluorescence dyes based on triphenylpyridine core

A series of novel triphenylpyridine-containing triphenylamine derivatives have been carefully designed and prepared in good yields using the stepwise route reactions. The relationship of photoluminescence property and structure of compounds 9–13 was systematically investigated via UV–vis, fluorescence, thermogravimetric and electrochemical analyzer. The highest occupied molecular orbital and the lowest unoccupied molecular orbital distributions of compounds 9–13 were calculated by density functional theory method. The high fluorescence quantum yields, desirable the highest occupied molecular orbital levels and high thermal stability of compounds 9–13 indicate that the linkage of triphenylpyridine and triphenylamine is an efficient means to enhance hole-transporting ability and fluorescent quantum yield.

Generalization of Einstein relation for organic semiconductor thin films

Under the basis of Gaussian energy distributions of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), analytical expressions of generalized Einstein relation for electron and hole transport in organic semiconductor thin films are developed. Numerical calculations show that, although traditional Einstein relation is still valid for low carrier concentrations, when the carrier concentration is high, the diffusion coefficient–mobility ratio increases rapidly with the carrier concentration. A relative turning carrier concentration, characterizing the upper limit of the validity of traditional Einstein relation is defined. The dependences of the relative turning concentration on the variance of LUMO or HOMO energy distributions as well as the sample temperature, and the applications of the generalized Einstein relation in the analysis of organic light-emitting device are discussed.