High-lying Highest Occupied Molecular Orbital Research Articles

Carbazole-Decorated Organoboron Emitters with Low-Lying HOMO Levels for Solution-Processed Narrowband Blue Hyperfluorescence OLED Devices.

The hyperfluorescence has drawn great attention in achieving efficient narrowband emitting devices based on multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters. However, achieving efficient solution-processed pure blue hyperfluorescence devices is still a challenge, due to the unbalanced charge transport and serious exciton quenching caused by that the holes are easily trapped on the high-lying HOMO (the highest occupied molecular orbital) level of traditional diphenylamine-decorated emitters. Here, we developed two narrowband blue organoboron emitters with low-lying HOMO levels by decorating the MR-TADF core with weakly electron-donating carbazoles, which could suppress the hole trapping effect by reducing the hole traps between host and MR-TADF emitter from deep (0.40 eV) to shallow (0.14/0.20 eV) ones for facilitating hole transport and exciton formation, as well as avoiding exciton quenching. And the large dihedral angle between the carbazole and MR-TADF core makes the carbazole act as a steric hindrance to inhibit molecular aggregation. Accordingly, the optimized solution-processed pure blue hyperfluorescence devices simultaneously realize record external quantum efficiency of 29.2 %, narrowband emission with a full-width at half-maximum of 16.6 nm, and pure blue color with CIE coordinates of (0.139, 0.189), which is the best result for the solution-processed organic light-emitting diodes based on MR-TADF emitters.

Influence of Tetraphenylbenzene on the OFET Behavior of Triarylamines

AbstractA series of new tetraphenylbenzene‐based triarylamines are successfully synthesized by employing the Diels–Alder cycloaddition reaction. The molecules exhibited good solubility in common organic solvents and thermal stability up to 500 °C. Molecular geometry and charge distribution of the molecules are envisioned by density functional theory. The high‐lying highest occupied molecular orbital (HOMO) level (‐5 eV) of compounds enables the p‐channel conductivity. The influence of tetraphenylbenzene units on charge‐delocalization of HOMO, crystalline packing, and organic field‐effect transistor (OFET) characteristics is analyzed. OFETs are fabricated under ambient conditions by solution‐processable method, and the devices displayed promising performance with charge carrier mobility µh up to 0.72 cm2 V−1 s−1, on/off ratio of 107, and lowest VT of −2 V.

Crystalline monometal-substituted free carbenes

<h2>Summary</h2> The rich chemistry of transition-metal (TM) carbyne complexes has fueled their application in a wide range of fields, from synthetic chemistry to materials science. The isomeric forms of these complexes are monometal-substituted carbenes and have so far been detected only in the gas phase. In this article, we describe the synthesis of crystalline monometal-substituted free carbenes. To isolate these molecules, we selected a bulky π-donor substituent and a closed-shell electron-rich TM center as the substituents around the carbene center. This combination allows these metallocarbenes to exhibit a nucleophilic carbenic character with a high-lying highest occupied molecular orbital (HOMO) arising from the metal substituent. Additionally, we report a formal rearrangement of the metallocarbene to a zwitterionic species containing a carbyne anion, which makes these free compounds not only synthetic astonishments but also attractive for the development of novel subvalent carbon-transfer reactions.

A Thiazole-Based Polymer Donor for Efficient Organic Solar Cells

The development of new materials plays a critical role in improving the efficiency of organic solar cells (OSCs). At present, the relatively high-lying highest occupied molecular orbital (HOMO) level of the high-efficiency polymer donor is regarded as one of the main reasons for the low open-circuit voltage (Voc). In this work, we introduced the strong electron-withdrawing thiazole unit into the construction of a polymer donor. We designed and prepared an alternating donor–acceptor material, namely PSZ, by copolymerizing 4-methyl thiazole with an electron-donating benzodithiophene unit and studied its application in high-efficiency OSCs. The optical and electrical properties of the new material were characterized by UV–Vis absorption spectroscopy and electrochemical cyclic voltammetry. Results show that PSZ is a typical wide-bandgap material with a high optical bandgap of 2.0 eV and a deep HOMO level of − 5.70 eV. When a non-fullerene BTP-eC9 was selected as the acceptor material, Voc reached 0.88 V in the resulting device, and the corresponding power conversion efficiency (PCE) was 8.15%. In addition, when PSZ was added as the third component to the binary photoactive combination with PBDB-TF as the donor and BTP-eC9 as the acceptor, Voc of the cell device could be increased, thereby obtaining a high PCE of 17.4%. These results indicated that introducing thiazole units into polymer donors can remarkably reduce the HOMO levels and improve Voc and PCE in OSCs.

The influence of fluorinated position on the performance of isoindigo-based polymer solar cells

Two novel donor-acceptor (D-A) polymers based on two-dimensional conjugated alkylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene and difluorine substituted isoindigo derivatives were synthesized with the structures as poly{6-(4,8-bis(4,5-dioctylthiophen-2-yl)-benzo[1,2-b:4,5-b′]-dithiophen-2-yl)−5,5′-difluoro-1,1′-dioctyl-isoindigo} (P1) and poly{5-(4,8-bis(4,5-dioctyl-thiophen-2-yl)-benzo[1,2-b:4,5-b′]-di-thiophen-2-yl)−4,6-difluoro-1,1′-dioctyl-isoindigo} (P2), respectively. Thermal stability, absorption properties, energy levels and photovoltaic properties of the polymers were intensively investigated. The symmetric polymer P1 showed broad absorption in comparison with the asymmetric polymer P2 both in solution and solid film, corresponding to their band gaps of 1.45 (P1) and 1.59 (P2) eV, respectively. The copolymers possessed similar high-lying highest occupied molecular orbital (HOMO) levels of – 5.34 (P1) and – 5.32 eV (P2). The performance of conventional solar cells based on copolymers/PCBM (PC61BM and PC71BM) films were investigated. All of P2-based PSCs delivered superior power conversion efficiencies (PCEs) and higher short circuit current (Jsc) than P1-based devices, where a best PCE up to 4.23% was obtained from P2/PC71BM device by THF solvent-vapor-annealing (SVA) treatment with an open-circuit voltage (Voc) of 0.88 V, a Jsc of 8.27 mA/cm2 and a fill factor (FF) of 0.578. The encouraging performance was attributed to its good miscibility, preferable nanophase separation and high mobilities, as well as efficient exciton dissociation with weak bimolecular and trap recombination. In this study, the incorporated position of fluorine atoms on IID-based copolymer influences not only the thin-film structure such as phase separation structure but also photophysical/electrochemical properties and photovoltaic performance, which highlights a new strategy for the design and synthesis of fluorinated D-A polymers.

Carbonyl-Terminated Quinoidal Oligothiophenes as p-Type Organic Semiconductors.

A series of quinoidal oligothiophenes terminated with carbonyl groups (nTDs, n = 2–4) are studied as p-type organic semiconductors for the active materials in organic field-effect transistors (OFETs) both by the theoretical and experimental approaches. The theoretical calculations clearly show their high-lying highest occupied molecular orbital (HOMO) energy levels (EHOMOs), small reorganization energies for hole transport (λholes), and large contribution of sulfur atoms to HOMOs, all of which are desirable for p-type organic semiconductors. Thus, we synthesized nTDs from the corresponding aromatic oligothiophene precursors and then evaluated their physicochemical properties and structural properties. These experimental evaluations of nTDs nicely proved the theoretical predictions, and the largest 4TDs in the series (4,4′′′-dihexyl- and 3′,4,4″,4′′′-tetrahexyl-5H,5′′′H-[2,2′:5′,2″:5″,2′′′-quaterthiophene]-5,5′′′-dione) can afford solution-processed OFETs showing unipolar p-type behaviors and hole mobility as high as 0.026 cm2 V−1 s−1.

Three-Dimensional Spirothienoquinoline-Based Small Molecules for Organic Photovoltaic and Organic Resistive Memory Applications.

A new electron-rich spirothienoquinoline unit, tBuSAF-Th, has been developed via incorporation of a thienyl unit instead of a phenyl unit into the six-membered ring of the spiroacridine (SAF) and utilized for the first time as a building block for constructing small-molecule electron donors in organic solar cells (OSCs) and as active layers in organic resistive memory devices. The resulting three-dimensional spirothienoquinoline-containing 1-4 exhibit high-lying highest occupied molecular orbital (HOMO) energy levels. By the introduction of electron-deficient benzochalcogenodiazole linkers, with the chalcogen atoms being varied from O to S and Se, a progressively lower lowest unoccupied molecular orbital (LUMO) energy level has been achieved while keeping the HOMO energy levels similar. This strategy has allowed an enhanced light-harvesting ability without compromising open-circuit voltage (Voc) in vacuum-deposited bulk heterojunction OSCs using 1-4 as donors and C70 as the acceptor. Good photovoltaic performances with power conversion efficiencies (PCEs) of up to 3.86% and high short-circuit current densities (Jsc) of up to 10.84 mA cm-2 have been achieved. In addition, organic resistive memory devices fabricated with these donor-acceptor small molecules exhibit binary logic memory behaviors with long retention times and high on/off current ratios. This work indicates that the spirothienoquinoline moiety is a potential building block for constructing multifunctional organic electronic materials.

Fused Bithiophene Imide Oligomer and Diketopyrrolopyrrole Copolymers for n-Type Thin-Film Transistors.

Diketopyrrolopyrrole (DPP)-based copolymers have received considerable attention as promising semiconducting materials for high-performance organic thin-film transistors (OTFTs). However, these polymers typically exhibit p-type or ambipolar charge-transporting characteristics in OTFTs due to their high-lying highest occupied molecular orbital (HOMO) energy levels. In this work, a new series of DPP-based n-type polymers have been developed by incorporating fused bithiophene imide oligomers (BTIn) into DPP polymers. The resulting copolymers BTIn-DPP show narrow band gaps as low as 1.27 eV and gradually down-shifted frontier molecular orbital energy levels upon the increment of imide group number. Benefiting from the coplanar backbone conformation, well-delocalized π-system, and favorable polymer chain packing, the optimal polymer in the series shows promising n-type charge transport with an electron mobility up to 0.48 cm2 V-1 s-1 in OTFTs, which is among the highest values for the DPP-based n-type polymers reported to date. The results demonstrate that incorporating fused bithiophene imide oligomers into polymers can serve as a promising strategy for constructing high-performance n-type polymeric semiconductors.

Naphthodithiophene Diimide-Based Copolymers: Ambipolar Semiconductors in Field-Effect Transistors and Electron Acceptors with Near-Infrared Response in Polymer Blend Solar Cells

New π-conjugated copolymers based on naphtho[2,3-b:6,7-b′]dithiophene-4,5,9,10-diimide (NDTI) combined with thiophene, thienothiophene, or dithienothiophene units are synthesized and used in field-effect transistors (FETs) and organic solar cells (OSCs). The low-lying lowest unoccupied molecular orbital (LUMO) and high-lying highest occupied molecular orbital (HOMO) levels of the polymers contribute to reducing injection barriers for both electrons and holes, resulting in ambipolar operation of FET devices. The charge mobilities were strongly affected by the molecular orientation of the copolymers, and the highest electron mobility of 0.26 cm2/(V s) was observed for the copolymer with thienothiophene unit with edge-on orientation. On the other hand, OSCs with PTB7 as the electron donor polymer and the copolymers as the acceptor showed a broad photoresponse extending to the near-IR region, and the highest power conversion efficiency of over 3.5% was obtained for the copolymer with dithienothiophene unit th...

A diketopyrrolopyrrole-based regular terpolymer bearing two different π-extended donor units and its application in solar cells

In this study, to adjust the desired molecular energy levels and bandgap energies of polymers for photovoltaic applications, a regular terpolymer structure was designed. A new regular terpolymer, poly(DPP4T-alt-TBP), containing diketopyrrolopyrrole (DPP), BT, and BP units in the repeating group was successfully synthesized. The DPP-BT monomeric unit in polymer backbone enhanced chain packing and induced a high-lying highest occupied molecular orbital (HOMO) level and the DPP-BP segment induced a deeper HOMO level. The lowest unoccupied molecular orbital (LUMO) level of the terpolymer was also controlled in a similar manner. The HOMO level of the terpolymer was similar to that of poly(DPP-alt-BP), and the energies of the LUMOs were governed by the DPP-BT unit. The polymer chain arrangement of the terpolymer on the substrate was observed to be a mix of face-on and edge-on orientations, which is a different chain arrangement mode to those shown in both poly(DPP-alt-BP) and poly(DPP-alt-BT). A TFT fabricated with poly(DPP4T-alt-TBP) had a charge carrier mobility of 0.59 cm2 V−1 s−1 and a moderately high current on/off ratio. Furthermore, a polymer solar cell containing the terpolymer and PC71BM had a power conversion efficiency of 4.54%, which is significantly higher than those of the PCEs of poly(DPP-alt-BP) and poly(DPP-alt-BT)-based solar cells with identical device configurations.

Series of Quinoidal Methyl-Dioxocyano-Pyridine Based π-Extended Narrow-Bandgap Oligomers for Solution-Processed Small-Molecule Organic Solar Cells

Small molecules with narrow bandgap of <1.6 eV can harvest the visible and near-infrared solar photons. In this Article, we report a new method to achieve narrow bandgap small molecule donors by using electron-deficient quinoidal methyl-dioxocyano-pyridine (MDP) to induce possible quinoidal resonance structure along the conjugated A-pi-D-pi-A backbone. Practically, two MDP moieties are covalently linked onto an electron-rich benzodithiophene (BDT) through the oligothiophene (OT-5T) pi-bridge. The affording small molecules, namely, nTBM, exhibit broad and strong absorption bands covering the visible and near-infrared region from 400 to 870 nm. The estimated optical bandgap is down to 1.4 eV. The narrow bandgap is associated with the low-lying lowest unoccupied molecular orbital (LUMO) energy level (about -3.7 eV) and the high-lying highest occupied molecular orbital (HOMO) energy level (around -5.1 eV). Density-functional theory calculations reveal that the HOMO and LUMO energy levels, with the increase of the size of the oligothiophene bridge, become localizations in different moieties, i.e., the central electron-donating and the terminal electron-withdrawing units, respectively, which provides necessary driving force for the delocalization of the excited electrons and formation of the quinoidal resonance structure. The quinoidal structure enhances the photoinduced intramolecular charge-transfer, leading to the absorbance enhancement of the low-energy absorption band. With the increase of the size of the oligothiophene from 0 to 5 thienyl units and the change of the direction of the alkyl chains on the bridged thiophene from outward to inward, the crystalline nature, fibril length, and phase size of the blend films as well as the cell performance are all fine-tuned, also. With the inward alkyl chains, the terthiophene bridged molecule is amorphous, while the pentathiophene bridged one is relatively crystalline. Both molecules form nanoscale interpenetrating networks with a phase size of 15-20 nm when blended with PC71BM, showing the higher hole mobility and promising electric performance.

Power Efficiency Improvement of White Phosphorescent Organic Light-Emitting Diode with Thin Double-Emitting Layers and Hole-Trapping Mechanism

This study is carried out to discuss how to reduce the driving voltage of blue phosphorescent organic light-emitting diodes (PHOLEDs) by using a thin double-emission layer. A hole transport-type host (TCTA) is inserted between the hole transport layer (TAPC) and the emitting layer (EML), constituting a buffer layer between them with the aim of improving charge carrier balance. Furthermore, in this study, we also utilize the interface between double light-emitting layers of devices by codoping them with a red phosphorescent dopant [Os(bpftz)2(PPh2Me)2]. An Os complex with a high-lying highest occupied molecular orbital (HOMO) energy level (trapping holes) is codoped at the interface between emitting layers and an exciton-formation zone is expanded to obtain a white PHOLED with high efficiency. From the results, the optimal structure of the white device exhibits a yield of 35 cd A-1, a power efficiency of 22 lm W-1, and CIE coordinates of (0.33,0.38) at a luminance of 1000 cd m-2. Furthermore, the power efficiency can be improved to 30 lm W-1 by attaching the outcoupling enhancement film.

Aromaticity of neutral and doubly charged polyacenes

The aromatic character of neutral and doubly charged polyacenes was explored in terms of the harmonic oscillator model of aromaticity (HOMA) and bond resonance energy (BRE). Doubly charged species of polyacenes are different in global and local aromaticity from the neutral species. Neutral species are fairly uniform in local aromaticity, whereas doubly charged species are more aromatic in the edge rings than in the central rings. Higher polyacenes have been predicted to exist as singlet-state diradicals. It was found that these polyacene diradicals are quite similar in global and local aromaticity to the closed-shell molecular dications. The possible instability of higher polyacenes in closed-shell electronic configuration is associated with the high-lying highest occupied molecular orbital (HOMO) that contributes little to global aromaticity.

Design of Novel Reducing Agents for Direct Thermographic Materials

We describe the systematic design and evaluation of new phenolic reducing agents (developers) for dry-processed, silver-based direct thermographic (TG) black-and-white films for medical diagnostic imaging. In TG media, heat exposure as short as 7 ms/pixel is used to induce development, thus highly active developers are needed. Our approach to this problem combined synthesis, coating evaluation, and electronic structure calculations to provide insights into the TG development mechanism. The computed highest occupied molecular orbital (HOMO) energy of the neutral developer did not correlate with maximum image density Dmax. Rather, the best developers (high Dmax, low fog) had relatively low phenol O-H deprotonation energies and high-lying phenolate anion HOMO energies, implying that, in the TG process, silver is reduced primarily by phenolate ion. This information allowed discovery of new categories of potentially useful TG developers, including ortho-aminophenol, para-aminophenol, halophenol, and resorcinol derivatives.