Low-lying LUMO Research Articles

Palladium-Catalyzed [3+2] Annulation of Naphthalimide Acceptors and Thiophene Donors.

Donor-acceptor (D-A) dyes constitute one of the fundamental structural motifs of functional organic materials. In most cases, the donor and acceptor moieties are connected by a single bond, which could potentially be replaced by a fused aromatic ring to enhance the rigidity and conjugation of the dye moieties. However, there is still a lack of synthetic methodologies for such fused D-A systems. Here we report the synthesis of D-A and A-D-A dyes that possess fully annulated donor and acceptor moieties based on palladium (Pd)-catalyzed [3+2] annulation reaction between bromo-chloro-naphthalene dicarboximide and thiophene- and indole-based boronic esters. Thus, a series of fused D-A and A-D-A conjugated dyes were synthesized in good to high yields by a cascade of Pd-catalyzed Suzuki-Miyaura cross-coupling and direct arylation reactions. The newly synthesized fused D-A and A-D-A dyes with one or two naphthalimide units fused to five-membered electron-rich heterocyles were systematically investigated by ultraviolet-visible spectroscopy, cyclic and square wave voltammetry, and density functional theory calculations. These dyes possess desirable optical and electrochemical properties for application as organic electronic materials as they show absorption up to the near-infrared region, undergo up to 4-fold reduction processes, and have low-lying LUMO energy levels down to -3.62 eV.

Insight into the optoelectronic characteristics of diimide-based acceptors in organic solar cells by performing DFT calculation and molecular dynamics simulation

In order to compare the main difference of two diimide derivatives on the modulation of electronic and optical properties of P3HT-based organic solar cell, the density functional theory and molecular dynamics simulations were implemented to achieve elementary data on geometrical structure, molecular orbital, open-circuit voltage, absorption spectra, energetic driving force, and interface parameter of P3HT/D1 and P3HT/D2 systems. According to the investigation, P3HT/D1 system not only exhibits higher open circuit voltage and enough energetic driving force than P3HT/D2 system, but also possesses low-lying LUMO +1 orbital which can favor the exciton separation efficiency. Moreover, on the basis of some typical interface models choose from MD simulation, the estimation of the interface rate manifests that the P3HT/D1 interface possesses the smaller charge recombination rates and larger charge separation rates than those of the P3HT/D2 interface. It is expect that this work can provide certain guidelines for the further develop the performance of organic solar cell. We hope this work can further study on non-fullerene acceptor materials as a certain guides.

Electronic Tuning of Mixed Quinoidal-Aromatic Conjugated Polyelectrolytes: Direct Ionic Substitution on Polymer Main-Chains.

The synthesis of conjugated polymers with ionic substituents directly bound to their main chain repeat units is a strategy for generating strongly electron-accepting conjugated polyelectrolytes, as demonstrated through the synthesis of a series of ionic azaquinodimethane (iAQM) compounds. The introduction of cationic substituents onto the quinoidal para-azaquinodimethane (AQM) core gives rise to a strongly electron-accepting building block, which can be employed in the synthesis of ionic small molecules and conjugated polyelectrolytes (CPEs). Electrochemical measurements alongside theoretical calculations indicate notably low-lying LUMO values for the iAQMs. The optical band gaps measured for these compounds are highly tunable based on structure, ranging from 2.30 eV in small molecules down to 1.22 eV in polymers. The iAQM small molecules and CPEs showcase the band gap reduction effects of combining the donor-acceptor strategy with the bond-length alternation reduction strategy. As a demonstration of their utility, the iAQM CPEs so generated were used as active agents in photothermal therapy.

Simple structured color tunable white organic light-emitting diodes utilizing an ambipolar anthracene derivative with low-lying LUMO

Abstract We demonstrated a simple structured color tunable white organic light-emitting diode (WOLED) with promising color rendering capability. The white light, comes from complementary colors, is contributed from a newly developed anthracene-based derivative doped with 0.5 w.t.% Ir(ppy)3, the exciplex emission form at the interface between hole-transporting layer and emission layer, and the electromer from the hole-transporting layer. By using the ambipolar carrier-transporting anthracene derivative with low-lying LUMO as the emission layer, the emission color of the WOLED can be easily adjusted by varying the applied voltage. The proposed WOLED has great advantages of easy fabrication and low dopant consumption. The correlated color temperature (CCT) of the WOLED is tunable from 2621 K to 4533 K with color rendering index ranging from 80 to 84.

Isothianaphthene diimide: an air-stable n-type semiconductor

Herein, we propose a new strategy to develop air-stable n-type organic semiconductors with non-classical thiophene aromatic diimide derivatives by replacing aromatic naphthalene with a heteroaromatic isothianaphthene core. We designed and successfully synthesized the isothianaphthene core based diimide material, N,N ′-bis( n -hexyl)isothianaphthene-2,3,6,7-tetracarboxylic acid diimide ( BTDI-C6 ) as an n-type semiconductor. Compared to N,N ′-bis( n -hexyl)naphthalene-1,4,5,8-tetracarboxylic acid diimide ( NDI-C6 ), BTDI-C6 possesses a deeper LUMO energy level of −4.21 eV, which is 0.32 eV lower than that of NDI-C6 . Both molecular modelling and experimental results elucidated that organic thin film transistors (OTFTs) based on both of these materials exhibit comparable mobilities; however, the threshold voltage of BTDI-C6 based device (+7.5 V) is significantly lower than that of NDI-C6 based counterpart (+34 V). Moreover, the low-lying LUMO energy level of BTDI-C6 ensures excellent air-stability which is further validated by the device performance. In addition, BTDI-C6 shows high luminescence while NDI-C6 is not luminescent at all in solution, which reveals the potential application of our newly synthesized material in n-type light-emitting transistors.

An A−D−Aʹ−Dʹ strategy enables perylenediimide-based polymer dyes exhibiting enhanced electron transport characteristics

In this work, we develop two new A−D−Aʹ−Dʹ type polymer dyes, namely PPDIDTBT and PPDIDTBT-2F, containing perylenediimide (PDI), thiophene, and benzothiadiazole units. Both polymer dyes have high thermal stability and broad absorption throughout the UV–vis region. More importantly, the introduction of the second electron-withdrawing units, benzothiadiazoles, endows both polymer dyes with low-lying LUMO energy levels of −3.85 eV for PPDIDTBT and −3.88 eV for PPDIDTBT-2F. The charge transport properties of the two polymer dyes were examined by fabricating organic field-effect transistors (OFETs) with top-gate/bottom-contact configuration. Both polymer dyes exhibit enhanced electron transport characteristics with the highest electron mobility of 0.070 cm2 V−1 s−1, which is among the highest values for PDI-based polymer semiconductors reported to date. Our results highlight that the A−D−Aʹ−Dʹ strategy is a useful approach in tuning orbital energetic and charge transport properties of polymer dyes for developing high-performance OFETs.

Low Bandgap Donor-Acceptor π-Conjugated Polymers From Diarylcyclopentadienone-Fused Naphthalimides.

Two novel aromatic imides, diarylcyclopentadienone-fused naphthalimides (BCPONI-2Br and TCPONI-2Br), are designed and synthesized by condensation coupling cyclopentadienone derivatives at the lateral position of naphthalimide skeleton. It has been found that BCPONI-2Br and TCPONI-2Br are highly electron-withdrawing acceptor moieties, which possess broad absorption bands and very low-lying LUMO energy levels, as low as −4.02 eV. On the basis of both building blocks, six low bandgap D-A copolymers (P1–P6) are prepared via Suzuki or Stille coupling reactions. The optical and electrochemical properties of the polymers are fine-tuned by the variations of donors (carbazole, benzodithiophene, and dithienopyrrole) and π-conjugation linkers (thiophene and benzene). All polymers exhibit several attractive photophysical and electrochemical properties, i.e., broad near-infrared (NIR) absorption, deep-lying LUMO levels (between −3.88 and −3.76 eV), and a very small optical bandgap as low as 0.81 eV, which represents the first aromatic diimide-based polymer with an of <1.0 eV. An investigation of charge carrier transport properties shows that P5 exhibits a moderately high hole mobility of 0.02 cm2 V−1 s−1 in bottom-gate field-effect transistors (FETs) and a typical ambipolar transport behavior in top-gate FETs. The findings suggest that BCPONI-2Br, TCPONI-2Br, and the other similar acceptor units are promising building blocks for novel organic semiconductors with outstanding NIR activity, high electron affinity, and low bandgap, which can be extended to various next-generation optoelectronic devices.

Dicyano-substituted 2,3-naphthalimide: Synthesis and optoelectronic properties

Organic semiconductors (OSCs) have been attracted intensive academic and commercial interest due to their intriguing optoelectronic properties and potential applications for electronics. However, the development of n-type OSCs has significantly lagged behind that of p-type counterparts. One general approach towards n-type OSCs is to attach electron-withdrawing cyano or/and imide groups onto the typical p-type OSCs. Here we report the synthesis and optoelectronic properties of three dicyano-substituted 2,3-naphthalimide derivatives. Their low-lying LUMO energy levels from −3.29 to −3.37 eV made them potential candidates for n-type OSCs.

(Invited) Synthesis of Benzothieno[60]Fullerenes through Fullerenyl Cation Intermediates

Benzothieno[60]fullerenes were synthesized by using fullerenyl cation key intermediates. The reaction proceeded through a nucleophilic attack of the sulfur atom as a weak nucleophile to the fullerenyl cation electrophile. A monoarylated fullerene, (2-methylthiophenyl)hydro[60]fullerene, C60ArH (Ar = C6H4-SMe-2, and so on; four derivatives) was subjected to deprotonation with KO t Bu to form a fullerenyl anion ArC60 –, followed by oxidation using I2to generate a fullerenyl cation ArC60 + leading to intramolecular demethylative cyclization via fullerene cation–S interaction to the product. Electrochemical study elucidated low-lying LUMO level of this compound. Computational calculation revealed high-lying HOMO level due to the fused thieno moiety, giving relatively narrower band gap than usual fullerene derivatives.

Arylacetylene end capped naphthodithiophene diimide (NDTI)-based semiconductors for air-stable, solution-processed n-channel organic field-effect transistors: Effect of terminal aryl groups on charge transport

Solution-processable n-channel organic field-effect transistors (OFETs) are crucial for realizing low-cost, large-area, and all organic logic/complementary circuits. Herein, we present the synthesis, characterization and OFET characteristics of a series of arylacetylene end capped NDTI-based n-channel small molecule semiconductors with four different arylacetylene groups, i.e., thiophenylethynyl (TE, NDTI-BTE), phenylethynyl (PE, NDTI-BPE), 2-pyridinylethynyl (2PdE, NDTI-B2PdE), 4-pyridinylethynyl (4PdE, NDTI-B4PdE). All four NDTIs exhibit excellent solubility behavior in common organic solvents. Electrochemical and optical studies reveal that the introduction of the arylacetylene groups at the thiophene α-positions of NDTI endowed these molecules have sufficiently low-lying LUMO energy levels from −4.12 to −4.24 eV, suitable for better air-stable electron transport. Under ambient atmosphere, we found that solution-processed bottom-gate top-contact (BGTC) transistors based on 120 °C-annealed thin-films of these NDTIs exhibit the best performance with the maximum electron mobilities of 0.049, 0.023, 0.011 and 0.033 cm2 V−1 s−1 for NDTI-BTE, NDTI-BPE, NDTI-B2PdE and NDTI-B4PdE, respectively, due to better thin film microstructures. AFM measurements reveal that the spin-coated films of these materials exhibited quite smooth and uniform morphology although these are small molecules forming crystalline structures. These results clearly demonstrate that arylacetylene groups are potential functional moieties for developing ambient-stable solution-processable small molecular electron-transporting materials for printable organic electronics.

Conjugated Macrocycles in Organic Electronics

This Account describes a body of research on the design, synthesis, and application of a new class of electronic materials made from conjugated macrocycles. Our macrocyclic design takes into consideration the useful attributes of fullerenes and what properties make fullerenes efficient n-type materials. We identified four electronic and structural elements: (1) a three-dimensional shape; (2) a conjugated and delocalized π-space; (3) the presence of an interior and exterior to the π-surface; and (4) low-energy unoccupied molecular orbitals allowing them to accept electrons. The macrocyclic design incorporates some of these properties, including a three-dimensional shape, an interior/exterior to the π-surface, and low-lying LUMOs maintaining the n-type semiconducting behavior, yet we also install synthetic flexibility in our approach in order to tune the properties further. Each of the macrocycles comprises perylenediimide cores wound together with linkers. The perylenediimide building block endows each macrocycle with the ability to accept electrons, while the synthetic flexibility to install different linkers allows us to create macrocycles with different electronic properties and sizes. We have created three macrocycles that all absorb well into the visible range of the solar spectrum and possess different shapes and sizes. We then use these materials in an array of applications that take advantage of their ability to function as n-type semiconductors, absorb in the visible range of the solar spectrum, and possess intramolecular cavities. This Account will discuss our progress in incorporating these new macrocycles in organic solar cells, organic photodetectors, organic field effect transistors, and sensors. The macrocycles outperform acyclic controls in organic solar cells. We find the more rigid macrocyclic structure results in less intrinsic charges and lower dark current in organic photodetectors. Our macrocyclic-based photodetector has the highest detectivity of non-fullerene acceptors. The macrocycles also function as sensors and are able to recognize nuanced differences in analytes. Perylenediimide-based fused oligomers are efficient materials in both organic solar cells and field effect transistors. We will use the oligomers to construct macrocycles for use in solar energy conversion. In addition, we will incorporate different electron-rich linkers in our cycles in an attempt to engineer the HOMO/LUMO gap further. Looking further into the future, we envision opportunities in applying these conjugated macrocycles as electronic host/guest materials, as concatenated electronic materials by threading the macrocycles with electroactive oligomers, and as a locus for catalysis that is driven by light and electric fields.

Azo-MICs: Redox-Active Mesoionic Carbene Ligands Derived from Azoimidazolium Dyes.

Azoimidazolium dyes were used as precursors for mesoionic carbene ligands (Azo-MICs). The properties of these ligands were examined by synthesizing RhI , AuI , and PdII complexes. Experimental (NMR, IR) and theoretical investigations show that Azo-MICs are potent σ-donor ligands. Yet, they feature a small singlet-triplet gap and very low-lying LUMO levels. The unique electronic properties of Azo-MICs allow for reversible one-electron reductions of the metal complexes, as evidenced by cyclic voltammetry.

Highly efficient and very robust blue-excitable yellow phosphors built on multiple-stranded one-dimensional inorganic-organic hybrid chains.

Inorganic-organic hybrid semiconductors are promising candidates for energy-related applications. Here, we have developed a unique class of multiple-stranded one-dimensional (1D) structures as very robust and efficient lighting phosphors. Following a systematic ligand design strategy, these structures are constructed by forming multiple coordination bonds between adjacent copper iodide inorganic building units Cu m I m (m = 2, 4, 6) (e.g. dimer, tetramer and hexamer clusters) and strong-binding bidentate organic ligands with low LUMO energies which give rise to infinite 1D chains of high stability and low bandgaps. The significantly enhanced thermal/photostability of these multiple-stranded chain structures is largely attributed to the multi-dentate nature and enhanced Cu-N bonding, and their excellent blue excitability is a result of using benzotriazole based ligands with low-lying LUMO energies. These facts are confirmed by Density Functional Theory (DFT) calculations. The luminescence mechanism of these compounds is studied by temperature dependent photoluminescence experiments. High internal quantum yields (IQYs) are achieved under blue excitation, marking the highest value reported so far for crystalline inorganic-organic hybrid yellow phosphors. Excellent thermal- and photo-stability, coupled with high luminescence efficiency, make this class of materials promising candidates for use as rare-earth element (REE) free phosphors in energy efficient general lighting devices.

Isolation of Tetracyano-Naphthalenediimide and Its Stable Planar Radical Anion.

Reported herein is the first isolation of tetracyano-naphthalenediimide [NDI(CN)4 ] and its radical anion, and structural elucidation through spectroscopic and X-ray diffraction studies. The radical anion shows remarkable stability and was purified by chromatography, which is unique for planar radical anions. The stability results from multiple hydrogen bonds to the counter ion and through an array of intramolecular noncovalent interactions. The radical anion revealed one of the strongest NDI π-π interactions (3.268 Å). Electrochemical studies of [NDI(CN)4 ] confirm its extraordinarily low-lying LUMO (-5.0 eV), rendering it one of the strongest electron-deficient planar π systems to be isolated. The manifold potential, which remained unknown to date, can now be explored for these open- and closed-shell planar π systems.

The Twisted Benzo[ghi]-Perylenetriimide Dimer as a 3D Electron Acceptor for Fullerene-Free Organic Photovoltaics.

A series of twisted N,N-linked benzo[ghi]-perylenetriimide dimers (t-BPTI) with various lengths of the α-branched alkyl side chain at the six-membered imide ring position was designed, synthesized, and characterized. These compounds showed the low-lying LUMO energy level of -3.78 eV, which was similar to that of PC61 BM (-3.71 eV), but with intensive optical absorption in the range 350-500 nm. The twisted molecular geometry with two nearly perpendicular BPTI planes achieved a favorable nanoscale phase separation by relieving the self-aggregation of rigid BPTI units in blend films. The acceptor t-BPTI-3 unit with the longest alkyl side chains has been demonstrated to be an efficient electron acceptor in solution-processed bulk heterojunction organic photovoltaics (OPV), giving a power conversion efficiency of 3.68 % when using conjugated polymer PTB7-Th as the donor and without additional treatments.

Optical materials: Studying the role of hetropolyacid to enhance the nonlinear optical responses of porphyrin in their hybrids system

Nonlinear optical responses are measured to investigate the role of hetropolyacid (HPA) in porphyrin-Lindqvist type {∼PM@HPA} hybrid systems, {where PM@HPA = [(TBA)8{(MTPPN4)(Mo6O18)4}]; M = 2H (1) and Mn (2)}. The well-known Z-scan technique with wavelength (λ) = 532 nm and pulse duration of τ = 6 ns is used. The results clearly show the significant enhancement in nonlinear self-defocusing effect and reverse saturable absorption under closed and open-aperture configuration for PM@HPA hybrids. It is also observed that the second hyperpolarizability γ values, {1 (4.639 × 10−29 esu) < 2 (5.214 × 10−29 esu)}, are directly proportional to the discrepancy of LUMO level (ΔE) values {1 (2.214 eV) < 2 (2.339 eV)} while they are inversely proportional to the HOMO−LUMO energy gap Eg values {1 (2.003 eV) > 2 (1.862 eV)} in the PM@HPA hybrid system where the porphyrin moiety acts as electron donor whilst HPA acts as electron acceptor. Therefore, the emergence of a facile electron transformation from the porphyrin moieties to the low-lying LUMO levels of HPA is mainly responsible for the enhancement in nonlinear optical responses of PM@HPA hybrids system. As a consequence the hybrid compounds can be fascinating for optical limiting applications.

NIR-Absorbing Dye Based on BF2-Bridged Azafulvene Dimer as a Strong Electron-Accepting Unit.

BF2-bridged azafulvene dimers designed to be strong electron-accepting units were selectively synthesized using a bulky base. Single-crystal X-ray diffraction analysis revealed that the high electron-accepting ability of this structure stems from the contribution of the π-conjugation mode of the azafulvene dimer upon formation of B-N coordination bonds. As a result of the low-lying LUMO energy of this electron-accepting unit, the corresponding D-A-D dye exhibits an intense NIR absorption band at 922 nm, which tails up to 1150 nm, while significant absorption bands in the visible region are absent. As a NIR dye this molecule exhibits moreover exceptional photostability and resistance to oxidation by atmospheric oxygen, even in dilute solution.

Toward Two-Dimensional π-Conjugated Covalent Organic Radical Frameworks.

Reported is the synthesis, characterization, and material properties of the first π-conjugated two-dimensional covalent organic radical framework (CORF), PTM-CORF, based on the stable polychlorotriphenylmethyl (PTM) radical. The covalent organic framework (COF) precursor (PTM-H-COF) was first synthesized by liquid/liquid interfacial acetylenic homocoupling of a triethynylpolychlorotriphenylmethane monomer, and showed crystalline features with a hexagonal diffraction pattern matching that of A-B-C stacking. Subsequent deprotonation and oxidation of the PTM units in PTM-H-COF gave PTM-CORF. Magnetic measurements revealed that the neighboring PTM radicals in the PTM-CORF are anti-ferromagnetically coupled each other, with a moderate exchange interaction (J=-375 cm-1 ). The PTM-CORF has a small energy gap (ca. 0.88 eV) and a low-lying LUMO energy level (-4.72 eV), and exhibits high electrocatalytic activity and durability toward the oxygen reduction reaction.

Peripherally Cyanated Subphthalocyanines as Potential n-Type Organic Semiconductors.

A series of peripherally dicyano-, tricyano-, and tetracyano-substituted subphthalocyanines (SubPcs) have been prepared through microwave-assisted, palladium-mediated cyanation of iodinated precursors. The introduction of π-accepting cyano groups in the macrocycle clearly influences its electronic and redox properties, which are dependent on the number and relative position of these substituents. Additional functionalization of the periphery of SubPcs with electron-donating or -withdrawing groups allows for a further fine-tuning of their features, leading to intensely absorbing, strongly electron-accepting panchromatic dyes with low-lying LUMO energy levels. Flash-photolysis time-resolved microwave conductivity measurements on vapor-deposited films demonstrate that some of these novel SubPc derivatives display remarkable intrinsic charge-carrier mobilities that are comparable to or larger than those of other known well-performing acceptor SubPcs; thus confirming their potential as n-type organic semiconductors for application in the fabrication of photovoltaic devices.

Reduction of nitroarenes, azoarenes and hydrazine derivatives by an organic super electron donor

Reduction of nitrobenzene by excess organic electron donor, 12, affords diphenylhydrazine in a reaction where azobenzene oxide and azobenzene are likely intermediates. No cleavage of the N-N σ-bond is seen under photoactivation conditions, whereas traces are seen under thermal activation. Hydrazone derivatives were prepared to explore the cleavage of N-N σ-bonds; the results show that a low-lying LUMO assists the transition state for accepting an electron, and the stabilisation that the potential fragments from N-N bond cleavage afford to the fragments is important in determining whether cleavage is observed.