Small HOMO-LUMO Gap Research Articles

A Divalent Pentastable Redox‐Switchable Donor–Acceptor Rotaxane

Donor-acceptor materials with small HOMO-LUMO gaps are important in molecular electronics, but are often difficult to synthesise. A simple and efficient way to position tetrathiafulvalene (TTF) as the donor and naphthalene diamide (NDI) as the acceptor in close proximity to each other in a divalent crown/ammonium pseudo[2]rotaxane is presented. The divalent design provides high chelate cooperativity and much stronger binding compared with a monovalent analogue. The pseudo[2]rotaxane was then doubly interlocked by stoppering it in a catalyst-free 1,3-dipolar cycloaddition. UV/Vis and cyclic voltammetry experiments with the resulting [2]rotaxane revealed the optoelectronic properties of an intramolecular charge transfer with a small HOMO-LUMO energy gap. Redox-switching experiments showed the rotaxane to be pentastable. DFT calculations provided insights into the electronic structures of the five redox states.

The Power of Packing: Metallization of an Organic Semiconductor.

Benzoquino-bis-1,2,3-dithiazole 5 is a closed shell, antiaromatic 16π-electron zwitterion with a small HOMO-LUMO gap. Its crystal structure consists of planar ribbon-like molecular arrays packed into offset layers to generate a "brick-wall" motif with strong 2D interlayer electronic interactions. The spread of the valence and conduction bands, coupled with the narrow HOMO-LUMO gap, affords a small band gap semiconductor with σRT = 1 × 10-3 S cm-1 and Eact = 0.14 eV for transport within the brick-wall arrays. Closure of the band gap to form an all-organic molecular metal with σRT > 101 S cm-1 can be achieved by the application of pressure to 8 GPa.

Extended germa[N]pericyclynes: synthesis and characterization.

We herein describe the syntheses and characterization of extended germa[N]pericyclynes, which are macrocycles composed of germanium-butadiyne units. The obtained novel extended germa[4]-[8]pericyclynes were characterized by X-ray crystallography, UV-Vis spectroscopy, fluorescence and phosphorescence emission spectroscopy, and cyclic voltammetry, and exhibited characteristic absorptions and emissions. Density functional theory (DFT) calculations suggested smaller HOMO-LUMO gap energy compared to that of general germapericyclynes.

Size dependent electronic and magnetic properties of ultra thin graphene nanoribbons

We present the results of systematic investigation into size dependent electronic and magnetic properties of ultra thin graphene nanoribbons (GNRs) of finite and infinite length using spin polarized density functional calculation. The rectangular GNRs with width up to 1.40 nm with finite (2.50 nm) and infinite length were investigated. The ultra thin GNRs are found less stable in comparison to larger GNRs with binding energy increasing with increase in the size. All GNRs have shown finite HOMO-LUMO gap which decreases oscillatory as a function of width. HOMO-LUMO gap in the Finite length armchair GNRs is in range of 0.01–0.36 eV, where as in infinite length GNR the energy gap is in the range of 0.30–1.41 eV. Zigzag GNRs have shown very small HOMO-LUMO gap in the range 50–80 meV. The zigzag GNR have shown opening of energy band gap. However, for N = 8 and 10 layers, GNRs with zigzag edges are found to be ferromagnetic. The edge C atom on zigzag edges contribute magnetic moment of 0.94 per C atom with total magnetic moment remaining constant with increase in the width. The energy difference between ferromagnetic and anti-ferromagnetic state decreases sharply with the increase in GNR width suggesting iso-energetic behavior in larger GNRs. The results are consistent with the reported experimental results.

An Azulene-Fused Tetracene Diimide with a Small HOMO-LUMO Gap.

A newly prepared tetraazulene-fused tetracene diimide (TA-fused TDI) showed absorption in the near-IR region owing to the effective extension of the π-conjugated system as well as a large two-photon absorption cross-section (σ(2) =2140 GM) at 950 nm. Four reversible reduction processes and n-type semiconductivity were also confirmed as attractive electronic properties of this compound.

A Systematic Study of Structure and E-H Bond Activation Chemistry by Sterically Encumbered Germylene Complexes.

A series of new germylene compounds has been synthesized offering systematic variation in the σ- and π-capabilities of the α-substituent and differing levels of reactivity towards E-H bond activation (E=H, B, C, N, Si, Ge). Chloride metathesis utilizing [(terphenyl)GeCl] proves to be an effective synthetic route to complexes of the type [(terphenyl)Ge(ERn )] (1-6: ERn =NHDipp, CH(SiMe3 )2 , P(SiMe3 )2 , Si(SiMe3 )3 or B(NDippCH)2 ; terphenyl=C6 H3 Mes2 -2,6=Ar(Mes) or C6 H3 Dipp2 -2,6=Ar(Dipp) ; Dipp=C6 H3 iPr2 -2,6, Mes=C6 H2 Me3 -2,4,6), while the related complex [{(Me3 Si)2 N}Ge{B(NDippCH)2 }] (8) can be accessed by an amide/boryl exchange route. Metrical parameters have been probed by X-ray crystallography, and are consistent with widening angles at the metal centre as more bulky and/or more electropositive substituents are employed. Thus, the widest germylene units (θ>110°) are found to be associated with strongly σ-donating boryl or silyl ancillary donors. HOMO-LUMO gaps for the new germylene complexes have been appraised by DFT calculations. The aryl(boryl)-germylene system [Ar(Mes) Ge{B(NDippCH)2 }] (6-Mes), which features a wide C-Ge-B angle (110.4(1)°) and (albeit relatively weak) ancillary π-acceptor capabilities, has the smallest HOMO-LUMO gap (119 kJ mol(-1) ). These features result in 6-Mes being remarkably reactive, undergoing facile intramolecular C-H activation involving one of the mesityl ortho-methyl groups. The related aryl(silyl)-germylene system, [Ar(Mes) Ge{Si(SiMe3 )3 }] (5-Mes) has a marginally wider HOMO-LUMO gap (134 kJ mol(-1) ), rendering it less labile towards decomposition, yet reactive enough to oxidatively cleave H2 and NH3 to give the corresponding dihydride and (amido)hydride. Mixed aryl/alkyl, aryl/amido and aryl/phosphido complexes are unreactive, but amido/boryl complex 8 is competent for the activation of E-H bonds (E=H, B, Si) to give hydrido, boryl and silyl products. The results of these reactivity studies imply that the use of the very strongly σ-donating boryl or silyl substituents is an effective strategy for rendering metallylene complexes competent for E-H bond activation.

β-to-β 2,5-Pyrrolylene-Linked Cyclic Porphyrin Oligomers.

β-to-β 2,5-Pyrrolylene linked cyclic porphyrin oligomers have been synthesized by Suzuki-Miyaura coupling of 2,5-diborylpyrrole and 3,7-dibromoporphyrin. The cyclic porphyrin oligomers exhibit roughly coplanar structures, strong excitonic coupling, small electrochemical HOMO-LUMO gaps, and ultrafast excitation energy transfer between the neighboring porphyrins via the pyrrolylene bridge.

Mechanistic Insight into the Intramolecular Benzylic C-H Nitrene Insertion Catalyzed by Bimetallic Paddlewheel Complexes: Influence of the Metal Centers.

The intramolecular benzylic C-H amination catalyzed by bimetallic paddlewheel complexes was investigated by using density functional theory calculations. The metal-metal bonding characters were investigated and the structures featuring either a small HOMO-LUMO gap or a compact SOMO energy scope were estimated to facilitate an easier one-electron oxidation of the bimetallic center. The hydrogen-abstraction step was found to occur through three manners, that is, hydride transfer, hydrogen migration, and proton transfer. The imido N species are more preferred in the Ru-Ru and Pd-Mn cases whereas coexisting N species, namely, singlet/triplet nitrene and imido, were observed in the Rh-Rh and Pd-Co cases. On the other hand, the triplet nitrene N species were found to be predominant in the Pd-Ni and Pd-Zn systems. A concerted asynchronous mechanism was found to be modestly favorable in the Rh-Rh-catalyzed reactions whereas the Pd-Co-catalyzed reactions demonstrated a slight preference for a stepwise pathway. Favored stepwise pathways were seen in each Ru-Ru- and Pd-Mn-catalyzed reactions and in the triplet nitrene involved Pd-Ni and Pd-Zn reactions. The calculations suggest the feasibility of the Pd-Mn, Pd-Co, and Pd-Ni paddlewheel complexes as being economical alternatives for the expensive dirhodium/diruthenium complexes in C-H amination catalysis.

Structures and structure-related electronic properties of new C60(CF3)10 isomers

Abstract The largest collection of well-characterized homoleptic fullerene derivatives C60(X)n is for X = CF3 and n = 2–18. The largest subset of these with a given composition is the set of 27 isomers of C60(CF3)10. The X-ray structures of five of them were reported previously. In this paper we report the X-ray structures of, and extensive DFT calculations on, eight additional C60(CF3)10 isomers and the electrochemical reduction potentials of eight of the (now) 13 structurally-characterized isomers. The connections between their molecular structures (i.e., their addition patterns), the positions, extents of delocalization, and relative energies of their DFT-predicted LUMOs, and their first reduction potentials are compared and contrasted with each other and with the previously reported isomers. It is shown that addition patterns that produce the greatest number of non-terminal double bonds in fullerene pentagons are the best electron acceptors and tend to exhibit relatively small HOMO–LUMO gaps. A detailed analysis of the nearest-neighbor packing patterns in the 13 X-ray structures reveals significant differences in the number of nearest neighbor fullerene molecules and differences in the ranges of fullerene centroid⋯centroid distances even when the number of nearest neighbors is constant. Nevertheless, ten solvent-free structures exhibit nearly equally-efficient lattice packing arrays as far as crystal density is concerned. When the addition pattern results in two sterically-accessible C(sp2)6 hexagons on the fullerene surface, a one-dimensional chain of electronically-coupled nearest-neighbor C60(CF3)10 molecules is formed.

Optimized geometry, vibration (IR and Raman) spectra and nonlinear optical activity of p-nitroanilinium perchlorate molecule: A theoretical study

AbstractThe molecular modeling of p-nitroanilinium perchlorate molecule was carried out by using B3LYP and HSEH1PBE levels of density functional theory (DFT). The IR and Raman spectra were simulated and the assignments of vibrational modes were performed on the basis of relative contribution of various internal co-ordinates. NBO analysis was performed to demonstrate charge transfer, conjugative interactions and the formation of intramolecular hydrogen bonding interactions within PNAPC. Obtained large dipole moment values showed that PNAPC is a highly polarizable complex, and the charge transfer occurs within PNAPC. Hydrogen bonding and charge transfer interactions were also displayed by small HOMO-LUMO gap and molecular electrostatic potential (MEP) surface. The strong evidences that the material can be used as an efficient nonlinear optical (NLO) material of PNAPC were demonstrated by considerable polarizability and hyperpolarizability values obtained at DFT levels.

Conformational Fixation of a Rectangular Antiaromatic [28]Hexaphyrin Using Rationally Installed Peripheral Straps.

A rectangular [28]hexaphyrin bearing outer straps at the long side has been synthesized by SN Ar reaction of [26]hexaphyrin with allyl alcohol, intramolecular olefin metathesis by using Hoveyda-Grubbs second-generation catalyst, and reduction with NaBH4 . The peripheral straps enforce a rectangular conformation for the [28]hexaphyrin, which shows Hückel antiaromatic character, as confirmed by its planar X-ray structure, a strong paratropic ring current, characteristic UV/Vis/NIR absorption features, small electrochemical HOMO-LUMO gap, and very fast S1 decay.

Computational insights into intriguing vibration-induced pulsing diradical character in perfluoropentacene and the perfluorination effect.

As an n-type organic semiconductor compound, perfluoropentacene has more widespread applications in organic electronics because of its higher electron mobility compared with its parent pentacene. Herein, we explore intriguing dynamic electronic properties of perfluoropentacene caused by structural vibrations using density functional theory calculations. Perfluoropentacene could exhibit diradical character because of the persistent vibrations, although it belongs to a closed-shell singlet molecule in its equilibrium configuration. Not all the vibration-induced structural changes can induce diradical character, but only those leading to a small singlet-triplet energy gap, especially the small HOMO-LUMO gap, as well as the short cross-linking C-C bonds and distorted carbon ring structures in polyacetylene chains make great contributions. Due to molecular vibrations, the diradical character of dynamic perfluoropentacene exhibits pulsing behavior. Compared with pentacene, its perfluorination can not only considerably stabilize two frontier molecular orbitals, but also reduce the HOMO-LUMO gap, thus leading to an increase of the number of vibrational modes which can make the diradical character appear. In particular, perfluorination makes 19 diradical vibrational modes appear in the low frequency region. These observations indicate that some low energy pulses can trigger perfluoropentacene molecular vibrations according to some low energy modes and thus the appearance of pulsing diradical character or molecular magnetism. Clearly, the observed novel characters of a molecule possessing hidden pulsing diradical character and tunable magnetism in this work would contribute to opening up promising areas for designing peculiar magnetic materials.

Synthesis and Electronic, Optical, and Electrochemical Properties of a Series of Tetracyanobutadiene‐Substituted Carbazoles

AbstractA series of tetracyanobutadiene (TCBD)‐substituted carbazole chromophores were synthesized by formal [2+2] cycloaddition between tetracyanoethylene (TCNE) and carbazole derivatives containing an acetylenic moiety, followed by retro‐electrocyclization. The reactivity of the carbazole varied depending on the conjugation connectivity between the carbazole and acetylenic moieties, the number of the carbazole moieties, and the length of the acetylenic moieties. All of the products feature intramolecular charge‐transfer bands in the visible region. TCBD substitution at the 1‐position of the carbazole gave rise to small HOMO–LUMO gaps compared to that at the 2‐ and 3‐positions. The relationship between the TCBD substitution pattern of the carbazole moiety and the effectiveness of donor–acceptor π‐conjugation was evaluated, and it was established that the π‐conjugation was effective in order of positions 1<2<3.

Exploiting Electrostatics To Generate Unsaturation: Oxidative GeE Bond Formation Using a Non π‐Donor Stabilized [R(L)Ge:]+ Cation

AbstractThe two‐coordinate germanium cation [(IDipp){(Me3Si)2CH}Ge:]+ has been synthesized, which lacks π‐donor stabilization of the metal center and consequently has a very small HOMO–LUMO gap (187 kJ mol−1). It undergoes a variety of facile oxidative bond‐forming reactions, most notably allowing access to the first examples of Group 14 metal cations containing ME multiple bonds (E=C, N). The use of an electrostatic (rather than purely steric) strategy to discourage aggregation means that less bulky systems (for example, containing a primary alkylidene fragment, CHR) are accessible.

Exploiting Electrostatics To Generate Unsaturation: Oxidative Ge=E Bond Formation Using a Non π-Donor Stabilized [R(L)Ge:](+) Cation.

The two-coordinate germanium cation [(IDipp){(Me3Si)2CH}Ge:](+) has been synthesized, which lacks π-donor stabilization of the metal center and consequently has a very small HOMO-LUMO gap (187 kJ mol(-1)). It undergoes a variety of facile oxidative bond-forming reactions, most notably allowing access to the first examples of Group 14 metal cations containing M=E multiple bonds (E = C, N). The use of an electrostatic (rather than purely steric) strategy to discourage aggregation means that less bulky systems (for example, containing a primary alkylidene fragment, =CHR) are accessible.

A Convenient N-Arylation Route for Electron-Deficient Pyridines: The Case of π-Extended Electrochromic Phosphaviologens.

A simple and representative procedure for the synthesis of N,N'-diarylated phosphaviologens directly from both electron-rich and electron-poor diaryliodonium salts and 2,7-diazadibenzophosphole oxide is reported. The latter are electron-deficient congeners of the widely utilized N,N'-disubstituted 4,4'-bipyridinium cations, also known as viologens, that proved to be inaccessible by the classical two-step route. The single-step preparation method for phosphaviologens described herein could be extended to genuine viologens but reached its limit when sterically demanding diaryliodonium salts were used. The studied phosphaviologens feature a significantly lowered reduction threshold as compared to all other (phospha)viologens known to date due to the combination of an extended π-system with an electron deficient phosphole core. In addition, a considerably smaller HOMO-LUMO gap was observed due to efficient π-delocalization across the phosphaviologen core, as well as the N-aryl substituents, which was corroborated by quantum chemical calculations. Detailed characterizations of the singly reduced radical species by EPR spectroscopy and DFT calculations verified delocalization of the radical over the extended π-system. Finally, to gain deeper insight into the suitability of the new compounds as electroactive and electrochromic materials, multicolored proof-of-concept electrochomic devices were manufactured.

Synthesis and Divergent Electronic Properties of Two Ring-Fused Derivatives of 9,10-Diphenylanthracene.

Two new contorted polycyclic aromatic hydrocarbons (PAHs) 1 and 2 were synthesized by acid-catalyzed benzannulation of a substituted anthracene. The isomers reveal dissimilar photophysical and redox properties with 2 having a much smaller HOMO-LUMO gap than 1. In the solid state, 2 packs in a unique two-dimensional herringbone motif that gives rise to efficient ambipolar charge transport in OFET devices, a feature not previously observed in contorted PAHs. On the other hand, 1 packs in one-dimensional dimerized π-stacks and displays insulating properties.

Improving DIIS convergence for metallic systems using Gaussian basis set

Self-consistent calculations on metallic systems in Gaussian basis sets are likely to encounter convergence problems because of the long-wavelength charge sloshing. Using a simple formula for the charge response of the Fock matrix, a correction to DIIS have been derived to suppress sloshing effects. The proposed method is similar in spirit to the Kerker preconditioner in band-structure calculations. The new method is applied to systems like Ru4(CO), Pt13, Pt55 and (TiO2)24 and the convergence is compared to previously available DIIS methods. For systems with small HOMO–LUMO gap, this new method manages to converge while previous DIIS methods do not.

A Triphenylamine with Two Phenoxy Radicals Having Unusual Bonding Patterns and a Closed-Shell Electronic State.

Reported herein is the structure and the electronic properties of a novel triphenylamine derivative having two phenoxy radicals appended to the amino nitrogen atom. X-ray single crystal analysis and the magnetic resonance measurements demonstrates the unexpected closed-shell electronic structure, even at room temperature, of the molecule and two unusual C-N bonds with multiple-bond character. The theoretical calculations support the experimentally determined molecular geometry with the closed-shell electronic structure, and predicted a small HOMO-LUMO gap originating from the nonbonding character of the HOMO. The optical and electrochemical measurements show that the molecule has a remarkably small HOMO-LUMO gap compared with its triphenylamine precursor.

A Triphenylamine with Two Phenoxy Radicals Having Unusual Bonding Patterns and a Closed‐Shell Electronic State

AbstractReported herein is the structure and the electronic properties of a novel triphenylamine derivative having two phenoxy radicals appended to the amino nitrogen atom. X‐ray single crystal analysis and the magnetic resonance measurements demonstrates the unexpected closed‐shell electronic structure, even at room temperature, of the molecule and two unusual CN bonds with multiple‐bond character. The theoretical calculations support the experimentally determined molecular geometry with the closed‐shell electronic structure, and predicted a small HOMO–LUMO gap originating from the nonbonding character of the HOMO. The optical and electrochemical measurements show that the molecule has a remarkably small HOMO–LUMO gap compared with its triphenylamine precursor.