Low HOMO-LUMO Gap Research Articles

Electronic, magnetic and optical properties of Cu, Ag, Au-doped Si clusters

The structural, optical and magnetic properties of Cu, Ag, Au-doped Si7 Clusters have been systematically investigated using density functional theory calculations. The global optimized structures of Cu, Ag, Au-doped Si clusters are predicted to have a lower HOMO-LUMO gap and higher magnetic moment. M-doping (M = Cu, Ag, Au) in Si cluster widens a range of adsorption wavelength, especially Au-doping. The characteristics in electronic density of states (DOSs) show that C5v-Si6Cu has a big asymmetrical spin-up and spin-down. The average atomic moment is 0.428 mμB per atom for the Si6Cu cluster with C5v symmetry, while the average paramagnetic moment is 0.143 mμB per atom for other M-doped (M = Cu, Ag, Au) Si7 clusters.

1,4‐Phenylene and 2,5‐Thienylene π‐Linkers in Charge‐Transfer Chromophores

AbstractFifteen model charge‐transfer chromophores with ferrocene donors, NO2 acceptors, and 1,4‐phenylene and 2,5‐thienylene π‐linkers were synthesized and their properties investigated by X‐ray analysis, electrochemistry, absorption spectra, and DFT calculations. Structure–property relationships were further evaluated to answer the question of suitability of the π‐linkers in the construction of chromophore backbone. Gradual replacement of 1,4‐phenylene with 2,5‐thienylene and π‐linker extension resulted in a lower HOMO–LUMO gap, a bathochromic shift of both charge‐transfer bands, and overall planarization of the molecule. Hence, thiophene, when incorporated into a chromophore backbone, is a more polarizable moiety that allows very efficient intramolecular charge transfer.

Back Cover: New Conjugated Oligothiophenes Containing the Unique Arrangement of Internal Adjacent [All]-S,S-Oxygenated Thiophene Fragments (Chem. Eur. J. 17/2013)

Oligothiophenes with inner sulfonated rings were synthesized by using HOF⋅CH3CN, which is easily prepared from diluted F2 and aqueous acetonitrile. This family of compounds exhibits more favorable properties, such as lower HOMO–LUMO gaps and higher reduction potentials, than other fully oxygenated, polyoxygenated, and nonoxygenated oligothiophenes. For more details, see the Full Paper by S. Rozen and S. Potash on p. 5289 ff.

Theoretical studies on the spectroscopic properties of a series of palladium (II) complexes with 1-allyl-3-(2-pyridyl)thiourea

To explore the spectroscopic properties, a series of palladium complexes are investigated theoretically through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Full geometry optimizations of 1-allyl-3-(2-pyridyl)thiourea (APTU) and its corresponding Pd (II) complexes [Pd(APTU)Cl2] (A), trans-[Pd(APTU)2]2+ (B), cis-[Pd(APTU)2]2+ (C), together with two thiolic tautomeric APTU-based Pd (II) complex cis-[Pd(APTU)2] (D) and trans-[Pd(APTU)2] (E) are carried out at the B3LYP, B3PW91, and M06 levels. All of them present slightly distorted square-planar geometries. On the basis of the optimized geometries, the absorption spectra in ethanol are evaluated by using three TD-DFT methods with the PCM model. Experimental absorption spectra are well reproduced by our time-dependent density functional theory calculations. Both UV–vis absorption peaks at 219.64 and 254.80nm for complex A are mainly attributed to (p,π,d)→π* with a mixed character of intraligand charge-transfer (ILCT)/ligand–ligand charge-transfer (LLCT)/metal-ligand charge-transfer (MLCT) transitions. Another weak absorption band of complex A is located at 313.17nm largely originated from HOMO−6 to LUMO transition. There are two isomers with small energy difference for [Pd(APTU)2]2+, i.e., trans-[Pd(APTU)2]2+ (B) and cis-[Pd(APTU)2]2+ (C). Only when both isomers are considered, four absorption peaks can be obtained, which is in line with the experimental results. The absorption peaks of complexes D and E present obvious red shift that is associated with their lower HOMO–LUMO gap (ΔE|HOMO–LUMO|) among five complexes. Differences and similarities in molecular orbitals and electronic transition characters for five complexes are discussed in detail.

Synthesis of π-Conjugated 2,2:6′,2″-Terpyridine-Substituted Oligomers Based on 3,4-Ethylenedioxythiophene

Dissymmetric π-conjugated monomers and oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) units and bearing terpyridine end groups were synthesized in good yields through Vilsmeyer-Haak formylation followed by a reaction with 2-acetylpyridine in basic media or, for the longest oligomers, direct C-H bond arylation. They have a low HOMO-LUMO gap and are easily oxidized at low potentials. Upon complexation with cobalt(II) and iron(II) they yield new hybrid materials that can be used in various applications ranging from photovoltaics to spintronics.

Electric field induced tuning of molecular conformation to acquire spintronics property in biphenyl systems

We have theoretically studied the effect of an external electric field on biphenyl, methylene substituted biphenyl monoradical and diradical systems. It has been found that the molecular conformation changes after the application of an external electric field. Thus, it is possible to tune molecular conformation reversibly only by external electrical stimuli. The HOMO–LUMO gap of the diradical reduces under an external electric field. The molecules with a low HOMO–LUMO gap can be used as unimolecular rectifiers if the HOMO and LUMO have a different spatial location on the molecule, or as a molecular conductor if the HOMO and LUMO are in the same spatial position. It has been observed that the diradical can be used as both a unimolecular rectifier and a molecular conductor depending upon the field strength. Now, the insulator molecule can be switched to a molecular rectifier or a molecular conductor and vice versa by an external electric field. The spin topology in a radical depends on the strength and direction of the applied electric field. Consequently, the extent of magnetic coupling in a diradical system changes with the external electric field. We have calculated the transmission spectra and I–V curve for the diradical at different dihedral angles. At a lower dihedral angle the diradical shows efficient spin filter behavior; within the range of bias voltage of 1 V and 1.5 V, the spin filtering efficiency of the diradical is achieved up to 99%. It is now possible to use a single molecule for multifarious applications like a unimolecular rectifier, spin filter and a molecular conductor depending upon the strength of the applied electric field.

Synthesis of nitro- and amino-functionalized π-conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) units

Dissymmetric π-conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) units and bearing nitro and amino end-groups were synthesized in good yields through Pd-catalyzed Suzuki coupling reactions and direct C–H bond arylation. Their spectroscopic properties show that they have a low HOMO–LUMO gap. They are easily oxidized at low potential and could be used in various applications ranging from photovoltaics to molecular electronics.

Density functional study on the adsorption of the drug isoniazid onto pristine and B-doped single wall carbon nanotubes

The current study explores a new strategy to incorporate single wall carbon nanotubes (SWNTs)/doped SWNTs as carrier modules in target-specific administration of antitubercular chemotherapeutics through covalent and noncovalent functionalization onto the nanotube sidewall. Density functional studies illustrate that noncovalent functionalization of isoniazid (INH) is preferred over covalent attachment, exhibiting low adsorption energy values, HOMO-LUMO gap and comparison of quantum molecular descriptors performed in (5,5) and (9,0) SWNT systems. Substitution doping of boron facilitates the adsorption of INH onto the otherwise inert nanotube. Frontier orbital analysis reveals reorientation of electronic charge in the nanotubes after functionalization, the effect being more pronounced in the case of doped nanotubes. The charge transfer is significant in covalent functionalization of INH via the B-dopant atom, whereas in noncovalent functionalization a small amount of charge transfer is noted. Solvation studies demonstrate the dissolution of INH in B-doped (5,5) and (9,0) SWNTs to be higher compared to pristine nanotube-INH complexes. Functionalization of nanotubes via covalent and noncovalent means can foster pioneering prospects especially for experimental studies in this area of research.

Encapsulation of TCNQ and the Acridinium Ion within a Bisporphyrin Cavity: Synthesis, Structure, and Photophysical and HOMO–LUMO‐Gap‐Mediated Electron‐Transfer Properties

The encapsulation of tetracyanoquinodimethane (TCNQ) and fluorescent probe acridinium ions (AcH(+)) by diethylpyrrole-bridged bisporphyrin (H(4)DEP) was used to investigate the structural and spectroscopic changes within the bisporphyrin cavity upon substrate binding. X-ray diffraction studies of the bisporphyrin host (H(4)DEP) and the encapsulated host-guest complexes (H(4)DEP⋅TCNQ and [H(4)DEP⋅AcH]ClO(4)) are reported. Negative and positive shifts of the reduction and oxidation potentials, respectively, indicated that it was difficult to reduce/oxidize the encapsulated complexes. The emission intensities of bisporphyrin, upon excitation at 560 nm, were quenched by about 65 % and 95 % in H(4)DEP⋅TCNQ and [H(4)DEP⋅AcH]ClO(4), respectively, owing to photoinduced electron transfer from the excited state of the bisporphyrin to TCNQ/AcH(+); this result was also supported by DFT calculations. Moreover, the fluorescence intensity of encapsulated AcH(+) (excited at 340 nm) was also remarkably quenched compared to the free ions, owing to photoinduced singlet-to-singlet energy transfer from AcH(+) to bisporphyrin. Thus, AcH(+) acted as both an acceptor and a donor, depending on which part of the chromophore was excited in the host-guest complex. The electrochemically evaluated HOMO-LUMO gap was 0.71 and 1.42 eV in H(4)DEP⋅TCNQ and [H(4)DEP⋅AcH]ClO(4), respectively, whilst the gap was 2.12 eV in H(4)DEP. The extremely low HOMO-LUMO gap in H(4)DEP⋅TCNQ led to facile electron transfer from the host to the guest, which was manifested in the lowering of the CN stretching frequency (in the solid state) in the IR spectra, a strong radical signal in the EPR spectra at 77 K, and also the presence of low-energy bands in the UV/Vis spectra (in the solution phase). Such an efficient transfer was only possible when the donor and acceptor moieties were in close proximity to one another.

NTCDA–TTF first axial fusion: emergent panchromatic, NIR optical, multi-state redox and high optical contrast photooxidation

The first synthetic entry into axially fused NTCDA/PMDA-TTF multipolar molecules demonstrates a high optical contrast photooxidation, panchromism, low HOMO-LUMO gap, generation of a stable radical cation, NIR absorption/emission beyond 2150/800 nm and theoretically calculated NLO activity.

Isomeric Complexes of [RuII(trpy)(L)Cl] (trpy = 2,2′:6′,2′′-Terpyridine and HL = Quinaldic Acid): Preference of Isomeric Structural Form in Catalytic Chemoselective Epoxidation Process

The present work deals with the isomeric complexes of the molecular composition [Ru(II)(trpy)(L)Cl] in 1 and 2 (trpy = 2,2':6',2''-terpyridine, L = deprotonated form of quinaldic acid, HL). Isomeric identities of 1 and 2 have been established by their single-crystal X-ray structures, which reveal that under the meridional configuration of trpy, O(-) and N donors of the unsymmetrical L are in trans, cis and cis, trans configurations, respectively, with respect to the Ru-Cl bond. Compounds 1 and 2 exhibit appreciable differences in bond distances involving Ru-Cl and Ru-O1/Ru-N1 associated with L on the basis of their isomeric structural features. In relation to isomer 2, the isomeric complex 1 exhibits a slightly lower Ru(II)-Ru(III) oxidation potential [0.35 (1), 0.38 (2) V versus SCE in CH(3)CN] as well as lower energy MLCT transitions [559 nm and 417 nm (1) and 533 nm and 378 nm (2)]. This has also been reflected in the DFT calculation where a lower HOMO-LUMO gap of 2.59 eV in 1 compared to 2.71 eV in 2 is found. The isomeric structural effect in 1 and 2 has also been prominent in their (1)H NMR spectral profiles. The relatively longer Ru-Cl bond in 1 (2.408(2) Å) as compared to 2 (2.3813(9) Å) due to the trans effect of the anionic O(-) of coordinated L makes it labile, which in turn facilitates the transformation of [Ru(II)(trpy)(L)(Cl)] (1) to the solvate species, [Ru(II)(trpy)(L)(CH(3)CN)](Cl) (1a) while crystallizing 1 from the coordinating CH(3)CN solvent. The formation of 1a has been authenticated by its single-crystal X-ray structure. However, no such exchange of "Cl(-)" by the solvent molecule occurs in 2 during the crystallization process from the coordinating CH(3)CN solvent. The labile Ru-Cl bond in 1 makes it a much superior precatalyst for the epoxidation of alkene functionalities. Compound 1 is found to function as an excellent precatalyst for the epoxidation of a wide variety of alkene functionalities under environmentally benign conditions using H(2)O(2) as an oxidant and EtOH as a solvent, while isomer 2 remains almost ineffective under identical reaction conditions. The remarkable differences in catalytic performances of 1 and 2 based on their isomeric structural aspects have been addressed.

Pentaceno[2,3-b]thiophene, a Hexacene Analogue for Organic Thin Film Transistors

Hexacene and larger fused rings remain elusive targets for chemists. Here, we report a hexacene-like molecule containing six linearly fused rings, specifically a pentacene molecule fused with a terminal thiophene ring, pentaceno[2,3-b]thiophene. It can be purified and isolated as a purple-black powder at ambient conditions. This molecule has a low HOMO-LUMO gap of 1.75 eV in o-DCB and an optical band gap of 1.58 eV in thin film. Top contact organic thin film transistors (OTFTs) were made, and atomic force microscopy (AFM) reveals a dendritic thin film growth characteristic of pentacene. An OTFT mobility of 0.574 cm(2)/V s was measured for pentaceno[2,3-b]thiophene under nitrogen.

New M3N@C2n Endohedral Metallofullerene Families (M=Nd, Pr, Ce; n=40–53): Expanding the Preferential Templating of the C88 Cage and Approaching the C96 Cage

Three new families of trimetallic nitride template endohedral metallofullerenes (TNT EMFs), based on cerium, praseodymium, and neodymium clusters, were synthesized by vaporizing packed graphite rods in a conventional Krätschmer-Huffman arc reactor. Each of these families of metallofullerenes was identified and characterized by mass spectroscopy, HPLC, UV/Vis-NIR spectroscopy, and cyclic voltammetry. The mass spectra and HPLC chromatograms show that these larger metallic clusters are preferentially encapsulated by a C(88) cage. When the size of the cluster is increased, the C(96) cage is progressively favored over the predominant C(88) cage. It is also observed that the smaller cages (C(80)-C(86)) almost disappear on going from neodymium to cerium endohedral metallofullerenes. The UV/Vis-NIR spectra and cyclic voltammograms confirm the low HOMO-LUMO gap and reversible electrochemistry of these M(3)N@C(88) metallofullerenes.

Predicting the UV−Vis Spectra of Tetraarylcyclopentadienones: Using DFT Molecular Orbital Energies to Model Electronic Transitions of Organic Materials

Tetraphenylcyclopentadienone, due to its intrinsically low HOMO-LUMO gap, has been suggested as a valuable repeat unit in conducting polymers for nanoscale electronics. The HOMO and LUMO of tetraphenylcyclopentadienone appear to be associated with the relevant pi orbitals of unsubstituted cyclopentadienone. Using previously developed carbonylative coupling reactions, a series of tetraarylcyclopentadienones was synthesized, accessing a range of substituents not previously available. The UV-vis spectra of these molecules were compared to their calculated wave functions and predicted transitions. A quantitative structure-activity relationship was discovered that may greatly simplify prediction of band gaps for oligomers and polymers built from these tetraarylcyclopentadienones.

New Chelating Stilbazonium-Like Dyes from Michler's Ketone

A series of "push-pull" salts substituted with an electron-donating bis(N,N-dimethyl)aniline unit and different electron-withdrawing methyl or chelating pyridinium units have been designed and synthesized from Michler's ketone. The spectroscopic and electronic properties were investigated and compared to their DAST homologues. The studies revealed that a lower HOMO-LUMO gap is obtained in all cases, showing the ability of our donor to increase the "push-pull" effect. Two chromophores with a terpyridine as acceptor end group have also been prepared.

A Highly Conjugated Polymerizable Fluorene-Based 1,3-Dithietane

Reaction of 2,7-(2-thienyl)fluoren-9-one with Lawessons reagent produces a low HOMO-LUMO gap 1,3-dithietane that can be electrochemically polymerised to yield a low band gap poly(fluorene-co-dithiophene) copolymer. Keywords: Dithietane, conducting polymer, fluorenone, band gap, copolymer, thiophene

Ruthenium Molecular Wires with Conjugated Bridging Ligands: Onset of Band Formation in Linear Inorganic Conjugated Oligomers

We have prepared and characterized a series of multimetallic oligomers of Ru using the pi-conjugated bridging ligand tetra-2-pyridyl-1,4-pyrazine (tppz), as well as mixed-ligand complexes with terpyridine end caps, and analyzed their electrochemical and spectroscopic properties, comparing them with modern computational electronic structure methods. The results suggest that the high degree of metal-metal interunit communication in these linear oligomers yields low HOMO-LUMO gaps, high delocalization, and the onset of "quasi-band" features, all indicative that these compounds should be excellent molecular wire materials. Recent spectroscopic and excited-state analyses of these and related compounds focus on optically accessible states, which ignore optically silent frontier electronic states more relevant to nanoelectronic applications.

Poly(Δ 4,4′-dicyclopenta[2,1-b:3,4-b′]dithiophene–co-3,4-ethylenedioxythiophene): Electrochemically generated low band gap conducting copolymers

A low HOMO–LUMO gap, alkene bridged bis-bithiophene (Δ 4,4′-dicyclopenta [2,1-b:3,4-b′]dithiophene) has been copolymerized with electron rich 3,4-ethylenedioxythiophene, to produce copolymers with reduced band gaps and enhanced conductivities. Electrochemical band gaps as low as 0.1 eV have been observed, but maximum conductivities were only ca. 0.3 mS cm −1. Poor matching of the HOMO energies of the two components, together with cross-conjugation at the alkene bridge appear to limit charge carrier mobilities. These results provide further evidence that the use of donor and acceptor moieties to decrease band gaps leads to materials with decreased charge carrier mobilities due to charge localization.

The First Studies of a Tetrathiafulvalene‐σ‐Acceptor Molecular Rectifier

Langmuir-Blodgett monolayers of a donor-acceptor diad TTF-sigma-(trinitrofluorene) (8) with an extremely low HOMO-LUMO gap (0.3 eV) have been used to create molecular junction devices that show rectification behavior. By virtue of structural similarities and position of molecular orbitals, 8 is the closest well-studied analogue of the model Aviram-Ratner unimolecular rectifier (TTF-sigma-TCNQ). Compressing the monolayer results in aligning the molecules, and is followed by a drastic increase in the rectification ratio. The direction of rectification depends on the electrodes used and is different in n-Si/8/Ti and Au/8/C16H33S-Hg junctions. The molecular nature of such behavior was corroborated by control experiments with fatty acids and by reversing the rectification direction with changing the molecular orientation (Au/D-sigma-A versus Au/A-sigma-D).

Organophosphorus π-conjugated materials: the rise of a new field

The synthesis and electronic properties of new linear organic π-conjugated systems incorporating phosphole rings are described. Well defined α,α′-(phosphole-thiophene) oligomers possess low HOMO–LUMO gaps and their optical and electrochemical properties can be tuned via chemical modifications of the P-atoms. The physical properties of these compounds make them valuable materials for OLED’s. The coordination ability of phosphole-based dipoles has been exploited for the synthesis of efficient multipolar NLO-phores. Lastly, phospholes have been used for the synthesis of assemblies exhibiting through-bond interaction between two π-systems via P–P σ-skeletons.