Low-energy Charge-transfer Bands Research Articles

Binding affinity of aniline-substituted dodecaborates to cyclodextrins.

A new set of hybrid guest molecules bearing organic and inorganic residues have been studied for their recognition by cyclodextrins in aqueous solution. The guest molecules consist of nitroanilines linked through their amino group to the dodecahydrido-closo-dodecaborate cluster B12H122-, which serves as an anchor group. They show sizable affinity to cyclodextrins, and unexpected photophysical properties, with a very strong and low-energy charge-transfer band. The dodecaborate cluster increases the pKa of the anilines by 5.0 to 5.7 pH units, and the deprotonated forms of the o- and p-nitroaniline derivatives show strong charge transfer absorption bands in the visible part of the spectrum.

Selecting nitride host for Yb3+ toward near-infrared emission with low-energy charge transfer band

Yb3+-doped phosphors have characteristic near-infrared (NIR) emissions, but their applications in phosphor-converted light-emitting-diodes (pc-LEDs) and Si solar cells are limited due to their mismatching excitation spectra. Here, we selected nitride La3Si6N11 (LSN) as host material to achieve Yb3+ NIR emission upon low-energy charge transfer (CT) excitation. The obtained phosphor LSN:Yb3+ has a broad CT excitation band ranging from 250 to 500 nm and narrowband NIR emissions ranging from 950 to 1100 nm centered at 983 nm. On the basis of spectral data, the vacuum referred binding energies (VRBE) schemes are constructed to locate energy levels of all lanthanide ions in LSN. We also fabricated NIR pc-LED device using 395 nm LED chip to demonstrate the potential applications of LSN:Yb3+ phosphors.

The Impact of Ligand Oxidation State and Fold Angle on the Charge Transfer Processes of MoIVO‐Dithione Complexes

AbstractWe report a series of mononuclear monooxo Mo(IV) complexes possessing either one or two fully oxidized dithiolene ligands; [MoOCl(R2Dt0)2][X], (1 and 2), and MoO(p‐SC6H4Y)2(R2Dt0), (3 and 4), (R=Me, iPr; X=PF6, SbF6, BF4; Y=H, Cl, F, CF3, Me, tBu, OMe). Either four or two quasi‐reversible ligand‐based redox couples are detected depending upon the number of fully oxidized dithiolene ligands present. The molecular structure of 3 and 4 exhibit a large (47° to 70°) fold angle along the S⋅⋅⋅S vector of the dithione ligand. The UV‐Vis spectra of 3 and 4 exhibit a low energy charge transfer band at ∼540 nm that are red‐shifted ∼200 nm compared to the spectra of 1 and 2. Density Functional Theory (DFT) calculations show that the low energy charge transfer band of 3 and 4 is heavily influenced by ligand fold angle. Reducing the fold angle decreases the charge transfer energy, and the transition becomes more ligand‐based.

Geometries and Dynamics of Photoinduced Electron-Hole Pairs in Polyhexylthiophene-Fullerene Systems

For developments of the organic light-energy conversion systems, it is essential to achieve efficient long-range charge-separations (CS). Light-induced electrons and holes generated by the electron-transfer (ET) reactions are thus required to escape from the electrostatic binding at the initial stage. In the natural photosynthetic reaction centers, the efficient distant CS is accomplished with ~100% efficiency by cascading the redox states by the sequential ET processes to a series of cofactors, leading to the light-to-chemical energy conversion. In contrast, the photoinduced CS at the interfaces of organic photovoltaic (OPV) cells generates electron-hole pairs, eventually achieving light-to-electricity conversion after conductions of the electron and the hole through the donor (D) and the acceptor (A) domains, respectively. However, the D/A interfaces in the OPV cells often suffer from the charge-recombination (CR) at the early stage, leading to losses of the input photon energies. Concerning the mechanism of the efficient photo-career generations in the OPV systems, role of the hot charge-transfer (CT) excitons is currently under intense debate. A recent ultrafast spectroscopic study of a polymer/fullerene (PCPDTBT/PC60BM) OPV films clearly showed that the vibrationally unrelaxed electron-hole pairs (PCPDTBT+/PC60BM-) are generated in < 50 fs before time scales of the internal conversions (IC) of the polymer excited states.[1] On the other hand, Vandewal et al.[2] have demonstrated for the several polymer:PCBM solar cells that the internal quantum efficiency (IQE) is essentially independent of the excitation energy of the lights; even if the low-energy CT band is excited, the IQE is higher than 90 % for some blend films, giving rise to very high OPV performances. This result strongly indicates that the hot CT states are not necessarily required to produce the highly separated photo-careers. To elucidate why and how the photo-carriers escape from the Coulomb binding, it is important to directly observe locations and orientations of the intermediate charges just after the CSs. Time-resolved electron paramagnetic resonance (TREPR) and pulsed EPR methods have been powerful to obtain the electron spin-spin dipolar interaction and the spin-spin exchange coupling in the photoinduced CS states and thus have been utilized to characterize the interspin distances and the electronic couplings of the transient CS states. The TREPR analyses have been useful to obtain the CS state geometries for several systems, since the electron spin polarization (ESP) as the microwave absorption (A) and the emission (E) is sensitive to the molecular conformations.[3] Recent spin polarization analyses have revealed representative geometries of the transient electron-hole pairs in the OPV blends.[4] However, the highly inhomogeneous molecular environments at the bulk-heteojunction (BHJ) interfaces induces the large special distribution in the solid-state OPV materials and would prevent us from understanding the mechanism of the charge conductions. To unveil fundamental molecular mechanisms of the light-induced charge conductions, it is highly desired to investigate the structure and dynamics of the initial photo-carriers generated in a conjugated polymer-backbone in which the intramolecular orientation and the electronic interaction are well-defined at the initial CS stage. Thus far, no study has characterized the geometry, electronic coupling and charge-conduction dynamics of the photo-carriers in a covalently linked polymer system, although the polymer-fullerene dyads were synthetized and investigated by transient spectroscopies. In this respect, we have synthesized a new series of donor-bridge-acceptor (P3HT-Phm-C60, m = 0, 1, 2, 3) linked polymers composed of regio-regular poly(3-hexylthiophene) (P3HT) as the electron donor, ful-leropyrrolidine (C60) as the acceptor and oligo-p-phenylene bridges as the spacers.[5] We have herein investigated the molecular geometries and the hole-conduction dynamics of the photoinduced intramolecular CS states by using the TREPR in tetrahydrofuran (THF) at room temperature. We clearly show that, without going through the hot CT, localized holes are generated in the conjugated polymer-backbones nearby the phenylene units by quenching triplet excitons and then undergo one-dimensional intramolecular dissocia-tions from the electrons situated at C60 in the dyads. Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H. J.; Brida, D.; Cerullo, G.; Lanzani, G. Nat. Mater. 2013, 12, 29-33.Vandewal, K.; Albrecht, S.; Hoke, E. T.; Graham, K. R.; Widmer, J.; Douglas, J. D.; Schubert, M.; Mateker, W. R.; Bloking, J. T.; Burkhard, G. F.; Sellinger, A.; Frechet, J. M. J.; Amassian, A.; Riede, M. K.; McGehee, M. D.; Neher, D.; Salleo, A. Nat. Mater. 2014, 13, 63-68.Kobori, Y.; Miura, T. J. Phys. Chem. Lett. 2015, 6, 113-123.Kobori, Y.; Noji, R.; Tsuganezawa, S. J. Phys. Chem. C 2013, 117, 1589-1599.Miura, T.; Tao, R.; Shibata, S.; Umeyama, T.; Tachikawa, T.; Imahori, H.; Kobori, Y. J . Am. Chem. Soc. 2016, 138, 5879-5885.

Oxo-carboxylato-molybdenum(vi) complexes possessing dithiolene ligands related to the active site of type II DMSOR family molybdoenzymes

Spectroscopic and kinetic studies indicate that oxo-carboxylato-molybdenum(VI) bis-dithiolene complexes, (Mo(VI)O(p-X-OBz)L2), have been generated at low temperature as active site structural models for the type II class of pyranopterin molybdenum DMSOR family enzymes. A DFT analysis of low energy charge transfer bands shows that these complexes possess a Mo-S(dithiolene) π-bonding interaction between the Mo(d(xy)) redox active molecular orbital and a cis S(p(z)) donor orbital located on one of the dithiolene ligands.

Synthesis, crystal and electronic structure, and theoretical investigations of charge-transfer, optical, and bonding properties of a [VF6]3− anion compound

A green-colored V(III) compound, imidazolium hexafluorovanadium(III), [C3H5N2)]3[VF6], has been prepared and characterized. The geometric and electronic structure, together with charge-transfer, optical, and bonding properties, were thoroughly investigated by X-ray crystallography, density functional theory (DFT), and time-dependent density functional theory (TDDFT) calculations. The low-energy charge-transfer bands responsible for its green color may be theoretically assigned to a F(2p) → V(4p) ligand-to-metal charge-transfer transition. The ligand-field charge-transfer bands (d → d bands) occur at a lower energy region; they are too weak and will be obscured by the quite intense ligand-to-metal bands in the optical spectrum. Partial density of states analysis clearly shows that the nature of metal–ligand interactions in [VF6]3− is mainly ionic. .

Postfunctionalization of aromatic polyamine by [2+2] cycloaddition of 7,7,8,8-tetracyanoquinodimethane with side chain alkynes

Electron-rich, side chain alkynes of an aromatic polyamine were functionalized by a [2+2] cycloaddition, followed by retro-cyclization with the electron-accepting 7,7,8,8-tetracyanoquinodimethane (TCNQ). 1H NMR studies were used to optimize the reaction conditions. Mild heating to >50 °C afforded the postfunctionalized aromatic polyamines with the desired acceptor amounts. The quantitative TCNQ addition was demonstrated by the MALDI-TOF mass spectrum and elemental analysis. Introduction of the cyano-based acceptor moieties into the polymer side chains resulted in unusually strong intermolecular interactions. In addition to the π–π interactions of the extended acceptor moieties, these intermolecular forces were supposed to improve the thermal stability of the aromatic polymers. Furthermore, the donor–acceptor chromophores formed by this postfunctionalization displayed low energy charge-transfer bands and redox activities in both the anodic and cathodic directions. The straightforward postfunctionalization technique using the alkyne–TCNQ addition is useful for the preparation of narrow band gap polymers in one step.

Expanding the chemical space for push-pull chromophores by non-concerted [2+2] and [4+2] cycloadditions: access to a highly functionalised 6,6-dicyanopentafulvene with an intense, low-energy charge-transfer band

Non-concerted [2+2] and [4+2] cycloadditions between N,N-dimethylanilino-substituted 1,1,2,4,4-pentacyanobuta-1,3-diene and 4-ethynyl-N,N-dimethylaniline are controlled by solvent polarity and provide access to a highly functionalised 6,6-dicyanopentafulvene featuring an intense, low-energy charge-transfer band and to an unusual spirocyclic zwitterion, characterised by X-ray analysis.

Luminescent platinum(II) bis(1,1-dithiolate) complexes

The bis(1,1-dithiolate) complexes (TBA)2[Pt(ecda)2], (TBA)2[Pt(i-mnt)2] and (PPN)2[Pt(ecda)2], where TBA = tetra-n-butylammonium, ecda = 1-(ethoxycarbonyl)-1-cyanoethylene-2,2-dithiolate, i-mnt = 1,1-dicyanoethylene-2,2-dithiolate and PPN = bis(triphenylhosphoranylidene)ammonium, have been prepared and their photoluminescent properties have been investigated. Intense emission is observed in frozen solvent glasses and in the solid state at 77 K, but not in fluid solution at 298 K. The emission is assigned to a charge-transfer-to-dithiolate phosphorescence, involving an excited state having the same orbital parentage as the strongly allowed, low energy charge-transfer band in the absorption spectrum. The counter-ion influences the energy of the charge-transfer absorption and emission bands slightly but significantly as a result of weak ion-pairing that occurs in solution. Photolysis with λ < 360 nm in halocarbon solvents leads to irreversible photooxidation of the complex, presumably via a charge transfer-to-solvent excitation. The photochemically promoted oxidation does not occur upon longer wavelength irradiation (λ > 400 nm) and therefore does not arise from the emissive excited state of the complex.

Complexes of dithiobenzoic acids. V. Observations on the nature of the molybdenum--Sulfur bond in tetrakis(dithiobenzoato)molybdenum(IV) complexes

Interpretations of data from infrared, visible-ultraviolet, and 13C NMR spectra lead to the conclusion that there is extensive covalent bonding in dithiocarboxylates, 4, of eight-coordinated molybdenum(IV), [Mo(4) 4], and suggest the presence of metal-to-ligand π backbonding, in contrast to the bonding in analogous dithiocarbamate complexes. Some dd bands have been observed in the latter compounds, but in the dithiocarboxylates the d-d transitions are buried beneath intense low energy charge transfer bands, implying very efficient mixing of metal and ligand orbitals. The first CT band (at least) is most likely metal(xy)-to-ligand(π*).

Coloured and colourless charge-transfer complexes of small and polymeric quaternary pyridinium bromides

The products of quaternization reactions involving alkyl bromides are, under certain conditions, coloured, e.g. 4-methyl-N-n-propylpyridinium bromide is red and poly(4-vinyl-N-n-butylpyridinium bromide) is green when prepared in benzene, say, at 333 K. Neither are coloured, however, when heated in sulpholane. These and other products have been shown to be coloured due to charge-transfer bands; the possible involvement of impurities or side reactions has been eliminated: important in establishing the reliability of kinetic measurements by the authors. Comparison has been made with several iodides under varying conditions: these are invariably colourless in the solid and pale yellow in ethanol. They lack the low-energy charge-transfer bands (colours), but show charge-transfer bands in the u.v.

Chemistry and optical properties of 4d and 5d transition metal compounds. IV. Magnetic circular dichroism characterization of the low-energy charge-transfer bands of [RuIII(NH3)5L]2+

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChemistry and optical properties of 4d and 5d transition metal compounds. IV. Magnetic circular dichroism characterization of the low-energy charge-transfer bands of [RuIII(NH3)5L]2+R. S. Evans and A. F. SchreinerCite this: Inorg. Chem. 1975, 14, 7, 1705–1708Publication Date (Print):July 1, 1975Publication History Published online1 May 2002Published inissue 1 July 1975https://doi.org/10.1021/ic50149a055RIGHTS & PERMISSIONSArticle Views54Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (545 KB) Get e-Alerts Get e-Alerts

A comparison of solution and solid state electronic absorption spectra of pyridinium iodides

Charge-transfer (CT) absorption spectra from CH2C12 solutions and Nujol mulls of a variety of pyridinium iodides have been compared. Factors such as hand grindings vs. mechanical milling, milling time, and support medium were examined with respect to effects on the position of the absorption bands in the solid state. Generally, the position of the low energy CT band appeared at lower wavelengths in mulls between CaF2 plates than in CH2Cl2 solutions. However, it was interesting to discover that the same mulls on filter paper or glass microfiber paper exhibit the CT band at energies higher than those found between CaF2 plates or in CH2Cl2 solution. As might be expected, small changes in structure such as differences in alkyl chain length influence the CT band position significantly more in the solid state than in solution.

Ultraviolet and Charge-Transfer Spectra of Benzylsilanes

Abstract The frequencies of low-energy charge-transfer bands (CT-II) of 14 benzylsilane–tetracyanoethylene complexes were correlated with σI constants of substituents on silicon. The CT-II frequencies of benzylsilanes and other related compounds were also linearly related to their 1La absorption frequencies, a σ-π conjugation mechanism being discussed.

The spectra of complexes of conjugated ligands. Part III. Charge-transfer spectra of bisnioximebis(substituted pyridine)iron(II) complexes

A series of bisnioximebis(substituted pyridine)iron(II) complexes has been prepared and analysed, and their visible and near-u.v. spectra have been measured in solution and in the solid state. The compounds exhibit low-energy charge-transfer bands from iron(II) to nioxime and to pyridine. The effects of methyl substituents on the energies of these bands are discussed by using the approach developed1 for phenanthroline complexes, in which the shifts are explained as due to a combination of changes in the ligand molecular-orbital and d-orbital energies. As in the case of phenanthroline, shifts on successive substitution are additive to within experimental error; their numerical values are quite consistent with those in the phenanthroline complexes. The pyridine-acceptor molecular orbital is identified. The areas of both the charge-transfer bands are used to obtain values for the iron-nitrogen resonance integral; the two values agree remarkably well. The charge-transfer band to pyridine shows a large solvent shift, explained by the changing orientation of the pyridine rings.