Intense Intervalence Charge Transfer Research Articles

Intense Photoinduced Intervalence Charge Transfer in High-Valent Iron Mixed Phenolate/Carbene Complexes.

We report high‐valent iron complexes supported by N‐heterocyclic carbene (NHC)‐anchored, bis‐phenolate pincer ligands that undergo ligand‐to‐metal charge transfer (LMCT) upon photoexcitation. The resulting excited states – with a lifetime in the picosecond range – feature a ligand‐based, mixed‐valence system and intense intervalence charge transfer bands in the near‐infrared region. Upon oxidation of the complex, corresponding intervalence charge transfer absorptions are also observed in the ground state. We suggest that the spectroscopic hallmarks of such LMCT states provide useful tools to decipher excited‐state decay mechanisms in high‐valent NHC complexes. Our observations further indicate that NHC‐anchored, bis‐phenolate pincer ligands are not sufficiently strong donors to prevent the population of excited metal‐centered states in high‐valent iron complexes.

Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption.

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we demonstrate that bimetallic Ni salen complexes form bis-ligand radicals upon two-electron oxidation, whose NIR absorption energies depend on the geometry imposed in the bis-ligand radical complex. Relative to the oxidized monomer [1˙]+ (E = 4500 cm-1, ε = 27 700 M-1 cm-1), oxidation of the cofacially constrained analogue 2 to [2˙˙]2+ results in a blue-shifted NIR band (E = 4830 cm-1, ε = 42 900 M-1 cm-1), while oxidation of 5 to [5˙˙]2+, with parallel arrangement of chromophores, results in a red-shifted NIR band (E = 4150 cm-1, ε = 46 600 M-1 cm-1); the NIR bands exhibit double the intensity in comparison to the monomer. Oxidation of the intermediate orientations results in band splitting for [3˙˙]2+ (E = 4890 and 4200 cm-1; ε = 26 500 and 21 100 M-1 cm-1), and a red-shift for [4˙˙]2+ using ortho- and meta-phenylene linkers, respectively. This study demonstrates for the first time, the applicability of exciton coupling to ligand radical systems absorbing in the NIR region and shows that by simple geometry changes, it is possible to tune the energy of intense low energy absorption by nearly 400 nm.

Bridged cyclometalated diruthenium complexes for fundamental electron transfer studies and multi-stage redox switching.

Four bridged cyclometalated diruthenium systems are highlighted in this Frontier article, including strongly-coupled diruthenium complexes with a short phen-1,4-diyl or a planar pyren-2,7-diyl bridge, redox asymmetric diruthenium complexes characterized by different terminal ligands on the two ends, diruthenium complexes with a urea bridge that allows modulating the degree of electronic coupling, and those with a redox-active amine bridge with varying electronic structures. These complexes posess redox couples with low potentials and intense intervalence charge transfer absorptions in the near-infrared region in the one-electron-oxidized mixed-valent state. They are appealing not only for providing a platform for fundamental electron transfer studies but also as molecular materials with multi-stage redox switching properties.

Push–Pull‐Type Polychlorotriphenylmethyl Radicals: New Two‐Photon Absorbers and Dyes for Generation of Photo‐Charges

Tri(4-iodo-2,3,5,6-tetrachlorophenyl)methane (2) is reported as a general building block for the synthesis of various π-conjugated polychlorotriphenylmethyl (PTM) radicals. Three push-pull-type triphenylamine-substituted PTM radicals with different substitution patterns were prepared and all exhibited intense inter-valence charge-transfer bands and large two-photon absorption (TPA) cross sections. Moreover, increase of solvent polarity also resulted in improved TPA response. The charge-transfer character of the relevant excited states provoked the efficient photo-generation of charges, anions in the PTM and cations in the amine arms, driven by the amphoteric redox character and the small coupling between donor and acceptor.

Strongly coupled cyclometalated ruthenium-triarylamine hybrids: tuning electrochemical properties, intervalence charge transfer, and spin distribution by substituent effects.

Nine cyclometalated ruthenium complexes with a redox-active diphenylamine unit in the para position to the RuC bond were prepared. MeO, Me, and Cl substituents on the diphenylamine unit and three types of auxiliary ligands-bis(N-methylbenzimidazolyl)pyridine (Mebip), 2,2':6',2''-terpyridine (tpy), and trimethyl-4,4',4''-tricarboxylate-2,2':6',2''-terpyridine (Me3 tctpy)--were used to vary the electronic properties of these complexes. The derivative with an MeO-substituted amine unit and Me3 tctpy ligand was studied by single-crystal X-ray analysis. All complexes display two well-separated redox waves in the potential region of +0.1 to +1.0 V versus Ag/AgCl, and the potential splitting ranges from 360 to 510 mV. Spectroelectrochemical measurements show that these complexes display electrochromism at low potentials and intense near-infrared (NIR) absorptions. In the one-electron oxidized form, the complex with the Cl-substituted amine unit and Mebip ligand shows a moderate ligand-to-metal charge transfer at 800 nm. The other eight complexes show asymmetric, narrow, and intense intervalence charge-transfer transitions in the NIR region, which are independent of the polarity of the solvent. The Mebip-containing complexes display rhombic or broad isotropic EPR signals, whereas the other seven complexes show relatively narrow isotropic EPR signals. In addition, DFT and time-dependent DFT studies were performed to gain insights into the spin distributions and NIR absorptions.

Near-infrared electrochromism in electropolymerized metallopolymeric films of a phen-1,4-diyl-bridged diruthenium complex.

A phen-1,4-diyl-bridged tris-bidentate diruthenium complex 3(PF6)2, [Ru2(dpb)(vbpy)4](PF6)2, has been designed and prepared, where dpb is 1,4-di(pyrid-2-yl)benzene and vbpy is 5-vinyl-2,2'-bipyridine. Upon reductive electropolymerization, metallopolymeric thin films of this complex have been deposited on platinum and ITO glass electrode surfaces. These films display two well-separated redox couples at +0.16 and +0.60 V versus Ag/AgCl. In the mixed-valent state, these films display intense intervalence charge transfer absorptions around 1300 nm. The electrochromic behavior at this wavelength has been examined by spectroelectrochemical measurements and double-potential-step chronoamperometry. A highest optical contrast ratio of 41% at 1300 nm with a coloration efficiency of 200 cm(2)/C has been achieved. The electrochromic behavior is highly dependent on the surface coverage. The highest contrast ratio was obtained with a film of 6.0 × 10(-9) mol/cm(2). In addition, a monoruthenium complex 2(PF6), [Ru(dpb)(vbpy)2](PF6), has been prepared and electropolymerized for a comparison study.

Near-infrared electrochromism in mixed-valent diruthenium complexes

The potential applications and evaluation parameters of electrochromic devices are first introduced, together with the basic requirements and current research status of near-infrared electrochromic materials. On the basis of the recent research progress in this laboratory, the design, synthesis, electrochemical and spectroscopic properties, and the metal-metal electronic coupling of cyclometalated diruthenium complexes are then presented. The presence of M-C bonds significantly decreases the redox potential of the metal center and enhances the intermetallic electronic coupling. In the mixed-valent state, these diruthenium complexes display intense intervalence charge-transfer transitions in the near-infrared region and the energy and intensity of these transitions can be modulated by changing the bridging and/or terminal ligands. By using reductive or oxidative electropolymerization, these dimetallic complexes can be deposited onto electrode surfaces as metallopolymeric films. The resulting films exhibit multistage near-infrared electrochromism with low operation potential, short response time, good contrast ratio, and long optical memory time.

Synthesis, Characterization, and Electron-Transfer Processes in Indium Ferrocenyl-Containing Porphyrins and Their Fullerene Adducts

Three new indium(III) tetra- and penta(ferrocenyl)-substituted porphyrins of the general formula XInTFcP [X = Cl(-), OH(-), or Fc(-); TFcP = 5,10,15,20-tetraferrocenylporphyrin(2-); Fc = ferrocene] have been prepared and characterized by UV-vis, magnetic circular dichroism (MCD), (1)H, (13)C, 2D, and variable-temperature NMR spectroscopy, as well as elemental analysis. Molecular structures of the ClInTFcP, FcInTFcP, and FcInTFcP@4C60 complexes were determined by X-ray crystallography with the last compound being not only the first example of a C60 adduct to the organometallic porphyrins but also the first structure in which organometallic porphyrin antennas intercalated into four electron-transfer channels. The electronic structures and relative energies of individual atropisomers, as well as prospective electron-transfer properties of fullerene adducts of XInTFcP complexes, were investigated by the Density Functional Theory (DFT) approach. Redox properties of XInTFcP complexes were investigated using electrochemical (CV and DPV), spectroelectrochemical, and chemical oxidation approaches. Electrochemical experiments conducted in low-polarity solvent using noncoordinating electrolyte were crucial for the sequential oxidation of ferrocene substituents in XInTFcP compounds. In agreement with DFT calculations, the axial ferrocene ligand in FcInTFcP, with direct In-C σ-bond has a 240 mV lower oxidation potential compared to the first oxidation potential for equatorial ferrocene substituents connected to the porphyrin core. The first equatorial ferrocene oxidation process in all XInTFcP complexes is separated by at least 150 mV from the next three ferrocene based oxidations. The second, third, and fourth redox processes in the ferrocene region are more closely spaced. The addition of the bulky axial ferrocene ligand results in significantly larger rotational barriers for equatorial ferrocene substituents in FcInTFcP compared to the other complexes and leads to better defined redox waves in cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments. Mixed-valence compounds of the general formula [XInTFcP](n+) (n = 1, 2) were observed and characterized by spectroelectrochemical and chemical oxidation approaches. In all cases, the presence of the intense intervalence charge transfer (IVCT) bands associated with the oxidation of a single equatorial ferrocene substituent were detected in the NIR region confirming the presence of the iron-based mixed-valence species and suggesting long-range metal-metal coupling in the target systems. The resulting data from the mixed-valence [XInTFcP](n+) (n = 1, 2) complexes matched very closely to the previously reported MTFcP and metal-free poly(ferrocenyl)porphyrins and were assigned as Robin and Day Class II mixed-valence compounds.

Class III Delocalization and Exciton Coupling in a Bimetallic Bis‐ligand Radical Complex

The geometric and electronic structure of an oxidized bimetallic Ni complex incorporating two redox-active Schiff-base ligands connected via a 1,2-phenylene linker has been investigated and compared to a monomeric analogue. Information from UV/Vis/NIR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, and density functional theory (DFT) calculations provides important information on the locus of oxidation for the bimetallic complex. The neutral bimetallic complex is conformationally dynamic at room temperature, which complicates characterization of the oxidized forms. Comparison to an oxidized monomer analogue 1 provides critical insight into the electronic structure of the oxidized bimetallic complex 2. Oxidation of 1 provides [1˙](+), which is characterized as a fully delocalized ligand radical complex; the spectroscopic signature of this derivative includes an intense NIR band at 4500 cm(-1). Oxidation of 2 to the bis-oxidized form affords a bis-ligand radical species [2˙˙](2+). Variable temperature EPR spectroscopy of [2˙˙](2+) shows no evidence of coupling, and the triplet and broken symmetry solutions afforded by theoretical calculations are essentially isoenergetic. [2˙˙](2+) is thus best described as incorporating two non-interacting ligand radicals. Interestingly, the intense NIR intervalence charge transfer band observed for the delocalized ligand-radical [1˙](+) exhibits exciton splitting in [2˙˙](2+), due to coupling of the monomer transition dipoles in the enforced oblique dimer geometry. Evaluating the splitting of the intense intervalence charge transfer band can thus provide significant geometric and electronic information in less rigid bis-ligand radical systems. Addition of excess pyridine to [2˙˙](2+) results in a shift in the oxidation locus from a bis-ligand radical species to the Ni(III) /Ni(III) derivative [2(py)4](2+), demonstrating that the ligand system can incorporate significant bulk in the axial positions.

Di-organoiron Mixed Valent Complexes Featuring “(η2-dppe)(η5-C5Me5)Fe” Endgroups: Smooth Class-III to Class-II Transition Induced by Successive Insertion of 1,4-Phenylene Units in a Butadiyne-Diyl Bridge

The synthesis and study of a new redox family of symmetric dinuclear iron(II/III) complexes featuring "(eta(2)-dppe)(eta(5)-C(5)Me(5))Fe(CC)" endgroups connected by a bis(diethynyl)-4,4'-biphenyl spacer are reported. The solid-state structures were determined (X-rays) for the homovalent Fe(II)/Fe(II) and Fe(III)/Fe(III) parents. In contrast, the mixed valent (MV) complex 5[PF(6)] has a low thermodynamic stability (Kc around 10) and cannot be isolated in a pure form, but was studied in solution. According to the Robin and Day classification, it constitutes a remarkable example of well-behaved weakly coupled class-II organometallic MV compound. The photodriven metal-metal electron-transfer process takes place over ca. 16 A and corresponds to an electronic coupling of ca. 150 cm(-1) with a reorganization energy of ca. 6250 cm(-1) in dichloromethane. A similar investigation was also conducted in the near-IR range for the known and much more stable MV analogue 3[PF(6)] featuring the 1,4-phenyl unit instead of the 4,4'-biphenyl one (K(c) = 2.6 10(4)). The latter also exhibits a localized valency, but presents a very intense intervalence charge-transfer band (IVCT) with a cutoff on the low-energy side. A much stronger electronic coupling is derived (ca. 1700 cm(-1)) from the band shape for this MV complex in the frame of the two-level model. Although slowed, the electron exchange is not disrupted by insertion of an additional para-phenylene moiety into a 1,4-diethynylaryl bridge. Thus, starting from a compound with a butadiyne-diyl spacer, stepwise para-phenylene insertions in the bridge produce a smooth Class-III to Class-II transition for the corresponding MV complexes.

Variable Coordination Mode of Chloranilic Acid. Synthesis, Structure, and Electrochemical Properties of Some Osmium Complexes

Reaction of chloranilic acid (H2ca) with [Os(bpy)2 Br2] (bpy = 2,2'-bipyridine) affords a dinuclear complex of type [{Os(bpy)2}2 (ca)]2+, isolated as the perchlorate salt. A similar reaction of H2ca with [Os(PPh3)2 (pap)Br2] (pap = 2-(phenylazo)pyridine) affords a dinuclear complex of type [{Os(PPh3)2 (pap)}2 (ca)]2+ (isolated as the perchlorate salt) and a mononuclear complex of type [Os(PPh3)2 (pap)(ca)]. Reaction of H2ca with [Os(PPh3)2(CO)2(HCOO)2] gives a dinuclear complex of type [{Os(PPh3)2(CO)2}2 (r-ca)], where r-ca is the two electron reduced form of the chloranilate ligand. The structures of the [{Os(PPh3)2 (pap)}2 (ca)](ClO4)2, [Os(PPh3)2 (pap)(ca)], and [{Os(PPh3)2(CO)2}2 (r-ca)] complexes have been determined by X-ray crystallography. In the [{Os(bpy)2}2 (ca)]2+ and [{Os(PPh3)2 (pap)}2 (ca)]2+ complexes, the chloranilate dianion is serving as a tetradentate bridging ligand. In the [Os(PPh3)2 (pap)(ca)] complex, the chloranilate dianion is serving as a bidentate chelating ligand. In the [{Os(PPh3)2(CO)2}2 (r-ca)] complex, the reduced form of the chloranilate ligand (r-ca(4-)) is serving as a tetradentate bridging ligand. All the four complexes are diamagnetic and show intense metal-to-ligand charge-transfer transitions in the visible region. The [Os(PPh3)2 (pap)(ca)] complex shows an Os(II)-Os(III) oxidation, followed by an Os(III)-Os(IV) oxidation on the positive side of a standard calomel electrode. The three dinuclear complexes show two successive oxidations on the positive side of SCE. The mixed-valent Os(II)-Os(III) species have been generated in the case of the two chloranilate-bridged complexes by coulometric oxidation of the homovalent Os(II)-Os(II) species. The mixed-valent Os(II)-Os(III) species show intense intervalence charge-transfer transitions in the near-IR region.

Intervalence charge-transfer bands in triphenylamine-based polymers

In this paper we show that bis(triarylamine) systems with conjugated bridges can be polymerised potentiodynamically. The resulting polymer films can be doped to a degree of ca. 75% per triarylamine unit. At 50% doping level the polymers show broad and intense intervalence charge-transfer bands. These IV-CT bands are quite similar to those in monomeric model compounds. Therefore, the polymers investigated can be regarded as polymeric mixed valence compounds.

Long-range electronic coupling in various oxidation states of a C4-linked tris(beta-diketonato)ruthenium dimer.

Getting the message across: Strong electronic coupling over a distance of 1.3 nm is observed for the Ru state of the complex 1n (n=+). The fully delocalized system 1+ is distinguished by an extremely intense intervalence charge-transfer (IVCT) band and a single intense vibrational band.

Organic mixed valence compounds with N,N-dihydrodimethylphenazine redox centres

The mixed-valence character of four bis(N,N-dihydrodimethylphenazine) radical cation derivatives with varying π-electron bridges was investigated. The electron transfer (ET) distance within these derivatives varies from ca. 12.5 A for 1,2-bis[2-(5,10-dihydro-5,10-dimethylphenazinyl)]acetylene (1) to ca. 19.3 A for 9,10-bis[2-(5,10-dihydro-5,10-dimethylphenazinyl)ethynyl]anthracene (4). All radical cation species show intense intervalence charge-transfer (IV-CT) bands in the NIR. The Mulliken–Hush analysis was used to derive the electronic coupling V, which ranges from 310 to 870 cm−1. Comparisons with analogous ET systems in which the dihydrodimethylphenazine redox centres have been replaced by triarylamine units show that the dihydrodimethylphenazine species have a significantly higher internal reorganisation energy associated with the ET. This behaviour is attributed to C–N stretching and C–C ring modes of the dihydrodimethylphenazine units.

The stable diiron(2.5) complex ion [(NC)(5)Fe(mu-tz)Fe(CN)5](5-), tz = 1,2,4,5-tetrazine, and its neighboring oxidation states.

The conceptually simple mixed-valent diiron compound (NEt(4))(5)[(NC)(5)Fe(mu-tz)Fe(CN)(5)] with the 1,2,4,5-tetrazine (tz) bridging ligand was obtained as a thermally and air-stable material that displays large and highly variable electrochemical comproportionation constants between about 10(8) (in water) and 10(19.0) (in acetonitrile). Strong metal-metal interaction is also evident from spectroscopic results obtained for the solid and for the dissolved species. The rather intense intervalence charge-transfer band occurs around 2400 nm; infrared and Mössbauer spectra reveal the high spectroscopic symmetry of the system according to an (Fe(2.5))(2) formulation. DFT calculations on the [(NC)(5)Fe(mu-tz)Fe(CN)(5)](6-) ion confirm the presence of very low-lying pi(tz) and high-lying d(Fe) orbitals.

One- and Two-Dimensional Electron Transfer Processes in Triarylamines with Multiple Redox Centers.

Radical monocations of symmetrical triarylamines such as 1 show intense intervalence charge-transfer (IV-CT) bands in the NIR spectra. These bands refer to photoinduced degenerate electron transfers (ET) between two amine redox centers. Hush analysis of the IV-CT bands revealed a very strong electronic coupling between the redox centers in 1+ compared to the total reorganization energy for the ET process. This results in a very fast thermal electron transfer.

Synthesis and characterisation of cyanide bridged heterobinuclear complexes based on group VI octacyanometalate(IV) and pentaammineosmium(III) ions: a comparison with group VIII hexacyano analogues

Two new mixed-valence complexes of the form, [(NC) 7M′ IV(μ-CN)Os III(NH 3) 5]Li·2H 2O (M′= Mo, W) have been synthesised and characterised by spectroscopic and electrochemical techniques. The complexes exhibit moderately intense intervalence charge transfer (IVCT) transitions for M′ =Mo at 638 nm (ϵ max ≈ 1000 M −1 cm −1) and for M′=W at 717 nm (ϵ max≈1500 M −1 cm −1). The Hush parameters reveal Robin and Day Class II behaviour (α 2<0.6%; H rp≈1000 cm −1). The IVCT bandwidths are unusually narrow (Δ v 1 2 ≈3400 M −1 cm −1) relative to the well-known Group VIII hexacyano analogues, [(NC) 5M′ II(μ-CN)Os III(NH 3) 5] − (aq) (M′=Ru, Os), and are attributed to a combination of reduced Franck-Condon barriers to electron transfer and the exclusion of spin-orbit broadening. Electrochemical measurements reveal an octacyanometalate(IV/V) based irreversible couple at +0.72 V (M′=Mo) and a near-reversible couple at +0.445 V (M′=W) in 0.2 M KCl(aq) (versus SCE) for the novel complexes.

Synthesis and spectroscopy of linear-chain palladium(II,IV) and platinum(II,IV) complexes involving quadridentate ligands

The effects of the in-plane ligand, the metal atom, the bridging halogen atom and the counter-anion on the electronic structures of a series of linear-chain, halogen-bridged, mixed-valence complexes of platinum and palladium have been investigated. Complexes of the type [ML][MLX2]Y4, where M = Pd or Pt, X = Cl, Br or I, L = 3, 7-diazanonane-1,9-diamine, 4,7-diazadecane-1,10-diamine, 1,4,8,11-tetraazacyclotetradecane or 1,4,8,12-tetraazacyclopentadecane and Y = BF4, ClO4 or PF6, have been examined by means of electronic, Raman and resonance-Raman spectroscopy. The electronic spectra of the complexes each show an intense intervalence charge-transfer band in the visible region, occurring at higher energy, Eg, than it does for analogous bis(ethylenediamine) complexes; this implies greater localisation of valences for the quadridentate than for the bis(ethylenediamine) complexes, in agreement with conclusions drawn from the X-ray structural results. Resonance-Raman spectra of the complexes display long progressions in the symmetric chain stretching mode ν1, νsym(X–MIV–X), corresponding members occurring at higher wavenumbers than for the analogous en complexes. Progressions in ν1 reach at most to v1= 12 for X = Cl, v1= 9 for X = Br and v1= 5 for X = I. Both the trends in Eg and also those in v1(max) vary in the order I < Br < Cl, Pd < Pt and BF4⩽ ClO4 < Pf6, implying increased valence localisation to the right in each series.