near-IR Transitions Research Articles

Tailoring the Properties of Single-Wall Carbon Nanotube Samples through Structure-Selective Near-Infrared Photochemistry.

As-produced samples of single-wall carbon nanotubes (SWCNTs) contain many structural forms with characteristic properties. In particular, semiconducting SWCNTs display distinct near-IR optical absorption and emission peaks. We show that the selective irradiation of these absorption features can induce structure-specific functionalization in unsorted SWCNT samples. This approach is demonstrated with an ambient temperature photoreaction involving dissolved O2 and irradiation at 955, 985, and 1130 nm, causing preferential covalent reactions of (8,3), (6,5), and (7,6) SWCNTs, respectively. Treated samples showed permanent fluorescence quenching and absorption bleaching near the irradiation wavelength and an increase in the Raman D/G intensity ratio, indicating the formation of covalent defects. The reaction has a very low photochemical quantum yield and was observed for samples suspended in single-stranded DNA and in conventional surfactants that gave incomplete coverage of the nanotube surface. The approach of exploiting sharp nanotube near-IR transitions for structure-selective photochemistry provides a path to tailor SWCNT optical properties for several potential applications without the need for physical sorting.

Magnetic Circular Dichroism of Substoichiometric Molybdenum Oxide (MoO3–x) Nanoarchitectures with Polaronic Defects

Magnetic circular dichroism (MCD) is demonstrated for the first time in substoichiometric molybdenum oxide (MoO₃–ₓ) nanoarchitectures to explore the origin of their near-IR (NIR) transitions. Various nanostructures of MoO₃–ₓ are synthesized by a simple hydrothermal process using an ionic Mo(VI) or metallic Mo(0) precursor. The phase, morphology, and spectroscopic properties of the obtained nanostructures are dependent both on the reaction temperature and the molybdenum precursor. In particular, hexagonal MoO₃–ₓ nanostructures are colored blue with different degrees. To better understand the nature of the electronic states in these nanomaterials, MCD spectroscopy is conducted. A derivative-like MCD response is detected in the vis-NIR region, but it is not attributed to surface magnetoplasmonic modes because of the absence of the peak-energy shift with the increasing refractive index of the dispersion medium. Then, the bisignate MCD signal can be attributed to Faraday B-terms for small-polaronic transitions, arising from two interacting polaronic states close in energy that would give opposite signs under an applied magnetic field. We believe that MCD evaluation for polaronic nanomaterials will expand promising new applications in fields such as semiconductor-based nanophotonics and magneto-optical devices.

Study of intersubband transitions in GaN-ZnGeN2 coupled quantum wells

In this work, we design and analyze a closely lattice-matched wide bandgap GaN-ZnGeN2 coupled quantum well (QW) structure targeting for near-infrared (IR) (λ ≤ 3 um) intersubband transition for quantum cascade laser applications. The coupled quantum well structure comprised two GaN wells separated by a thin ZnGeN2 barrier layer. The QW active region is surrounded by thick ZnGeN2 layers as barriers. The computations of the electron-phonon and electron-photon scattering rates are carried out by employing the Fermi's golden rule for transitions. The calculation takes into consideration the conservation of energy and momentum in scattering processes. The coupled QW structure is optimized through tuning the confined subband energy levels in the conduction band to achieve (1) electron-LO phonon resonant scattering when the energy separation between the first and second conduction subband levels matches the phonon energy of GaN (92 meV); and (2) dominant electron-photon transition in near-IR between the third and second conduction subband levels.

Ruthenium Derivatives of in Situ Generated Redox-Active 1,2-Dinitrosobenzene and 2-Nitrosoanilido. Diverse Structural and Electronic Forms.

The article describes one-pot synthesis and structural elucidation of tc-[RuII(pap)2(L•-)]ClO4 [1]ClO4 and tc-[RuII(pap)2(L'-)]ClO4 [2]ClO4, which were obtained from tc-[RuII(pap)2(EtOH)2](ClO4)2 and benzofuroxan (L = 1,2-dinitrosobenzene, an intermediate tautomeric form of the biologically active benzofuroxan, L'- = 2-nitrosoanilido, pap = 2-phenylazopyridine, tc = trans and cis corresponding to pyridine and azo nitrogen donors of pap, respectively). The same reaction with the newly synthesized and structurally characterized metal precursor cc-RuII(2,6-dichloropap)2Cl2, however, affords isomeric ct-[RuII(2,6-dichloropap)2(L•-)]+ (3a+) and tc-[RuII(2,6-dichloropap)2(L•-)]+ (3b+) (cc, ct, and tc with respect to pyridine and azo nitrogens of 2,6-dichloropap) with the structural authentication of elusive ct-isomeric form of {Ru(pap)2} family. The impact of trans or cis orientation of the nitroso group of L/L' with respect to the N═N (azo) function of pap in the complexes was reflected in the relative lengthening or shortening of the latter distance, respectively. The redox-sensitive bond parameters of 1+ and 3+ reveal the intermediate radical form of L•-, while 2+ involves in situ generated L'-. The multiple redox processes of the complexes in CH3CN are analyzed via experimental and density functional theory (DFT) and time-dependent DFT calculations. One-electron oxidation of the electron paramagnetic resonance-active radical species (1+ and 3+) leads to [RuII(pap)2(L)]2+ involving fully oxidized L0 in 12+ and 32+; the same in 2+ results in a radical species [RuII(pap)2(L'•)]2+ (22+). Successive reductions in each case are either associated with pap or L/L'--based orbitals, revealing a competitive scenario relating to their π-accepting features. The isolated or electrochemically generated radical species either by oxidation or reduction exhibits near-IR transitions in each case, attributing diverse electronic structures of the complexes in accessible redox states.

Unsymmetric (μ-oxido)/(μ-pyrazolato) and Symmetric (μ-pyrazolato)2 Bridged Diosmium Frameworks: Electronic Structure and Magnetic Properties.

The present article deals with the structurally characterized unsymmetric oxido/pyrazolato-bridged [(bpy)2Os(III)(μ-oxido)(μ-pz)Os(III)(bpy)2](ClO4)3 ([1](ClO4)3) and symmetric dipyrazolato-bridged [(bpy)2Os(II)(μ-pz)2Os(II)(bpy)2](ClO4)2 ([2](ClO4)2) (pz = pyrazolato, bpy = 2,2'-bipyridine) complexes with the Os···Os separations of 3.484 and 4.172 Å, respectively. The anti-ferromagnetically coupled Os(III) centers [E(S = 1)-E(BS(1,1) S = 0) = 322.504 cm(-1)] in 1(3+) and diamagnetic (S = 0) 2(2+) exhibit well-resolved (1)H NMR resonances. [1](ClO4)3 shows temperature- and magnetic field-dependent paramagnetism at low magnetic field and diamagnetism at high magnetic field. 1(3+) and 2(2+) display successive metal-based oxidation processes involving the intermediate mixed-valent states and isovalent congeners: Os(IV)Os(IV) (1(5+))→Os(III)Os(IV) (1(4+))⇌Os(III)Os(III) (1(3+))⇌Os(III)Os(II) (1(2+)) and Os(III)Os(III) (2(4+))→Os(II)Os(III) (2(3+))⇌Os(II)Os(II) (2(2+)) as well as bpy-centered reductions. The effect of π donor O(2-) and σ/π-donating pz(-) in 1(3+) and 2(2+), respectively, leads to varying oxidation state of the metal ions in the isolated complexes: Os(III)Os(III) versus Os(II)Os(II). UV-visible-near-IR-electron paramagnetic resonance spectro-electrochemistry and density functional theory (DFT)/time-dependent DFT calculations collectively reveal overlapping of the metal- and ligand (pz, O, bpy)-based frontier orbitals in the delocalized mixed-valent states in 1(4+) and 1(2+) with comproportionation constant (Kc) value > 1 × 10(14) as well as in isovalent 1(3+), resulting in mixed metal/ligand to metal/ligand near-IR transitions in all the three states. The mixed-valent Os(II)Os(III) state in 2(3+) exhibits high Kc value of 1 × 10(22) corresponding to a strong electrochemical coupling situation. However, closeness of the bandwidth (Δν1/2, 4861 cm(-1)) of broad and weak intervalence charge transfer transition of 2(3+) at 1360 nm (ε/M(-1) cm(-1): 490) with the calculated Δν1/2 of 4121 cm(-1) based on the Hush formula as well as spin-density distributions of Os1: 0.811/0.799, Os2: 0.045/0042, and pz: 0.162/0.173 in meso and rac diastereomeric forms, respectively, attribute its localized class II state.

Nonradiative Deactivation of Lanthanoid Excited States by Inner-Sphere Carboxylates.

The vibrational deactivation of metal-centered excited states is one of the fundamental processes that governs the luminescence of inorganic luminophores. In molecular lanthanoid luminescence, the most reliable way to modulate and systematically investigate these processes is deuteration of X-H stretching modes (X = O, N, C). Apart from the effect of these high-energy vibrational motifs, very little is known about the impact of other oscillator fragments present in lanthanoid complexes. We have developed a synthetic protocol to efficiently and selectively label the popular chelator motif "pyridine-2-carboxylic acid" with stable (13)C/(18)O isotope at the carboxylate group. The corresponding isotopologic lanthanoid complexes (Ln = Sm, Eu, Ho) show a decrease of the local-mode carbonyl stretching frequency of up to 5% after isotopic substitution. While this does not seems to have any effect on the luminescence of lanthanoids with medium- to high-energy gaps (Sm and Eu), we have found the first example of a quantifiable luminescence isotope effect for one of the near-IR transitions of holmium ((3)K8 → (5)I5) that only involves the isotopic editing of the vibrational environment at the four carbonyl oscillators.

Unusual Low-Energy Near-Infrared Bands for Ferrocenyl–Naphthalimide Donor–Acceptor Dyads with Aromatic Spacer Groups: Prediction by Time-Dependent DFT and Observation by OTTLE Spectroscopy

Time-dependent density functional theory (TDDFT) calculations for a series of donor–spacer–acceptor (D–S–A) molecules with phenyl (1), biphenyl (2), and anthryl (3) spacers interpolated between the ferrocenylalkene donor and −C≡C-4-naphthalimido acceptor components predicted the presence of weak, very low energy near-IR (NIR) transitions in the electronic spectra of the oxidized species Fc+–S–A. Subsequent optically transparent thin layer electrolysis (OTTLE) spectroscopy experiments in the spectral range 1500–3200 nm reveal that, in each case, very low energy NIR bands are indeed observed at approximately 2500 nm. The TDDFT calculations not only allow the prediction of these low energy bands but also, in concert with recently developed natural transition orbital (NTO) analysis, give detailed insights into the nature of the transitions involved. It is found that these bands around 2500 nm correspond to forbidden e1″ → e1″ transitions localized on the ferrocenium subunit with very low oscillator strengths.

Recognition of fractional non-innocent feature of osmium coordinated 2,2'-biimidazole or 2,2'-bis(4,5-dimethylimidazole) and their interactions with anions.

Mononuclear complexes [Os(II)(bpy)2(H2L1)](ClO4)2 ([](ClO4)2) and [Os(II)(bpy)2(H2L2)](ClO4)2 ([](ClO4)2) incorporating two free NH protons at the back face of the coordinated H2L, and deprotonated L(2-) bridged symmetric dinuclear complexes [(bpy)2Os(II)(μ-L1(2-))Os(II)(bpy)2)](ClO4)2 ([](ClO4)2) and [(bpy)2Os(II)(μ-L2(2-))Os(II)(bpy)2)](ClO4)2 ([](ClO4)2) (bpy = 2,2'-bipyridine, H2L1 = 2,2'-biimidazole and H2L2 = 2,2'-bis(4,5-dimethylimidazole)) have been characterised. Crystal structures of [](ClO4)2 and the meso (ΔΛ) diastereomeric form of [](ClO4)2 have been determined. The crystal structure of [](ClO4)2 also reveals the hydrogen bonding interactions between its free acidic NH protons and the oxygen atoms of the perchlorate anion in the nearby asymmetric unit. Experimental and DFT/TD-DFT calculations have divulged the non-innocent feature of the doubly deprotonated L(2-) in (3+) and (3+), leading to the resonating formulation of {Os(II)(μ-L(2-))Os(III)} ↔ {Os(II)(μ-L˙(-))Os(II)}, instead of a simple mixed valent situation {Os(II)(μ-L(2-))Os(III)}. The dinuclear complexes (3+) and (3+) display one broad and moderately intense near-IR transition near 1000 nm corresponding to a mixed Os(dπ)/L(π) → Os(dπ)/L(π*) MLLMCT (metal/ligand to ligand/metal charge transfer) transition. Different experimental studies have also established the interaction of (2+) and (2+) with the selective anions.

X-ray Structural Characterization of Charge Delocalization onto the Three Equivalent Benzenoid Rings in Hexamethoxytriptycene Cation Radical

Definitive X-ray crystallographic evidence is obtained for a single hole (or a polaron) to be uniformly distributed on the three equivalent 1,2-dimethoxybenzenoid (or veratrole) rings in the hexamethoxytriptycene cation radical. This conclusion is further supported by electrochemical analysis and by the observation of an intense near-IR transition in its electronic spectrum, as well as by comparison of the spectral and electrochemical characteristics with the model compounds containing one and two dimethoxybenzene rings.

CNO ABUNDANCES OF HYDROGEN-DEFICIENT CARBON AND R CORONAE BOREALIS STARS: A VIEW OF THE NUCLEOSYNTHESIS IN A WHITE DWARF MERGER

We present high-resolution (R~50,000) observations of near-IR transitions of CO and CN of the five known hydrogen-deficient carbon (HdC) stars and four R Coronae Borealis (RCB) stars. We perform an abundance analysis of these stars by using spectrum synthesis and state-of-the-art MARCS model atmospheres for cool hydrogen-deficient stars. Our analysis confirms reports by Clayton and colleagues that those HdC stars exhibiting CO lines in their spectrum and the cool RCB star S Aps are strongly enriched in 18O (with 16O/18O ratios ranging from 0.3 to 16). Nitrogen and carbon are in the form of 14N and 12C, respectively. Elemental abundances for CNO are obtained from CI, C2, CN, and CO lines. Difficulties in deriving the carbon abundance are discussed. Abundances of Na from NaI lines and S from SI lines are obtained. Elemental and isotopic CNO abundances suggest that HdC and RCB stars may be related objects and that they probably formed from a merger of a He white dwarf with a C-O white dwarf.

Electronic structure and near-IR transitions of FcC 2R and FcC 4R dyads

This paper describes the orbital configuration which provides a basis for the understanding of the electronic structure and spectroscopic properties of 17e and 18e FcC 2R and FcC 4R dyads, where R is H, 1-naphthyl, 9-anthryl, 3-pyrenyl, perylenyl. DFT calculations show that destabilisation of the ferrocenyl π orbitals upon binding a C CR group to a Cp ring leads to the metal-based a 1 orbital dropping below the e 1-a so that the frontier orbital configuration is ( e 2 ′ -a, π ) 2 ( e 2 ′ -b,metal ) 2 ( e 1 ′ -a, π ) 2 , (a 1, metal) 2. The contribution of the aryl group to the π e 2-a and e 1-a orbitals varies with the annelation of the ring. The LUMO is aryl based. The calculations are consistent with the spectroscopic data for the 18e species. Oxidation to the 17e cations does not change the orbital configuration but the orbital energies are lowered by the positive charge centred on the Fe. A strongly solvatochromic transition in the near-IR, a signature for the 17e cations, is best described as an LMCT transition but the contribution of C 2R and C 4R to the donor and acceptor levels depends on the ionization energy of the aryl π orbital. LMCT energies decrease from FcC 2R to FcC 4R dyads.

High-resolution IR cavity ring-down spectroscopy of jet-cooled free radicals and other species

Initial spectral results are reported from a newly constructed cavity ringdown spectrometer. The apparatus incorporates a slit-jet expansion, with or without a discharge, to produce cold sample molecules. High spectral resolution in both the near- and mid-IR is obtained by using stimulated Raman scattering of the pulsed amplified output of a cw Ti:Sa ring laser. Molecular spectra presented include the electronic near-IR transitions a (1)Delta(g)(-)<-- X (3)Sigma(g)(-) of O(2) and B (3)Pi(g)<-- A (3)Sigma(u)(+) of metastable N(2) and vibrational overtones of H(2)O (polyad 2) and the OH radical. Fundamental vibrational transitions of CH(3) (nu(3)) in the mid-IR are also observed. This apparatus has demonstrated the potential for obtaining high-resolution spectra of both reactive and non-reactive species throughout the entire IR region.

Near-IR [$\ion{Fe}{ii}$] emission diagnostics applied to cold disk winds in young stars

We investigate the emissivity properties of the main near-IR transitions of the Fe+ ion in the conditions prevailing in the inner regions of jets from young stars, based on a simplified 16-level atom model. We present new diagnostic diagrams involving prominent near-IR line ratios that allow us to constrain the electronic density, temperature, and Fe gas phase abundance ratio, independently of the heating process. Comparison with recent near-IR observations of a sample of HH objects indicates gas phase Fe abundances ranging from 15–50% up to 100% of the solar value (depending on the assumed temperature and on the HH object), in agreement with the moderate depletions previously derived from optical line ratios or shock models. Hence, it appears that Fe-bearing dust is efficiently destroyed in stellar jets. We then use our Fe+ emissivity model to predict near-IR [Fe ii] emission maps for self-similar, cold MHD disk wind models. We show that near-IR [Fe ii] lines are stronger than [S ii] λ6731 and [O i] λ6300 in the cool regions ( K) near the wind base, and that observations in [Fe ii] with AMBER on the VLTI could severely constrain the MHD solution and the inner launch radius of the jet. We also compare theoretical predictions with recent observations in the near-IR [Fe ii] lines of the L1551-IRS5 and DG Tau jets. The cold disk wind model reproduces quite well the two velocity components observed at -100 and -300 km s-1, although the high velocity component appears overestimated by a factor of 1.5 in the DG Tau jet. However, the model predicts too little emission at intermediate velocities and insufficient densities. Similar problems were encountered in previous model comparisons in the optical range with jets from T Tauri stars. Denser disk winds with stronger heating at the jet base, which have been invoked for optical jets, also appear needed in younger, embedded Class I jet sources.

A dinuclear bis(bipyridine)ruthenium(ii) complex, [(bpy)2RuII{L2–}RuII(bpy)2]2+, incorporating an unusual non-innocent bridging ligand containing a p-benzoquinonediimine fragment: synthesis, structure, redox, and UV/VIS/NIR and EPR spectroelectrochemical properties

The reaction of [Ru(bpy)2(EtOH)2]2+ (bpy = 2,2′-bipyridine) with the symmetrical binucleating phenolatodiimine ligand HOC6H4NCHC6H4CHNC6H4OH (H2L1) in ethanol under dinitrogen results in the unexpected formation of the diruthenium complex [(bpy)2RuII{OC6H4NC6H3(NH)O}RuII(bpy)2](ClO4)21. In this complex, the bridging ligand (Lsq,sq)2− contains two inequivalent o-iminophenolate N,O-chelating binding sites, each formally at the semiquinone oxidation level, linked such that there is a p-benzoquinonediimine bridge between the two Ru centres. The crystal structures of H2L1 and of 1 have been determined. Complex 1 is electrochemically active and undergoes two reversible oxidations and two reversible reductions which, on the basis of UV/VIS/NIR and EPR spectroelectrochemical evidence, comparison with related systems and consideration of the redox potentials, we assign as centred on the bridging ligand. The two oxidations (at +0.12 and + 0.35 V versus SCE) result in formation of a neutral, fully quinonoidal bridging ligand Lq,q; in the ‘mixed-valence’ mono-oxidised state, a near-IR transition (1570 nm) is ascribed to an intra-ligand charge transfer from the non-oxidised (semiquinone-like) to the oxidised (quinone-like) terminus. The two reductions (at −0.98 and −1.38 V versus SCE) are localised on the central p-benzoquinonediimine unit of the bridging ligand, affording the p-benzosemiquinone radical and then a p-diamide unit. In addition, at more extreme potentials, two oxidations at +1.49 and +1.70 V versus SCE are ascribed to Ru(II)/Ru(III) couples, and the reduction at −1.81 V versus SCE is bpy-based.

Structural and spectroscopic analysis of the F393H mutant of flavocytochrome P450 BM3.

In the preceding paper in this issue [Ost, T. W. B., Miles, C. S., Munro, A. W., Murdoch, J., Reid, G. A., and Chapman, S. K. (2001) Biochemistry 40, 13421-13429], we have established that the primary role of the phylogenetically conserved phenylalanine in flavocytochrome P450 BM3 (F393) is to control the thermodynamic properties of the heme iron, so as to optimize electron-transfer both to the iron (from the flavin redox partner) and onto molecular oxygen. In this paper, we report a detailed study of the F393H mutant enzyme, designed to probe the structural, spectroscopic, and metabolic profile of the enzyme in an attempt to identify the factors responsible for causing the changes. The heme domain structure of the F393H mutant has been solved to 2.0 A resolution and demonstrates that the histidine replaces the phenylalanine in almost exactly the same conformation. A solvent water molecule is hydrogen bonded to the histidine, but there appears to be little other gross alteration in the environment of the heme. The F393H mutant displays an identical ferric EPR spectrum to wild-type, implying that the degree of splitting of the iron d orbitals is unaffected by the substitution, however, the overall energy of the d-orbitals have changed relative to each other. Magnetic CD studies show that the near-IR transition, diagnostic of heme ligation state, is red-shifted by 40 nm in F393H relative to wild-type P450 BM3, probably reflecting alteration in the strength of the iron-cysteinate bond. Studies of the catalytic turnover of fatty acid (myristate) confirms NADPH oxidation is tightly coupled to fatty acid oxidation in F393H, with a product profile very similar to wild-type. The results indicate that gross conformational changes do not account for the perturbations in the electronic features of the P450 BM3 heme system and that the structural environment on the proximal side of the P450 heme must be conformationally conserved in order to optimize catalytic function.

Synthesis and structure of tris(1,2-benzoquinone diimine)osmium(II) perchlorate, and its redox and spectroelectrochemical series ‡

Reaction of [NH4]2[OsIVBr6] with 1,2-diaminobenzene (H2opda) in aerobic conditions afforded [Os(bqdi)3][ClO4]2 (bqdi = 1,2-benzoquinone diimine), a diamagnetic osmium(II) complex containing three redox non-innocent α,α′-diimine ligands. In the crystal structure the relatively long CN distances together with short Os–N distances indicate the existence of considerable metal(dπ)–ligand(pπ) interactions. Cyclic voltammetry of [Os(bqdi)3][ClO4]2 shows one electron redox processes at −0.24, −0.67 and −1.78 V versus Fc–Fc+ (Fc = ferrocene) which are assigned as successive reductions of the coordinated bqdi, and an irreversible OsII–OsIII couple at +0.99 V. The solution electronic spectra of the mixed-valent (ligand) compounds, generated by electrolysis of [Os(bqdi)3]2+ in an optically transparent thin-layer electrode cell at potentials past the first and the second cathodic responses, show near-IR transitions at 1405 and 1164 nm, respectively, assignable to inter-valence charge transfer between the reduced 1,2-diiminosemiquinone monoanion (dsq) and non-reduced (bqdi) ligands. The spectrum of the triply reduced form is more consistent with the formulation [OsIII(dsq)2(opda)]− than with the expected formulation [Os(dsq)3]−, suggesting that an internal redox isomerism has occurred.

Spectroelectrochemical studies and molecular orbital calculations on mononuclear complexes [Mo(TpMe,Me)(NO)Cl(py)] (where py is a substituted pyridine derivative): electrochromism in the near-infrared region of the electronic spectrum

Reaction of [Mo(TpMe,Me)(NO)Cl2] [where TpMe,Me is hydrotris(3,5-dimethylpyrazolyl)borate] with substituted pyridine derivatives (py-R) affords the complexes [Mo(TpMe,Me)(NO)Cl(py-R)] [1, R=4-(1-butylpentyl); 2, R=4-Ph; 3, R=3-Ph; 4, R=4-benzoyl; 5, R=3-benzoyl; 6, R=4-acetyl; 7, R=3-acetyl; 8, R=4-cyano; 9, R=3-cyano; 10, R=4-Cl; 11, R=3-Cl; 12, py-R=isoquinoline]. Two of the complexes, 5 and 8, have been crystallographically characterised. These neutral, 17-electron complexes [formally Mo(I)] were examined by UV/VIS/NIR spectroelectrochemistry. All of them undergo a chemically reversible one-electron reduction to the 18-electron monoanion at potentials which are sensitive to the nature of the pyridyl substituent R. Whereas the neutral (17-electron) forms of the complexes only show transitions in the UV and visible regions of the spectrum of the type we have described before, the reduced forms of the complexes show intense, low-energy transitions ascribable to MLCT transition from the electron-rich metal centre [formally Mo(0)] to the π* orbitals of the pyridyl ligands. The 4-substituted pyridines show only one such low-energy transition (e.g. for [1]-, λmax=830 nm; Iµ=12000 d mol-1 cm-1), but those complexes in which the pyridyl ligand has a strongly electron-withdrawing substituent at the C3 position of the pyridyl ligand develop two new MLCT transitions on reduction of which one is well into the near-IR region: e.g. for complex [5]-, λmax=717 nm; Iµ=5300 d mol-1 cm-1 and λmax=1514 nm; Iµ=2500 d mol-1 cm-1. These spectroscopic results have been rationalised by ZINDO calculations which were used both to calculate the frontier molecular orbitals of the complexes, and to calculate the electronic spectra of the reduced complexes. The strong near-IR transitions in the reduced complexes with 3-substituted pyridines, especially [5]- and [7]-, are of particular significance for the development of electrochromic dyes for use in electro-optic switching in this region of the spectrum.

A very strong metal-metal interaction in a dinuclear ruthenium(II) complex with the dianion of 2,3-dihydroxy-but-2-enal as a compartmental bridging ligand

An unexpected reaction between [Ru(bipy) 2Cl 2] (bipy=2,2′-bipyridine) and ethylene glycol at high temperatures affords the dinuclear complex [{Ru(bipy) 2} 2(μ-C 4H 4O 3)][PF 6] 2 ( 1), in which the compartmental bridging ligand is the dianon of 2,3-dihydroxy-but-2-enal. Electrochemical and spectroscopic properties indicate a very strong electronic interaction between the metals across the delocalised bridge, with a 0.77 V separation between the Ru(II)/Ru(III) couples and a strong near-IR transition ( λ max=1210 nm; ϵ=5000 dm 3 mol −1 cm −1) in the mixed-valence state.

Excited-State Distortions and Electron Delocalization in Mixed-Valence Dimers: Vibronic Analysis of the Near-IR Absorption and Resonance Raman Profiles of [Fe2(OH)3(tmtacn)2]2+

The near-IR transition associated with valence delocalization in the class III mixed-valence dimer [Fe(2)(OH)(3)(tmtacn)(2)](2+) is studied using variable-temperature (VT) electronic absorption and resonance Raman (RR) spectroscopies to gain insight into the properties of electron delocalization in this dimer. Laser excitation into this absorption band leads to dominant resonance Raman enhancement of totally-symmetric [Fe(2)(OH)(3)](2+) core vibrational modes at 316 and 124 cm(-)(1), descriptions of which are calculated from a normal coordinate analysis. Vibronic analysis of the near-IR resonance Raman excitation profiles and VT-absorption bandshapes using an anharmonic excited-state model provides a description of the geometric distortions accompanying this excitation. The excited-state distortion is dominated by expansion of the [Fe(2)(OH)(3)](2+) core along the Fe.Fe axis, reflecting the significant Fe-Fe sigma --> sigma character of this transition. The ground-state sigma-interaction between the two metals has been identified as the orbital pathway for valence delocalization, and the sigma --> sigma distortion analysis is used to quantify the structural dependence of the electronic-coupling matrix element, H(AB), associated with this pathway. The dominant role of totally-symmetric nuclear coordinates in the absorption and RR spectroscopies of [Fe(2)(OH)(3)(tmtacn)(2)](2+) is also discussed in relation to the Q(-) vibrational coordinate and the vibronic spectroscopies of other class II and class III mixed-valence dimers. It is shown that intensity contributions from the Q(-) coordinate to the absorption and RR spectra of [Fe(2)(OH)(3)(tmtacn)(2)](2+) are small relative to those of the totally-symmetric coordinates due to the inefficient change-in-curvature mechanism by which the Q(-) coordinate gains intensity, compared to the efficient excited-state displacement mechanism allowed for totally-symmetric coordinates. This is in contrast with the dominance of the Q(-) coordinate over other totally-symmetric coordinates observed in intervalence transfer (IT) absorption and RR spectroscopies of class II mixed-valence complexes.

β-carotene triplet state absorption in the near-IR range

The β-carotene absorption from the lowest triplet state was investigated over a wide spectral range. A new absorption transition in the near-IR region was found. The absorption band exhibited a 0-0 transition at about 8100 cm −1 with a low intensity, lower by a factor of approximately 300 than the intensity of the known triplet-triplet absorption in the visible range. The vibronic structure of the absorption band was similar to that of the steady state β-carotene absorption. The energy of the triplet state, resulting from this near-IR transition, was close to the β-carotene S 1 state energy. Correspondingly, the interconversion of the carotenoid should depend on the features of this triplet state. Moreover, this relatively low-lying triplet state could also take part in the quenching of molecular triplet states by carotenoids.