near-IR Absorption Spectra Research Articles

Core-modified porphyrins. Part 4: Steric effects on photophysical and biological properties in vitro

21,23-Dithiaporphyrins ( 2– 10) were designed and prepared as analogues of 5,20-diphenyl-10,15-bis(4-carboxylatomethoxy)phenyl-21,23-dithiaporphyrin ( 1) to examine the impact of steric bulk at the 5- and 20- meso positions as well as the impact of symmetry. Changes at the meso positions had minimal impact on the UV–vis–near-IR absorption spectra, quantum yields for the generation of singlet oxygen, and quantum yields for fluorescence and some impact on values of the octanol/water partition coefficient. Of the compounds 1– 10, 5-phenyl-20-(2-thienyl)-10,15-bis-(4-carboxylatomethoxy-phenyl)-21,23-dithiaporphyrin ( 3) showed the greatest phototoxicity toward cultured R3230AC cells, with 68% cell kill at 1 × 10 −7 M and irradiation with 5 J cm −2 of 350–750 nm light. Results in this study suggest that smaller substituents on the meso ring and less symmetrical compounds are more effective as photosensitizers than compounds with two bulky substituents at adjoining meso sites and a higher symmetry. The mitochondria appear to be involved in the process of phototoxicity as determined by the inhibition of whole cell cytochrome c oxidase activity in cells treated with 3 and light. No impact upon mitochondrial cytochrome c oxidase activity was observed in cells treated with 3 and no light. Fluorescence microscopy studies suggest that the mitochondria are not initial sites of accumulation of 3.

Spectral Manifestations of d–π Exchange and Charge Transfer Interactions in Cu(II) Porphyrins

New absorption bands due to spin-forbidden transitions from the ground state to excited triplet states are found at 295 K in the near-IR absorption spectra of a number of Cu(II) complexes of octaethylporphyrin (OEP) that differ in the nature, number, and position of their side substituents. The frequency distribution, number, and nature of these transitions are analyzed using computer decomposition of complex contours into Gaussian components and additional data from the phosphorescence and phosphorescence excitation spectra (295-77 K). The d-π exchange integrals, determining the energy splitting ΔE = E(2T1)–E(4T1) in the compounds under study, are calculated on the basis of the experimental data obtained. It is shown that, in addition to the formally spin-allowed 2S0 → 2T1 transition (λmax = 833 nm), the absorption spectrum of the nonplanar CuOETPP molecule at 295 K exhibits the low-intensity spin-forbidden 2S0 → 4T1 transition (λmax = 937 nm). For this compound at 77 K, phosphorescence from the 4T1 state is observed (λmax = 955 nm), with a quantum yield equal to ϕPh = 0.0015 and a decay time amounting to τPh = 190 ns. For the CuOEP-Ph(o-NO2) molecule, which contains the electron-acceptor nitro group, direct absorption from the ground state S0 to a charge transfer (CT) state (λmax = 845 nm) is observed at 295 K. The extinction coefficient of this absorption and the energy of the CT state are determined.

Carbon‐Chain Oxides in Proton‐Irradiated CO Ice Films

After H2 and water, CO and CO2 are the most common molecules in stellar and interstellar environments. They are often dominant components in icy mantles of comets and interstellar dust grains. These ice mantles are subject to a varying degree of cosmic radiation, leading to chemical alterations. This paper presents a study of the near-IR absorption spectrum (1900-4000 cm-1) of proton-irradiated CO thin films. The spectra of such films display several features arising in the vicinity of the CO fundamental that are attributed to carbon oxides such as CnO and CnO2 for n ≤ 7. The formation mechanisms of carbon chains in interstellar environments are of interest since carbon chains (or molecules derived from them) have been considered to be carriers of some of the diffuse interstellar bands.

Multifrequency EPR Study of Metallofullerenes: Eu@C82 and Eu@C74

In this work, the cage structure of a family of europium metallofullerenes, Eu@C74 and three isomers of Eu@C82, were investigated. The analogy of the electronic states of three isomers of Eu@C82 with those of three isomers of Ca@C82 was confirmed by UV−vis−near-IR absorption spectra. The symmetries Cs, C2, and C2v were assigned for three isomers of Eu@C82 by comparing each corresponding isomer of Ca@C82. The D3h cage structure of Eu@C74, which is expected only for the C74 from the isolated pentagon rule, was also confirmed by comparing its photoabsorption spectrum with that of Ca@C74. Multifrequency electron paramagnetic resonance (EPR) spectroscopy was employed to determine the zero-field splitting (ZFS) parameters of the metallofullerenes, which are closely related to the surrounding cage structures. The experimental X- and W-band EPR spectra were completely reproduced by computer simulations based on a spin Hamiltonian considering the ZFS terms up to fourth order. The introduction of the fourth-order ZFS terms into the Hamiltonian resulted in a precise determination of the second-order ZFS terms. A nonvanishing rhombicity parameter E of Eu@C74 demonstrated the reduction of the symmetry from D3h to C2v due to the positioning of Eu2+ ion at the off-center of C74 cage. Moreover, in this work, the semiphenomenological superposition-exchange model (SPEM) was invoked to examine a relationship between the molecular structures and ZFS parameters. All of the experimental ZFS parameters were satisfactory reproduced in terms of the SPEM, leading to confirmation of the molecular symmetries. The semiphenomenological analysis also enabled us to select the most appropriate cage for Eu@C82 with C2 symmetry from among three possible isomers.

Diminished band-gap transitions of single-walled carbon nanotubes in complexation with aromatic molecules.

The near-IR absorption spectrum of semiconducting single-walled carbon nanotubes is characterized by the transitions associated with the first (S11) and second (S22) pairs of van Hove singularities in the electronic density of states. We report that a significant effect on the transitions can be caused by non-covalent complexation of the nanotube with planar aromatic molecules such as pyrenes in solution, resulting in the absence of S11 and S22 bands in the near-IR absorption spectrum. Since the complexation is reversible, the characteristic absorption bands can be turned on and off with the complexation in a reversible fashion.

Neptunium(VI) Chromate Complex with 1,2-Diammoniopropane {H3NCH2CH(NH3)CH3}[(NpO2)2(CrO4)3(H2O)] · 3H2O: Synthesis, Crystal Structure and Properties

New neptunium(VI) complex {H3NCH2CH(NH3)CH3}[(NpO2)2(CrO4)3(H2O)] · 3H2O is synthesized; its crystal structure is determined and IR and near-IR absorption spectra are recorded. The crystallographic data are: a = 10.805(2) A, b = 11.238(2) A, c = 17.615(8) A, space group P212121, Z = 4, V = 2139(1) A3, R = 0.051, wR(F2) = 0.109. The crystal structure of the compound is built of the anionic layers of [(NpO2)2(CrO4)3(H2O)]2n– n . The {H3NCH2CH(NH3)CH3}2+ cations and crystallization water molecules are arranged between the layers. Coordination polyhedron of two crystallographically independent Np atoms has the shape of a pentagonal bipyramid. The equatorial plane in one Np polyhedron is formed by the oxygen atoms of four chromate ions and water molecule and by the oxygen atoms of five chromate ions in the other one.

Efficiencies of Electron Injection from Excited Sensitizer Dyes to Nanocrystalline ZnO Films as Studied by Near-IR Optical Absorption of Injected Electrons

Near-IR absorption spectra of electrons injected from excited sensitizer dyes to nanocrystalline ZnO films have been studied. The absorption steeply increases with increasing wavelength and can be assigned to the optical transition of conducting electrons to higher energy levels of the conduction band. From the measurements in various sensitizer dyes, the relative efficiencies of the electron injection can be obtained as a function of the free energy change (ΔG) for the electron injection process. Around ΔG = 0 eV, the efficiencies increase gradually with increasing −ΔG and they are constant at negative ΔG. We succeeded in explaining the tendency by considering the site heterogeneity of the surface. The relation between the injection efficiencies and the solar cell performance will also be discussed.

Measurement of the populations of metastable levels in gadolinium vapor by diode laser-based UV and near-IR absorption spectroscopy

The metastable populations of gadolinium atoms, which were generated by an electron beam gun, were measured by diode laser-based absorption spectroscopy. Transitions originating out of the ground and low-lying metastable levels were investigated in the spectral range of 390–394 nm. A tapered semiconductor power-amplifier which was seeded by a single-mode external-cavity diode laser was used as a fundamental light source for the second-harmonic generation in the ultraviolet wavelength range. The population appears to be frozen at an atomic excitation temperature of ∼600 K, which is about 1/4 of the heated surface temperature. The results obtained from the ultraviolet absorption spectra are compared to those obtained from the near-IR absorption spectra with wavelength ranges of 673–692 and 773–775 nm.

Fabrication of Various Ordered Films of Oxotitanium(IV) Phthalocyanine by Vacuum Deposition and Their Spectroscopic Behavior

Different types of ordered solid films of oxotitanium(IV) phthalocyanine (OTiPc) were grown in 50−500 nm thickness by vacuum deposition onto such conventional substrates as metal- and ITO-coated glass and neat glass at a rate of ∼0.05 nm s-1. Under 10-3−10-4 Pa isotropic films with short-range molecular organization and preferential molecular orientation were grown on the substrates kept at ∼25 °C but ordered α (phase II) or β (phase I) crystal films on the substrates heated at 150 °C. The deposition onto surface-oxidized Al, Ti, and Cu/glass, ITO/glass, and neat glass at ∼25 or 150 °C selectively gave ordered isotropic or α-crystal films which have preferential molecular alignments with “standing” orientations of the molecular plane with respect to the substrate surface. On the other hand, another type of ordered isotropic or β-crystal films with preferential “lying” molecular orientations was formed by the deposition onto Au, Ag, and unoxidized Cu on glass at ∼25 or 150 °C. In the case of 150 °C-heated Pt/glass, the α-crystal film with “standing” molecular orientations was grown under 10-3−10-4 Pa, but another type of α-crystal film with “lying” orientations under 2.7 × 10-5 Pa. Scanning electron microscopic observations of the ordered crystalline films indicated that single-crystal-like domains are grown from the bottom to the top of the film and are densely packed to give monolithic morphologies with little grain boundary. Visible−near-IR absorption and fluorescence spectra of the ordered solid films were recorded, showing unique dependences on the molecular alignments involving anisotropic behavior in the polarized absorption spectra of the α-crystal films with different molecular alignments.

Sol−Gel Syntheses and Spectroscopic Characterization of Chromium-Doped Silicates and Germanates

To avoid the technical difficulties encountered with high-temperature processing techniques, low-temperature sol−gel processes have been examined as routes to the Cr-doped olivine, Mg2SiO4, and the structural analogues Ca2SiO4 and Ca2GeO4. Gelation yields transparent, lightly colored gels that are converted to microcrystalline ceramic powders on heating to 850 °C. Initially doped with CrCl3·6H2O (0.01−1%), conversion to the ceramic is accompanied by changes in the coordination and oxidation state of the chromium. Consistent with size considerations, EPR, EXAFS, and XANES data indicate chromium exists principally as tetrahedrally coordinated Cr4+ in Ca2SiO4 and Ca2GeO4, and as a mixture of octahedrally coordinated Cr3+ and tetrahedrally coordinated Cr4+ in Cr-Mg2SiO4. Visible and near-IR electronic absorption and emission spectra reflect the composition of the material, and a comparison of the absorption of randomly oriented microcrystalline ceramics shows a surprising similarity in both band position and ...

Isolation and characterization of two Pr@C 82 isomers

The second isomer of Pr@C 82 (Pr@C 82-II) has been isolated by a two-step HPLC method, separately from the known major isomer (Pr@C 82-I). Visible and near-IR absorption spectra, as well as cyclic and differential pulse voltammograms show that the characteristic features of Pr@C 82-II are significantly different from those of Pr@C 82-I. The absorption peaks of Pr@C 82-II have blue-shifts and the redox potentials have negative shifts, relative to those of Pr@C 82-I. Chemical derivatization of both isomers with disilirane have also been achieved.

Structural Control of the Reversible Dimerization of π-Conjugated Oligomeric Cation Radicals

The reversible dimerization of the electrochemically or chemically generated cation radicals of diversely substituted tetrathienylenevinylenes (1−6) has been analyzed by concentration and temperature-dependent cyclic voltammetry, UV−vis, near-IR, and ESR spectroscopies, and time-resolved spectroelectrochemistry. For unsubstituted and end-substituted oligomers 1 and 2 the apparent redox potential corresponding to the formation of the cation radical (Eapp1) shifts negatively when increasing substrate concentration or when decreasing temperature, as expected for an ECDimE scheme. Such effects are not observed for compounds 3−6 containing β-substituted thiophene rings suggesting that the corresponding cation radicals do not dimerize. UV−vis and near-IR absorption spectra of the chemically generated cation radical 2•+ show, in addition to the two absorption bands of the monomeric cation radical, two hypsochromically shifted transitions confirming that 2•+ dimerizes already at room temperature. These additional...

Control of the Photophysical Properties of Polyatomic Molecules by Substitution and Solvation: The Second Excited Singlet State of Azulene

The UV−visible−near-IR absorption spectra, S2 → S0 fluorescence quantum yields and S2 fluorescence lifetimes of 1-fluoroazulene, 1,3-difluoroazulene, and several of their alkyl-substituted derivatives have been measured at room temperature in up to six solvents, benzene, dichloromethane, ethanol, acetonitrile, n-hexane, and perfluoro-n-hexane. The quantum yields (up to 0.2) and lifetimes (up to 9.5 ns) of the S2 state of 1,3-difluoroazulene are exceptionally largethe largest ever reported for an upper excited singlet state of a polyatomic molecule with a closed-shell ground state. The nonradiative rate constants for the decay of the S2 states of these molecules in these solvents and of azulene, 1,3-dichloroazulene and 1,3-dibromoazulene, determined previously, have been analyzed in terms of the weak coupling case of radiationless transition theory. The data show that the nonradiative rate constants for the S2 states of azulene, 1-fluoroazulene, and 1,3-difluoroazulene in the nonpolar solvents follow the l...

Near-IR Absorption Spectrum of Aromatic Excimers

The absorption spectrum of excimers in fluid media has been measured with 1-methylnaphthalene, anthracene, pyrene, and perylene. In all compounds studied an absorption band characteristic of an excimer has been observed in the near-IR. It is shown that the IR band can be understood by assuming that an excimer is essentially a charge-transfer complex: The near-IR band is to be assigned to a transition from the lowest excimer state to a higher state which has an ion-pair character.

Experimental Evidence of the Symmetry Breakdown in 4,8-Diethoxy-2,7-bis(benzo-1‘,3‘-dithiol-2‘-ylidene)-1,3,5,7-tetrathia-s-indacene Compounds

The visible−near-IR absorption spectra of a bis(TTF) molecule, 4,8-diethoxy-2,7-bis(benzo-1‘,3‘-dithiol-2‘-ylidene)-1,3,5,7-tetrathia-s-indacene (1), and of the corresponding monocationic species 1•+ generated by chemical or electrochemical oxidation are reported. IR and Raman studies of 1 and the derived 1xPF6 salt and ESR studies of 1•+ are described. The results of this experimental investigation are compared with those of semiempirical calculations performed using the PM3 or AM1 modeling and simulation programs, and of a normal coordinate analysis. This comparison affords additional evidence for a symmetry breakdown in radical cations based on such bis(TTF) systems. An asymmetrical spin density distribution in the 1•+ radical cation is also confirmed by ESR measurements.

Chemical behaviour of oxotitanium(IV) phthalocyanine (OTiPc) solutions associated with the preparation of OTiPC monolayers and multilayers

The chemical behaviour of oxotitanium(IV) phthalocyanine (OTiPc) in mixtures of dichloromethane and trihalogenoacetic acids was studied by means of spectroscopic measurements. In dichloromethane–trifluoroacetic acid mixtures, OTiPc was relatively unstable, undergoing gradual decomposition via the radical cation of OTiPc; phtalimide and 3-iminoisoindolin-1-one were isolated. In dichloromethane–trichloroacetic acid mixed solvent, on the other hand, a stable solution of OTiPc was obtained with no formation of the radical cation. A monolayer of OTiPc was formed upon spreading this solution onto the water surface and was deposited on substrates with a horizontal lifting method to form a multilayer film. Polarized visible and near-IR absorption spectra and X-ray diffraction of the film indicated a specific order of molecular orientation.

Near-infrared detection of flow injection analysis by acoustooptic tunable filter-based spectrophotometry.

The instrumentation development of a near-infrared (near-IR) spectrophotometer based on an acoustooptic tunable filter (AOTF) and its application as a detector for flow injection analysis (FIA) are reported. In addition to being compact and all solid state, this AOTF-based instrument is very sensitive, has high resolution, and can be rapidly scanned. The latter advantage make it uniquely suited as a detector for FIA, in that it can rapidly record the whole near-IR absorption spectrum of a mixture passing through the FIA flow cell. Subsequent treatment of the recorded spectra with multivariate calibration methods makes it possible to use the FIA, for the first time, for such applications as the simultaneous determination of trace amounts of water and benzene in ethanol. Because all organic compounds absorb light in the near-infrared region, this AOTF-based near-IR detector can serve as a universal detector for FIA; as a consequence, applications of the FIA techniques can be expanded to other areas which are not possible otherwise.

Electronic Structure of Ce@C82: An Experimental Study

Ce@C82 has been isolated with a purity of ∼99.5% by an efficient solvent extraction procedure, followed by HPLC separation. The UV−vis−near-IR absorption spectrum of the metallofullerene was measured at a wavelength range from 300 to 2100 nm. X-ray photoelectron spectroscopy (XPS) of Ce@C82 has been studied. The results suggest that, analogous to the La in La@C82, the Ce in Ce@C82 donates three valence electrons to the carbon cage as opposed to two electrons suggested by a recent ab initio calculation.

SINGLET OXYGEN LUMINESCENCE SPECTRA: A COMPARISON OF INTERFEROMETER‐ AND GRATING‐BASED SPECTROMETERS

The current trend in methodology for determining IR and near-IR absorption spectra is to employ interferometer-based instruments to replace the monochromator-based devices used heretofore. As a dispersion element, the interferometer offers major improvements in spectral resolution (Connes advantage), light throughput (Jacquinot advantage) and data acquisition through multiplexing (Felgett advantage). We have compared signal-to-noise (S/N) ratios of grating-based and interferometer-based instruments for making spectral determinations of near-IR luminescence. Our results show that under identical excitation and detector conditions the interferometer instrument easily outperforms the grating, giving a 10-fold improvement in S/N at high signal amplitude (A488nm = 0.97) and a 20-fold improvement when the signal amplitude is low (A488nm = 0.06). Although some spectral resolution is sacrificed when scan times on the Fourier transform (FT)IR are significantly shortened, the S/N ratio was found only to decrease by a factor of 2 for a 10-fold decrease in scan time. This adds to the advantages of the FTIR technique because the S/N will thus improve for the same total acquisition time.

Isolation and Characterization of an ESR-Active La@C82 Isomer

A minor isomer of LaC[sub 82] has been isolated with an efficient HPLC method. The isolated minor LaC[sub 82] was well identified as a stable metallofullerene molecule. Both mass and ESR spectra confirmed that it was LaC[sub 82] with a 0.836-G [sup 139]La hyperfine splitting. Visible and near-IR absorption spectra, cyclic and differential pulse voltammograms, and [sup 13]C hyperfine splitting structures in an ESR spectrum for the sample showed features quite different from those for the major isomer of LaC[sub 82] with a 1.159-G [sup 139]La hyperfine splitting. We concluded that isolated species is a carbon-cage isomer of the major LaC[sub 82]. 12 refs., 4 figs.