Spectroscopic properties of dipicolinic acid and its dianion

If robust spectroscopic techniques are to be developed for the detection and identification of pathogens, one must understand the relevant spectroscopic properties of the target molecules. In this paper, we employ density functional theory (DFT) to study the structural, electronic, vibrational, optical, and magnetic properties of dipicolinic acid (DPA) and its dianion DPA⁻². Our full geometrical optimization and Mulliken charge analysis show that DFT does not lead to the significant discrepancies between charges on symmetric carbon, hydrogen, and oxygen atoms that are found in less accurate calculations based on the complete active space MCSCF method. Our calculated vibrational frequencies, Raman spectra, and infrared spectra for ground-state DPA and DPA⁻² are in good agreement with experiment, and this is also true of the four calculated ¹³C NMR spectral lines (for α, β, γ, and carboxyl sites). Our time-dependent DFT study of the optical excitation and absorption of both DPA and DPA⁻² provides the first interpretation of the observed near ultraviolet absorption and fluorescence spectra. Finally, we discuss for the first time the effect of a solvent on the spectral properties of DPA.     

Spectroscopic,crystallographic and photochemical properties of atrans-cinnamonitrile — Copper(I) chloride adduct

Summary The first copper(I) complex oftrans-cinnamonitrile,trans- (PhCHCHCN)₃ · CuCl, has been prepared. Its infrared and Raman spectra indicate that bonding of the ligands to the metal occursvia the nitrogen atoms and that coordination induces a slight strengthening of the CN bond and weakening of the C=C bond. Single crystals of the compound and the isostructural bromide have been obtained, and the structure of the latter determined by x-ray methods. A photochemical study, performed on CuCl solutions in pure nitrile or in cinnamonitrile diluted in MeOH have shown the photoassistance of thetrans-cis nitrile isomerization by thetrans- (PhCHCHCN)₃ · CuCl complex. The quantum yields of this isomerization were compared.     

Spectroscopic and photochemical studies of model visual pigment chromophores

The primary step in the bleaching sequence of visual pigments has been shown to occur in picoseconds, while the isomerization of model visual pigment chromophores, protonated 11-cis retinylidene Schiff bases, takes place on a time scale several orders of magnitude slower. Thus, the well-accepted notion that the primary step in visual transduction involves a simple isomerization deserves closer examination. Studies of visual pigments and compounds which mimic visual pigment chromophores are discussed in terms of several alternatives for the nature of the primary step. In addition to discussion of photochemical studies, spectroscopic experiments are discussed. To fundamentally understand the nature of the primary processes in visual transduction it is important to understand the photochemistry, and therefore the electronic structure, of pigment chromophores. Spectroscopic studies aimed at elucidating the electronic structure of polyenic systems are thus discussed.     

Conformational properties and photochemistry of tetrazolylpyridines in low temperature matrices. Spectroscopic evidence for the photochemical carbon-to-nitrogen rearrangement

The most stable conformers of 2-(tetrazol-1-yl)-, 3-(tetrazol-1-yl)- and 2-(tetrazol-5-yl)pyridines undergo photolysis in Ar matrices at cryogenic temperatures to yield pyridin-2-ylcarbodiimide or pyridin-3-ylcarbodiimide. Spectroscopic evidence of carbon-to-nitrogen rearrangement in the case of the 2-(tetrazol-5-yl)pyridine molecule is provided. For the latter molecule a second pathway leads to the 1-cyclopenta-2,4-dienylketenimine formation. The experimental findings are supported by extensive B3LYP/6-311++G(2d,2p) calculations.     

Synthesis and photochemical properties of oligo-ortho-azobenzenes

Azobenzenes have attracted great interest in recent years because of their ability to change conformation upon irradiation. This property has been featured in several applications not only in organic chemistry but also in biology. Even though monoazobenzenes have been extensively studied and documented in the literature, only a few methods are available for the synthesis of oligo-ortho-azobenzenes. Also, their photochemical properties have not been reported so far. This study shows an efficient strategy for the preparation of oligo-ortho-azobenzenes and the investigation of their photochemical properties. It is demonstrated that the absorption spectra are highly influenced by the substituents. Interestingly, none of the ortho-bis-, tris-, or tetra-azobenzenes showed any E → Z isomerization. Only the ortho-nitrogen-substituted monoazobenzenes' photochromic behavior upon UV irradiation was observed.     

Synthesis and photochemical properties of azidohemicyanine

Photochemical activity of azidohemicyanine (1-methyl-4-(4-azidostyryl)quinolinium iodide) was predicted by quantum chemical calculations and confirmed experimentally. The azidohemicyanine, which was synthesized, is characterized by a long-wavelength absorption band (LWAB) in the spectral region 350–500 nm with a maximum at 417 nm; it decomposes with a quantum yield of 0.84±0.17 upon irradiation within the LWAB, the quantum yield being independent of the presence of oxygen. The reaction products identified by ESI mass spectrometry include the corresponding primary amine as well as azo, hydrazo, nitroso, and nitro compounds, some of them are unidentified. The azidohemicyanine possesses the longest-wavelength visible light sensitivity among aromatic azides known so far. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1402–1408, July, 2008.     

Spectroscopic and quantum chemical study of the novel compound cyclopropylmethylselenol

An investigation into the properties of the novel compound cyclopropylmethylselenol has been undertaken by use of Stark-modulation microwave spectroscopy and high-level quantum chemical calculations. Ground-state spectra belonging to six isotopomers of a single conformer of the molecule were recorded and assigned. This conformer, predicted to be the lowest in energy by a series of quantum chemical calculations, possesses a synclinal arrangement of the H-C-C-Se atoms. In addition to the assignment of these ground-state spectra, transitions attributable to vibrationally excited states of the 78Se- and 80Se-containing isotopomers were identified. A tentative assignment of these excited-state spectra to specific vibrational modes has been made with the assistance of a density functional theory calculation at the B3LYP/6-311++G(3df,2pd) level of theory. Close agreement was found between experimental ground-state rotational constants and ab initio equilibrium values calculated at the MP2/aug-cc-pVTZ level of theory. Good agreement was also noted between certain r(s) principal axis coordinates of atoms in the molecule and the corresponding ab initio r(e) values. Limited evidence in favor of the formation of a weak intramolecular hydrogen bond between the H atom of the selenol group and electron density associated with the cyclopropyl ring is discussed.     

Protocetraric acid: an excellent broad spectrum compound from the lichen Usnea albopunctata against medically important microbes

The aim of this study was to investigate the antimicrobial property of the compounds present in the lichen Usnea albopunctata. Ethyl acetate extract of the lichen was purified by column chromatography to yield a major compound which was characterised by spectroscopic methods as protocetraric acid. In this study, protocetraric acid recorded significant broad spectrum antimicrobial property against medically important human pathogenic microbes. The prominent antibacterial activity was recorded against Salmonella typhi (0.5 μg/mL). Significant antifungal activity was recorded against Trichophyton rubrum (1 μg/mL), which is significantly better that the standard antifungal agent. Protocetraric acid is reported here for the first time from U. albopunctata. Thus the results of this study suggest that protocetraric acid has significant antimicrobial activities and has a strong potential to be developed as an antimicrobial drug against pathogenic microbes.     

Photophysical, photochemical, and thermodynamic properties of shikimic acid derivatives: calycin and rhizocarpic acid (lichens)

Photophysical and photochemical parameters of the lichen metabolites calycin and rhizocarpic acid were determined. Experiments were carried out in micellar solutions of 3% Brij 35, at pH 2 and 12, and in acetonitrile. Both metabolites absorb in the UV-A and UV-B regions, and emit fluorescence in the visible region of the solar spectrum. Shifts were not observed in the absorption spectra, at pH 2 and 12. The low phi(c), between 10(-5) and 10(-2), shows that both compounds are photostable in the experimental conditions. For rhizocarpic acid, two values of pK(a) were obtained: 5.1 corresponding to the hydroxyl group, and 9.0 corresponding to the protonated nitrogen. Calycin presents only one value of pK(a): 4.9, that is attributed to the hydroxyl group. L-(+)-Gluconic-gamma-lactonic acid was used as a reference model; the compound showed greater photoinstability, demonstrating that the photodegradation observed occurs mainly in the oxolane carbonylic ring.     

Synthesis and photochemical properties of the indoline series fulgides

Using the Stobbe condensation fulgides that undergo E, Z-isomerization on exposure to UV radiation are obtained from diethyl [2-(1,3,3-trimethyindoline-2-ilidene)ethylidene]succinate, and benzaldehyde and its derivatives; and from 1,2-dimethyl-3-formylindole and -formyl-1,3,3-trimethyl-2-methyleneindoline. The structure and photochemical changes of the fulgides were investigated by means of PMR.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 55–60, January, 1992.     

Luminescent and photochemical properties of the lanthanide-containing polymer materials

The light-transforming polymer materials have been obtained based on the mixed europium cinnamates, cinnamic acid, and high density polyethylene. The materials exhibit an intensive luminescence in the range of 400–700 nm. The photostability of the materials has been higher than that of polymer materials activated solely with europium compounds. The photolysis of the luminescent polymers has been studied. For the obtained polymer compositions the build-up of europium ion and cinnamic acid luminescence has been observed.     

Optical and photochemical properties of N-benzalaminophenylpyridinium perchlorates

The electronic spectra of a number of benzylidene derivatives of N-aminoarylpyridinium perchlorates are associated with absorption localized on the individual pyridinium and azomethine fragments and with intramolecular charge transfer (ICT) from the azomethine fragment to the pyridinium ring. The luminescence is associated with the latter process. The photochromism is due to proton transfer in the excited state of the azomethine fragment, which competes with ICT, and is also due to structural factors.     

Phototoxic and photochemical properties of sanguinarine.

Sanguinarine, a commercial drug exhibiting antimicrobial and antitumor properties, was studied with respect to its basic photochemical characteristics and also with regard to its phototoxicity to mosquito larvae (Aedes atropalpus). Sanguinarine proved to be clearly phototoxic to larvae, with an LD50 of 0.096 mg/mL with near UV exposure as compared with 23.3 mg/mL without. Flash photolysis experiments enabled the study of the triplet state of sanguinarine to be undertaken. Quenching by oxygen occurs with a rate constant of 6 x 10(9) M-1s-1 and time-resolved emission studies indicate that sanguinarine produces a significant amount of singlet oxygen (phi delta = 0.16) as does the isoquinoline alkaloid, berberine (phi delta = 0.25). These values represent the first direct quantitative measurements of photosensitization parameters of these compounds. Additionally, sanguinarine exhibits efficient electron donation properties, undergoing reaction with methyl viologen with a rate constant greater than 10(10) M-1s-1, but is a poor electron acceptor. Phototoxicity of sanguinarine can thus be explained in terms of its photosensitization properties.     

Synthesis and photochemical properties of porphyrie-quinone compounds

The results of studies on the synthesis of porphyrin-quinone compounds and investigation of their photochemical properties are summarized. Effects of various factors (the redox potential, the distance between donor and acceptor moieties, their spatial orientation, the free energy of the reaction, and solvents) on the photoinduced electron transfer in these model systems are discussed. The dyad and triad model systems have been compared. The possibility of using these systems for modeling the primary steps of photosynthesis is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1. pp. 9–24, January, 1996.     

Synthesis and photochemical properties of photo-cleavable crosslinkers

We report herein the synthesis and the development of homo-bifunctional photo-cleavable sulfhydryl group cross-linkers that are able to react and then to photorelease two cysteines leading to the photoregulation of cross-linkers cleavage.     

Spectral and photochemical properties of aromatic nitrenes

Absorption and emission spectra of para-substituted phenyl nitrenes have been studied in glassy ethanol at 77 K. Absorption spectra of nitrenes with neutral and donor substituents are similar to benzyl radical absorption and show three bands at 400–450, 320 and 250 nm. The emission is assigned to T₁-T₀ fluorescence. Nitrenes could be photoreduced to substituted anilines at 77 K.     

Spectral and photochemical properties of isomeric styrylbenzoquinolines

Photochemical properties of the annelated 2-styrylquinoline (2SQ) derivatives 2-styryl-benzo[g]quinoline (2SBQ) and 3-styrylbenzo[f]quinoline (3SBQ) and 3-styrylacridine (3SA) have been investigated by experimental and quantum-chemical methods. It has been found that the trans-isomer of 3SBQ undergoes the photoisomerization reaction with a quantum yield of φ_tc= 0.19, which increases to 0.44 for the protonated form, with the fluorescence quantum yield decreasing from 0.74 to 0.09. In contrast, the trans-isomers of 2SBQ and 3SA are photochemically inert in both neutral and protonated forms, φ_tc < 10^?3. Quantum-chemical calculation by the PM3 method show that the difference in photochemical properties of 2SBQ and 3SBQ is due to the difference in the structure of frontier molecular orbitals, which, in turn, results in the difference of the shape of the potential energy surface of excited S1 states. Comparison of the properties of 2SBQ and 3SBQ with those of 2SQ reveals size and topological effects in the photochemistry of aza-diarylethylenes.     

Synthesis and photochemical properties of porphyrin-azobenzene triad

Porphyrin-azobenzene triad E-2, having ester spacers between the chromophores, was synthesized and its photochemical and thermal isomerization properties were investigated. Triad E-2 showed little electronic communication among the chromophores according to a comparison of the steady-state absorption and fluorescence spectra of E-2 and their model compounds. E-2 showed photochemical E-Z isomerization and subsequent thermal Z-to-E isomerization. The porphyrin chromophore in Z-2, obtained by photoisomerization, did not strongly affect the transient state of thermal Z-to-E isomerization of the azobenzene unit. The high E-isomer composition at the photostationary state indicates the occurrence of triplet energy transfer between porphyrin and azobenzene. E-2 forms a 1:1 complex with 4,4′-bipyridyl (Bipy). The stability constant for E-2 with Bipy was determined (log K = 4.20 mol⁻¹ dm³) by iterative least-squares fitting to a 1:1 binding model.