Photophysical Changes Research Articles

Structure elucidation of laser-dye coumarin-540A by joint application of X-ray diffraction, X-ray fluorescence, prompt fluorescence, UV and visible spectroscopy

X-ray, ultraviolet, and visible light induced photophysical changes of coumarin-540A in ethanol has been studied by the joint applications of X-ray, ultraviolet, and visible spectroscopy. Some impurities were found by X-ray fluorescence measurements. During the high power optical pumping, coumarin showed photochemical changes. Photoproduct emission spectra characteristics showed that photoproduct molecules could also be used as a laser-dye at a different emission frequency in the ultraviolet region.

Structure elucidation of laser dye coumarin-540A by joint application of X-ray diffraction, X-ray fluorescence, prompt fluorescence, UV and visible spectroscopy

X-ray, ultraviolet, and visible light induced photophysical changes of coumarin-540A in ethanol have been studied by the joint applications of X-ray, ultraviolet, and visible spectroscopy. Some impurities were found by X-ray fluorescence measurements. During the high power optical pumping, coumarin showed photochemical changes. Photoproduct emission spectra characteristics showed that photoproduct molecules can also be used as a laser dye at a different emission frequency in the ultraviolet region.

Polypyridine ligands with extended π-conjugation: highly tunable fluorophores

Abstract This account is intended to give a brief overview on the potential of π-conjugated oligomers incorporating polypyridine ligand subunits in the molecular skeleton. The case of 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen) and the terdentate 2,2′:6′2″-terpyridine (tpy) as coordinating building-blocks is emphasized. These structures display intense visible absorption and fluorescence emission properties in solution that can be actively modulated by addition of exogenous species, such as protons or metal ions. The mechanisms underlying the photophysical changes upon protonation or metal complexation are described. Such tunable fluorophores are of tremendous interest in supramolecular chemistry and molecular materials science. To cite this article: S. Leroy-Lhez, F. Fages, C. R. Chimie 8 (2005).

Photophysical and photochemical study of laser-dye coumarin-481 and its photoproduct in solution

An ultraviolet light-induced photophysical and Photochemical changes of coumarin-481 in cyclohexane have been studied by photolysis technique at room temperature, due to its potential applications in photonics, photochemistry, and electronic spectroscopy. During the optical pumping, coumarin-481 showed photochemical changes, therefore as the concentration of coumarin-481 decreased, a photoproduct concentration increased rapidly. An absorption band of the product was observed at around 250 nm. Photoproduct emission spectra characteristics show that photoproduct molecules can also be used as a laser-dye at different emission frequency.

Photodoping of PPV/TiO 2 solar cells

Photophysical changes of MDMO-PPV due to irradiation in air have been studied. Current-voltage (i-V) and capacitance-voltage (C-V) of PPV | TiO 2 | F:SnO 2 devices are analyzed in detail. Both i-V and C-V behavior indicate that initially PPV is slightly oxidized. The degree of oxidation increases further by irradiation. Photodoping increases the acceptor density 2 orders of magnitude and is fully reversible; by keeping the sample in darkness the original response is retrieved.

The synthesis of the ligand 5-bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine-1,10-phenanthroline and of its ruthenium(II) and rhenium(I) complexes. Binuclear species with Cu(I) and some photophysical properties†

The synthesis and characterization of the new polyfunctional ligand phen-pza (5-bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine-1,10-phenanthroline) are described. Also reported are the preparations and some photophysical properties of the ruthenium(II) and rhenium(I) complexes [Ru(bpy)2(phen-pza)](PF6)2 (bpy = 2,2′-bipyridine) and [Re(CO)3Br(phen-pza)], respectively, and the supramolecular copper(I) adducts [Ru(bpy)2(phen-pzaCuI)](PF6)3 and [Re(CO)3Br(phen-pzaCuI)](PF6). Limited studies of copper(II) adducts and photophysical changes upon reaction with NO are also described. The synthesis, structural characterization and some redox chemistry of the model copper(II) complex [Cu(pza)](CF3SO3)2 (pza = bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine) are reported as are the syntheses and characterizations of 5-diethylamino-1,10-phenanthroline (phen-dea) and rhenium(I) complex Re(CO)3Br(phen-dea).

Photophysics and photochemistry of colloidal CdS–TiO 2 coupled semiconductors — photocatalytic oxidation of indole

The photophysical changes taking place by coupling of Cd(OH) 2-coated Q-CdS with colloidal TiO 2 have been examined. The coupling of TiO 2 causes the quenching of the band gap emission of CdS and the red emission is not affected appreciably. For a typical 0.22×10 −3 mol dm −3 of TiO 2 the average emission lifetime of Cd(OH) 2-coated CdS is reduced from 26.4 to 6.8 ns by trapping of the conduction band electron by TiO 2. Cd(OH) 2-coated Q-CdS does not sensitize the reaction of indole, however, the coupled CdS–TiO 2 catalyst photoinitiated this reaction efficiently. The photoactivity of the coupled semiconductor is suggested to increase due to chemical interaction between Cd(OH) 2-coated Q-CdS and TiO 2 which removes Cd(OH) 2 layer to form possibly [CdS–TiO 2(OH) 2]. The photogenerated hole in the coupled catalyst CdS(h +) forms an emissive exciplex with indole. The exciplex is long lived and results in the decomposition of indole ( φ=0.18) to yield indigo with a quantum efficiency of 0.08.The concentration of both TiO 2 and redox couple controls the extent of charge separation in the photocatalyst. The application of coupled semiconductors as catalysts for photosynthetic work is suggested.

Photophysical Properties Changes Caused by Hypercoordination of Organosilicon Compounds: From Trianthrylfluorosilane to Trianthryldifluorosilicate

Photophysical Properties Changes Caused by Hypercoordination of Organosilicon Compounds: From Trianthrylfluorosilane to Trianthryldifluorosilicate

The complexation of flavone derivatives with alkali and alkaline earth metal cations studied by spectroscopic methods

A new kind of fluoroionophore, typified by flavone quasi-crown ether [4′-(dimethylamino)-3,6-(tetraethylene glycol)flavone] and flavone lariat-crown ether, were synthesized and characterized. The photophysical changes upon complexation with alkali and alkaline earth metal cations are explained in terms of the enhancement of a cation-induced intramolecular charge transfer process, which results from the interaction between the bound cation and the carbonyl group of the flavone. The size and shape of the quasi-crown or quasi-cryptand and the charge density of the bound cation control the magnitude of the photophysical changes. Based on the determination of stability constants between different cations and the macrocyclic ligand, our results indicate that the selectivity of flavone quasi-crown ether and flavone lariat-crown ether is better for the Ca2+ cation.

Photophysical Analysis of Class I Major Histocompatibility Complex Protein Assembly Using a Xanthene-Derivatized β2-Microglobulin

Spectral changes and a sixfold increase in the emission intensity were observed in the fluorescence of a single xanthene probe (Texas red) attached to β 2m-microglobulin ( β 2m) upon assembly of β 2m into a ternary complex with mouse H-2K d heavy chain and influenza nuclear protein peptide. Dissociation of the labeled β 2m from the ternary complex restored the probe’s fluorescence and absorption spectra and reduced the emission intensity. Thus changes in xanthene probe fluorescence upon association/dissociation of the labeled β 2m molecule with/from the ternary complex provide a simple and convenient method for studying the assembly/dissociation mechanism of the class I major histocompatibility complex (MHC-I) encoded molecule. The photophysical changes in the probe can be accounted for by the oligomerization of free labeled β 2m molecules. The fluorescence at 610 nm is due to β 2m dimers, where the probes are significantly separated spatially so that their emission and excitation properties are close to those of xanthene monomers. Fluorescence around 630 nm is due to β 2m oligomers where xanthene probes interact. Minima in the steady-state excitation (550 nm) and emission (630 nm) anisotropy spectra correlate with the maxima of the high-order oligomer excitation and emission spectra, showing that their fluorescence is more depolarized. These photophysical features are explained by splitting of the first singlet excited state of interacting xanthene probes that can be modeled by exciton theory.

Onset of Carbonization: Spatial Location Via Simultaneous LIF-LII and Characterization Via TEM

Soot formation/ growth processes include fuel pyrolysis reactions resulting in PAHs, the coalescence/condensation of PAHs and other fuel pyrolysis intermediates ultimately leading to soot. Here simultaneous LIF-LII images identify the fuel pyrolysis ( PAH containing) and soot containing regions along the axial flow streamline within a laminar ethylene/ air diffusion flame. Spectrally resolved emission, excitation wavelength dependence and temporal decay characterize and identify the LIF and LII signals within the simultaneous LIF-LII images. The boundary between the PAH and soot containing regions is postulated to mark the onset of carbonization of condensed/ coalesced soot precursor material. Support for this postulate is based on the observed photophysical changes resulting in a decline in the LIF intensity and emergence of LII signal along the axial streamline between 33 and 34 mm HAB. Bright and dark field transmission electron microscopy images of material sampled from the axial center-line at this a...

The photophysics of disulfonated metallophthalocyanines upon complexation with fluoride

The photophysics of the disulfonated metallophthalocyanines (MPcS2) AlPcS2, GaPcS2 and ZnPcS2 upon complexation with the fluoride ion are characterised. The addition of fluoride to AlPcS2 produces a decrease in the fluorescence lifetime and yield, whereas the triplet lifetime, triplet yield and singlet-oxygen yield increased. The measured enhancement for the triplet yield in aqueous and methanol solvents was 47 and 37% respectively, with a concomitant increase in the singlet-oxygen yield observed in both cases. The photophysical changes are due solely to the intersystem crossing process which is seen to increase on addition of fluoride. The fluoride ion is believed to act as an axial ligand to the AlPcS2 molecule; an equilibrium constant of 4.2 × 103 dm3 mol–1 was determined for this complexation. Addition of excess fluoride to aqueous solutions of AlPcS2 at low and high pH gave a common absorption spectrum. This suggests the H2O axial ligand of the aluminium complex is substituted by fluoride rather than OH–. The photophysics of methanol solutions of GaPcS2 and ZnPcS2 are unchanged upon addition of fluoride, although a blue shift in the absorption spectrum for the gallium species is indicative of complex formation. The consequences of these results for the effect of added fluoride upon the efficacy of these agents as PDT sensitisers is discussed. Also, several properties of triplet-state AlPcS2 in the absence of fluoride ion have been found to be different from previously reported values. The measured lower triplet quantum yields for AlPcS2 in buffered solutions of H2O and D2O were 0.17 and 0.22 respectively. These differences are attributed to the triplet extinction coefficient (ΔIµT) used to calculate literature values. Using the singlet depletion technique we have shown that ΔIµT= 36 000 dm3 mol–1 cm–1 at the triplet absorption maximum of 490 nm, in all solvents. Similarly, the singlet-oxygen yield, determined in D2O buffered solution was found to be 0.17, giving an SΔ of 0.77.

Photophysical and photochemical processes in fullerenes under high-intensity illumination

The origins and structure of molecular and solid-state fullerenes are reviewed. Comparison of the optical properties of solution and solid state indicates strongly that the molecular nature is preserved in the solid state. Picosecond time resolved photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high-intensity illumination on the properties of Fullerene single crystals. A highly non-linear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This non-linear increase is associated with a dramatic shift to the red of the emission maximum. Under similar conditions, the photoconductive response of the fullerenes is also seen to increase non-linearly with input intensity. Temperature-dependent measurements indicate that the non-linear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies. At room temperature, in many samples, the shifting becomes irreversible, and a high molecular weight, insoluble material is formed. The photochemical process is proposed to be a polymerisation-like reaction of the fullerene molecules in the triplet excited state. This is supported by the observation that the rate of the reaction is reduced greatly in the presence of oxygen, an efficient triplet quencher. In conclusion, the response of Fullerene crystals to light is divided into three categories. At low intensities the photophysical processes are characteristic of those of a molecular insulator, the electronic wavefunctions being molecularly localised. At higher intensities, the material undergoes an optically-induced Mott-like transition to a semiconductor/metal, in which the electrons become delocalised in three dimensions. Thirdly, the material is found to be photochemically unstable under some conditions but analysis of the temperature and intensity dependence of Raman spectroscopy shows that the photodegradation process can be predicted and therefore controlled.

Excited State Phenomena in Solid State Fullerene

Photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high intensity illumination on the properties of Fullerenes. A highly nonlinear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This nonlinear increase is associated with a dramatic shift to the red of the emission maximum. The photoconductive response of the fullerenes is also seen to increase nonlinearly with input intensity. Temperature dependent measurements indicate that the nonlinear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies

Ion-responsive fluorescent compounds. 4. Effect of cation binding on the photophysical properties of a coumarin linked to monoaza- and diaza-crown ethers

The properties of fluoroionophores consisting of coumarin 153 linked to monoaza-12-crown-4, monoaza-15-crown-5, and diaza-18-crown-6 are described. The photophysical changes upon complexation with alkali and alkaline-earth metal cations can be described in terms of enhancement of the intramolecular charge transfer in the coumarin dye owing to the direct interaction between the bound cation and the carbonyl group of the coumarin. The magnitude of the photophysical changes is controlled by the charge density of the bound cation. In the case of the diaza-18-crown-6 linked to two coumarins, an interesting specific increase of the fluorescence quantum yield upon binding of potassium and barium ions is observed

Phase-modulated holography: a new technique for investigation of solid-state photochemistry and hologram formation mechanisms

A new holographic technique, called phase-modulated holography (PMH), has been developed. With this method, amplitude and phase gratings generated by photochemical or photophysical changes of molecules dissolved in, e.g. solid matrices, can be monitored separately and simultaneously. Accordingly, the hologram formation mechanism and the underlying photoreaction can be monitored separately and simultaneously allowing conclusions to be made on photoinduced matrix effects and direct measurements of photochemical quantum yields. This feature and an up to eight orders of magnitude improvement of detection sensitivity over conventional phase-insensitive holographic techniques makes PMH the ideal tool for the investigation of new holographic recording materials and of photochemistry and photophysics of molecules in the condensed phase.

PHOTOPHYSICAL PROPERTIES OF NUCLEIC ACID COMPONENTS—1. THE PYRIMIDINES: THYMINE, URACIL, N, N‐DIMETHYL DERIVATIVES AND THYMIDINE

Abstract— Low‐temperature (and some room temperature) absorption and emission, fluorescence and phosphorescence, data including quantum yields and lifetimes have been obtained from the title pyrimidine bases as a function of the nature of the solvent environment. Modest vibrational resolution has been observed for the first time in the absorption spectra, particularly for thymine and uracil. The excitation spectra also show structure. The quantum yields of fluorescence (φF) and phosphorescence are independent of the excitation wavelength. Thymine, thymidine and uracil have profoundly different photophysical properties in polar‐aprotic vs polar‐protic solvents. The N, N‐dimethyl substitution of thymine and uracil produces photophysical changes comparable to the solvent change for the unsubsti‐tuted bases. The species involved in the emission processes is the keto (lactam) form. It is probable that 1,3(n,π*) state(s) has(have) changed order relative to a lowest 1(π,π*) state as a consequence of both the solvent change and N, N‐dimethyl substitution. The lowest triplet state is assigned as 3(n π*). We propose that an important factor contributing to the previously reported excitation wavelength dependence of φF and φT1 (φisc) for nucleic‐acid components is the equilibrium coexistence of H‐bonded and non‐H‐bonded forms each having different photophysical properties. Consideration is given of the impact of the significantly different photophysical properties of nucleic‐acid bases as a function of the nature of the solvent upon the photochemical properties.