Short-wavelength Band Research Articles

Titanium dioxide: a catalyst for deterioration mechanisms in the third millennium

Titanium dioxide has been used in acrylic dispersions for both house paints and artists' materials, largely for its superior hiding power. While the use of the pigment in canvas paintings attracts critical comment on account of its cloudiness and deadness of colour, its use in outdoor murals is nothing short of catastrophic. Titanium dioxide absorbs ultraviolet (UV) light at wavelengths directly damaging to plastics but facilitates photocatalytic oxidation at shorter wavelengths, leading to degradation of the acrylic binder. These reactions are at their peak when the glass transition temperature (Tg) of the medium is exceeded and moisture is present. Normal window glass will screen the directly damaging UV wavelengths but not the shorter wavelength band responsible for photocatalytic oxidation. This paper discusses observations at an outdoor painting executed in the early 1980s by the New York artist Keith Haring.

Initial results from the XUV Doppler Telescope

We developed a unique telescope to obtain simultaneous XUV images and the velocity maps by measuring the line-of-sight Doppler shifts of the Fe XIV 211 Å line (T = 1.8 MK): the Solar XUV Doppler Telescope (hereafter XDT). The telescope was launched by the Institute of Space and Astronautical Science with the 22nd S520 rocket on January 31, 1998, and took 14 XUV whole sun images during 5 minutes. Simultaneous observations of XDT with Yohkoh (SXT), SOHO (EIT, CDS, LASCO and MDI) were successfully carried out. The images taken with EIT, XDT and SXT are able to cover the wide temperature ranging from 1 to 10 MK, and clearly show the multi-temperature nature of the solar corona. Indeed, we notice that both the cool (1–2 MK) loops observed with EIT and XDT, and the hot (>3 MK) loops observed with SXT exist in the same active regions but in a spatially exclusive way. The XDT red-blue ratio between longer- and shorter-wavelength bands of Fe XIV 211 Å line indicates a possible down-flow of 1.8 MK plasma near the footpoints of multiple cool loops.

Flavonols as metal-ion chelators: complex formation with Mg 2+ and Ba 2+ cations in the excited state

The excited state complexation of flavonol derivatives with alkali and alkali-earth metal ions were investigated. It was found that in the excited state chelating magnesium complexes are more stable than in the ground state, and they are produced both from normal and from phototautomer forms of flavonols. The ‘external’ barium complexes with flavonols containing electron donor groups in 4′-position behave similarly in the excited state. The ‘external’ barium complexes of unsubstituted and 4′-methoxy substituted flavonols dissociate in the excited state. Their disruption occurs as a result of the excited state intramolecular proton transfer (ESIPT) in flavonol molecules. Ejection of metal ion from the crown-cycle was found for the magnesium and barium complexes of azacrown-flavonol – M 2L. In the case of Ba 2L complex the process of cation ejection results in the formation of a complex which does not exist in the ground state. The authors assumed that the short-wavelength band in the emission spectrum of unsubstituted flavonol in acetonitrile might be due to the fluorescence of flavonol molecules forming a hydrogen bond with the solvent.

Synthesis and photonics of ketocyanine dyes, 2,6-bis(4-dimethylaminoalka-1,3-dienyl)-4H-pyran-4-ones and ethoxytridecamethine salts based on them

The reactions of β-dimethylaminoacrolein aminals with 2,6-dimethyl-γ-pyrone lead to 2,6-bis(4-dimethylaminoalka-1,3-dienyl)-4H-pyran-4-ones, whose alkylation affords ethoxytridecamethine salts. The spectral and fluorescence properties of the synthesized compounds were studied. Their absorption spectra are unusual; along with the long-wavelength band in the visible spectral region, they contain a much more intense short-wavelength band in the near UV region. This pattern of the absorption spectra is explained in terms of the model of chromophore interaction, assuming an acute angle between the chromophore “halves” of the polyene chain of the dye molecule. The central pyran ring in the ethoxytridecamethine salts can hamper conjugation in the polymethine chain. Thermochromism of 2,6-bis(4-dimethylaminoalka-1,3-dienyl)-4H-pyran-4-ones (the long-wavelength shift of the absorption spectra on cooling of the solutions) is observed; only the long-wavelength absorption band undergoes a pronounced thermochromic shift. The introduction of methyl or phenyl substituents into the polyene chains of substituted 4H-pyranones decreases the fluorescence quantum yield.

Molecular Distortion and Exciton Coupling Effects in β Metal-Free Phthalocyanine

As pointed out in titanylphthalocyanine, molecular distortion (reduction in molecular symmetry) upon crystallization exerts a significant influence on the optical absorption in the near-IR region. The electronic spectra of β metal-free phthalocyanine (MfPc) has been investigated from the standpoint of molecular distortion and exciton coupling effects. Close examination of the crystal structure of MfPc revealed that the molecular symmetry is reduced from D2h to Ci on going from solution to the solid state. Molecular distortion is found to enhance the band splitting in such a way that the longer-wavelength band is more displaced toward longer wavelengths while the shorter-wavelength band is more shifted toward shorter wavelengths. The electronic spectra of MfPc can mostly be interpreted in terms of the distortion of molecular framework. However, some exciton coupling effect is also observed, as shown by polarized reflection spectra on single crystals as well as the temperature dependence of absorption spectra.

Molecular Distortion and Exciton Coupling Effects in β Metal-Free Phthalocyanine

As pointed out in titanylphthalocyanine, molecular distortion (reduction in molecular symmetry) upon crystallization exerts a significant influence on the optical absorption in the near-IR region. The electronic spectra of β metal-free phthalocyanine (MfPc) has been investigated from the standpoint of molecular distortion and exciton coupling effects. Close examination of the crystal structure of MfPc revealed that the molecular symmetry is reduced from D2h to Ci on going from solution to the solid state. Molecular distortion is found to enhance the band splitting in such a way that the longer-wavelength band is more displaced towards longer wavelengths while the shorter-wavelength band is more shifted towards shorter wavelengths. The electronic spectra of MfPc can mostly be interpreted in terms of the distortion of molecular framework. However, some exciton coupling effect is also observed as shown by polarized reflection spectra on single crystals as well as temperature dependence of absorption spectra.

Photochemistry of Methylglyoxal in the Vapor Phase

The photolysis of methylglyoxal (CH3COCHO) in the presence of synthetic air was studied by laboratory experiments in a static reactor in order to determine its atmospheric lifetime. Quantum yields of the molecular photolysis products CO and HCHO were determined at 298 K as a function of wavelength (260 ≤ λ ≤ 440 nm) and pressure (30 ≤ P ≤ 900 Torr) using an optical resolution of 8.5 nm. The results can be distinguished with respect to both UV/VIS-absorption bands of methylglyoxal. For the short-wavelength band (260 ≤ λ ≤ 320 nm) photolysis quantum yields were found to be unity, independent of wavelength and pressure, consistent with a dissociation mechanism yielding peroxy radicals and CO according to CH3COCHO CH3COO2 + HO2 + CO (P1). For the long-wavelength band (380 ≤ λ ≤ 440 nm) two different processes were distinguished. The major process is photodissociation (P1) with quantum yields (φD) decreasing with increasing wavelength and pressure following the Stern−Volmer relationship: 1/φD(λ) = 1/φ0(λ) + P...

Nonlinear spectroscopy of phosphate glasses containing cadmium selenide nanoparticles

An investigation was made of the photoluminescence and differential absorption spectra of phosphate glasses containing embedded CdSe nanoparticles. These spectra were determined by the pump-and-probe method. The photoluminescence spectrum included a short-wavelength band resulting from direct recombination of electron — hole (eh) pairs and a long-wavelength band associated with carrier recombination via deep levels of defects. Two peaks, attributed to the 1Se → 1S3/2 and 1Se → 2S3/2 transitions, were observed in the short-wavelength photoluminescence band at high excitation rates. The kinetics of the differential absorption spectra was explained by saturation of the absorption of the exciton (eh) transitions and by the appearance, in the excited nanoparticles, of the absorption corresponding to two-pair (2eh) states.

An INDO/S study of the second ππ* transition in the spectra of 1,4-diaminoanthraquinones responsible for their green hue

The INDO/S approach is used to study the structure/spectra relationship for some N,N′-disubstituted 1,4-diaminoanthraquinones. The calculated intensity and position of the short-wavelength absorption band depend on the molecular conformation, in agreement with the experimental data. The reflection spectra of crystalline 1,4-bisanilinoanthraquinone and 1,4-bis(mesitylamino)anthraquinone are reported and the crystal structure of the latter compound has been determined.

Femtosecond dynamics of excitons in π-conjugated oligomers: the role of intrachain two-exciton states in the formation of interchain species

We report femtosecond transient absorption results for solutions and thin films of a substituted oligomer of poly(para-phenylene vinylene) performed over wide spectral and pump-intensity ranges. Solutions and films exhibit a photoinduced absorption (PA) band with dynamics matching those of the stimulated emission, demonstrating unambiguously that these features originate from intrachain singlet excitons. Thin films exhibit an additional short-wavelength PA band with pump-independent dynamics, indicating the formation of non-emissive interchain excitons. Correlations in the dynamics of the two PA features, as well as the intensity-dependence, provide strong evidence that the formation of interchain excitons is mediated by intrachain two-exciton states.

Modification of surface morphology and optoelectronic response in porous Si films by electrochemical methods

The effect of varied H+concentration on surface morphology of porous silicon (PS) is observed by atomic force microscopy technique. A new mechanism of PS formation concerning H+ effect is proposed. The photoluminescence (PL) spectra of two typical PS samples have different responses in the short-wavelength region. Surface photovoltage spectra results imply that the short-wavelength PL band of the PS sample prepared in high H+ concentration may be related to direct band-gap transition.

Multiple emission of N-(1-anthryl)-pyridinium

The acceptor-substituted anthracene N-(1-anthryl)-2,4,6-trimethylpyridinium shows dual fluorescence with a precursor-successor relationship which is most prominent in the intermediate temperature range. At 77 K, only the short-wavelength band (anthracene type fluorescence originating formally from S 2) remains. The additional red-shifted fluorescence band observed at higher temperatures is of charge transfer type. A comparison of the radiative transition rates in absorption and emission suggests that structural relaxation leading to increased orbital overlap takes place in the excited state. This behaviour is unusual for a charge transfer state andits rationalized by the proposal of a bending motion (rehybridization of the nitrogen) induced by the increased electron density within the pyridinium ring.

Luminescence of fluctuation tails of disordered solid solutions

The form of the stationary luminescence spectra of excitons, localized by composition fluctuations, in disordered solid solutions under weak excitation is calculated. The tail states for which there are no nonradiative transition channels are distinguished by means of continuum percolation theory. Such states are responsible for the “zero-phonon” luminescence band. The shape of the short-wavelength luminescence band edge is determined mainly by the number of isolated localizing clusters and their smallest complexes, which decreases rapidly near the mobility threshold. The real luminescence spectrum is due to the simultaneous emission of phonons. The phonon emission determines the form of the long-wavelength wing of the emission band. The computed shape of the emission spectrum is compared with the experimental luminescence spectra of the solid solution CdS(1−c)Sec.

Photoinhibition in the Mediterranean green alga Acetabularia mediterranea measured in the field under solar irradiation

Photoinhibition of photosynthesis was investigated in the Mediterranean green macroalga Acetabularia mediterranea using pulse amplitude modulated (PAM) chlorophyll fluorescence and oxygen evolution measurements in situ under solar radiation. Fluorescence parameters measured during the course of the day showed that moderate photoinhibition occured even in the natural environment of this alga when the sun was at high angles, but photosynthetic capacity was almost fully restored until sunset. A drastic decline in effective quantum yield was monitored when plants collected at 5 m depth were exposed to high irradiance close to the surface. Removal of the short wavelength band from solar radiation using cut-off filters revealed that UV has an overproportional inhibitory effect on photosynthesis. Especially the UV-B part of the spectrum contributes to photoinhibition whereas UV-A seems to be less effective in A. mediterranea. Total recovery of PAM chlorophyll fluorescence was obtained after 2 h of shading in all samples, indicating reversible photoinhibition rather than non-reversible photodamage. Oxygen evolution showed slower kinetics of inhibition.

Bragg gratings with efficient and wavelength-selective fiber out-coupling

A new type of out-coupling Bragg grating has been fabricated in a standard depressed-cladding telecommunication fiber (3% Ge). In addition to the usual Bragg reflection, a narrow shorter wavelength-band is coupled out of the core guide. High out-coupling efficiency, more than 50%, and high spectral selectivity, 3 dB bandwidths as small as 0.4 mm, have simultaneously been achieved. The performances of such outcoupling elements are increased because of strong variations of the index change perpendicularly to the fiber axis. These are due to the fact that the gratings are narrower than the fiber core diameter. Based on the same principle, enhancement of the out-coupling efficiency is also obtained for gratings fabricated in fibers having a triangular core index profile.

Photoinhibition in the Mediterranean Green Alga Halimeda tuna Ellis et Sol Measured in situ

Abstract— Photoinhibition of photosynthesis was investigated in the Mediterranean green alga Halimeda tuna measuring pulse amplitude modulation (PAM) chlorophyll fluorescence and oxygen evolution in situ under solar radiation. Exposure to solar radiation at the surface caused a drastic decline in the photosynthetic quantum yield accompanied by a decline in the photochemical quenching, while the nonphotochemical quenching dramatically increased. The algae recovered from these effects within a few hours indicating that these are mainly due to reversible photoinhibition and only to a smaller extent to nonreversible photodamage. Individuals harvested from deeper waters were more affected than those from shallower waters. Photoinhibition occurs in this alga even in its natural habitat when the sun is at high angles as shown by measuring the fluorescence parameters at hourly intervals during the course of the day. Photoinhibition was less pronounced when the short wavelength band was increasingly removed from solar radiation using cut‐off filters. After exposure of thalli to solar radiation at the water surface, oxygen production decreased drastically within 30 min; this inhibition was even more pronounced in algae harvested from deeper waters. Oxygen measurements at different depths showed optimal photosynthesis at a depth of 1 m. Also for photosynthetic oxygen production inhibited by high solar irradiance at least partial recovery could be observed within several hours. Despite the fact that UVB accounts for a very small fraction of solar radiation, it has a considerable effect on photosynthesis, whereas UVA seems to contribute only little to photoinhibition in H. tuna.

Time-resolved fluorescence of intramolecular charge-transfer systems: experimental results and theoretical predictions

Abstract The time-resolved emission spectrum of dual fluorescent pyrrolidino-benzonitrile in glycerol acetate is interpreted by employing a stochastic model. The model system is made up of an internal coordinate describing the intramolecular adiabatic charge-transfer reaction and a polarization coordinate describing the dynamic behaviour of the polar solvent. The time evolution of the system is governed by a coupled Smoluchowski equation plus source and sink terms to account for the excitation from the ground state and subsequent decay. The model reproduces the dynamic Stokes shift of the frequency maximum of the emission band related to the charge-transfer (CT) state towards the red region, and it provides interpretation of the kinetic behaviour manifested by the system after pulse excitation at the short-wavelength absorption band, corresponding to the locally excited (LE) state. The observation of a dynamic Stokes shift accompanying the time evolution of the CT band implies a kinetic pathway which deviates from steepest descent to the CT state, the rate-determining step being the solvent relaxation.

Room-temperature, short-wavelength (400–500 nm) photoluminescence from silicon-implanted silicon dioxide films

Experiments are reported which explore the possibility of using low-temperature, multiple-energy Si+ ion implantation into thin SiO2 films on Si and subsequent short-time thermal processing to form silicon nanostructures capable of yielding a high-intensity emission in the short-wavelength part of the visible spectrum. A room-temperature short-wavelength PL band of high intensity was found after double implantation with energies of 200 and 100 keV at a temperature of −20 °C to a total dose of 4.8×10 16 cm−2 (atomic concentration about 2×1021 cm−3) and subsequent furnace annealing at 400 °C for 0.5 h in forming gas or by flash lamp annealing at 1050 °C for 20 ms.

Effect of Base on the Solution Behavior of Oxo(d-penicillaminato)(l-penicillaminato)rhenate(V)

The effect of base on the solution behavior of oxo(d-penicillaminato)(l-penicillaminato)rhenate(V) (1) was investigated by 1H NMR, resonance Raman, and UV−visible spectroscopy. The 1H NMR spectrum of 1 consisted of two sets of sharp pen signals in DMSO-d6 and D2O below pH 8 (penicillamine = penH4, the subscript on H indicating the number of dissociable protons present). The data were consistent with ReO(d-penH3)(l-penH2) and its enantiomer ReO(l-penH3)(d-penH2), with penH3 coordinated by N, S, and carboxyl O. These enantiomers have a cis-N2,cis-S2 coordination of the pen ligands, both CO2 groups anti to the oxo ligand and one CO2 coordinated trans to the oxo ligand (form I). In D2O, between pH 8.6 and 10.1, the signals broadened, collapsed, and re-emerged as one set of signals, but no accompanying changes were observed in either the resonance Raman or the UV−visible spectra. The NMR spectral changes were attributed to base-catalyzed interconversion between two enantiomers; both CO2 groups are deprotonated in D2O, and they alternate between ligated and deligated states. Near pH 11, a new form (II) in slow exchange with form I was detected by NMR spectroscopy. The ReO Raman band of I (963 cm-1) was replaced by a new midfrequency band (930 cm-1). The UV−visible bands (346 and 492 nm) decreased in intensity. With increasing pH and near pH 12, the 1H NMR signals of II shifted. The midfrequency ReO band was replaced by a low-frequency band (845 cm-1), and both the UV and visible bands continued to decline as new shorter wavelength bands emerged. These spectral changes were consistent with deprotonation of II to give II‘. The 1H NMR spectra of 1 and ReO(d-penH3)(d-penH2) were nearly identical at pH 12. At this pH, the latter complex is a trans-dioxo species with a ReO band at 846 cm-1; the similar NMR spectrum and ReO band of 1 suggest that II‘ is a trans-dioxo species also. This conclusion was supported by studies in methanol which showed that [ReO(OCH3)(d-penH2)(l-penH2)]2- did not deprotonate.

Porphyridium purpureum (Rhodophyta) from red and green light: characterization of photosystem I and determination of in situ fluorescence spectra of the photosystems

Recent investigations have shown the existence of a light-harvesting complex (LHC) of photosystem I (PSI) with a fluorescence maximum at 680 nm in the red alga Porphyridium purpureum (Wolfe et al., Nature, 367 (1994) 566–568). We have attempted to determine whether the composition of this red algal LHC is invariable or subject to light acclimatization and whether the 680–690 nm fluorescence band of intact algae, which is usually ascribed to photosystem II (PSII), could be partially due to the LHC of PSI. For this purpose, we characterized PSI core complexes and PSI holocomplexes consisting of core and LHC from algae grown under red or green light conditions and calculated the in situ spectra of the photosystems by normalizing the thylakoid spectra of red and green light algae to an identical PSI or PSII concentration. The composition of the LHC seems to be independent of the light quality, since no significant difference was found in the antenna size and the pigment composition of the PSI holocomplexes from red and green light algae; both holocomplex preparations contained 150–160 chlorophylls, 21–22 zeaxanthins and 23–25 β-carotenes per reaction centre, while the core complexes contained approximately 90 chlorophylls, 15 carotenes and no zeaxanthin. Both holocomplex preparations had a fluoroscence maximum (77 K) at 718 nm. A short-wavelength emission band, as reported by Wolfe et al., was not found in the spectra of the isolated PSI complexes or in the PSI spectrum calculated from thylakoid spectra. This indicates that the LHC does not contribute to the fluorescence spectrum of intact cells, but transfers energy very effectively to the PSI core complex.