S2 Emission Research Articles

Synthesis and spectroscopic properties of a novel zinc phthalocyanine with four 17‐membered crown ether voids

AbstractA novel zinc phthalocyanine containing four 17‐crown‐5 ether voids(17C5ZnPc) has been synthesized and characterized. UV‐visible absorption and fluorescence emission spectra and as sociated photophysical parameters have been determined. In contrast to most of the crown ether substituted phthalocyanines, no cofacial dimer formation is observed in the presence of alkali metal salts. In addition to the fluorescence at 710 nm from S1, a strong upper excited state (Soret S2) emission around 424 nm has been detected for the first time in the phthalocyanine series. Fluorescence decay of S1 and S2 emission can be analyzed by mono‐ and biexponential fits respectively. X‐ray structure analysis showed that the crown ether unit is conformationally deformed and oblate that may account for the unusual spectroscopic properties.

Time-Resolved Photoluminescence Measurements on ZnSe-ZnTe Superlattices

Time-resolved photoluminescence of ZnSe–ZnTe superlattices with different Te concentrations has been measured in the picosecond time region. The samples show typical S1 (blue band) and S2 (green band) emissions in the time-integrated spectra. In the time-resolved spectra, both samples exhibit an asymmetrically broadened shallow component at the high-energy side of the luminescence band, indicating the presence of shallow localized excitons due to long-range potential fluctuations.

Spectroscopic Properties of Spheroidene Analogs Having Different Extents of π-Electron Conjugation

The spectroscopic properties of spheroidene and a series of spheroidene analogs with extents of π-electron conjugation ranging from 7 to 13 carbon−carbon double bonds were studied using steady-state absorption, fluorescence, fluorescence excitation, and time-resolved absorption spectroscopy. The spheroidene analogs studied here were 5‘,6‘-dihydro-7‘,8‘-didehydrospheroidene, 7‘,8‘-didehydrospheroidene, and 1‘,2‘-dihydro-3‘,4‘,7‘,8‘-tetradehydrospheroidene and taken together with data from 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene already published (DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lugtenburg, J.; Farhoosh, R.; Frank, H. A. Biochim. Biophys. Acta 1992, 1102, 107) provide a systematic series of molecules for understanding the molecular features that control energy transfer to bacteriochlorophyll in photosynthetic bacterial light-harvesting complexes. All of the molecules were purified by high-pressure liquid chromatographic techniques prior to the spectroscopic experiments. The absorption spectra of the molecules were observed to red-shift with increasing extent of π-electron conjugation. The room temperature fluorescence data show a systematic crossover from dominant S1 → S0 (21Ag → 11Ag) emission to dominant S2 → S0 (11Bu → 11Ag) with increasing extent of conjugation. The S2 fluorescence quantum yields of all the carotenoids in the series were measured here and indicate that 3,4-dihydrospheroidene with nine carbon−carbon double bonds has an S2 quantum yield of (2.7 ± 0.3) × 10-4 which is the highest value in the series. The lifetimes of the S1 states of the molecules were determined from time-resolved transient absorption spectroscopy and found to decrease as the conjugated chain length increases. The transient data are discussed in terms of the energy gap law for radiationless transitions which allows a prediction of the S1 energies of the molecules. The implications of these results for the process of light harvesting by carotenoids in photosynthesis are discussed.

Pressure Dependence of the Electronic Band Gap in 6HSic

AbstractPhotoluminescence experiments on 6H‐SiC doped with nitrogen have been performed at low temperature (T = (29 ± 2) K) under hydrostatic pressure up to 5 GPa. The pressure coefficients of the S0, R0, P0, S02, and R02 emission lines related to the neutral nitrogen donor bound excitons were determined. The pressure coefficient of the indirect gap of 6H‐Sic deduced from the P0 line turns out to be +2.0 meV/GPa. Nonrelativistic band structure calculations within the density‐functional theory based on the local density approximation are used to calculate the pressure coefficients of the indirect band gaps of the 6H, 4H, and 3C SiC polytypes. The comparison with available experimental data shows good agreement with the theoretical results.

Vibronic excitations of large molecules in solution studied by two-color pump–probe experiments on the 20 fs time scale

The ultrafast vibronic response of organic dye molecules in solution is studied in pump–probe experiments with 30 fs excitation pulses resonant to S0–Sn transitions. The molecular dynamics is probed either by pulses at the same spectral position or by 20 fs pulses overlapping with both the S0–S1 absorption and emission bands. Three contributions on distinctly different time scales are observed in the temporally and spectrally resolved two-color measurements. In the regime below 50 fs, a strong coherent coupling of the S0–Sn and the S0–S1 transitions occurs that is due to coherent vibrational motions in the electronic ground state. This signal is superimposed on the fast bleaching of the electronic ground state, resulting in a steplike increase of transmission. In the range of the S0–S1 emission band, one finds a subsequent picosecond rise of transmission that is due to stimulated emission from vibronic S1 states. The data demonstrate that the relaxation of Sn states directly populated by the pump pulses is much faster than the buildup of stimulated emission. This gives insight into different steps of intramolecular vibronic redistribution and is compared to the Sn–S1 relaxation in other molecules.

Continuous and time-resolved luminescence spectroscopy of sulfonatoporphyrin dopants in sol-gel hosts

Organic chromophores are of interest for use in remote luminescent chemical sensor applications, as they can be highly sensitive to conditions such as oxygen concentration, pH, temperature, metal ion activity, etc. Small fluctuations in local environmental parameters can lead to substantial perturbations in the optical properties of select organic probe molecules. Lifetime studies were performed on the S1 emission of tetra-4-sulfonatophenylporphyrin and the zinc (II) and cadmium (II) adducts in sol-gel derived aluminosilicate hosts. Vibronically-coupled quenching was observed to occur as a function of the complexed transition metal ion.

Thiophosgene monomer–dimer equilibria in perfluoroalkane solutions: spectroscopic and photophysical implications of self-association in the ground state

The S2–S0 absorption, emission, and emission excitation spectra of thiophosgene in a perfluoroalkane solvent have been measured as a function of the analytical concentration, C, of the solute. Strong evidence of the formation of van der Waals dimers at 10−4 M < C < 10−1 M and of higher oligomers at C > 10−1 M has been found. Variations in the absorption spectra at C < 10−1 M have been analyzed successfully in terms of a single [Formula: see text] equilibrium, for which the value of K is (5.0 ± 0.4) × 102 M−1 at 295 K. The mole fractions of monomer and dimer have been calculated at each C, and have been used to construct the absorption spectrum of the dimer. Both the monomer and dimer have similar S0 → S1, and S0 → S2 absorption spectra, but the dimer has an oscillator strength that is about twice that of the monomer. The effects of self-aggregation on the S2–S0 emission and excitation spectra have been analyzed. Absorption by the dimer in the ultraviolet also leads to emission, but with a different quantum efficency compared with direct excitation of the monomer. The implications of these observations on the interpretation of the spectroscopy, photophysics, and photochemistry of thiophosgene in solution are assessed.

Interpretation of the temperature-dependent behaviour of the emission from isoelectronic tellurium centres in epitaxial ZnSe1-xTex

The emission from isoelectronic tellurium centres in metal-organic chemical vapour deposition ZnSe1-xTex has been investigated with photoluminescence and photoluminescence excitation spectroscopy. The emission is dominated by two bands, previously called the S1 and S2 bands and attributed to excitonic emission at isolated tellurium ions or tellurium ion pairs or clusters. The authors account for the temperature-dependent behaviour of these bands with a model in which the tellurium isoelectronic centres behave as spherical potential wells which trap holes via low lying resonant states in the valence band. Changes in intensity of the emission when both bands are observed together have been satisfactorily accounted for within the framework of this model and indicate that the S1 emission results from exciton recombination at tellurium pairs whilst the S2 emission results from recombination at tellurium triplets or larger clusters.

S0 Emission Trading Rights: A Model for Other Pollutants

Beginning in 1995, electric utilities will have to reduce sulfur dioxide emissions under new rules that not only will revolutionize the regulation of sulfur dioxide but also provide a model to reduce a wide variety of other pollutants. Under authorization of the Clean Air Act of 1990, the Environmental Protection Agency will issue tradable emissions allowances to electric generating utilities. Actual emissions will have to match allowances EPA assigns to utilities, giving them a right to emit a predetermined amount of sulfur dioxide. Each allowance will be equivalent to the right to emit 1 ton of sulfur dioxide per year. The allowances will be based on utilities' average emissions in the reference years 1985 through 1987. EPA hopes thereby to harness the free enterprise system and reduce emissions of a major precursor to acid rain to 9 million tons annually—a 10 million ton reduction from 1980. The chemical process industry would do well to pay ...

Spectroscopic and dynamical studies of the S1 and S2 states of decatetraene in supersonic molecular beams

Fluorescence and fluorescence excitation spectra of all-trans-2,4,6,8-decatetraene have been obtained in free jets and in inert-gas clusters. In isolated decatetraene, excitation into 1 1Bu (S2) results in emission from both S2 (1 1Bu→1 1Ag) and S1 (2 1Ag→1 1Ag) on time scales that are faster than the 10 ns experimental resolution. In clusters, rapid electronic and vibrational relaxation leads to long-lived (360 ns) emission from thermally relaxed levels of S1. Direct excitation of low-lying, S1 vibronic levels in cold, isolated molecules also results in long-lived S1→S0 fluorescence, as expected for this symmetry-forbidden transition. The detection of S1 emission in free decatetraene has permitted the first detailed study of the vibronic structure and kinetics of the 2 1Ag state of an isolated, all-trans linear polyene. The S1←S0 fluorescence excitation spectrum is rich in low-frequency vibronic progressions. Analysis of this spectrum suggests that the transition not only is made allowed by vibronic coupling involving low-frequency bu skeletal modes (Herzberg–Teller coupling), as for polyenes in condensed phases, but also gains intensity from interactions between the electronic motion and the hindered rotations (torsions) of the terminal methyl groups. Preliminary analysis suggests that the barriers to internal rotation of the methyl groups must be substantially reduced in the 2 1Ag (S1) state. For isolated decatetraene, the 2 1Ag fluorescence lifetimes show a monotonic decrease with increasing vibrational energy, presumably due to increased mixing with the 1 1Bu state.

Changes in canine ventricular refractoriness induced by trains of subthreshold high-frequency stimuli

Epicardial electrodes were applied to 12 thoractomized dogs to determine the effects of trains of subthreshold conditioning stimuli (TSc) on ventricular refractoriness when delivered preceding a premature suprathreshold stimulus (S2). Several factors were analyzed: (1) the influence of TSc pulse frequency (100–900 Hz); (2) the delay between TSc and S2 (1 or 10 ms); (3) the distance between the electrodes for the emission of TSc and S2 (same electrodes or different electrodes at 3 mm); and (4) S2 current intensity (two- or fourfold diastolic threshold). The TSc (mean current intensity 0.33 mA, range 0.1–0.7) were found to significantly prolong the effective ventricular refractory period (EVRP) at all train pulse frequencies. The EVRP increment was progressively greater as pulse frequency was increased, the maximum EVRP increment being at 900 Hz (mean 50.8 ± 32.3 ms; maximum increment 130 ms). On increasing S2 current intensity, the EVRP increment was less (maximum value 35 ms) and less consistent (in four of six dogs); in two cases the EVRP was shortened. The increase in delay between TSc and S2 attenuated the EVRP prolongation, which was present in only three of six dogs tested, and the EVRP was shortened in two dogs. There was no EVRP prolongation at any TSc pulse frequency when TSc and S2 were delivered at different electrodes. Thus TSc decreases myocardial ventricular excitability, prolonging EVRP in direct proportion to TSc pulse frequency. However, this property is limited by S2 current intensity as well as the time and distance between TSc and S2.

Determination of thiamine using continuous flow molecular emission cavity analysis

A continuous flow method for the determination of thiamine hydrochloride (20.0-240.0 microgram ml(-1), 5.9 x 10(-5)-7.1 x 10(-4)m) is described. The sample was mixed with an excess of sodium hydroxide and remained in the delay coil for 20 min at 90 degrees C. The solution was then mixed with an excess of orthophosphoric acid and the hydrogen sulphide evolved was transferred continuously into the cavity to generate a molecular++ emission of S2. The analysis is completely automated, requires no sample pre-treatment and samples can be analysed at a rate of 30 samples h(-1) with a relative error of 1-2%. The method was evaluated by carrying out an interference study with common excipients and other water-soluble vitamins, a recovery study and by the analysis of commercial formulations. Results compared well with those obtained using the official method. The method was also applied to content uniformity tests.

Red and near-infrared laser-induced emission of S2 in an Ar matrix

The S2 emission in the red and near-infrared regions has been reinvestigated using the laser-induced emission technique. Four progressions of S2 in solid Ar were observed in the emission spectra following excitation in the UV region with a pulsed Nd-YAG laser system. 34S-isotopic shifts allowed the determination of spectroscopic parameters for these progressions. Among them, ν00=19 757 and 15 417 cm−1 for the two progressions with distinct zero-phonon lines (ZPL); ν00=19 384 and 15 003 cm−1 for the other two with no ZPL, respectively. The progressions are assigned as the c 1Σ−u→X 3Σ−g and the c 1Σ−u →a 1Δg transitions of S2 in two matrix sites, respectively. The data yield the T0 values for the c 1Σ−u and the a 1Δg states. The relative intensities of these four progressions varied with excitation wavelengths and isotopic species.

Application of photon count statistical measurements to the stationary fluorescence of fluoranthene

AbstractThe broad S1 → S0 emission spectra of fluoranthene at room temperature in solution and in solid state down to T = 25 K have been recorded in a standard photon counting experiment. The wavelength dependence of the photon count statistics have been measured. In time scales of several microseconds deviations from poission statistics are found which exhibit a fine structure not or less resolved in the broad luminescence spectrum.

Ultraviolet and visible fluorescence produced by controlled electron impact on SO2

The emission spectrum in the ultraviolet and visible region of the optical spectrum (2000–8000 Å) produced by electron impact on SO2 has been studied under single-collision conditions. Two major features have been observed: the A3Π → X3Σ− system of the SO fragment (2400–2650 Å) and a broad, unstructured emission extending from 2500 to 4600 Å, which has been identified as arising from the excitation–de-excitation of the SO2 molecule. Absolute emission cross sections for the SO system have been measured for incident electron energies from threshold to 500 eV, and a value of 5.2 × 10−18 cm2 has been found at 100 eV with an uncertainty of approximately ± 18%. The maximum in the emission cross section is at 30 eV (6.5 × 10−18 cm2). The measured threshold of 11.5 eV is about 1 eV above the minimum energy required for partial fragmentation of SO2 into SO (A3Π) and a ground-state oxygen atom, indicating that very little translational kinetic energy is carried away by the dissociation fragments in the near-threshold region. A complex excitation–de-excitation scheme involving three different excited states of the SO2 molecule, e.g., the 8a1 → 3b1, 1.3B1 states and the 5b2 → 3b11A2 state, produces the broad 2500–4600 Å emission feature. At 100 eV, an emission cross section of 12.2 × 10−18 cm2 has been measured for the SO2 system. Several O1 and S1 emission lines have also been identified, yet they are found to be extremely weak with estimated emission cross sections smaller than 10−21 cm2 at 100 eV impact energy.

Characterization of the flame photometric detector in the sulphur mode

The effect of experimental conditions such as gas flow rates, detector and column temperatures on sulphur response are discussed. It is concluded that the response of the flame photometric detector is affected by the O/H ratio and the total gas flow. Column temperature has an important influence upon S2 emission. It is shown that the sulphur response of the FPD is dependent on the compound containing the sulphur.

Luminescence of some metalloporphins including the complexes of the IIIb metal group

Luminescence yields and decay lifetimes were measured on some metalloporphyrins including the complexes of the IIIb metal group. The S2 emission was observed for AlClTPP (TPP: tetraphenylporphin), GaClTPP, and InClTPP as well as some other TPP metal complexes, ZnP (P:porphin) and H4P2+ (diacid), while it was not detected for H2TPP, H2OEP (OEP: octaethylporphin), AlClOEP as well as other OEP metal complexes. The rate of S2→S1 internal conversion is dependent upon the peripheral substituents rather than the central metal ion and is enhanced in the porphyrins which do not emit the S2 fluorescence. On the other hand, the intersystem crossing plays a more important part in the relaxation process of the S1 state. With increase in the atomic number of the central metal ion as well as its axial ligand, the yield and decay lifetime of the S1 emission are reduced. In the relaxation of the T1 state, the radiative decay is also accelerated for the central metal ion with higher atomic number.

Effects of Intermolecular Hydrogen-bonding on the Luminescence Properties of Acetophenone, Characterization of Emission States

Abstract The effects of intermolecular hydrogen-bond formation by protic solvents on the absorption and luminescence properties of acetophenone have been investigated. It has been concluded from analysis of the T←S0 absorption, phosphorescence excitation spectra and the emission characteristics that the lowest triplet state of acetophenone in its vapor phase and in nonpolar as well as in moderately polar solvents (e.g., ethanol, EPA etc.) is of n-π* character. In rigid polar media, acetophenone exhibits a very weak 1π-π*→S0 fluorescence and a weak 3π-π*→S0 phosphorescence superposed with the principal 3π-π*→S0 emission. A progressive enhancement of both of these emissions with increasing polarity of the solvent has been observed. It is shown that the fluorescence and the dual phosphorescence emissions from acetophenone in rigid polar media arise as a result of decreased efficiencies of the spin-orbit and vibronic coupling interactions due to intermolecular hydrogen-bond formation by the protic solvents.

ChemInform Abstract: THE SPECTROSCOPY OF A BENZIL‐BIBENZYL MIXED CRYSTAL SYSTEM. EFFECTS OF ISOTOPIC SUBSTITUTION

AbstractIn einer vorangegangenen Arbeit wurde über Anregungs‐ und Emissionsspektren von in Bibenzyl eingebettetem Benzil berichtet (hochaufgelöste Niedertemp.‐ Spektren).

The spectroscopy of a benzil–bibenzyl mixed crystal system. Effects of isotopic substitution

We report the T = 4.2 K, S0→S1 excitation and T1→S0 emission spectra of five different isotopic derivatives of benzil doped into bibenzyl single crystals. Selective excitation of the individual components of the isotopic mixtures has been observed. The 0–0 transition energy shifts with isotopic substitutions are additive, as may be expected for systems where geometry changes upon excitation consist primarily of linear displacements of the normal coordinates.