Density dependence of the Stokes shift and solvent reorganization energy in supercritical fluids

The Stokes shifts of coumarin 153 (C153) in CF₃H, CO₂ and C₂H₆ have been measured at several reduced densities (0.3–1.8). For C153 in CF₃H, the shifts increase with a decrease in reduced density and show a maximum value at a reduced density of 0.5. In non-polar solvents, the shifts are not dramatically altered as a function of reduced density but slightly increase with a decrease in the reduced density.     

Isomeric boron-fluorine complexes with donor-acceptor architecture: strong solid/liquid fluorescence and large Stokes shift

Novel isomeric fluorine-boron complexes with donor-acceptor architecture have been efficiently synthesized and well characterized. Significant features, such as strong solid and liquid fluorescence, unusual large Stokes shifts, and lower LUMO levels and higher HOMO levels, are observed in these potential multifunctional molecules.     

Separation and analysis of dynamic Stokes shift with multiple fluorescence environments: coumarin 153 in bovine beta-lactoglobulin A

We use time-dependent fluorescence Stokes shift (TDFSS) information to study the fluctuation rates of the lipocalin, beta-lactoglobulin A in the vicinity of an encapsulated coumarin 153 molecule. The system has three unique dielectric environments in which the fluorophore binds. We develop a method to decompose the static and dynamic contributions to the spectral heterogeneity. This method is applied to temperature-dependent steady-state fluorescence spectra providing us with site-specific information about thermodynamic transitions in beta-lactoglobulin. We confirm previously reported transitions and discuss the presence of an unreported transition of the central calyx at 18 degrees C. Our method also resolves the contributions to the TDFSS from the coumarin 153 centrally located in the calyx of beta-lactoglobulin despite overlapping signals from solvent exposed dyes. Our experiments show dynamics ranging from 3-1200 ps. The analysis shows a decrease in the encapsulated dye's heterogeneity during the relaxation, which is taken as evidence of the breakdown of linear response.     

Shift of emission band upon excitation at the long-wavelength absorption edge. Time-dependent fluorescence shift of a fluid ethanol solution of 6-methoxyquinoline

The fluorescence spectrum of 6-methoxyquinoline in fluid ethanol solution at 294 K shifts progressively to the red with time. The results are discussed in connection with the excitation-energy-dependent fluorescence shift previously studied.     

On the origin of the dynamic Stokes shift of Laurdan molecules in lipid membranes

Frequency-resolved time correlated single photon counting experiments have been performed to investigate the time scales and molecular mechanisms for solvation dynamics of 6-dodecanoyl-2-dimethylaminonaphthalene (Laurdan) embedded in liquid-crystalline bilayers of unilamellar dimyristoyl-phosphatidylcholine (DMPC) vesicles. The reconstructed fluorescence spectra as a function of time exhibit a pronounced double-exponential Stokes shift, which is strongly correlated with the rotational diffusion dynamics of the chromophore as evidenced by experiments on transient fluorescence depolarization.     

Free energy gap laws for the pulse-induced and stationary fluorescence quenching by reversible charge transfer in polar solutions

The Stern-Volmer constants for either pulse-induced or stationary fluorescence being quenched by a contact charge transfer are calculated and their free energy dependencies (the free energy gap laws) are specified. The reversibility of charge transfer is taken into account as well as spin conversion in radical ion pairs, followed by their recombination in either singlet or triplet neutral products. The natural decay of triplets as well as their impurity quenching by ionization are accounted for when estimating the fluorescence quantum yield and its free energy dependence.     

Formation of the background component of the analytical signal in the long-wavelength region of X-ray fluorescence spectrum

Components of X-ray background in the long-wavelength spectral region of a crystal diffraction X-ray fluorescence spectrometer were calculated. The calculations took into account the bremsstrahlung radiation of free electrons, diffuse scattering and fluorescence of the crystal analyzer, and high-order reflections of the scattered radiation of the fluorescent sample by the crystal analyzer. The results of calculations were compared with the intensities of background samples measured in the region of the NaK _α fluorescence line on an SRM-25 wave X-ray spectrometer. The experimental background intensities (response function) well correlate with those found by the regression equation with calculated factors. The importance of particular processes in the formation of X-ray background was assessed.     

Selective Hg(II) sensing with improved Stokes shift by coupling the internal charge transfer process to excitation energy transfer

Versatile chemistry of the Bodipy chromophore allows modular assembly of an excitation energy donor, acceptor, and a cation selective ligand in just a couple of steps. The new approach should be applicable in other designs which target molecular sensors with large Stokes shifts and red to near IR emission.     

Time and energy resolved fluorescence of CIS-glyoxal

A pulsed, tunable dye laser has been used to excite the 0---0 band of cis-glyoxal near 4875 Å, at pressures from 30 to 150 mtorr. Fluorescence was resolved through a spectrometer and lifetines measured. Rate constants for the quenching of the vibrationless levels of the first excited singlet states of cis- and trans-glyoxal by argon, cyclohexane and glyoxal are presented. Quenching rates of cis-glyoxal are generally two to three times faster than the corresponding trans rates. A zero-pressure extrapolated lifetime of 0.96 ± 0.02 μsec is obtained for cis-glyoxal.     

The vibrational Stokes shift of water (HOD in D2O)

The vibrational Stokes shift of the OH stretching transition nu(OH) of water is the shift between the ground-state absorption and the excited-state (v=1) emission. A recent measurement on HOD in D(2)O solvent [S. Woutersen and H. J. Bakker, Phys. Rev. Lett. 83, 2077 (1999)] of a 70 cm(-1) redshift, and a subsequent calculation of a 57 cm(-1) redshift using equilibrium molecular dynamics simulations [C. P. Lawrence and J. L. Skinner, J. Chem. Phys. 117, 8847 (2002)] were in good agreement. We now report extensive measurements of the vibrational Stokes shift in HOD/D(2)O using an ultrafast IR pump, Raman probe method. The vibrational Stokes shift is seen to depend on the pump pulse frequency and on time delay; by varying these parameters it can be made to range from 112 to -32 cm(-1) (negative values indicate a blueshift in the excited state). The equilibrium vibrational Stokes shift is actually a negative rather than a positive quantity. Possible reasons for the disagreement between experiment and theory are briefly discussed.     

Temperature-dependent energy gap of the primary charge separation in photosystem I: study of delayed fluorescence at 77-268 K

The dynamics of fluorescence decay and charge recombination were studied in the ether-extracted photosystem I reaction center isolated from spinach with picosecond resolution over a wide time range up to 100 ns. At all temperatures from 268 to 77 K, a slow fluorescence decay component with a 30-40 ns lifetime was detected. This component was interpreted as a delayed fluorescence emitted from the singlet excited state of the primary donor P700*, which is repopulated through charge recombination that was increased by the lack of secondary acceptor phylloquinone in the sample. Analysis of the fluorescence kinetics allowed estimation of the standard free-energy difference -DeltaG between P700* and the primary radical pair (P700(+)A0(-)) state over a wide temperature range. The values of -DeltaG were estimated to be 160/36 meV at 268/77 K, indicating its high sensitivity to temperature. A temperature-dependent -DeltaG value was also estimated in the delayed fluorescence of the isolated photosystem I in which the secondary acceptor quinone was partially prereduced by preillumination in the presence of dithionite. The results revealed that the temperature-dependent -DeltaG is a universal phenomenon common with the purple bacterial reaction centers, photosystem II and photosystem I reaction centers.     

Efficient energy transfer from the long-wavelength antenna chlorophylls to P700 in photosystem I complexes from Spirulina platensis

To study the role of the long-wavelength chlorophylls (Chl) in photosystem I (PSI), the action spectra of P700 photooxidation at 293 and 77 K have been measured for PSI trimeric and monomeric complexes isolated from Spirulina platensis. The long-wavelength Chls which absorb in the region 710dash740 nm transfer excitation energy to the reduced P700 with the same efficiency as bulk antenna Chls, causing the oxidation of P700. The relative quantum yield of P700 photooxidation is about unity (293-77 K) even under the direct excitation of Chl absorbing at 735 nm (Chl735). At 77 K Chl735 exhibits a fluorescence band at 760 nm (F760) whose intensity is quenched under illumination of the PSI trimeric complexes from Spirulina. The relative quantum yield of F760 quenching is not dependent on the wavelength of excitation in the region 620–750 nm. Since the value of the overlap integral between the band of F760 and the absorption band of the cation radical of P700 (P700⁺) is higher than that of the P700 band, it is suggested that Chl735 transfers energy to P700⁺ more efficiently than to reduced P700; energy transfer to P700⁺ causes the quenching of F760. A linear relationship between the photooxidation rate of P700 and the fraction of P700⁺ at 293 K indicates that the energy exchange between PSI subunits of the trimer is negligible. Thus, the antenna of PSI trimers of Spirulina is organized in separate photosynthetic units.     

Quantum vs. classical models of the nitro group for proton chemical shift calculations and conformational analysis

A model based on classical concepts is derived to describe the effect of the nitro group on proton chemical shifts. The calculated chemical shifts are then compared to ab initio (GIAO) calculated chemical shifts. The accuracy of the two models is assessed using proton chemical shifts of a set of rigid organic nitro compounds that are fully assigned in CDCl3 at 700 MHz. The two methods are then used to evaluate the accuracy of different popular post-SCF methods (B3LYP and MP2) and molecular mechanics methods (MMX and MMFF94) in calculating the molecular structure of a set of sterically crowded nitro aromatic compounds. Both models perform well on the rigid molecules used as a test set, although when using the GIAO method a general overestimation of the deshielding of protons near the nitro group is observed. The analysis of the sterically crowded molecules shows that the very popular B3LYP/6-31G(d,p) method produces very poor twist angles for these, and that using a larger basis set [6-311++G(2d,p)] gives much more reasonable results. The MP2 calculations, on the other hand, overestimate the twist angles, which for these compounds compensates for the deshielding effect generally observed for protons near electronegative atoms when using the GIAO method at the B3LYP/6-311++G(2d,p) level. The most accurate results are found when the structures are calculated using B3LYP/6-311++G(2d,p) level of theory, and the chemical shifts are calculated using the CHARGE program based on classical models.     

Measurement of the enhancement effect in different series in X-ray fluorescence analysis

In this study the enhancement effect depending on the changing of the sample of the matrix element has been investigated using energy dispersive X-ray fluorescence analysis. The present measured values of the enhancement effect factors for eight series (La₂O₃−Sm₂O₃, La₂O₃−Eu₂O₃, La₂O₃−Gd₂O₃, La₂O₃−Tb₄O₇, La₂O₃−Dy₂O₃, La₂O₃−Ho₂O₃, La₂O₃−Er₂O₃ and La₂O₃−Tm₂O₃) at 59.5 keV incident photon energy have been studied. It has been observed that changing of the matrix element influence the enhancement effect.     

HOMO energy gap dependence of hole-transfer kinetics in DNA

DNA consists of two type of base-pairs, G-C and A-T, in which the highest occupied molecular orbital (HOMO) localizes on the purine bases G and A. While the hole transfer through consecutive Gs or As occurs faster than 10(9) s(-1), a significant drop in the hole transfer rate was observed for G-C and A-T mixed random sequences. In this study, by using various natural and artificial nucleobases having different HOMO levels, the effect of the HOMO-energy gap between bases (Δ(HOMO)) on the hole-transfer kinetics in DNA was investigated. The results demonstrated that the hole transfer rate can be increased by decreasing the Δ(HOMO) and can be finely tuned over 3 orders of magnitude by varying the Δ(HOMO).     

Stokes/anti-stokes intensity ratio in the resonance region

Relative intensities of the Stokes and anti-Stokes Raman lines associated with the I-I stretching mode of I₂ and symmetric stretching mode of MnO^?₄ are presented. The data indicate that the maxima in the excitation profile of the anti-Stokes scattering are shifted from those of the Stokes scattering. The experimental Stokes/anti-Stokes intensity ratios agree with the theoretical values obtained with parameters from the electronic absorption spectra.     

Remarkable temperature shift of the fluorescence maximum of a crystalline anthracene derivative

The large temperature dependence of the fluorescence maximum of (-)-bornyl 9-anthryl-3-propanoate is interpreted in terms of anharmonicity of thermally populated excimer vibrational levels.