Showing papers on "Stokes shift published in 2002"

Interband and Intraband Optical Studies of PbSe Colloidal Quantum Dots

Abstract: PbSe nanocrystal colloids exhibit a well-defined excitonic structure with the lowest energy exciton tuning from 0.5 to 1 eV, as a function of size. Band-edge fluorescence is observed from 1.2 to 2 μm with a small Stokes shift, sub-μs lifetime, and near-unity quantum yield. Upon pumping at 1.064 μm, the first exciton decay is consistent with radiative relaxation at low pump intensity and with Auger recombination at higher pump intensities. Optically induced absorption is observed at approximately midgap. These transitions exhibit strengths similar to the interband exciton and are size-tunable. They are assigned to the 1Se,h−1Pe,h and 1Pe,h−1De,h intraband excitations. Intraband pump−probe measurements of the 1Se,h−1Pe,h transition reveal a short lifetime and the absence of the phonon bottleneck.

Ultrafast Dynamics of Porphyrins in the Condensed Phase: II. Zinc Tetraphenylporphyrin†

Abstract: Femtosecond spectroscopic studies of zinc tetraphenylporphyrin (ZnTPP) in benzene and dichloromethane are reported, combining both fluorescence up-conversion and transient absorption measurements. The purpose is to investigate the initial electronic and vibrational relaxation of the S_1 and S_2 excited states, in a system in which interference from solvent rearrangement is insignificant as evidenced by the small Stokes shift in the fluorescence. Excitation of the low-lying singlet excited state (S_1) results in nanosecond relaxation, while excitation to S_2, the Soret band, leads to multiple electronic and vibrational relaxation time scales of S_2 and S_1 populations, from hundreds of femtoseconds to tens of picoseconds. The systematic and detailed studies reported here reveal that the Soret fluorescence band decays with a lifetime in benzene of 1.45 ps for excitation at 397 nm, while emission monitored at the same wavelength, but for two-photon 550 nm excitation, decays biexponentially with 200 fs and 1.0 ps time constants. In addition, the Soret fluorescence decay lifetime for 397 nm excitation is distinctly longer than the rise time of S_1 fluorescence for the same excitation, which varies with wavelength. These observations are consistent with the model presented here in which the Soret band structure consists of absorption from S_0 to two manifolds of states with distinct electronic and vibrational couplings to S_1 and higher electronic states. To compare with literature, we also measured the S_2 lifetime in dichloromethane and found it to be 1.9 ps, a lengthening from its value in benzene. However, the transient fluorescence intensity is greatly reduced. These observations in dichloromethane provide evidence of an ultrafast (<100 fs) channel for electron transfer from ZnTPP to dichloromethane for a subset of excited molecules in favorably oriented contact with the solvent, that is, a bifurcation of population. Finally, solvent-induced vibrational relaxation of the S_1 population following internal conversion from S_2 occurs over a range of time scales (picoseconds to tens of picoseconds) depending on the wavelength (fluorescence or transient absorption), and the observed rate indeed changes with solvent.

Absorption and emission in oligo-phenylene vinylene nanoaggregates: The role of disorder and structural defects

Abstract: The impact of exciton-phonon coupling and defect states on the photophysical properties of p-distyrylbenzene nanoaggregates is studied numerically. Molecular packing within aggregates is based on the known crystal structures of poly-p-phenylene vinylene (Type I) and the five phenyl group oligomer (Type II). Calculations of absorption and emission are conducted using a reduced basis set consisting of all one- and two-particle vibronic states. The calculated spectra are very similar for both aggregate types, the only substantial difference being the polarization directions for the J-band and 0-0 emission line. Under the noninteracting domains approximation the calculated nanoaggregate absorption spectrum is in excellent agreement with experiment, assuming an exciton coherence length of approximately 20 A. In the calculated emission spectrum the 0-0 emission is uniquely polarized compared with the rest of the vibronic progression, also in agreement with experiment. The 0-0 emission intensity in defect-free Type I and II aggregates is linearly proportional to the total number of molecules, becoming superradiant beyond a certain size threshold. The 0-0 emission is highly sensitive to stacking faults and dislocations. These defects account for the measured Stokes shift, but quench the 0-0 emission (and superradiance) while only slightly affecting the rest of the vibronic progression. Adding orientational point defects to an aggregate with stacking faults and/or dislocations enhances the 0-0 oscillator strength, bringing the 0-0 emission intensity into good agreement with experiment.

Time-resolved fluorescence and absorption spectroscopies of porphyrin J-aggregates

Abstract: Dynamics of excited states in porphyrin J-aggregates has been investigated using femtosecond time-resolved fluorescence and absorption spectroscopies. An ultrafast relaxation process due to internal conversion (IC) from the S2-exciton state to the S1-exciton state is observed as an S2-fluorescence and a recovery from a bleaching of the S2-exciton state. The S2-fluorescence shows a sharp spectrum with almost no Stokes shift with a decay-time constant of 360±70 fs. In the transient absorption spectrum, the bleaching of the S2-exciton state disappears with a time constant of about 300 fs, which is in agreement with the result of the time-resolved fluorescence data. Relaxation dynamics of the S1-exciton following S2→S1IC is also studied and several relaxation processes such as an intra-aggregate vibrational energy redistribution, vibrational and phase-space coolings are investigated.

Large and Fast Relaxations inside a Protein: Calculation and Measurement of Reorganization Energies in Alcohol Dehydrogenase

Abstract: CIS electronic structure calculations have been employed to derive the fluctuation in the ground to excited state energy gap of dihydroNicotinamide Adenine Dinucleotide (NADH) embedded in the Horse Liver Alcohol Dehyrogenase (LADH figure 1.) protein matrix as a function of time. Classical molecular dynamics trajectories of 130 ps in length have been generated for an LADH+NADH dimer in water to elucidate the dynamics of equilibrated LADH over a 5ps timescale. Steady-state absorption and emission spectra have then been calculated to an accuracy within 10 % of experiment (figure 2) by utilising ca. 10,000 CIS QM/MM calculations per simulation coupled to our framework for predicting optical spectra based on linear response theory. 1,2,3 It has been found that the accuracy of the results obtained are very dependent on the quality of the molecular dynamics trajectory, especially in the parameterisation of the nicotinamide moiety. This can be explained by considering the orbitals involved in the excitation leading to the observed emission. Figure 3 shows a snapshot from one of the CIS calculations. The orbitals are localised on the nicotinamide moiety and so it is fluctuations in this section of the NADH that are responsible for the emission spectra of LADH. The results have shown that applications of linear response theory can work for systems, such as NADH, which have very large reorganisation energies (Stokes Shift ≈ 1 eV) yielding predicted spectra that are within 10 % of experiment. Previous work with different NADH parameters yielded poor results implying that the method employed in this work provides an independent method for validating certain force field parameters.

Time-dependent vibration stokes shift during solvation: Experiment and theory

Abstract: Peak shift dynamics of CO stretching modes of [Re(R2-bpy)(CO)3Cl] [R = COOH and COOEt] complexes in their MLCT excited state were studied in polar solvents using femtosecond visible pump and IR probe spectroscopy. For Re complex in dimethyl formamide, peak shift magnitudes for the three CO stretching modes are different but their dynamics are identical. Comparison in different alcohols showed that peak shift magnitude increases with solvent dielectric constant and peak shift time follows the trend of solvation time. The observed peak shift is attributed to solvation-induced time-dependent vibrational Stokes shift. A general theory based on the Onsager dielectric continuum model of solute solvent interaction and time-dependent reaction field was developed to qualitatively describe time-dependent vibrational Stokes shift. The theory predicts that the peak shift correlation function is determined by solvent dielectric relaxation at the vibration frequency, and the peak shift magnitudes are solvent and normal...

Synthesis, structure and properties of a new two-photon photopolymerization initiator

Abstract: A new two-photon free-radical photopolymerization initiator, (E,E)-4-{2-[p′-(N,N-di-n-butylamino)stilben-p-yl]vinyl}pyridine (abbreviated to DBASVP), has been synthesized. Quantum chemistry calculations showed that the new initiator possesses a large delocalized π electron system, a large change in dipole moment on transition to the excited state and a large transition moment. The calculated two-photon absorption cross-section is as high as 881.34 × 10−50 cm4 s photon−1. The single-photon and two-photon absorption and fluorescence properties in various solvents have been investigated carefully. The new initiator exhibits outstanding solvent-sensitivity, which experimentally interprets the excellent electron delocalized properties of the molecule. A microstructure has been fabricated under irradiation at 800 nm using a 200 fs, 76 MHz Ti:sapphire femtosecond laser.

Two-color three pulse photon echo peak shift spectroscopy

Abstract: Two-color three pulse photon echo peak shift spectroscopy (2C3PEPS) has been used to probe correlation in electronic transitions in two different regions of the electronic spectrum of a chromophore in the condensed phase. The 2C3PEPS can be done in an “uphill” or a “downhill” sense, where the first two interactions are of lower (higher) frequency than the final interaction with the radiation field. In both cases no correlation between the two spectral regions is observed at very short times. Different parts of the spectrum gain correlation owing to ultrafast solvent motion and the accompanying Stokes shift. We propose a model for the spectral shift that incorporates a conditional probability distribution for transition frequencies in the two spectral regions. The model qualitatively reproduces all the features of complete numerical simulations. Our results show that memory is partially conserved during the spectral diffusion process from the pump to the probe region. The downhill difference peak shift is very sensitive to the time scale of the inertial component of solvation and seems very promising for an accurate determination of this time scale.

Low Stokes shift in thick and homogeneous InGaN epilayers

Abstract: The optical properties of thick InxGa1−xN layers have been studied using optical absorption and cathodoluminescence techniques. The indium composition x of the layers ranged from 0.03 to 0.17 as determined by Rutherford backscattering measurements. The difference between the band gap and the peak emission energy (Stokes shift) was found to be considerably smaller than reported in the past for these alloys. Monochromatic images show that light emission from most of the film is homogeneous and is associated with a low Stokes shift. A second emission band at longer wavelengths is observed for x⩾0.08. This band originates from indium-rich regions in the vicinity of extended defects, and exhibits a larger Stokes shift. Our observations indicate that it is possible to grow InGaN epilayers with high indium composition, high homogeneity, and lower Stokes shift.

Gap bowing and Stokes shift in InxGa1−xN alloys: First-principles studies

Abstract: First-principles pseudopotential plane-wave calculations are used to investigate the electronic properties of InxGa1−xN alloys The alloys are described within a cluster-expansion method considering configurations in large 64-atom supercells The unusual behavior of the gap bowing is discussed as a function of composition We show that the strongest contribution to the gap bowing is due to a structural effect, ie, the composition-induced disorder in the bond lengths We explain the observed huge Stokes shifts by band gap variations due to composition fluctuations and phase separation

Electronic structure transformation from a quantum-dot to a quantum-wire system: Photoluminescence decay and polarization of colloidal CdSe quantum rods

Abstract: Radiative lifetime, polarization, and the global Stokes shift of colloidal CdSe quantum rods with aspect ratios from 1.9 to 3.8 are measured at room temperature. The radiative lifetime and the global Stokes shift show nonmonotonous dependence on the aspect ratio of the nanorods, and strong linear polarization in emission sharply appears as the aspect ratio crosses a turning point. The features of radiative lifetime and polarization versus aspect ratio in these nanorods indicate a transformation of the electronic structure from a zero-dimensional quantum-dot system to a one-dimensional quantum-wire system.

Photophysical Studies on the Mono- and Dichromophoric Hemicyanine Dyes III. Ultrafast Fluorescence Up-conversion in Methanol: Twisting Intramolecular Charge Transfer and "Two-State Three-Mode" Model

Abstract: A series of hemicyanine dyes, including monomer ([(E)-N-methyl-4-(2-(4-N,N-dimethyl phenyl) ethenyl) pyridinium] iodide; M) and dimers (1,n-Bis [(E)- 4-(2-(4-N,N-dimethyl phenyl) ethenyl) pyridinyl]-alkane dibromide; n = 3, alkane = propane, B3; n = 5, alkane = pentane, B5; n = 12, alkane = dodecane, B12), are synthesized and their ultrafast fluorescence up-conversion behaviors are studied. The fluorescence decay curves of these dyes can be well fitted by the sum of several exponential decays in the shorter wavelengths and by the sum of the exponential rise(s) with decay(s) in the longer wavelengths. The fact that the values of longest lifetime component at longer wavelengths tend to be a constant proves the existence of a “sink” region on the potential energy surface of excited state. This sink region is around the TICT state, by which the nonradiative transition dominates the deactive decay paths of excited state. The time dependent Stokes shift function analysis of these four dye molecules in methanol ...

Femtosecond spectroscopy of p-dimethylaminocyanostilbene in solution—no evidence for dual fluorescence

Abstract: The photo-induced relaxation of p-dimethylaminocyanostilbene (DCS) in acetonitrile was studied with broadband transient fluorescence and absorption spectroscopy. The dynamic Stokes shift of fluorescence was measured to be 4000 cm−1. The ‘dual fluorescence’ of DCS, reported previously, is shown to be an artefact resulting from excited state re-absorption of fluorescence. The shift has a characteristic time of 100 fs and is governed by solvation in acetonitrile. Together with results from quantum chemical calculations it is concluded that twisted intramolecular charge transfer does not contribute measurably to the evolution of fluorescence and that the dynamic Stokes shift reflects pure polar solvation.

Optical characterization of AlxGa1-xN alloys (x < 0.7) grown on sapphire or silicon

Abstract: The optical properties of Al x Ga 1-x N samples (x < 0.7) have been studied by photoluminescence (PL) and reflectivity in the 10-300 K temperature range. Various physical properties have been studied as a function of composition, such as Stokes shift, alloy broadening, exciton localization, and Huang-Rhys factor. Up to x 0.3, a band gap bowing factor of ∼0.9 eV accounts for the variation of PL and reflectivity energies. At higher compositions, luminescence energies deepen with regard to this behaviour. This is interpreted as a consequence of the Γ 9 -Γ 7 crossover of the valence band maxima. This is confirmed by the linear polarization of the luminescence studied under oblique observation.

Optical spectroscopy of Al2O3:Ti3+ single crystal under hydrostatic pressure : The influence on the Jahn-Teller coupling

Abstract: This work investigates the effect of hydrostatic pressure on the excitation and emission spectra, as well as on the lifetime, of Al2O3:Ti3+ at room temperature. The aim is to establish correlations between the pressure-induced band shifts and the corresponding local structural changes undergone by the TiO6 complex. A blue-shift of 8.52 and 6.86 cm−1 kbar−1 was found for the lower (E1) and upper (E2) energy components of the excitation band at 17 760 and 20 500 cm−1, respectively, and blue-shifts of 5.93 and 5.40 cm−1 kbar−1 for the two overlapping bands of the emission spectrum located at 12 680 and 14 210 cm−1. We explain these results on the basis of a reduction of the TiO6 Jahn–Teller distortion upon increasing the pressure. In contrast, the increase of the overall Stokes shift, which is mainly associated with electron–vibration coupling to the totally symmetric a1g vibration, is explained by the increase of the excited-state stabilization energy, Sa1g hωa1g , with increasing pressure. The luminescence lifetime is also found to be pressure dependent, varying from 2.6 μs at ambient conditions to 3.2 μs at 80 kbar. This increase is caused by a diminution of the transition oscillator strength that is related to the odd vibration assistance mechanism. The softening of the transition mechanism is interpreted in terms of the blue-shift experienced by the O2− -> Ti3+ charge transfer transition energy upon increasing the pressure.

Poly(silyleneethynylenephenylene) and Poly(silylenephenyleneethynylenephenylene)s: Synthesis and Photophysical Properties Related to Charge Transfer

Abstract: Novel poly(dimethylsilyleneethynylenephenylene) [5] and poly(dimethyl (or diphenyl) silylenephenyleneethynylenephenylene)s [6's] without diyne defects were synthesized by using the Pd/Cu-catalyzed AB-type coupling reaction. 6's showed small but clear π-to-σ charge-transfer absorption bands at longer wavelengths (>340 nm) than those of π−π* absorption bands (<320 nm), whereas the charge-transfer absorption band was not found in 5. When 6's were excited at the π−π* absorption wavelengths, two type of emission were observed: One is the vibronically structured emission with a maximum around 325 nm due to the locally excited π* state, and the other is the intramolecular charge-transfer emission having a broad and structureless band (emission λmax ca. 370 nm) with a large Stokes shift. On the other hand, when excited at the charge-transfer absorption wavelengths, 6's exhibited only the intramolecular charge-transfer emission with a maximum above 390 nm. This suggests that the indigo blue light emission of 6's ...

Water-Induced Quenching of Salicylic Anion Fluorescence

Abstract: Salicylic anion absorption and emission are studied in a variety of solvents and solvent mixtures. The large Stokes shift observed for this anion is taken to be indicative of a rapid excited state proton transfer reaction to its keto form. The changes in the Stokes shift in the various solvents can be well-correlated with changes in polarity/polarizability and hydrogen-bonding acidity. The time-resolved data can for the most part also be well-correlated with these properties. A notable exception is the behavior in water and water-rich mixtures. A significant decrease in fluorescence lifetime is observed, and the influence of temperature in pure water is much larger than in other neat liquids. As an explanation for these effects, an excited state intermolecular proton transfer reaction is suggested, from larger-sized water clusters, to the anion.

ABA-C15: A new dye for probing solvent relaxation in phospholipid bilayers

Abstract: We synthesized and studied N-palmitoyl-3-aminobenzanthrone (ABA-C 1 5 ), which we proved to be an advantageous new fluorescent phospholipid membrane label. While the absorption of ABA-C 1 5 in protic solvents shows negative solvatochromism, its fluorescence emission is substantially red-shifted when the polarity of the solvent is increased. ABA-C 1 5 is excitable by lasers emitting in the range between 390 and 490 nm; it exhibits reasonable quantum yields in protic solvents and binds with high affinity to small unilamellar phospholipid vesicles. Absorption, steady state fluorescence, and solvent relaxation data indicate that the aminobenzanthrone chromophore is located in the headgroup region of phospholipid bilayers in the liquid crystalline state of small unilamellar vesicles. The solvent relaxation kinetics probed by ABA-C 1 5 in the liquid crystalline state is characterized by three solvent relaxation times in the order of 0.05, 0.2, and 1.5 ns, respectively. We observed that the relative contribution of the 0.05 ns component and the overall Stokes shift became larger with increasing difference between the experimental temperature and the main phase transition temperature; this suggests that the chromophore becomes more accessible by water molecules. In the gel phase, a component faster than 30 ps significantly contributes to the solvent relaxation kinetics. However, the solvent relaxation on the nanosecond time scale appears to be slower than in the liquid crystalline phase. The shape and time evolution of the time-resolved emission spectra suggest that two distinct microenvironments of the dye might be responsible for the atypical solvent relaxation characteristics in the gel phase.

One-dimensional continuum and exciton states in quantum wires

Abstract: High-quality T-shaped quantum wires are fabricated by cleaved-edge overgrowth with the molecular beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal. Characterization by micro-photoluminescence (PL) and PL excitation (PLE) spectroscopy at 5 K reveals high uniformity, a sharp spectral width, and a small Stokes shift of one-dimensional (1-D) excitons. The PLE spectrum for 1-D states shows a large peak of ground-state excitons and a small absorption band ascribed to 1-D continuum states with an onset at 11 meV above the exciton peak.

Synthesis, magnetic properties, and electronic spectra of mixed ligand bis(β-diketonato)chromium(III) complexes with a chelated nitronyl nitroxide radical: X-ray structure of [Cr(dpm)2(NIT2py)]PF6

Abstract: A new series of nine nitronyl nitroxide Cr(III) complexes with various β-diketonates, [Cr(β-diketonato)2(NIT2py)]PF6, have been synthesized where NIT2py is 2-(2′-(pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxyl, and their structures, magnetic and optical properties have been examined. X-Ray analysis of [Cr(dpm)2(NIT2py)]PF6 (monoclinic, space group P21/a, a = 13.960(4), b = 31.19(1), c = 13.940(4), Z = 4) demonstrated that NIT2py coordinated to Cr(III) as a bidentate six-membered chelate. Variable-temperature magnetic susceptibility measurements indicated the antiferromagnetic interaction between Cr(III) and NIT2py with a variety of the magnetic coupling constant J values for these complexes. From the variable temperature or solvent dependent UV–vis spectra and MCD, and/or the resonance Raman spectra of the bis(β-diketonato)(NIT2py) Cr(III) complexes, the absorption components centered around 13.3 × 103 cm−1 were assigned to the formally spin-forbidden d–d transition within the t2g subshell associated with the intensity enhancement and the newly appeared vibronic bands around (16.0–18.0) × 103 cm−1 were due to the metal–ligand charge transfer [t2g–SOMO(π*) MLCT] transitions. The change of their spectroscopic characteristics with varying the β-diketonato ligands is discussed in connection with the antiferromagnetic coupling constant J values on the basis of the exchange mechanism along with the coligand effect. The luminescence spectra which show a large Stokes shift suggest the antiferromagnetic interaction in the lowest excited states originates from the 2E or 2T1 level of the Cr(III) moiety.

Self-absorption effects in a LEC with low Stokes shift

Abstract: The electroluminescence spectra both in forward and reverse directions of a light-emitting electrochemical cell prepared from a methyl-substituted ladder-type poly(paraphenylene) (mLPPP), blended with a crown ether, Dicyclohexano18crown6 (DCH18C6), and a Li salt, lithium trifluoromethanesulfonate (litriflate, LiTf for short) are compared. Different from the forward direction, in reverse direction the spectrum is strongly influenced by self-absorption effects. We discuss this behavior by different degrees of doping of the p- and n-type doped zones. This effect can be applied for a new method to realize bias-dependent color emission from single-layer devices at low voltages that can be prepared by only one simple and easy spin-coating process.

Exciton Luminescence of Hydrogen-Bonded Ferroelectric Pbhpo 4

Abstract: Luminescence properties of hydrogen-bonded ferroelectrics PbHPO 4 (LHP) have been studied in the temperature range of 10 to 320 v K. A broad luminescence band with a large Stokes shift is observed at 2.8 v eV, stimulated efficiently at the first exciton band region. This result strongly suggests that the luminescence band is intrinsic in origin and it is due to annihilation of self-trapped excitons. This is in agreement with the small value of the steepness parameter ( † 0 =0.78) of the Urbach tail. The time-resolved photoluminescence measurement indicates that the 2.8 v eV luminescence consists of composite bands which decay with lifetimes of 6 v ns, 600 v ns and 60 v µs.

Deformation of the Raman scattering spectrum of Ih ice under local laser heating near 0°C

Abstract: The Raman band of Nd:YAG laser second-harmonic scattering from the O-H stretching vibrations of hexagonal ice was observed to broaden asymmetrically by 90 cm−1 near 0°C with the pulse repetition frequency increasing from 1 to 8 Hz. The center of this band was found to undergo simultaneously a Stokes shift by 25 cm−1. The observed spectral features can be accounted for by the ice being heated by pulsed laser radiation through the electrocaloric effect, because the one-photon absorption mechanism produces a negligible contribution.

New fluorescent pigment and method for measuring nucleic acid

Abstract: PROBLEM TO BE SOLVED: To provide a new fluorescent intercalative dye or the like which shows a large fluorescent enhancement upon intercalation into a double-stranded nucleic acid when used in detection of the nucleic acid, shows a great difference between the excitation wavelength and the fluorescence wavelength (i.e., has a large Stokes shift). SOLUTION: The purpose is attained by providing a compound represented by general formula 1, particularly e.g. compound 1 represented by formula 1A or a salt, hydrate, solvate or stereoisomer thereof, a method for producing these compounds and a nucleic acid probe obtained by bringing these compounds into contact with a nucleic acid or a nucleic acid probe obtained by chemically bonding these compounds to a single-stranded oligonucleotide.