Showing papers on "Stokes shift published in 1996"

Band-edge exciton in quantum dots of semiconductors with a degenerate valence band: Dark and bright exciton states

Abstract: We present a theoretical analysis of the band-edge exciton structure in nanometer-size crystallites of direct semiconductors with a cubic lattice structure or a hexagonal lattice structure which can be described within the framework of a quasicubic model. The lowest energy exciton, eightfold degenerate in spherically symmetric dots, is split into five levels by the crystal shape asymmetry, the intrinsic crystal field (in hexagonal lattice structures), and the electron-hole exchange interaction. Transition oscillator strengths and the size dependence of the splittings have been calculated. Two of the five states, including the ground state, are optically passive (dark excitons). The oscillator strengths of the other three levels (bright excitons) depend strongly on crystal size, shape, and energy band parameters. The relative ordering of the energy levels is also heavily influenced by these parameters. The distance between the first optically active state and the optically forbidden ground exciton state increases with decreasing size, leading to an increase of the Stokes shift in the luminescence. Our results are in good agreement with the size dependence of Stokes shifts obtained in fluorescence line narrowing and photoluminescence experiments in CdSe nanocrystals. Mixing of the dark and bright excitons in an external magnetic field allows the direct optical recombination of the dark exciton ground state. The observed shortening of the luminescence decay time in CdSe nanoncrystals in a magnetic field is also in excellent agreement with the theory, giving further support to the validity of our model. \textcopyright{} 1996 The American Physical Society.

Solvation Dynamics of Coumarin 480 in Reverse Micelles. Slow Relaxation of Water Molecules

Abstract: The picosecond time-resolved Stokes shift of the laser dye coumarin 480 (I) is studied in Aerosol OT-heptane reverse micelles at various concentrations of water. On addition of water to a n-heptane solution of I containing 0.09 M AOT, the emission spectrum of I exhibits a prominent shoulder at 480 nm which is ascribed to the dye molecules in the water pool of the reverse micelles. In n-heptane, the fluorescence decays of I at short and long wavelengths are identical, which rules out any time-resolved Stokes shift. On addition of water to the AOT/heptane system, the decay at long wavelengths exhibit a distinct growth on the nanosecond time scale. This and the time-dependent Stokes shift indicate that the water molecules in the water pool of the AOT reverse micelles undergo slow relaxation on the nanosecond time scale.

The Integrated Photon Echo and Solvation Dynamics

Abstract: The time-integrated three-pulse photon echo is discussed in terms of the line-shape functions which are shown to be directly related to the solvation dynamics. By using the short-time approximation to the line-shape function, an algebraic form of the three-pulse photon echo signal SPE (τ, T) is obtained as a function of two delay times, τ and T, between pulses. On the basis of the approximate form of the photon echo signal, the echo peak shift with respect to the second delay time (population evolution period) is derived and found to be linearly proportional to the time-dependent fluorescence Stokes shift function for times greater than the bath correlation time, τc. In the case of intermediate inhomogeneous broadening, the asymptotic echo peak shift magnitude is found to be useful in estimating the static inhomogeneous width. To illustrate the theoretical results, an analysis of the echo peak shift measurement of IR144 in a room temperature glass, PMMA, is presented.

Solvation Dynamics of Coumarin 480 in Micelles

Abstract: The picosecond time-resolved Stokes shift of the laser dye, coumarin 480 (I) is studied in neutral (Triton X-100, TX), cationic (cetyltrimethylammonium bromide, CTAB), and anionic (sodium dodecyl sulfate, SDS) micelles. Above critical micellar concentration (cmc) for all three micelles I exhibits wavelength dependent fluorescence decays and a distinct growth at the long wavelengths. The time dependent Stokes shift studies indicate that the water molecules in the Stern layer of the micelles relax on the time scale 180−550 ps, which is slower than the subpicosecond relaxation dynamics observed in ordinary bulk water.

Evolution of Photophysical and Photovoltaic Properties of Perylene Bis(phenethylimide) Films upon Solvent Vapor Annealing

Abstract: A detailed study is presented of the changes in photophysical and photovoltaic properties of thin films of perylene bis(phenethylimide), PPEI, as the film structure evolved from amorphous to polycrystalline under the influence of methylene chloride vapor. A strong absorption band grew in on the low-energy side of the original spectrum as the film order increased, and the peak of the emission spectrum shifted to higher energy, decreasing the Stokes shift by 283 meV. The singlet exciton diffusion length increased by 76%, leading to enhanced quenching of the PPEI fluorescence by a surface film of titanyl phthalocyanine, TiOPc. The heterojunction barrier height in TiOPc/PPEI photovoltaic cells decreased by 88 mV after 12 min exposure to methylene chloride vapor and subsequently remained constant. The series resistance of the cells increased monotonically as the films crystallized. Photocurrents increased by a factor of 10 after brief exposure and decreased thereafter. The active depth from which photocurrent ...

Polarized site-selective fluorescence spectroscopy of the long-wavelength emitting chlorophylls in isolated Photosystem I particles of Synechococcus elongatus.

Abstract: Isolated trimeric Photosystem I complexes of the cyanobacterium Synechococcus elongatus have been studied with absorption spectroscopy and site-selective polarized fluorescence spectroscopy at cryogenic temperatures. The 4 K absorption spectrum exhibits a clear and distinct peak at 710 nm and shoulders near 720, 698 and 692 nm apart from the strong absorption profile located at 680 nm. Deconvoluting the 4 K absorption spectrum with Gaussian components revealed that Synechococcus elongatus contains two types of long-wavelength pigments peaking at 708 nm and 719 nm, which we denoted C-708 and C-719, respectively. An estimate of the oscillator strengths revealed that Synechococcus elongatus contains about 4–5 C-708 pigments and 5–6 C-719 pigments. At 4 K and for excitation wavelengths shorter than 712 nm, the emission maximum appeared at 731 nm. For excitation wavelengths longer than 712 nm, the emission maximum shifted to the red, and for excitation in the far red edge of the absorption spectrum the emission maximum was observed 10–11 nm to the red with respect to the excitation wavelength, which indicates that the Stokes shift of C-719 is 10–11 nm. The fluorescence anisotropy, as calculated in the emission maximum, reached a maximal anisotropy of r=0.35 for excitation in the far red edge of the absorption spectrum (at and above 730 nm), and showed a complicated behavior for excitation at shorter wavelengths. The results suggest efficient energy transfer routes between C-708 and C-719 pigments and also among the C-719 pigments.

Subpicosecond Solvation Relaxation of 4-(Dicyanomethylene)-2-methyl-6-(p-(dimethylamino)styryl)-4H-pyran in Polar Liquids

Abstract: The solvation dynamics of 4-(dicyanomethylene)-2-methyl-6-(p-(dimethylamino)styryl)-4H-pyran (DCM), dissolved in a polar solvent methanol, ethylene glycol, ethyl acetate, or acetonitrile, has been studied using the fluorescent up-conversion technique with a time resolution of ≈100 fs. Typically a bimodal dynamic Stokes shift behavior is found, with time constants of about 100 fs and a few picoseconds, respectively. The initial component in the solvation relaxation is attributed to the effect of free streaming motions of the solvent molecules, whereas the second component is typical of the rotational diffusion motions of the solvent molecules. From the initial rapid rise in the integrated emission intensity it is concluded that electron transfer is preferably to a higher lying (less emissive) charge transfer state which is formed in less than 100 fs.

Fluorescence loss mechanism due to large-amplitude motions in derivatives of 2,2′-bipyridyl exhibiting excited-state intramolecular proton transfer and perspectives of luminescence solar concentrators

Abstract: Fluorescence quantum yields and lifetimes as functions of temperature and solvent polarity are compared for two different hydroxy derivatives of 2,2′-bipyridyl. Both dyes show strongly red-shifted excited-state intramolecular proton transfer (ESIPT) fluorescence at all temperatures investigated. In protic solvents, the ESIPT emission is of a more allowed nature and somewhat blue shifted, owing to specific solvatation. A strong difference in behaviour can, however, be observed for the non-radiative losses. The symmetric dye 1 possesses large fluorescence quantum yields, whereas sizeable fluorescence quantum yields from 2 could only be obtained under high viscosity conditions. Increased solvent polarity leads to enhanced fluorescence losses. These facts and the comparison with quantum chemical calculations for planar and twisted structures of 1 and 2 are interpreted as evidence for a photochemically reached low-lying state with charge separation (twisted intramolecular charge transfer (TICT)), responsible for these fluorescence losses and especially active for 2. For 1 in aprotic solvents, this reaction is discussed within the quantum-chemical extension of electron transfer theory and shown to involve nuclear tunnelling. In addition to this viscosity-dependent decay channel, 2 also decays via a viscosity-independent (presumably n −π ∗ ) channel. The identification of the TICT and n −π ∗ non-radiative channels allows a new approach to the development of highly fluorescent ESIPT dyes with very large Stokes shift for use in fluorescence solar collectors or other devices utilizing the principle of fluorescence-based light-pipes.

Structures, Spectra, and Lasing Properties of New (Aminostyryl)pyridinium Laser Dyes

Abstract: Recent work from our Photonics Research Laboratory has shown that a new class of hydroxy amino styryl pyridinium derivatives exhibit both one- and two-photon pumped lasing. Because of the exciting prospect of up-conversion lasing by direct two-photon excitation, we have conducted a thorough study of the spectroscopic and lasing properties of a group of related compounds with the objective of understanding the structure−spectroscopic properties relationship. These dyes were found to have two mesomeric forms, one predominant in the ground-state and the other in the excited-state, leading to a large Stokes shift. A low fluorescence quantum yield was observed for all the studied dyes and could be possibly attributed to two factors: (1) the presence of a counterion, iodide, which increases the singlet-to-triplet intersystem crossing transition, and (2) a twisted intramolecular charge-transfer (TICT) geometry derived from the rotation of the amino moiety. However, significant lasing efficiencies were observed ...

Torsional and Proton Transfer Dynamics in Substituted 3-Hydroxyflavones.

Abstract: The role of torsional motion in proton transfer dynamics of 4‘-(N,N-dimethylamino)-α-naphthyl-3-hydroxychromone has been studied using time-resolved emission spectroscopy. Excited state proton transfer (ESPT) rates in hydrocarbon solvents of varying viscosities are obtained. The viscosity is varied so that ESPT rate prior to, and following, torsional relaxation may be determined. The results show that as the aryl torsional relaxation proceeds, the exothermicity of proton transfer decreases and the proton transfer rate increases; thus, the system exhibits “Marcus inverted” behavior. These results are interpreted in terms of torsional potentials. These potential energy surfaces are constructed from solvent-dependent static absorption and emission spectra and time-dependent Stokes shift measurements. Potential energy surfaces are constructed for the normal and tautomeric forms of the molecule in both torsionally relaxed and unrelaxed states. From these surfaces, a ratio of reaction rates is obtained, which i...

Spectral properties and dynamics of the excited state structural relaxation of the ortho analogues of POPOP — Effective abnormally large Stokes shift luminophores

Abstract: The spectral behaviour of the sterically hindered, substantially non-planar molecules of the ortho analogues of POPOP was studied by steady state absorption and fluorescence methods, time-resolved fluorescence methods and quantum chemical calculations. It was shown that the investigated molecules undergo noticeable flattening in their excited state, which leads to the essential lowering of the energy of the fluorescent term and, in turn, enlargement of the Stokes shift of the fluorescence. The flattening of the ortho analogues of POPOP in the excited state is a very effective process (estimated rate constants are of the order of 108–109 s−1); it has a low activation energy and mainly intramolecular nature with a minor influence of the environment. The replacement of one of the oxazole rings in the ortho-POPOP molecule by an oxadiazoleling causes a near doubling of the flattening activation energy.

Influence of crystal structure on the luminescence properties of bismuth(III), europium(III) and dysprosium(III) in Y2SiO5

Abstract: The luminescence properties of Bi3+, Eu3+, Dy3+ and energy transfer from Bi3+ to Dy3+ and Eu3+ have been studied in two modifications of Y2SiO5(low-temperature X1 type and high-temperature X2 type) and discussed in relation to their crystal structures. The Bi3+ ion luminesces in the blue region of the spectrum in X1-Y2SiO5 but in the UV region in X2-Y2SiO5. Two obviously different luminescent centres have been observed for Bi3+ and Eu3+ in X1-Y2SiO5, but only one has been seen in X2-Y2SiO5. The Stokes shift (9200 cm–1) for Bi3+ in X1-Y2SiO5 is much larger than that (5000 cm–1) in X2-Y2SiO5. This suggests that the host lattice is more rigid in X2-Y2SiO5 than in X1-Y2SiO5. As a result, the Bi3+, Eu3+ ions show higher emission intensity in the former than in the latter type. X1-Y2SiO5 is more suitable for Bi3+→ Eu3+ energy transfer and X2-Y2SiO5 is more suitable for Bi3+→ Dy3+ energy transfer.

Protein-Matrix Solvation Dynamics in the α Subunit of C-Phycocyanin

Abstract: We have used femtosecond transient hole-burning (THB) spectroscopy to study single-chromophore dynamics at room temperature in the α subunit of C-phycocyanin, a cyanobacterial light-harvesting protein. The α subunit contains a single phycocyanobilin (open-chain tetrapyrrole) chromophore. A reasonable description of the THB spectra requires the presence of a broad excited-state absorption (ESA) line shape underlying the entire photobleaching/stimulated emission (PB/SE) region. The time evolution of the THB spectrum involves line broadening on the 100−200-fs time scale and a <100-fs time scale dynamic Stokes shift of the SE component. We suggest that the dynamic Stokes shift can be attributed to transient solvation by the protein matrix. The solvent correlation function, C(t), exhibits a sub-100-fs component that describes 90% of the total C(t) response, with the remaining 10% of response occurring on a much longer time scale than the longest delay employed in these experiments (10 ps). We assign the ultraf...

Stokes shift in quantum wells : trapping versus thermalization

Abstract: Low temperature photoluminescence and photoluminescence excitation measurements have been performed in a set of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs samples with a different indium molar fraction, well width, growth conditions, and post-growth treatment. This has allowed to change in a controlled way the degree and source of disorder in the samples, thus resulting in an excitonic absorption linewidth varying between 1 and 18 meV, and an ensuing Stokes shift changing between zero and 8 meV. The conditions of validity of two different models relating the Stokes shift to the linewidth broadening have been established in terms of different regimes of disorder and temperature. A continuous transition between those regimes has been demonstrated. \textcopyright{} 1996 The American Physical Society.

Optical properties of a quantum wire crystal, C5H10NH2PbI3

Abstract: We report optical properties of a single crystal of C5H10NH2PbI3, which can be considered as arrays of 0.4 nm thick quantum wires isolated by organic barrier materials. The lowest exciton absorption band is located at 3.3 eV and mainly polarized along the wire direction, while the photoluminescence band is located at 2.1 and 0.3 eV wide. The large Stokes shift (1.2 eV) and the broad width suggests the strong exciton-lattice interaction in this one-dimensional crystal. The luminescence decays single exponentially in time, and the time constant decreases with temperature, ranging from 15 μs at 13 K to 32 ns at 250 K.

The charge transfer state of excited bianthryl and a derivative: solvatochromism, emission CT spectra broadening in homogeneous solvents

Abstract: The photophysical properties of a recently synthesized molecule, the bis-9,9′-(2-ethyl-10-hexyl)anthryl (BOA), have been studied with the parent molecule, the 9,9′-bianthryl (BA), in solvents of various polarity The comparative analysis of the photophysical behaviour of these two probes has been achieved in order to emphasise environmental effects of the alkyl substituent on the aromatic chromophore The results obtained in perfluoroalkane, reported for the first time in this paper, suggest that the charge transfer state (CT) of BA or BOA contributes to the fluorescence emission even in non polar solvents like cyclohexane The fluorescence spectra of these two probes in perfluoroalkane have been introduced as standard spectra of the locally excited state form (LE) and used to obtain a new decomposition of the other overall fluorescence spectra into two emission components: one coming from the CT state and the second from the initial excited singlet state (LE) We have analysed the solute-solvent interactions not only in term of Stokes shift but also using the inhomogeneous band broadening parameter The vibrational band decomposition found its validation in the agreement observed between the thermodynamic and spectroscopic properties of both excited states A correlation between the inhomogeneous broadening and the Stokes shift of the charge transfer spectrum is exhibited The existence of up to 50% of the CT state even in non polar solvents like cyclohexane has been shown The dipole moment of the CT state of BA and of BOA can be estimated in the range of 62 to 103 D depending on the solvatochromic model used It has been shown that the alkyl substituent of BOA destabilises the CT state and only four methylene groups of each lateral chain of BOA contribute to the local environment of the aromatic chromophore

Optical spectroscopy of Ce3+, Tb3+ and Eu3+ in new scintillating glasses

Abstract: Optical absorption, luminescence and excitation spectra of new silicate scintillating glasses doped with Eu 3+ , Ce 3+ and Tb 3+ have been measured at room temperature. The spectra have been assigned and the lifetimes of the emitting states measured. In the case of the Ce 3+ doped glass, a low Stokes shift, probably responsible for the relatively high emission quantum yield induced by the predominantly radiative decay, has been evidenced. In the case of the Eu 3+ doped glasses, information about the distortion of the local geometry around the impurity has been obtained through the intensity ratio I( 5 D 0 → 7 F 2 )/I( 5 D 0 → 7 F 1 ) The Tb 3+ doped glass shows a very efficient emission in the green spectral region characterised by a long lifetime around 2.7 ms with no evidence of concentration quenching.

Subpicosecond studies of the solvation dynamics of fluoroprobe in liquid solution

Abstract: The dynamic Stokes shift of fluoroprobe in its lowest excited charge transfer state has been studied in a series of solvents, at room temperature, with a time resolution of about 300 fs. Using the fluorescent upconversion method dynamic Stokes shifts of up to 3000 cm −1 have been resolved. The Stokes shift is attributed to the solvent response to the large photoinduced dipole moment of fluoroprobe (∼ 30 D) in the fluorescent charge transfer state. The solvation dynamics is determined by the rotational diffusional motions of the ethereal solvent molecules.

Exciton dynamics and trapping in J-aggregates of carbocyanine dyes

Abstract: Exciton relaxation processes in J-aggregates of two thiacarbocyanine dyes (TDC and THIATS) in water/ethylene glycol low temperature glass have been studied by means of steady-state, site-selective and time-resolved spectroscopy. The results of different experiments can be rationalized within the framework of exciton self-trapping (ST) in a 1D molecular chain. The measured Stokes shift is found to be proportional to the exciton - phonon coupling strength squared that is in accordance with the model of large radius ST excitons. A negative fluorescence anisotropy was observed for J-aggregates of both investigated dyes upon excitation outside the J-band. The observed monotonic dependence of the fluorescence anisotropy on the excitation wavelength is interpreted quantitatively within the framework of intraband absorption and Davydov splitting.

UV-visible absorption and fluorescence studies of 4,4′-diamino-trans-stilbene and its protonated species

Abstract: 4,4′-Diaminostilbene (DAS), its monoprotonated and diprotonated forms exhibit fluorescence. Their UV absorption and fluorescence emission spectra in different solvents and proton concentrations have been measured. 4-Trimethylammonium-4′-aminostibene hydrogen sulfate has been synthesized as a model for the monoprotonated DAS form. p K a values in the ground state have been measured spectrophotometrically. The Stokes shifts in solvents of different polarities, the quantum yields and the apparent rate constants of trans - cis isomerization have been determined. Solvent polarity has a strong influence on spectroscopic properties.

Luminescence from samples: substitutional ions are `silent'

Abstract: Through this work it is shown that the luminescence observed in samples is related to the formation of microcrystalline precipitates This conclusion is supported by the experimental emission and excitation spectra as well as by the temperature dependence displayed by the emission intensity Also the EPR spectrum of most doped samples clearly shows the presence of a signal (superimposed on that coming from isolated ions) whose temperature dependence is similar to that displayed by crystals The absence of luminescence associated with isolated ions in the host lattice is discussed It is pointed out that luminescence quenching is probably related to a multiphonon non-radiative process rather than to a (G) excited state close to the bottom of the conduction band Although luminescence quenching appears for a Dexter - Klick - Russell parameter probably close to 01, this is qualitatively related to a high radiative lifetime ( ms) The analysis made also shows that firstly increases by a factor of about 17 on going from -doped fluoroperovskites to in fluoride-type crystals, and secondly increases on going from to Both variations mainly reflect a parallel increase in the Stokes shift