Memory effects in single-molecule spectroscopy

From the time series of LH2 optical single-molecule fluorescence excitation spectra of Rhodospirillum molischianum the memory function of the Mori-Zwanzig equation for the optical intensity is derived numerically. We show that the time dependence of the excited states is determined by at least three different non-Markovian stochastic processes with decay constants for the Mori-Zwanzig kernel on the order of . We suggest that this decay stems from the conformational motion of the protein scaffold of LH2.     

Excitonic relaxation in a conjugated polymer: Initial coherence beyond energy-dispersive population transfer

In this contribution, we deal with the pathways of femtosecond excitonic relaxation in conjugated polymers, focusing on the prominent, ground-state nondegenerate model system poly(p-phenylenevinylene) (PPV). After a brief discussion of the molecular exciton picture of optical excitations in organic semiconductors, we present exemplary results, categorized in two regimes. On early picosecond time scales, the dynamics of incoherent population relaxation is investigated by combining selectively tuned femtosecond excitation pulses with the technique of fluorescence up-conversion. As an initially prepared excitonic distribution gradually lowers its energy by diffusion to the bottom of the density of states (DOS), an excitation energy-dependent bathochromic shift dynamics is observed, accompanied by spectral intensity rearrangements. Motivated by the perception that intersite electronic coupling is absent in the lowest energy regime of the DOS, we further devise a strategy that enables us to measure the very early electronic oscillations and their quantum-stochastic relaxation within approximately the first 200 fs. By using femtosecond wave packet interferometry with appropriately tuned but otherwise freely propagating pulses, we observe fluorescence interferograms with strongly damped, low-frequency beatings, which stem from the spatial interference of two wave packets launched by two-pulse excitation. The results can be explained, semiquantitatively, in terms of a second-order perturbational approach and open up a new perspective on the complex puzzle of PPV optical dynamics.     

Fluorescence quenching by charge carriers in indolo[3,2-b]carbazole-based polymer

The fluorescence quenching by the applied voltage in a new indolocarbazole-based polymer poly{5,11-di-2,3-epoxypropyl-6-pentyl-5,11-dihydroindolyl[3,2-b]carbazolyl-alt-4,4′-tiobisbenzentiole} sandwiched between ITO and Al electrodes has been investigated. Time-resolved fluorescence measurements have shown that the excitons are quenched during their entire life-time. The fluorescence intensity showed complex dependence on the applied voltage and the excitation light intensity, memory effects and dynamics on time scale of tens of seconds. These effects are explained by the exciton quenching by mobile and trapped charge carriers. Charge carrier density dynamics calculated by using a simplified model evaluating carrier injection, photogeneration, recombination and trapping was used to model variations of the fluorescence intensity induced by the periodically applied voltage. These effects point out the importance of the fluorescence quenching by trapped carriers in organic materials possessing high density of carrier traps or with broad distribution of density of states.     

Analysis of the local conformation of proteins with two-dimensional fluorescence techniques

Two 2D fluorescence techniques are described which allow the study of conformational changes in proteins in their native form in μM solutions using aromatic amino acids (tryptophan, tyrosine) as intrinsic fluorescence markers. Simultaneous time- and wavelength-resolved fluorescence spectra are measured using a 80 ps laser source in conjunction with streak detection in the exit plane of an astigmatism-corrected spectrometer. This approach allows identification of different photophysical processes by their associated lifetime and spectral intensity distribution; errors due to the more common integration over a wider spectral range are avoided. Time-resolved spectra are sensitive to changes in the collisional environment (dynamic quenching) and can thus be used to monitor local conformation changes close to the respective fluorophors. This is demonstrated for the Ras protein which undergoes a drastic conformation change while binding to different nucleotides. Excitation-emission spectra are two-dimensional fluorescence images with one axis corresponding to the excitation and the other to the emission wavelength. Thus, they contain all conventional excitation and fluorescence spectra of a given substance. The 2D structure facilitates the interpretation of these spectra and allows the direct identification of resonance effects, scattering and the isolation of the contribution of different fluorophors to the complete spectrum. This is demonstrated for mixtures of tyrosine and tryptophan. In this case, both wavelength-resolved spectra and temporal decays are affected by energy transfer processes between the two amino acids. In a last example, both static and time-resolved spectral methods are combined to determine the respective contribution of static and dynamic quenching in calsequestrin. Evaluation of the fluorescence data is in good agreement with a recent crystallographic analysis which shows that all tryptophans are located in a conserved domain of the protein. Addition of Ca²⁺ ions leads to a more compact form of calsequestrin and to polymers. This information would not be obtainable from either of the two techniques alone. Received: 10 February 2000 / Published online: 13 September 2000     

Single Molecule Fluorescence of Native and Refolded Peridinin–Chlorophyll–Protein Complexes

Single molecule spectroscopy was applied to study the optical properties of native and refolded peridinin-chlorophyll-protein (PCP) complexes. The native system is a trimer with six chlorophyll a (Chl a) molecules, while the refolded one contains two Chl a and resembles structurally and spectroscopically the PCP monomer. The fluorescence emission of single PCP complexes strongly broadens with increasing excitation power. Simultaneously, the distribution of fluorescence maximum frequencies is also broadened. These spectral changes are attributed to photoinduced conformational changes of the protein that influence the fluorescence of embedded chromophores. Comparison of fluorescence intensities measured for PCP complexes in two different solvents indicates that the native PCP trimers are preserved in EDTA Tris buffer, while in PVA polymer matrix only monomers are stable.     

Fine-structure spectroscopy of 2-methylnaphthalene cooled in a supersonic jet

The fluorescence and fluorescence excitation spectra of 2-methylnaphthalene molecules cooled in a supersonic jet are measured. The frequencies of vibrations in the S ₀ and S ₁ states, as well as the relative intensities of electronic-vibrational transitions in the fluorescence and fluorescence excitation spectra, are calculated with the semiempirical MO/M8ST method. The intensities are calculated in the Franck-Condon approximation taking into account the mixing of all the 38 totally symmetric normal vibrations. Based on the calculations, most observed spectral lines are assigned. It is shown that the calculation accuracy of the method is high enough for it to be used to interpret the spectra of molecules of aromatic compounds such as substituted naphthalenes. It is found that the main contribution to the fluorescence spectrum is made by four optically active vibrations.     

A quantum chemical approach towards the electronically excited states of helium clusters

We have investigated the electronic energies of the ground and excited states of an octahedral helium cluster by quantum chemical ab initio calculations. The excited levels were calculated for the central atom for a set of different inter atomic separations. Our approach yields potential energy curves which are suited to describe a density dependence of the spectral features as previously observed in photo excitation experiments. The potential energy curves of the 2s and 2p states show a hump at ? caused by the strong perturbation of neighbouring atoms. The existence of this hump explains the experimentally observed blue shift and its dependence on the cluster size or density, respectively. The potential curves of the higher levels show almost constant energies. Perturbations of these levels are small, because the overlap between the Rydberg orbital and the orbitals of the surrounding atoms is small. This is the case for both small R values where the Rydberg orbital is well outside the cluster as well as for large R where the density drastically decreases. These findings coincide with the un-shifted features of small clusters observed in experiments.     

Fluorescence Intermittency in Monolayer WSe_2

Fluorescence intermittent dynamics of single quantum emitters in monolayer WSe2 are investigated via measuring spectrally resolved time traces and time-dependent fluorescence intensity trajectories. Analysis of fluorescence trajectories and spectral shifting reveal a correlation between the fluorescence intermittency and spectral diffusion.A model of an inverse power law can be used to understand the observed blinking dynamics.     

Observation of coherent phonon states in porous silicon films

The dynamics of differential transmission and reflectance spectra of porous silicon films was studied using the femtosecond excitation technique (τ≈50 fs, ?ω_pump=2.34 eV) with supercontinuum probing (?ω_probe=1.6–3.2 eV) and controlled time delay with a step of Δt=7 fs between the pump and probe pulses. A short-lived region of photoinduced bleaching was observed in the differential transmission spectra at wavelengths shorter than the pump wavelength. The excitation of coherent phonon states with a spectrum corresponding to nanocrystalline silicon with an admixture of a disordered phase was observed. The relaxation of electronic excitation was found to slow down in the spectral region where the amplitude of excited coherent vibrations was maximal.     

Picosecond fluorescence studies by intracavity gain spectroscopy in a modelocked dye laser

This article presents measurements which combine modelocking technique with intracavity spectroscopy. To test this approach, a sample (10^–5 m ethanolic solution of 1,4-dihydroxyanthranquinone) was inserted in a modelocked Ar⁺ ion laser and probed by intracavity pulses of a synchronously pumped dye laser. The probing of the sample results in an amplification of the dye laser output. Maximum output was measured if the pulses of the dye laser temporally overlapped with those of the Ar⁺ ion laser inside the sample. Under this condition, the spectral laser intensity was shaped by the spectrum of stimulated fluorescence which originated from a molecular vibronic state populated by pump laser excitation.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Specific Fluorescence Properties and Picosecond Transient Absorption of 8-Azasteroids

The specific fluorescence properties as well as picosecond transient absorption features have been studied for two 8-azasteroids. It is shown that at various excitation wavelengths the essentially different final excited electronic states are realized. Because of the multicenter character of 8-azasteroids the spectroscopic data obtained may be analyzed on a basis of the mesomeric tautomerism model taking into account the dynamic combination of cis- and trans-configurations. The dependence of fluorescence spectral characteristics on the solvent nature is a manifestation of intermolecular H-bond interactions.     

Excitation-wavelength Dependent Fluorescence of Ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate

The excitation wavelength dependence of the steady-state and time-resolved emission spectra of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in tetrahydrofuran (THF) at room temperature has been examined. It is found that the ratio of the fluorescence intensity of the long-wavelength and short-wavelength fluorescence bands strongly depends on the excitation wavelength, whereas the wavelengths of the fluorescence excitation and fluorescence bands maxima are independent on the observation/excitation wavelengths. The dynamic Stokes shift of fluorophore in locally excited (LE) and intramolecular charge transfer (ICT) states has been studied with a time resolution about 30 ps. The difference between Stokes shift in the LE and ICT states was attributed to the solvent response to the large photoinduced dipole moment of EAADCy in the fluorescent charge transfer state. On this base we can state that, the relaxation of the polar solvent molecules around the fluorophore was observed.     

Steady-State and Time-Resolved Spectroscopic Characteristics of Novel Silicon-Containing Organopolymers

The spectroscopic characteristics of novel -conjugated polymers containing four-coordinated silicon, acetylene groups and either 1,4-biphenylene or 2,7-fluorene in the main chain were investigated by steady-state and picosecond laser spectroscopy. The spectral features of absorption, fluorescence excitation spectra, fluorescence lifetime, and fluorescence polarization were explained by the existence of two kinds of inhomogeneously broadened electronic states formed in the disordered polymeric chain. The dynamics of photoinduced absorption was measured in the 400–900 nm spectral range with picosecond time resolution. The long-wavelength band with _max 710 nm was ascribed to excited-state absorption from higher-lying electronic states created in short polymeric segments with essential conformational distortion of the subunits. The short-wavelength band with _max 580 nm and a shoulder at 500 nm was interpreted as photoinduced absorption from a lower-lying state arisen in more planar, longer -conjugated segments populated via direct excitation and energy migration between disordered segments of the polymeric chain. For the fluorene-containing polymer, the smaller Stokes shift and the greater degree of fluorescence polarization are consistent with more extensive electron delocalization along the backbone.     

Effect of magnetic field on the character of fluorescence of dense and cold atomic ensembles excited by pulsed radiation

Specific features of fluorescence of dense and cold nondegenerate atomic ensembles in an external constant magnetic field are analyzed theoretically. The angular distribution, polarization properties, as well as the spectral composition of fluorescence radiation are calculated. The time variation of these characteristics after the end of the excitation pulse is analyzed. The dependence of the properties of secondary radiation on the duration and carrier frequency of the pulse is investigated. It is shown that, for dense clouds in which the free path length of quasiresonance photons is commensurate with the interatomic distance, the magnetic field significantly modifies all the observable properties of the radiation. Under these conditions, the trapping time may increase by tens of times. Magnetic field enhances the effect of quantum beats observed on time scales commensurate with the lifetime of the excited states of atoms. For individual polarization channels, this field also intensifies the phenomenon of coherent backscattering (CBS). The phenomena found are explained by the effect of magnetic field on the character of resonance dipole–dipole interaction and, as a result, on the specific features of collective phenomena in dense atomic ensembles.     

Low-temperature dynamics of amorphous polymers and evolution over time of spectra of single impurity molecules: I. Experiment

The optical dynamics of a doped amorphous system, tetra-tert-butylterrylene in amorphous polyisobutylene, has been experimentally studied by the spectra of single impurity molecules measured at temperatures of 2, 4.5, 7, and 15 K. The study of the temporal evolution of the fluorescence excitation spectra of the molecules under consideration made it possible to unambiguously establish the individual identity of the spectra of particular molecules and to analyze their multiplet structure. Repeated scanning of a selected spectral range with subsequent summation of the data made it possible to considerably reduce the errors that arise upon single scanning of the spectra of single molecules. The majority of the spectral trails detected were in agreement with the model of two-level systems. Jumps of spectral lines due to transitions in such systems were observed at all temperatures.     

Optical absorption of 1,3-diphenyl-1H-Pyrazolo[3,4-b]quinoline and its derivatives

Paper deals with the experimental investigations and quantum chemical calculations of the absorption spectra of newly synthesized 1,3-diphenyl-1H-Pyrazolo[3,4-b]quinoline and its 6-Vinyl, 6-N,N-Diphenyl, 6-Methyl, 6-Fluoro, 6-Bromo, and 6-Chloro derivatives. The calculations are performed by means of the semiempirical quantum chemical methods AM1 or PM3 combined with: (a) equilibrium molecular conformation (EMC) in vacuo; (b) the molecular conformation model considering a dynamical rotation of phenyl rings only (T = 300 K); and (c) the most general model of the conformational molecular dynamics (MD) at T = 300 K. It is shown that the phenyl dynamics appears to be not important in the spectral position of absorption thresholds as well as in a broadening of most absorbtion bands. On the other hand, the MD simulations reproduce a broadening of the absorbtion spectra as well as the electron-vibronic coupling leading to a red-shift of absorption bands with increasing of temperature. The conformational MD model in combination with the quantum chemical AM1 method gives in most cases the best agreement with the experimental data, namely in the sense of spectral positions and width of the absorption bands including first oscillators (absorption thresholds).     

Laser and fluorescence properties of dye-doped sol-gel silica from 400 nm to 800 nm

Lasing and fluorescence properties of ten dyes covering the spectral range from 400 nm to 800 nm were studied in a silica matrix prepared by the sol-gel technique. The dye-doped sol-gel silica samples fluoresced strongly under laser excitation. Many of the samples were also induced to superradiate. In comparison to dyes in alcohol solvents, significant red-shifting was observed in the sol-gel silica fluorescence spectra. Plausible causes of the red-shift are discussed.     

Effects of long-wavelength fluorescence excitation in N, N′-dimethylaminobenzonitrile liquid solutions

We have studied the properties of the emission, absorption, and excitation of dual fluorescence of N,N??-dimethylaminobenzonitrile in a set of solvents of different polarity under selective irradiation of solutions by light with different energies of quanta in the range of the long-wavelength absorption band. In all cases, dual fluorescence is observed, which is caused by emission from locally excited Franck-Condon and charge-transfer states. A change in the energy of excitation quanta has no effect on the position of the fluorescence bands; however, the intensity ratio between the bands noticeably changes in favor of the intensity of the long-wavelength band, which belongs to the charge-transfer state. To explain the observed effects, we involve data of quantum-mechanical calculations, which show that there is a considerable probability of occurrence in solutions of these systems of rotational isomers that differ in the orientation of the dimethylamino group with respect to benzonitrile. In the excited state, these rotamers have different charge-transfer reaction rates, which leads to a change in the intensity ratio of the observed fluorescence bands upon using the selective excitation.     

Photon distribution function in blinking fluorescence of individual molecules

A photon distribution function w_N(T) for blinking fluorescence with bright on- and dark off-intervals is derived. The function w_N(T) is expressed via few Poissonian functions each of which relates to corresponding exponential process in quantum dynamics of a given individual molecule. The distribution of photons is calculated for short, middle and long time intervals as compared to off-intervals. The distributions are much broader than Poissonian distribution and have rather complicated shape. If time resolution of an experiment does not permit us to see off-interval and, therefore, fluorescence looks like CW emission, the distribution of photons gives a signal about existence of hidden off- intervals in such CW fluorescence.