Dye Molecules In Solution Research Articles

Dual wavelength optical sampling technique for ultrafast transient bleaching spectroscopy

The ultrafast optical sampling technique has been successfully applied to the non-degenerate subpicosecond pump-probe spectroscopy by using two independently oscillating tunable femtosecond lasers with slightly different pulse repetition rates ( f 1, f 2). The choice of the pump and probe wavelengths is fully arbitrary within the tunability range of the two lasers with time resolution (∼300 fs) limited only by the width of the cross correlation. In our method the difference frequency f 1 – f 2 is locked to the frequency of an external reference for better temporal accuracy and introduction of the homodyne detection revealed a great improvement of sensitivity. The transient absorption bleaching measurements have been performed for organic dye molecules in solution to demonstrate the abilities of this technique. Signals of the time evolution of the photoexcited molecules ranging from subpicosecond intramolecular relaxation to nanosecond ground-state recovery, as well as picosecond solvation dynamics and subnanosecond orientational relaxation, are simultaneously detected in such a simple manner as varying the sweep time of the oscilloscope.

Fluorescence decay dynamics of organic dye molecules in solution

A picosecond laser system consisting of two tuneable dye lasers pumped synchronously by a mode locked argon-ion laser has been used for studying the relaxation of the S1 excited states of dye molecules Nile Blue and Oxazine 720. The dependence of the excited state lifetimes on the solvent and on the temperature are discussed. Both intramolecular and intermolecular decay channels contribute to the radiationaless de-excitation. The mechanism of intramolecular decay is different, while that of intermolecular decay is very similar for the two molecules.

Femtosecond spectral evolution monitoring the bond-twisting event in barrierless isomerization in solution

Using ∼ 100 femtosecond laser pulses we have studied the spectra evolution in time associated with the bonnd-twisting event in the barrierless isomerization of a cyanine dye molecule in solution. The experimental result are compared with Monte Carlo simulations based on a modified Smoluchowski equation. The results shwo that the actual bond-twisting event is observed as a delayed onset of ground state recovery and as very fast spectral shifts of stimulated emission and excited state absorption.

Transient hole-burning and time-resolved fluorescence spectra of dye molecules in solution: Evidence for ground-state relaxation and hole-filling effect

A picosecond transient hole-burning (THB) spectroscopy has been performed for organic dyes in solution. The THB spectra of rhodamine 640 have been found to show a time-dependent spectral change. This phenomenon corresponds to the solvent relaxation effect observed in the time-resolved fluorescence (TRF) spectrum. Although TRF spectrum is related only to the excited-state relaxation, THB spectrum is affected by both ground- and excited-state relaxations. Comparing with the TRF spectrum measured under the same exciting energy, we have clarified the presence of the ground-state relaxation. Further, the THB spectrum of styryl-8 gives an antihole between well-separated two holes corresponding to ground- and excited-state contributions. This may originate from the hole-filling effect due to the nonadiabatic relaxation from the nonequilibrium excited state to the ground state. The analysis based on a configuration coordinate model gives an overall understanding for these phenomena.

The energy relaxation in intermediate state of second-order optical processes in dye molecules in solution

The excitation energy dependence of the peak energy of the 0-0 luminescence band of β-carotene in isopentane has been measured at various temperatures. It is analyzed using the two-dimensional configuration coordinate model. Assuming that the nature of the intermediate state interaction for a second-order optical process in this system is in slow modulation, we calculate the amplitude of the energy modulation using the value of the observed Stokes shift and compare it to that derived from the analysis of the excitation profiles of Raman scattering and luminescence. They are in good agreement with each other, which indicates that the observed energy shift of the luminescence and the excitation profiles of second-order optical processes can be understood by the stochastic motion on the adiabatic potential curves.

Wavelength-dependent rotation of dye molecules in a polar solution

Investigation of rotation movement of 3-amino-N-methylphthalimide in glycerol was carried out, taking into consideration the fluctuation of solvate structure. It was shown theoretically and experimentally that structural relaxation of the solvate shell, which follows excitation of the dye molecule, causes not only shift of the fluorescence spectrum in time but also additional rotation of the dye molecule. This effect, which may be called "wavelength-dependent rotation", depends on the light frequency of both excitation and fluorescence. In particular, at excitation near the maximum of the absorption band, when the relaxation process is followed with the red shift of the fluorescence maximum, the anisotropy of fluorescence decreases faster in the red part of the fluorescence band than in the blue part. On the contrary, in the case of far anti-Stokes excitation, when the temporal shift of fluorescence is going to the blue, the anisotropy in the red part of the spectrum drops more slowly than in the blue part. Finally, there is a special excitation frequency which causes neither change of the fluorescence maximum nor acceleration of the rotational movement of the dye molecule. It is also shown that the temporal evolution of the spectrum and anisotropy of fluorescence in a polar dye solution may be quantitatively described using the socalled inhomogeneous broadening function (IBF). This function gives the distribution of dye molecules in a solution over frequencies of pure electronic transition due to fluctuations of the surrounding shell structure. Measurements of IBF changes in time carried out for 3-amino-N-methylphthalimide showed that during first 3 ns after excitation, the half-width of the IBF grows, and at the same time its maximum quickly shifts to the red. At the later time period there are only small changes of IBF position but considerable exponential decrease in its half-width. The IBF during this period preserves the Gaussian shape.

Ultrafast nonlinear spectroscopy with chirped optical pulses.

Ultrafast four-wave mixing experiments using chirped-pulse excitation of a dye molecule in solution are reported and analyzed. Novel time-delayed signals are observed which are assigned to coherent Stokes-Raman scattering involving the excited state. Numerical calculations of four-wave mixing with linearly chirped pulses based on a stochastic dynamical model show that the signal near zero delay time is extremely sensitive to the presence or absence of slow fluctuations in the solute-solvent interaction.

Analysis of microviscosity and reaction coordinate concepts in isomerization dynamics described by Kramers’ theory

In this work we have studied the isomerization dynamics of a cyanine dye molecule in solution. The viscosity and temperature dependencies of the isomerization rate have been measured in the series of n-alcohols for three different sizes of the isomerizing group. From these measurements we conclude that the shear viscosity of the solvent is not a good measure of the microscopic friction experienced by the isomerizing groups. The friction is varying in a nonhydrodynamic manner with viscosity, which shows that the relative volume of the isomerizing group and solvent molecules (Vp/Vs) is a critical parameter determining the microscopic friction. When the microscopic friction is calculated using a model for molecular rotational relaxation proposed by Dote, Kievelson, and Schwartz [J. Phys. Chem. 85, 2169 (1981)], good fits to Kramers’ equation is obtained. Similar models for microscopic rotational and translational friction combined with Kramers’ equation also yield an apparent improvement over the hydrodynamic Kramers description. The measurements also show that the non-Kramers behavior of the reaction rates have a more complex origin than the (Vp/Vs) dependence of the microscopic friction, that possibly can be traced back to a more general failure of the hydrodynamic description of friction (frequency dependent friction), or to a temperature and solvent dependence of the potential surface parameters. The results also suggest that the detailed nature of the reaction coordinate plays an important role in determining the detailed viscosity dependence of the isomerization. Thus a reactive motion mainly experiencing rotational friction is much more sensitive to the molecular size and free-volume effects, than is the isomerization controlled by translational friction.

Femtosecond photon echoes via non-Markovian dephasing

The photon echo effect in dye molecules in solution has not been studied since effective mechanisms for inhomogeneous line broadening are not expected. We show that photon echoes may occur on a femtosecond time scale when the observation times are shorter than the memory time of the heat bath, and hence non-Markovian features of the molecular dephasing may become relevant. The properties and the dependence of the echo on the characteristics of the non-Markovian dephasing process are studied.

Femtosecond photon echoes from molecules in solution.

Two pulse photon echoes were observed in organic dye molecules in solution using 6-fs optical pulses. The results show an initial rapid dephasing of the echo due to level multiplicity followed by quantum beaks. The temporal decay of the quantum beats is characterized by a ${\mathrm{T}}_{2}$ of about 60 fs.

ChemInform Abstract: Dynamics of Fluorescence of a Dye Molecule in Solution

Transient and stationary fluorescence properties of rhodamine 6G in liquid ethanol have been investigated in a wide temperature range between 110 and 300 K. Various phenomena concerning the interaction between the dye molecule and the surroundings such as dynamic Stokes shift, stepwise temperature dependence of the fluorescence peak energy, and excitation‐wavelength dependent fluorescence peak shift, have been observed. To explain these phenomena in a unified manner, a stochastic theory based on the Onsager cavity model has been developed. It is shown that the fluorescence as well as the absorption characteristics are explained well by this treatment. Various parameters characterizing the optical properties of the molecule, i.e., the dipole moments in the ground and excited states, transition energy in vacuum, temperature‐dependent relaxation time of the dynamic Stokes shift and so on, can be derived from the comparison between the theory and experiment. Finally, the origin of the relaxation processes found in the present work is discussed in connection with the dielectric properties of the solvent and also with the resonance Raman scattering process.

Dynamics of fluorescence of a dye molecule in solution

Transient and stationary fluorescence properties of rhodamine 6G in liquid ethanol have been investigated in a wide temperature range between 110 and 300 K. Various phenomena concerning the interaction between the dye molecule and the surroundings such as dynamic Stokes shift, stepwise temperature dependence of the fluorescence peak energy, and excitation-wavelength dependent fluorescence peak shift, have been observed. To explain these phenomena in a unified manner, a stochastic theory based on the Onsager cavity model has been developed. It is shown that the fluorescence as well as the absorption characteristics are explained well by this treatment. Various parameters characterizing the optical properties of the molecule, i.e., the dipole moments in the ground and excited states, transition energy in vacuum, temperature-dependent relaxation time of the dynamic Stokes shift and so on, can be derived from the comparison between the theory and experiment. Finally, the origin of the relaxation processes found in the present work is discussed in connection with the dielectric properties of the solvent and also with the resonance Raman scattering process.

Concentration depolarization of fluorescence in the presence of molecular rotation

A theory is presented to describe the observables in fluorescence depolarization experiments when both rotational depolarization and concentration depolarization mechanisms are active. The latter mechanism arises from electronic excitation migration among chromophores and becomes more important as their concentration is increased. We treat the simplest case of randomly distributed chromophores, with fixed centers of mass, undergoing independent rotational Brownian motion. Our formalism reduces to existing theories if either depolarization mechanism is turned off, but makes new predictions in the situation where both are operative. Because electronic excitation transfer and molecular orientation are dynamically correlated, we find that previous ad hoc expressions that neglected such correlations are inappropriate. Approximate expressions for the fluorescence anisotropy are provided that can be tested by time-resolved or photostationary experiments on dye molecules in solution. Extensions of the theory to cases of correlated chromophore positions are straightforward and will allow interpretation of experiments on labeled macromolecular systems.

Phase Conjugation by Degenerate Four-wave Mixing in Inverted Dye Solutions

Abstract Phase conjugation has been observed in an amplifying medium of rhodamine 6G dye molecules in solution, excited either by flashlamps or by a laser. It is found that the mechanism involved in phase conjugation is degenerate four-wave mixing through gain saturation, and reasonable agreement obtained with the theory of saturable amplifiers. A phase-conjugate signal was observed over the tuning range of the dye system (580–605 nm) and a reflectivity up to a maximum of 25% was obtained.

溶液中色素分子のスペクトル形状と励起状態ダイナミクス

溶液中色素分子のスペクトル形状と励起状態ダイナミクス

Role of electronic excitation energy migration in tunable lasers utilizing multicomponent dye solutions

The results of studies of electronic excitation energy transfer in dye solutions used as laser active media are summarized. Experimental results are presented, confirming that excitation energy may be transferred during vibrational relaxation. The influence of inhomogeneous broadening of dye spectra on transfer of electronic excitation and the lasing characteristics of dye solutions is discussed. Fluorescence quenching processes that take place as a result of energy migration between dye molecules in solutions and the role of triplet states in these processes are discussed. The results of studies of the influence of energy transfer, association of dye molecules, and their triplet states on the lasing properties of solutions are presented.

Spatial properties of electronic excited-state energy transport in three-dimensional disordered systems: picosecond transient grating studies

The picosecond transient grating technique is used to measure directly the excited state energy transport diffusion constant of dye molecules in solution. Extremely narrow grating fringe spacings (< 980 Å) enable measurement of the excitation's spatial mobility, in the diffusive limit. Results are compared to theoretical predictions for the concentration dependence of the diffusive propagation of the excitation.

Femtosecond excited state relaxation of dye molecules in solution

The bleaching dynamics of organic dye molecules in solution have been investigated using 70 fs pulses from a colliding pulse mode-locked ring dye laser. In addition to ground state relaxation on a nanosecond time scale, a fast partial recovery is observed. For the dyes Nile blue, oxazine 720, cresyl violet and rhodamine 640, this recovery is exponential, with relaxation times in the range 190–480 fs.

High-pressure studies of rotational isomerism of triphenylmethane dye molecules

Hydrostatic pressures to 120 kbar have been used to examine rotational isomerism in the hydrogen, methyl and ethyl derivatives of triphenylmethane dye molecules in solution at room temperature. Peak locations of the A and B isomers generally red-shift with increasing pressure. However, steric interactions in the B isomer of the ethyl derivative result in a blue-shift with pressure. The partial molar volume of the system is found to increase on isomerization from the A to the B form for all three derivatives; the largest Δ V is for the ethyl derivative and the smallest for the hydrogen.

Picosecond measurements using photoacoustic detection

We report experimental results on picosecond time-resolved photoacoustic measurements of excited-state lifetimes, cross sections, and polarization properties for organic dye molecules in solution, using a new technique in which we detect the total photoacoustic impulse produced by two ultrashort optical pulses with variable time delay between them. The picosecond photoacoustic detection technique reported here appears to be a promising new way to observe weak excited-state cross sections and to perform picosecond lifetime measurements in a large variety of weakly absorbing and/or nonfluorescing atomic and molecular systems.