Dynamic Stokes Shift Research Articles

Polar Solvation Dynamics in Zn(II)-Substituted Cytochrome c: Diffusive Sampling of the Energy Landscape in the Hydrophobic Core and Solvent-Contact Layer

We employed picosecond time-resolved fluorescence spectroscopy to characterize polar diffusive protein and solvent dynamics in Zn(II)-substituted cytochrome c. The solvent-response function, obtained from the dynamic Stokes shift of the Zn(II) porphyrin's fluorescence 0-0 transition energy over the 100-ps to 12-ns time scale, exhibits a double-exponential decay (250 ps and 1.45 ns time constants). The addition of glycerol to the external solvent medium slows only the slower of the two exponential components; in the presence of 50% (v/v) glycerol, the second time constant was lengthened to 2.2 ns. The magnitude of the decay accounts for 85% of the reorganization energy and leaves room for only a minor inertial component on the sub-ps time scale. The results show that two types of polypeptide motions are sensed by the intrinsic Zn(II) porphyrin. The slower of the motions arises from polypeptide motions in the interfacial, solvent-contact portion of the protein. The faster, solvent-independent part of the response arises from motions of the polypeptide in the hydrophobic core of the protein. The two classes of motion are apparently not strongly coupled, nor is the dynamic Stokes shift coupled to the nonpolar reorganization that accompanies the excited-state photodissociation of the Zn(II) porphyrin's protein-derived axial ligands. The finding that the inertial response is limited to a small fraction of the total reorganization energy suggests that the hydrophobic core of the protein is largely incapable of the free-rotor or librational character of polypeptide and solvent motions that is observed near the solvent interface.

Excitation Wavelength Dependence of Solvation Dynamics in a Water Pool of a Reversed Micelle

Abstract We have investigated the excitation wavelength dependence of solvation dynamics in the water pool of a reversed micelle using dynamic Stokes shift method. The solvation dynamics displays nanosecond and subnanosecond components, which are almost independent of the excitation wavelength. On the other hand, the total dynamics Stoke shift decreases with increasing the excitation wavelength.

Study of Solvation Dynamics in an Ormosil: CTAB in a Sol−Gel Matrix

Solvation dynamics of coumarin 480 (C480) is studied in a nanoporous sol-gel matrix in the presence of cetyltrimethylammonium bromide (CTAB). In the presence of CTAB, the solvation dynamics is described by two components of 120 ps (25%) and 7200 ps (75%) with a total dynamic Stokes shift of 1200 cm - 1 . This is substantially slower than the solvation dynamics in the sol-gel matrix and in CTAB micelles. The anisotropy decay indicates hindered rotation of the probe (C480) inside the sol-gel matrix in the presence of CTAB.

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.

Solute Dependence of Polar Solvation Dynamics Studied by RISM/Mode-Coupling Theory

We apply the combination of the reference interaction-site model (RISM) theory and mode-coupling theory (MCT) for the description of solvation dynamics in acetonitrile. The RISM theory is used for the static features of the solute–solvent system, whereas MCT is employed to capture solvent dynamics. We calculate the dynamic response function of the average-energy relaxation of the system, SS(t), a corresponding quantity which is experimentally measured through the dynamic Stokes shift of fluorescence spectra, for example. The RISM–MCT framework turns out to be very applicable for realistic estimation of SS(t). We vary the model solutes from a simple ion, to dumbbell dipoles, to quadrupole, to octapole. As a general feature, SS(t) begins to relax with the initial Gaussian decay, followed by the underdamped oscillation. Higher multiplicity of the pole or larger charges put on the solute-site causes SS(t) to relax more slowly. We suggest this behavior is related to an increased complexity of solvent–solvent interactions just around the charged sites on the solute. On the other hand, the amplitude of the underdamped oscillation becomes smaller as the multiplicity of the pole increases.

Femtosecond fluorescence studies of Auramine O in hybrid sol-gel derived films

ABSTRACTFemtosecond fluorescence upconversion experiments have been performed on Auramine O (a diphenylmethane dye) in polymethylmethacrylate (PMMA) and in hybrid organic/inorganic solgel based films.All the investigated samples showed a fast decay (few picoseconds) and a long decay (hundreds of picoseconds). The fast components are representative of the rapid cooling of the excess excitation energy to the matrices. The long components are representative of the excited-state lifetime of the probed molecules. Auramine O in solid matrices showed lifetimes longer than in liquid solutions. Torsional diffusion motions of the two phenyl rings of the molecule are held responsible for the excited-state dynamics.A dynamic Stokes shift has been observed for all samples. The lack of a rise time when detection was on the red side is explained in terms of an adiabatic coupling between emissive and nonemissive excited states, as is the case of liquid solutions. Different spectral shifts in PMMA and hybrid glasses have been measured. A different coupling between the emissive and nonemissive excited states for the two types of matrices is considered.

Photoluminescence decay lifetime measurements of hemicyanine derivatives of different alkyl chain lengths

The fluorescence upconversion setup for the detection of photoluminescence (PL) decay lifetime with subpicosecond time resolution was constructed, and the photoluminescence phenomena of several hemicyanine dyes with alkyl chains of different chain lengths tethered to the N atom of the pyridine moiety (HC- n, n=6, 15, 22) in methanol were investigated. The average decay lifetimes of the solutions determined from the measured data by multi-order exponential decay curve fitting were ∼27 ps at the PL peak wavelength. It was found that the PL decay properties did not depend on the alkyl chain length in the molecule, implying that the twist of the alkylpyridinium ring of the molecule is not possible as a nonfluorescing relaxation pathway. The time-dependent PL spectra constructed from the PL lifetime data showed the dynamic Stokes shift of ∼1000 cm −1.

Bond length alternation in ground and HOMO?LUMO excited states in polyenes. Dynamic Stokes shift?

Complete-active-space self-consistent-field calculation of the reorganisation energy, λ, corresponding to the strongly allowed HOMO→LUMO transition in planar polyenes in the trans form (C 2 h symmetry), gives λ>0.5 eV. This large λ depends on the fact that the short and long bond lengths of the excited 1B u (or 3B u ) state compared to the 1A g ground state are almost cancelled. The emission redshift (Stokes shift) in molecules with the same type of π system is quite small, however, which suggests that the Stokes shift may be dynamic, owing to the presence of another excited state at lower or about the same energy.

Solvation dynamics in the plastic crystal and supercooled liquid state ofethanol

The dynamics of ethanol in its plastic crystalline and supercooled liquid state hasbeen measured using triplet state solvation dynamics techniques. This studyfocuses on temperatures near the respective glass transitions Tgof the two distinct phases, where the structural relaxation times are between 1ms and 10 s. In the plastic crystal, the correlation times of the Stokesshift dynamics match the longitudinal time constants of the system asderived from dielectric relaxation data. Additionally, a maximum of thetime resolved optical linewidths is indicative of heterogeneous dynamicsin the plastic crystal. The supercooled liquid state is obtained afterquenching the liquid from room temperature to below Tgand displays relaxation times that are a factor of 100 faster compared withthe plastic crystal at the same temperature. The results do not revealdifferent translational contributions to solvation for the two phases.

Ultrafast photoreactions in protein nanospaces as revealed by fs fluorescence dynamics measurements on photoactive yellow protein and related systemsDedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.

We have investigated primary processes of ultrafast photoreactions of various photoresponsive proteins by means of fs fluorescence dynamics measurements. Based on these studies, the effects of the protein nanospace (PNS), containing the chromophore, on the dynamics and mechanisms of the ultrafast and highly efficient reactions of these proteins have been elucidated. In this article, we discuss mainly the results of our studies on ultrafast photoisomerization of photoactive yellow protein (PYP), its mutants and analogues. The chromophore of the PYP, deprotonated coumaric acid thioester (O−-phenyl-CHCH–CO–S–CH2–), fixed in PNS by hydrogen (H) bonding interactions at the head part, O−-phenyl-, and by covalent bonding at the tail part, –CO–S–, undergoes ultrafast twisting by flipping the thioester bond, owing to the intrachromophore head to tail charge transfer caused by the photoexcitation. In the site-directed mutants where the PNS structure is looser and more disordered, the photoinduced twisting reaction becomes slowed compared with the wild-type PYP and moreover, the twisting becomes much slower in the denatured PYP, showing the supreme importance of more regulated PNS for the fast twisting. We found also coherent vibrations in the fluorescence decay curves which might be coupled with the twisting. Furthermore, we found for a PYP analogue with replaced chromophore ultrafast dynamic Stokes shift of fluorescence rather than the quenching due to twisting, indicating the importance of chromophore-PNS fine adjustment for the ultrafast twisting.

Computer simulations of the solvation dynamics of Coumarin 153 in dimethylsulfoxide

We report molecular dynamics (MD) simulations of the solvation dynamics of Coumarin 153 in liquid dimethylsulfoxide using two distinct sets of partial charges for the coumarin probe. The excited state dipole moment of the coumarin and the dynamic Stokes shift in solution depend significantly on the type of charge distributions used. Nevertheless, the overall characteristics of the solvation responses obtained from both sets of charges are very similar and show good agreement with time-dependent Stokes shift experiments. Microscopic details of the solvent reorganization around the probe are discussed in light of the charge transfer upon photoexcitation.

Ultrafast Photoinduced Reaction Dynamics and Coherent Oscillations in Photoresponsive Proteins.

We have studied primary processes of ultrafast photoisomerization reactions of photoactive yellow protein (PYP) and visual rhodopsin (Rh) as well as ultrafast photoinduced electron transfer reactions of flavoproteins (FP) by means of the fs fluorescence up-conversion method. On the basis of these studies, we have examined the effects of the protein nanospace (PNS), where the chromophore is held, on the dynamics and mechanisms of the ultrafast and highly efficient photoinduced reactions of these proteins. In this article, we discuss mainly results of our comparative fluorescence dynamics studies on wild type (w. -t.) PYP, it’s site-directed mutants, analogues with replaced chromophore and FP, including the coherent vibrations in the ultrafast fluorescence decays of w. -t. PYP and mutants in the course of the photoisomerization, ultrafast dynamic Stokes shift of fluorescence of PYP analogues, and ultrafast electron transfer of FP.

Femtosecond fluorescence studies of two-dimensional dynamics in photoexcited Michler's ketones

Femtosecond fluorescence upconversion experiments are reported for two diphenyl ketone dye molecules, the unbridged Michler's ketone [4,4 ′-bis( N,N-dimethylamino)-benzophenone] and its bridged derivative compound 3,6-bis(dimethylamino)-10,10-dimethylanthrone (BMK). For Michler's ketone (MK), in alcoholic solution, anomalously fast dynamic Stokes shifts and fluorescence decay transients are observed. It is discussed that phenyl-group twisting as well as solvation determine the excited-state relaxation dynamics of MK. In the case of BMK, twisting is absent and the excited-state dynamics is determined solely by (fast) initial solvation followed by population decay to the ground state.

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

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.

Temperature Dependence of Charge-Transfer Fluorescence from Extended and U-shaped Donor−Bridge−Acceptor Systems in Glass-Forming Solvents

The behavior is reported of three fluorescent D−bridge−A systems that display a fascinating temperature dependence in glass forming solvents over the temperature range between 77 and 293 K. In two of these systems, a rigid, saturated alkane bridge maintains an extended conformation, and as a result, the charge-transfer (CT) state is of giant dipolar nature. This causes the position of the CT fluorescence to be an extremely sensitive probe for the reorientation polarization of the surrounding medium. As a result, the thermochromism of the continuous CT fluorescence maximum in 2-methyltetrahydrofuran (MTHF) covers the full visible region. In the higher temperature domain (above ca. 145 K), this thermochromism can be quantitatively described via the Lippert−Mataga relation. At lower temperatures, solvent relaxation slows down sufficiently to detect exceptionally large dynamic Stokes shifts of the fluorescence maximum on time scales up to ≥40 ns. The third D−bridge−A system studied features a U-shaped ground state conformation. Remarkably, this system displays a significant thermochromic shift over a narrow temperature region around 175 K in the nonpolar methylcyclohexane (MCH) in which the other systems display only very minor thermochromism. In this U-shaped system therefore, one monitors the temperature dependence of an internal reorganization instead of a medium relaxation. Extensive ab initio calculations indicate that this internal reorganization must be related to an electrostatically driven conformational collapse of the U-shaped system in the CT state.

Two-dimensional Fourier transform electronic spectroscopy

Two-dimensional Fourier transform electronic spectra of the cyanine dye IR144 in methanol are used to explore new aspects of optical 2D spectroscopy on a femtosecond timescale. The experiments reported here are pulse sequence and coherence pathway analogs of the two-dimensional magnetic resonance techniques known as COSY (correlated spectroscopy) and NOESY (nuclear Overhauser effect spectroscopy). Noncollinear three pulse scattering allows selection of electronic coherence pathways by choice of phase matching geometry, temporal pulse order, and Fourier transform variables. Signal fields and delays between excitation pulses are measured by spectral interferometry. Separate real (absorptive) and imaginary (dispersive) 2D spectra are generated by measuring the signal field at the sample exit, performing a 2D scan that equally weights rephasing and nonrephasing coherence pathways, and phasing the 2D spectra against spectrally resolved pump–probe signals. A 3D signal propagation function is used to correct the 2D spectra for excitation pulse propagation and signal pulse generation inside the sample. At relaxation times greater than all solvent and vibrational relaxation timescales, the experimental 2D electronic spectra can be predicted from linear spectroscopic measurements without any adjustable parameters. The 2D correlation spectra verify recent computational predictions of a negative region above the diagonal, a displacement of the 2D peak off the diagonal, and a narrowing of the 2D cross-width below the vibrational linewidth. The negative region arises from 4-level four-wave mixing processes with negative transition dipole products, the displacement off the diagonal arises from a dynamic Stokes shift during signal radiation, and the narrow 2D cross-width indicates femtosecond freezing of vibrational motion.

Solvation dynamics of nile blue in ethanol confined in porous sol–gel glasses

We report on solvation dynamics measurements of the chromophore nile blue in ethanol confined to sol–gel glasses with 50 Å and 75 Å average pore size and compare them with the dynamics of the respective bulk solution. Both the amplitude of the dynamic Stokes shift as well as the dynamics of the solvation process are drastically changed upon confinement. In both confined solutions the dynamic Stokes shift is reduced by a factor of about 2. As the large majority of the chromophores is adsorbed at the inner pore surfaces the solute molecules interact with only a “half-space” of solvent molecules. In a first approximation this decreases the stabilization energy by a factor of 2. The solvation dynamics in the confined solutions show nonexponential behavior comparable to the bulk. However, the whole solvation process slows down and the single decay time constants characterizing it increase with decreasing pore size of the sol–gel glass. We have introduced two phenomenological models to rationalize this behavior. The enhanced polarization field model takes into account the confinement and predicts a strengthening of the solvent’s polarization field in an extended solvation cage induced by the electrical field of the chromophore. The steric hindrance model focuses on the interaction of the liquid with the surface reducing molecular mobility resulting in longer relaxation times. Probably both effects are of relevance in the confined solutions investigated. Confinement affects the steady-state spectra as well leading to a redshift of the absorption and a blueshift of the fluorescence. Additional data on the reorientational dynamics of the chromophore are consistent with the fact that the molecules are predominantly adsorbed at the pore surfaces.

Femtosecond Fluorescence Anisotropy Studies of Solvation-Induced Intraligand Charge Transfer in Photoexcited Aluminum(III) Tris(8-hydroxyquinoline)

For the organic light emitting diode compound aluminum(III) tris(8-hydroxyquinoline) (Alq3), dissolved in dimethylformamide (DMF), the time dependence of the fluorescence anisotropy has been studied using the femtosecond fluorescence upconversion technique. Upon excitation within the first absorption band, near 364 nm, with polarized laser pulses of duration less than 150 fs, an initial fluorescence anisotropy of about 0.2 is found to rapidly decay with a time constant of 2.0 ± 0.2 ps. The observed fast anisotropy decay component is concomitant with the solvation-induced dynamic Stokes shift of about 1000 cm-1. When excitation is at wavelengths below 330 nm, the fluorescence of Alq3 in liquid solution does not show any anisotropy effect. It is discussed that the emissive lowest excited electronic state of Alq3 is ligand localized and that its electronic wave function is affected by solvation. More specifically, the electronic wave function is considered to be an admixture of wave functions belonging to se...

Excited-state dynamics of rhodopsin probed by femtosecond fluorescence spectroscopy

Nonexponential fluorescence dynamics of bovine rhodopsin were measured at various wavelengths from 530 to 780 nm. No dynamic Stokes shift was detected but the fastest part of the fluorescence at both blue and red sides of the spectrum decayed faster than that at the center, which has led to the conclusion that the Franck–Condon state→fluorescence state conversion occurs within time-resolution of apparatus (<100 fs) owing to coupling with intra-chromophore high frequency modes and faster initial decay is due to sharpening of band shape caused by a decrease of amplitudes of hfm along the reaction coordinate of twisting.

Nanosecond Fluorescence Dynamic Stokes Shift of Tryptophan in a Protein Matrix

The fluorescent dynamic Stokes shift (FDSS) method has emphasized a time-dependent dipolar relaxation process around the single tryptophan residue (Trp31) in cytidine monophosphate kinase from E. coli (CMPK). This Trp residue, located close to the protein surface in a hydrophobic pocket, is weakly accessible to acrylamide, a water-soluble quencher. It exhibits fluorescence characteristics suitable for a detailed study of dipolar relaxation: (i) a fluorescence decay almost monoexponential and (ii) a fluorescence emission maximum of 329 nm, in a wavelength range intermediate between those of a completely polar environment and a strongly apolar one. This emission maximum is shifted to 320 nm by decreasing the temperature to 230−240 K with glycerol as cryoprotectant. A time constant (∼100 ps) affected by a negative preexponential, evidenced in the red-edge fluorescence intensity decays, supports the existence of an excited-state reaction. A multiphasic FDSS (with time constants ranging from ∼100 ps to severa...