Mohsen Sajadi

Femtosecond stimulated Raman spectroscopy of flavin after optical excitation.

In blue-light photoreceptors using flavin (BLUF), the signaling state is formed already within several 100 ps after illumination, with only small changes of the absorption spectrum. The accompanying structural evolution can, in principle, be monitored by femtosecond stimulated Raman spectroscopy (FSRS). The method is used here to characterize the excited-state properties of riboflavin and flavin adenine dinucleotide in polar solvents. Raman modes are observed in the range 90-1800 cm(-1) for the electronic ground state S(0) and upon excitation to the S(1) state, and modes >1000 cm(-1) of both states are assigned with the help of quantum-chemical calculations. Line shapes are shown to depend sensitively on resonance conditions. They are affected by wavepacket motion in any of the participating electronic states, resulting in complex amplitude modulation of the stimulated Raman spectra. Wavepackets in S(1) can be marked, and thus isolated, by stimulated-emission pumping with the picosecond Raman pulses. Excited-state absorption spectra are obtained from a quantitative comparison of broadband transient fluorescence and absorption. In this way, the resonance conditions for FSRS are determined. Early differences of the emission spectrum depend on excess vibrational energy, and solvation is seen as dynamic Stokes shift of the emission band. The nπ* state is evidenced only through changes of emission oscillator strength during solvation. S(1) quenching by adenine is seen with all methods in terms of dynamics, not by spectral intermediates.

Dynamic Polar Solvation Is Reported by Fluorescing 4-Aminophthalimide Faithfully Despite H-Bonding

Solvation dynamics of 4-aminophthalimide (4AP) in methanol is measured by broadband upconversion of the fluorescence band. The peak emission frequency nu(t) is determined from 100 fs onward with 85 fs time resolution. Polar solvation based on simple continuum theory, including solute polarizability, describes the temporal shape of nu(t) quantitatively. Extrapolation nu(t-->0) points to an initial emission frequency which agrees with the result from stationary spectroscopy in a nonpolar solvent. The extent (4300 cm(-1)) of the dynamic Stokes shift is largely due (50%) to H-bonding, however. The observations imply that H-bonds with 4AP adiabatically follow the dielectric relaxation of the methanol network. The stimulated emission band is also used to measure solvation dynamics. The evolving band is monitored by transient absorption spectroscopy of supercontinuum probe pulses. But the excited-state absorption spectrum, its relative amplitude, and its evolution are needed to extract nu(t) from such measurements. These key data are obtained by comparison with the upconversion results. Thus calibrated photometrically, 4AP transient absorption can be used to monitor solvation dynamics in any solvent. The excited-state absorption spectrum is assigned with the help of time-dependent density-functional calculations. Fluorescence excitation and double-resonance spectroscopy of isolated 4AP, cooled in a supersonic jet, is used to determine optically active modes. An intramolecular reorganization energy is inferred which is consistent with the value in 2-methylbutane (2025 cm(-1)). The crystal structure is also provided.

Femtosecond broadband fluorescence upconversion spectroscopy: Improved setup and photometric correction

A setup for fluorescence upconversion spectroscopy (FLUPS) is described which has 80 fs temporal response (fwhm) for emission in the spectral range 425-750 nm. Broadband phase matching is achieved with tilted gate pulses at 1340 nm. Background from harmonics of the gate pulse is removed and sensitivity increased compared to previous designs. Photometric calibration of the upconversion process is performed with a set of fluorescent dyes. For Coumarin 153 in methanol the peak position, bandwidth, and asymmetry depending on delay time are reported.

Terahertz Absorption Spectroscopy of a Liquid Using a Polarity Probe: A Case Study of Trehalose/Water Mixtures

Water is important for the structure, stability, and function of biomolecules. It can simplify the energy landscape for molecular recognition or protein folding and often controls the native stability. 2] Proton transfer through local water networks requires correlated movement of H-bonds, and conformational changes of proteins appear to be coupled to the dynamics of bulk and hydration water. To understand such processes the vibrational absorption spectrum of the biomolecule–water interface must be observed. The underlying dynamics are widely distributed in time, from fast vibrational modes to slow diffusive reorientation. In lowviscosity liquids, like water, the diffusive regime comprises processes on the picosecond to nanosecond timescale (corresponding to wavenumbers n< 1.5 cm ), which are captured by microwave dielectric spectroscopy. On the high-frequency side (in water above 1000 cm ) intramolecular vibrations are observed by infrared absorption spectroscopy. What is usually lacking is information on the intermolecular vibrational and librational dynamics, which are reflected in the intermediate segment of the spectrum, in the THz (up to 30 cm ) and farinfrared (FIR, 30–250 cm ) regions. It is just this intermediate regime in which processes associated with H-bond dynamics are expected. Unfortunately, the generation and detection of light is difficult here. Furthermore, the response of the biomolecule–water interface is generally not so different from that of bulk water. There is thus a clear need to develop local spectroscopic schemes which avoid contributions from the bulk and confine absorption measurements to the interfacial region. Using the polarity probe N-methyl-6-quinolone (MQ, inset Figure 3) we recently showed that the time-resolved Stokes shift (TRSS) of fluorescence reflects the infrared spectrum of the surrounding liquid. The effective distance for the interaction ranges up to approximately 15 ; spatial resolution of this size may therefore be achieved by linking the probe to the supramolecular structure of interest. What is missing to date is how to extract the (unknown) dielectric properties from a measured TRSS curve. This key step is introduced herein and tested with aqueous trehalose solutions. The disaccharide trehalose (inset Figure 1) is synthesized by some organisms in dry climates for protection against osmotic pressure and freezing; it alters the Hbonding structure of water and modifies the collective dynamics. Trehalose is therefore an intriguing biomolecular model solute for demonstrating the practical use and spectroscopic potential of MQ.

Observing the Hydration Layer of Trehalose with a Linked Molecular Terahertz Probe.

The terahertz (THz) absorption bands of biomolecular hydration layers are generally swamped by absorption from bulk water. Using the disaccharide trehalose, we show that this limitation can be overcome by attaching a molecular probe. By time-resolving the fluorescence shift of the probe, a local THz spectrum is obtained. From the dependence on temperature and H2O/D2O exchange, it is concluded that the trehalose hydration layer is being observed. The region of dynamic water perturbation by the disaccharide encompasses the probe and is therefore larger than the first two solvation layers.

An Ab Initio Microscope: Molecular Contributions to the Femtosecond Time-Dependent Fluorescence Shift of a Reichardt-Type Dye†

The molecular probe N-methyl-6-quinolone (MQ) gives spectroscopic access to its local environment.[1] Its experimentally observed time-dependent Stokes shift can be explained by molecular dynamics simulations in combination with DFT calculations. Decomposition of the MD trajectories shows that an important contribution to the time-dependent Stokes shift originates from a group of water molecules that strongly interact with the molecular dipole of MQ.[2]

Excess dynamic Stokes shift of molecular probes in solution.

The solvation dynamics of molecular probes is studied by broad-band fluorescence upconversion. The time-dependent position of the S1 → S0 emission band or of a vibronic line shape is measured with ~80 fs, 10 cm(-1) resolution. Polar solutes in acetonitrile and acetone, when excited into S1 with excess vibrational energy, show a dynamic Stokes shift which extends to the red beyond the quasistationary state. Equilibrium is then reached by a slower blue shift on a 10 ps time scale. In methanol, excess vibrational energy as large as ~14,000 cm(-1) shows no such effect. Nonpolar solutes exhibit an excess red shift of the emission band in both polar and nonpolar solvents even upon excitation near the vibronic origin. The observed dynamics are discussed in terms of transient heating of the excited chromophore, conformational change, and changes of the molecular cavity size. For solvation studies the optical excitation should be chosen close to the band origin.

Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm−1 from a metallic spintronic emitter

We explore the capabilities of metallic spintronic thin-film stacks as a source of intense and broadband terahertz electromagnetic fields. For this purpose, we excite a W/CoFeB/Pt trilayer (thickness of 5.6 nm) on a large-area glass substrate (diameter of 7.5 cm) by a femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, and wavelength 800 nm). After focusing, the emitted terahertz pulse is measured to have a duration of 230 fs, a peak field of 300 kV cm−1, and an energy of 5 nJ. In particular, the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of its amplitude maximum, thereby facilitating nonlinear control over matter in this difficult-to-reach frequency range on the sub-picosecond time scale.

Terahertz-field-induced optical birefringence in common window and substrate materials.

We apply intense terahertz (THz) electromagnetic pulses with field strengths exceeding 2 MV cm(-1) at ~1 THz to window and substrate materials commonly used in THz spectroscopy and determine the induced optical birefringence. Materials studied are diamond, sapphire, magnesium oxide (MgO), polymethylpentene (TPX), low-density polyethylene (LDPE), silicon nitride membrane (SiN) and crystalline quartz. We observe a Kerr-effect-type transient birefringence in all samples, except in quartz and Si, where, respectively, a linear electrooptic signal and a response beyond the perturbative regime are found. We extract the nonlinear refractive indices and the electrooptic coefficient (in the case of quartz) of these materials and discuss implications for their use as windows or substrates in THz pump-optical probe spectroscopy.

Covalent linkage of N-methyl-6-oxyquinolinium betaine to trehalose.

The common route to link quinolinium and pyridinium fluorophores to biomolecules via bromoacetic acid has failed in labeling the disaccharide trehalose with N-methyl-6-oxyquinolinium betaine: the unexpected, extremely high instability of the N-carboxymethyl ester was overcome by direct N-alkylation of the quinoline derivative with trehalose triflate.