Excited-State Dynamics of Thioflavin T: Planar Stable Intermediate Revealed by Nuclear Wave Packet Spectroscopies

Electroabsorption and Stark Fluorescence Spectroscopies of Thioflavin T

Thioflavin T (ThT) is a typical fluorescent marker for detecting the formation of amyloid fibrils, because its fluorescence intensity increases by more than 2 orders of magnitude upon complexation with the fibrils. Strong electrostatic fields on protein surfaces are known to be a significant factor in chemical reactions and biological functions. Therefore, ThT bound to amyloid fibrils must experience strong electric fields. This study employed electroabsorption and Stark fluorescence spectroscopies to clarify the effects of external electric fields on the photophysics of ThT. The absorption spectrum shows two bands ascribed to locally excited (LE) and charge transfer (CT) states. Coupling between the LE and CT states is enhanced in the presence of an external electric field, resulting in fluorescence quenching. The electric field strength of the amyloid fibril surface was inferred from the fluorescence quenching efficiency of ThT.

Coherent Two-Dimensional and Broadband Electronic Spectroscopies

Over the past few decades, coherent broadband spectroscopy has been widely used to improve our understanding of ultrafast processes (e.g., photoinduced electron transfer, proton transfer, and proton-coupled electron transfer reactions) at femtosecond resolution. The advances in femtosecond laser technology along with the development of nonlinear multidimensional spectroscopy enabled further insights into ultrafast energy transfer and carrier relaxation processes in complex biological and material systems. New discoveries and interpretations have led to improved design principles for optimizing the photophysical properties of various artificial systems. In this review, we first provide a detailed theoretical framework of both coherent broadband and two-dimensional electronic spectroscopy (2DES). We then discuss a selection of experimental approaches and considerations of 2DES along with best practices for data processing and analysis. Finally, we review several examples where coherent broadband and 2DES were employed to reveal mechanisms of photoinitiated ultrafast processes in molecular, biological, and material systems. We end the review with a brief perspective on the future of the experimental techniques themselves and their potential to answer an even greater range of scientific questions.

Coherent internal conversion from high lying electronic states to S1 in boron-dipyrromethene derivatives

Internal conversion is the first step after photoexcitation to high lying electronic states, and plays a central role in many photoinduced processes. In this report, we demonstrate a truly ultrafast internal conversion (IC) in large molecules by time-resolved fluorescence (TF). Following photoexcitation to the S_n (n ≥ 2) state, TF of the S₁ state was recorded for two boron-dipyrromethene (BODIPY) derivatives in solution. IC to S₁ takes place nearly instantaneously within 20 fs for both molecules. Abundant nuclear wave packet motions in the S₁ state are manifest in the TF signals, which demonstrates that the IC in these BODIPY molecules is coherent with respect to most of the vibrational modes. Theoretical calculations assuming impulsive IC to S₁ account for the wave packet dynamics accurately.

Intramolecular Charge Transfer of 1-Aminoanthraquinone and Ultrafast Solvation Dynamics of Dimethylsulfoxide

The intramolecular charge transfer (ICT) of 1-aminoanthraquinone (AAQ) in the excited state strongly depends on its solvent properties, and the twisted geometry of its amino group has been recommended for the twisted ICT (TICT) state by recent theoretical works. We report the transient Raman spectra of AAQ in a dimethylsulfoxide (DMSO) solution by femtosecond stimulated Raman spectroscopy to provide clear experimental evidence for the TICT state of AAQ. The ultrafast (~110 fs) TICT dynamics of AAQ were observed from the major vibrational modes of AAQ including the ν_C-N + δ_CH and ν_C=O modes. The coherent oscillations in the vibrational bands of AAQ strongly coupled to the nuclear coordinate for the TICT process have been observed, which showed its anharmonic coupling to the low frequency out of the plane deformation modes. The vibrational mode of solvent DMSO, ν_S=O showed a decrease in intensity, especially in the hydrogen-bonded species of DMSO, which clearly shows that the solvation dynamics of DMSO, including hydrogen bonding, are crucial to understanding the reaction dynamics of AAQ with the ultrafast structural changes accompanying the TICT.

Solvent effect on excited state potential energy surfaces of Thioflavin T. Qualitatively different results by TDDFT and SA-2-CASSCF methods

Thioflavin T (ThT) is a viscosity-sensitive fluorescent dye and its emission intensity undergoes a significant enhancement upon binding to DNA or amyloid fibrils. This fluorescence light-up feature has been attributed earlier to restriction of structural rearrangements in the excited state that are coupled to an intramolecular charge transfer (ICT) reaction. In this work TDDFT (using B3LYP and CAM-B3LYP functionals) and SA-2-CASSCF calculations were carried out to obtain relaxed excited-state potential energy surfaces (PES) along twisting φ and wagging δ angles that describe mutual orientation of benzothiazole (BTZ) and dimethylaniline (DMA) fragments in ThT. For isolated ThT molecule both methods predict that during structural rearrangements of the initially excited Franck-Condon state, besides twisting along CC bond which connects BTZ and DMA fragments, a considerable wagging motion is expected to occur. Account for solvent effect using polarized continuum model showed qualitative differences in the excited state PES features calculated by SA-2-CASSCF and TDDFT methods. Single-reference TDDFT calculations failed to describe solvation of TICT state and predicted increase of its energy in more polar media.

Time-Resolved Impulsive Stimulated Raman Spectroscopy with Synchronized Triple Mode-Locked Lasers

A complete understanding of a photochemical reaction dynamics begins with real-time measurements of both electronic and vibrational structures of photoexcited molecules. Time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS) with femtosecond actinic pump, Raman pump, and Raman probe pulses is one of the incisive techniques enabling one to investigate the structural changes of photoexcited molecules. Herein, we demonstrate that such femtosecond TR-ISRS is feasible with synchronized triple mode-locked lasers without using any time-delay devices. Taking advantage of precise control of the three repetition rates independently, we could achieve automatic scanning of two delay times between the three pulses, which makes both rapid data acquisition and wide dynamic range measurement of the fifth-order TR-ISRS signal achievable. We thus anticipate that the present triple mode-locked laser-based TR-ISRS technique will be of critical use for long-term monitoring of photochemical reaction dynamics in condensed phases and biological systems.

Revisiting thioflavin T (ThT) fluorescence as a marker of protein fibrillation - The prominent role of electrostatic interactions

Thioflavin T (ThT), a benzothiazole-based fluorophore, is a prominent dye widely employed for monitoring amyloid fibril assembly. Despite the near-universal presumption that ThT binds to β-sheet domains upon fibrillar surface via hydrophobic forces, the contribution of the positive charge of ThT to fibril binding and concomitant fluorescence enhancement have not been thoroughly assessed. Here we demonstrate a considerable interdependence between ThT fluorescence and electrostatic charges of peptide fibrils. Specifically, by analyzing both fibril-forming synthetic peptides and prominent natural fibrillar peptides, we demonstrate pronounced modulations of ThT fluorescence signal that were solely dependent upon electrostatic interactions between ThT and peptide surface. The results further attest to the fact that fibril ζ-potential rather than pH-dependent assembly of the fibrils constitute the primary factor affecting ThT binding and fluorescence. This study provides the first quantitative assessment of electrostatically driven ThT fluorescence upon adsorption to amyloid fibrils.

Non-Born-Oppenheimer Molecular Dynamics Observed by Coherent Nuclear Wave Packets

The reaction dynamics of a photochemical reaction is typically described by reaction coordinates based on the Born-Oppenheimer (BO) approximation. A strong interaction between electrons and nuclei, conventionally occurring at conical intersections, however, breaks the BO approximation and has major consequences for the efficiency of a photochemical reaction. Despite its importance, related studies into the non-BO dynamics are scarce. Here, we investigate the non-BO dynamics of excited-state intramolecular proton transfer (ESIPT) occurring in 10-hydroxybenzo[h]quinoline (HBQ). Two coherent vibrational modes at 237 and 794 cm^-1 representing molecular dynamics on a diabatic surface in HBQ are identified by a wave packet analysis based on a transient absorption measurement with a time resolution of 11 fs and with a density functional theory-based model calculation. It is also revealed that the strong Coulomb field effect in HBQ leads to the completion of ESIPT within about two cycles of the OH stretching mode. The work paves the way for time-domain studies of molecular dynamics beyond the BO approximation in other photochemical reactions.

Role of coherent nuclear motion in the ultrafast intersystem crossing of ruthenium complexes

Ultrafast intersystem crossing (ISC) of ruthenium complexes is time-resolved directly. Coherent nuclear wave packets suggest that metal–ligand stretching or symmetry breaking vibrational modes are strongly coupled with the ISC.

Interferometric Measurement of Transient Absorption and Refraction Spectra with Dual Frequency Comb

We demonstrate that a dual frequency comb-transient absorption (DFC-TA) technique can be combined with a time-domain interferometric detection to measure both the transient absorption and refraction spectra of molecules in solution. To do this, the pump-probe signal field of DFC-TA is allowed to interfere with a time-delayed local oscillator field in a time domain. We show that this DFC interferometric pump-probe spectroscopy (DFC-IPS) technique has a unique ability to extract the phase and amplitude information on the pump-probe signal using just a single-scan data, while conventional techniques require an independent signal measured without the pump field for the normalization of the pump-probe spectrum. As a proof-of-principle experiment, we here show that the DFC-IPS enables us to simultaneously measure the frequency-resolved (from 650 to 950 nm) transient absorption and refraction signals with an exceptionally broad dynamic range from femtosecond to nanosecond without using a mechanical translational stage for pump-probe time-scanning. We anticipate that our DFC-IPS technique with femtosecond time-resolution capability will be useful to investigate photoinduced chemical and biological reactions covering broad dynamic ranges.

Dual-Frequency Comb Transient Absorption: Broad Dynamic Range Measurement of Femtosecond to Nanosecond Relaxation Processes

We experimentally demonstrate a dual-frequency comb-based transient absorption (DFC-TA) technique, which has a 12 fs time resolution and an ultrafast scan rate. Here, the fast scan rate is achieved by employing asynchronous optical sampling (ASOPS), which utilizes two independent mode-locked lasers with a slightly detuned repetition rates. The ASOPS approach is advantageous because photodegradation damage of optical sample during TA measurements can be minimized by a gated sampling. We show that the vibrational and electronic population relaxations of near-IR dye molecules in solution that occur in the time range from femtoseconds to nanoseconds can be resolved even with a single time scan measurement. The phase coherent nature of our dual-frequency comb lasers is shown to be the key for successful coherent averaging with femtosecond time resolution preserved over many data acquisitions. We anticipate that the present DFC-TA method without using any pump-probe time delay devices could be of use in developing ultrafast TA-based microscopy and time-resolved coherent multidimensional spectroscopy.