https://pubs.rsc.org/en/content/articlepdf/2019/NP/C8NP00092A

Marine natural products.

ConspectusFluorescent environment-sensitive probes are specially designed dyes that change their fluorescence intensity (fluorogenic dyes) or color (e.g., solvatochromic dyes) in response to change in their microenvironment polarity, viscosity, and molecular order. The studies of the past decade, including those of our group, have shown that these molecules become universal tools in fluorescence sensing and imaging. In fact, any biomolecular interaction or change in biomolecular organization results in modification of the local microenvironment, which can be directly monitored by these types of probes. In this Account, the main examples of environment-sensitive probes are summarized according to their design concepts. Solvatochromic dyes constitute a large class of environment-sensitive probes which change their color in response to polarity. Generally, they are push–pull dyes undergoing intramolecular charge transfer. Emission of their highly polarized excited state shifts to the red in more polar solven...

Solvatochromic and Fluorogenic Dyes as Environment-Sensitive Probes: Design and Biological Applications

Twisted intramolecular charge transfer (TICT) is an electron transfer process that occurs upon photoexcitation in molecules that usually consist of a donor and acceptor part linked by a single bond. Following intramolecular twisting, the TICT state returns to the ground state either through red-shifted emission or by nonradiative relaxation. The emission properties are potentially environment-dependent, which makes TICT-based fluorophores ideal sensors for solvents, (micro)viscosity, and chemical species. Recently, several TICT-based materials have been discovered to become fluorescent upon aggregation. Furthermore, various recent studies in organic optoelectronics, non-linear optics and solar energy conversions utilised the concept of TICT to modulate the electronic-state mixing and coupling on charge transfer states. This review presents a compact overview of the latest developments in TICT research, from a materials chemistry point of view.

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Recent advances in twisted intramolecular charge transfer (TICT) fluorescence and related phenomena in materials chemistry

Vibrational and vibronic relaxation and related energy redistribution play an important role in many photoinduced processes of molecules. In the condensed phase, in which collision-induced relaxation is of particular relevance, these phenomena occur on a very rapid time scale between several tens of femtoseconds and 100 ps (1, 2). Spectroscopy with ultrashort laser pulses has been widely applied to elucidate the dynamics and physical mechanisms of intraand intermolecular energy redistribution (3, 4). Indeed, time resolved experiments have provided extensive information not accessible by other experimental techniques. In this article, we present a review on vibrational and vibronic relaxation of large molecules in liquids. Photoinduced redistribution processes of molecules, which consist typically of 20 or more atoms, are considered. We discuss. femtoand picosecond spectroscopy of the intramolecular redistribution and the intermolecular energy transfer from vibrationally hot molecules to their (cold) surrounding. We focus on measurements of vibrational and vibronic lifetimes, as well as on studies of energy redis­ tribution and transport. However, we will not consider dephasing of vibranic or vibrational states, relaxation processes connected to the

VIBRATIONAL AND VIBRONIC RELAlXATION OF LARGE POL-yrATOMIC MOLECULES IN LIQUIDS

Formation of amyloid fibrils underlies a wide range of human disorders, including Alzheimer's and prion diseases. The amyloid fibrils can be readily detected thanks to thioflavin T (ThT), a small molecule that gives strong fluorescence upon binding to amyloids. Using the amyloid fibrils of Aβ40 and Aβ42 involved in Alzheimer's disease, and of yeast prion protein Ure2, here we study three aspects of ThT binding to amyloids: quantification of amyloid fibrils using ThT, the optimal ThT concentration for monitoring amyloid formation and the effect of ThT on aggregation kinetics. We show that ThT fluorescence correlates linearly with amyloid concentration over ThT concentrations ranging from 0.2 to 500 µM. At a given amyloid concentration, the plot of ThT fluorescence versus ThT concentration exhibits a bell-shaped curve. The maximal fluorescence signal depends mostly on the total ThT concentration, rather than amyloid to ThT ratio. For the three proteins investigated, the maximal fluorescence is observed at ThT concentrations of 20-50 µM. Aggregation kinetics experiments in the presence of different ThT concentrations show that ThT has little effect on aggregation at concentrations of 20 µM or lower. ThT at concentrations of 50 µM or more could affect the shape of the aggregation curves, but this effect is protein-dependent and not universal.

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Thioflavin T as an amyloid dye: fibril quantification, optimal concentration and effect on aggregation.

Thioflavin-T (ThT) can bind to amyloid fibrils and is frequently used as a fluorescent marker for in vitro biomedical assays of the potency of inhibitors for amyloid-related diseases, such as Alzheimer's disease, Parkinson's disease, and amyloidosis. Upon binding to amyloid fibrils, the steady-state (time-integrated) emission intensity of ThT increases by orders of magnitude. The simplicity of this type of measurement has made ThT a common fluorescent marker in biomedical research over the last 50 years. As a result of the remarkable development in ultrafast spectroscopy measurements, researchers have made substantial progress in understanding the photophysical nature of ThT. Both ab initio quantum-mechanical calculations and experimental evidence have shown that the electronically excited-state surface potential of ThT is composed of two regimes: a locally excited (LE) state and a charge-transfer (CT) state. The electronic wave function of the excited state changes from the initial LE state to the CT state as a result of the rotation around a single C-C bond in the middle of the molecule, which connects the benzothiazole moiety to the dimethylanilino ring. This twisted-internal-CT (TICT) is responsible for the molecular rotor behavior of ThT. This Account discusses several factors that can influence the LE-TICT dynamics of the excited state. Solvent, temperature, and hydrostatic pressure play roles in this process. In the context of biomedical assays, the binding to amyloid fibrils inhibits the internal rotation of the molecular segments and as a result, the electron cannot cross into the nonradiative "dark" CT state. The LE state has high oscillator strength that enables radiative excited-state relaxation to the ground state. This process makes the ThT molecule light up in the presence of amyloid fibrils. In the literature, researchers have suggested several models to explain nonradiative processes. We discuss the advantages and disadvantages of the various nonradiative models while focusing on the model that was initially proposed by Glasbeek and co-workers for auramine-O to be the best suited for ThT. We further discuss the computational fitting of the model for the nonradiative process of ThT.

Molecular rotors: what lies behind the high sensitivity of the thioflavin-T fluorescent marker.

A computational model of nonradiative decay is developed and applied to explain the time-dependent emission spectrum of thioflavin T (ThT). The computational model is based on a previous model developed by Glasbeek and co-workers (van der Meer, M. J.; Zhang, H.; Glasbeek, M. J. Chem. Phys. 2000, 112, 2878) for auramine O, a molecule that, like ThT, exhibits a high nonradiative rate. The nonradiative rates of both auramine O and ThT are inversely proportional to the solvent viscosity. The Glasbeek model assumes that the excited state consists of an adiabatic potential surface constructed by adiabatic coupling of emissive and dark states. For ThT, the twist angle between the benzothiazole and the aniline is responsible for the extensive mixing of the two excited states. At a twist angle of 90°, the S(1) state assumes a charge-transfer-state character with very small oscillator strength, which causes the emission intensity to be very small as well. In the ground state, the twist angle of ThT is rather small. The photoexcitation leads first to a strongly emissive state (small twist angle). As time progresses, the twist angle increases and the oscillator strength decreases. The fit of the experimental results by the model calculations is good for times longer than 3 ps. When a two-coordinate model is invoked or a solvation spectral-shift component is added, the fit to the experimental results is good at all times.

Modeling the nonradiative decay rate of electronically excited thioflavin T.

Steady-state absorption and emission as well as time-resolved emission spectroscopies were employed to study the photophysics and photochemistry of d-luciferin, the firefly active bioluminescent co

Excited-state intermolecular proton transfer of the firefly's chromophore D-luciferin.

Optical steady-state and time-resolved spectroscopic methods were used to study the photoprotolytic reaction of oxyluciferin, the active bioluminescence chromophore of the firefly’s luciferase-cata...

Comparative study of the photoprotolytic reactions of D-luciferin and oxyluciferin.

Steady-state and time-resolved techniques were employed to study the nonradiative process of curcumin dissolved in ethanol and 1-propanol in a wide range of temperatures. We found that the nonradiative rate constants at temperatures between 175–250 K qualitatively follow the same trend as the dielectric relaxation times of both neat solvents. We attribute the nonradiative process to solvent-controlled proton transfer. We also found a kinetic isotope effect on the nonradiative process rate constant of ∼2. We propose a model in which the excited-state proton transfer breaks the planar hexagonal structure of the keto–enol center of the molecule. This, in turn, enhances the nonradiative process driven by the twist angle between the two phenol moieties.

Yuval Erez

Papers

Molecular rotors: what lies behind the high sensitivity of the thioflavin-T fluorescent marker.

Modeling the nonradiative decay rate of electronically excited thioflavin T.

Excited-state intermolecular proton transfer of the firefly's chromophore D-luciferin.

Comparative study of the photoprotolytic reactions of D-luciferin and oxyluciferin.

Temperature dependence of the fluorescence properties of curcumin.