Ultrafast excited state reaction dynamics in light-driven unidirectional rotary molecular motors and fluorescent protein chromophores

Excited state dynamics on an ultrafast timescale can provide insight into primary events in photochemical and photobiological processes. In this work, excited state dynamics of two important systems are characterized: unidirectional molecular rotary motors and HBDI derivatives (synthetic chromophores of the green fluorescent protein (GFP)). In both cases, the excited state is selectively probed by ultrafast fluorescence up-conversion with a time resolution better than 50 fs. Molecular motors have biphasic (sub-picosecond and picosecond) fluorescence decays and oscillations attributed to excitation of coherently excited vibrational modes. The fluorescence data were contrasted with excited state decay and ground state recovery kinetics recorded using ultrafast transient absorption. Combining these experimental data with substituent dependence and solvent dependence studies, as well as existing calculations, we proposed a coupled two-state model for dynamics on the excited state potential energy surface. These data have implications for the design and optimisation of optically driven molecular motors. A ‘molecular propeller’ was also studied and shown to be more sensitive to medium friction than the motor. The GFP experiments focused on determining the effect of alkyl substitution upon excited state dynamics of HBDI. HBDI in solution exhibits a very low quantum yield compared to the chromophore in its protein environment. Large alkyl substituents were found to shift the spectra but to exhibit only small retardation effects upon the excited state decay time, even in highly viscous solvents. This supports an assignment of a volume conserving isomerization mechanism promoting radiationless decay. Substituents which distort the planar structure of the chromophore lead to an enhanced radiationless decay. This provides further evidence for a link between radiationless decay of the excited state and twisting of HBDI

Ultrafast dynamics in light-driven molecular rotary motors probed by femtosecond stimulated raman spectroscopy

Photochemical isomerization in sterically crowded chiral alkenes is the driving force for molecular rotary motors in nanoscale machines. Here the excited state dynamics and structural evolution of the prototypical light driven rotary motor are followed on the ultrafast timescale by femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption (TA). TA reveals a sub 100 fs blue shift and decay of the Franck-Condon bright state arising from relaxation along the reactive potential energy surface. The decay is accompanied by coherently excited vibrational dynamics which survive the excited state structural evolution. The ultrafast Franck-Condon bright state relaxation is to a dark excited state, which FSRS reveals to have a rich spectrum compared to the electronic ground state, with the most intense Raman active modes shifted to significantly lower wavenumber. This is discussed in terms of a reduced bond order of the central bridging bond and overall weakening of bonds in the dark state, which is supported by electronic structure calculations. The observed evolution in the FSRS spectrum is assigned to vibrational cooling accompanied by partitioning of the dark state between the product isomer and the original ground state. Formation of the product isomer is observed in real time by FSRS. It is formed vibrationally hot and cools over several picoseconds, completing the characterization of the light driven half of the photocycle

PRACTICE OF CAD AND CAE DESIGN IN THE FIELD OF PLASMA TECHNOLOGIES

The effectiveness of automated plasma torch design methods can be improved by integrating design and engineering analysis technologies. The features of CAD and CAE technologies for designing plasma torches are considered. Shows examples of the design of plasma torches for cutting metals and waste treatment with the use of digital technologies.Эффективность автоматизированных методов проектирования плазмотронов можно повысить за счет интеграции технологий проектирования и инженерного анализа. Рассмотрены особенности CAD и CAE технологий проектирования плазмотронов. Показаны примеры проектирования плазмотронов для резки металлов и обезвреживания отходов с применением цифровых технологий

Reactive Dynamics in Confined Liquids: Interfacial Charge Effects on Ultrafast Torsional Dynamics in Water Nanodroplets

The excited-state dynamics of a reactive dye molecule, auramine O, have been studied in nanoscale water droplets stabilized by a nonionic surfactant. Spectral dynamics were measured as a function of the radius of the water nanodroplet with 50 fs time resolution using time-resolved fluorescence up-conversion method. Qualitatively, the effect of confinement is to dramatically slow the rate of the reaction compared to that of bulk water. Data were quantitatively analyzed using the one-dimensional generalized Smoluchowski equation assuming a time-dependent diffusion coefficient. The results were contrasted with our earlier analysis of auramine O in aqueous nanodroplets stabilized by the ionic surfactant AOT. The excited-state reaction is slower in the nonionic surfactant, showing that interfacial charge is not required to suppress reactions in nanoscale water droplets. The location of the dye in the heterogeneous micelle is investigated by comparing the absorption spectra of AO in the micelle with those of a water- polyethyleneglycol mixture (to mimic the surfactant head group). The results suggest that the charged dye is located in the water phase

Chemically Optimising Operational Efficiency of Molecular Rotary Motors

Unidirectional molecular rotary motors that harness photoinduced cis-trans (E-Z) isomerization are promising tools for the conversion of light energy to mechanical motion in nanoscale molecular machines. Considerable progress has been made in optimizing the frequency of ground-state rotation, but less attention has been focused on excited state processes. Here the excited state dynamics of a molecular motor with electron donor and acceptor substituents located to modify the excited-state reaction coordinate, without altering its stereochemistry, are studied. The substituents are shown to modify the photochemical yield of the isomerization without altering the motor frequency. By combining 50 fs resolution time-resolved fluorescence with ultrafast transient absorption spectroscopy the underlying excited-state dynamics are characterized. The Franck-Condon excited state relaxes in a few hundred femtoseconds to populate a lower energy dark state by a pathway that utilizes a volume conserving structural change. This is assigned to pyramidalization at a carbon atom of the isomerizing bridging double bond. The structure and energy of the dark state thus reached are a function of the substituent, with electron-withdrawing groups yielding a lower energy longer lived dark state. The dark state is coupled to the Franck Condon state and decays on a picosecond time scale via a coordinate that is sensitive to solvent friction, such as rotation about the bridging bond. Neither subpicosecond nor picosecond dynamics are sensitive to solvent polarity, suggesting that intramolecular charge transfer and solvation are not key driving forces for the rate of the reaction. Instead steric factors and medium friction determine the reaction pathway, with the sterically remote substitution primarily influencing the energetics. Thus, these data indicate a chemical method of optimizing the efficiency of operation of these molecular motors without modifying their overall rotational frequency

Ultrafast photophysics of the environment-sensitive 4′-methoxy-3-hydroxyflavone fluorescent dye

The ESIPT reaction speed of 4′-methoxy-3-hydroxyflavone varies by 3 orders of magnitude depending on the H-bonding capabilities of its environment.</p

Ultrafast ignition of a uni-directional molecular motor

Light-driven molecular motors convert light into mechanical energy via excited state reactions. In this work we follow sub-picosecond primary events in the cycle of a two-stroke unidirectional motor by fluorescence up-conversion and transient absorption.

Ultrafast isomerization dynamics of a unidirectional molecular rotor revealed by femtosecond stimulated raman spectroscopy (FSRS)

Unidirectional molecular rotors based on chiral overcrowded alkenes operate via sequential photochemical- and thermal-activated steps. Over the last decade the rotation rate limiting thermal step has been optimized through modification of the molecular structure. In recent years we have shown the photochemical step proceeds on an ultrafast timescale via a barrierless isomerization reaction. Here we reveal for the first time the excited state vibrational structure and associated ultrafast dynamics for a unidirectional molecular rotor, providing insight into the structural and electronic evolution involved in the rotary motion