The excited-state dynamics of the aminoazobenzene derivative, Metanil Yellow (MY), were studied by ultrafast Transient Absorption (TA) spectroscopy and state-of-the-art XUV tTme-Resolved Photoelectron Spectroscopy (TRPES). Experiments were carried out with two different excitation wavelengths, λ=370 nm and λ=490 nm, to investigate the non-hydrated and hydrated forms of the molecule and reveal differences in their dynamics. The dynamics were also studied in two solvents, water and ethanol, to investigate the effect of hydrogen bonding with the solvent. In TRPES experiments the dynamics were studied in water solution, using a λ=400 nm pump, thus exciting both forms. The timescales from the TRPES experiments are in good agreement with the results from the TAS measurements. Based on quantum chemical calculations the dynamics are tentatively assigned to the S2→S1 conversion followed by relaxation to a long-lived state, the nature of which (possibly a twisted intramolecular charge transfer – TICT – state) remains to be confirmed.
We use femtosecond UV-Vis absorption spectroscopy to investigate the photoreaction mechanism of a recently synthesized oxindole-based molecular switch showing a large C=C double bond photoisiomerization quantum yield (50%), and promising applications e.g. in photopharmacology. Due to an electron-donating phenol moeity, the molecular switch exhibits a push-pull electronic effect which affects its photophysical properties. In solvents of various polarities and hydrogen bonding capabilities, we observe a faster (sub-ps) photoisomerization dynamics of the deprotonated phenolate form of the compound, where the push-pull effect is enhanced. This work aims at unraveling the synthetic design strategies towards optimizing the photoreaction dynamics and quantum yiled of such molecular switches.
Ultrafast photoinduced isomerization is a fundamental process governing molecular dynamics both in biologically relevant chromophores and in functional materials, e.g. those based on molecular switches and motors. It is widely accepted that the efficiency of isomerization is governed by the dynamics through conical intersections, the regions of the potential energy landscape, where two potential energy surfaces cross. Recent developments in computational chemistry can help identifying conical intersections in isolated chromophores molecules and describe the relaxation dynamics. However, the complex environments hosting the chromophores have a profound influence on the dynamics through conical intersection making direct application of these methods to “real life” problems a very challenging task and underscoring the importance of experimental investigations.
Most all-optical time-resolved spectroscopic techniques cannot directly capture the dynamics at conical intersections both because it is very fast and because the gap between the two electronic states vanishes at the intersection. However, the XUV-based spectroscopic techniques and, in particular, XUV time-resolved photoelectron spectroscopy (TRPES) give promise in delivering detectable signals from the regions of conical intersections. TRPES of molecular chromophores requires application of photoemission methods to the liquid phase samples (molecular solutions). Our group has recently become the first to demonstrate liquid phase TRPES of organic molecules by combining an ultrafast tunable XUV source with a microliquid jet sample delivery method and time-of-flight photoelectron detection.
In this contribution we will report on the recent results applying this method to the prototypical molecules, Methyl Orange and Metanil Yellow. The experimental results are complemented by high-level time-dependent density functional theory (TDDFT) surface hopping calculations to reveal electronic state involved in ultrafast relaxation of the molecules. We will further show preliminary results for several bio-mimetic chromophores and will discuss the experimental challenges of the techniques, when working with samples of low concentration and using different solvents.
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