Introduction of Substituents on the 2-Oxo-piperazine Skeleton by [3+2] Cycloaddition and Subsequent Transformation

The 3,4-substituted 2-oxo-piperazines 5 - 9 are obtained by [3+2] cycloaddition from nitrone 1 and a variety of alkenes. Subsequent functionalization of the bicyclic adducts involves reductive N-O bond cleavage. A route towards libraries of immobilized 1,3-aminoalcohols with a 3,4-substituted 2-oxo-piperazine scaffold is briefly discussed for adducts derived from N-substituted maleic imide

Discrimination between FRET and non-FRET quenching in a photochromic CdSe quantum dot/dithienylethene dye system

A photochromic Förster resonance energy transfer (FRET) system was employed to disentangle the fluorescence quenching mechanisms in quantum dot/photochromic dye hybrids. In the off-state of the dye the main quenching mechanism is FRET whereas the moderate quenching in the on-state is due to non-FRET pathways opened up upon assembly

Ultrafast coherent oscillations reveal a reactive mode in the ring-opening reaction of fulgides

The ultrafast ring-opening reaction of photochromic fulgides proceeds via conical intersections to the ground state isomers involving activation barriers in the excited state. The coherent oscillations observed in the femtosecond transient absorption signal of a methyl-substituted indolylfulgide were analysed in the framework of vibrational wavepackets to expose a dominant low-frequency mode at similar to 80 cm(-1). The quantum chemical calculations in the relaxed excited state geometry of this fulgide revealed that the experimentally observed vibrational normal mode has a dominant contribution to the relevant ring-opening reactive coordinate

Chemical control of Hemithioindigo-photoisomerization – Substituent-effects on different molecular parts

Hemithioindigo-molecules show a photochromic behavior due to a photo-induced Z/E-isomerization on a picosecond timescale. Changes in the reaction kinetics, caused by polar substituents attached to the thioindigo-moiety, are studied using time-resolved absorption spectroscopy. The experiments reveal that substituents in the thioindigo-part influence the reaction dynamics in a different manner as previously found for stilbene-substitution. The results are discussed in the context of inductive- and resonance-effects and lead to an improved understanding of the molecular mechanisms of the Hemithioindigo-photoreaction

A New Class of Ultrafast Photoswitchable Chromopeptides

The photochemical properties of a new ultrafast photoswitch based on hemithioindigo derivatives are investigated by transient absorption spectroscopy. The applicability as a trigger molecule for fast structural changes in small peptides is tested on a biologically relevant structure.

The Hammett Relationship and Reactions in the Excited Electronic State: Hemithioindigo Z/E-Photoisomerization

The photochemical reaction dynamics of a set of photochromic compounds based on thioindigo and stilbene molecular parts (hemithioindigos, HTI) are presented. Photochemical Z/E isomerization around the central double bond occurs with time constants of 216 ps (Z → E) and 10 ps (E → Z) for a 5-methyl-hemithioindigo. Chemical substitution on the stilbene moiety causes unusually strong changes in the reaction rate. Electron-donating substituents in the position para to the central double bond (e.g., para-methoxy) strongly accelerate the reaction, while the reaction is drastically slowed by electron-withdrawing groups in this position (e.g., para-nitrile). We correlate the experimental data of seven HTI-compounds in a quantitative manner using the Hammett equation and present a qualitative explanation for the application of ground-state Hammett constants to describe the photoisomerization reaction.

Quantitative IR Readout of Fulgimide Monolayer Switching on Si(111) Surfaces

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