TY - JOUR A1 - Goulet-Hanssens, Alexis A1 - Utecht, Manuel A1 - Mutruc, Dragos A1 - Titov, Evgenii A1 - Schwarz, Jutta A1 - Grubert, Lutz A1 - Bleger, David A1 - Saalfrank, Peter A1 - Hecht, Stefan T1 - Electrocatalytic Z -> E Isomerization of Azobenzenes JF - Journal of the American Chemical Society N2 - A variety of azobenzenes were synthesized to study the behavior of their E and Z isomers upon electrochemical reduction. Our results show that the radical anion of the Z isomer is able to rapidly isomerize to the corresponding E configured counterpart with a dramatically enhanced rate as compared to the neutral species. Due to a subsequent electron transfer from the formed E radical anion to the neutral Z starting material the overall transformation is catalytic in electrons; i.e., a substoichiometric amount of reduced species can isomerize the entire mixture. This pathway greatly increases the efficiency of (photo)switching while also allowing one to reach photostationary state compositions that are not restricted to the spectral separation of the individual azobenzene isomers and their quantum yields. In addition, activating this radical isomerization pathway with photoelectron transfer agents allows us to override the intrinsic properties of an azobenzene species by triggering the reverse isomerization direction (Z -> E) by the same wavelength of light, which normally triggers E -> Z isomerization. The behavior we report appears to be general, implying that the metastable isomer of a photoswitch can be isomerized to the more stable one catalytically upon reduction, permitting the optimization of azobenzene switching in new as well as indirect ways. Y1 - 2017 U6 - https://doi.org/10.1021/jacs.6b10822 SN - 0002-7863 VL - 139 IS - 1 SP - 335 EP - 341 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Bleger, David A1 - Dokic, Jadranka A1 - Peters, Maike V. A1 - Grubert, Lutz A1 - Saalfrank, Peter A1 - Hecht, Stefan T1 - Electronic decoupling approach to quantitative photoswitching in linear multiazobenzene architectures JF - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry N2 - A strategy to optimize the photoswitching efficiency of rigid, linear multiazobenzene constructs is presented. It consists of introducing large dihedral angles between azobenzene moieties linked via aryl-aryl connections in their para positions. Four bisazobenzenes exhibiting different dihedral angles as well as three single azobenzene reference compounds have been synthesized, and their switching behavior has been studied as well as experimentally and theoretically analyzed. As the dihedral angle between the two azobenzene units increases and consequently the electronic conjugation decreases, the photochromic characteristics improve, finally leading to individual azobenzene switches operating independently in the case of the perpendicular ortho,ortho,ortho',ortho'-tetramethyl biphenyl linker. The electronic decoupling leads to efficient separation of the absorption spectra of the involved switching states and hence by choosing the appropriate irradiation wavelength, an almost quantitative E -> Z photoisomerization up to 97% overall Z-content can be achieved. In addition, thermal Z -> E isomerization processes become independent of each other with increasing decoupling. The electronic decoupling could furthermore be proven by electrochemistry. The experimental data are supported by theory, and calculations additionally provide mechanistic insight into the preferred pathway for the thermal Z,Z -> Z,E -> E,E isomerization via inversion on the inner N-atoms. Our decoupling approach outlined herein provides the basis for constructing rigid rod architectures composed of multiple azobenzene photochromes, which display practically quantitative photoswitching properties, a necessary prerequisite to achieve highly efficient transduction of light energy directly into motion. Y1 - 2011 U6 - https://doi.org/10.1021/jp2044114 SN - 1520-6106 VL - 115 IS - 33 SP - 9930 EP - 9940 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Goulet-Hanssens, Alexis A1 - Rietze, Clemens A1 - Titov, Evgenii A1 - Abdullahu, Leonora A1 - Grubert, Lutz A1 - Saalfrank, Peter A1 - Hecht, Stefan T1 - Hole Catalysis as a General Mechanism for Efficient and Wavelength-Independent Z -> E Azobenzene Isomerization JF - CHEM N2 - Whereas the reversible reduction of azobenzenes has been known for decades, their oxidation is destructive and as a result has been notoriously overlooked. Here, we show that a chain reaction leading to quantitative Z -> E isomerization can be initiated before reaching the destructive anodic peak potential. This hole-catalyzed pathway is accessible to all azobenzenes, without exception, and offers tremendous advantages over the recently reported reductive, radical-anionic pathway because it allows for convenient chemical initiation without the need for electrochemical setups and in the presence of air. In addition, catalytic amounts of metal-free sensitizers, such as methylene blue, can be used as excited-state electron acceptors, enabling a shift of the excitation wavelength to the far red of the azobenzene absorption (up to 660 nm) and providing quantum yields exceeding unity (up to 200%). Our approach will boost the efficiency and sensitivity of optically dense liquid-crystalline and solid photo-switchable materials. Y1 - 2018 U6 - https://doi.org/10.1016/j.chempr.2018.06.002 SN - 2451-9294 VL - 4 IS - 7 SP - 1740 EP - 1755 PB - Cell Press CY - Cambridge ER - TY - JOUR A1 - Herder, Martin A1 - Utecht, Manuel Martin A1 - Manicke, Nicole A1 - Grubert, Lutz A1 - Pätzel, Michael A1 - Saalfrank, Peter A1 - Hecht, Stefan T1 - Switching with orthogonal stimuli electrochemical ring-closure and photochemical ring-opening of bis(thiazolyl) maleimides JF - Chemical science N2 - The photochemistry as well as electrochemistry of novel donor-acceptor bis(morpholinothiazolyl)maleimides has been investigated. Proper substitution of these diarylethene-type molecular switches leads to the unique situation in which their ring-closure can only be accomplished electrochemically, while ring-opening can only be achieved photochemically. Hence, these switches operate with orthogonal stimuli, i.e. redox potential and light, respectively. The switch system could be optimized by introducing trifluoromethyl groups at the reactive carbon atoms in order to avoid by-product formation during oxidative ring closure. Both photochemical and electrochemical pathways were investigated for methylated, trifluoromethylated, and nonsymmetrical bis(morpholinothiazolyl) maleimides as well as the bis(morpholinothiazolyl) cyclopentene reference compound. With the aid of the nonsymmetrical "mixed" derivative, the mechanism of electrochemically driven ring closure could be elucidated and seems to proceed via a dicationic intermediate generated by two-fold oxidation. All experimental work has been complemented by density functional theory that provides detailed insights into the thermodynamics of the ring-open and closed forms, the nature of their excited states, and the reactivity of their neutral as well as ionized species in different electronic configurations. The particular diarylethene systems described herein could serve in multifunctional (logic) devices operated by different stimuli (inputs) and may pave the way to converting light into electrical energy via photoinduced "pumping" of redox-active meta-stable states. Y1 - 2013 U6 - https://doi.org/10.1039/c2sc21681g SN - 2041-6520 VL - 4 IS - 3 SP - 1028 EP - 1040 PB - Royal Society of Chemistry CY - Cambridge ER -