@article{GouletHanssensUtechtMutrucetal.2017, author = {Goulet-Hanssens, Alexis and Utecht, Manuel and Mutruc, Dragos and Titov, Evgenii and Schwarz, Jutta and Grubert, Lutz and Bleger, David and Saalfrank, Peter and Hecht, Stefan}, title = {Electrocatalytic Z -> E Isomerization of Azobenzenes}, series = {Journal of the American Chemical Society}, volume = {139}, journal = {Journal of the American Chemical Society}, number = {1}, publisher = {American Chemical Society}, address = {Washington}, issn = {0002-7863}, doi = {10.1021/jacs.6b10822}, pages = {335 -- 341}, year = {2017}, abstract = {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.}, language = {en} } @article{GouletHanssensRietzeTitovetal.2018, author = {Goulet-Hanssens, Alexis and Rietze, Clemens and Titov, Evgenii and Abdullahu, Leonora and Grubert, Lutz and Saalfrank, Peter and Hecht, Stefan}, title = {Hole Catalysis as a General Mechanism for Efficient and Wavelength-Independent Z -> E Azobenzene Isomerization}, series = {CHEM}, volume = {4}, journal = {CHEM}, number = {7}, publisher = {Cell Press}, address = {Cambridge}, issn = {2451-9294}, doi = {10.1016/j.chempr.2018.06.002}, pages = {1740 -- 1755}, year = {2018}, abstract = {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.}, language = {en} } @article{KnieUtechtZhaoetal.2014, author = {Knie, Christopher and Utecht, Manuel Martin and Zhao, Fangli and Kulla, Hannes and Kovalenko, Sergey and Brouwer, Albert M. and Saalfrank, Peter and Hecht, Stefan and Bleger, David}, title = {ortho-Fluoroazobenzenes: visible light switches with very long-lived Z isomers}, series = {Chemistry - a European journal}, volume = {20}, journal = {Chemistry - a European journal}, number = {50}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0947-6539}, doi = {10.1002/chem.201404649}, pages = {16492 -- 16501}, year = {2014}, abstract = {Improving the photochemical properties of molecular photoswitches is crucial for the development of light-responsive systems in materials and life sciences. ortho-Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing sigma-electron-withdrawing F atoms ortho to the N=N unit leads to both an effective separation of the n -> pi* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z iso-merizations, and greatly enhanced thermal stability of the Z isomers. Additional para-electron-withdrawing groups (EWGs) work in concert with ortho-F atoms, giving rise to enhanced separation of the n -> pi* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho-fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.}, language = {en} } @article{BlegerDokicPetersetal.2011, author = {Bleger, David and Dokic, Jadranka and Peters, Maike V. and Grubert, Lutz and Saalfrank, Peter and Hecht, Stefan}, title = {Electronic decoupling approach to quantitative photoswitching in linear multiazobenzene architectures}, series = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, volume = {115}, journal = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, number = {33}, publisher = {American Chemical Society}, address = {Washington}, issn = {1520-6106}, doi = {10.1021/jp2044114}, pages = {9930 -- 9940}, year = {2011}, abstract = {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.}, language = {en} } @article{HerderUtechtManickeetal.2013, author = {Herder, Martin and Utecht, Manuel Martin and Manicke, Nicole and Grubert, Lutz and P{\"a}tzel, Michael and Saalfrank, Peter and Hecht, Stefan}, title = {Switching with orthogonal stimuli electrochemical ring-closure and photochemical ring-opening of bis(thiazolyl) maleimides}, series = {Chemical science}, volume = {4}, journal = {Chemical science}, number = {3}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {2041-6520}, doi = {10.1039/c2sc21681g}, pages = {1028 -- 1040}, year = {2013}, abstract = {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.}, language = {en} } @article{DokicGotheWirthetal.2009, author = {Dokic, Jadranka and Gothe, Marcel and Wirth, Jonas and Peters, Maike V. and Schwarz, Jutta and Hecht, Stefan and Saalfrank, Peter}, title = {Quantum chemical investigation of thermal cis-to-trans isomerization of azobenzene derivatives : substituent effects, solvent effects, and comparison to experimental data}, issn = {1089-5639}, doi = {10.1021/jp9021344}, year = {2009}, abstract = {Quantum chemical calculations of various azobenzene (AB) derivatives have been carried out with the goal to describe the energetics and kinetics of their thermal cis -> trans isomerization. The effects of substituents, in particular their type, number, and positioning, on activation energies have been systematically studied with the ultimate goal to tailor the switching process. Trends observed for mono- and disubstituted species are discussed. A polarizable continuum model is used to study, in an approximate fashion, the cis -> trans isomerization of azobenzenes in solution. The nature of the transition state(s) and its dependence on substituents and the environment is discussed. In particular for push-pull azobenzenes, the reaction mechanism is found to change from inversion in nonpolar solvents to rotation in polar solvents. Concerning kinetics, calculations based on the Eyring transition state theory give usually reliable activation energies and enthalpies when compared to experimentally determined values. Also, trends in the resulting rate constants are correct. Other computed properties such as activation entropies and thus preexponential rate factors are in only moderate agreement with experiment.}, language = {en} } @article{SchmidtHagenBreteetal.2010, author = {Schmidt, Roland and Hagen, Sebastian and Brete, Daniel and Carley, Robert and Gahl, Cornelius and Dokic, Jadranka and Saalfrank, Peter and Hecht, Stefan and Tegeder, Petra and Weinelt, Martin}, title = {On the electronic and geometrical structure of the trans- and cis-isomer of tetra-tert-butyl-azobenzene on Au(111)}, issn = {1463-9076}, doi = {10.1039/B924409c}, year = {2010}, abstract = {Near edge X-ray absorption. ne structure and X-ray photoelectron spectroscopy have been employed to follow the reversible trans to cis isomerization of tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). For one monolayer the molecules adopt an adsorption geometry characteristic of the trans-TBA isomer. The azo-bridge (N = N) is aligned nearly parallel to the surface and the phenyl rings exhibit a planar orientation with a small tilt angle <= 4 degrees with respect to the surface normal. Illumination of the molecular layer at 455 nm triggers the trans to cis isomerization which is associated with a pronounced change of the geometrical and electronic structure. The N1s to pi* transition of the central azo-bridge shifts by 0.45 +/- 0.05 eV to higher photon energy and the transition dipole moment (TDM) is tilted by 59 +/- 5 degrees with respect to the surface normal. The pi-system of one phenyl ring is tilted by about 30 degrees with respect to the surface normal, while the second ring plane is oriented nearly perpendicular to the surface. This reorientation is supported by a shift and broadening of the C-H resonances associated with the tert-butyl legs of the molecule. These findings support a configuration of the photo-switched TBA molecule on Au(111) which is comparable to the cis-isomer of the free molecule. In the photo-stationary state 53 +/- 5\% of the TBA molecules are switched to the cis configuration. Thermal activation induces the back reaction to trans-TBA.}, language = {en} }