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  - 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  -