TY  - JOUR
A1  - Maass, Friedrich
A1  - Utecht, Manuel Martin
A1  - Stremlau, Stephan
A1  - Gille, Marie
A1  - Schwarz, Jutta
A1  - Hecht, Stefan
A1  - Klamroth, Tillmann
A1  - Tegeder, Petra
T1  - Electronic structure changes during the on-surface synthesis of nitrogen-doped chevron-shaped graphene nanoribbons
JF  - Physical review : B, Condensed matter and materials physics
N2  - Utilizing suitable precursor molecules, a thermally activated and surface-assisted synthesis results in the formation of defect-free graphene nanoribbons (GNRs), which exhibit electronic properties that are not present in extended graphene. Most importantly, they have a band gap in the order of a few electron volts, depending on the nanoribbon width. In this study, we investigate the electronic structure changes during the formation of GNRs, nitrogen-doped (singly and doubly N-doped) as well as non-N-doped chevron-shaped CGNRs on Au(111). Thus we determine the optical gaps of the precursor molecules, the intermediate nonaromatic polymers, and finally the aromatic GNRs, using high-resolution electron energy loss spectroscopy and density functional theory calculations. As expected, we find no influence of N-doping on the size of the optical gaps. The gap of the precursor molecules is around 4.5 eV. Polymerization leads to a reduction of the gap to a value of 3.2 eV due to elongation and thus enhanced delocalization. The CGNRs exhibit a band gap of 2.8 eV, thus the gap is further reduced in the nanoribbons, since they exhibit an extended delocalized pi-electron system.
Y1  - 2017
U6  - https://doi.org/10.1103/PhysRevB.96.045434
SN  - 2469-9950
SN  - 2469-9969
VL  - 96
PB  - American Physical Society
CY  - College Park
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  - 
TY  - JOUR
A1  - Dokic, Jadranka
A1  - Gothe, Marcel
A1  - Wirth, Jonas
A1  - Peters, Maike V.
A1  - Schwarz, Jutta
A1  - Hecht, Stefan
A1  - Saalfrank, Peter
T1  - Quantum chemical investigation of thermal cis-to-trans isomerization of azobenzene derivatives : substituent effects, solvent effects, and comparison to experimental data
N2  - 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.
Y1  - 2009
UR  - http://pubs.acs.org/journal/jpcafh
U6  - https://doi.org/10.1021/jp9021344
SN  - 1089-5639
ER  -