@article{LuoUtechtDokicetal.2011, author = {Luo, Ying and Utecht, Manuel Martin and Dokic, Jadranka and Korchak, Sergey and Vieth, Hans-Martin and Haag, Rainer and Saalfrank, Peter}, title = {Cis-trans isomerisation of substituted aromatic imines a comparative experimental and theoretical study}, series = {ChemPhysChem : a European journal of chemical physics and physical chemistry}, volume = {12}, journal = {ChemPhysChem : a European journal of chemical physics and physical chemistry}, number = {12}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1439-4235}, doi = {10.1002/cphc.201100179}, pages = {2311 -- 2321}, year = {2011}, abstract = {The cis-trans isomerisation of N-benzylideneaniline (NBA) and derivatives containing a central C=N bond has been investigated experimentally and theoretically. Eight different NBA molecules in three different solvents were irradiated to enforce a photochemical trans (hv) -> cis isomerisation and the kinetics of the thermal backreaction cis (Delta)-> trans were determined by NMR spectroscopy measurements in the temperature range between 193 and 288 K. Theoretical calculations using density functional theory and Eyring transition-state theory were carried out for 12 different NBA species in the gas phase and three different solvents to compute thermal isomerisation rates of the thermal back reaction. While the computed absolute rates are too large, they reveal and explain experimental trends. Time-dependent density functional theory provides optical spectra for vertical transitions and excitation energy differences between trans and cis forms. Together with isomerisation rates, the latter can be used to identify "optimal switches" with good photochromicity and reasonable thermal stability.}, language = {en} } @article{UtechtKlamrothSaalfrank2011, author = {Utecht, Manuel Martin and Klamroth, Tillmann and Saalfrank, Peter}, title = {Optical absorption and excitonic coupling in azobenzenes forming self-assembled monolayers a study based on density functional theory}, series = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, volume = {13}, journal = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, number = {48}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1463-9076}, doi = {10.1039/c1cp22793a}, pages = {21608 -- 21614}, year = {2011}, abstract = {Based on the analysis of optical absorption spectra, it has recently been speculated that the excitonic coupling between individual azobenzene-functionalized alkanethiols arranged in a self-assembled monolayer (SAM) on a gold surface could be strong enough to hinder collective trans-cis isomerization-on top of steric hindrance [Gahl et al., J. Am. Chem. Soc., 2010, 132, 1831]. Using models of SAMs of increasing complexity (dimer, linear N-mers, and two-dimensionally arranged N-mers) and density functional theory on the (TD-) B3LYP/6-31G* level, we determine optical absorption spectra, the nature and magnitude of excitonic couplings, and the corresponding spectral shifts. It is found that at inter-monomer distances of about 20 angstrom and above, TD-B3LYP excitation frequencies (and signal intensities) can be well described by the frequently used point-dipole approximation. Further, calculated blue shifts in optical absorption spectra account for the experimental observations made for azobenzene/gold SAMs, and hint to the fact that they can indeed be responsible for reduced switching probability in densely packed self-assembled structures.}, language = {en} } @article{BronnerLeyssnerStremlauetal.2012, author = {Bronner, C. and Leyssner, F. and Stremlau, S. and Utecht, Manuel Martin and Saalfrank, Peter and Klamroth, Tillmann and Tegeder, P.}, title = {Electronic structure of a subnanometer wide bottom-up fabricated graphene nanoribbon: End states, band gap, and dispersion}, series = {Physical review : B, Condensed matter and materials physics}, volume = {86}, journal = {Physical review : B, Condensed matter and materials physics}, number = {8}, publisher = {American Physical Society}, address = {College Park}, issn = {1098-0121}, doi = {10.1103/PhysRevB.86.085444}, pages = {5}, year = {2012}, abstract = {Angle-resolved two-photon photoemission and high-resolution electron energy loss spectroscopy are employed to derive the electronic structure of a subnanometer atomically precise quasi-one-dimensional graphene nanoribbon (GNR) on Au(111). We resolved occupied and unoccupied electronic bands including their dispersion and determined the band gap, which possesses an unexpectedly large value of 5.1 eV. Supported by density functional theory calculations for the idealized infinite polymer and finite size oligomers, an unoccupied nondispersive electronic state with an energetic position in the middle of the band gap of the GNR could be identified. This state resides at both ends of the ribbon (end state) and is only found in the finite sized systems, i.e., the oligomers.}, 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{UtechtPanKlamrothetal.2014, author = {Utecht, Manuel Martin and Pan, Tianluo and Klamroth, Tillmann and Palmer, Richard E.}, title = {Quantum chemical cluster models for chemi- and physisorption of chlorobenzene on Si(111)-7x7}, series = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, volume = {118}, journal = {The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment \& general theory}, number = {33}, publisher = {American Chemical Society}, address = {Washington}, issn = {1089-5639}, doi = {10.1021/jp504208d}, pages = {6699 -- 6704}, year = {2014}, abstract = {Motivated by recent atomic manipulation experiments, we report quantum chemical calculations for chemi- and physisorption minima of chlorobenzene on the Si(111)-7x7 surface. A density functional theory cluster approach is applied, using the B3LYP hybrid functional alongside Grimme's empirical dispersion corrections (D3). We were able to identify chemisorption sites of binding energies of 1.6 eV and physisorption energies of 0.6 eV, both in encouraging agreement with the trend of experimental data. The cluster approach opens up the possibility of a first-principles based dynamical description of STM manipulation experiments on this system, the interpretation of which involves both the chemi- and physisorbed states. However, we found that special care has to be taken regarding the choice of clusters, basis sets, and the evaluation of the dispersion corrections.}, language = {en} } @article{BronnerUtechtHaaseetal.2014, author = {Bronner, Christopher and Utecht, Manuel Martin and Haase, Anton and Saalfrank, Peter and Klamroth, Tillmann and Tegeder, Petra}, title = {Electronic structure changes during the surface-assisted formation of a graphene nanoribbon}, series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, volume = {140}, journal = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, number = {2}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-9606}, doi = {10.1063/1.4858855}, pages = {7}, year = {2014}, abstract = {High conductivity and a tunability of the band gap make quasi-one-dimensional graphene nanoribbons (GNRs) highly interesting materials for the use in field effect transistors. Especially bottom-up fabricated GNRs possess well-defined edges which is important for the electronic structure and accordingly the band gap. In this study we investigate the formation of a sub-nanometer wide armchair GNR generated on a Au(111) surface. The on-surface synthesis is thermally activated and involves an intermediate non-aromatic polymer in which the molecular precursor forms polyanthrylene chains. Employing angle-resolved two-photon photoemission in combination with density functional theory calculations we find that the polymer exhibits two dispersing states which we attribute to the valence and the conduction band, respectively. While the band gap of the non-aromatic polymer obtained in this way is relatively large, namely 5.25 +/- 0.06 eV, the gap of the corresponding aromatic GNR is strongly reduced which we attribute to the different degree of electron delocalization in the two systems.}, 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{SchulzeUtechtMoldtetal.2015, author = {Schulze, Michael and Utecht, Manuel Martin and Moldt, Thomas and Przyrembel, Daniel and Gahl, Cornelius and Weinelt, Martin and Saalfrank, Peter and Tegeder, Petra}, title = {Nonlinear optical response of photochromic azobenzene-functionalized self-assembled monolayers}, series = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, volume = {17}, journal = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, number = {27}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1463-9076}, doi = {10.1039/c5cp03093e}, pages = {18079 -- 18086}, year = {2015}, abstract = {The combination of photochromic and nonlinear optical (NLO) properties of azobenzene-functionalized self-assembled monolayers (SAMs) constitutes an intriguing step towards novel photonic and optoelectronic devices. By utilizing the second-order NLO process of second harmonic generation (SHG), supported by density-functional theory and correlated wave function method calculations, we demonstrate that the photochromic interface provides the necessary prerequisites en route towards possible future technical applications: we find a high NLO contrast on the order of 16\% between the switching states. These are furthermore accessible reversibly and with high efficiencies in terms of cross sections on the order of 10(-18) cm(2) for both photoisomerization reactions, i.e., drivable by means of low-power LED light sources. Finally, both photostationary states (PSSs) are thermally stable at ambient conditions.}, language = {en} } @article{SchulzeUtechtHebertetal.2015, author = {Schulze, Michael and Utecht, Manuel Martin and Hebert, Andreas and R{\"u}ck-Braun, Karola and Saalfrank, Peter and Tegeder, Petra}, title = {Reversible Photoswitching of the Interfacial Nonlinear Optical Response}, series = {The journal of physical chemistry letters}, volume = {6}, journal = {The journal of physical chemistry letters}, number = {3}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/jz502477m}, pages = {505 -- 509}, year = {2015}, abstract = {Incorporating photochromic molecules into organic/inorganic hybrid materials may lead to photoresponsive systems. In such systems, the second-order nonlinear properties can be controlled via external stimulation with light at an appropriate wavelength. By creating photochromic molecular switches containing self-assembled monolayers on Si(111), we can demonstrate efficient reversible switching, which is accompanied by a pronounced modulation of the nonlinear optical (NLO) response of the system. The concept of utilizing functionalized photoswitchable Si surfaces could be a way for the generation of two-dimensional NLO switching materials, which are promising for applications in photonic and optoelectronic devices.}, language = {en} } @article{SchulzeUtechtMoldtetal.2015, author = {Schulze, Michael and Utecht, Manuel Martin and Moldt, Thomas and Przyrembel, Daniel and Gahl, Cornelius and Weinelt, Martin and Saalfrank, Peter and Tegeder, Petra}, title = {Nonlinear optical response of photochromic azobenzene-functionalized self-assembled monolayers}, series = {Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies}, volume = {27}, journal = {Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies}, number = {17}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1463-9076}, doi = {10.1039/c5cp03093e}, pages = {18079 -- 18086}, year = {2015}, abstract = {The combination of photochromic and nonlinear optical (NLO) properties of azobenzene-functionalized self-assembled monolayers (SAMs) constitutes an intriguing step towards novel photonic and optoelectronic devices. By utilizing the second-order NLO process of second harmonic generation (SHG), supported by density-functional theory and correlated wave function method calculations, we demonstrate that the photochromic interface provides the necessary prerequisites en route towards possible future technical applications: we find a high NLO contrast on the order of 16\% between the switching states. These are furthermore accessible reversibly and with high efficiencies in terms of cross sections on the order of 10-18 cm2 for both photoisomerization reactions, i.e., drivable by means of low-power LED light sources. Finally, both photostationary states (PSSs) are thermally stable at ambient conditions.}, language = {en} }