@article{Titov2021, author = {Titov, Evgenii}, title = {On the low-lying electronically excited states of azobenzene dimers}, series = {Molecules : a journal of synthetic chemistry and natural product chemistry / Molecular Diversity Preservation International}, volume = {26}, journal = {Molecules : a journal of synthetic chemistry and natural product chemistry / Molecular Diversity Preservation International}, number = {14}, publisher = {MDPI}, address = {Basel}, issn = {1420-3049}, doi = {10.3390/molecules26144245}, pages = {24}, year = {2021}, abstract = {Azobenzene-containing molecules may associate with each other in systems such as self-assembled monolayers or micelles. The interaction between azobenzene units leads to a formation of exciton states in these molecular assemblies. Apart from local excitations of monomers, the electronic transitions to the exciton states may involve charge transfer excitations. Here, we perform quantum chemical calculations and apply transition density matrix analysis to quantify local and charge transfer contributions to the lowest electronic transitions in azobenzene dimers of various arrangements. We find that the transitions to the lowest exciton states of the considered dimers are dominated by local excitations, but charge transfer contributions become sizable for some of the lowest pi pi* electronic transitions in stacked and slip-stacked dimers at short intermolecular distances. In addition, we assess different ways to partition the transition density matrix between fragments. In particular, we find that the inclusion of the atomic orbital overlap has a pronounced effect on quantifying charge transfer contributions if a large basis set is used.}, language = {en} } @article{TitovSharmaLomadzeetal.2021, author = {Titov, Evgenii and Sharma, Anjali and Lomadze, Nino and Saalfrank, Peter and Santer, Svetlana and Bekir, Marek}, title = {Photoisomerization of an azobenzene-containing surfactant within a micelle}, series = {ChemPhotoChem}, volume = {5}, journal = {ChemPhotoChem}, number = {10}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {2367-0932}, doi = {10.1002/cptc.202100103}, pages = {926 -- 932}, year = {2021}, abstract = {Photosensitive azobenzene-containing surfactants have attracted great attention in past years because they offer a means to control soft-matter transformations with light. At concentrations higher than the critical micelle concentration (CMC), the surfactant molecules aggregate and form micelles, which leads to a slowdown of the photoinduced trans -> cis azobenzene isomerization. Here, we combine nonadiabatic dynamics simulations for the surfactant molecules embedded in the micelles with absorption spectroscopy measurements of micellar solutions to uncover the reasons responsible for the reaction slowdown. Our simulations reveal a decrease of isomerization quantum yields for molecules inside the micelles. We also observe a reduction of extinction coefficients upon micellization. These findings explain the deceleration of the trans -> cis switching in micelles of the azobenzene-containing surfactants.}, language = {en} } @article{SharmaBekirLomadzeetal.2020, author = {Sharma, Anjali and Bekir, Marek and Lomadze, Nino and Santer, Svetlana}, title = {Photo-Isomerization Kinetics of Azobenzene Containing Surfactant Conjugated with Polyelectrolyte}, series = {Molecules}, volume = {29}, journal = {Molecules}, number = {1}, publisher = {MDPI}, address = {Basel}, issn = {1420-3049}, doi = {10.3390/molecules26010019}, pages = {12}, year = {2020}, abstract = {Ionic complexation of azobenzene-containing surfactants with any type of oppositely charged soft objects allows for making them photo-responsive in terms of their size, shape and surface energy. Investigation of the photo-isomerization kinetic and isomer composition at a photo-stationary state of the photo-sensitive surfactant conjugated with charged objects is a necessary prerequisite for understanding the structural response of photo-sensitive complexes. Here, we report on photo-isomerization kinetics of a photo-sensitive surfactant in the presence of poly(acrylic acid, sodium salt). We show that the photo-isomerization of the azobenzene-containing cationic surfactant is slower in a polymer complex compared to being purely dissolved in aqueous solution. In a photo-stationary state, the ratio between the trans and cis isomers is shifted to a higher trans-isomer concentration for all irradiation wavelengths. This is explained by the formation of surfactant aggregates near the polyelectrolyte chains at concentrations much lower than the bulk critical micelle concentration and inhibition of the photo-isomerization kinetics due to steric hindrance within the densely packed aggregates.}, language = {en} } @article{GeorgievGrafmuellerBlegeretal.2018, author = {Georgiev, Vasil N. and Grafm{\"u}ller, Andrea and Bl{\´e}ger, David and Hecht, Stefan and Kunstmann, Sonja and Barbirz, Stefanie and Lipowsky, Reinhard and Dimova, Rumiana}, title = {Area increase and budding in giant vesicles triggered by light}, series = {Advanced science}, volume = {5}, journal = {Advanced science}, number = {8}, publisher = {Wiley}, address = {Hoboken}, issn = {2198-3844}, doi = {10.1002/advs.201800432}, pages = {9}, year = {2018}, abstract = {Biomembranes are constantly remodeled and in cells, these processes are controlled and modulated by an assortment of membrane proteins. Here, it is shown that such remodeling can also be induced by photoresponsive molecules. The morphological control of giant vesicles in the presence of a water-soluble ortho-tetrafluoroazobenzene photoswitch (F-azo) is demonstrated and it is shown that the shape transformations are based on an increase in membrane area and generation of spontaneous curvature. The vesicles exhibit budding and the buds can be retracted by using light of a different wavelength. In the presence of F-azo, the membrane area can increase by more than 5\% as assessed from vesicle electrodeformation. To elucidate the underlying molecular mechanism and the partitioning of F-azo in the membrane, molecular dynamics simulations are employed. Comparison with theoretically calculated shapes reveals that the budded shapes are governed by curvature elasticity, that the spontaneous curvature can be decomposed into a local and a nonlocal contribution, and that the local spontaneous curvature is about 1/(2.5 mu m). The results show that exo- and endocytotic events can be controlled by light and that these photoinduced processes provide an attractive method to change membrane area and morphology.}, language = {en} } @article{RyabchunSakhnoStumpeetal.2017, author = {Ryabchun, Alexander and Sakhno, Oksana and Stumpe, Joachim and Bobrovsky, Alexey}, title = {Full-Polymer Cholesteric Composites for Transmission and Reflection Holographic Gratings}, series = {Advanced optical materials}, volume = {5}, journal = {Advanced optical materials}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {2195-1071}, doi = {10.1002/adom.201700314}, pages = {376 -- 379}, year = {2017}, abstract = {A new type of self-organized materials based on cholesteric networks filled with photoactive side-chain copolymer is being developed. Supramolecular helical structure of cholesteric polymer network resulting in the selective reflection is used as a photonic scaffold. Photochromic azobenzene-containing nematic copolymer is embedded in cholesteric scaffold and utilized as a photoactive media for optical pattering. 1D and 2D transmission diffraction gratings are successfully recorded in composite films by holographic technique. For the first time the possibility to create selective reflection gratings in cholesteric material mimicking the natural optical properties of cholesteric mesophase is demonstrated. That enables the coexistence of two selective gratings, where one has an intrinsic cholesteric periodic helical structure and the other is a holographic grating generated in photochromic polymer. The full-polymer composites provide high light-induced optical anisotropy due to effective photo-orientation of side-chain fragments of the azobenzene-containing liquid crystalline polymer, and prevent the degradation of the helical superstructure maintaining all optical properties of cholesteric mesophase. The proposed class of optical materials could be easily applied to a broad range of polymeric materials with specific functionality. The versatility of the adjustment and material preprogramming combined with high optical performance makes these materials a highly promising candidate for modern optical and photonic applications.}, language = {en} } @article{RyabchunRaguzinStumpeetal.2016, author = {Ryabchun, Alexander and Raguzin, Ivan and Stumpe, Joachim and Shibaev, Valery and Bobrovsky, Alexey}, title = {Cholesteric Polymer Scaffolds Filled with Azobenzene-Containing Nematic Mixture with Phototunable Optical Properties}, series = {Scientific reports}, volume = {8}, journal = {Scientific reports}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/acsami.6b09642}, pages = {27227 -- 27235}, year = {2016}, abstract = {The past two decades witnessed tremendous progress in the field of creation of different types of responsive materials. Cholesteric polymer networks present a very promising class of smart materials due to the combination of the unique optical properties of cholesteric mesophase and high mechanical properties of polymer networks. In the present work we demonstrate the possibility of fast and reversible photocontrol of the optical properties of cholesteric polymer networks. Several cholesteric photopolymerizable mixtures are prepared, and porous cholesteric network films with different helix pitches are produced by polymerization of these mixtures. An effective and simple method of the introduction of photochromic azobenzene-containing nematic mixture capable of isothermal photoinducing the nematic isotropic phase transition into the porous polymer matrix is developed, It is found that cross-linking density and degree of polymer network filling with a photochromic nematic mixture strongly influence the photo-optical behavior of the obtained composite films. In particular, the densely cross-linked films are characterized by a decrease in selective light reflection bandwidth, whereas weakly cross-linked systems display two processes: the shift of selective light reflection peak and decrease of its width. It is noteworthy that the obtained cholesteric materials are shown to be very promising for the variety applications in optoelectronics and photonics.}, language = {en} } @article{FeldmannMaduarSanteretal.2016, author = {Feldmann, David and Maduar, Salim R. and Santer, Mark and Lomadze, Nino and Vinogradova, Olga I. and Santer, Svetlana}, title = {Manipulation of small particles at solid liquid interface}, series = {Scientific reports}, volume = {6}, journal = {Scientific reports}, publisher = {Nature Publishing Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/srep36443}, pages = {10}, year = {2016}, abstract = {The strong adhesion of sub-micron sized particles to surfaces is a nuisance, both for removing contaminating colloids from surfaces and for conscious manipulation of particles to create and test novel micro/nano-scale assemblies. The obvious idea of using detergents to ease these processes suffers from a lack of control: the action of any conventional surface-modifying agent is immediate and global. With photosensitive azobenzene containing surfactants we overcome these limitations. Such photo-soaps contain optical switches (azobenzene molecules), which upon illumination with light of appropriate wavelength undergo reversible trans-cis photo-isomerization resulting in a subsequent change of the physico-chemical molecular properties. In this work we show that when a spatial gradient in the composition of trans- and cis- isomers is created near a solid-liquid interface, a substantial hydrodynamic flow can be initiated, the spatial extent of which can be set, e.g., by the shape of a laser spot. We propose the concept of light induced diffusioosmosis driving the flow, which can remove, gather or pattern a particle assembly at a solid-liquid interface. In other words, in addition to providing a soap we implement selectivity: particles are mobilized and moved at the time of illumination, and only across the illuminated area.}, language = {en} } @article{YadavalliKoenigSanter2015, author = {Yadavalli, Nataraja Sekhar and K{\"o}nig, Tobias and Santer, Svetlana}, title = {Selective mass transport of azobenzene-containing photosensitive films towards or away from the light intensity}, series = {Journal of the Society for Information Display}, volume = {23}, journal = {Journal of the Society for Information Display}, number = {4}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {1071-0922}, doi = {10.1002/jsid.306}, pages = {154 -- 162}, year = {2015}, abstract = {Here, we report on two photosensitive amorphous polymers showing opposite behavior upon exposure to illumination. The first polymer (PAZO) consists of linear backbone to which azobenzene-containing side chains are covalently attached, while in the second polymer (azo-PEI), the azobenzene side chains are attached ionically to a polyelectrolyte backbone. When irradiated through a mask, the PAZO goes away from the intensity maxima, leaving behind topography trenches, while the direction of the mass transport of the azo-PEI polymer points towards the intensity maxima. This kind of behavior has been reported only for certain liquid crystalline polymers that exhibit in-phase reaction on illumination, that is, topography maxima coincides with the intensity maxima. Furthermore, flat nanocrystals placed on top of azo-PEI film was found to be moved together with the mass transport of the underlying polymer film as visualized using in situ atomic force microscopy (AFM) measurements. It was also demonstrated that the two polymer films respond differently on irradiation with the polarization and intensity interference patterns (IPs). To record the kinetic of the surface relief grating formation within two polymers during irradiation with different IPs, we utilized a homemade setup combining the optical part for the generation of IP and AFM. A possible mechanism explaining different responses on the irradiation of amorphous polymers is discussed in the frame of a theoretical model proposed by Saphiannikova et al. (J. Phys. Chem. B 113, 5032-5045 (2009)).}, language = {en} } @article{YadavalliKorolkovMoulinetal.2014, author = {Yadavalli, Nataraja Sekhar and Korolkov, Denis and Moulin, Jean-Francois and Krutyeva, Margarita and Santer, Svetlana}, title = {Probing opto-mechanical stresses within azobenzene-containing photosensitive polymer films by a thin metal film placed above}, series = {ACS applied materials \& interfaces}, volume = {6}, journal = {ACS applied materials \& interfaces}, number = {14}, publisher = {American Chemical Society}, address = {Washington}, issn = {1944-8244}, doi = {10.1021/am501870t}, pages = {11333 -- 11340}, year = {2014}, abstract = {Azo-modified photosensitive polymers offer the interesting possibility to reshape bulk polymers and thin films by UV-irradiation while being in the solid glassy state. The polymer undergoes considerable mass transport under irradiation with a light interference pattern resulting in the formation of surface relief grating (SRG). The forces inscribing this SRG pattern into a thin film are hard to assess experimentally directly. In the current study, we are proposing a method to probe opto-mechanical stresses within polymer films by characterizing the mechanical response of thin metal films (10 nm) deposited on the photosensitive polymer. During irradiation, the metal film not only deforms along with the SRG formation but ruptures in a regular and complex manner. The morphology of the cracks differs strongly depending on the electrical field distribution in the interference pattern, even when the magnitude and the kinetics of the strain are kept constant. This implies a complex local distribution of the opto-mechanical stress along the topography grating. In addition, the neutron reflectivity measurements of the metal/polymer interface indicate the penetration of a metal layer within the polymer, resulting in a formation of a bonding layer that confirms the transduction of light-induced stresses in the polymer layer to a metal film.}, language = {en} } @article{MiasnikovaBenitezMontoyaLaschewsky2013, author = {Miasnikova, Anna and Benitez-Montoya, Carlos Adrian and Laschewsky, Andr{\´e}}, title = {Counterintuitive photomodulation of the thermal phase transition of poly(methoxy diethylene glycol acrylate) in aqueous solution by trans-cis isomerization of Copolymerized Azobenzenes}, series = {Macromolecular chemistry and physics}, volume = {214}, journal = {Macromolecular chemistry and physics}, number = {13}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1022-1352}, doi = {10.1002/macp.201300203}, pages = {1504 -- 1514}, year = {2013}, abstract = {The non-ionic monomer (methoxy diethylene glycol) acrylate is copolymerized with its azodye-functionalized acrylate analogue using reversible addition-fragmentation chain transfer (RAFT) polymerization. Copolymerization is increasingly difficult with increasing amounts of the azo-dye-bearing monomer. The resulting water-soluble polymers are thermosensitive, exhibiting lower critical solution temperature (LCST) behavior, which can be modulated by the photoinduced trans-cis isomerization of the dye. While already small contents of the hydrophobic azobenzene group reduce the phase-transition temperatures of the copolymers strongly, photoisomerization of the apolar trans-state to the more-polar cis-state has only a small effect, and decreases rather than increases the cloud points.}, language = {en} }