Dr. rer. nat. Stefan Kirstein
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Forschungsthemen4
Ecopotfilter – Modulare, biopolymerbasierte Wasserfiltration zur Entfernung von Schwermetallionen
Quelle ↗Förderer: Andere inländische Stiftungen Zeitraum: 10/2025 - 12/2026 Projektleitung: Mohammad Fardin Gholami, Dr. rer. nat. Stefan Kirstein
Entwicklung eines Gerätes zur Untersuchung sub-mikroskopischer Partikel
Quelle ↗Zeitraum: 04/2006 - 03/2007 Projektleitung: Dr. rer. nat. Stefan Kirstein
SFB 448 III: Hybridaggregate: Struktur und Energieleitung (Teilprojekt C 6)
Quelle ↗Förderer: DFG Sonderforschungsbereich Zeitraum: 01/2004 - 12/2009 Projektleitung: Dr. rer. nat. Stefan Kirstein
SFB 951/3: Optische Anregung von TMDC/Farbstoff-Hybriden unter mechanischer Beanspruchung (TP A06)
Quelle ↗Förderer: DFG Sonderforschungsbereich Zeitraum: 07/2019 - 06/2023 Projektleitung: Dr. rer. nat. Stefan Kirstein, Prof. Dr. rer. nat. Jürgen P. Rabe
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Publikationen25
Top 25 nach Zitationen — Quelle: OpenAlex (BAAI/bge-m3 embedded für Matching).
The Journal of Physical Chemistry B · 683 Zitationen · DOI
CdTe nanoclusters were prepared in aqueous solution by the reaction between Cd2+ and NaHTe in the presence of thioglycolic acid. Under reflux, the clusters start to crystallize and show a narrow band emission. The photoluminescence efficiency of CdTe nanocrystals strongly depends on the pH value of the colloidal solution. The maximum quantum yield at room temperature is approximately 18% when the pH value of the CdTe solution is brought to 4.5 by using thioglycolic acid. The optical spectroscopy studies imply that the pH-dependent behavior of the CdTe nanocrystals' fluorescence is caused by structural changes on the surface rather than the size of the nanocrystals. Systematic absorption and fluorescence studies on dialyzed samples suggest that in the acidic range a shell of cadmium thiol complexes is formed around the CdTe core. Thus, the fluorescence quantum yield is enhanced dramatically when the solution is made acidic. In contrast, such a shell can also be produced in the alkaline range, but only after the CdTe nanocrystal crude solution is purified by dialysis.
Journal of Applied Physics · 308 Zitationen · DOI
Water soluble thiol capped CdTe nanocrystals are assembled into ultrathin films in combination with poly(diallyldimethylammonium chloride) (PDDA) by the self-assembly method of layer-by-layer adsorption of oppositely charged polyelectrolytes. Electroluminescent devices, which produce different color emissions, are fabricated by sandwiching CdTe/PDDA films between indium–tin–oxide (ITO) and aluminum electrodes using CdTe nanocrystals of different sizes. It is shown that the electroluminescence (EL) spectra of the CdTe/polymer films are nearly identical to the photoluminescence spectra of the corresponding CdTe nanocrystals in aqueous solutions. The devices produce room-light visible light output with an external quantum efficiency up to 0.1%. Light emission is observed at current densities of 10 mA/cm2 and at low onset voltages of 2.5–3.5 V, which depends on the thickness of the film indicating field-dependent current injection. A variation of the EL efficiency with the size of the CdTe particles is observed and explained by the size dependent shift of the CdTe energy levels with respect to the work function of the electron injecting Al electrode. This is confirmed by the behavior of two-layer devices prepared from two differently sized CdTe particles being spatially separated, i.e., one size CdTe near ITO and the other size CdTe near Al by using the self-assembly method.
Uniform exciton fluorescence from individual molecular nanotubes immobilized on solid substrates
2009Nature Nanotechnology · 219 Zitationen · DOI
The Journal of Physical Chemistry B · 215 Zitationen · DOI
Electrooptical and structural studies on self-assembled films composed of CdSe nanoparticles, poly(p-phenylene vinylene) (PPV), and different nonconjugated polyelectrolytes are reported. It is demonstrated by optical spectroscopy and X-ray reflectivity measurements that CdSe nanoparticles and PPV can successfully be incorporated into homogeneous ultrathin films by the self-assembly method. The surface roughness obtained from the X-ray measurements is 2.7 and 1.3 nm respectively for CdSe/PAH (PAH, poly(allylamine) hydrochloride) and PSS/PPV (PSS, poly(styrenesulfonic acid)) multilayer films. This allows us to stack a (PSS/PPV)*n film on top of a (CdSe/PAH)*n film to build up well-defined two-layer composite film devices. Electroluminescence studies show that pure (PSS/PPV)*n film devices exhibit green light emission but with a very short lifetime (several seconds) if operated in ambient air. During operation, the PPV emission shifts toward the blue, which indicates that the mean conjugation length of PPV is shortened due to oxidation. Oxidation of CdSe particles is also observed in (CdSe/PAH)*n devices during operation. However, the stability of CdSe particles is enhanced if they are combined alternately with PPV, and a (CdSe/PPV)*n device gives a broad, nearly white emission. The turn-on voltage of it is much smaller and better defined than that for a (CdSe/PAH)*20 device. This proves that PPV works like a charge-transportation layer rather than an emitting layer in the (CdSe/PPV)*n film device. In an ITO//PEI(CdSe/PAH)*10/(PSS/PPV)*10//Al two-layer composite film device the lifetime of PPV electroluminescence can be prolonged for at least 1 order of magnitude only after the device is first operated under backward bias (positive pole on Al electrode). This suggests that the oxygen within the film is removed by this operation due to oxidation of the particles. Afterward, this two-layer composite film device presents emission from PPV and CdSe, respectively, when the sign of the external voltage is changed.
The Journal of Physical Chemistry B · 202 Zitationen · DOI
Light- and small-angle neutron scattering as well as cryo-transmission electron microscopy (cryo-TEM) studies were performed to probe the structure of J-aggregates formed by a series of achiral dye molecules of the 5,5‘,6,6‘-tetrachlorobenzimidacarbocyanine chromophore having 1,1‘-dialkyl substituents combined with 3,3‘-bis(4-sulfobutyl)-3,3‘-bis(4-carboxybutyl) or 3,3‘-bis(3-carboxypropyl) substituents. Assemblies that display a dependence on the substituents different complex supramolecular structures of nanometer-to-micrometer size have been directly visualized by cryo-TEM. The superstructures span from monomolecular layers formed by the 1,1‘-diethyl-3,3‘-bis(4-sulfobutyl) derivative and stacks of bilayer ribbons in the case of the 1,1‘-dioctyl-3,3‘-bis(4-carboxybutyl) derivative to twisted ropelike structures for the chiral aggregate of the 1,1‘-dioctyl-3,3‘-bis(3-carboxypropyl)-substituted chromophore.
Nano Letters · 153 Zitationen · DOI
We demonstrate trench channeling of mono- and multilayer graphenes with silver nanoparticles with high speed in ambient environment and at elevated temperatures. A silver nanoparticle located at a graphene edge catalyzes oxidation of neighboring carbon atoms, thereby burning a trench into the graphene layer. High-resolution scanning tunneling microscopy imaging reveals that the trench edges are very smooth with a peak-to-peak roughness below 2 nm. We discuss the channeling mechanism and demonstrate that channeling speeds of up to 250 nm/s and the smoothness of the resulting trenches indicate the prospect of a "catalytic pen" for high-precision lithography on graphenes.
Advanced Materials · 152 Zitationen · DOI
Inorganic nanoparticles in organic/inorganic hybrid devices are expected to have properties of interest for electroluminescence applications. The preparation of films of CdSe nanoparticles and poly(phenylenevinylene), PPV, by means of self‐assembly is presented and photoluminescence and electroluminescence studies of these films are described. It is shown that nearly white‐light emission is achieved from a twenty‐layer film and that the emission orgininates most probably from the nanoparticles alone. The role of the PPV is discussed.
Journal of the American Chemical Society · 149 Zitationen · DOI
Alternating thin films of the anionic dye pyrenetetrasulfonic acid (4-PSA) and poly(allylamine hydrochloride) (PAH) were formed by the squential deposition of 4-PSA and PAH from aqueous solution onto solid substrates. Film growth was followed by absorption spectroscopy, and the resulting polyelectrolyte-dye multilayer assemblies were characterized by using fluorescence spectroscopy, X-ray reflectivity, and atomic force microscopy (AFM) measurements. 4-PSA was successfully deposited in alternation with PAH when the ionic strength of the PAH solution was greater than 0.1 M. At each PAH adsorption step a given amount of previously adsorbed 4-PSA was extracted. The amount of dye released depended on the film thickness and the salt concentration in the PAH solution from which PAH was subsequently adsorbed. However, the total amount of dye remaining in the film after PAH deposition was essentially independent of the salt content in the PAH solution. Both X-ray reflectivity and AFM measurements revealed a high roughness of the 4-PSA/PAH multilayer films (ca. 20 Å). To gain a better understanding of the parameters that control dye adsorption and extraction, dye adsorption was also investigated by immersion of preformed PAH and poly(styrene sulfonate) (PSS) multilayer films of various thickness into 4-PSA solutions. The amount of 4-PSA absorbed by the films increased with film thickness, and saturated at a thickness above about 20 nm. For these films, subsequent adsorption of a PAH layer extracted most of the dye, except for a small amount approximately equal to that present in one 4-PSA/PAH bilayer prepared by the alternate adsorption process. These observations indicate that the dye molecules are adsorbed to a finite depth of about 20 nm, and after deposition of the oppositely charged PAH, a small amount that is independent of film thickness remains. The presence of this remaining 4-PSA is necessary to complex and bind the next PAH layer, hence allowing continued multilayer film growth. It was also found that the 4-PSA adsorption/desorption process depended on the drying process that was employed during the formation of the PSS/PAH multilayer films. The amount of dye as well as the penetration depth was reduced when the film was dried between deposition of each layer, and minor changes in the film structure were observed by X-ray reflectivity measurements.
J‐aggregates of amphiphilic cyanine dyes: Self‐organization of artificial light harvesting complexes
2006International Journal of Photoenergy · 127 Zitationen · DOI
The simultaneous chemical linkage of cyanine dye chromophores with both hydrophobic and hydrophilic substituents leads to a new type of amphiphilic molecules with the ability of spontaneous self‐organization into highly ordered aggregates of various structures and morphologies. These aggregates carry the outstanding optical properties of J‐aggregates, namely, efficient exciton coupling and fast exciton energy migration, which are essential for the build up of artificial light harvesting systems. The morphology of the aggregates depends sensitively on the molecular structure of the chemical substituents of the dye chromophore. Accordingly, lamellar ribbon‐like structures, vesicles , tubes, and bundles of tubes are found depending on the dyes and the structure can further be altered by addition of surfactants, alcohols, or other additives. Altogether the tubular structure is the most noticeable structural motif of these types of J‐aggregates. The optical spectra are characterized in general by a complex exciton spectrum which is composed of several electronic transitions. The spectrum is red‐shifted as a total with respect to the monomer absorption and exhibits resonance fluorescence from the lowest energy transition. For the tubular structures, the optical spectra can be related to a structural model. Although the molecules itself are strictly achiral, a pronounced circular dichroism (CD) is observed for the tubular aggregates and explained by unequal distribution of left‐ and right‐handed helicity of the tubes. Photo‐induced electron transfer (PET) reactions from the dye aggregates to electron acceptor molecules lead to superquenching which proves the delocalization of the excitation. This property is used to synthesize metal nanoparticles on the aggregate surface by photo‐induced reduction of metal ions.
Journal of the American Chemical Society · 116 Zitationen · DOI
The polarity of polyelectrolyte (PE) multilayer films is investigated with pyrene as a polarity-sensitive probe. Multilayer films of poly(styrene sulfonate) (PSS) and various polycations were prepared by the layer-by-layer self-assembly technique. Pyrene (PY) molecules were inserted into the films by exposing the multilayers to pyrene solutions. By this method a homogeneous distribution of pyrene molecules at low concentration within the film was obtained. The ratio of the fluorescence intensities of the first (I) to the third (III) vibronic band (Py-value) of the pyrene emission spectrum is employed here to determine the polarity of the PE films. PSS and poly(allylamine hydrochloride) (PAH) multilayer films yielded a pyrene value close to the solvent polarity of acetone, while multilayers of PSS and poly(diallyldimethylammonium chloride) (PDADMAC) displayed a value higher than the one corresponding to water. The pyrene values of the polyelectrolyte films were independent from the solvent employed for probe dissolving. Although no direct relationship between solvent polarity and dielectric constant (epsilon) is available, an estimate of the static dielectric constant of the films can be provided by comparing the Py-values of the films with those of various solvents. Changes in the humidity conditions of the film environment in a closed cell did not affect the film polarity. However, a drastic and irreversible reduction of polarity could be induced by actively drying the samples by a nitrogen flow.
The Journal of Physical Chemistry B · 108 Zitationen · DOI
Achiral molecules of 5,5‘,6,6‘-tetrachlorobenzimidacarbocyanine dyes having 1,1‘-di-n-alkyl substituents longer than hexyl combined with 3,3‘-bis(2-acidoethyl) or 3,3‘-bis(3-acido-n-propyl) substituents form J-aggregates whose absorption spectrum exhibits Davydov-split subbands that display strong circular dichroism, indicating the enantioselective formation of chiral J-aggregates. In this way, for the first time, in the liquid phase self-organization of achiral dye molecules to chiral supramolecular aggregates has been realized which can be controlled by the molecular structure of the monomeric precursors. A helix-like, cylindric structure of the chiral J-aggregates is suggested. The results expose an interesting model for studying and understanding enantioselective processes in the biosphere.
Journal of Applied Physics · 106 Zitationen · DOI
The influence of a viscous fluid on the dynamic behavior of a vibrating scanning near-field optical microscopy fiber tip is investigated both theoretically and experimentally. A continuum mechanical description of a cylindric cantilever is used to calculate the resonance frequencies and the widths of the resonance bands. The linearized Naviers–Stokes equations are analytically solved and describe the interaction of the beam with the viscous fluid. The contribution of the liquid to the shift and the broadening of the resonance lines is summarized by two constants that can be derived from a master function and the kinetic Reynolds number. The theoretical values are compared with experimental data collected from an optical fiber which is used as a probe in a scanning near-field microscope. Agreement, with a relative error of less than 1%, is achieved. The theory is further developed for the application to atomic force microscopy cantilevers with a rectangular cross section. Experimental data taken from literature are in good agreement with the theory.
The Journal of Physical Chemistry B · 95 Zitationen · DOI
The amphiphilic dye 3,3'-bis(2-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine (C8S3) self-aggregates in aqueous solution to form tubular J-aggregates with a diameter of 17.0 +/- 0.5 nm, a wall thickness of approximately 4 nm, and a length exceeding several hundred nanometers. The absorption spectrum shows the typical features expected for tubular J-aggregates with several sharp and red-shifted absorption bands. Morphological investigations using cryo-transmission electron microscopy (cryo-TEM) and spectroscopic investigations reveal a high stability of the tubular morphology but a tendency of the aggregates to assemble into ropelike bundles after several weeks of storage. It is found that aggregation in solutions containing additives such as alcohols or surfactants results in the formation of new types of aggregates. A second type of tubular aggregate with a diameter of 13.0 +/- 0.5 nm is observed when the solutions contain more than 10 wt % MeOH. On the time scale of days these tubular aggregates transform into ribbonlike structures characterized by a new absorption spectrum, and they convert after several weeks into giant tubes with diameters of up to 500 nm.
Langmuir · 94 Zitationen · DOI
The morphology of the J-aggregates formed after the addition of ionic surfactants into solutions of the 3,3‘-bis(3-carboxypropyl)-5,5‘,6,6‘-tetrachloro-1,1‘-dioctylbenzimidacarbocyanine dye (C8O3) was characterized by absorption and circular dichroism spectroscopy, cryogenic temperature transmission electron microscopy, scanning force microscopy, and small-angle neutron scattering. The optical activity of the C8O3 J-aggregates, existing in aqueous solution as superhelices composed of tubular single strands, disappears upon addition of both cationic and anionic surfactants. This is accompanied by distinct spectral changes in the visible region as well as changes of the aggregates' morphology. The anionic surfactant sodium dodecyl sulfate induces the formation of single-walled tubules of 15 nm diameter and 300−600 nm length, which completely transform after several days into thick multilamellar tubes of micrometer length. The cationic surfactant trimethyltetradecylammonium bromide first produces vesicles that later transform again into tubular aggregates of nanometer thickness and micrometer length.
ChemPhysChem · 88 Zitationen · DOI
The self-assembly of helical supramolecular structures from chiral building units is a basic principle of biological materials. The mesoscopic structure of a chiral molecular aggregate, which formed spontaneously from a nonchiral J-aggregating cyanine dye in aqueous solution, is presented. In single crystals (as shown in the picture) a coexistance of planar molecules with left- and right-handed twisted conformers of the same dye are found; the latter may act as templates to build up the helical superstructures.
Journal of the American Chemical Society · 82 Zitationen · DOI
Self-assembled supramolecular nanotubes of J-aggregated amphiphilic cyanine dye in aqueous solution are employed as chemically active templates for the photoinitiated formation of silver nanowires with a very small and homogeneous diameter of (6.4 +/- 0.5) nm. Key features of the template are (1) its small and well-defined diameter; (2) its photochemical activity, which allows photoinitiation of the structure formation; and (3) the processability in aqueous solution. The latter includes the potential to remove the template after the reaction, or to functionalize it further, e.g. with optoelectronically active polycations, providing access to quasi one-dimensional hybrid structures with well-defined metallic nanowires as a core.
Journal of Applied Physics · 67 Zitationen · DOI
Neutron and x-ray reflectivity studies of modulated heterostructures consisting of alternate layers of conjugated and nonconjugated polymers is described. Such heterostructures are currently being used to fabricate polymer-based light emitting diodes. The heterostructures were prepared by the layer-by-layer self-assembly technique using the precursor of the conjugated polymer, deuterated poly(phenylenevinylene) (D-PPV), and other polyelectrolyte spacers. Heat treatment after the layer assembly converted the pre-D-PPV to a conjugated semiconducting polymer. For the first time in such heterostructures, we were able to observe quasi-Bragg reflections (up to the third order) due to the formation of ordered modulated structures. Both the neutron and the x-ray data were analyzed using the same layer-by-layer model and the same fitting procedure with consistent results. Most importantly, the model and the fitting procedure yield the buried interlayer roughness at the D-PPV/spacer interface. This roughness parameter, of the order of 12±3 Å, was found to be smaller than the thickness of the D-PPV and the spacer layers, suggesting that the length over which interdigitation between neighboring polymer layers occurs is significantly smaller than the spacer layer. We demonstrate that the conversion to conjugated polymer by the heat treatment leads to ∼7% reduction of the repeat unit and the film thickness without significant changes of other structural properties; in fact, the interfacial roughness was somewhat improved. The fabrication of high quality modulated structures with controlled layer thickness and relatively small interfacial roughness may be a first step towards polymer-based multiquantum wells analogous to such devices in inorganic heterostructures.
European Journal of Organic Chemistry · 64 Zitationen · DOI
Abstract A new class of dyes, in which the self‐assembling property of surfactants is combined with the capability for light energy propagation over long distances in dye J‐aggregates, is described. This has been achieved by the syntheses of achiral 5,5′,6,6′‐tetrachlorobenzimidacarbocyanine dyes 1 , possessing systematically varied hydrophobic and hydrophilic substituents at their nitrogen atoms. These substituents are introduced into the 5,6‐dichlorobenzimidazole precursor 3 by substitution, either firstly by nucleophilic reaction with ψ‐bromoalkylnitriles and secondly by quaternization with alkyl bromides (route II) or firstly by nucleophilic reaction with alkyl bromides and secondly by quaternization with ψ‐bromoalkylnitriles or ψ‐bromoalkyl esters (route III) and subsequent saponification. The UV/Vis spectra of 20 differently substituted dyes containing the same chromophore 1 have been investigated. The spectra of the dye monomers in dimethyl sulfoxide are nearly identical, with no signs of optically activity, whereas in aqueous alkaline solutions quite different spectra are obtained for the dyes, indicating the formation of different aggregates depending on the nitrogen substituents. One of these types of J‐aggregate is optically inactive and displays a single red‐shifted (with respect to the dye monomers) absorption band, resembling the behaviour of J‐aggregates of common cyanine dyes. In the cases of strongly amphiphilic 5,5′,6,6′‐benzimidacarbocyanines 1 with 1,1′‐dialkyl substituents longer than hexyl and 3,3′‐bis(2‐carboxyethyl), 3,3′‐bis(3‐carboxypropyl) or 3,3′‐bis(3‐sulfopropyl) substituents, a new type of J‐aggregate is formed, and is distinguished by a doubly or even triply split J‐absorption band that displays optical activity. A third type of aggregate showing different spectral behaviour occurs when the dyes contain very short 3,3′‐bis(carboxymethyl) substituents or strongly hydrophobic fluorinated octyl groups, or when all four nitrogen atoms are identically substituted by hydrophilic 3‐carboxypropyl groups. The various types of dye aggregates have been characterized through UV/Vis spectroscopic parameters such as the positions and widths of the absorption and fluorescence bands, the Stokes′ shifts, the coupling constants, and the strength of the J‐band splitting. The results provide new prospects for the development of new artificial light‐harvesting systems as well as for the understanding of the evolution of asymmetry in the biosphere. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
Langmuir · 61 Zitationen · DOI
The morphology of J-aggregates formed by 3,3‘-bis(3-carboxypropyl)-5,5‘,6,6‘-tetrachloro-1,1‘-dioctylbenzimidacarbocyanine dye (C8O3) in aqueous solution upon addition of alcohols was characterized by spectroscopic methods, optical microscopy, and cryo-transmission electron microscopy (cryo-TEM). With increasing concentration of methanol, ethylene glycol, 1-octanol, or 1-decanol, the well-known 3-fold split excitonic absorption spectrum of the superhelical J-aggregates obtained in the absence of alcohols is replaced by a 2-fold split spectrum (von Berlepsch et al. J. Phys. Chem. B 2000, 104, 5255), while the circular dichroism signal generally increases. The spectral changes indicate alcohol-induced modifications of the aggregates' molecular packing. Cryo-TEM revealed that these novel J-aggregates are composed of individual bilayer-walled tubules having a diameter of 11 ± 1 nm, which are assembled to form ropelike superhelices of several micrometers in length. The superhelices' diameter is markedly increased if compared with that in the alcohol-free system.
Chemical Physics Letters · 58 Zitationen · DOI
The Journal of Physical Chemistry C · 52 Zitationen · DOI
The amphiphilic cyanine dye 3,3‘-bis(2-sulfopropyl)-5,5‘,6,6‘-tetrachloro-1,1‘-dioctylbenzimidacarbocyanine (C8S3) self-assembles in aqueous solution to form double-walled, tubular J-aggregates with ∼13 nm diameters and lengths up to several hundred nanometers. The redox and light absorption properties of immobilized J-aggregates on transparent, conductive indium tin oxide (ITO) electrodes have been studied directly using cyclic voltammetry (CV) in conjunction with UV−vis spectroscopy to elucidate unique mechanistic features of J-aggregate oxidation. Morphological properties were examined using in situ atomic force microscopy (AFM). Irreversible J-aggregate oxidation appears to occur primarily along the outer wall of the tubular structure as evidenced by the potential-induced irreversible bleaching of J-band absorption. Voltammetric studies as a function of scan rate and pH indicate that J-aggregate oxidation involves both electrochemical and chemical steps in which dimerization and subsequent dehydrogenation of the J-aggregate leads to the formation of a new dehydrogenated dimer oxidation product. This dehydrogenated dimer exhibits an absorbance band near 560 nm along with a reversible reduction peak characteristic of a surface-confined, redox-active species. Excellent correlation of J-aggregate redox potentials with spectroelectrochemical data is obtained that allows us to understand energetic thresholds for electron transfer in C8S3 tubular J-aggregates.
Chemical Physics Letters · 52 Zitationen · DOI
The Journal of Physical Chemistry B · 51 Zitationen · DOI
The achiral 3,3‘-bis(3-carboxypropyl)-5,5‘,6,6‘-tetrachloro-1,1‘-dioctylbenzimidacarbocyanine dye (C8O3) self-aggregates in aqueous solution to form optically active helical J-aggregates. A three-phasic circular dichroism (CD) spectrum with positive Cotton effect at 568 nm, negative Cotton effect at 582 nm, and positive Cotton effect at 605 nm (+,−,+) is usually found and ascribed to molecular excitons. The predominance of the (+,−,+) signature means that the generation of chirality is enantioselective. The CD signal increases by a factor of about 100 when the chiral (R)-2-octanol is added to an aggregated solution, and the CD signal is further amplified by a factor of about 5 when the alcohol is present in the solvent prior to aggregation. The signature of the CD signal reverses if the enantiomeric (S)-2-octanol is used instead, and for a molar alcohol/dye ratio R of about 20, the spectra become mirror images in good approximation; i.e., the chirality of the alcohol and the CD spectra of aggregates are correlated. The net effect of chiral alcohols is an increased excess of aggregates with a particular handedness due to induced circular dichroism. Samples containing (S)-2-octanol showed that their “reversed” CD spectra ((−,+,−) signature) are not fully stable when stored for longer times at ambient temperature. The spectral changes indicate a gradually increasing contribution from aggregates with the opposite sense of handedness ((+,−,+) signature). Electron micrographs reveal left helix handedness for the majority of aggregates in the presence of (R)-2-octanol ((+,−,+) signature), while right helix handedness predominates for the aggregates in the presence of (S)-2-octanol ((−,+,−) signature).
Nanotubular J-Aggregates and Quantum Dots Coupled for Efficient Resonance Excitation Energy Transfer
2015ACS Nano · 49 Zitationen · DOI
Resonant coupling between distinct excitons in organic supramolecular assemblies and inorganic semiconductors is supposed to offer an approach to optoelectronic devices. Here, we report on colloidal nanohybrids consisting of self-assembled tubular J-aggregates decorated with semiconductor quantum dots (QDs) via electrostatic self-assembly. The role of QDs in the energy transfer process can be switched from a donor to an acceptor by tuning its size and thereby the excitonic transition energy while keeping the chemistry unaltered. QDs are located within a close distance (<4 nm) to the J-aggregate surface, without harming the tubular structures and optical properties of J-aggregates. The close proximity of J-aggregates and QDs allows the strong excitation energy transfer coupling, which is around 92% in the case of energy transfer from the QD donor to the J-aggregate acceptor and approximately 20% in the reverse case. This system provides a model of an organic-inorganic light-harvesting complex using methods of self-assembly in aqueous solution, and it highlights a route toward hierarchical synthesis of structurally well-defined supramolecular objects with advanced functionality.
Physical Chemistry Chemical Physics · 49 Zitationen · DOI
The simulation of the optical properties of supramolecular aggregates requires the development of methods, which are able to treat a large number of coupled chromophores interacting with the environment. Since it is currently not possible to treat large systems by quantum chemistry, the Frenkel exciton model is a valuable alternative. In this work we show how the Frenkel exciton model can be extended in order to explain the excitonic spectra of a specific double-walled tubular dye aggregate explicitly taking into account dispersive energy shifts of ground and excited states due to van der Waals interaction with all surrounding molecules. The experimentally observed splitting is well explained by the site-dependent energy shift of molecules placed at the inner or outer side of the double-walled tube, respectively. Therefore we can conclude that inclusion of the site-dependent dispersive effect in the theoretical description of optical properties of nanoscaled dye aggregates is mandatory.
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- Dr. rer. nat. Stefan Kirstein
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- Mathematisch-Naturwissenschaftliche Fakultät
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- Institut für Physik
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- Experimentelle Physik (Physik von Makromolekülen)
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