Dye Monolayer Research Articles

Efficient Light Harvesting with LB Films for Application in Crystalline Silicon Solar Cells

AbstractWe have carried out fluorescence lifetime measurements using time correlated single photon counting (TCSPC) for a cyanine dye near the silicon surface. The measurements have been carried out for both (100) and (111) crystal orientations of the silicon surface, showing the dependence of energy transfer rate as a function of the separation between the dye monolayer and the silicon surface. Langmuir Blodgett fatty acid layers were used to create a multistep structure and a monolayer of a cyanine dye was deposited on top of the stepped structure. Spectroscopic ellipsometry has been used to measure the thickness of the fatty acid steps and provide an accurate estimate of the distance of the dye monolayer to the silicon surface. Time resolved emission spectra and fluorescence decay curves were measured with a single photon picosecond time correlated system. We find that the fluorescence lifetime of the dye monolayer is significantly shortened when present close to the silicon surface signifying efficient energy transfer. The dissipation of the excitation energy near silicon is explained using the classical theory developed for metals and a deviation is observed for distances close to the silicon surface (d<5nm). The model can be reconciled with the observed data by modifying the value of the silicon extinction coefficient which can provide an insight into the energy transfer process in the near field dye-silicon interaction.

Decolorization of organic dyes on Pilkington Activ™ photocatalytic glass

The air–solid photocatalytic degradation of organic dye films Acid Blue 9 (AB9) and Reactive Black 5 (RBk5) is studied on Pilkington Activ™ glass. The Activ™ glass comprises of a colorless TiO2 layer deposited on clear glass. The Activ™ glass is characterized using atomic force microscopy (AFM) and X-ray diffraction (XRD). Using AFM, the TiO2 average agglomerate particle size is 95nm, with an apparent TiO2 thickness of 12nm. The XRD results indicate the anatase phase of TiO2, with a calculated crystallite size of 18nm.Dyes AB9 and RBk5 are deposited in a liquid film and dried on the Activ™ glass to test for photodecolorization in air, using eight UVA blacklight-blue fluorescent lamps with an average UVA irradiance of 1.4mW/cm2. A novel horizontal coat method is used for dye deposition, minimizing the amount of solution used while forming a fairly uniform dye layer. About 35–75 monolayers of dye are placed on the Activ™ glass, with a covered area of 7–10cm2. Dye degradation is observed visually and via UV–vis spectroscopy.The kinetics of photodecolorization satisfactorily fit a two-step series reaction model, indicating that the dye degrades to a single colored intermediate compound before reaching its final colorless product(s). Each reaction step follows a simple irreversible first-order reaction rate form. The average k1 is 0.017 and 0.021min−1 for AB9 and RBk5, respectively, and the corresponding average k2 is 2.0×10−3 and 1.5×10−3min−1. Variable light intensity experiments reveal a p=0.44±0.02 exponent dependency of initial decolorization rate on the UV irradiance. Solar experiments are conducted outdoors with an average temperature, water vapor density, and UVA irradiance of 30.8°C, 6.4gwater/m3 dry air, and 1.5mW/cm2, respectively. For AB9, the average solar k1 is 0.041min−1 and k2 is 5.7×10−3min−1.

Influence of Monolayer State on Spectroscopy and Photoisomerization of an Amphiphilic Styryl-Pyridinium Dye on a Solid Substrate

The spectroscopy and photochromic properties of transferred monolayers of the amphiphilic styryl-pyridinium dye 4-(3',4'-dimethoxystyryl)-N-octadecylpyridinium perchlorate (DMPOP) were studied at different conditions during their transfer. The emission maxima of the monolayers transferred from the air-water interface in the liquid-expanded phase are strongly dependent on the surface pressure applied during the transfer process, even at values when the area per molecule is 2-3 times larger than the area occupied by a chromophore. In monolayers transferred from the liquid-condensed phase, the presence of a different kind of aggregates was observed. The fluorescence emission properties of the monolayers can be reversibly modulated by photoinduced E-Z isomerization. A blue shift up to 72 nm in the emission maximum, depending on the transfer conditions of the films, can be obtained by irradiation with blue light, and partially recovered (a red shift of up to 26 nm) with UV radiation. The rate at which the first process (E-->Z) takes place is drastically reduced in monolayers transferred from the liquid-condensed phase as compared to those transferred from the liquid-expanded one. However, the rate of the reverse reaction (Z-->E) is not significantly altered. These properties make DMPOP a promising material for the preparation of Langmuir-Blodgett films, whose properties can be effectively controlled by the transfer conditions and subsequently optically modulated, for potential applications as photonics devices for data storage.

Fabrication of Gradient Mesostructures by Langmuir−Blodgett Rotating Transfer

In this letter, we present a simple yet novel method, Langmuir-Blodgett (LB) rotating transfer, to achieve a continuous gradient mesostructure in a well-ordered fashion over large areas. A mixed monolayer of phospholipid and dye is chosen as a model system to test the feasibility of LB rotating transfer to fabricate continuous gradient structures, which is confirmed by the simulation and experimental results. The technique presented here to obtain gradient structures is low-cost and high-throughput and can be extended to other systems of LB patterning.

Photoinduced Dynamics of Azobenzene Dyes in Langmuir Films Investigated by Optical Second Harmonic Generation

Photoinduced molecular dynamics of a Langmuir film of an azobenzene dye is investigated by means of optical second harmonic generation. The dynamics is induced by turning on and off irradiation with monochromatic light at a wavelength in the dye absorption range. We find different dynamical behaviors depending on the surface pressure of the film. At the lowest pressures, we observe a fully reversible photoinduced dynamics which is most likely related to cis-trans photoisomerization. However, the measured relaxation rate of the dye monolayer on the water surface is somewhat faster than the reported cis-trans rate of the same dye in dilute solutions, probably reflecting the effect of dye-dye or dye-water intermolecular interactions. At the highest pressures, we observe an irreversible or slowly reversible dynamics of still unidentified nature. These results, although still very preliminary, point to a strong influence of the molecular state of aggregation on the photoisomerization processes and possibly also on the related photoinduced reorientation phenomena, in accordance with previous observations on the same dye in Langmuir-Blodgett multilayer films.

Enhancement of Adsorbed Dye Monolayer Fluorescence by a Silver Nanoparticle Overlayer

Deposition of silver nanoparticles directly onto a Rhodamine Red monolayer covalently bound to glass is found to increase the photoluminescence by as much as 20 times depending on excitation wavelength and nanoparticle density. At the same time, the excited state lifetime is reduced by more than a factor of 2, the fluorescence spectrum is blue-shifted, and greater polarization anisotropy in the emission is observed. We attribute these effects to local enhancement of the optical fields near the molecules by interactions with silver plasmons. We approximately separate the relative contributions of increased absorption and increased emissive rate to the observed enhancement of fluorescence. A study of the spatial inhomogeneity of the effect using scanning confocal microscopy demonstrates that average enhancements at least twice as large are possible.

Structural Analysis of a J-aggregate Monolayer at the Air-water Interface by a Grazing Incidence X-ray Diffraction Method and Calculation of the Energy Level of Its Excitonic Absorption Band

The monolayer of amphiphilic merocyanine dye (MD) molecules at the air-water interface was observed by the grazing incidence X-ray diffraction (GIXD) method. At the air-water interface, the MD molecules formed two-dimensional crystallites (J-aggregates), whose excitonic absorption bands were located at longer wavelength than the monomer band. The structural analysis on the MD J-aggregate was carried out, and, using the determined structure, the relationship between the molecular arrangement and the energy shift of the visible absorption band upon crystallization was clarified. For the calculation of the energy shift, simple formulas were derived on the basis of molecular exciton theory. It was revealed that not only the transition dipole moment interaction but also the electric dipole moment interaction among the molecules plays an important role in the energy shift.

The reuse of dried activated sludge for adsorption of reactive dye

Adsorption processes are alternative effective methods for removal of textile dyes from aqueous solutions. The adsorption ability of adsorbent affects by physico-chemical environment for this reason in this paper effect of initial pH, dye concentrations, temperature and dye hydrolyzation were determined in a batch system for removal of reactive dye by dried activated sludge. The Langmuir isotherm model was well described of adsorption reactive dye and maximum monolayer adsorption capacity (at pH 2) of activated sludge was determined as 116, 93 and 71 mg g −1 for 20°, 35° and 50 °C, respectively. Initial pH 2, 20 °C and 30 min contact time are suitable for removal of reactive dyes from aqueous solutions. Activated sludge was characterized by FT-IR analysis and results showed that active sludge has different functional groups and functional groups of activated sludge are able to react with dye molecules in aqueous solution. The pseudo first-order, second-order and intraparticle diffusion kinetics were used to describe the kinetic data. The pseudo second-order kinetic model was fit well over the range of contact times and also an intra particle diffusion kinetic model was fit well but in the first 30 min. The dye hydrolyzation was affected adsorption capacity of biomass and adsorption capacity of biomass decreased with dye hydrolyzation from 74 to 38 mg g −1.

Molecular Ordering in Monolayers of an Alkyl-Substituted Perylene-Bisimide Dye by Attenuated Total Reflectance Ultraviolet—Visible Spectroscopy

Surface-relative orientational parameters were determined for monolayer films of N, N'-ditridecylperylenetetracarboxylic dianhydridediimide (C13-PTCDI) in terms of the relative electronic transition dipole strengths, providing a three-dimensional view of the absorption dipole distribution. In order to obtain a macroscopically ordered film, C13-PTCDI was deposited by (1) horizontal Lang-muir-Blodgett (LB) transfer onto methyl- and phenyl-silanized glass, and (2) vapor deposition onto oriented films of poly(tetrafluoroethylene) (PTFE) on glass. Films of LB-deposited C13-PTCDI were found to be completely isotropic prior to annealing. After annealing, these films remained isotropic in the plane of the substrate while the out-of-plane anisotropy was significantly enhanced. In contrast, films of C13-PTCDI vapor deposited onto oriented poly(tetrafluoroethylene) (PTFE)-modified substrates yielded films with a high degree of both in- and out-of-plane anisotropy. Atomic force microscopy (AFM) images of both the LB- and vapor-deposited films show substantial differences in film morphology and long-range order. These results demonstrate that molecular orientation in C13-PTCDI films can be controlled by varying substrate surface chemistry and post-deposition processing.

Organisation in monolayers at the air–water interface of butadienyl dyes containing benzodithiacrown-ether or dimethoxybenzene

The novel amphiphilic butadienyl dye, containing a dimethoxybenzene unit instead of benzocrown-ether moiety (dye 1) and the amphiphilic benzodithia-15-crown-5 butadienyl dye (dye 2) form stable insoluble monolayers on distilled water. In order to evaluate the influence of the ionophore size on the molecular organisation, the known amphiphilic benzodithia-18-crown-6 butadienyl dye (dye 3) has also been used. The monolayers have been characterized by surface pressure–area and surface potential–area isotherms as well as Brewster angle microscopy and reflection spectroscopy. Besides monomers of the dyes two types of associates exist in the pure dye monolayers, i.e. dimers and aggregates. The spectroscopic properties of the monomers were obtained by studying mixed monolayers of the dyes with the lipid dimyristoyl-phosphatidyl-cholin (DMPC). The dimers may be organised as head-to-tail dimers with the intermolecular distances 0.38, 0.45, and 0.49 nm for dyes 1, 2, and 3, respectively. According to the extended dipole model, we propose formation of aggregates in which the chromophores are parallel to each other with the same intermolecular distances as in the dimers, and the centers of their transition moments shifted by 0.95 nm (dye 1) and 1.2 nm (dyes 2 and 3).

Time-Domain Raman Measurement of Molecular Submonolayers by Time-Resolved Reflection Spectroscopy

Femtosecond time-resolved photoinduced reflectance change was measured on a spin-coated dye (cresyl violet) of submonolayer coverage (4 × 1013 cm-2) on a fused silica substrate. We found that the sensitivity of the reflection measurement was much higher than that of the absorption measurement for thin monolayers of dye. A prominent beating structure due to vibrational coherence of the dye was observed on the time-resolved reflection signal. A vibrational spectrum of the dye at submonolayer coverage, which corresponded to its resonance Raman spectrum, was obtained in the range 0−800 cm-1 by a Fourier transform analysis of the reflection signal.

In Situ Observation of the Thermochromic Phase Transition of the Merocyanine J-Aggregates Monolayer at the Air−Water Interface Using External Infrared Reflection Absorption Spectroscopy

To understand the mechanism of thermochromic phase transition of a J-aggregate monolayer of a merocyanine dye (MD, 3-caboxymethyl-5-[2-(3-octadecyl-benzothiazolin-2-ylidene)ethylidene]rhodanine) on aqueous subphases containing Mg2+ and Cd2+ ions, the molecular vibrational investigation was performed by using external infrared (IR) reflection absorption spectroscopy. The in situ observation indicates that, on increasing the subphase temperature from 19.5 to 34.5 °C, IR bands observed at 1177 and 1126 cm-1 for the J-aggregate monolayer shift to 1188 and 1146 cm-1, respectively; the shift is reversible and exhibits a thermal-hysteresis. These spectral changes correspond well to the thermochromic behavior of the visible absorption band of the J-aggregate (J-band), where the J-band shift is observed between 620 and 595 nm. The ab initio calculation of the normal modes were carried out on the MD molecule to assign the IR bands near 1183 and 1136 cm-1, indicating that they are attributed by the vibrational modes...

Monolayers of a novel ionoselective butadienyl dye

The novel amphiphilic benzodithia-18-crown-6 butadienyl dye ( 1) forms relatively stable insoluble monolayers on distilled water (collapse pressure of 41 mN/m) and on aqueous subphases containing alkali metal or heavy metal salts (collapse pressures in the range of 27–38 mN/m, respectively). The dye 1 monolayer organization depends on chromophore association and interactions (especially complex formation) with heavy and alkali metal ions as deduced from surface pressure–area and surface potential–area isotherms as well as reflection spectra and Brewster angle microscopy observations. Dye 1 undergoes specific interactions with Hg 2+ and Ag +, respectively (formation of different complexes). Nonspecific interactions have been observed with other salts, such as KClO 4 or Pb(ClO 4) 2. Further, dye 1 monolayers on 1 mM Hg(ClO 4) 2 solution undergo reversible photoisomerization, in contrast to monolayers on water and other aqueous salt subphases.

Morphological and optical studies for a multi component monolayer of cyanine dye and fatty acid

The aggregation state of a mixed monolayer of cyanine dye and fatty acid during compression was investigated on the basis of surface pressure–area isotherm measurements, atomic force microscopic observations and spectroscopic measurements. The mixed monolayer was in a miscible state at all surface pressure values. The domains were formed in a matrix region by compression, which is probably due to ordering of molecular conformation. Compression at a surface pressure higher than the plateau pressure causes regularization of stacking of cyanine rings, resulting in the formation of J-aggregates.

Different J-Aggregates of an Amphiphilic Cyanine Dye in Monolayers at the Air−Water Interface

J-Aggregates of the amphiphilic symmetrical cyanine dye 5,5‘-6,6‘-tetrachloro-1,1‘-diethyl-3,3‘-dioctadecyl-benzimidazolocarbocyanine toluenesulfonate are formed in monolayers of the pure dye at the air−water interface as well as in mixtures with the coaggregates hexadecane (HD) and methyl octadecanoate (MOD), respectively, at a molar ratio of dye/coaggregate of 1:1. The monolayers have been characterized by surface pressure−area and surface potential−area isotherms as well as reflection spectroscopy. The position of the J-aggregate band that is red-shifted with respect to the monomer band depends on the kind of coaggregate. This in the case of symmetrical cyanines surprising phenomenon was analyzed by applying the extended dipole model according to the model of two-dimensional “brickstone” structure assuming that the coaggregate MOD in contrast to HD modifies chromophore packing.

Second harmonic generation from insoluble films of a rhodamine dye at the air–water interface: effect of sodium dodecylsulfate

The second harmonic generation (SHG) from the insoluble monolayers of bis-( N-ethyl, N-octadecyl)rhodamine perchlorate (RhC 18) formed on the surface of sodium dodecylsulfate (SDS) solutions of different concentrations has been studied. An enhancement of the second harmonic response was observed in the mixed films of RhC 18/SDS compared to the pure-dye layer. To clarify the origin of the phenomenon, the films were characterized by surface pressure–area isotherm and reflection–absorption spectroscopy studies. The analysis of surface pressure–area isotherms of RhC 18/SDS layers showed that incorporated SDS molecules essentially influence the rheological properties of the dye monolayer. The film parameters, such as the molecular surface area, maximum surface pressure, and solid-condensed phase composition, are the functions of SDS bulk concentration. A joint analysis of the SHG results and the reflection–absorption spectra revealed that the structural ordering within films was responsible for the enhancement of the nonlinear optical response, whereas the contributions from the spectral shifts and increased absorption upon aggregate formation are of less importance.

Supramolecular assemblies of photochromic benzodithia-18-crown-6 ethers in crystals, solutions, and monolayersElectronic supplementary information (ESI) available: crystal data, data collection, and structure solution and refinement parameters. See http://www.rsc.org/suppdata/nj/b1/b110630a/

We studied the assembly of dithiacrown ether styryl dye (CSD) molecules in crystals, solutions, and films in the presence of metal cations. X-Ray diffraction data allowed us to conclude that the anion affects the supramolecular architecture of CSDs in the crystal, specifically, the type of stacking of the dye molecules. In solution, in the presence of Pb2+, CSD molecules with the betaine structure spontaneously form dimeric complexes consisting of two dye molecules and two metal cations, with a fixed mutual arrangement of the double bonds. The dimer complex is stable due to coordination between the anion substituent of one molecule and the metal cation located in the crown ether cavity of the other molecule. Irradiation of the dimer complexes leads to regio- and stereoselective [2 + 2]-cycloaddition, giving only one cyclobutane derivative of the eleven theoretically possible products. The other photoreaction studied for CSDs is reversible Z–E isomerization. Due to its specific structure, the betaine-type CSD is able to form the ‘anion-capped’ Z-isomer. Intramolecular coordination in the ‘anion-capped’ isomer enhances its stability and causes a sharp deceleration of its dark Z–E isomerization. The amphiphilic CSD forms relatively stable monolayers on distilled water and various aqueous salt subphases. The results obtained indicate that it is possible to distinguish between two types of the dye monolayer structures based on the presence of alkali or heavy metal cations in the aqueous subphase.

Phase transitions in Langmuir monolayers of a rhodamine dye as studied by a second harmonic generation technique

Phase transitions in the Langmuir monolayer of a xanthene dye alkylated with two long hydrophobic chains have been studied by a second harmonic generation (SHG) technique at various temperatures. Owing to the sensitivity of the SHG technique to the molecular ordering at an interface, transitions between phases of different orientational organizations of dye chromophores have been observed in the shape of SHG signals. The method of the derivative of an SHG signal has been proposed to determine the points of phase transitions in the Langmuir monolayer. Obtained results are in good agreement with those derived from ( π– A) isotherms. Several additional orientational phase transitions have been found upon compression of a liquid phase of the layer. Detailed comparison of the SHG signals with the ( π– A) isotherms has allowed us to conclude that the phase transitions at a low surface pressure may not involve the rearrangement of dye chromophores. Formation of a denser phase likely occurs by means of the orientational rearrangement of flexible alkyl chain network of the layer. However, a transition to the solid phase is accompanied by profound changes in the chromophore arrangement. The amplitude of SHG signals generated by the solid phase increases with a decrease in the temperature, suggesting the formation of more ordered layers at lower temperatures.

Anion-capped benzodithia-18-crown-6 styryl dye monolayers

Abstract The anion-capped amphiphilic benzodithia-18-crown-6 styryl dye forms relatively stable insoluble monolayers on distilled water (collapse pressure is 25.5 mN m−1) and aqueous solutions containing alkali (25.3–26.4 mN m−1) and heavy metals (49.0 mN m−1 for Ag+). Two types of the dye monolayer structures (depending on alkali metal or heavy metal cations present in the aqueous subphase) have been discriminated from surface pressure–molecular area and surface potential–molecular area isotherms. The ionoselective and photosensitive properties of the amphiphilic dye in monolayers have been studied in the presence of K+, Na+, Mg2+, Ca2+ or Ag+ cations in the aqueous subphase.

Phase transition between different J-aggregate states of amphiphilic merocyanine dye molecules

The two-dimensional phase transition between different J-aggregate states in the amphiphilic merocyanine dye monolayer was investigated using a multi-purpose nonlinear optical microscope. Precise in-situ observations of the transition on cooling revealed the nucleation and growth process of the low temperature phase in the matrix of high temperature phase.