Orientation Of Chromophores Research Articles

Photorefractive effect in an azobenzene chromophore side groups polymer

We have analyzed the photorefractive (PR) effect of a polymer composite was reported which combines a novel bi-functional poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl (PVNPAK) and 2,4,7-trinitro-9-fluorenone (TNF). PVNPAK was synthesized by a post-azo-coupling reaction, with an azo derivative as the electrooptic chromophore and carbazolyl as photoconductive moiety. The asymmetric two-beam coupling gain of 13.9 cm−1 and diffraction efficiency of 1.2% for poled polymer film fabricated using a corona poling are obtained at the wavelength of 647.1 nm, confirming photorefractivity. We interpreted this result as the orientational enhancement, in which the spatial charge field may enhance the modulated orientation of the azobenzen chromophore. It is unexpected that the photorefractive gain of 9.5 cm−1 for the unpoled polymer film also was observed without external field in two-beam coupling (TBC) experiment. This phenomenon is attributed to a light-induced orientational grating when the azobenzene groups are illuminated by polarized light.

TERS microscopy as a probe for visualizing the orientation of chromophores embedded in the glassy polymers

Photo-induced effects on azobenzene-functionalized polymer thin films under exposure to strongly focused higher laser modes are investigated. Tip-enhanced Raman scattering microscopy provides a possibility for probing and visualizing the orientation of anisotropic chromophores embedded in the glassy environment at room temperature.

Molecular dynamics simulation of configurational ensembles compatible with experimental FRET efficiency data through a restraint on instantaneous FRET efficiencies.

Förster resonance energy transfer (FRET) measurements are widely used to investigate (bio)molecular interactions or/and association. FRET efficiencies, the primary data obtained from this method, give, in combination with the common assumption of isotropic chromophore orientation, detailed insight into the lengthscale of molecular phenomena. This study illustrates the application of a FRET efficiency restraint during classical atomistic molecular dynamics simulations of a mutant mastoparan X peptide in either water or 7 M aqueous urea. The restraint forces acting on the donor and acceptor chromophores ensure that the sampled peptide configurational ensemble satisfies the experimental primary data by modifying interchromophore separation and chromophore transition dipole moment orientations. By means of a conformational cluster analysis, it is seen that indeed different configurational ensembles may be sampled without and with application of the restraint. In particular, while the FRET efficiency and interchromophore distances monitored in an unrestrained simulation may differ from the experimentally-determined values, they can be brought in agreement with experimental data through usage of the FRET efficiency restraining potential. Furthermore, the present results suggest that the assumption of isotropic chromophore orientation is not always justified. The FRET efficiency restraint allows the generation of configurational ensembles that may not be accessible with unrestrained simulations, and thereby supports a meaningful interpretation of experimental FRET results in terms of the underlying molecular degrees of freedom. Thus, it offers an additional tool to connect the realms of computer and wet-lab experimentation.

Energy flow dynamics within cofacial and slip-stacked perylene-3,4-dicarboximide dimer models of π-aggregates.

Robust perylene-3,4-dicarboximide (PMI) π-aggregates provide important light-harvesting and electron-hole pair generation advantages in organic photovoltaics and related applications, but relatively few studies have focused on the electronic interactions between PMI chromophores. In contrast, structure-function relationships based on π-π stacking in the related perylene-3,4:9,10-bis(dicarboximides) (PDIs) have been widely investigated. The performance of both PMI and PDI derivatives in organic devices may be limited by the formation of low-energy excimer trap states in morphologies where interchromophore coupling is strong. Here, five covalently bound PMI dimers with varying degrees of electronic interaction were studied to probe the relative chromophore orientations that lead to excimer energy trap states. Femtosecond near-infrared transient absorption spectroscopy was used to observe the growth of a low-energy transition at ~1450-1520 nm characteristic of the excimer state in these covalent dimers. The excimer-state absorption appears in ~1 ps, followed by conformational relaxation over 8-17 ps. The excimer state then decays in 6.9-12.8 ns, as measured by time-resolved fluorescence spectroscopy. The excimer lifetimes reach a maximum for a slip-stacked geometry in which the two PMI molecules are displaced along their long axes by one phenyl group (~4.3 Å). Additional displacement of the PMIs by a biphenyl spacer along the long axis prevents excimer formation. Symmetry-breaking charge transfer is not observed in any of the PMI dimers, and only a small triplet yield (<5%) is observed for the cofacial PMI dimers. These data provide structural insights for minimizing excimer trap states in organic devices based on PMI derivatives.

Supramolecularly Engineered Functional π-Assemblies Based on Complementary Hydrogen-Bonding Interactions

Abstract As details of exciting photonic and electronic properties arising from self-assembled functional dyes and related π-conjugated molecules become clearer, strategies for control of short and long-range chromophore–chromophore orientations, regulation of π–π stacking arrangements, construction of self-organized nano-to-microscale architectures, diversification of materials properties, analysis of self-assembly pathways, and impartment of stimuli-responsive properties, are all becoming important issues in the research area of organic functional materials. This article focuses on the supramolecular assemblies of dyes and related π-conjugated systems whose organization processes are rationally controlled by complementary multiple hydrogen bonding (CMHB) interactions between melamine and barbituric acid/cyanurate functional units. The article includes an overview of the construction principle of such systems, but special attention is focused on the work carried out in the authors’s group (since 2002) using azobenzene, diarylethene, merocyanine, perylene bisimide dyes, and π-conjugated oligomers including oligo(p-phenylenevinylene), oligo(p-phenyleneethynylene), and oligothiophenes. The design principles discussed in this account could open up new avenues to novel functional aspect and application of functional dyes and small-molecular π-conjugated systems.

Theoretical Sum Frequency Generation Spectroscopy of Peptides.

Vibrational sum frequency generation (SFG) has become a very promising technique for the study of proteins at interfaces, and it has been applied to important systems such as anti-microbial peptides, ion channel proteins, and human islet amyloid polypeptide. Moreover, so-called “chiral” SFG techniques, which rely on polarization combinations that generate strong signals primarily for chiral molecules, have proven to be particularly discriminatory of protein secondary structure. In this work, we present a theoretical strategy for calculating protein amide I SFG spectra by combining line-shape theory with molecular dynamics simulations. We then apply this method to three model peptides, demonstrating the existence of a significant chiral SFG signal for peptides with chiral centers, and providing a framework for interpreting the results on the basis of the dependence of the SFG signal on the peptide orientation. We also examine the importance of dynamical and coupling effects. Finally, we suggest a simple method for determining a chromophore’s orientation relative to the surface using ratios of experimental heterodyne-detected signals with different polarizations, and test this method using theoretical spectra.

Triphenylamine photoconductive polymers for high performance photorefractive devices

Abstract Photorefractive performances of the composites using two kinds of photoconductive triphenylamine-based polymer have been compared and investigated. One polymer is poly(4-(diphenylamino)benzyl acrylate) (PDAA). The other is newly synthesized one of photoconductive acrylate polymer with methoxy substituted triphenylamine pendant, poly(4-((4-methoxyphenyl)(phenyl)amino)benzyl acrylate) (PMPAA). The methoxy substituent in PMPAA does not only shift the highest occupied molecular orbital (HOMO) level of the polymer, but also effectively enhances the chromophore orientation. Larger phase shift is confirmed by using the modified photoconductive polymer of PMPAA. The plasticizer of (4-(diphenylamino)phenyl)methanol (TPAOH) (IP = −5.64 eV) works as an effective trap in the PDAA (IP = −5.69 eV)-based composite, resulting in higher diffraction efficiency. Diffraction efficiency of 70% and fast response time of 25 ms (dominant) is measured at 532 nm under the moderate electric field of 45 V/μm.

Effect of Nanostructures on Corona Poling Assisted Second Harmonic Generation in Hybrid Organic-Inorganic Films

Hybrid organic-inorganic films are attractive materials for an alternative manufacture of photonic devices based on non-linear second-order optical processes. However, their efficiency and stability, closely related to their chromophores non-centrosymmetric orientation, still need some improvements. The use of nanostructures in this kind of materials gives place to a change in the chromophores surrounding medium; it can be used to improve the efficiency and the stability of the chromophores orientation in the films. In this work we show that some nanostructures are able to improve significantly the organic-inorganic SiO2 films second-order non-linear optical responses. Two kinds of nanostructures were studied: some nanostructures were just immersed, but the other ones were spontaneously induced in the films. The immersed nanostructures were multi-walled carbon nanotubes and the spontaneously induced nanostructures were formed in the films by means of surfactants self-assembly. The films were synthesized by the sol-gel method and all of the films were doped with Disperse Red 1 (DR1) dipolar chromophores. The induced nanostructures were lamellar and hexagonal long-range ordered nanostructures, detected by X-ray diffraction (XRD). The second-order non-linear optical properties of the films were studied in resonance through Corona poling assisted second harmonic generation. The results show significant improvements, for some kinds of nanostructures, in the non-linear optical responses of the films. In particular, those films containing Triton X-100 showed, at a poling temperature of 120°C, an improved second-order non-linear coefficient d33 with respect to equivalent non-nanostructured films; the d33 value of those films, determined by the Maker fringes technique, was equal to 21.4 pm/V.

Nonlinear optical materials by electrospinning technique

ABSTRACTThe possibility of using electrospinning to deposit, and simultaneously electrically pole, polymeric mats containing nonlinear optical (NLO) molecules is studied. Disperse Red 1 (DR1) chromophore dispersed into polymethyl methacrylate (PMMA) fibers is chosen as test system. Aligned fibers of DR1/PMMA are deposited by means of the rotating collector electrospinning technique. The processing parameters are optimized to obtain an oriented texture of fibers and the NLO activity is characterized by means of second harmonic generation measurements. Comparison with spin coating and corona poling samples of the same material is discussed. The results point toward a low efficiency of chromophore orientation for the electrospun samples in comparison to the poled ones. An improvement in the electrospinning apparatus is proposed to enhance the chromophore orientation in the polymeric fibers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40913.

On-off switching properties of one-dimensional photonic crystals consisting of azo-functionalized polymer liquid crystals having different methylene spacers and polyvinyl alcohol

In this paper, photoresponsive behavior of multi-bilayered films having precisely controlled layer thickness prepared by stacking an azo-functionalized polymer liquid crystal, PMAzXAc, and polyvinyl alcohol alternatively, PVA, is described. The multi-bilayered films were found to reflect a light of specific wavelength depending on the layer thickness and refractive index, and showed the reversible change in the reflection intensity by irradiation with visible and UV lights. The change in the reflection intensity was brought about by change in the molecular orientation of PMAzXAc between an out-of-plane orientation and a photo-induced isotropic state, and was strongly dependent on the number of methylene spacer of PMAzXAc linking the azobenzene side group with the acrylate polymer main chain. PMAz6Ac with hexa-methylene spacer showed the largest change in the reflection intensity, while smaller change in the reflection intensity was observed for PMAzXAc having shorter or longer methylene spacer than 6. The effect of the methylene spacers on the photochemical change in the molecular orientation of azobenzene chromophores in the multi-bilayered films will be discussed.

Polarized 3D Raman and nanoscale near-field optical microscopy of optically inscribed surface relief gratings: chromophore orientation in azo-doped polymer films

We have used polarized confocal Raman microspectroscopy and scanning near-field optical microscopy with a resolution of 60 nm to characterize photoinscribed grating structures of azobenzene doped polymer films on a glass support. Polarized Raman microscopy allowed determining the reorientation of the chromophores as a function of the grating phase and penetration depth of the inscribing laser in three dimensions. We found periodic patterns, which are not restricted to the surface alone, but appear also well below the surface in the bulk of the material. Near-field optical microscopy with nanoscale resolution revealed lateral two-dimensional optical contrast, which is not observable by atomic force and Raman microscopy.

Metal-ion induced FRET in macrocyclic dynamic tweezers

We report here about effective FRET process (73–99%) in mono-Mg2+ complexes of symmetrical crown ether bis(styryl) dyes. The FRET process has not been observed in the free state and in binuclear complexes. The formation of mononuclear complex provides two styrylic fragments with appropriate positions of absorption and emission bands for FRET. The other important parameter for FRET is the proper geometric orientation of both chromophores, which attain sandwich conformation with close positioning of complexed and free styryl fragments induced by ion-modulated geometry reorganization of the bis-dye.

Villain's fractal growth of poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt] J-aggregates onto layer-by-layer films and its effect on film absorbance spectrum

Morphology of poly(allylamine hydrochloride) and poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) layer-by-layer (LBL) films is shown to influence the orientation of PAZO chromophores with respect to solid support surface, which in turn is related with observed red-shifts changes of the chromophore absorbance peak position relative to that of solution spectrum, as the bilayers are being deposited. For the first bilayers, an increase of red shift values is observed, while roughness and grain radius are kept practically constant; after the 5th bilayer, the red-shift values decrease, while grain sizes increase and the number of grains decreases. This behavior is consistent with adsorption of coiled PAZO molecules, treated as pseudo-particles, with the chromophores head-to-head oriented-J aggregates. These aggregates adsorb perpendicularly to the substrate surface for the first layers and, as roughness and grain radius increase, the adsorption of the J aggregates takes place parallel to the solid support surface, which gives rise to a decrease in the red shift value. Moreover, the adsorption of these pseudo-particles follows a fractal growth characterized by a scaling exponent of α = 0.80 ± 0.02 and a temporal growth exponent of β = 0.17 ± 0.02. These values suggest a layer growth according with Villain model, which accounts for the interactions between deposited particles and the surface. This is in accordance with the electrostatic forces driving LbL film formation and accounts for the observed morphology behavior for the different number of layers.

All-optical poling and second harmonic generation diagnostic of layer-by-layer assembled photoactive polyelectrolytes

Layer-by-layer (LBL) self-assembling was employed to produce dye-containing multilayer films with a nonlinear optical maleic acid copolymer containing Disperse Red 1 moieties. By alternate adsorption of maleic acid copolymer derivative and polyethyleneimine, films made from 1 to 30 bilayers were prepared and characterized. The effect of salt addition to the polyelectrolyte on the LBL self-assembly process was also studied. Second order optical nonlinearity χ(2) of LBL films was studied by second harmonic generation and its stability was assessed by all-optical poling. A ‘restoring force’ model for the chromophore orientation in the LBL films is proposed to simulate the rotational motion. It clarifies the origin of the stability of the noncentrosymmetry in LBL films.

Singlet Oxygen Triplet Energy Transfer (STET) Based Imaging Technologies for Mapping Protein-Protein Proximity and Interactions in Intact Cells

Forster resonance energy transfer (FRET) has been widely used for the detection of protein-protein interactions but its detection distance is <10 nm. To overcome the short detection range of FRET, we have developed a new concept for molecular imaging: Singlet oxygen Triplet Energy Transfer (STET). We have engineered genetically encoded singlet oxygen generator (SOG) and singlet oxygen sensor (SOS). The proximity of protein X to protein Y can be assayed by fusing X to SOS and Y to SOG. Upon illumination, SOG is excited and converts molecular oxygen into singlet oxygen through triplet energy transfer. The singlet oxygen transfers to SOS through diffusion and chemically reacts with the sensor. The closer X and Y are, the faster SOS will respond to illumination of SOG. STET has a distance range (∼50-100 nm) an order of magnitude greater than that of FRET and does not depend on the orientation of chromophores. The utility of STET in mapping proximity is illustrated in a three-membrane-spanning protein complex that physically tethers ER and mitochondria in yeast. In fixed yeast cells, STET can detect any pair-wise protein proximity spanning ER, mitochondrial outer and inner membranes, and mitochondrial matrix (i.e. a detection range of >40nm). The strength of STET robustly reflects the relative distances between the protein pairs. In a separate set of experiments, we have demonstrated that STET can monitor the sub-second time course of the association of two proteins upon ligand induction inside live cells, highlighting the temporal resolution and single-molecule sensitivity of STET. Taken together, STET will find broad applications in (1) estimating the topology of a stable protein complex and (2) identifying and monitoring transient protein-protein interactions.

High levels of molecular orientation of surface azo chromophores can be optically induced even in a wet biological environment

We have developed polyelectrolyte multilayer bio-films containing azobenzene chromophores that enhance reversible photo-orientation upon irradiation with linearly polarized light, to effect surface photo-switching of adjacent biological systems. When conditions of film preparation and irradiation were optimized, we could observe the highest measured birefringence to date in amorphous systems (Δn > 0.2). This birefringence change to probe orientation was also for the first time measured and determined to be stable completely underwater, permitting optimization for in situ applications immersed in biological conditions.

Spectroscopic signatures of an Fmoc–tetrapeptide, Fmoc and fluorene

The hydrophobic Fmoc [N-(fluorenyl)-9-methoxycarbonyl] unit is often conjugated to peptides to confer amphiphilicity on the molecules, and to introduce the potential for aromatic stacking interactions. In this paper we report spectroscopic data for fluorene, Fmoc and Fmoc conjugated to a small peptide (GRDS in this case) to enable interpretation of spectroscopic data collected on complex structures that can be created with Fmoc–peptide molecules. We report absorbance data (ultra violet and infrared), circular dichroism, linear dichroism (both film and Couette flow orientation) and Raman spectra. Orientations of the Fmoc chromophores in the linear dichroism studies are deduced.

Molecular orientation and photoswitching kinetics on single-walled carbon nanotubes by optical second harmonic generation

Electrical conduction through chromophore-functionalized nanotubes can be modulated by light with wavelengths expected to isomerize the chromophores. Here, we use second harmonic generation to directly measure the orientation and photoisomerization kinetics of azo-benzene chromophores on single-walled carbon nanotubes. We find a net chromophore orientation with an average chromophore tilt angle of 40° ± 3°. We show that this angle can be reduced effectively to zero with an applied corona field. Periodic illumination with unpolarized 495 nm light induces reversible trans-cis switching, enabling the extraction of switching time scales both with and without an applied electric field.

Influence of the Continuous Application of an External DC Electric Field on Chromophore Orientations of Low T g-photorefractive Polymers

The influence of the continuous DC electric field on the chromophore orientations is investigated in a typical PVK based photorefractive polymer. The result from the modulation measurement with stabilized Mach-Zehnder interferometer shows that the degree of the chromophore orientation is not deteriorated by the migration of carrier in the case of our sample.

Relative Phase Change of Nearby Resonances in Temporally Delayed Sum Frequency Spectra.

Surface-selective sum frequency generation (SFG) spectroscopy has been previously shown to benefit from a finite time delay between two input laser pulses, which suppresses the nonresonant background and improves spectral resolution. Here we demonstrate another consequence of the time delay in SFG: depending on the magnitude of the delay, nearby resonances (e.g., vibrational modes) can "flip" their relative phase, i.e., appear either in-phase or out-of-phase with one another, resulting in either constructive or destructive interference in SFG spectra. This is significant for interpretation of the SFG spectra, in particular because the sign of the resonant amplitude provides the absolute molecular orientation (up vs down) of the vibrational chromophore. We present results and model calculations for symmetric and asymmetric CH-stretch modes of the methyl-terminated Si(111) surface, showing that the phase flip occurs when the delay matches half-cycle of the difference frequency between the two modes.