Orientation Of Transition Moments Research Articles

Designing a Propellane-based Nonlinear Optically Active System Absorbing in Three Different Wavelength Regions.

The aim of this work is to demonstrate the possibility of using propellane in designing a molecule that can absorb in three different wavelength regions and their nonlinear optical (NLO) activity can be fine-tuned by varying the three wings. We considered 22 tailor-made propellane derivatives consisting of phenyl, naphthyl, and biphenyl wings for this purpose. Using the state-of-the-art linear and quadratic response methods within TD-DFT and RI-CC2 theories and a suitable generalized few-state model that quantifies the effect of orientation of different transition moments on NLO properties, we discussed how and why the linear and nonlinear optical activity of propellane vary when the three wings are assembled successively to construct a full-propellane.

Determining the orientation of transition moments and depolarization by fluorescence polarizing angle spectrum.

In this paper, fluorescence polarizing angle spectrum, combined with degree of polarization(DOP), is proposed to determine the spatial orientation of transition dipole moments (TDMs) and depolarization of chlorophyll in solution. It is found that, due to the oriented TDMs under polarized laser excitation, the projections of angle of polarization(AOP) and DOP on the three orthogonal planes are different from each other. Experiments demonstrate that we can acquire the spatial orientation by detecting the projections of AOP on two orthogonal planes (xOz and yOz). Meanwhile, The depolarization can also be determined by the DOP spectrums. The validity of this method has been verified by another projection on the xOy plane.

Identifiability analysis of rotational diffusion tensor and electronic transition moments measured in time-resolved fluorescence depolarization experiment.

The subject of this paper is studies of the deterministic identifiability of molecular parameters, such as rotational diffusion tensor components and orientation of electronic transition moments, resulting from the time-resolved fluorescence anisotropy experiment. In the most general case considered, a pair of perpendicularly polarized emissions enables the unique determination of all the rotational diffusion tensor's principal components. The influence of the tensor's symmetry and the associated degeneration of its eigenvalues on the identifiability of the electronic transitions moments is systematically investigated. The analysis reveals that independently of the rotational diffusion tensor's symmetry, the transition moments involved in photoselection and emission processes cannot be uniquely identified without a priori information about their mutual orientation or their orientation with respect to the principal axes of the tensor. Moreover, it is shown that increasing the symmetry of the rotational diffusion tensor deteriorates the degree of the transition moments identifiability. To obtain these results analytically, a novel approach to solve bilinear system of equations for Markov parameters is applied. The effect of the additional information, obtained from fluorescence measurements for different molecular mobilities, to improve the identifiability at various levels of analysis is shown. The effectiveness and reliability of the target analysis method for experimental determination of the molecular parameters is also discussed.

Influence of shear processing on morphology orientation and mechanical properties of styrene butadiene triblock copolymers

The influence of ram extrusion on structure and mechanical properties of a triblock copolymer consisting of polystyrene (S) outer blocks and poly(styrene–stat–butadiene) (S/B) middle block is studied for a wide range of shear rates. Structural features on the mesoscale (10–100 nm) are investigated by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Transition Moment Orientation Analysis (TMOA) is applied to quantify the orientation on the molecular (segmental) scale (<1 nm). All extruded samples microphase-separate and show a lamellar morphology with periodicities of about 33 nm. Significant orientation is observed on the mesoscale where the surface normals of the lamellae are preferentially perpendicular to the extrusion direction. The corresponding degree of orientation drops slightly at elevated shear rates of about 600 s−1. Interestingly, Chevron-like pattern with two preferred orientations of the lamellae are observed in cross-sections probably due to shear velocity gradients in the rectangular die. In contrast, significant orientation on the molecular scale is absent for styrene and butadiene units indicating basically random orientation of the chain segments. The mechanical properties are, however, strongly anisotropic. Uniaxial tensile tests performed parallel and perpendicular to the extrusion direction reveal higher E∥ moduli (1.1 – 0.6 GPa) along with yielding but significantly smaller E⊥ moduli (100−250 MPa) without pronounced yielding. Main trends in both moduli, E∥ and E⊥, can be explained based on mesoscale orientation using the analytical composite model. In general, the results demonstrate that orientation effects on the mesoscale have a strong influence on the mechanical properties and must be considered during the optimization of extruded or injection-molded components made from microphase-separated block polymers.

The effects of annealing process influence on optical properties and the molecular orientation of selected organometallic compounds thin films

The paper presents the optical properties of four metallophtalocyanines (MPcs, M=Cu, Co, Mg and Zn) and two metallophtalocyanine chlorides (MClPcs, M=Al, Ga) thin films. Investigated films were fabricated by Physical Vapor Deposition (PVD) in high vacuum onto quartz substrates. After fabrication both MPcs and MClPcs thin films were undergone an annealing process in ambient atmosphere for 12h at temperature equal 150°C or 250°C. The absorbance spectra were measured in range 190–1100nm to investigate the optical and structural properties. Theoretical model of physical dimer was used to explain experimental results. The position and shape of the main absorbance peak (Q-band) in these materials are compared and discussed, taking into consideration the molecular arrangement and the longitudinal contribution which depends on the transition moment orientation. It was found that annealing process changing both optical and structural properties of MPcs and MClPcs comparing to samples without applying the process.

IR transition moment orientational analysis on semi-crystalline polyethylene films

The recently developed approach of Transition Moment Orientation Analysis is employed to determine the three dimensional orientation and order of the different molecular moieties in semi-crystalline polyolefins. Based on IR transmission spectra for varying polarisation and inclination the quadratically averaged orientation distribution of the different IR transition moments is deduced. By that, the full order parameter tensor and its orientation with respect to the sample coordinate system is obtained.

Long-Distance FRET Analysis: A Monte Carlo Simulation Study

A new method for extending the utilizable range of Förster resonance energy transfer (FRET) is proposed and tested by the Monte Carlo technique. The obtained results indicate that the efficiency of FRET can be significantly enhanced at a given distance if the energy transfer takes place toward multiple acceptors that are closely located on a macromolecule instead of a single acceptor molecule as it is currently used in FRET analysis. On the other hand, reasonable FRET efficiency can be obtained at significantly longer distances than in the case of a single acceptor. Randomly distributed and parallel orientated acceptor transition moments with respect to the transition moment of the donor molecule have been analyzed as two extreme cases. As expected, a parallel orientation of donor and acceptor transition moments results in a more efficient excitation energy transfer. This finding could be used to directly reveal the assembly/deassembly of large protein complexes in a cell by fluorescence microscopy.

On the Excited State Dynamics of Vibronic Transitions. High-Resolution Electronic Spectra of Acenaphthene and Its Argon van der Waals Complex in the Gas Phase

Rotationally resolved fluorescence excitation spectroscopy has been used to study the dynamics, electronic distribution, and the relative orientation of the transition moment vector in several vibronic transitions of acenaphthene (ACN) and in its Ar van der Waals (vdW) complex. The 0(0)(0) band of the S(1) ← S(0) transition of ACN exhibits a transition moment orientation parallel to its a-inertial axis. However, some of the vibronic bands exhibit a transition moment orientation parallel to the b-inertial axis, suggesting a Herzberg-Teller coupling with the S(2) state. Additionally, some other vibronic bands exhibit anomalous intensity patterns in several of their rotational transitions. A Fermi resonance involving two near degenerate vibrations has been proposed to explain this behavior. The high-resolution electronic spectrum of the ACN-Ar vdW complex has also been obtained and fully analyzed. The results indicate that the weakly attached argon atom is located on top of the plane of the bare molecule at ~3.48 Å away from its center of mass in the S(0) electronic state.

Direct energy transfer from conjugated polymer to DNA intercalated dye: Label-free fluorescent DNA detection

The development of methods for DNA detection is of importance in disease diagnosis, gene-targeted drug discovery and molecular biology field. In this paper, we synthesize a new cationic water-soluble CP containing fluorene moiety and flexible ethylenic moiety in the backbone (PFV) for label-free DNA detection. The conformational freedom of PFV provides stronger interactions with double-stranded DNA (dsDNA) and optimizes the orientation of transition moments between PFV and ethidium bromide (EB) intercalated in dsDNA. The efficient FRET from PFV (donor) to EB (acceptor) intercalated in dsDNA is observed and the emission of EB is amplified by the good light-harvesting ability of conjugated polymers. The interactions between PFV and DNA can also be probed by measuring the FRET ratio between PFV and EB intercalated in DNA. In comparison to other DNA detection assays based on FRET and conjugated polymers, synthesis of dye-labeled DNA probe is avoided in our method, which significantly reduces the cost and the synthetic complexity. The PFV/dsDNA/EB system provides promising applications on DNA detection with a simply, fast and label-free manner.

Tryptophan orientation in model lipid membranes

Tryptophans in membrane proteins display strong preference for the lipid membrane interface and are important for anchoring proteins at the proper longitudinal level. Linear dichroism spectroscopy on indoles in shear-deformed liposomes has been used to show that this positioning is accompanied by an intrinsically adopted orientation, also observed for tryptophans in membrane-bound peptides. Similarities in orientation of different indoles suggest that tryptophan will adopt this orientation independent of the protein it is part of. From the orientation of indole electronic transition moments L a, L b and B b, a binding model is proposed where the indole long axis is ∼60–65° from the membrane normal and the indole plane is at an oblique angle. We propose that dipole–dipole interactions and steric constraints in the membrane hydrocarbon region determine positioning and orientation of tryptophans whereas hydrogen bonding and cation-π interactions with lipid head-groups, though contributing to the membrane affinity of indoles, are less important.

Determination of the average orientation of 4-phenylpyridine in nematic solvent by means of infrared linear dichroism: Study of its conformational dependence on the dihedral angle between aromatic rings

A method using IR linear dichroic measurements for determination of the average orientation of the twisted 4-phenylpyridine in a nematic solvent is proposed. By means of this approach the relationship between orientational characteristics of solute molecule and its conformational state in nematic phase has been studied. This method combines an interactive subtraction procedure of IR polarized spectra of 4-phenylpyridine for determination of its vibrational transition moment orientation and DFT quantum chemical calculations as additional source of information of mutual orientation of aromatic subunits of the guest molecule.

Van der Waals Bonding to a Molecule with π-Electron Conjugation: Styrene-Argon Complex Studied by Mass Selective High Resolution R2PI UV Spectroscopy and Ab-initio Calculations

Combining high-resolution mass-selective resonance-enhanced two-photon ionization spectroscopy analysed by genetic-algorithm-based computer-assisted rotational fit and high-level ab initio calculations we were able to determine the structure and the transition moment orientation of the styrene-Ar complex. The results demonstrate that Ar binds to the benzene ring of styrene. The binding site of Ar is 3.42 Å above the benzene-ring plane and is close to the C 6 symmetry axis of the aromatic ring, but slightly shifted to the vinyl group, which implies that the binding pattern is perturbed by the presence of the bonded to the benzene ring vinyl group. The transition moment ratio of the styrene-Ar complex is considerably different from the one of bare styrene this being a purely mass effect stemming from the reorientation of the principal axes of inertia upon the cluster formation. The red shift of the 00 0 origin band of the styrene-Ar2 complex is almost twice that of the single complex, indicating that the red-shift additivity rule holds in this case. On this basis we conclude that the second Ar atom is bound to the opposite side of the benzene ring.

Investigation of the peculiarities in the polarized reflectance spectra of some Tutton salt monoclinic single crystals using dispersion analysis

Dispersion analysis in the ν 3(SO 4 2−) frequency region of two Tutton salt monoclinic single crystals, K 2Ni(SO 4) 2·6H 2O and (NH 4) 2Ni(SO 4) 2·6H 2O, was performed. Employing analytical expressions for the dielectric tensor and the reflectance function valid for monoclinic crystals, oscillator parameters like, transversal mode frequencies and the corresponding attenuation constants, transition moments orientation and oscillator strengths as well as longitudinal mode frequencies and the corresponding transition moment orientations were obtained. These oscillator parameters were used in simulations in order to explain certain peculiarities in the reflectance spectra occurring with directional changes of the polarization vector. From the simulations it follows that the appearance of spectra with two totally overlapped bands (having quite different oscillator strengths) depends particularly on the orientation of the longitudinal transition moment of the mode with the smaller LO–TO splitting.

Fluorescence anisotropy decay for intramolecular two-state excited-state processes with added quencher in the presence of rotational diffusion: An identifiability analysis

A deterministic identifiability analysis of kinetic models of a reversible intramolecular excited-state process involving species-dependent rotational Brownian diffusion is performed for spherical and cylindrically symmetric rotors. It is assumed that the axes of rotation do not change upon interconversion, and that the rate constants are independent of orientation. Use of three quencher concentrations allows the determination of two internally consistent sets of quenching rate constants, combinations of excited-state deactivation/exchange rate constants, individual rotational diffusion coefficients, relative orientations of transition moments, and restricted spectral information. Assignment of each set obtained to a particular species is not possible without additional prior information.

Evidence for a C–H⋯π type weak interaction: 1 : 1 complex of styrene with acetylene studied by mass selective high-resolution UV spectroscopy and ab initio calculations

The 1 : 1 complex of styrene with acetylene has been studied by mass selective low- and high-resolution UV resonance-enhanced two-photon ionisation (R2PI) spectroscopy combined with genetic-algorithm-based computer-aided fit of the spectra with partial rotational resolution, and high level ab initio quantum chemistry calculations. Two stable conformeric geometries of the 1 : 1 complex of styrene and acetylene have been theoretically found: one with acetylene binding to styrene as a proton donor, and one with acetylene acting as a proton acceptor. From the analysis of the vibronic structure of the S1<-- S0 spectrum and the fit of the highly resolved spectrum of the 0 origin band of the complex it is shown that the favoured conformation is the one in which acetylene binds to the benzene ring of styrene through formation of a non-conventional hydrogen bond of C-H...pi type with no marked change of the transition moment orientation of styrene. The styrene moiety remains planar and the acetylene molecule is tilted by a small angle of 4 degrees relative to the C6 symmetry axis of the benzene ring, most likely due to the reduced symmetry of the benzene ring pi electrons rather than to a direct interaction with the vinyl group.

Specific and nonspecific interactions in a molecule with flexible side chain: 2-phenylethanol and its 1:1 complex with argon studied by high-resolution UV spectroscopy

Using high-resolution resonance-enhanced two-photon ionization spectroscopy in combination with genetic-algorithm-based computer-aided rotational fit analysis and ab initio quantum chemistry calculations we determined the conformational structure and transition moment orientation in 2-phenylethanol and its 1:1 clusters with argon. The results clearly demonstrate that the gauche structure of 2-phenylethanol, which is stabilized by the intramolecular pi-hydrogen bond between the folded side chain and the benzene ring, is the most abundant in the cold molecular beam. In this conformer the transition moment is rotated by 18 degrees from the short axis of the aromatic ring. Two distinct 1:1 complexes of 2-phenylethanol with argon in a cis- and trans-configuration with respect to the side chain have been found. Employing the Kraitchman [Am. J. Phys. 21, 17 (1953)] analysis we have found that the structure of the 2-phenylethanol moiety and the orientation of the transition moment do not change after the complexation with argon within the experimental accuracy. From the measured band intensities we conclude that in addition to the dispersion interaction of the argon atom with the aromatic ring a hydrogen-bond-type interaction with the terminal -OH group of the side chain stabilizes the cis-structure of the 1:1 complex of 2-phenylethanol with argon.

Measured Davydov splitting in oligothiophene crystals

The polarized absorption spectra of single crystals of oligothiophenes in a wide spectral range are reported. The experimental procedure is discussed, underlying several details which are relevant to obtain reliable spectra particularly for samples of increasing thickness. On the basis of these considerations, it has been possible to fully detect the transition to the upper Davydov exciton originating from the first molecular state. The position and shape of the main exciton peak in these materials are compared and discussed, taking into consideration the molecular arrangement and the longitudinal contribution which depends on the transition moment orientation. The Davydov splitting values as deduced from the experimental data at room temperature are also reported either for the first vibronic replica or for the electronic transition as a whole. The difference between the purely transverse and the measured Davydov splitting is discussed.

Twisted intramolecular charge transfer states: Rotationally resolved fluorescence excitation spectra of 4,4′-dimethylaminobenzonitrile in a molecular beam

We report the observation at high resolution of seven vibronic bands that appear within approximately 200 cm(-1) of the electronic origin in the S(1)-S(0) fluorescence excitation spectrum of 4,4'-dimethylaminobenzonitrile (DMABN) in a molecular beam. Surprisingly, each band is found to be split into two or more components by a (coordinated) methyl group tunneling motion which significantly complicates the analysis. Despite this fact, high quality [(Observed-Calculated)< or =30 MHz] fits of each of the bands have been obtained, from which the rotational constants, inertial defects, torsion-rotation interaction constants, methyl group torsional barriers, and transition moment orientations of DMABN in both electronic states have been determined. The data show that DMABN is a slightly pyramidalized (approximately 1 degree) but otherwise (heavy-atom) planar molecule in its ground S(0) state, and that its electronically excited S(1) state has both a more pyramidalized (approximately 3 degrees) and twisted (approximately 25 degrees) dimethylamino group. Large reductions in the methyl group torsional barriers also show that the S(1)<--S(0) electronic transition is accompanied by significant charge transfer from the nitrogen atom to the pi* orbitals of the aromatic ring. Thereby established is the participation of all three vibrational coordinates in the dynamics leading to the "anomalous" emissive behavior of DMABN in the condensed phase.

Optical Characterization of Oligo(phenylene−ethynylene) Self-Assembled Monolayers on Gold

Vibrationally resonant sum-frequency generation (VR-SFG) and spectroscopic ellipsometry (SE) have been used to characterize self-assembled monolayer films of unsubstituted and mononitro-substituted oligo(phenylene−ethynylene) molecules on vapor-deposited Au substrates. When combined with quantum chemical calculations of the relevant transition moment directions, orientation distributions and electronic excitation spectra are obtained. The orientation distribution from VR-SFG is in good agreement with previous IR reflection studies, indicating both molecules are tilted from the surface normal by ∼30°. The calculated resonant hyperpolarizabilities are in good agreement with experimental spectra. The optical polarizability extracted from SE suggests strong intermolecular interactions, consistent with molecular exciton theory.

Infrared Dichroism of Isotope-edited α-Helices and β-Sheets

Isotope editing of amide infrared bands not only localises secondary structural elements within the protein but also yields conformational information that is not available from the linear dichroism of aligned samples without isotope editing. The additional information that can be derived on the orientational distribution of α-helices in membranes by the combined use of different amide bands and several positions of labelling is presented here. Also, the relationship between the azimuthal orientation of the transition moment and the protein structure is treated explicitly. A comprehensive analysis of the infrared dichroism for β-sheets and β-barrels is given here, for the first time. The orientation of the individual transition moments in a β-sheet that is essential for this analysis is derived for the different amide bands.