Optical Transition Moments Research Articles

Molecular orientation and optimization of membrane dyes based on conjugated oligoelectrolytes

Conjugated oligoelectrolytes (COEs) are amphiphilic, fluorogenic molecules that spontaneously associate with lipid bilayer membranes and are gaining attention as molecular reporters, particularly for exosome detection by flow cytometry. Questions nonetheless remain on how to best design COEs for optimal performance and on the geometry of lipid bilayer intercalation. In response, we designed a series of oligo-phenylenevinylene COEs with varying lengths and numbers of charged groups to address these uncertainties. Examination of the organization within lipid bilayers through polarized fluorescence microscopy shows that the optical transition moments are perpendicular to the bilayer plane, with the conjugated segment flanked by hydrophobic phospholipid tails. COEs initially form a disorganized layer on the vesicle periphery, reflecting electrostatic association before intercalation. Uptake experiments show that longer dimensions and increased numbers of charges allow for a higher degree of cellular association. Both shorter core length and increased number of charges accelerate the rate needed to achieve emission saturation.

Ordered arrangement of F4TCNQ anions in three-dimensionally oriented P3HT thin films

An ordered arrangement of electron-accepting molecular dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), in three-dimensionally (3D) oriented poly(3-hexylthiophene) (P3HT) film was clarified. The 3D oriented P3HT thin films prepared by the friction-transfer technique were doped with F4TCNQ by dipping into an acetonitrile solution. The presence of F4TCNQ anions in the 3D oriented P3HT thin films was investigated by polarized ultraviolet/visible/near-infrared absorption spectroscopy, grazing incidence X-ray diffractometry, polarized Fourier transform infrared spectroscopy (FT-IR), and infrared p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS). The F4TCNQ-doped 3D oriented P3HT films showed anisotropic properties in all characterizations. In particular, the anisotropic molecular vibrations from polarized FT-IR and pMAIRS have clearly revealed orientations of polymeric chains and molecular dopant molecules. Considering the results from several independent techniques indicated that F4TCNQ anions in the 3D oriented P3HT were orderly arranged in a 3D manner with respect to the 3D oriented P3HT such that their molecular long-axis parallel to the P3HT backbone, with in-plane molecular orientation. Additionally, the direction of the optical transition moment of the F4TCNQ anion was found to be parallel to the molecular short-axis.

Triplet state delocalization in a conjugated porphyrin dimer probed by transient electron paramagnetic resonance techniques.

The delocalization of the photoexcited triplet state in a linear butadiyne-linked porphyrin dimer is investigated by time-resolved and pulse electron paramagnetic resonance (EPR) with laser excitation. The transient EPR spectra of the photoexcited triplet states of the porphyrin monomer and dimer are characterized by significantly different spin polarizations and an increase of the zero-field splitting parameter D from monomer to dimer. The proton and nitrogen hyperfine couplings, determined using electron nuclear double resonance (ENDOR) and X- and Q-band HYSCORE, are reduced to about half in the porphyrin dimer. These data unequivocally prove the delocalization of the triplet state over both porphyrin units, in contrast to the conclusions from previous studies on the triplet states of closely related porphyrin dimers. The results presented here demonstrate that the most accurate estimate of the extent of triplet state delocalization can be obtained from the hyperfine couplings, while interpretation of the zero-field splitting parameter D can lead to underestimation of the delocalization length, unless combined with quantum chemical calculations. Furthermore, orientation-selective ENDOR and HYSCORE results, in combination with the results of density functional theory (DFT) calculations, allowed determination of the orientations of the zero-field splitting tensors with respect to the molecular frame in both porphyrin monomer and dimer. The results provide evidence for a reorientation of the zero-field splitting tensor and a change in the sign of the zero-field splitting D value. The direction of maximum dipolar coupling shifts from the out-of-plane direction in the porphyrin monomer to the vector connecting the two porphyrin units in the dimer. This reorientation, leading to an alignment of the principal optical transition moment and the axis of maximum dipolar coupling, is also confirmed by magnetophotoselection experiments.

Ultrafast multiphoton pump-probe photoemission excitation pathways in rutileTiO2(110)

We investigate the spectroscopy and photoinduced electron dynamics within the conduction band of reduced rutile $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surface by multiphoton photoemission $(\mathrm{mPP})$ spectroscopy with wavelength tunable ultrafast $(\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{fs})$ laser pulse excitation. Tuning the $\mathrm{mPP}$ photon excitation energy between 2.9 and 4.6 eV reveals a nearly degenerate pair of new unoccupied states located at $2.73\ifmmode\pm\else\textpm\fi{}0.05$ and $2.85\ifmmode\pm\else\textpm\fi{}0.05$ eV above the Fermi level, which can be analyzed through the polarization and sample azimuthal orientation dependence of the $\mathrm{mPP}$ spectra. Based on the calculated electronic structure and optical transition moments, as well as related spectroscopic evidence, we assign these resonances to transitions between $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}3d$ bands of nominally ${t}_{2g}$ and ${e}_{g}$ symmetry, which are split by crystal field. The initial states for the optical transition are the reduced $\mathrm{T}{\mathrm{i}}^{3+}$ states of ${t}_{2g}$ symmetry populated by formation oxygen vacancy defects, which exist within the band gap of $\mathrm{Ti}{\mathrm{O}}_{2}$. Furthermore, we studied the electron dynamics within the conduction band of $\mathrm{Ti}{\mathrm{O}}_{2}$ by three-dimensional time-resolved pump-probe interferometric $\mathrm{mPP}$ measurements. The spectroscopic and time-resolved studies reveal competition between 2PP and 3PP processes where the ${t}_{2g}\text{\ensuremath{-}}{e}_{g}$ transitions in the 2PP process saturate, and are overtaken by the 3PP process initiated by the band-gap excitation from the valence band of $\mathrm{Ti}{\mathrm{O}}_{2}$.

Measurement of weak optical transition moments through two-pathway coherent control

We present a full description of the (ω, 2ω) two-pathway coherent control interference technique for measurement of weak, highly forbidden optical transitions. We have recently applied this technique to the measurement of the magnetic dipole transition moment M1 of the 6s 2S1/2 → 7s 2S1/2 transition in atomic cesium. We also demonstrate detailed methods by which this technique may be used for measurement of the electroweak-induced parity nonconserving amplitude in atomic cesium. The principal benefits of this technique include reduced systematic errors related to field reversals often encountered in related measurements, reduced interference from adjacent, noninterfering transitions, and the absence of electric quadrupole transitions. We present a critical evaluation of the effects induced by stray fields and other systematic effects.

H− − H Collision Induced Radiative Transitions

Exchange interaction leads to the formation of gerade and ungerade states of temporary molecules (quasimolecules) formed during the H− +H slow collisions. The work deals with the radiation produced by optical transitions between those states. The main characteristics involved in the description of optical transitions in quasimolecules, i.e., energy terms, an optical dipole transition moments, have been calculated in the frame of zero-range potentials model. The main feature of calculations is that the results can be expressed analytically in closed forms via the Lambert W function.

Polarization Analysis of Microscopic Faraday Rotation of Thin Solid Ferrocene Aggregates

The Faraday rotation angles of ferrocene aggregates formed on a glass plate and a water surface were measured by a pulsed magnetic field Faraday-effect measurement system. The observed Faraday rotation of the aggregates showed a dependency on the incident polarization angle with a period of π/2, in contrast to the case of polarized absorption in the absence of a magnetic field, which gave a period of π. The polarization dependency of the Faraday rotation was analyzed in terms of the products of trigonometric functions with the angle of the transition moment of the ferrocene aggregate, inspired by the equations for the Faraday A and B terms. From the polarization angle dependencies in the presence and absence of pulsed magnetic fields, the differences in angles between the optical transition moment and the magneto-optical transition moment were evaluated, and they suggested that the ferrocene molecules in the aggregate formed on a glass plate were oriented more parallel to the horizontal glass surface than those in the aggregate formed on a water surface.

An isolated line-shape model based on the Keilson and Storer function for velocity changes. I. Theoretical approaches

This paper presents new results for the modeling of isolated line shapes from the Doppler to the collisional regime, thus including the effects of confinement (Dicke narrowing) and of the speed dependence of collisional parameters. They are obtained within a classical description of the time evolution of the autocorrelation function of the optical transition moment, combined with the use of the Keilson and Storer model for the changes in the radiator translational velocity. A purely numerical solution to the subsequent differential equations, which uses discretized grids for the radiating-molecule velocity vector, is first described. An alternative approach, using projections onto generalized Laguerre polynomials (for the velocity modulus) and spherical harmonics (for the velocity orientation), is also presented. A first test of these approaches is made in the particular case of the Q(1) Raman line of the fundamental band of pure H(2) at room temperature (others will be presented in paper II). It is shown that the two models lead to exactly the same results, as expected, and to satisfactory agreement with measured values of the linewidth at various densities.

Magnetoelectric emission in rare-earth doped ferroelectric crystalsLa2Ti2O7:R3+(R=Er, Eu, and Nd)

The optical magnetoelectric (OME) effect, i.e., the change of optical response on the reversal of light propagation vector $(\mathbit{k})$, has been investigated for the emission of rare earth ${R}^{3+}$ ion ($R=\text{Nd}$, Eu, Er) doped in a ferroelectric ${\text{La}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ single crystal under magnetic field $(\mathbit{H})$. The symmetry condition for the appearance of the OME effect for $\mathbit{H}\ensuremath{\perp}\mathbit{k}$ was confirmed by varying the relative angle between the electric polarization and magnetic field. Another tensor component of the second-order magnetolectric tensor ${\ensuremath{\beta}}_{ijk}$ for $\mathbit{H}\ensuremath{\parallel}\mathbit{k}$, i.e., the magnetochiral effect, is allowed in the Faraday configuration but found to be small compared with the OME effect in the Voigt configuration. The importance of the spin-orbit coupling, the magnetic dipole transition, and the noncentrosymmetric crystal structure is discussed as the origin of the OME effect on the basis of the observed signal magnitude depending on the species of the rare-earth ion and its optical transition moment.

Light absorption spectrum of two-dimensional Mott insulators

We theoretically investigate the light absorption spectrum of two-dimensional (2D) Mott insulators. We employ the numerical diagonalization method to calculate the light absorption spectrum with using the effective Hamiltonian for the extended Hubbard model, which is valid when the on-site Coulomb interaction energy $U$ is much larger than the nearest-neighbor transfer integral $t$ and the nearest-neighbor Coulomb interaction energy $V$. For $V=0$, the absorption spectrum consists of a broad band with a width of about $16t$ and a sharp central peak, and the energy eigenstates contributing to the absorption spectrum do not have the antiferromagnetic (AF) spin order, when $U$ is sufficiently larger than $t$. These features result from the spin-charge interplay inherent in the very strong correlation region where charge transfer term is dominant. With decreasing $U∕t$, the peak structure in absorption spectrum becomes unclear and some low energy eigenstates have the AF spin order, as a result of the spin-spin interaction term. For large $V∕t$, the dominant peak arises in the lower energy region of the spectrum. A large number of holon-doublon bound states, which have nearly the same charge but different spin structures, are responsible for this peak, in contrast to the conventional exciton state. This is also in contrast to the charge bound states in one-dimensional (1D) Mott insulators, where a single energy eigenstate dominates the optical transition moment as a result of the spin-charge separation. The essentially different absorption spectra between the 1D and 2D Mott insulators originate from the difference in the coupling between spin and charge degrees of freedom.

Cation substitution effects on structural, electronic and opticalproperties of nonlinear optical AgGa(SxSe1−x)2crystals

The structural, electronic and optical properties of tetragonalnonlinear optical (NLO) crystals, AgGa(SxSe1−x)2(x = 0.0,0.25, 0.5, 0.75, and 1.0), were investigated theoretically and experimentally. Theresults obtained indicated that the electronic bandgaps, optical properties andbulk moduli of these compounds were linearly dependent on the substitutionconcentration of cations. From partial density of state analysis, it was found thatthe electronic states near the band edges of AgGa(SxSe1−x)2were a simple proportional mixture of the atomic orbitals of sulfur andselenium. A cell-volume effect was proposed as the major cause of the lineardependence of material properties on the substitution concentration. It wascalculated that the second-order NLO susceptibilities were scaled withthe cubic power of bandgap, although a minor deviation existed. Thisdeviation arose from the optical transition moment products.

Novel Push−Pull Phthalocyanines as Targets for Second-Order Nonlinear Applications

The second harmonic generation (SHG) hyperpolarizabilities of a family of peripherally substituted push−pull phthalocyanines 1, 2, and 3, having an ethynediyl subunit in the linking bridge between the acceptor and donor moieties, have been measured. Electric field induced second harmonic (EFISH) generation experiments at 1.064 and 1.900 μm and hyper-Rayleigh scattering (HRS) experiments at 1.064 μm have been carried out. The quadratic hyperpolarizabilities values derived from these experiments are quite significant and superior to those found for similar push−pull compounds lacking of a triple C-bond in the donor−acceptor path. The main electronic parameters of the three compounds (electric dipole moments, orbital electronic distributions, and optical transition moments) have been calculated using the ZINDO/S method for molecular geometries that have been optimized through the PM3 method. Excellent agreement with spectroscopic data has been achieved. The SHG results have been analyzed through a 3-level model associated to the ground level and two split levels responsible for the Q-band, assuming either strict (for compound 2) or approximate (for compounds 1 and 3) C2v planar symmetry. The parameters used in the model were those obtained from the electronic calculations. For molecule 2, both the theoretical diagonal and off-diagonal elements of the beta tensor, as well as βEFISH and βHRS, have been determined. Good agreement with the experimental HRS values is obtained, whereas EFISH values considerably differ from those obtained from the simple expression for γEFISH ignoring the electronic contributions.

Raman Anisotropy Measurements: An Effective Probe of Molecular Orientation in Conjugated Polymer Thin Films

AbstractUnderstanding the molecular alignment of conjugated polymers within thin‐film samples is essential for a complete picture of their optical and transport properties, and hence for the continued development of optoelectronic device applications. We report here on the efficacy of Raman anisotropy measurements as a probe of molecular orientation, presenting results for aligned polyfluorene nematic glass films. Comparison is made with the results of optical dichroism measurements performed on the same samples. We show that in many cases molecular orientation can be more directly characterized by Raman anisotropy, and that it can have a greater sensitivity to the degree of molecular orientation than conventional optical dichroism. The fact that the Raman measurements can be readily performed on the same thin films (∼ 100 nm thickness) that are required for optical dichroism means that there is no ambiguity in a direct comparison of results. This situation differs from that for standard X‐ray diffraction measurements (these require film thicknesses of several μm) and electron diffraction or electron energy loss spectroscopy measurements (these require film thicknesses of 10 nm or less). The Raman data allow the angle (relative to the chain axis) for the optical dipole transition moment to be deduced from the dichroic ratio, and confirm the role that its off‐axis component plays in limiting this ratio. The added fact that Raman anisotropy data can be collected in situ, in reflection geometry for standard device structures, and with microscopic resolution and chemical specificity makes the technique even more attractive as a non‐invasive device probe.

Magnetophotoselection Study of the Carotenoid Triplet State inRhodobacter sphaeroidesReaction Centers

The triplet state of the carotenoid spheroidene, a pigment cofactor of reaction centers (RCs) from the photosynthetic bacterium Rhodobacter sphaeroides 2.4.1., was studied with time-resolved direct-detection EPR under polarized light excitation (magnetophotoselection). Four types of magnetophotoselection experiments, differing by excitation wavelength and temperature, enabled determination of the orientations of the optical transition moments of the reaction center spheroidene and the primary donor relative to the principal triplet axes of these molecules. Making use of the projections of the primary donor principal triplet axes onto the X-ray structural coordinate system determined earlier, we were able to calculate for the first time the orientation of the spheroidene optical transition moment and its principal triplet axes relative to the RC molecular frame. The same type of experiment on carotenoidless reaction centers from Rb. sphaeroides R26 reconstituted with spheroidene demonstrated a significant difference between the native and reconstituted molecules with regard to the orientation of the optical transition moment of the carotenoid relative to its triplet principal axes frame.

Deconvolution and assignment of different optical transitions of the blue copper protein azurin from optically detected electron paramagnetic resonance spectroscopy.

Magnetic circular dichroism is a powerful spectroscopic tool for the assignment of optical resonance lines. An extension of this technique, microwave-modulated circular dichroism, provides additional details, in particular information about the orientation of optical transition moments. It arises from magnetization precessing around the static magnetic field, excited by a microwave field, in close analogy to electron paramagnetic resonance (EPR). In this paper we investigate the visible and near-infrared spectrum of the blue copper protein Pseudomonas aeruginosa azurin. Using a nonoriented sample (frozen solution), we apply this technique to measure the variation of the optical anisotropy with the wavelength. A comparison with the optical anisotropies of the possible ligand-field and charge-transfer transitions allows us to identify individual resonance lines in the strongly overlapping spectrum and assign them to specific electronic transitions. The technique is readily applicable to other proteins with transition metal centers.

Absorbance Detected Magnetic Resonance Spectra of the FMO Complex ofProsthecochlorisaestuariiReconsidered: Exciton Simulations

The isotropic and linear dichroic triplet−singlet difference spectra of the Fenna−Matthews−Olson (FMO) complex of the green sulfur bacterium Prosthecochloris aestuarii measured with absorbance detected magnetic resonance (ADMR) are presented. The absorption, linear dichroic (LD), circular dichroic (CD), triplet-minus-singlet (T−S), and LD(T−S) spectra are simultaneously analyzed by means of an exciton model. In this analysis the dependence of the T−S(α) spectrum on the angle α between the optical and microwave transition moments was taken into account. The present simulation procedure yields fits of the absorption and LD spectra of FMO that are about equally good as those in the literature, while the fits of the CD, T−S, and LD(T−S) spectra show significant improvements. We find that the interaction energies are roughly 25% higher than in previous analyses. The simultaneous fit of all available optical (difference) spectra with correct procedures offers a precise and detailed description of pigment−pigment interactions in the FMO complex.

The influence of conformation and energy gaps on optical transition moments in donor–acceptor biphenyls

Transition moments of absorption, M a, and fluorescence, M f, of three donor–acceptor biphenyls ( I– III) are compared with those obtained by quantum chemical calculations to verify the previous conclusions of photoinduced twisting relaxations. The configurational analysis in terms of a state interaction model (analysis of electronic structure using reference states) reveals that existing experimental formalisms are insufficient to quantitatively explain the remarkably strong coupling of the electron transfer state ( 1 ET ) with 1 L a and S 0. Using AM1/CI optimized geometries, excited state structural relaxations other than twisting (e.g. bond-length shortening) are shown to lead to a better π-conjugation connected with increased M f values. On the other hand, ZINDO/S-CI calculations show that solvent polarity induced energy gap dependent S 1 decoupling from 1 L a reduces M f especially for highly twisted rotamers. Nevertheless, in the experimental range of energy gaps between 1L a and 1ET of around 10 000 cm −1, the polarity induced decrease of M f plays a minor role. Due to this result, the experimental M f values in polar solvents are indeed consistent with a photoinduced relaxation of II towards planarity and of III towards perpendicularity.

Electric linear dichroism. A powerful method for the ionic chromophore–colloid system as exemplified by dye and montmorillonite suspensions

The electric linear dichroism (ELD) measures the anisotropic absorption of a chromophore in solution under externally applied electric pulse field, which orients the solute molecule (particle) rotationally to the field direction. The ELD method is used to study at 20°C the binding mode and electrooptic properties of a nonlinear optical molecule (DAES), 4-[2-[4-(dimethylamino)phenyl]ethenyl]-1-ethylpyridinium, in the presence of montmorillonite K10 (MK-10) in the absorption region at mixing ratios ( D/ S) between 0.1 and 0.97, where D is the concentration of DAES and S is the cation exchange capacity of MK-10. Descriptions are given on a new home-built ELD apparatus. The measured quantities are the wavelength (600–300 nm) and field-strength (0–3 kV cm −1) dependence of the reduced dichroism, Δ A/ A. The optical transition moment directions of bound DAES is determined from the observed ELD spectra relative to the symmetry axis of the field-oriented disklike MK-10 particle. Discussion is given both on the field-strength dependence of Δ A/ A and on the ELD spectral analysis in terms of the tilt, roll, and inclination angles of the molecular plane of DAES bound to the surface and/or interlayer of MK-10 particle. The DAES molecule is not bound flatly on the surface or between the layers, but inclined at 35–39°, tilted by ±21°, and rolled by ±(27–33)° relative to the clay plane. The DAES–MK-10 system is no more associative than the clay itself on the basis of the electric-dichroism-average relaxation time, evaluated from the field-off decay signal, of DAES bound to MK-10 particle in solution.

Magnetophotoselection Study of the Lowest Excited Triplet State of the Primary Donor in Photosynthetic Bacteria

The first time-resolved magnetophotoselection study of the primary donor triplet state (3P) in bacterial reaction centers from Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis is reported. With direct excitation of the primary donor, this approach provides the orientation (spherical coordinates δ, colatitude, and γ, longitude) of the excited optical transition moment in the principal triplet axis system. It is essentially free from the effects of spin−lattice relaxation of 3P, enabling magnetophotoselection measurements over a wide temperature range. Two independently measured triplet-state spectra, excited with light polarized parallel and perpendicular to the EPR magnetic field, are simulated with the same set of parameters. This procedure results in a high precision (ca. ±5° with sufficient signal/noise ratio) of the obtained spherical coordinates of the optical transition moment vector. We find δ = 80 ± 5°, γ = 70 ± 5° and δ = 75 ± 5°, γ = 70 ± 5° for Rb. sphaeroides R26 and Rps. viridis, respectively. We demonstrate that excitation of the sample with nonpolarized light is essentially nonisotropic. Neglect of this effect in spectral simulations of light-induced signals may lead to considerable error in the parameters determined.

Circular Dichroic Triplet−Singlet Difference Spectroscopy. 1. Analysis of Dichroic Components in Optically Detected Magnetic Resonance

Linear and circular dichroic triplet-minus-singlet spectroscopy (LD(T−S) and CD(T−S)) are highly sensitive methods for determining the angles between the optical and microwave transition moments, the strength of molecular interactions, and band assignments of a wide range of biological systems. Both LD(T−S) and CD(T−S) spectra can in principle be measured via dichroic optically detected magnetic resonance (ODMR), a complex measurement method involving a sample with both linear and circular anisotropies, imperfect light modulation, possibly misaligned components, optical elements at low temperatures, and a partially polarized light source. We report a theoretical analysis of the influence of these effects on the measured spectra, via a Stokes−Mueller analysis of the optical system used for recording dichroic-ODMR spectra. From the theoretical results we develop and test experimental strategies to ensure the reliability and measurability of LD(T−S) and CD(T−S) spectra.