Polarized Emission Spectroscopy Research Articles

Perpendicular Alignment of 2D Nanoplatelet Emitters in Electrospun Fibers: A Result of the Barus Effect?

AbstractStable jet electrospinning (SJES) is a special form of optical fiber generation that prevents chaotic fiber whipping typical for conventional electrospinning procedures. Incorporation of highly emissive semiconductor nanoplatelets (NPLs) in such fibers has very high potential in optical data transmission, optological circuits, fiber lasers, solar light concentrators and many other fields because NPLs exhibit strongly directed emission from their surface plane due to various in‐plane transition dipole moments. However, potential orientation control of 2D‐NPLs in SJES is entirely unknown as electric fields and various mechanical forces contribute in a complex manner simultaneously. Here, the observation of counter‐intuitive yet very beneficial orientation of rectangular CdSe/CdS 2D‐NLP in SJES perpendicular to the fiber drawing axis is reported. Scanning electron microscopy, 3D‐single particle excitation polarization microscopy, 3D‐photogoniometry, polarized emission spectroscopy and small angle X‐ray scattering (SAXS) demonstrate aggregation free perpendicular alignment of the NPLs in poly(methyl methacrylate) (PMMA) fibers, resulting in dominant emission in directions parallel to the fiber. It is suggested that the observed vertical alignment is due to normal forces resulting from viscoelastic expansion when the polymer solution leaves the cannula (Barus effect) and that using such perpendicular nano‐emitter alignment forces allows for the generation of novel materials also beyond fibers.

Hyperspectral Imaging and Optical Trapping: Complementary Tools for Assessing Direction‐Dependent Polarized Emission from Single Upconverting LiYF4:Yb3+/Er3+ Microparticles

AbstractSingle‐particle fluorescent probes with the capacity to infer specific intracellular conditions, for instance, have great application potential in the realm of biomedicine. Imaging techniques that improve our understanding of the fluorescence processes at a single‐particle level are thus instrumental in actualizing this potential. This study demonstrates the importance of implementing synergistic single‐particle spectroscopic techniques to gain a more comprehensive understanding of the optical anisotropy exhibited by upconverting erbium and ytterbium co‐doped lithium yttrium tetrafluoride (LiYF4:Yb3+/Er3+) microparticles. More specifically, optical trapping and single‐particle polarized emission spectroscopy is herein leveraged to provide a plausible explanation for the spatial emission intensity distribution variation exhibited by LiYF4:Yb3+/Er3+ microparticles during hyperspectral imaging. By probing the polarized emission stemming from a single, optically trapped LiYF4:Yb3+/Er3+ microparticle, it is possible to find evidence that the emission intensity anisotropy exhibited by the respective microparticles during hyperspectral imaging arises as a consequence of the selection rules governing the emission probability in rare‐earth (RE3+) ions doped into a uniaxially birefringent host matrix such as LiYF4.

Dinuclear Triple-Stranded Helicates Comprising Al(III), Ga(III), or In(III) and a Hydrazine-Linked Bisiminopyrrolyl Ligand: Synthesis, Structure, Optical Resolution, and Chiroptical Properties

Abstract The synthesis of helical complexes is attracting increased attention because their chiral properties lead to a variety of functional applications. In this work, a series of neutral, dinuclear M(III) (M = Al, Ga, or In) triple-stranded helicates were prepared using a tetradentate bisiminopyrrole ligand (H2L). An X-ray crystallographic analysis revealed that both right- (P) and left-handed (M) helical structures exist in the solid state for Al2L3, Ga2L3, and In2L3. The enantiomers were successfully resolved by chiral column chromatography, and the chiroptical properties of the complexes were characterized by circular dichroism, circularly polarized emission spectroscopy, and time-dependent density functional theory calculations.

A Europium Complex That Selectively Stains Nucleoli of Cells

A europium complex selectively staining the nucleolus of NIH 3T3, HeLa, and HDF cells is reported. This complex possesses not only the advantage of the long lifetime of europium emission (0.3 ms), but also a chromophore that allows excitation at a relatively long wavelength (lambda(max) = 384 nm) and gives rise to an acceptable quantum yield (9%). The complex can be used both in live cell and fixed cell imaging, giving an average intracellular concentration on the order of 0.5 microM. Strong binding to serum albumin has been demonstrated by examination of the analogous gadolinium complex, studying relaxivity changes with increasing protein concentration. The intracellular speciation of the complex has been examined by circularly polarized emission spectroscopy and is consistent with the presence of more than one europium species, possibly protein bound.

Tungsten and molybdenum oxo complexes of tetrakis(phenyldiazenyl)calix[4]arene substituted derivatives: EHMO calculations, spectroscopic characterization, and perturbations of the photophysical properties by neutral guest molecules

Bowl-shaped chromophores fabricated with tungsten and molybdenum oxo complexes of calix[4]arenes substituted at the para positions with phenyldiazenyl (R2C6H3N2) derivatives are investigated. The nature of the lowest energy excited states has been addressed theoretically using Extended Hückel Molecular Orbital (EHMO) calculations and experimentally using UV-visible, luminescence, and polarized emission spectroscopy at 77 K. The lowest energy excited states are intraligand (azobenzene fragment) ππ* (S2 and T2) and ππ*states (S1, and T1). The compounds are weakly luminescent in the 600–630 nm range exhibiting emissions arising from the 1(ππ*) states [Formula: see text]. The region located above 900 nm was not investigated. Evidence for guest–host chemistry has been demonstrated by1H-NMR spectroscopy, and the binding constants for two of the compounds with pyridine in toluene as a solvent were obtained by UV–visible spectroscopy. The presence of a guest molecule inside the luminescent metal oxo complex of a calix[4]arene hosts decreases the emission lifetimes.Key words: host–guest, photophysics, calix[4]arene, EHMO, fluorescence

Mueller Matrix Approach to Fluorescence Spectroscopy. Part I: Mueller Matrix Expressions for Fluorescent Samples and Their Application to Problems of Circularly Polarized Emission Spectroscopy

Mueller matrixes are presented for optically inactive and fluorescent samples with and without photo-selection, and for optically active and fluorescent ones with and without photo-selection, in order to analyze phenomena observed in fluorescence polarization. Absorption-fluorescence transformation matrices are introduced in the derivation of formula to take account of the process occurring during the interval between excitation and emission. Mueller matrix expressions derived are used for investigating the problems of circularly polarized emission (CPE) and linearly polarized emission (LPE) measurements. It is found that they are useful not only in analyzing the effects of experimental systems on fluorescence polarization spectroscopy but also in the design, analysis, and evaluation of CPE and LPE spectrometers.

Polarization effects in resonance Raman scattering from coupled optically bright states

This paper extends the McClain formulation of two-photon absorption to show what observables can be independently determined in an emission experiment for which incident and scattered photon polarizations are resolved. Interelectronic state coupling contributions and quantum interference can be extracted from the polarized emission spectrum by determining the three linearly independent contributions to the scattering intensity. This enhances the value of polarized emission spectroscopy as a tool for understanding transitions between coupled electronic states that occur during dissociation dynamics. We demonstrate that the three linearly independent contributions to the polarized emission spectrum can be determined by performing three polarization experiments, at least one of which must involve circularly polarized light. Furthermore, the three polarization experiments must be performed at a scattering angle other than π/2. We also present sample calculations on a model of the methyl iodide system which serve as an illustration of the general theory and demonstrate the sensitivity of this technique to the details of the potential-energy surfaces.

Quantum calculations of the polarized emission spectrum of methyl iodide: The importance of interference effects

We present detailed quantum calculations of the polarized emission spectrum from methyl iodide after excitation at 266 nm. Methyl iodide has two dipole accessible and mutually coupled repulsive electronic surfaces at this excitation energy. Thus it is expected that there are interference contributions to the angularly resolved polarized emission spectrum. Using a one-dimensional model and time-independent quantum computations of the emission into ground state C–I overtone vibrations, these interference contributions are shown to significantly influence the polarized emission spectrum and, for example, to produce a counterintuitive dependence on final state overtone number. The calculations demonstrate that the influence of the interference terms is enhanced by the formation of polarized emission intensity ratios because of the differing signs of geometric coefficients associated with the interference terms. We also study the effect of varying the strength of the coupling potential and the range over which it acts. The nonintuitive nature of the interference terms and the amplification of their influence by the formation of polarized emission intensity ratios demonstrate why quantum computations are important for a complete understanding of the polarized emission spectroscopy experiment for methyl iodide.

Polarized emission spectroscopy of photodissociating nitromethane at 200 and 218 nm

We report the polarized emission spectra from photodissociating nitromethane excited at 200 and 218 nm. At both excitation wavelengths, the emission spectra show a strong progression in the NO2 symmetric stretch; at 200 nm a weak progression in the NO2 symmetric stretch in combination with one quantum in the C–N stretch also contributes to the spectra. We measure the angular distribution of emitted photons in the strong emission features from the relative intensity ratio between photons detected perpendicular to versus along the direction of the electric vector of the excitation laser. We find the anisotropy is substantially reduced from the 2:1 ratio expected for the pure CH3NO2 X(1A1)→1B2(ππ*)→X(1A1) transition with no rotation of the molecular frame. The intensity ratios for the features in the NO2 symmetric stretching progression lie near 1.5 to 1.6 for 200 nm excitation and 1.7 for 218 nm excitation. The analysis of the photon angular distribution measurements and consideration of the absorption spectrum indicate that the timescale of the dissociation is too fast for molecular rotation to contribute significantly to the observed reduction in anisotropy. The detailed analysis of our results in conjunction with electron correlation arguments and previous work on the absorption spectroscopy and final products’ velocities results in a model which includes two dissociation pathways for nitromethane, an electronic predissociation pathway and a vibrational predissociation pathway along the 1B2(ππ*) surface. Our analysis suggests a reassignment of the minor dissociation channel, first evidenced in photofragment velocity analysis experiments which detected a pathway producing slow CH3 fragments, to the near threshold dissociation channel CH3 + NO2(2 2B2).

Evolution of molecular dissociation through an electronic curve crossing: Polarized emission spectroscopy of CH3I at 266 nm

We present a study of the dissociation of CH3I on coupled repulsive electronic potential energy surfaces by the technique of polarized emission spectroscopy. We excite CH3I at 266 nm and disperse the photons emitted from the dissociating molecule by both frequency and angular distribution with respect to the polarization direction of the excitation laser. We thus measure the polarization of the first 12 C–I stretching emission features, corresponding to the spectral region between 266 and 317 nm. We also obtain the rotational envelope of selected emission features in higher resolution scans and model the lineshapes with parameters derived from the polarization results. The polarization measurements show the emission into the first few low-lying C–I stretching vibrational levels is via a transition moment parallel to the absorbing one, consistent with excitation to and emission from the 3Q0(2A1) repulsive surface. Emission to higher C–I stretching overtones shows an increasing contribution from emission via a transition moment perpendicular to the absorbing one, consistent with emission from a repulsive surface of E symmetry following excitation to the 3Q0(2A1) state. We extract from the data the fraction of photons emitted via a perpendicular transition for each of the C–I stretch emission features. The analysis includes the derivation of analytic expressions for the angular distribution of the photons, with and without integration over the rotational contour, when the detector has a finite acceptance angle. We discuss the results in relation to a simple model where photoabsorption excites the molecule to the 3Q0(2A1) repulsive surface (parallel transition moment) and amplitude develops on the 1Q1(3E) repulsive surface as the molecule dissociates through a curve crossing. The changes in amplitude of the molecular wavefunction on the A1 vs the E repulsive surfaces during dissociation is thus probed. We outline a crude classical quasidiatomic approximation for roughly extracting from our data the electronic energy at which the ‘‘curve crossing’’ occurs. This derived energy is compared to that given in model and ab initio calculations of the excited electronic potential energy surfaces. Finally, we discuss the results in relation to the simple quasidiatomic Landau–Zener crossing model utilized by other workers, a model which does not fully explain the collection of experimental results over the last decade on the iodoalkane curve crossing.

Order parameters of fluorophores in ground and excited states. Probe molecules in a lyotropic liquid crystal

Perylenc, 9,10-dimethylanthracene and 1,6-diphenyl-1,3,5-hexatriene (DPH) solubilized in a macroscopically aligned lyotropic nematic phase have been studied with linear dichroism (LD), steady-state and time-resolved fluorescence spectroscopy. From LD an order parameter of the fluorophore in its electronic ground state can be obtained, and from time-resolved polarized emission spectroscopy for its excited state. The order parameters of perylene in the ground and excited states are the same within experimental accuracy, but they are significantly different for 9,10-dimethylanthracene and DPH. It is concluded that the orientational distributions of fluorescent probe molecules in ground and excited states are generally not equal. Hence the order parameter of a fluorophore in a microscopically anisotropic and macroscopically isotropic system, e.g. a vesicle suspension, cannot generally be determined directly from the emission anisotropy.

On the Artifacts in Circularly Polarized Emission Spectroscopy

Circularly polarized emission (CPE) is observed from achiral fluorescent molecules dissolved in optically inactive solvents. An extensive investigation is carried out to determine whether or not the signal observed is a true solvent-induced CPE signal. As a result, the following three decisive factors responsible for the origin of the artifact in CPE spectroscopy are presented: (1) the amount of linear polarization in emission; (2) the residual static birefringence of the optical system due mainly to an electro-optical modulator; and (3) the second harmonics response of the lock-in amplifier used in the electronic system.

Interpretation of intramolecular energy transfer experiments

The measurement of the efficiency of resonance energy transfer between two luminophores attached to the same macromolecular substrate is a promising tool for determining intramolecular distances if reasonable bounds can be placed upon the orientation factor k 2, To date, the analysis of such experiments has been based upon assumed average values of k 2. A critical review of this procedure is offered along with a method for estimating upper and lower limits for k 2 which derive from the freedom of motion of the luminophores as determined by polarized emission spectroscopy.