Energy Transfer Polarization Research Articles

Electrically tunable polarized luminescence from guest-host composites via interaction between rare earth complexes and liquid crystals.

Polarized luminescence has many applications in energy saving, displays, communication and other fields. Here, electrically tunable polarized luminescence of aligned Eu(tta)3phen/E7 composites is successfully obtained based on the interaction between liquid crystal (LC) hosts and rare earth (RE) complex guests. The generation mechanism of the polarization effect is thoroughly explored from three different aspects. The polarized luminescence of Eu(tta)3phen/E7 composites is closely related to the polarized energy absorption of incident light, the site symmetry of Eu3+ ions for oriented Eu(tta)3phen complex molecules, and the polarized energy transfer from E7 to Eu(tta)3phen. The convenient control of the well-aligned Eu(tta)3phen complex by simply embedding the Eu(III) complex guest in the E7 host and utilizing small-magnitude electric field forces is explicitly confirmed. The E7 host not only provides the orientation condition for the electrically tunable polarized luminescence but also promotes the degree of polarization through polarized energy transfer. The polarized spectral properties and the symmetry of the Eu3+ site of the aligned Eu(tta)3phen complexes are described in detail. The analysis of the generation mechanism of polarized luminescence supplements the research content in the field of RE/LC composites and paves the way for exciting novel advances in the field of polarized emission.

Rotationally Gated Fluorescence: Common Theme Behind Molecular Rotors, Photoactivation and Förster Resonance Energy Transfer (FRET)

About the common features behind the mechanism of molecular rotors, photoswitching of engineered fluorescent proteins (GFPs), and Forster resonance energy transfer (FRET). Besides explining the common mechanism, we also offer a new method for increasing sensitivity of viscosity determination even in living cells, by the combination of polarization energy transfer (polFRET) method and a molecular rotor dye either as an energy donor or acceptor.

Ultrasensitive Polarized Up-Conversion of Tm3+–Yb3+ Doped β-NaYF4 Single Nanorod

Up-conversion luminescence in rare earth ions (REs) doped nanoparticles has attracted considerable research attention for the promising applications in solid-state lasers, three-dimensional displays, solar cells, biological imaging, and so forth. However, there have been no reports on REs doped nanoparticles to investigate their polarized energy transfer up-conversion, especially for single particle. Herein, the polarized energy transfer up-conversion from REs doped fluoride nanorods is demonstrated in a single particle spectroscopy mode for the first time. Unique luminescent phenomena, for example, sharp energy level split and singlet-to-triplet transitions at room temperature, multiple discrete luminescence intensity periodic variation with polarization direction, are observed upon excitation with 980 nm linearly polarized laser. Furthermore, nanorods with the controllable aspect ratio and symmetry are fabricated for analysis of the mechanism of polarization anisotropy. The comparative experiments suggest that intraions transition properties and crystal local symmetry dominate the polarization anisotropy, which is also confirmed by density functional theory calculations. Taking advantage of the REs based up-conversion, potential application in polarized microscopic multi-information transportation is suggested for the polarization anisotropy from REs doped fluoride single nanorod or nanorod array.

A Study of the Photoorientation Phenomena in Cholesteric Polymer Systems Containing Photochromic Diarylethene Derivatives

AbstractSummary: Photoorientation processes in two cholesteric mixtures were studied. Mixture I contained a nematogenic phenylbenzoate homopolymer, a photochromic diarylethene dopant and a chiral dopant. Mixture II consisted of a binary comb‐shaped nematogenic copolymer with fluorescent perylene side groups and the same chiral and photochromic dopants. Irradiation of the mixtures with a polarized UV light did not induce photoorientation of the photoinduced “closed” form of the diarylethene dopant. However, the action of a polarized visible light on films of mixtures irradiated with UV light led to the appearance of dichroism. This phenomenon is associated with the photoselection process of the photochromic diarylethene molecules in the cholesteric matrix. An investigation of the kinetics of the photosensitized back photoreaction of the photochromic molecules in the cholesteric mixture showed that the polarized energy transfer from the fluorescent groups to the photochromic diarylethene dopant caused an increase in the photoselection rate. The thermal stability of dichroism in films of the mixtures was studied and it was shown that the disappearance of the dichroism of the diarylethene molecules occurred with a noticeable rate even at room temperature.Scheme of photophysical and photochemical processes under the indirect excitation of diarylethene molecules.imageScheme of photophysical and photochemical processes under the indirect excitation of diarylethene molecules.

Light-Induced Electron Spin Polarization of a Weakly Coupled Triplet−Doublet Spin Pair in a Covalently Linked Porphyrin Dimer

The spin-polarization patterns of a weakly coupled triplet−doublet spin pair are studied in a covalently linked copper (II)−free base porphyrin dimer partially oriented in a liquid crystalline solvent. The triplet−doublet pair is generated in the dimer by either spin−orbit intersystem crossing (ISC) within the free-base moiety or via energy transfer (EnT) from the copper porphyrin. The two pathways result in two different spin-polarization patterns, which can be observed separately by selectively exciting either the free base porphyrin at 640 nm or the copper porphyrin at 540 nm. The time development and orientation dependence of the two spin-polarization patterns is investigated at two microwave frequencies. An analysis of the EPR transients reveals that the polarization associated with the EnT pathway rises monoexponentially, while the ISC polarization rises rapidly and decays biexponentially. The rise of the EnT polarization pattern and the fast decay component of the polarization from ISC are governed by the same characteristic lifetime of 2 μs independent of the molecular orientation. However, the ratio of the two decay components of the ISC polarization is strongly orientation dependent. Separately, a general treatment of the spin-polarized transient EPR spectra of coupled triplet−doublet spin pairs is presented and two possible mechanisms for the kinetic behavior involving spin-selective depopulation or relaxation between the spin sublevels of the triplet−doublet pair are investigated. It is shown that all the observed features of the polarization patterns including orientation and excitation wavelength dependence as well as the kinetics can be explained in terms of the model and are consistent with either of the two mechanisms. In addition to the weakly coupled triplet−doublet pair, narrow signal components are observed near the center of the spectrum. The microwave frequency dependence of the spectra confirms the previous assignment that these components are the quartet and doublet states of a strongly coupled triplet−doublet pair generated in conformations of the dimer in which the two porphyrins are in close proximity.

Depolarizing energy transfer in photoluminescent polymer blends

The occurrence of a depolarizing polarizing energy transfer effect is demonstrated in uniaxially oriented polymer blend films. These blends are based on ultra-high molecular weight polyethylene as the matrix polymer, and additionally comprise 1,4-bis{(2-ethylhexyloxy)phenylethynyl} benzene (EHO-BPB) as a sensitizer, and either a regioregular HT sexi(3- n-octylthiophene) (6OT) or 7-(dimethylamino)-4-(trifluoromethyl) coumarin (C152) as emitter. The EHO-BPB sensitizer molecules are highly oriented in tensile-deformed blend films, and, thus, cause a highly anisotropic, i.e. linearly polarized optical absorption. The absorbed energy is, subsequently, transferred to the emitter molecules, which are essentially unoriented and, consequently, emit unpolarized light.

Polarizing energy transfer in photoluminescent polymer blends

The occurrence of polarizing energy transfer in uniaxially oriented polymer blend films is investigated. A poly(2,5-dialkoxy-p-phenylene ethynylene) derivative (EHO-OPPE) and poly[2-methoxy-5-[2′-ethyl-hexyloxy]-p-phenylene vinylene] (MEH-PPV) were used as the acceptors, and various sensitizers were used as donors. Some of the properties of the chromophores required for polarizing energy transfer to occur efficiently are elucidated, such as form-isotropy and thermal characteristics. The energy transfer efficiency is quantified, and for the present, optimized systems, values as high as 85% were demonstrated.

Polarizing Energy Transfer in Photoluminescent Conjugated Polymers with Covalently Attached Sensitizers

A new class of poly(p-phenyleneethynylene) (PPE) polymers, COU-OPPE and ANT-OPPE, have been prepared, in which coumarin- and anthracene-based sensitizer molecules are covalently linked to the conjugated polymer backbone via a flexible spacer. In dilute solutions of these polymers, efficient resonance energy transfer is observed from the sensitizer moieties to the PPE backbone, resulting in enhanced luminescence of the PPE macromolecules. Also, when incorporated as guests in oriented polyethylene films, the novel polymers, COU-OPPE and ANT-OPPE, show efficient energy transfer from the pendent sensitizer to the PPE backbone. Especially in the case of ANT-OPPE, we found that the PPE backbone is efficiently oriented while the anthracene moiety remains essentially isotropic, which results in a high degree of polarizing energy transfer for this system.

Photoluminescent Polymer Films for Display Applications

ABSTRACTNovel poly(p-phenylene ethynylene) polymers, ANT-OPPE and COU-OPPE, have been prepared, in which anthracene and coumarin sensitizer molecules are covalently attached to the conjugated polymer backbone via a flexible spacer. These polymers show efficient energy transfer from the pendent sensitizer molecule to the PPE backbone, both in dilute solution as well as in an oriented polyethylene matrix. In case of ANT-OPPE, we found that the PPE backbone adapts the orientation of the matrix and is efficiently oriented, while the anthracene moiety remains essentially isotropic, resulting in a state-of-the-art polarizing energy transfer for this system.

Time-resolved fluorescence study on the mechanism of polarizing energy transfer in uniaxially oriented polymer blends

A time-resolved study of polarizing energy transfer in oriented blends of a conjugated polymer [a dialkoxy-substituted poly(p-phenyleneethynylene) derivative] and an organic laser dye (7-diethylamino-4-methylcoumarin) in ultra-high molecular weight polyethylene is presented. The transfer is described in terms of a Forster mechanism, based on long-range dipole–dipole interactions. Forster radii were determined in oriented blend films and in chloroform solutions. It was found that the transfer process is critically influenced by the phase behavior of the system under investigation. A depolarizing homotransfer between donor molecules was found to be a key step in the polarizing nature of the transfer which, ultimately, allows excitation light polarized perpendicularly to the film orientation direction to be emitted with the polarization direction parallel to its orientation.

Photoluminescent Polymer Films for Display Applications

ABSTRACTNovel poly(ρ-phenylene ethynylene) polymers, ANT-OPPE and COU-OPPE, have been prepared, in which anthracene and coumarin sensitizer molecules are covalently attached to the conjugated polymer backbone via a flexible spacer. These polymers show efficient energy transfer from the pendent sensitizer molecule to the PPE backbone, both in dilute solution as well as in an oriented polyethylene matrix. In case of ANT-OPPE, we found that the PPE backbone adapts the orientation of the matrix and is efficiently oriented, while the anthracene moiety remains essentially isotropic, resulting in a state-of-the-art polarizing energy transfer for this system.

Polarizing energy transfer in photoluminescent materials for display applications

Combinations of sheet polarizers and colour filters form the basis of numerous products1,2,3,4 — most notably colour liquid-crystal displays2,4 — that require polarized chromatic light. But this combination of elements does not make efficient use of light, as a substantial fraction of the incident light is converted into thermal energy3,4, limiting the brightness and energy efficiency of the resulting devices. Here we show that these limitations can be overcome by using polymer-based photoluminescent polarizers. Our polarizers operate on a two-step principle: randomly orientated ‘sensitizer’ molecules harvest the incident light by isotropic absorption and then efficiently transfer the energy to a uniaxially orientated photoluminescent polymer, from which coloured light with a high degree of linear polarization is emitted. In principle, isotropic-to-polarized conversion efficiencies approaching unity could be attainable by this approach.

Polarized Electronic Energy Transfer in Langmuir−Blodgett Films

Aggregation of cyanine dyes within Langmuir−Blodgett (LB) films is exploited to form heterodimers within mixed dye monolayers. The geometrical constraints imposed on the amphiphilic dye molecules within an LB monolayer are utilized to facilitate polarized energy transfer. Time-correlated single-photon counting is used to measure the residual polarization of electronic energy transfer from donor oxacyanine molecules to acceptor oxacarbocyanine molecules. The residual polarization decay profiles are fit to a biexponential and are contrasted for two types of LB matrixes. In a neutral tripalmitin LB matrix, the residual polarization of this mixed cyanine dye system is as high as 0.13 and decays with decay times of 170 ps and 2.8 ns to 0.03. In an ionic fatty acid LB matrix the residual polarization reaches 0.11 and decays to 0.06 with decay times of 620 ps and 5.3 ns. The concentration depolarization of these residual polarizations suggests that heterodimers are randomly distributed throughout the LB monolayer.

Dynamic membrane studies in individuals at risk for Huntington's disease

Lymphocyte plasma membrane dynamics were studied by energy-transfer polarization in twenty-three neurologically normal individuals at-risk for Huntington's disease (HD). The results were compared to 10 normal controls and 10 known HD patients. The normal and HD subjects segregated into two distinct groups. The at-risk group had findings distributed along a continuum with values similar to known HD patients or to normal controls. These findings suggest that further studies of membrane dynamics will contribute to understanding the molecular defect in HD and to the development of a potential molecular marker.