Tunable room-temperature phosphorescence and circularly polarized luminescence encoding helical supramolecular polymer

Supramolecular polymers with different functionalities have been continuously developed in the past decade because of their indispensable role in soft materials.However,pure organic supramolecular polymers with stable room temperature phosphorescence(RTP)emission were very rarely reported for the difficulties of synthesis and achieving RTP in solution.Herein,soluble helical supramolecular polymers with circularly polarized room-temperature phosphorescence were developed via a facile hostguest strategy.Through assembly,a transition from pure fluorescence to almost pure RTP emission was achieved.Adjusting the asymmetry of guest could easily control the chiroptical property of supramolecular polymers.This work provides new opportunities for the design and development of intelligent soft functional soft materials.     

Liquid crystalline polyfluorenes for blue polarized electroluminescence

A series of 9,9‐dialkyl‐poly(fluorene‐2,7‐diyl)s containing linear and branched alkyl substituents with a M_n of up to 200000 g/mol has been synthesized. Moreover, some of the polymers were end capped with a suitable hole transport functionality, such as a triphenylamine derivative, to improve their charge transport properties and to control the molecular weight. The thermal alignment of these novel polymers on a rubbed polyimide layer led to highly anisotropic film formation with dichroic ratios (absorption parallel and perpendicular to the rubbing direction) of up to 26 in absorption and 21 in emission.     

Chiral exciplex dyes showing circularly polarized luminescence: extension of the excimer chirality rule

A series of axially chiral binaphthyls and quaternaphthyls possessing two kinds of aromatic fluorophores, such as pyrenyl, perylenyl, and 4-(dimethylamino)phenyl groups, arranged alternately were synthesized by a divergent method. In the excited state, the fluorophores selectively formed a unidirectionally twisted exciplex (excited heterodimer) by a cumulative steric effect and exhibited circularly polarized luminescence (CPL). They are the first examples of a monomolecular exciplex CPL dye. This versatile method for producing exciplex CPL dyes also improved fluorescence intensity, and the CPL properties were not very sensitive to the solvent or to the temperature owing to the conformationally rigid exciplex. This systematic study allowed us to confirm that the excimer chirality rule can be applied to the exciplex dyes: left- and right-handed exciplexes with a twist angle of less than 90° exhibit (−)- and (+)-CPL, respectively.

Axially chiral binaphthyls and quaternaphthyls possessing two kinds of fluorophores were synthesized. In the excited state, the fluorophores formed a twisted exciplex and exhibited CPL. This study gave us named the exciplex chirality rule.     

Chiral sensing using an achiral europium(III) complex by induced circularly polarized luminescence

[Eu(bda)(2)](-) (bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) produces intense circularly polarized luminescence (CPL) in aqueous solutions in the presence of (S)-2-pyrrolidone-5-carboxylic acid upon UV irradiation, although the molecular structure of the europium(III) complex is achiral. The mechanism for the induction of CPL was preliminarily attributed to distortions induced by association with an amino acid to generate chirality in the achiral complex. The optical anisotropy factor (g(lum) value) for the (5)D(0) → (7)F(1) transition was 0.03 in the presence of 1.0 mol dm(-3) of the amino acid. Analysis of the CPL intensity as a function of the amino acid concentration gave an association constant between those of [Eu(bda)(2)](-) and the amino acid, K(aso) = 0.55 ± 0.09 mol(-1) dm(3). These results demonstrate the potential of [Eu(bda)(2)](-) to act as a luminescent chiral-sensing reagent in microscopic spectroscopy.     

Cholesteric polymer guest-host mixture with circularly polarized fluorescence: two ways for phototuning of polarization and its intensity

A photosensitive fluorescent cholesteric guest-host mixture consisting of a nematic polyacrylate, a chiral, photochromic dopant sensitive to UV light, and a fluorescent dopant was prepared. The nematic polyacrylate contains 4-phenyl-4'-methoxybenzoate nematogenic side groups and photochromic 4-cyanoazobenzene side groups. The chiral-photochromic dopant formed by isosorbide and cinnamic acid is capable of E-Z photoisomerization and [2 + 2] photo-cycloaddition under light irradiation. The planarly oriented films possess a selective light reflection in the visible spectral region coinciding with the emission peak of the fluorescent dopant. The fluorescence emitted by the planarly oriented films of the mixture is strongly circularly polarized and characterized by a large value of the dis-symmetry factor. At temperatures below glass transition (T(g)) the polarized light action of an Ar(+) laser (488 nm) leads to the photo-orientation of the azobenzene fragments resulting in a strong and reversible disruption of the selective reflection and a decrease of the dis-symmetry factor of fluorescence. UV irradiation leads to E-Z isomerization and/or [2 + 2] cycloaddition of the chiral-photochromic dopant, causing an irreversible shift of the maximum of the dis-symmetry factor to a long-wavelength spectral region under subsequent annealing at temperatures higher than T(g). Such multifunctional glass-forming guest-host mixtures combining photosensitive and fluorescent properties with the unique optical properties of cholesteric liquid crystals can be considered as promising material for optical data processing technologies and photonic applications.     

Absolute asymmetric synthesis driven by circularly polarized light

Circularly polarized light (CPL) is an inherently chiral entity and is regarded as one of the possible deterministic signals that led to the evolution of homochirality in earth. Thus, CPL as an external physical field has been widely used in a technique known as absolute asymmetric synthesis, because a product enriched in one enantiomer is formed from racemic precursor molecules without the intervention of a chiral catalyst. In this review, we retrospect the historical research of CPL-induced absolute asymmetric synthesis, including chiral organic molecules, helical polymers, supramolecular assemblies, noble metal nanostructures. However, based on these results, we concluded that the chiral photon-matter interaction is very faint due to the arrangement of molecular bonds giving rise to chiral features, is over a smaller distance than the helical pitch of CPL, leading extremely small enantiomeric excess for product. Therefore, we highlight the recently emerged technology called superchiral field, in which the superchiral far-field and near-field could enhance the dissymmetry of optical field and near-field, respectively. In sum, we hope this review could bring some enlightenment to researchers and further improve the enantioselectivity of CPL-induced absolute asymmetric synthesis.     

High-efficiency circularly polarized emission from liquid-crystalline platinum complexes

Realizing both a high emission efficiency and luminescence dissymmetry factor (g_lum) in circularly polarized solution processable organic light-emitting diodes (CP-OLEDs) remains a significant challenge. In this contribution, two chiral phosphorescent liquid crystals based on cyclometalated platinum complexes are prepared, in which the chiral s-2-methyl-1‑butyl group is introduced into the cyclometalating ligand and the mesogenic fragment is attached to the periphery of the ancillary ligand. The platinum complexes exhibit both smectic and chiral nematic phases as evidenced by polarized optical microscopy, differential scanning calorimetry and small-angle X-ray diffraction. Remarkably, a high photoluminescent quantum efficiency of over 78% and clear circularly polarized luminescent signal with g_PL of about 10^–2 are observed for the complexes. Further, solution-processed CP-OLEDs show maximum external quantum efficiencies (EQE) of over 15% and strong circularly polarized electroluminescent signals with a g_EL ≈ 10^–2. This research demonstrates that both liquid crystallinity and the number of chiral centers play key roles in improving the chiroptical property, paving the way for a new approach for the design of high-efficiency CPL emitters.     

Highly contorted superhelicene hits near-infrared circularly polarized luminescence

Herein we describe a novel superhelicene structure consisting of three hexa-peri-hexabenzocoronene (HBC) units arranged in a helical geometry and creating two carbo[5]helicenes and a carbo[7]helicene. The central HBC bears a tropone moiety, which induces a saddle-helix hybrid geometry into the 3D structure of the prepared nanographene. The introduction of multiple helicenes and the position of the tropone unit trigger near-infrared circularly polarized luminescence (NIR-CPL, up to 850 nm, |g_lum| = 3.0 × 10⁻³) combined with good photoluminescence quantum yields (ϕ_F = 0.43) and upconverted emission based on two-photon absorption (TPA). Compared to previously reported superhelicenes of similar size, higher quantum yields, CPL brightness, and red-shifted absorption and emission spectra are achieved. Besides, chiroptical properties of enantiopure thin films were evaluated. These findings place this novel superhelicene as the first NIR-CPL superhelicene ever reported and make it a promising candidate for use as a chiral luminescent material in optoelectronic devices.

This twisted superhelicene employed its twists and loops to shine chiral emission into the NIR region. The proof of its promising applicability in optoelectronic devices is the analogous chiroptical properties obtained in solution and thin films.     

Synthesis of UV-curable liquid crystalline diacrylates for the application of polarized electroluminescence

Two series of UV-curable rod-like diacrylates containing bis-tolane (M1-M5) and bis-stilbene (M6-M10) mesogens were synthesized. All of the diacrylates were prepared through convergent syntheses involving palladium-catalysed coupling reactions. The thermal transitions and mesomorphic properties of the diacrylates were characterized by differential scanning calorimetry and optical microscopy. All the bis-tolane-based monomers reveal nematic phases, except M5 which shows no mesomorphic behaviour. For the bis-stilbene based monomers, most show no mesomorphic behavior and only M7 and M9 exhibit a SmA phase.     

White light-emitting devices based on carbon dots' electroluminescence

We demonstrate the first white light-emitting device originating from single carbon dot components. A maximum external quantum efficiency of 0.083% at a current density of 5 mA cm(-2) with a color-rendering index of 82 is realized, indicating that carbon dots have great potential to be an alternative phosphor for fabricating white light electroluminescent devices.     

Organic electroluminescence devices based on anthracene sulfide derivatives

A series of anthracene derivatives are synthesized and fabricated as light-emitting materials in OLED devices. The incorporation of the chalcogen atoms, either oxygen or sulfur, in between the anthracene moiety and the alkyl or aryl substituents affected drastically the photo- and electroluminescence properties of the materials, especially the HOMO-LUMO band gap and the emitting color of the devices. The new anthracene sulfide derivatives represent a new design for further modification of other light-emitting doped materials.     

Aggregation-amplified circularly polarized luminescence from axial chiral boron difluoride complexes

The development of small organic molecules with intense and switchable circularly polarized luminescence(CPL) is currently attracting great interest due to their promising applications in chiroptical devices and sensors. In this paper, CPL-active BF2-bridged azaanthracene dimers(BA1 and BA2) were facilely synthesized by incorporating boron difluoride unit to the binaphthalene. BA1 and BA2 show moderate CPL in diluted solutions, however, BA2 exhibited aggregation-amplified red CPL with large dissymmetry factor up to 1.6×10~(-2). Moreover, acid-/base-triggered CPL switch off/on were also realized via disaggregation/aggregation of BA2 in tetrahydrofuran(THF)/water binary solvents.     

Resonance-enhanced multiphoton ionization with circularly polarized light: chiral carbonyls

An introduction to the principle and possibilities of the new method of circular dichroism laser mass spectrometry is given and its state of development is reviewed. This method allows enantiosensitive, mass-selective probing of chiral molecules. It is based on the combination of resonance-enhanced multiphoton ionization with circularly polarized light and specially modified time-of-flight mass spectrometry. As an example, application to carbonyls is presented.

Small-angle scattering of circularly polarized light from an anisotropic sphere

A theory of the small-angle scattering of circularly polarized light from an anisotropic sphere has been derived. The validity of the theory has been verified, and a relationship between the structural information thus obtained and the structural information obtained with linearly polarized light has been demonstrated.     

Detection of zinc ions by differential circularly polarized fluorescence excitation

A new chiroptical spectroscopic approach, differential circularly polarized fluorescence excitation (CPE), can be used to provide a selective method for detecting the presence of zinc ions. The approach utilizes the same instrumentation as fluorescence-detected circular dichroism and provides strong contrast in metal detection due to response to both chelation-enhanced fluorescence and circular dichroism upon metal ion binding. The observed contrast is therefore better than either of the parent spectroscopic detection methods. CPE also provides a strategy to reduce interference from background such as protein-based tryptophan fluorescence.     

Mechanistic insight into circularly polarized photoluminescence from a chiral-nematic film

Circularly polarized photoluminescence (CPPL) was characterized for rod-like molecules of Exalite 428 helically arranged in a chiral-nematic liquid crystalline film. With an unpolarized excitation at 370nm, CPPL intensities measured at 428nm, and a selective reflection wavelength ranging from 7.2 to 72.6mum, the observed dissymmetry factor, g, was found to be in excellent agreement with theoretical prediction, without resorting to adjustable parameters. As a result, new insight into liquid crystal-induced CPPL has emerged. Specifically, it was found that circular dichroism and circular polarization of the excitation beam prior to inducing linearly polarized photoluminescence (LPPL) at quasi-nematic layers play an insignificant role. The relatively large g value in the spectral region far removed from the selective reflection band was attributed to the circular polarization of LPPL emanating from all quasi-nematic layers comprising the chiral-nematic film. In the absence of a fluorescent dye, the propagation of unpolarized light through the chiral-nematic film under otherwise identical conditions results in no circular polarization. The present study has provided a foundation for generating circularly polarized light by way of photo-excitation with unpolarized light of a chiral-nematic film containing a fluorescent dye. e e     

Infrared electroluminescence of organic nanocrystals in polymer composites

Electroluminescent polymeric nanocomposite structures that are based on aromatic polyimides and cyanine dye nanosized crystals known as J aggregates and emit light in the IR region were prepared. For the first time, doped polymer systems were found to display IR luminescence whose spectrum had the form of a very narrow band that peaked at 1100 nm. Nanosized J-aggregate crystals in these new polymer materials act not only as effective acceptors of energy of excitonic states but also as active electron-hole transport sites.     

Quasienergy formalism for intense field multiphoton ionization of atoms induced by circularly polarized radiation

A practical non-perturbative approach is presented for multiphoton ionization of atoms induced by circularly polarized radiation. By use of co-ordinate rotation transformation and L²-discretization of the atomic continuum, the complex energy spectrum of a stationary quesienergy operator can be located and multiphoton ionization rates determined as a function of time and arbitrary photon intensity. The theory is applied to the two-photon ionization of the H atom in intense fields.     

Molecular photoelectron interference effects by intense circularly polarized attosecond x-ray pulses

We present photoelectron interference effects in molecular photoionization by intense circularly polarized attosecond X-ray laser pulses. Simulations are performed on single electron molecular systems, H\(_{2}^{+}\), HeH²⁺, and H\(_{3}^{2+}\), by numerically solving the corresponding three-dimensional time dependent Schrödinger equations. Photoelectron spectra show that both momentum ring and stripe patterns are obtained, arising respectively from the interference of the direct ionized and scattered electron wave packets. Multi-center electron interference models in molecular ultrafast photoionization are used to describe the ionized electron dynamics. The interference patterns are shown to be dependent on molecular orbital symmetry, thus offering tools for attosecond imaging molecular structure at different molecular geometries.     

Laser mass spectrometry with circularly polarized light: two-photon circular dichroism

Extensive studies of two-photon circular dichroism (TPCD) on 3-methylcyclopentanone are presented following the first TPCD experiment of gas phase molecules by Compton et al. [J. Chem. Phys.125 (2006) 144304]. (2 + 1)-Multiphoton ionization in a specially designed time-of-flight mass analyzer has been used to perform these studies. CD of two-photon transitions from the molecular ground state to low lying Rydberg states is strongly enhanced with respect to corresponding one-photon transitions in good agreement with Compton. Differences between CD values determined via the molecular ion and via fragment ions indicate strong molecular ion CD effects. This would be the first time that circular dichroism of isolated molecular ions has been measured.