Boost the Circularly Polarized Phosphorescence of Chiral Organometallic Platinum Complexes by Hierarchical Assembly into Fibrillar Networks.

Circularly polarized luminescence (CPL)-active molecular materials have drawn increasing attention due to their promising applications for next-generation display and optoelectronic technologies. Currently, it is challenging to obtain CPL materials with both large luminescence dissymmetry factor (glum) and high quantum yield (Φ). A pair of enantiomeric N N C-type Pt(II) complexes (L/D)-1 modified with chiral Leucine methyl ester are presented herein. Though the solutions of these complexes are CPL-inactive, the spin-coated thin films of (L/D)-1 exhibit giantly-amplified circularly polarized phosphorescences with |glum| of 0.53 at 560 nm and Φair of ~50 %, as well as appealing circular dichroism (CD) signals with the maximum absorption dissymmetry factor |gabs| of 0.37-0.43 at 480 nm. This superior CPL performance benefits from the hierarchical formation of crystalline fibrillar networks upon spin coating. Comparative studies of another pair of chiral Pt(II) complexes (L/D)-2 with a symmetric N C N coordination mode suggest that the asymmetric N N C coordination of (L/D)-1 are favorable for the efficient exciton delocalization to amplify the CPL performance. Optical applications of the thin films of (L/D)-1 in CPL-contrast imaging and inducing CP light generation from achiral emitters and common light-emitting diode lamps have been successfully realized.

Endowing Metal-Organic Coordination Materials with Chiroptical Activity by a Chiral Anion Strategy.

Recently, chiral metal-organic coordination materials have emerged as promising candidates for a wide range of applications in chiroptoelectronics, chiral catalysis, and information encryption, etc. Notably, the chiroptical effect of coordination chromophores makes them appealing for applications such as photodetectors, OLEDs, 3D displays, and bioimaging. The direct synthesis of chiral coordination materials using chiral organic ligands or complexes with metal-centered chirality is very often tedious and costly. In the case of ionic coordination materials, the combination of chiral anions with cationic, achiral coordination compounds through noncovalent interactions may endow molecular materials with desirable chiroptical properties. The use of such a simple chiral strategy has been proven effective in inducing promising circular dichroism and/or circularly polarized luminescence signals. This concept article mainly delves into the latest advances in exploring the efficacy of such a chiral anion strategy for transforming achiral coordination materials into chromophores with superb photo- or electro-chiroptical properties. In particular, ionic small-molecular metal complexes, metal clusters, coordination supramolecular assemblies, and metal-organic frameworks containing chiral anions are discussed. A perspective on the future opportunities on the preparation of chiroptical materials with the chiral anion strategy is also presented.

Dual-Emissive Monoruthenium Complexes of N(CH3)-Bridged Ligand: Synthesis, Characterization, and Substituent Effect.

Three monoruthenium complexes 1(PF6)2-3(PF6)2 bearing an N(CH3)-bridged ligand have been synthesized and characterized. These complexes have a general formula of [Ru(bpy)2(L)](PF6)2, where L is a 2,5-di(N-methyl-N'-(pyrid-2-yl)amino)pyrazine (dapz) derivative with various substituents, and bpy is 2,2'-bipyridine. The photophysical and electrochemical properties of these compounds have been examined. The solid-state structure of complex 3(PF6)2 is studied by single-crystal X-ray analysis. These complexes show two well-separated emission bands centered at 451 and 646 nm (Δλmax = 195 nm) for 1(PF6)2, 465 and 627 nm (Δλmax = 162 nm) for 2(PF6)2, and 455 and 608 nm (Δλmax = 153 nm) for 3(PF6)2 in dilute acetonitrile solution, respectively. The emission maxima of the higher-energy emission bands of these complexes are similar, while the lower-energy emission bands are dependent on the electronic nature of substituents. These complexes display two consecutive redox couples owing to the stepwise oxidation of the N(CH3)-bridged ligand and ruthenium component. Moreover, these experimental observations are analyzed by computational investigation.

Electrically Amplified Circularly Polarized Luminescence by a Chiral Anion Strategy.

The development of circularly polarized electroluminescence (CPEL) is currently hampered by the high difficulty and cost in the syntheses of suitable chiral materials and the notorious chirality diminishment issue in electrical devices. Herein, diastereomeric IrIII and RuII complexes with chiral (±)-camphorsulfonate counteranions are readily synthesized and used as the active materials in circularly polarized light-emitting electrochemical cells to generate promising CPELs. The addition of the chiral ionic liquid (±)-1-butyl-3-methylimidazole camphorsulfonate into the active layer significantly improves the device performance and the electroluminescence dissymmetry factors (≈10-3 ), in stark contrast to the very weak circularly polarized photoluminescence of the spin-coated films of these diastereomeric complexes. Control experiments with enantiopure IrIII complexes suggest that the chiral anions play a dominant role in the electrically-induced amplification of CPELs.

Molecular Cocrystals with Hydrogen-Bonded Polymeric Structures and Polarized Luminescence.

Crystalline materials with appealing luminescent properties are attractive materials for various optoelectronic applications. The in situ bicomponent reaction of 1,2-ethylenedisulfonic acid with 1,4-di(pyrid-2-yl)benzene, 1,4-di(pyrid-3-yl)benzene, or 1,4-di(pyrid-4-yl)benzene affords luminescent crystals with hydrogen-bonded polymeric structures. Variations in the positions of the pyridine nitrogen atoms lead to alternating polymeric structures with either a ladder- or zigzag-type of molecular arrangement. By using a nanoprecipitation method, microcrystals of these polymeric structures are prepared, showing polarized luminescence with a moderate degree of polarization.

Selective, Anisotropic, or Consistent Polarized-Photon Out-Coupling of 2D Organic Microcrystals.

Nano- and micromaterials with anisotropic photoluminescence and photon transport have widespread application prospects in quantum optics, optoelectronics, and displays. But the nature of the polarization information of the out-coupled light, with respect to that of the source luminescence, has never been explored in active optical-waveguiding organic crystals. Herein, three different modes (selective, anisotropic, and consistent) of polarized-photon out-coupling are proposed and successfully implemented in a set of 2D organic microcrystals with highly linearly-polarized luminescence. It is found that the polarization direction and degree of the luminescence out-coupled through different waveguiding channels can either be essentially retained or distinctly changed with respect to those of the original luminescence, depending on the molecular arrangement and the orientation of transition dipole moments of the crystal. This work demonstrates the promising potential of 2D emissive microcrystals in multi-channel polarized photon transport.

Organoplatinum(II) Cruciform: A Versatile Building Block to Fabricate 2D Microcrystals with Full-Color and White Phosphorescence and Anisotropic Photon Transport.

Conventional square-planar platinum complexes typically form one-dimensional assemblies as a result of unidirectional metallophilic and/or π⋅⋅⋅π intermolecular interactions. Organoplatinum(II) complexes with a cruciform shape are presented herein to construct two-dimensional (2D) microcrystals with full-color and white phosphorescence. These 2D crystals show unique monocomponent π⋅⋅⋅π stacking, from either the cyclometalating or noncyclometalating ligand, and the bicomponent alternate π⋅⋅⋅π stacking from both ligands along different facet directions. Anisotropic tri-directional waveguiding is further implemented on a single hexagonal microcrystal. These results demonstrate the great capability of the organoplatinum(II) cruciform as a general platform to fabricate 2D phosphorescent micro-/nanocrystals for advanced photonic applications.

Dual-Emissive Tris-Heteroleptic Ruthenium Complexes: Tuning the DNA-Triggered Ratiometric Emission Response by Ancillary Ligands.

Three tris-heteroleptic mononuclear Ru(II) complexes with dual fluorescence and phosphorescence-[Ru(dpma)(bpy)(phen)]2+ (12+), [Ru(dpma)(bpy)(dppz)]2+ (22+), and [Ru(dpma)(phen)(dppz)]2+ (32+)-have been designed and used as ratiometric light-response probes for DNA, where dpma is di(pyrid-2-yl)(methyl)-amine, bpy is 2,2'-bipyridine, phen is 1,10-phenanthroline, and dppz is dipyridophenazine, respectively. Single crystals of complex 2(PF6)2 have been obtained and studied by X-ray analysis. The interactions of these complexes with different DNAs are investigated by means of spectroscopic methods, viscosity measurements, and molecular modeling. In the presence of calf thymus DNA, complexes 2(PF6)2 and 3(PF6)2 show the emergence of a new lower-energy phosphorescence emission band; meanwhile, the higher-energy fluorescence emission band is essentially unchanged, functioning as an intrinsic internal reference. These two complexes exhibit stronger preference for calf thymus DNA over single-strand DNA (d(A)16 and d(C)16). In contrast, no binding interaction between 1(PF6)2 and calf thymus DNA is observed. The intrinsic binding constants (Kb) of 2(PF6)2 and 3(PF6)2 with calf thymus DNA are determined to be (1.4 ± 0.4) × 105 and (9.5 ± 0.15) × 104 M-1, respectively. In addition, these spectroscopic results are compared with those of the prototype complex [Ru(bpy)2(dppz)]2+ (42+), and density functional theory and time-dependent density functional theory calculations are employed to elucidate these experimental findings.

A Carbazole-Bridged Biscyclometalated Diplatinum Complex: Synthesis, Characterization, and Dual-Mode Aggregation-Enhanced Phosphorescence.

A cationic carbazole-bridged biscyclometalated diplatinum complex 4 has been synthesized and characterized. Single-crystal X-ray analysis demonstrates that complex 4 displays a dimeric structure with noncovalent π-π stacking and unique double Pt-Pt interactions. In aerated dilute CH3CN, complex 4 is characterized by a very weak monomeric yellow emission (λemi = 547 nm; Φ = 0.51%), which is attributed to the triplet intraligand (3LC) excited state mixing with some charge transfer characters. In contrast, under aerated conditions, the dispersion of 4 in a mixed solvent of CH3CN/Et2O (1/9, v/v) or CH3CN/H2O (1/9, v/v) displays intense yellow (λemi = 550 nm; Φ = 35.5%; τ = 11.10 µs) and red emission (λemi = 635 nm; Φ = 14.1%; τ = 7.00 µs), respectively. These aggregation-induced phosphorescent emission enhancements are considered being caused by the oxygen-shielding effect and the molecular rigidification-induced decrease of nonradiative decays in the aggregate state. The morphology and size of the aggregates under these two conditions are examined by scanning electron microscope and dynamic light scattering analysis. The absorption and emission properties of 4 are further rationalized by time-dependent density functional theory calculations on a model compound.

Full-Color and White Circularly Polarized Luminescence of Hydrogen-Bonded Ionic Organic Microcrystals.

A simple and general method is presented herein for the in situ preparations of circularly polarized luminescence (CPL)-active microcrystals with a large luminescence dissymmetry factor glum , high fluorescence quantum efficiency (ΦFL ), wide emission color tenability, and well-ordered morphology. The reactions of pyridine-containing achiral molecules 1-7 with chiral camphor sulfonic acid ((±)-CSA) gave crystalline microplates formed by hydrogen bonding interactions between the protonated pyridinium units and the sulfonic anions. The chiral information of CSA are effectively transferred to the microcrystals by hydrogen bonding to afford full-color CPL from deep-blue to red with glum in the order of 10-2 and ΦFL up to 80 %. Moreover, organic microcrystals with high-performance white CPL (ΦFL =46 %; |glum |=0.025) are achieved via the light-harvesting energy transfer between blue and yellow emitters.

Effect of the Fluoro-Substituent Position on the Crystal Structure and Photoluminescence of Microcrystals of Platinum ß-Diketonate Complexes.

Molecular packing has an important effect on the photophysical properties of crystalline materials. We demonstrate in this work the modulation of molecular packing and emission properties of microcrystals by minor molecular structural variations. Four platinum ß-diketonate complexes, with two fluoro substituents (1) or one fluoro atom substituted on different positions of the auxiliary phenylpyridine ligand (2-4) have been synthesized. These complexes were used to prepare one-dimensional microcrystals with well-defined shapes and uniform sizes. Although 1-4 display similar emission spectra in the solution state, the corresponding microcrystals display different emission colors from green to yellow and orange. In addition, different temperature-responsive (80-298 K) emission spectral changes have been observed from these microcrystals, including the intensity variation of the locally excited (LE) emission without obvious wavelength shifts, competition between the LE and metal-metal-to-ligand charge-transfer emissions, and the sole wavelength shift of the π-π excimer emissions. These differences in emission properties are rationalized by different molecular packings of these materials, as revealed by single-crystal X-ray analyses.

Ratiometric detection of amyloid-ß aggregation by a dual-emissive tris-heteroleptic ruthenium complex.

A dual-emissive tris-heteroleptic ruthenium complex is designed, synthesized and applied for the ratiometric photoluminescent detection of amyloid-ß (Aß) aggregation in both steady and transient states. The Aß aggregation is supported by transmission electron microscopy and confocal laser scanning microscopy analysis. In addition, molecular docking calculations have been performed to gain insights into the interaction mode between the ruthenium complex and Aß fibrils.

Wavelength-Tunable Single-Mode Microlasers Based on Photoresponsive Pitch Modulation of Liquid Crystals for Information Encryption.

Information encryption and decryption have attracted particular attention; however, the applications are frequently restricted by limited coding capacity due to the indistinguishable broad photoluminescence band of conventional stimuli-responsive fluorescent materials. Here, we present a concept of confidential information encryption with photoresponsive liquid crystal (LC) lasing materials, which were used to fabricate ordered microlaser arrays through a microtemplate-assisted inkjet printing method. LC microlasers exhibit narrow-bandwidth single-mode emissions, and the wavelength of LC microlasers was reversibly modulated based on the optical isomerization of the chiral dopant in LCs. On this basis, we demonstrate phototunable information authentication on LC microlaser arrays using the wavelength of LC microlasers as primary codes. These results provide enlightenment for the implementation of microlaser-based cryptographic primitives for information encryption and anticounterfeiting applications.

Temperature-Triggered Self-Assembled Structural Transformation: From Pure Hydrogen-Bonding Quadrilateral Nanonetwork to Trihexagonal Structures.

Adequate control over the structures of molecular building blocks plays an important role in the fabrication of desired supramolecular nanostructures at interfaces. In this study, the formation of a pure hydrogen-bonding co-assembly supramolecular nanonetwork on a highly oriented pyrolytic graphite surface was demonstrated by means of a scanning tunneling microscope. The thermal annealing process was conducted to monitor the temperature-triggered structural transformation of the self-assembled nanonetwork. On the basis of the single-molecule-level resolution scanning tunneling microscopy images, together with the density functional theory calculations, the formation mechanisms of the formed nanoarrays were proposed. The results have great significance with regard to controlled construction of complex nanostructures on the surface.

Temperature-Responsive Fluorescent Organoplatinum(II) Metallacycles.

The synthesis, characterization, and temperature-responsive properties of two fluorescent organoplatinum(II) metallacycles are reported. Metallacycles M1 and M2 were prepared via the coordination-driven self-assembly of a 120° triarylamine ligand L1 and a 120° diplatinum(II) acceptor Pt-1 or 180° diplatinum(II) acceptor Pt-2, respectively. M1 and M2 are hexagonal metallacycles, comprising of three or six freely rotating anthracene pendants on their periphery, respectively. In response to the temperature variation between -20 and 60 °C, the ligand displays irregular emission changes, whereas both metallacycles show reversible absorption and emission spectral changes in THF. The changes in their green emission intensity also exhibit a linear correlation with the temperature variation, with an average sensitivity of -0.67% and -0.77% per °C for M1 and M2, respectively. Furthermore, in coordinating solvents, such as DMF and CH3CN, M1 and M2 show different behaviors: in the lower temperature range, i.e., below 30 °C, their spectral changes are similar to those observed in THF; however, at a higher temperature the metallacycles were presumably destroyed by the solvents and displayed ratiometric fluorescent responses, including a cyan emission of the ligand L1.

Bridged cyclometalated diruthenium complexes for fundamental electron transfer studies and multi-stage redox switching.

Four bridged cyclometalated diruthenium systems are highlighted in this Frontier article, including strongly-coupled diruthenium complexes with a short phen-1,4-diyl or a planar pyren-2,7-diyl bridge, redox asymmetric diruthenium complexes characterized by different terminal ligands on the two ends, diruthenium complexes with a urea bridge that allows modulating the degree of electronic coupling, and those with a redox-active amine bridge with varying electronic structures. These complexes posess redox couples with low potentials and intense intervalence charge transfer absorptions in the near-infrared region in the one-electron-oxidized mixed-valent state. They are appealing not only for providing a platform for fundamental electron transfer studies but also as molecular materials with multi-stage redox switching properties.

Tuning the dual emissions of a monoruthenium complex with a dangling coordination site by solvents, O2, and metal ions.

A polypyridyl monoruthenium complex with a dangling coordination site shows dual fluorescence/phosphorescence emissions at room temperature. The emission properties can be modulated by multiple stimuli including solvents, O2, and metal ions.

Resistive memory devices based on a triphenylamine-decorated non-precious cobalt(ii) bis-terpyridine complex.

The ITO/active material/Au sandwiched devices of a cobalt(ii) bis-terpyridine complex decorated with two triphenylamine motifs display appealing flash-type resistive switching with a large ON/OFF ratio (>103) and low operating voltages (<±3 V). In contrast, devices with the triphenylamine-appended terpyridine ligand show WORM-type memory behaviour.

Anion-regulated electronic communication in a cyclometalated diruthenium complex with a urea bridge.

A combined study of electrochemical measurements, intervalence charge transfer analysis, and DFT calculations suggests that the degree of urea-mediated electronic coupling between two cyclometalated ruthenium sites is enhanced by the coordination of urea with Br- or Cl-via hydrogen bonding. In contrast, the redox waves of the diruthenium complex become highly irreversible in the presence of relatively strong basic anions such as H2PO4-, F-, or OAc-. This work demonstrates that the anion-urea interaction can be employed to regulate the electronic coupling and electron transfer between redox-active sites, suggesting the potential applications of the urea-functionalized diruthenium complex in anion sensing and stimuli-responsive molecular electronics.

Temperature-Triggered Chiral Self-Assembly of Achiral Molecules at the Liquid-Solid Interface.

Temperature triggered chiral nanostructures have been investigated on two-dimensional (2D) surfaces by means of scanning tunneling microscopy. Achiral molecules 1 and 2 tend to self-assemble into strip structures on graphite before heating. However, R and S flower-like structures are observed when heated to certain temperature. The transition temperatures of 1 and 2 systems are 55 and 60 °C, respectively. The density functional theory calculations demonstrate that R and S flower-like structures are more stable than strip structures. The coexistence of flower-like structures and strip structures demonstrates the thermodynamic equilibrium. Further, when chiral solvent is added to the sample with other conditions remaining the same, the racemic phenomenon disappears and homochirality emerges. This is an efficient method to control the chirality of 2D molecular assemblies.