Amplified circularly polarized phosphorescence from co-assemblies of platinum(ii) complexes

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.

Dual Enhancement of Phosphorescence and Circularly Polarized Luminescence through Entropically Driven Self-Assembly of a Platinum(II) Complex

Addressing the dual enhancement of circular polarization (glum) and luminescence quantum yield (QY) in circularly polarized luminescence (CPL) systems poses a significant challenge. In this study, we present an innovative strategy utilizing the entropically driven self-assembly of amphiphilic phosphorescent platinum(II) complexes (L-Pt) with tetraethylene glycol chains, resulting in unique temperature dependencies. The entropically driven self-assembly of L-Pt leads to a synergistic improvement in phosphorescence emission efficiency (QY was amplified from 15 % at 25 °C to 53 % at 60 °C) and chirality, both in the ground state and the excited state (glum value has been magnified from 0.04×10-2 to 0.06) with increasing temperature. Notably, we observed reversible modulation of phosphorescence and chirality observed over at least 10 cycles through successive heating and cooling, highlighting the intelligent control of luminescence and chiroptical properties by regulating intermolecular interactions among neighboring L-Pt molecules. Importantly, the QY and glum of the L-Pt assembly in solid state were measured as 69 % and 0.16 respectively, representing relatively high values compared to most self-assembled CPL systems. This study marks the pioneering demonstration of dual thermo-enhancement of phosphorescence and CPL and provides valuable insights into the thermal effects on high-temperature and switchable CPL materials.

Linker-dependent control of the chiroptical properties of polymethylene-vaulted trans-bis[(β-iminomethyl)naphthoxy]platinum(II) complexes

The effects of polymethylene bridges on the chiroptical properties of trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms were examined both experimentally and theoretically using newly designed planar chiral Pt analogues (1) having three-dimensional superstructures. A series of optically pure polymethylene-vaulted Pt complexes (R)- and (S)-1 were synthesized and characterized with regard to the chiroptical behaviour of the trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms. These complexes were found to exhibit structure-dependent chiroptical characteristics in solution, such that the absolute values of specific rotation, the circular dichroism dissymmetry factor (gabs) and the circularly polarized luminescence dissymmetry factor (glum) all increased upon shortening the polymethylene bridges. Density functional theory and time dependent density functional theory calculations were used to analyse vaulted and non-vaulted complexes, which demonstrated that the present linker-dependent chiroptical properties resulted from constraint-induced changes in the square planar Pt coordination centres rather than from chiral distortion along the coordination platforms.

SHG-active luminescent thermometers based on chiral cyclometalated dicyanidoiridate(iii) complexes

Multifunctional optical materials can be realized by combining stimuli-responsive photoluminescence (PL), e.g., optical thermometry, with non-linear optical (NLO) effects, such as second-harmonic generation (SHG). We report a novel approach towards SHG-active luminescent thermometers achieved by constructing unique iridium(iii) complexes, cis-[IrIII(CN)2(R,R-pinppy)2]- (R,R-pinppy = (R,R)-2-phenyl-4,5-pinenopyridine), bearing both a chiral 2-phenylpyridine derivative and cyanido ligands, the latter enabling the formation of a series of molecular materials: (TBA)[IrIII(CN)2(R,R-pinppy)2]·2MeCN (1) (TBA+ = tetrabutylammonium) and (nBu-DABCO)2[IrIII(CN)2(R,R-pinppy)2](i)·MeCN (2) (nBu-DABCO+ = 1-(n-butyl)-1,4-diazabicyclo-[2.2.2]octan-1-ium) hybrid salts, (TBA)2{[LaIII(NO3)3(H2O)0.5]2[IrIII(CN)2(R,R-pinppy)2]2} (3) square molecules, and {[LaIII(NO3)2(dmf)3][IrIII(CN)2(R,R-pinppy)2]}·MeCN (4) coordination chains. Thanks to the chiral pinene group, 1-4 crystallize in non-centrosymmetric space groups leading to SHG activity, while the N,C-coordination of ppy-type ligands to Ir(iii) centers generates visible charge-transfer (CT) photoluminescence. The PL characteristics are distinctly temperature-dependent which was utilized in achieving ratiometric optical thermometry below 220 K. The PL phenomena were rationalized by DFT/TD-DFT calculations indicating an MLCT-type of the emission in obtained Ir(iii) complexes with the rich vibronic structure providing a few emission bands that variously depend on temperature due to the role of thermally activated vibrations. As these crucial vibrational modes depend on the crystal lattice, the thermometry performance differs within 1-4 being the most efficient in 4 while the SHG is by far the best also for 4. This proves that pinene-functionalized cyclometalated dicyanidoiridates(iii) are great prerequisites for tunable PL-NLO conjunction with the most effective multifunctionality ensured by the insertion of these anions into bimetallic frameworks.

Structurally Diverse Pyrene-decorated Planar Chiral [2,2]Paracyclophanes with Tunable Circularly Polarized Luminescence between Monomer and Excimer

We reported the synthesis of a series of structurally diverse CPL-active molecules, in which pyrene units were installed to chiral pm/po-[2,2]PCP scaffolds either with or without a triple bond spacer for pm/po-PCP-P1 and pm/po-PCP-P2, respectively. The X-ray crystallographic analyses revealed that these pyrene-based [2,2]PCP derivatives exhibited diverse structures and crystal packings in the solid phases. The pyrene-based [2,2]PCP derivatives exhibit various (chir)optical properties in organic solutions, depending on their respective structures. In a mixture of dioxane and water, pm/po-PCP-P1 emit green excimer fluorescence, whereas pm/po-PCP-P2 emit blue one. The chiroptical investigation demonstrated that Rp-pm-PCP-P1 and Rp-pm-PCP-P2 exhibited completely opposite CD and CPL signals even they possess the same chiral Rp-[2,2]PCP core. The same argument also holds for other chiral pyrene-based [2,2]PCP derivatives. The theoretical calculation revealed that these unusual phenomena were attributed to different orientation between transition electric dipole moments and the magnetic dipole moments originating from the presence or absence of a triple bond spacer. These pyrene-based [2,2]PCP derivatives display various colours and fluorescence emissions in the solid state and PMMA films, possibly due to the different packings as observed in the crystal structure. Moreover, these compounds also can interact with perylene diimide through π-π interactions, leading to near-white fluorescence.

Circularly polarized phosphorescence with a large dissymmetry factor from a helical platinum(II) complex

A chiral platinum(II) complex with a helical Schiff-base [4]helicene ligand exhibits intense red circularly polarized phosphorescence (CPP) with a glum of 0.010 in the dilute solution state. The intense CPP was caused by a change in the electronic transition character based on the induction of the helical structure.

Kinetically Controlled Structural Modulation of the Self-Assembled Silver Nanoclusters

Metal nanoclusters (NCs) with atomic precision are growing into a fascinating class of building blocks for supramolecular chemistry. What makes it more interesting is the enhanced optical properties of the ordered structures, including aggregation-induced emission (AIE). However, algorithm dictating the self-assembly of metal NCs in multicomponent environment remains largely unknown, and effective means to manipulate the self-assembly is still lacking, especially under kinetic control. Herein, nanofibers which contain sub-1 nm nanowires and exhibit circularly polarized phosphorescence (CPP) are obtained from crystallization-induced self-assembly (CISA) of water-soluble, negatively charged silver NCs (Ag9 -NCs) in the presence of glutamic acid (Glu). By the introduction of a positively-charged additive (choline chloride, CC), the structure of the nanowires is modulated and the lateral interaction between adjacent nanofibers is adjusted, leading to simultaneous improvement of the phosphorescence and chirality which finally enhances CPP. Importantly, changing the time at which CC is introduced altered the kinetic pathway of the CISA, which enables to effectively manipulate both the final structures of the self-assembled Ag9 -NCs and the output of the optical signals.

A supramolecular assembly-based strategy towards the generation and amplification of photon up-conversion and circularly polarized luminescence

For the molecular properties in which energy transfer/migration is determinantal, such as triplet-triplet annihilation-based photon up-conversion (TTAUC), the overall performance is largely affected by the intermolecular distance and relative molecular orientations. In such scenarios, tools that may steer the intermolecular interactions and provide control over molecular organisation in the bulk, become most valuable. Often these non-covalent interactions, found predominantly in supramolecular assemblies, enable pre-programming of the molecular network in the assembled structures. In other words, by employing supramolecular chemistry principles, an arrangement where molecular units are arranged in a desired fashion, very much like a Lego toy, could be achieved. This leads to enhanced energy transfer from one molecule to other. In recent past, chiral luminescent systems have attracted huge attention for producing circularly polarized luminescence (CPL). In such systems, chirality is a necessary requirement. Chirality induction/transfer through supramolecular interactions has been known for a long time. It was realized recently that it may help in the generation and amplification of CPL signals as well. In this review article we have discussed the applicability of self-/co-assembly processes for achieving maximum TTA-UC and CPL in various molecular systems.

X/Y platinum(II) complexes: some features of supramolecular assembly via halogen bonding

Four series of new luminescent cyclometalated complexes [Pt(C^N)(IPy)Y] (HC^N = 2-phenylpyridine (Hppy), 2-(1-benzofuran-3-yl)pyridine (Hbfpy), methyl-2-phenylquinoline-4-carboxylate (Hmpqc), 2-(1-benzothiophen-3-yl)pyridine (Hbtpy), IPy = 4-iodopyridine, and Y = Cl, Br, I) have been investigated as X/Y 'building blocks' for the construction of a supramolecular network utilizing the I atom in IPy as a halogen bond (XB) donor (the X atom). The σ-hole of the X atom was found to provide non-covalent X⋯Y, X⋯Pt and X⋯π (π system of the metalated chelate ring) interactions for the complexes in the crystal state. NBO analysis confirms donation of the platinum electron density to iodine upon the X⋯Pt interaction. The nature of the X counterpart in XB depends on the nature of the Y atom and the cyclometalating ligand of the Pt(II) complex. DFT calculations show that the HOMO of [Pt(C^N)(IPy)Y] in the S0 state is delocalized over Pt, Y and a C-coordinating fragment of C^N, while the LUMO in most complexes is formed by the Py orbitals of IPy. However, the α-HOMO in the lowest triplet state of [Pt(C^N)(IPy)Y] contains no contribution of the IPy wavefunctions. All Pt(II) complexes exhibited triplet luminescence in solution and in the solid state (Φ up to 0.129), which is determined by the nature of the C^N ligand. The emission profile is independent of the nature of the ligand Y, while the quantum yield decreases from Cl to I. Accordingly, on the basis of DFT calculation, this emission is interpreted as a C^N intraligand charge transfer predominantly. The XB formation did not show an effect on the luminescence of the complexes in the solid phase, however grinding of crystals results in an increase of brightness.

Vapor-Induced Assembly of a Platinum(II) Complex Loaded on Layered Double Hydroxide Nanoparticles

Controlled self-assembly of PtII complexes is key to the development of optical and stimuli-responsive materials, but designing and precisely controlling them is still difficult owing to weak intermolecular interactions. Herein, we report the successful water-vapor-induced assembly of an anionic PtII complex [Pt(CN)2 (ppy)]- (Hppy=2-phenylpyridine) electrostatically loaded onto cationically charged layered double hydroxide (LDH) nanoparticles consisting of Mg2+ and Al3+ ions. When the PtII complexes were densely loaded onto the LDH nanoparticles, the assembly was maintained, even in dilute aqueous media. In the case of sparse loading, the PtII complexes were loaded discretely in the dry state; however, when water vapor was adsorbed, the increased mobility of the PtII complexes led to their assembly on the LDH nanoparticles. The presence of water vapor led to a drastic change in luminescence from green to orange.

Enhancing Circularly Polarized Phosphorescence via Integrated Top-Down and Bottom-Up Approach

In the dynamic domain of chiroptical technologies, it is imperative to engineer emitters endowed with circularly polarized luminescence (CPL) properties. This research demonstrates an advancement by employing a combined top-down and bottom-up strategy for the simultaneous amplification of photoluminescence quantum yield (Φ) and the luminescence dissymmetry factor (glum ). Square-planar Pt(II) complexes form helical assemblies, driven by torsional strain induced by bis(nonyl) chains. Integration of chiral anions leads these assemblies to prefer distinct helical sense. This arrangement activates the metal-metal-to-ligand charge transfer (MMLCT) transition that is CPL-active, with Φ and |glum | observing an upswing contingent on the charge number and aryl substituents in chiral anions. Utilizing the soft-lithographic micromolding in capillaries technique, we could fabricate exquisitely-ordered, one-dimensional co-assemblies to achieve the metrics to Φ of 0.32 and |glum | of 0.13. Finally, our spectroscopic research elucidates the underlying mechanism for the dual amplification, making a significant stride in the advancement of CPL-active emitters.

Chiroptical Response Control of Planar and Axially Chiral Polymethylene-Vaulted Platinum(II) Complexes Bearing 1,1'-Binaphthyl Frameworks

In this study, the synthesis, structure, and chiroptical response control of planar chiral polymethylene-vaulted trans-bis[(β-iminomethyl)aryloxy]platinum(II) complexes bearing axially chiral 1,1'-binaphthyl ligands are described. A series of enantiopure polymethylene (n = 4-10)-vaulted complexes were prepared in 6 steps using commercially available (R)- or (S)-BINOL as the starting material without an optical resolution process. The trans-coordination and three-dimensional vaulted structures of the platinum complexes were elucidated from X-ray diffraction (XRD) studies. The complexes were found to show structural dependence of chiroptical responses in the dilute solution state such that the absolute values of [α]D, dissymmetry factors gabs in circular dichroism (CD), and glum in circularly polarized luminescence (CPL) increased upon shortening the length of the polymethylene bridges. The enhanced chiroptical responses were theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, and the results are discussed in terms of the molecular structures and transition dipole moments of the ground states. The structural dependence of the chiroptical responses was ascribed to the distortion of the coordination platforms caused by restriction of the vaulting methylene linkers.

Chiral B-N AIEgens: Intense Blue Circularly Polarized Luminescence and Piezochromism

We present a new class of blue circularly polarized luminescent emitters based on tetraarylaminoborane (TAAB) with considerable dissymmetry factor in the solid state. The chiral pendant 1-phenylethylamine in BN-RR and BN-SS imparts chirality to the core chromophore, resulting in circularly polarized luminescence signals (glum = 0.8 × 10-3) with a quantum yield of 33% in the crystalline state. This novel set of compounds also showcases intriguing thermally reversible piezochromism.

Enantiopure cycloplatinated pentahelicenic N-heterocyclic carbenic complexes that display long-lived circularly polarized phosphorescence

The preparation of the first enantiopure cycloplatinated complexes bearing a bidentate, helicenic N-heterocyclic carbene and a diketonate ancillary ligand is presented, along with their structural and spectroscopic characterization based on both experimental and computational studies. The systems exhibit long-lived circularly polarized phosphorescence in solution and in doped films at room temperature, and also in a frozen glass at 77 K, with dissymmetry factor g_lum values ≥10^-3 in the former and around 10^-2 in the latter.

Aggregation-induced circularly polarized phosphorescence of Pt(II) complexes with an axially chiral BINOL ligand

A pair of chiral Pt(II) complexes coordinated by simple BINOL and bipyridine ligands displaying aggregation-induced phosphorescence and circularly polarized luminescence were characterized by X-ray crystallography and absorption and emission spectroscopies. The emission of the powder sample was reddish whereas the thin film dispersed in PMMA (f_Pf = 1 wt%) exhibited a white emission.

Mechano-Stimulus and Environment-Dependent Circularly Polarized TADF in Chiral Copper(I) Complexes and Their Application in OLEDs

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exciton decay are highly attractive for electroluminescent devices (OLEDs) or next-generation photonic applications, such as spintronics, quantum computing, cryptography, or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure {Cu(Cbz^R)[(S/R)-BINAP]} [R = H (1), 3,6-tBu (2)] are efficient thermally activated delayed fluorescence (TADF) emitters with high radiative rate constants of k_TADF up to 3.1 × 10⁵ s^-1 from ^1/3LLCT states according to our temperature-dependent time-resolved luminescence studies. The efficiency of the TADF process and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupted by grinding of the crystalline materials. The origin of this pronounced mechano-stimulus photophysical behavior is a thermal equilibrium between the ^1/3LLCT states and a ³LC state of the BINAP ligand, which depends on the relative energetic order of the excited states and is prone to inter-ligand C-H···π interactions. The copper(I) complexes are also efficient CPL emitters displaying exceptional dissymmetry values g_lum of up to ±0.6 × 10^-2 in THF solution and ±2.1 × 10^-2 in the solid state. Importantly for application in electroluminescence devices, the C-H···π interactions can also be disrupted by employing sterically bulky matrices. Accordingly, we have investigated various matrix materials for successful implementation of the chiral copper(I) TADF emitters in proof-of-concept CP-OLEDs.

Vortex Flow-controlled Circularly Polarized Luminescence of Achiral Pt(II) Complex Aggregates Assembled at the Air-Water Interface

Circularly polarized luminescence (CPL) has been researched for various applications by control of characteristics such as chirality and magnitude. Supramolecular chirality has been prepared by vortex motion as a mechanical stimulus; however, CPL has yet to be controlled precisely and reproducibly. In this work, the first precise control of CPL under vortex flow conditions at an air-water interface is reported. The supramolecular chirality of aggregates consisting of an achiral trans-bis(salicylaldiminato)Pt(II) complex bearing hexadecyl chains is induced and controlled with vortex flow at the air-water interface, whereas the complex naturally forms an achiral amorphous solid with non-chiroptical properties under non-vortex conditions. The CPL direction and magnitude (g_lum value) of the Pt(II) complex aggregates can be adjusted precisely according to the vortex conditions, including the rotatory direction and flow rate. Vortex-flow-induced emission enhancement is also observed upon an increase in the rate of the vortex flow.

Circularly polarized luminescence of pinene-modified tetradentate platinum(II) enantiomers containing fused 5/6/6 metallocycles

In this study, a couple of tetradentate Pt(II) enantiomers ((−)-1 and (+)-1) and a couple of tetradentate Pt(IV) enantiomers ((−)-2 and (+)-2) containing fused 5/6/6 metallocycles have been synthesized by controlling reaction conditions. Two valence forms could transform into each other through mild chemical oxidants and reductants. Single-crystal X-ray diffraction confirms the structures of (−)-1 and (−)-2. The coordination sphere of the Pt(II) cation in (−)-1 displays a distorted square-planar geometry and a platinum centroid helix chirality. In contrast, the structure of (−)-2 reveals a distorted octahedral geometry. The solution and the solid of (−)-1 are highly luminescent. Complex (−)-1 shows a prominent aggregation-induced emission enhancement (AIEE) behavior in DMSO/water solution with emission quantum yield (Φ_em) up to 73.2%. Furthermore, highly phosphorescent Pt(II) enantiomers exhibit significant circularly polarized luminescence (CPL) with a dissymmetry factor (g_lum) of order 10⁻³ in CH₂Cl₂ solutions at room temperature. Symmetrically appreciable CPL signals are observed for the enantiomers (−)-1 and (+)-1.

In Situ Regulation of Microphase Separation-Recognized Circularly Polarized Luminescence via Photoexcitation-Induced Molecular Aggregation

Circularly polarized luminescence (CPL) has attracted great interest owing to its extensive optical information and chiral structural dependence. However, rationally regulating solid-phase CPL signals remains difficult because of the close packing of molecules in solid-state materials and the lack of structural visualization. In this work, we proposed a microphase-separation-recognized CPL regulation strategy via coassembly of a hexathiobenzene-based luminophore and chiral block copolymer (cBCP) with in situ photocontrollability. As a consequence to the continuous increase in the luminophore-to-cBCP ratio, the CPL signal of the supramolecular system exhibited an increasing trend until a critical point. Then, further increasing the ratio stretched the helical pitch of cBCP, which led to CPL reduction. With the photoexcitation-induced molecular aggregation of the luminophore, which was implemented using in situ photoirradiation, the helical pitch was retracted along with the restoration of the CPL signal. These processes were fully recognized and monitored by the microphase-separated nanomorphological change of the coassembled system, which indicated that such a structural contrast could be an effective method for rationally regulating the supramolecular chiropticity of solid-state materials.

Circularly Polarized Luminescence of Cyclometalated Platinum(II) Complex Excimers: Large Difference between Isomers

A series of platinum(II) complexes bearing a chiral β-diketonato ligand and a cyclometalated ligand have been prepared. The platinum(II) complexes, (SP-4-3)-[Pt(ppy)(D-tac)] (ppy = 2-phenylpyridine, D-tac = 3-trifluoroacetyl-(D)-camphor), (SP-4-4)-[Pt(ppy)(D-tac)], (SP-4-3)-[Pt(ppy)(D-pbc)] (D-pbc = 3-perfluorobutyryl-(D)-camphor), and (SP-4-4)-[Pt(ppy)(D-pbc)], and their enantiomers were isolated and characterized by elemental analysis, NMR, and X-ray structural analysis. Photoisomerization between SP-4-3 (trans) and SP-4-4 (cis) isomers was observed. Green emission due to the monomer was observed in diluted solutions for all complexes. Higher quantum yields and longer lifetimes of green emission were observed in nonpolar solvents compared to polar solvents. The two geometrical isomers had surprisingly different excimer formation efficiencies. For the trans isomers, orange emission due to the excimers was observed in nonpolar solvents at high concentrations, whereas negligible intensities of the excimer emission were observed for the cis isomers. The formation of the excimers was evaluated by emission decay and time-resolved emission spectra. For the trans isomers, the green emission due to the monomer showed negligible CPL signals, but the orange emission gave pronounced CPL signals. The dissymmetry factors, g-values, of the excimer CPL (g_lum = 0.002) were enhanced over those of the circular dichroism (g_abs = 0.0002, g_lum/g_abs = 10). The intensities of the emission and the CPL of the excimer under oxygen were very small, although those under an argon atmosphere were very strong. Therefore, the emission color of the trans-isomers was changed from green to orange by deoxygenation.

Multistimuli-Responsive Properties of Aggregated Isocyanide Cycloplatinated(II) Complexes

Here, we describe the neutral cyclometalated tert-butylisocyanide Pt^II complexes, [Pt(C^∧N)Cl(CNBu^t)] 1, the double salts [Pt(C^∧N)(CNBu^t)₂][Pt(C^∧N)Cl₂] 2, and the cationic complexes [Pt(C^∧N)(CNBu^t)₂]ClO₄ 3 [C∧N = difluorophenylpyridine (dfppy, a), 4-(2-pyridyl)benzaldehyde (ppy-CHO, b)]. A comparative study of the pseudopolymorphs 1a, 1a·CHCl₃, 1b, 1b·0.5Toluene, 1b·0.5PhF, and 3a·0.25CH₂Cl₂ reveals strong aggregation through Pt···Pt and/or π···π stacking interactions to give a variety of distinctive one-dimensional (1D) infinite chains, which modulate the photoluminescent properties. This intermolecular long-range aggregate formation is the main origin of the photoluminescent behavior of 1a and 1b complexes, which exhibit highly sensitive and reversible responses to multiple external stimuli including different volatile organic compounds (VOCs), solvents, temperatures, and pressures, with distinct color and phosphorescent color switching from green to red. Furthermore, complex 1b undergoes supramolecular self-assembly via Pt···Pt and/or π···π interactions into a polymer thin polystyrene (PS) film 10 wt % in response to toluene vapors, and 3a exhibits vapochromic and vapoluminescent behavior. Theoretical simulations on the dimer, trimer, and tetramer models of 1a and 1b have been carried out to get insight into the photophysical properties in the aggregated solid state.

Chiral Arylene Diimide Phosphors: Circularly Polarized Ambient Phosphorescence from Bischromophoric Pyromellitic Diimides

Chiral organic phosphors with circularly polarized room-temperature phosphorescence (CPP) provide new prospects to the realm of circularly polarized luminescence (CPL) materials, owing to the long-lived triplet states and persistent emission. Although several molecular designs show efficient room-temperature phosphorescence (RTP), realization of ambient organic CPP remains a formidable challenge. Herein, we introduce a chiral bischromophoric phosphor design to realize ambient CPP emission by appending molecular phosphors to a chiral diaminocyclohexane core. Thus, solution-processable polymer films of the trans-1,2-diaminocyclohexane (DAC) chiral cores with heavy-atom substituted pyromellitic diimide phosphors, exhibits one of the most efficient exclusive CPP emissions with high phosphorescence quantum yield (≈18 % in air and ≈46 % under vacuum) and significant luminescence dissymmetry factor (|g_lum |≈4.0×10^-3 ).

Phosphorescent Cyclometalated Platinum(II) Enantiomers with Circularly Polarized Luminescence Properties and Their Assembly Behaviors

Platinum(II) complexes as supramolecular luminescent materials have received considerable attention due to their unique planar structures and fascinating photophysical properties. However, the molecular design of platinum(II) complexes with impressive circularly polarized luminescence properties still remains challenging and rarely explored. Herein, we reported a series of cyclometalated platinum(II) complexes with benzaldehyde and its derived imine-containing alkynyl ligands to investigate their phosphorescent, chiroptical, and self-assembly behaviors. An isodesmic growth mechanism is found for their temperature-dependent self-assembly process. The chiral sense of the enantiomers can be transferred from the chiral alkynyl ligands to the cyclometalated platinum(II) dipyridylbenzene N^C^N chromophore and further amplified through supramolecular assembly via intermolecular noncovalent interactions. Notably, distinctive phosphorescent properties and nanostructured morphologies have been found for enantiomers 4R and 4S. Their intriguing self-assembled nanostructures and phosphorescence behaviors are supported by crystal structure determination, ¹H NMR, emission, and UV-vis absorption spectroscopy, scanning electron microscopy, and X-ray powder diffraction studies.

Ruthenium Complexes Bearing Axially Chiral Bipyridyls: The Mismatched Diastereomer Showed Red Circularly Polarized Phosphorescence

Ruthenium^II complexes bearing three axially chiral bipyridyl ligands were synthesized as a new family of chiral complex dyes, and Δ-(S)- and Λ-(S)-diastereomers were obtained. The X-ray crystal structure analyses, spectroscopy, and DFT calculations suggested that all the bipyridyls maintained chirality in both the ground and excited states, and the Δ-(S)- and Λ-(S)-isomers are the matched (more relaxed) and mismatched (more constrained) pairs, respectively. The mismatched Λ-(S)-isomer exhibited red circularly polarized phosphorescence (CPP) both in solution and in the solid state. The solution state CPP is the most intense of ruthenium complexes, while the solid state CPP is the first example of them. It is supposed that, for the Λ-(S)-isomer, the six cumulative CH/π interactions suppress further distortion in the T₁ state.

Bamboo-like π-Nanotubes with Tunable Helicity and Circularly Polarized Luminescence

We report the fabrication of an exotic bamboo-like π-nanotube via the hierarchical self-assembly of a dipeptide-substituted naphthalenediimide gelator with tunable helicity and circularly polarized luminescence (CPL). It was found that in the presence of trifluoroacetic acid (TFA) the gelator molecules self-assembled into a bamboo-like π-nanotube, which is composed of truncated nanocones and CPL active. When defining the diameter ratio of the lower to upper edge of each nanocone as a parameter to express the helicity of different nanotubes, it was found that both the helicity and CPL of these nanotubes can be adjusted by the amount of TFA. Moreover, the helicity of the nanotube can be conveyed to the achiral quantum dots (QDs) and produce a hybrid nanotube/QDs CPL active materials with adjustable dissymmetry factor. This work finds a new type self-assembled bamboo-like π-nanotube and unveils their helicity and CPL control.

Twist to Boost: Circumventing Quantum Yield and Dissymmetry Factor Trade-Off in Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) enables promising applications in asymmetric photonics. However, the performances of CPL molecules do not yet meet the requirements of these applications. The shortcoming originates from the trade-off in CPL between the photoluminescence quantum yield (PLQY) and the photoluminescence dissymmetry factor (g_PL). In this study, we developed a molecular strategy to circumvent this trade-off. Our approach takes advantage of the strong propensity of [Pt(N^C^N)Cl], where the N^C^N ligand is 1-(2-oxazoline)-3-(2-pyridyl)phenylate, to form face-to-face stacks. We introduced chiral substituents, including (S)-methyl, (R)- and (S)-isopropyl, and (S)-indanyl groups, into the ligand framework. This asymmetric control induces torsional displacements that give homohelical stacks of the Pt(II) complexes. X-ray single-crystal structure analyses for the (S)-isopropyl Pt(II) complex reveal the formation of a homohelical dimer with a Pt···Pt distance of 3.48 Å, which is less than the sum of the van der Waals radii of Pt. This helical stack elicits the metal-metal-to-ligand charge-transfer (MMLCT) transition that exhibits strong chiroptical activity due to the electric transition moment making an acute angle to the magnetic transition moment. The PLQY and g_PL values of the MMLCT phosphorescence emission of the (S)-isopropyl Pt(II) complex are 0.49 and 8.4 × 10^-4, which are improved by factors of ca. 6 and 4, respectively, relative to the values of the unimolecular emission (PLQY, 0.078; g_PL, 2.4 × 10^-4). Our photophysical measurements for the systematically controlled Pt(II) complexes reveal that the CPL amplifications depend on the chiral substituent. Our investigations also indicate that excimers are not responsible for the enhanced chiroptical activity. To demonstrate the effectiveness of our approach, organic electroluminescence devices were fabricated. The MMLCT emission devices were found to exhibit simultaneous enhancements in the external quantum efficiency (EQE, 9.7%) and the electroluminescence dissymmetry factor (g_EL, 1.2 × 10^-4) over the unimolecular emission devices (EQE, 5.8%; g_EL, 0.3 × 10^-4). These results demonstrate the usefulness of using the chiroptically active MMLCT emission for achieving an amplified CPL.

Constructing chiral gold nanorod oligomers using a spatially separated sergeants-and-soldiers effect

A sergeants-and-soldiers (S&S) effect is very useful to the fabrication of supramolecular chirality. This strategy has not yet been explored in the construction of chiral plasmonic superstructures. Herein, we demonstrate a spatially separated S&S effect in fabricating plasmonic superstructures and modulating their chiroptical responses. Specifically, chiral cysteine (Cys) molecules, acting as sergeants, are sandwiched between a gold nanorod (AuNR) core and a Au shell via AuNR-templated Au overgrowth. Cationic surfactants, CTAB (cetyltrimethylammonium bromide) or CPC (cetylpyridinium chloride), are modified on the AuNR@Cys@Au shell surface, thus spatially separating from the chiral sergeants. During the assembly process, the surfactants act as soldiers which could transfer and amplify the local chirality induced by the adsorbed chiral molecules from the plasmonic monomers to the oligomers. Huge PCD signals could be achieved in the plasmonic oligomers by finely tuning chiral sergeants and achiral soldiers, indicating the feasibility of the S&S effect in fabricating chiral plasmonic superstructures.

Supramolecular Assembly of An Organoplatinum(II) Complex with Ratiometric Dual Emission for Two-Photon Bioimaging

The organoplatinum(II) complex [Pt(C^N^N)(Cl)] (C^N^N=5,6-diphenyl-2,2'-bipyridine, Pt1) can assemble into nanoaggregates via π-π stacking and complementary hydrogen bonds, rather than Pt-Pt interactions. Pt1 exhibits ratiometric dual emission, including rare blue emission (λ_em =445 nm) and assembly-induced yellow emission (λ_em =573 nm), under one- and two-photon excitation. Pt1 displays blue emission in cells with an intact membrane due to its low cellular uptake. In cells where the membrane is disrupted, uptake of the complex is increased and at higher concentrations yellow emission is observed. The ratio of yellow to blue emission shows a linear relationship to the loss of cell membrane integrity. Pt1 is, to our knowledge, the first example of an assembly-induced two-photon ratiometric dual emission organoplatinum complex. The excellent and unique characteristics of the complex enabled its use for the tracking of cell apoptosis, necrosis, and the inflammation process in zebrafish.

Helically chiral Pd(ii) complexes containing intramolecular PdPd metallophilicity as circularly polarized molecular phosphors

Carbon-centred chirality of the pincer-type cyclometalated ligands is transferred to the helical chirality of dinuclear and tetranuclear Pd(ii) arylacetylide complexes, and hence phosphorescence with quantum yields up to 50% and dissymmetry factors in the 10^-3 scale from the metal-metal-to-ligand charge-transfer excited states has been recorded in diluted solutions.

Aggregation-Induced Circularly Polarized Luminescence: Chiral Organic Materials for Emerging Optical Technologies

Chirality is becoming increasingly important in the design of organic materials with functional properties, when bulk anisotropy is needed. In the past decades, a plethora of chiral organic materials have been studied and developed. Nanostructures have brought substantial advancement to the realization of organic-molecule-based devices, and the possibilities for solid-state light emission are very promising in view of potential applications. Scientific approaches to the realization of chiral emissive materials are indeed growing exponentially. The chiral nanostructures discussed are related both to the way in which luminescence is generated and the way in which it is detected. As to the former, the focus will be on organic chromophores with aggregation-induced emission properties, so that emission is present, or at least largely amplified, when the molecules are in the aggregated state. As to the latter, the focus will be on the ability and a quantitative comparison of organic nanostructures capable of circularly polarized emission.

Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes

Chiral molecules are essential for the development of advanced technological applications in spintronic and photonic. The best systems should produce large circularly polarized luminescence (CPL) as estimated by their dissymmetry factor (g _lum), which can reach the maximum values of -2 ≤ g _lum ≤ 2 when either pure right- or left-handed polarized light is emitted after standard excitation. For matching this requirement, theoretical considerations indicate that optical transitions with large magnetic and weak electric transition dipole moments represent the holy grail of CPL. Because of their detrimental strong and allowed electric dipole transitions, popular chiral emissive organic molecules display generally moderate dissymmetry factors (10^-5 ≤ g _lum ≤ 10^-3). However, recent efforts in this field show that g _lum can be significantly enhanced when the chiral organic activators are part of chiral supramolecular assemblies or of liquid crystalline materials. At the other extreme, chiral Eu^III- and Sm^III-based complexes, which possess intra-shell parity-forbidden electric but allowed magnetic dipole transitions, have yielded the largest dissymmetry factor reported so far with g _lum ~ 1.38. Consequently, 4f-based metal complexes with strong CPL are currently the best candidates for potential technological applications. They however suffer from the need for highly pure samples and from considerable production costs. In this context, chiral earth-abundant and cheap d-block metal complexes benefit from a renewed interest according that their CPL signal can be optimized despite the larger covalency displayed by d-block cations compared with 4f-block analogs. This essay thus aims at providing a minimum overview of the theoretical aspects rationalizing circularly polarized luminescence and their exploitation for the design of chiral emissive metal complexes with strong CPL. Beyond the corroboration that f-f transitions are ideal candidates for generating large dissymmetry factors, a special attention is focused on the recent attempts to use chiral Cr^III-based complexes that reach values of g _lum up to 0.2. This could pave the way for replacing high-cost rare earths with cheap transition metals for CPL applications.

New Perspectives to Trigger and Modulate Circularly Polarized Luminescence of Complex and Aggregated Systems: Energy Transfer, Photon Upconversion, Charge Transfer, and Organic Radical

Chiral functional materials with circularly polarized luminescence (CPL) have risen rapidly in recent years because of their fascinating characteristics and potential applications in various research fields. CPL refers to the differential spontaneous emission of left (L)- and right (R)-handed circularly polarized light upon photon or electron excitation. Generally, an outstanding CPL-active material needs to possess a high luminescence dissymmetry factor (g_lum) (defined as 2(I_L - I_R)/(I_L + I_R) where I is the emission intensity), which is between -2 and +2. Although the exciting development in CPL-active materials was achieved, the modulation of CPL signs is still a challenge. For small organic systems, a relatively small g_lum value, one of the key parameters of CPL, limits their practical applications. Searching for efficient approaches for amplifying g_lum is important. Therefore, over the past decades, besides optimizing the structure of small molecules, many other strategies to obtain efficient CPL-active materials have been developed. For instance, self-assembly has been well demonstrated as an effective approach to amplify the supramolecular chirality as well as the g_lum values. On the other hand, chiral liquid crystals (CLCs), which are capable of selective reflection of left- and right-handed circularly polarized light, also to serve as a host matrix for endowing guest emitters with CPL activity and high g_lum values. However, self-assembly focuses on modulating the conformation and spatial arrangement of chiral emitters. And the CPL of a luminophore-doped CLC matrix depends on the helix pitch and band gap positions. Lately, novel photophysical approaches to modulate CPL signs have gradually emerged.In this Account, we discuss the recent progress of excited-state-regulation involved CPL-active materials. The emergence, amplification, and inversion of CPL can be adjusted through regulation of the excited state of chiral emitters. For example, Förster resonance energy transfer (FRET) can amplify the g_lum values of chiral energy acceptors in chiral supramolecular assemblies. By combining the concepts of photon upconversion and CPL, high-energy upconverted circularly polarized emission was achieved under excitation of low-energy light, accompanied by an amplified g_lum. In addition, the organic systems with unpaired electrons, i.e., charge transfer (CT) system and open-shell π-radical, show favorable CPL properties, which can be flexibly tuned with an applied magnetic field. It should be noted that these photophysical process are associated with the excited state of chiral emitters. So far, while the main focus is on the regulation of the molecular and supramolecular nanostructures, direct regulation of the excited state of the chiral system will serve as a new platform to understand and regulate the CPL activity and will be helpful to develop smart chiroptical materials.