Circularly Polarized Luminescence of Chiral Pt(pppb)Cl (pppbH=1-pyridyl-3-(4,5-pinenopyridyl)benzene) Aggregate in the Excited State

Photodynamic Effect of Amphiphilic N∧C∧N-Coordinated Platinum(II) Complexes in Human Umbilical Vein Endothelial Cells

Here, we report a photodynamic therapy (PDT) photosensitizer of N∧C∧N-coordinated Pt(II) complexes: [Pt(L)(solv)]+ (HL = 1,3-(2-dipyridyl)benzene) and [Pt(L)]+@HSA, which is the Pt(II) complex encapsulated in human serum albumin (HSA). The quantum yield of singlet oxygen production for [Pt(L)(solv)]+ is more than 50%, while that for [Pt(L)]+@HSA is much lower. Photoimages of human umbilical vein endothelial cells (HUVECs) treated with the Pt(II) complexes suggest that [Pt(L)(solv)]+ is delocalized in the entire cell after the fast uptake by diffusion and [Pt(L)]+@HSA is taken up by endocytosis and localized on organelles and the cell membrane. [Pt(L)(solv)]+ shows high photocytotoxicity for HUVECs, while [Pt(L)]+@HSA does not show photocytotoxicity.

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.

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.

Kinetic Analysis of Excited-State Dynamics of Emissive Oligomers of Pt(II) Complex in Solution

[Pt(NCN)MeCN]+ (NCN = 1,3-di(2-pyridyl)benzene, MeCN = acetonitrile) forms oligomers, such as dimers and trimers, in solutions due to metallophilic interactions. The emission and absorption spectra in the visible region are considerably changed by the concentrations of the solutions because the excitation energy of the oligomers is dependent on the degree of oligomerization. In this study, excited-state dynamics of [Pt(NCN)MeCN]+ in acetonitrile were investigated by time-resolved emission spectroscopy in time regions from microseconds to nanoseconds at various concentrations. The time-resolved emission spectra recorded with 355 nm photoexcitation showed the decay of the blue-green emission and the rise of the red emission in the microsecond time region. Stern-Volmer analysis of the time-resolved data at various concentrations and wavelengths provides two bimolecular rate constants (4.1 × 109 and 8.2 × 108 M-1 s-1) for the formation processes of the excited-state T1 dimer and T1 trimer, respectively. Kinetic parameters, such as the intrinsic decay rate constants of the T1 monomer, T1 dimer, and T1 trimer, and the association and dissociation rate constants of the T1 dimer and T1 trimer were estimated by fitting the time-resolved emission data at various concentrations.

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.

Assembling of a Water-Soluble N^C^N-Coordinated Pt(II) Complex Aggregate Assisted by Carbon Dioxide in Basic Aqueous Solution

We report an unprecedented result of self-aggregation of [Pt(L1 )Cl] (HL1 =1,3-di(5-carboxy-2-pyridyl)benzene) triggered by CO2 in basic aqueous solution. The color of basic aqueous solution containing [Pt(L1 )Cl] changes from yellow to blue-green during the aggregation resulted from a reaction with CO2 in air. Upon CO2 gas bubbling, strong and broad absorption bands of aggregate assigned to the metal-metal-to-ligand charge-transfer transition appeared at 701 and 1152 nm. Recrystallization of [Pt(L1 )Cl] from Na2 CO3 aqueous solution afforded polymorphic crystals of red and blue-green forms. A single X-ray crystallography revealed that the red form of crystal consists of a Pt-Pt stacked dimer bridged by CO3 2- ion and one of the carboxy groups of L1 is deprotonated. An elemental analysis provided evidence that the blue-green crystal is constructed by linear array consisting of the [Pt(L2 )(CO3 )]3- (HL2 =1,3-di(5-carboxylate-2-pyridyl)benzene) units. The formation process of blue-green aggregate in aqueous solution was monitored through a transient absorption spectrum, and the absorption of aggregates involved in the spectral change were examined by a global analysis. A singular value decomposition and kinetic analysis provide that there are four species resulted from the self-assembling reaction in the solution and the maximal degree of aggregation is at least 32-mer.

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.

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.

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.

Aggregation-induced chirality amplification of optically active fluorescent polyurethane and a cyclic dimer in the ground and excited states

Optically active linear polyurethane and a cyclic dimer were synthesized from 2,7-diisocyanatofluorene and 2,2'-dihydroxy-1,1'-binaphthyl. The circular dichroism (CD) spectral intensity of the polymer was amplified at a higher concentration through aggregate formation, while circularly polarized light (CPL) emission was not enhanced. The cyclic dimer's CPL emission was largely amplified (g_lum 1.1 × 10^-2) due to intermolecular excimer formation through aggregation, while the CD intensity was not affected.

Mechanochromic luminescence properties of fluoro-substituted pinene-containing cyclometalated platinum(II) complexes with multiple triplet excited states

The structure-mechanochromism relationship is explored with respect to packing patterns and corresponding intermolecular interactions that are affected by the number and location of -F. The distinct and reversible mechanochormic luminescence (Δλem up to ca. 90 nm) of yellow solids (-)-1-Yg, (-)-2-Yg, and (-)-3-Yg was displayed with a simultaneous crystal-to-amorphous transformation. The change of multiple triplet excited states accounted for the mechanochormic luminescence, and a switch from the 3π,π* monomer to the excimer/3MMLCT occurred in the grinding process. The mechanical force led to perturbation in the molecular packing, and aggregates with effective PtPt and π-π interactions were formed in the amorphous phase, leading to the variation of excited states. The mechanochromic luminescence could be reverted by dropping in CH2Cl2 and could be cycled multiple times without perceivable performance degradation. This work gives a reference for designing mechanochromic luminescent materials toward multicolor and multicomponent responses.

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.

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.

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.

Enhanced Circularly Polarized Luminescence Activity in Chiral Platinum(II) Complexes With Bis- or Triphenylphosphine Ligands

Distinct circularly polarized luminescence (CPL) activity was observed in chiral (C^∧N^∧N)Pt(II) [(C^∧N^∧N) = 4,5-pinene-6'-phenyl-2,2'-bipyridine] complexes with bis- or triphenylphosphine ligands. Compared to the pseudo-square-planar geometry of chiral (C^∧N^∧N)Pt(II) complexes with chloride, phenylacetylene (PPV) and 2,6-dimethylphenyl isocyanide (Dmpi) ligands, the coordination configuration around the Pt(II) nucleus of chiral (C^∧N^∧N)Pt(II) complexes with bulk phosphine ligands is far more distorted. The geometry is straightforwardly confirmed by X-ray crystallography. The phosphines' participation enhanced the CPL signal of Pt(II) complexes profoundly, with the dissymmetry factor (g _lum) up to 10^-3. The distorted structures and enhanced chiroptical signals were further confirmed by time-dependent density functional theory (TD-DFT) calculations.

Rational Design of Axially Chiral Platinabinaphthalenes with Aggregation-Induced Emission for Red Circularly Polarized Phosphorescent Organic Light-Emitting Diodes

Circularly polarized luminescent (CPL) materials have received a lot of interest due to their potential applications in next-generation displays. However, the development of easily accessible red circularly polarized phosphorescent emitters for practical organic light-emitting diodes fabrication remains a grand challenge. In this paper, we report a new family of CPL-active platinum complexes based on the binaphthalene chiral platform. These axially chiral platinabinaphthalenes were facile synthesized by directly incorporating platinum(II) into the π-conjugated backbone of a commercially available enantiopure binaphthalene derivate. These complexes exhibit aggregation-induced circularly polarized phosphorescence enhancement with high quantum yields of up to 66% and luminescence dissymmetry factors of around 2.6 × 10^-3. Moreover, solution-processable circularly polarized organic light-emitting diodes (CPOLEDs) using these complexes as emitters show good performance with the maximum luminance of up to 3500 cd m^-2 and dissymmetry factor values of around 1.0 × 10^-3. These findings by the rational design of axially chiral platinabinaphthalenes are important for the development of high-performance CPL complexes for CPOLEDs.

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

Molecules capable of producing zero-field circularly polarized phosphorescence (CPP) are highly valuable for chiroptoelectronic applications that rely on triplet exciton. However, the paucity of tractable molecular design rules for obtaining CPP emission has inhibited full utilization. We report amplification of CPP by the formation of helical co-assemblies consisting of achiral square planar cycloplatinated complexes and small fractions of homochiral cycloplatinated complexes. The latter has a unique Pfeiffer effect during the formation of superhelical co-assemblies, enabling versatile chiroptical control. Large dissymmetry factors in electronic absorption (g _abs, 0.020) and phosphorescence emission (g _lum, 0.064) are observed from the co-assemblies. These values are two orders of magnitude improved relative to those of individual molecules. In addition, photoluminescence quantum yields (PLQY) also increase by a factor of ten. Our structural, photophysical, and quantum chemical investigations reveal that the chiroptical amplification is attributable to utilization of both the magnetically allowed electronic transition and asymmetric coupling of excitons. The strategy overcomes the trade-off between g _lum and PLQY which has frequently been found for previous molecular emitters of circularly polarized luminescence. It is anticipated that our study will provide new insight into the future research for the exploitation of the full potential of CPP.

Pt⋯Pt interaction triggered tuning of circularly polarized luminescence activity in chiral dinuclear platinum(ii) complexes

Two couples of chiral binuclear Pt(ii) complexes are capable of displaying distinct CPL activities in the green to red region through tuning Pt⋯Pt interactions.

Solvent-tuned charge-transfer properties of chiral Pt(ii) complex and TCNQ˙− anion adducts

A new pair of adducts comprising one chiral Pt(ii) complex cation, [Pt((−)-L₁)(Dmpi)]⁺ ((−)-1) or [Pt((+)-L₁)(Dmpi)]⁺ ((+)-1) [(−)-L₁ = (−)-4,5-pinene-6′-phenyl-2,2′-bipyridine, (+)-L₁ = (+)-4,5-pinene-6′-phenyl-2,2′-bipyridine, Dmpi = 2,6-dimethylphenylisocyanide], together with one TCNQ˙⁻ anion have been obtained, and the structures have been confirmed via single-crystal X-ray crystallography and infrared (IR) spectroscopy. The chiral Pt(ii) cation and TCNQ˙⁻ anion are dissociated in MeOH solution, while charge transfer adducts are formed in H₂O solution, leading to perturbation of the electronic structure and alteration of the chiral environment, as evidenced by the differences in the UV-vis absorption and electronic circular dichroism spectra. The solvent-tuned charge-transfer properties also have been validated through emission and resonance light scattering spectra. The interesting findings may have potential applications in the development of black absorbers and wide band gap semiconductors.

A new couple of charge transfer adducts comprising of one chiral Pt(ii) complex cation together with one TCNQ˙⁻ anion have been prepared, and solvent-induced variances of absorption, luminescence as well as chiral spectra have been investigated.