Chromic soft crystals based on luminescent platinum(II) complexes

Platinum(II) complexes of square-planar geometry are interesting from a crystal engineering viewpoint because they exhibit strong luminescence based on the self-assembly of molecular units. The luminescence color changes in response to gentle stimuli, such as vapor exposure or weak mechanical forces. Both the molecular and the crystal designs for soft crystals are critical to effectively generate the chromic luminescence phenomenon of Pt(II) complexes. In this topical review, strategies for fabricating chromic luminescent Pt(II) complexes are described from a crystal design perspective, focusing on the structural regulation of Pt(II) complexes that exhibit assembly-induced luminescence via metal-metal interactions and structural control of anionic Pt(II) complexes using cations. The research progress on the evolution of various chromic luminescence properties of Pt(II) complexes, including the studies conducted by our group, are presented here along with the latest research outcomes, and an overview of the frontiers and future potential of this research field is provided.

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.

Solution-state mechanochromic luminescence of Pt(ii)-complexes displayed within micellar aromatic capsules

Mechanochromic luminescence (MCL) is an intrinsic phenomenon in the solid state and thus has been hardly observed in solution so far. Here we report that arylethynyl Pt(ii)-complexes with an NCN-pincer ligand are efficiently encapsulated by micellar aromatic capsules in water, through a simple grinding protocol with bent amphiphiles. When a bent pentamethylbenzene-based amphiphile is employed as an optimized capsule component, the resultant host-guest composite, with an average diameter of ∼4 nm, is obtained in water at room temperature. Notably, the nanocomposite displays strong red emission (Φ = 33%, λmax = 700 nm) derived from MCL via intermolecular Pt(ii)⋯Pt(ii) interactions even under aerobic aqueous conditions, in sharp contrast to the free Pt(ii)-complex with weak green emission (Φ = 4%, λmax = 500 nm) in CH2Cl2. Moreover, enhancement of the solution-state MCL (up to Φ = 48%) can be achieved by coencapsulation of the Pt(ii)-complexes with carbazole derivatives by the capsule in water. This study provides the first example of "solution-state" mechanochromic luminescence, capable of facilely tuning its intensity and wavelength, among the intensive studies of various solid-state MCL reported previously.

Functional Molecular Crystals from the Arylation of a Halogenoplatinum Complex: Stimuli Responsiveness, Comproportionation, and π-Bridged Dimerization

A chloroplatinum complex was arylated to obtain stimuli-responsive molecular crystals. The resulting arylplatinum complex showed polymorph-dependent emission, mechano- and thermochromic luminescence as well as comproportionation and π-bridged dimerization. Simple mixing of structurally similar arylplatinum complexes at room temperature resulted in the transfer of their aryl groups (comproportionation), which allowed their mechanochromic profiles to be tuned. We also found that recrystallization of the complex afforded a dimerized product in which two platinum ions are bridged by aryl groups resulting in a very short (3.0466(10) Å) Pt-Pt distance.

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.

Structural modifications to platinum(II) pincer complexes resulting in changes in their vapochromic and solvatochromic properties

There is a need to develop rapidly responsive chemical sensors for the detection of low concentrations of volatile organic solvents (VOCs). Platinum pincer complexes have shown promise as sensors because of their colours and vapochromic and solvatochromic properties, that may be related to the non-covalent interactions between the pincer complexes and the guest VOCs. Here we report an investigation into a series of Pt(II) complexes based on the 1,3-di(pyridine)benzene tridentate (N⁁C⁁N) skeleton with the formula [Pt(N⁁C(R)⁁N)(CN)] (R = C(O)Me 2, C(O)OEt 3, C(O)OPh 4) with the fourth coordination site occupied by a cyanide ligand. Solid-state samples of the complexes have been tested with a range of volatiles including methanol, ethanol, acetone, dichloromethane and water, and while 2 displays thermochromism, 3 and 4 display rapidly reversible vapochromism and solvatochromism. These results are correlated with X-ray powder and single crystal X-ray structural data including an assessment of the crystal packing and the void space in the crystalline space. The cyanide ligand and the R substituents are involved in hydrogen bonding that creates the voids within the structures and interact with the solvent molecules that influence the Pt⋯Pt separation in the crystalline state.

cis/trans-[Pt(C∧N)(C≡CR)(CNBut)] Isomers: Synthesis, Photophysical, DFT Studies, and Chemosensory Behavior

cis/trans Isomerism can be a crucial factor for photophysical properties. Here, we report the synthesis and optical properties of a series of trans- and cis-alkynyl/isocyanide cycloplatinated compounds [Pt(C^∧N)(C≡CR)(CNBu^t)] [R = C₆H₄-4-OMe 1, 3-C₄H₃S 2; C^∧N = 2-(2,4-difluorophenyl)pyridine (dfppy) (a), 4-(2-pyridyl)benzaldehyde (ppy-CHO) (b)]. The trans-forms do not isomerize thermally in MeCN solution to the cis forms, but upon photochemical irradiation in this medium at 298 K, a variable isomerization to the cis forms was observed. This behavior is in good agreement with the theoretically calculated energy values. The trans/cis configuration, the identity of the cyclometalated, and the alkynyl ligand influence on the absorption and emission properties of the complexes in solution, polystyrene (PS) films, and solid state are reported. All complexes are efficient triplet emitters in all media (except for trans-1a and trans-2a in CH₂Cl₂ solution at 298 K), with emission wavelengths depending mainly on the cyclometalated ligand in the region 473-490 nm (dfppy), 510-550 (ppy-CHO), and quantum yields (ϕ) ranging from 18.5 to 40.7% in PS films. The combined photophysical data and time-dependent density functional theory calculations (TD-DFT) at the excited-state T₁ geometry reveal triplet excited states of ³L'LCT (C≡CR → C^∧N)/³IL (C^∧N) character with minor ³MLCT contribution. The dfppy (a) complexes show a greater tendency to aggregate in rigid media than the ppy-CHO (b) and the cis with respect to the trans, showing red-shifted structureless bands of ³MMLCT and/or excimer-like nature. Interestingly, trans-1a,2a and cis-1a,2a undergo significant changes in the ultraviolet (UV) and emission spectra with Hg²⁺ ions enabling their use for sensing of Hg²⁺ ions in solution. This is clearly shown by the hypsochromic shift and substantial decrease of the low-energy absorption band and an increase of the intensity of the emission in the MeCN solution upon the addition of a solution of Hg(ClO₄)₂ (1:5 molar ratio). Job's plot analysis estimated a 1:1 stoichiometry in the complexation mode of Hg²⁺ by trans-2a. The binding constant (log K) calculated for this system from absorption titration data resulted to be 2.56, and the limit of the detection (LOD) was 6.54 × 10^-7 M.

15N CP/MAS NMR as a Tool for the Mechanistic Study of Mechanical Stimuli-Responsive Materials: Evidence for the Conformational Change of an Emissive Dimethylacridane Derivative

Mechanochromic luminescent molecules are currently attracting considerable attention because of their promising technological applications, but understanding their mechanism of action is challenging and is thus hindering our deeper understanding of these materials. The conformational change of 9,9'-dimethyl-9,10-dihydroacridane derivative 1 was examined using solid-state ¹⁵N nuclear magnetic resonance (NMR) spectroscopic techniques without using a specifically ¹⁵N-labeled compound. A difference between the two conformers was clearly observed in the measurements and was assigned to the ⟨pl⟩ and ⟨bf⟩ spatial structures. The results were supported by quantum chemical calculations on ¹⁵N NMR chemical shifts of each isomer. The technique presented here can clearly identify the structural changes caused by crushing a powder sample. Such structural changes are difficult to determine using X-ray diffraction (XRD) measurements.

Elastic and bright assembly-induced luminescent crystals of platinum(II) complexes with near-unity emission quantum yield

Molecular crystals of Pt(II) complexes with metallophilic interactions can provide bright assembly-induced luminescence with colour tunability. However, the brittleness of many of these crystals makes their application in flexible optical materials difficult. Herein, we have achieved the elastic deformation of crystals of polyhalogenated Pt(II) complexes exhibiting bright assembly-induced luminescence. A crystal of [Pt(bpic)(dFppy)] (Hbpic = 5-bromopicolinic acid, HdFppy = 2-(2,4-difluorophenyl)pyridine) and a co-crystal of [Pt(bpic)(dFppy)] and [Pt(bpic)(ppy)] (Hppy = 2-phenylpyridine) were found to exhibit significant elastic deformation due to their highly anisotropic interaction topologies. While the crystal of [Pt(bpic)(dFppy)] exhibited monomer-based ligand-centred ³ππ* emission with an emission quantum yield of 0.40, the co-crystal exhibited bright, triplet metal-metal-to-ligand charge transfer (³MMLCT) emission owing to Pt⋯Pt interactions, thereby achieving a significantly higher emission quantum yield of 0.94.

Luminescent Pt(II) Complexes Containing (1-Aza-15-crown-5)dithiocarbamate and (1-Aza-18-crown-6)dithiocarbamate: Mechanochromic and Solvent-Induced Luminescence

The strong tendency to stack in the solid state and rich luminescence for the Pt(II) complexes makes them potential candidates as new mechanochromic materials and sensing applications. Six mononuclear complexes [Pt(ppy)(O₄NCS₂)] (1), [Pt(bpy)(O₄NCS₂)]ClO₄ (2), [Pt(ppy)(O₅NCS₂)] (3), [Pt(phen)(O₄NCS₂)]ClO₄·CH₃OH (5a), [Pt(phen)(O₄NCS₂)]ClO₄ (5b), and [Pt(phen)(O₅NCS₂)]ClO₄ (6a), one dinuclear complex [Pt₂(phen)₂(NaO₅NCS₂)₂(ClO₄)₃]ClO₄ (6b), and one one-dimensional (1-D) coordination polymer {[Pt₂(bpy)₂(NaO₅NCS₂)₂(ClO₄)₂](ClO₄)₂}_n (4) were synthesized by reacting [Pt(ppy)Cl]₂, Pt(bpy)Cl₂, and Pt(phen)Cl₂ (ppy = 2-phenylpyridine, bpy = 2,2'-bipyridine, and phen = 1,10-phenanthroline) with (1-aza-15-crown-5)dithiocarbamate (O₄NCS₂) or (1-aza-18-crown-6)dithiocarbamate (O₅NCS₂), respectively, which have been isolated and structurally characterized by X-ray diffraction. Neutral complexes 1 and 3 contain no intermolecular Pt(II)···Pt(II) contact, whereas cationic complexes 2, 5a, 5b, and 6a with ClO₄^- as counteranions show alternative intermolecular Pt(II)···Pt(II) contacts of 3.535/4.091, 3.480/5.001, 3.527/4.571, and 3.446/4.987 Å in the solid state, respectively. Interestingly, complex 4 forms a 1-D coordination polymer through coordination between the encapsulated Na⁺ ions inside the azacrown ether rings of O₅NCS₂ and ClO₄^- anions with respective intra- and intermolecular Pt(II)···Pt(II) contacts of 3.402 and 3.847 Å in crystal lattices, whereas a dinuclear complex 6b was surprisingly formed and also connected by the encapsulated Na⁺ ions and ClO₄^- anions with alternative intra- and intermolecular Pt(II)···Pt(II) contacts of 3.650 and 3.677/4.4.372 Å, respectively. Upon excitation, complexes 1 and 3 showed similar vibronic luminescence at 507, 534, and 502, 532 nm, respectively, and the other complexes 2 and 4-6 showed broad luminescence with maxima at 537-567 nm. The B3LYP/LanL2DZ calculation was carried out and used to clarify their excited-state properties. In addition, the powder samples for complexes 1-4 almost showed no energy shift for the luminescence and significantly those of complexes 5-6 exhibited the mechanochromic luminescence upon grinding. It is noted that complexes 5a and 6a only showed minor red shifts (i.e., from 544 to 556 nm for complex 5a and from 551 to 565 nm for complex 6a), whereas complex 6b exhibited a remarkable red shift from 558 to 603 nm upon grinding. Besides, their luminescence reversibility was also examined toward various solvents.

Theoretical Study on the Vapochromic Ni(II)-Quinonoid Complex: One-Dimensional Stacking Structure-Based Color Switching

Vapochromic crystals of Ni(II)-quinonoid complexes were theoretically investigated using density functional theory (DFT) calculations. Kato et al. previously reported that the purple crystals of a four-coordinate Ni(II)-quinonoid complex (1P) exhibited vapochromic characteristics upon exposure to methanol gas, resulting in orange crystals of the six-coordinate methanol-bound complex (1O) [Angew. Chem., Int. Ed.2017, 56, 2345-2349]. However, the authors did not characterize the crystal structure of 1P. In the present study, we computationally predicted the crystal structure of 1P by performing a crystal structure search with classical force-field computations followed by optimization using DFT calculations. The simulated powder X-ray diffraction pattern of the DFT-optimized structure agreed with experimental observations, indicating that our predicted crystal structure is reliable. Investigation of the optimized crystal structure of 1P revealed that its color change arose from changes in its 1D-band structure, which consists of Ni 3d orbitals and quinonoid π-orbitals. Intermolecular interactions were weakened upon the binding of methanol to the Ni(II) center in 1O. Consequently, the intermolecular 3d-π interaction in 1P lowered the band gap and induced the red-shifting of the monomeric four-coordinate Ni(II)-quinonoid complex. Meanwhile, the obtained absorption spectrum of 1O closely corresponded to that of the monomeric six-coordinate Ni(II)-quinonoid complex. Our study provides a new strategy for accurately predicting molecular crystal structures and reveals a new insight into vapochromism based on band structure color switching.

A Series of D-A-D Structured Disilane-Bridged Triads: Structure and Stimuli-Responsive Luminescence Studies

A series of σ-π extended octamethyltetrasilanes, which have phenothiazine, 9,9-dimethyl-9,10-dihydroacridine, or phenoxazine (1, 2, and 3) groups as donor moieties and thienopyrazine or benzothiadiazole (a and b) groups as acceptor fragments, has been prepared, and their optical properties have been studied as an extension of our work. All six compounds exhibited fluorescence in the solid state with maximum wavelengths centered in the range of 400 and 650 nm upon excitation by a UV lamp. Compound 2b showed apparent dual emission behavior in solution, which depends on solvent polarity, and a reversible photoluminescent change under mechanical and thermal stimuli in the solid state. Quantum chemical calculations suggest the contribution of a quasi-axial conformer of the 9,9-dimethyl-9,10-dihydroacridine moiety in 2b to the dual emission in solution and the mechanofluoroluminescence in the solid state, similarly to 1a. These studies provide new insight into the preparation of disilane-bridged triads capable of responding to multiple stimuli.

Cyclometalated Platinum(II) Complexes in a Cis-N,N Configuration: Photophysical Properties and Isomerization to Trans Isomers

Synthesis, isomerization, and photophysical properties of novel cis-N,N-cyclometalated complexes [Pt(C^∧N)(l-Phe)] (C^∧N = benzoquinolinate (1), phenylpyridinate (2), 2-(p-tolyl)pyridinate (3), and 1-phenylpyrazolate (4); l-Phe^- = l-phenylalaninate) are reported herein. In solution, the cis forms of the complexes isomerize to their trans forms via an associative mechanism. This cis/trans isomerism barely influences the absorption and luminescence properties of the complexes in solution, except for a characteristic absorption at approximately 340 nm in the absorption spectra of the cis complexes that is not observed for the trans complexes. Interestingly, the cis complexes are spontaneously assembled in a crystalline phase and show bathochromic absorption and emission colors compared with those of the corresponding trans isomers, which are aggregated in an amorphous phase. cis-1 and cis-2 demonstrate hypsochromic luminescence mechanochromism. The influence of the geometrical isomerism on the photophysical properties and the isomerization mechanism are supported by density functional theory calculations.

Luminescent Behavior Elucidation of a Disilane-Bridged D-A-D Triad Composed of Phenothiazine and Thienopyrazine

A σ-π extended aryldisilane, comprising a thienopyrazine group as an acceptor fragment and phenothiazine groups as the donor moiety, has been prepared through the introduction of two Si-Si bridges (compound 1). X-ray diffraction analysis determined the crystal structure of 1, and experimental and theoretical approaches investigated its optical properties. Solvatochromic studies revealed the dual emission of 1 in all solvents tested. Compound 1 also exhibited fluorescence in the solid state upon excitation with a hand-held UV lamp, as well as mechanochromic luminescent properties. The packing mode in the crystal structure, variation of phenothiazine conformation, morphological changes between crystalline and amorphous phases are the major factors showing reversible fluorescence under external stimuli. A theoretical conformer study found that 1 exists in distinct conformational groups differing in Gibbs free energy by less than 3 kcal mol^-1 . The conformer in the crystalline state of 1 can promote the complete separation of the HOMO and LUMO between the phenothiazine donor and the thienopyrazine acceptor, linked by the disilane linker. HOMO-LUMO energy transition in the crystalline state is forbidden due to the lack of frontier orbital overlap. Crystalline state emission showed LUMO → HOMO-1 transition (locally excited (LE) state). In the amorphous state, the partial presence of quasi-axial conformers allows intramolecular charge-transfer type emission via energy transfer from dominant quasi-equatorial conformers. The strategy proposed in this work provides important guidance for developing stimuli-responsive materials with controlled excited states.

Half-lantern Cyclometalated Platinum(II) Complexes as Anticancer Agents: Molecular docking, Apoptosis, Cell Cycle Analysis and Cytotoxic Activity Evaluations

BACKGROUND AND OBJECTIVE

In the design of modern metal-based anticancer drugs, platinum-based complexes have gained growing interest. In this study, the anticancer activity of half-lantern cyclometalated Pt(II)‒Pt(II) complexes were was evaluated using MTT, apoptosis, cell cycle analysis, and DNA binding studies.

MATERIALS AND METHODS

The cytotoxicity of Pt(II)‒Pt(II) complexes were evaluated against different cancer cell lines such as human lung (A549), breast (MCF-7, and MDA-MB-231), ovarian (SKOV-3), and colon (HT-29) as well as normal breast (MCF-10A), and human lung fibroblast MRC-5 cells using MTT assay. BioLegend's PE Annexin V Apoptosis Detection Kit with 7AAD was applied to assess the apoptotic effects of 1A, and 1B compound against MCF-7, and A549 cell lines. Cell cycle analysis was determined using the flowcytometry method. The interaction of compounds with four different DNA structures with PDB codes (1BNA, 1LU5, 3CO3, and 198D) has been investigated by molecular docking. To achieve binding to DNA experimentally, the electrophoresis mobility shift assay and comet assay was applied.

RESULTS

In the evaluation of cytotoxic effects, 1A showed the highest cytotoxicity among the studied compounds, and it showed higher potency with more selectivity against normal cell lines than cisplatin. This compound had IC50 of 7.24, 2.21, 1.18, 2.71, 10.65, 18.32 and 49.21 μM against A549, SKOV3, HT29, MCF-7, MDA-MB-231, MRC-5, and MCF-10A, respectively, whereas cisplatin had IC50 of 9.75, 19.02, 107.23, 15.20, 18.09, 14.36, and 24.21 μm, respectively, on the same cell lines. In order to check the DNA binding activity of 1A, and 1B, electrophoretic mobility was also conducted, which indicated that the binding of these compounds led to a slight change in electrophoretic mobility to DNA. The migration of chromosomal DNA from the nucleus in the form of a tail or comet was executed in the comet assay of 1A on MCF-7. Examination of apoptosis of 1A, and 1B on the MCF-7 cancer cell line, showed that it could increase induction of apoptosis in this cancerous cell in a concentration-dependent manner. Investigating the effect of 1A using cell cycle analysis on MCF-7 cancer cell line showed that this complex affects the stage G1 and S of the cell cycle.

CONCLUSION

1A has the potential to play a significant role in future biopharmaceutical studies.

Vapochromic behaviour of a nickel(II)-quinonoid complex with dimensional changes between 1D and higher

The nickel(ii)-chloranilato complex {Ni(ca)(VM)2}n (H2ca = chloranilic acid, VM = coordinated vapour molecules, such as water) shows reversible vapochromism upon exposure to various vapours and subsequent drying by heating. In contrast to the Ni(ii)-quinonoid complex, [Ni(HLMe)2] (H2LMe = 4-methylamino-6-methyliminio-3-oxocyclohexa-1,4-dien-1-olate), which was reported to exhibit vapochromic spin-state switching between high and low spin states, the chloranilato complex does not change its spin state even after the removal of coordinated vapour molecules. X-ray absorption fine structure (XAFS) analysis revealed that the six-coordinate geometry of {Ni(ca)(VM)2}n was maintained even after the removal of vapour molecules, in contrast to the [Ni(HLMe)2] complex. The unique vapochromism that follows the dimensional change between 1D and higher is influenced by the relatively weaker ligand field of the chloranilate ligand.

Platinum(II) Complexes with 10-(Aryl)phenoxarsines: Synthesis, Cis/Trans Isomerization, and Luminescence

Synthesis and structural and photophysical characterization of platinum dihalogenide complexes formulated as [PtHal₂L₂], where Hal = Cl and I, with different 10-(aryl)phenoxarsine ligands such as 10-(p-chlorophenyl)phenoxarsine, 10-(p-tolyl)phenoxarsine, and 10-(phenyl)phenoxarsine are reported. The structures of complexes were determined by NMR spectroscopy, mass spectrometry, and X-ray analysis. Cis/trans isomerism of the complexes in solution was studied by NMR spectroscopy. In the solid state, under UV irradiation, platinum diiodide trans complexes exhibit an intense orange-red emission, which was attributed to a metal halide-centered triplet state. The UV/vis absorption and emission properties were studied and rationalized by density functional theory (DFT) and time-dependent DFT calculations.

Cooperative phenomenon of vapochromism and proton conduction of luminescent Pt(ii) complexes for the visualisation of proton conductivity

The luminescent and proton conductive Pt(ii) complex [PtCl(tpy-o-py)]Cl and its HCl adduct [PtCl(tpy-o-pyH)]Cl₂ (o-Pt and o-Pt·HCl, respectively; tpy-o-py = 2,2':6',2''-terpyridine-6',2'''-pyridine) were synthesised and their crystal structures, vapochromic behaviour, and proton conduction, were investigated and compared to those of the para isomers [PtCl(tpy-p-py)]Cl and [PtCl(tpy-p-pyH)]Cl₂ (p-Pt and p-Pt·HCl, respectively; tpy-p-py = 2,2':6',2''-terpyridine-4',4'''-pyridine). X-ray structure analysis revealed that the intermolecular metallophilic (PtPt) interaction was negligible in o-Pt but effective in o-Pt·HCl. Reversible transformation between o-Pt and o-Pt·HCl coupled with significant colour and luminescence changes was achieved by four different external stimuli, namely: exposure of o-Pt to humid HCl gas to form o-Pt·HCl, heating, exposure to MeOH vapour, and finally drying in air to regenerate the original o-Pt. The intraligand π-π* orange emission observed for o-Pt exhibited negligible dependence on the relative humidity (RH). Conversely, o-Pt·HCl exhibited red metal-metal-to-ligand charge-transfer (MMLCT) phosphorescence at 725 nm, originating from effective intermolecular Pt-Pt interactions, and interesting vapochromic behaviour that was dependent on the RH. Notably, o-Pt·HCl presented higher conductivity than the p-Pt·HCl isomer at RH < 80%. This trend was reversed at RH values > 80%, probably owing to the second water-adsorption-induced transformation of p-Pt·HCl. The cooperative phenomenon between the proton conduction and vapochromic behaviour observed for both o-Pt·HCl and p-Pt·HCl should allow the visualisation of the proton-conducting pathway, without the need for a bulk electrode, via the absorption and emission colours at both macroscopic and microscopic levels.

Elastic deformability and luminescence of crystals of polyhalogenated platinum(ii)–bipyridine complexes

A series of polyhalogenated platinum(ii)–bipyridine complexes showed solvent-of-crystallisation-dependent elastic flexibility and luminescence.

Long-wavelength NIR luminescence of 2,2′-bipyridyl-Pt(ii) dimers achieved by enhanced Pt–Pt interaction

Benefiting from the very strong intermolecular Pt–Pt interaction, 2,2′-bipyridyl-Pt(ii) dimers have achieved long-wavelength near-infrared luminescence exceeding 1000 nm for the first time.

Vapochromic luminescence of a spin-coated copper(I) complex thin film by the direct coordination of vapour molecules

A homogeneous thin film of a simple and highly luminescent Cu(i) complex, [CuI(PPh3)2(py)] (PPh3 = triphenylphosphine, py = pyridine) (Cu-py), was fabricated via spin coating using polyvinylpyrrolidone (PVP) and pyridine without destroying the complex. The thin film (Cu-py@PVP), with a thickness of 1 μm, exhibited efficient response to vapour, exhibiting reversible luminescence changes between blue-green and yellow upon exposure to vapours of N-heteroaromatic compounds such as py and 2-methylpyrazine (Mepyz). Vapochromic luminescence colour change resulting from ligand substitution was also observed in the crystal state, but the response of the thin film was remarkably faster than that of the crystalline samples. The vapour-induced ligand exchange on the thin film was fully characterised by comparing the luminescence properties of the Cu-py crystal with the newly prepared Cu(i) complex, [CuI(Mepyz)(PPh3)2] (Cu-Mepyz).

Multinuclear PtII Complexes: Why Three is Better Than Two to Enhance Photophysical Properties

The self-assembly of platinum complexes is a well-documented process that leads to interesting changes of the photophysical and electrochemical behavior as well as to a change in reactivity of the complexes. However, it is still not clear how many metal units must interact in order to achieve the desired properties of a large assembly. This work aimed to clarify the role of the number of interacting PtII units leading to an enhancement of the spectroscopic properties and how to address inter- versus intramolecular processes. Therefore, a series of neutral multinuclear PtII complexes were synthesized and characterized, and their photophysical properties at different concentration were studied. Going from the monomer to dimers, the growth of a new emission band and the enhancement of the emission properties were observed. Upon increasing the platinum units up to three, the monomeric blue emission could not be detected anymore and a concentration independent bright-yellow/orange emission, due to the establishment of intramolecular metallophilic interactions, was observed.

Steric Engineering of Cyclometalated Pt(II) Complexes toward High-Contrast Monomer-Excimer-Based Mechanochromic and Vapochromic Luminescence

Ligands play a crucial role in the supramolecular photoluminescence properties of Pt(II) square-planar complexes. To improve the luminescence color responses of N∧C∧N cyclometalated Pt(II) complexes to external stimuli such as mechanical stress and chemical vapors, we have conducted a steric engineering of the previous systems 1a-1d [Inorg. Chem. 2017, 56, 4978-4989] by introducing two tert-butyl groups to the tridentate ligand to form complexes 2a-2c. Unlike the "too low" or "too high" steric hindrance of the NCNPt core in 1a-1d, the combined steric effects of the tert-butyl groups at one side and the pentiptycene group at the other side of the NCNPt core in 2b are "just right" for generating as-prepared powders with pure monomer (green) emission or pure excimer (red) emission, depending on the rate of precipitation from solutions. The synergistic steric effects are also beneficial to the solid-state luminescence quantum efficiency (30-36%). As a result of the differences in steric interactions and thus in the relative monomer vs excimer emission intensity, each complex of 2a-2c performs a two-step luminescence mechanochromism and vapochromism with different color patterns. This work provides an intriguing example of steric engineering of Pt(II) complexes toward highly emissive molecular solids with high-contrast mechanochromic and vapochromic luminescence.

Bright Luminescent Platinum(II)-Biaryl Emitters Synthesized Without Air-Sensitive Reagents

Transition-metal complexes bearing biaryl-2,2'-diyl ligands tend to show intense luminescence. However, difficulties in synthesis have prevented their further functionalization and practical applications. Herein, a series of platinum(II) complexes bearing biaryl-2,2'-diyl ligands, which have never been prepared in air, were synthesized through transmetalation and successive cyclometalation of biarylboronic acids. This approach does not require any air- or moisture-sensitive reagents and features a simple synthesis even in air. The resulting (Et₄ N)₂ [Pt(m,n-F₂ bph)(CN)₂ ] (m,n-F₂ bph=m,n-difluorobiphenyl-2,2'-diyl) complexes exhibit intense green emissions with high quantum efficiencies of up to 0.80 at 298 K. The emission spectral fitting and variable-temperature emission lifetime measurements indicate that the high quantum efficiency was achieved because of the tight packing structure and strong σ-donating ability of bph.

Luminescence of mononuclear Pt(ii) complexes with glycolate: external stimuli-induced excimer emission changes to oligomer emissions

Trihydrate crystals of novel Pt^II complexes [Pt^II(bpy)(gl)] (bpy: 2,2′-bipyridine; Hgl⁻: glycolate) show excimer emission changes to two kinds of oligomer emissions depending on the type of external stimuli.

Fast and stable vapochromic response induced through nanocrystal formation of a luminescent platinum(II) complex on periodic mesoporous organosilica

A hybrid vapoluminescent system exhibiting fast and repeatable response was constructed using periodic mesoporous organosilica with bipyridine moieties (BPy-PMO) and a Pt(II) complex bearing a potentially luminescent 2-phenylpyridinato (ppy) ligand. An intense red luminescence appeared when the Pt(II)-complex immobilised BPy-PMO was exposed to methanol vapour and disappeared on exposure to pyridine vapour. The ON-OFF vapochromic behaviour occurred repeatedly in a methanol/pyridine/heating cycle. Interestingly, a rapid response was achieved in the second cycle and cycles thereafter. Scanning and transmission electron microscopies (SEM/TEM), absorption and emission, and nuclear magnetic resonance spectroscopies, mass spectrometry, and powder X-ray diffraction indicated that methanol vapour induced Si-C cleavage and thus liberated [Pt(ppy)(bpy)]Cl (bpy = 2,2′-bipyridine) from the BPy-PMO framework. Furthermore, the self-assembling properties of the Pt(II) complex resulted in the formation of highly luminescent micro/nanocrystals that were homogeneously dispersed on the porous support. The unique vapoluminescence triggered by the unprecedented protodesilylation on exposure to protic solvent vapour at room temperature is attributable to BPy-PMO being a giant ligand and an effective vapour condenser. Consequently, this hybrid system presents a new strategy for developing sensors using bulk powdery materials.

Reversible Aggregation of Chlorophyll Derivative Induced by Phase Transition of Lipid

Controlling the supramolecular organization of pigment molecules will provide innovative materials that exhibit variable optical properties. In nature, photosynthetic systems employ chlorophyllous supramolecules in which each pigment molecule is suitably organized in proteins, and their properties are adequately optimized by changing the structures of the surrounding amino acid residues. Here, we report a strategy for varying the aggregation behavior of a chlorophyll derivative by using a phase-transition phenomenon of lipid bilayers. Methyl pyropheophorbide a (MPP) was employed as a chlorophyllous pigment in our artificial system, and synthetic phosphatidylcholines with saturated acyl chain(s) were also used. The MPP molecules successfully accumulated within the lipid bilayer of liposomes without changing the vesicular structure. When the lipid bilayer was in a gel form (under the phase-transition temperature, T_m), the embedded MPP aggregated to yield a dimeric form showing red-shifted absorption bands and circular dichroism signals. When the solutions of MPP-containing liposomes were heated to higher temperatures than their T_m, MPP disaggregated to monomeric form as the absorption spectrum changed into its original fashion in dichloromethane. The reversible thermochromic (dis)aggregation of the MPP molecules had good cyclability. Additional careful examination of the phase transition in the MPP-lipid co-assemblies clarified that the critical temperatures of the MPP (dis)aggregation were in good agreement with the phase-transition temperatures of the pigment-containing bilayers. The reversible MPP aggregation in the lipid bilayers occurred in a wide range of temperatures (around 10-55 °C) by changing the length of the diacyl side chains of phospholipids. The reversible thermochromism of the chlorophyllous system was established by varying the nature of the surrounding lipid bilayer. This study can provide a useful strategy for making variable tetrapyrrolic aggregate systems induced by mild extrinsic stimuli.

Soft Crystals: Flexible Response Systems with High Structural Order

A new material concept of soft crystals is proposed. Soft crystals respond to gentle stimuli such as vapor exposure and rubbing but maintain their structural order and exhibit remarkable visual changes in their shape, color, and luminescence. Various interesting examples of soft crystals are introduced in the article. By exploring the interesting formation and phase-transition phenomena of soft crystals through interdisciplinary collaboration, new materials having both the characteristics of ordered hard crystals and those of flexible soft matter are expected.

Vapoluminescent Behavior and the Single-Crystal-to-Single-Crystal Transformations of Chloroform Solvates of [Au2 (μ-1,2-bis(diphenylarsino)ethane)2 ](AsF6 )2

The mono- and di-chloroform solvates of [Au₂ (μ-1,2-bis(diphenylarsino)ethane)₂ ](AsF₆ )₂ undergo single-crystal-to-single-crystal transformations that result in gain (over 12 hours) or slow loss (over five years) of only one chloroform molecule. The change in solvation results in changes in the structure and luminescence of the digold cation. The cation consists of a pair of slightly bent As-Au-As units that are connected through the two bridging dpae ligands and by aurophilic interactions with Au⋅⋅⋅Au contacts of 3.05152(15) Å in the disolvate or 2.9570(5) Å in the monosolvate.

Half-lantern cyclometalated Pt(ii) and Pt(iii) complexes with bridging heterocyclic thiolate ligands: synthesis, structural characterization, and electrochemical and photophysical properties

Half-lantern Pt(ii) and Pt(iii) cyclometalated binuclear complexes, bridged with various heterocyclic thiolate ligands, were synthesized and studied by electrochemical and photophysical techniques.