Exciplex Emission and Förster Resonance Energy Transfer in Polycyclic Aromatic Hydrocarbon-Based Bischromophoric Cyclophanes and Homo[2]catenanes.

Energy transfer and exciplex emission are not only crucial photophysical processes in many living organisms but also important for the development of smart photonic materials. We report, herein, the rationally designed synthesis and characterization of two highly charged bischromophoric homo[2]catenanes and one cyclophane incorporating a combination of polycyclic aromatic hydrocarbons, i.e., anthracene, pyrene, and perylene, which are intrinsically capable of supporting energy transfer and exciplex formation. The possible coconformations of the homo[2]catenanes, on account of their dynamic behavior, have been probed by Density Functional Theory calculations. The unique photophysical properties of these exotic molecules have been explored by steady-state and time-resolved absorption and fluorescence spectroscopies. The tetracationic pyrene-perylene cyclophane system exhibits emission emanating from a highly efficient Förster resonance energy transfer (FRET) mechanism which occurs in 48 ps, while the octacationic homo[2]catenane displays a weak exciplex photoluminescence following extremely fast (<0.3 ps) exciplex formation. The in-depth fundamental understanding of these photophysical processes involved in the fluorescence of bischromophoric cyclophanes and homo[2]catenanes paves the way for their use in future bioapplications and photonic devices.

Divinylanthracene-Containing Tetracationic Organic Cyclophane with Near-Infrared Photoluminescence.

Near-infrared (NIR) light is known to have outstanding optical penetration in biological tissues and to be non-invasive to cells compared with visible light. These characteristics make NIR-specific light optimal for numerous biological applications, such as the sensing of biomolecules or in theranostics. Over the years, significant progress has been achieved in the synthesis of fluorescent cyclophanes for sensing, bioimaging, and making optoelectronic materials. The preparation of NIR-emissive porphyrin-free cyclophanes is, however, still challenging. In an attempt for fluorescence emissions to reach into the NIR spectral region, employing organic tetracationic cyclophanes, we have inserted two 9,10-divinylanthracene units between two of the pyridinium units in cyclobis(paraquat-p-phenylene). Steady-state absorption, fluorescence, and transient-absorption spectroscopies reveal the deep-red and NIR photoluminescence of this cyclophane. This tetracationic cyclophane is highly soluble in water and has been employed successfully as a probe for live-cell imaging in a breast cancer cell line (MCF-7).

Helically Chiral Hybrid Cyclodextrin Metal-Organic Framework Exhibiting Circularly Polarized Luminescence.

Three achiral polycyclic aromatic fluorophores─namely, 1-pyrenecarboxylic acid, 9-anthracenecarboxylic acid, and perylene-3,9-dicarboxylic acid─were chosen based on their desired properties before being incorporated into the construction of a K+-carrying gamma-cyclodextrin (γ-CD)-based metal-organic framework (CD-MOF-1) and γ-CD-containing hybrid frameworks (CD-HFs). Among these fluorophores, only the pyrene-carrying one shows significant noncovalent bonding interactions with γ-CD in solution. This fluorophore is encapsulated in a CD-HF with a trigonal superstructure instead of the common cubic CD-MOF-1 found in the case of the other two fluorophores. Single-crystal X-ray diffraction analysis of the trigonal CD-HF reveals a π-stacked chiral positioning of the pyrene-carrying fluorophore inside the (γ-CD)2 tunnels and held uniformly around an enantiomorphous 32 screw axis along the c direction in the solid-state structure. This helix-like structure demonstrates an additional level of chirality over and above the point-chiral stereogenic centers of γ-CD and the axial chirality associated with the self-assembled π-stacked fluorophores. These arrangements result in specifically generated photophysical and chiroptical properties, such as the controlled emergence of circularly polarized luminescence (CPL) emission. In this manner, a complete understanding of the mechanism of chirality transfer from a chiral host (CD-HF) to an encapsulated achiral fluorophore has been achieved, an attribute which is often missing in the development of materials with CPL.

Thermally Controlled Exciplex Fluorescence in a Dynamic Homo[2]catenane.

Motion-induced change in emission (MICE) is a phenomenon that can be employed to develop various types of probes, including temperature and viscosity sensors. Although MICE, arising from the conformational motion in particular compounds, has been studied extensively, this phenomenon has not been investigated in depth in mechanically interlocked molecules (MIMs) undergoing coconformational changes. Herein, we report the investigation of a thermoresponsive dynamic homo[2]catenane incorporating pyrene units and displaying relative circumrotational motions of its cyclophanes as evidenced by variable-temperature 1H NMR spectroscopy and supported by its visualization through molecular dynamics simulations and quantum mechanics calculations. The relative coconformational motions induce a significant change in the fluorescence emission of the homo[2]catenane upon changes in temperature compared with its component cyclophanes. This variation in the exciplex emission of the homo[2]catenane is reversible as demonstrated by four complete cooling and heating cycles. This research opens up possibilities of using the coconformational changes in MIMs-based chromophores for probing fluctuations in temperature which could lead to applications in biomedicine or materials science.

Aggregation-Induced Emission and Circularly Polarized Luminescence Duality in Tetracationic Binaphthyl-Based Cyclophanes.

Here, we report an approach to the synthesis of highly charged enantiopure cyclophanes by the insertion of axially chiral enantiomeric binaphthyl fluorophores into the constitutions of pyridinium-based macrocycles. Remarkably, these fluorescent tetracationic cyclophanes exhibit a significant AIE compared to their neutral optically active binaphthyl precursors. A combination of theoretical calculations and time-resolved spectroscopy reveal that the AIE originates from limited torsional vibrations associated with the axes of chirality present in the chiral enantiomeric binaphthyl units and the fine-tuning of their electronic landscape when incorporated within the cyclophane structure. Furthermore, these highly charged enantiopure cyclophanes display CPL responses both in solution and in the aggregated state. This unique duality of AIE and CPL in these tetracationic cyclophanes is destined to be of major importance in future development of photonic devices and bio-applications.

Viologen Tweezers to Probe the Force of Individual Donor-Acceptor π-Interactions.

Donor-acceptor (DA) π-interactions are weak attractive forces that are exploited widely in molecular and supramolecular chemistry. They have been characterized extensively by ensemble techniques, providing values for their energies that are useful for the design of soft materials. For implementation of motions or operations based on these DA π-interactions in wholly synthetic molecular machines, the mechanical strength and force associated with their out-of-equilibrium performance are the key parameters, in addition to their energies obtained at thermodynamic equilibrium. In this context, we have used single-molecule force spectroscopy as a nonequilibrium technique to determine the mechanical strength of individual DA π-interactions in solution. We designed and synthesized a molecular tweezer that is able to encapsulate π-donors and also demonstrated a precise opening extension. The mechanical breaking of the noncovalent interactions between viologen units-π-acceptors commonly employed in mechanically interlocked molecules-and several π-donors afforded a characteristic force-distance signature, revealing the opening of individual viologen tweezers with an unambiguous extension. Single-tweezer host-exchange experiments performed in situ demonstrated the sensitivity of the technique. This simple design could be exploited in quantifying the force of a large range of weak noncovalent bonding interactions as well as the potential work that molecular machines can generate at the single-molecule level.

Host-Guest Complexation-Mediated Supramolecular Photon Upconversion.

The phenomenon of photon upconversion, in which a system absorbs two or more photons of lower energy and emits a photon of higher energy, has been used in numerous applications, including non-destructive bioimaging, deep-penetrating photodynamic therapy, catalysis, and photovoltaic devices. To date, photon upconversion has been observed typically in inorganic nanocrystals, nanoparticles, metal-organic frameworks, supramolecular assemblies, and organic dyads. Herein, we demonstrate a new strategy for harnessing photon upconversion-supramolecular upconversion-based on host-guest chemistry. We have identified a box-like fluorescent tetracationic host incorporating a thiazolothiazole emitter, which can accommodate a guest-sensitizer, 5,15-diphenylporphyrin, inside its cavity, and demonstrated that the host-guest inclusion complex displays triplet-fusion upconversion when the guest is excited with low-energy light. The strategy of supramolecular upconversion has been employed successfully in two other host-guest complexes-with hosts comprised of anthracene emitters and a 5,15-diphenylporphyrin guest-corroborating the fact that this strategy is a general one and can be applied to the design of a new family of host-guest complexes for photon upconversion. More importantly, supramolecular upconversion is accessible in solution under dilute conditions (µM) compared to most of the existing approaches that require significantly higher concentrations (mM) of emitters and/or sensitizers. Transient absorption spectroscopy and density functional theory have been employed in order to confirm a triplet-fusion upconversion mechanism. Host-guest complexation-mediated supramolecular photon upconversion eliminates multiple issues in the existing systems related to high working concentrations, high incident laser power, and low optical penetration depths.

Mechanical-Bond-Induced Exciplex Fluorescence in an Anthracene-Based Homo[2]catenane.

Collisional intermolecular interactions between excited states form short-lived dimers and complexes that lead to the emergence of excimer/exciplex emission of lower energy, a phenomenon which must be differentiated from the photoluminescence (PL) arising from the monomeric molecules. Although the utilization of noncovalent bonding interactions, leading to the generation of excimer/exciplex PL, has been investigated extensively, precise control of the aggregates and their persistence at very low concentrations remains a rare phenomenon. In the search for a fresh approach, we sought to obtain exciplex PL from permanent structures by incorporating anthracene moieties into pyridinium-containing mechanically interlocked molecules. Beyond the optical properties of the anthracene moieties, their π-extended nature enforces [π···π] stacking that can overcome the Coulombic repulsion between the pyridinium units, affording an efficient synthesis of an octacationic homo[2]catenane. Notably, upon increasing the ionic strength by adding tetrabutylammonium hexafluorophosphate, the catenane yield increases significantly as a result of the decrease in Coulombic repulsions between the pyridinium units. Although the ground-state photophysical properties of the free cyclophane and the catenane are similar and show a charge-transfer band at ∼455 nm, their PL characters are distinct, denoting different excited states. The cyclophane emits at ∼562 nm (quantum yield ϕF = 3.6%, emission lifetime τs = 3 ns in MeCN), which is characteristic of a disubstituted anthracene-pyridinium linker. By contrast, the catenane displays an exciplex PL at low concentration (10-8 M) with an emission band centered on 650 nm (ϕF = 0.5%, τs = 14 ns) in MeCN and at 675 nm in aqueous solution. Live-cell imaging performed in MIAPaCa-2 prostate cancer cells confirmed that the catenane exciplex emission can be detected at micromolar concentrations.

Efficient and Rapid Mechanochemical Assembly of Platinum(II) Squares for Guanine Quadruplex Targeting.

We present a rapid and efficient method to generate a family of platinum supramolecular square complexes, including previously inaccessible targets, through the use of ball milling mechanochemistry. This one-pot, two-step process occurs in minutes and enables the synthesis of the squares [Pt4(en)4(N∩N)4][CF3SO3]8 (en= ethylenediamine, N∩N = 4,4'-bipyridine derivatives) from commercially available precursor K2PtCl4 in good to excellent yields. In contrast, solution-based assembly requires heating the reagents for weeks and gives lower yields. Mechanistic investigations into this remarkable rate acceleration revealed that solution-based assembly (refluxing for days) results in the formation of large oligomeric side-products that are difficult to break down into the desired squares. On the other hand, ball milling in the solid state is rapid and appears to involve smaller intermediates. We examined the binding of the new supramolecular squares to guanine quadruplexes, including oncogene and telomere-associated DNA and RNA sequences. Sub-micromolar binding affinities were obtained by fluorescence displacement assays (FID) and isothermal titration calorimetry (ITC), with binding preference to telomere RNA (TERRA) sequences. ITC showed a 1:1 binding stoichiometry of the metallosquare to TERRA, while the stoichiometry was more complex for telomeric quadruplex DNA and a double-stranded DNA control.

Exploiting natural products to build metalla-assemblies: the anticancer activity of embelin-derived Rh(III) and Ir(III) metalla-rectangles.

Six new pentamethylcyclopentadienyl Rh(III) and Ir(III) metalla-rectangles ([3](CF3SO3)4-[8](CF3SO3)4) have been prepared by a self-assembly strategy using the embelin-derived metalla-clips (η5-C5Me5)2M2(µ4-C6HRO4-κO)Cl2 (M = Rh, 1; M = Ir, 2; R = (CH2)10CH3) and the linear ditopic ligands, pyrazine, 4,4'-bipyridine and 1,2-bis (4-pyridyl)ethylene. These new metalla-rectangles have been obtained in high yield and isolated as their triflate salts. The complexes have been fully characterized by standard spectroscopic techniques and the antiproliferative activity of these tetranuclear complexes was evaluated in vitro on cancerous (DU-145, A-549, HeLa) and noncancerous (HEK-293) cell lines. The biological study has showed a better activity for the rhodium derivatives over the iridium analogs and for all complexes a very good selectivity for cancerous over noncancerous cells. The presence of lipophilic side chains coupled with the positive charge of the tetranuclear complexes suggested a cytotoxic activity involving the mitochondrial machinery, as demonstrated by multiple biological experiments.

2,5-Di-bromo-3,6-dimeth-oxycyclo-hexa-2,5-diene-1,4-dione.

In the structure of the title compound, C8H6Br2O4, the complete mol-ecule is generated by the application of a centre of inversion. The mol-ecule is planar (r.m.s. deviation for all non-H atoms but methyl C = 0.0358 Å), with only the methyl groups being deviated from the plane [by ±0.321 (4) Å]. In the crystal packing, Br⋯O(methoxy) halogen bonds [3.2407 (19) Å] connect molecules into supramolecular layers parallel to (101).

Synthesis, characterisation and in vitro anticancer activity of hexanuclear thiolato-bridged arene ruthenium metalla-prisms.

Hexanuclear thiolato-bridged arene ruthenium metalla-prisms of the general formula [(p-cymene)(6)Ru(6)(SR)(6)(tpt)(2) ](6+) (R=CH(2)Ph, CH(2)C(6)H(4)-p-tBu, CH(2)CH(2)Ph; tpt=2,4,6-tris(4-pyridyl)-1,3,5-triazine), obtained from the dinuclear precursors [(p-cymene)(2)Ru(2)(SR)(2)Cl(2)], AgCF(3)SO(3) and tpt, have been isolated and fully characterised as triflate salts. The metalla-prisms are highly cytotoxic against human ovarian cancer cells, especially towards the cisplatin-resistant cell line A2780cisR (IC(50) <0.25 µM).

Dichlorido(furfuryl-amine-κN)(η-hexa-methyl-benzene)-ruthenium(II).

The single-crystal X-ray structure analysis of [RuCl(2)(C(12)H(18))(C(5)H(7)NO)] reveals a distorted piano-stool geometry around the Ru(II) atom, with a hexa-methyl-benzene ligand, two chloride ligands and a furfuryl-amine ligand, the latter coordinating through the amine group. In the crystal, a dimeric structure is observed as a result of N-H⋯Cl inter-actions between two symmetry-related mol-ecules.

Anticancer Activity of Half-Sandwich RhIII and IrIII Metalla-Prisms Containing Lipophilic Side Chains.

Two hexacationic pentamethylcyclopentadienyl rhodium(III) and iridium(III) metalla-prisms, [(η5 -C5 Me5 )6 M6 (µ3 -tpt-κN)2 (µ4 -C6 HRO4 -κO)3 ]6+ (tpt=2,4,6-tri-(pyridin-4-yl)-1,3,5-triazine; R=(CH2 )10 CH3 ; M=Rh, [3]6+ ; M=Ir, [4]6+ ) isolated as their triflate salts, have been synthesised from the dinuclear complexes (η5 -C5 Me5 )2 M2 (µ4 -C6 HRO4 -κO)Cl2 (M=Rh, 1; M=Ir, 2) and AgCF3 SO3 . The antiproliferative activity of the neutral and cationic complexes has been evaluated in vitro in human cancer cell lines. The positively charged metalla-prisms appear to target mitochondria, which ultimately induce apoptosis in cancer cells. All biological studies suggest that the rhodium derivative [3][CF3 SO3 ]6 possesses excellent activities, not only in vitro but also in vivo in tumour-induced C57L6/J mice.

Synthesis, molecular structure, computational study and in vitro anticancer activity of dinuclear thiolato-bridged pentamethylcyclopentadienyl Rh(III) and Ir(III) complexes.

Neutral dinuclear dithiolato-bridged pentamethylcyclopentadienyl Rh(III) complexes of the type (C5Me5)2Rh2(µ-SR)2Cl2 (R = CH2Ph, 1; R = CH2CH2Ph, 2) and cationic dinuclear trithiolato-bridged pentamethylcyclopentadienyl Rh(III) and Ir(III) complexes of the type [(C5Me5)2M2(µ-SR)3](+) (M = Rh, R = CH2Ph, 3; M = Rh, R = CH2CH2Ph, 5; M = Rh, R = CH2C6H4-p-(t)Bu, 7: M = Ir, R = CH2Ph, 4; M = Ir, R = CH2CH2Ph, 6; M = Ir, R = CH2C6H4-p-(t)Bu, 8) have been synthesized from the chloro-bridged pentamethylcyclopentadienyl Rh(III) and Ir(III) dimers (C5Me5)2M2(µ-Cl)2Cl2 by reaction with the corresponding thiol derivative (RSH). Complexes 3-8 were isolated as chloride salts. All complexes were obtained in good yield and were fully characterized by spectroscopic methods. The molecular structures of the neutral complexes (1 and 2) show interesting features: the two rhodium atoms are bridged by two thiolato ligands with no metal-metal bonds and the pentamethylcyclopentadienyl and chlorido ligands are oriented syn to each other, an uncommon conformation for such dinuclear complexes. These structural features were rationalized using DFT calculations. Additionally, the antiproliferative activity of the complexes was evaluated against the cancerous A2780 (cisplatin sensitive) and A2780cisR (cisplatin resistant) human ovarian cell lines and on the noncancerous HEK293 human embryonic kidney cells. All complexes were found to be active and the cationic iridium complexes , and are particularly cytotoxic, with IC50 values in the nanomolar range (IC50 < 0.1 µM). The catalytic activity of the complexes for the oxidation of glutathione (GSH) to GSSG was evaluated by NMR spectroscopy.