Modulating the Optical Properties of BODIPY Dyes by Noncovalent Dimerization within a Flexible Coordination Cage

Self-assembly of boc-protected diphenylalanine functionalized with boron-dipyrromethane

Peptides functionalized with fluorescent dyes can form aggregated structures that show potential for photonic and/or theranostic applications. In this study, we used the good-bad solvent method to induce the self-assembly of chromopeptides into various products, which were characterized using a multi-technique spectroscopic approach. Two hydrophobic chromopeptides were designed by attaching boron-dipyrromethane dyes to Boc-protected diphenylalanine, resulting in spherical aggregates with tunable dimensions, influenced by the compound's molar concentration and solvent composition. These aggregates exhibit distinct optical properties, including bathochromic shifts in absorption and emission bands, as well as induced chirality, suggesting different chromophore orientations within the microspheres. Moreover, the aggregates show accelerated excited-state relaxation compared to monomeric chromopeptides, primarily through non-radiative processes. This work establishes a foundation for future studies focused on optimizing the desired photophysical properties of similar chromopeptide aggregates, depending on their intended application.

Modulation of Photoluminescence of BODIHY Dye Using Water-Soluble Coordination Cages With Different Shapes

The confinement of guest molecules within supramolecular hosts can alter their photophysical properties. However, the shapes of the hosts in regulating the guest's emission remains underexplored. Herein, we investigate how the shape of the host alters the emission behavior of boron difluoride hydrazone (BODIHY) (G1) dye encapsulated within two iso-stoichiometric water-soluble coordination cages: MC1 (double-square cage) and MC2 (octahedral cage). Encapsulation of G1 within MC1 results in a highly emissive solution, whereas similar confinement in MC2 leads to a non-emissive host-guest solution. A similar trend was observed with different sets of iso-stoichiometric cages MC3 (double-square cage) and MC4 (octahedral cage). Using a combination of femtosecond transient absorption and time-resolved fluorescence spectroscopy, we observed that the disparity in fluorescence behavior of BODIHY is attributed to charge transfer interactions between the guest and ligand panels of cages. The shape of the coordination cage dictates the preorganization of the guest within the cavity, thereby suppressing or promoting this charge transfer interactions. Moreover, we demonstrate a turn-on emission of BODIHY dye due to its preferential binding to a double-square cage. These findings provide fundamental insights into host-mediated modulation of the guest's photophysics and offer a blueprint for designing supramolecular systems with tunable emissive behavior.

Tracking host-guest recognition in cells by a BODIPY·CB[7] complex

Interested in host·guest binding events in cellular environments, intracellular recognition between fluorescent BODIPY+ and cucurbit[7]uril was explored for lysosome tracking in living cancer cells. Sequential deaggregation from spontaneously dimerized BODIPY+ was unusually discovered upon complexation with CB[7].

Precise Control of Noncovalent Dimerization via Cucurbit[n]uril-Based Host-Guest Complexation

Upon complexation between differently sized cucurbit[n]uril (n = 7, 8, or 10) hosts and a fluorenone derivative with various binding modes, the aggregation state of the fluorenone guest is precisely switched from J-aggregates to the monomer, H-dimer, and separated "H-dimer", thus enabling the construction of multicolor luminescent materials in an aqueous solution.

Tetra-coordinated organoboron complexes with triaminoguanidine-salicylidene based ligands: aggregation induced enhanced emission and mechanoresponsive features

Organoboron complexes have garnered significant attention due to their remarkable optical properties and diverse applications. However, synthesizing stable fused five-, six- and seven-membered organoboron complexes possess significant challenges. In this study, we successfully developed novel mono-nuclear (6-8 & 10) and di-nuclear (9) organoboron complexes supported by triaminoguanidine-salicylidene based C3-symmetric Schiff base ligands via one-step condensation reaction with excess phenylboronic acid. Single-crystal X-ray diffraction analysis revealed that in the mononuclear complexes (6-8 & 10), boron atoms adopt tetrahedral geometry with fused five-membered N-B-N and six-membered O-B-N chelate ring whereas in the dinuclear complex (9), two boron atoms exist in distinct coordination environment, forming four fused boron-incorporated rings, including six-membered N-B-N, six-membered O-B-N, seven-membered N-B-O and five-membered N-B-N chelate rings. Our findings provide a unique family of mononuclear organoboron complexes and dinuclear organoboron complex with ESIPT unit. Aggregation induced enhanced emission features of the compounds were established in THF-water mixture and supported by DLS and SEM analyses. Interestingly, compound 6, 9 and 10 shows interesting mechanoresponsive features upon grinding.

Surface Tension Manipulation with Visible Light through Sensitized Disequilibration of Photoswitchable Amphiphiles

Azoarene isomerization lies at the heart of numerous applications, from catalysis or energy storage to photopharmacology. While efficient switching between their E and Z isomers predominantly relies on UV light, a recent study by Klajn and co-workers introduced visible light sensitization of E azoarenes and their subsequent isomerization as a tool coined disequilibration by sensitization under confinement (DESC) to obtain high yields of the Z isomer. This host-guest approach is, however, still constrained to minimally substituted azoarenes with limited applicability in advanced molecular systems. Herein, we expand DESC for the assembly of surfactants at the air-water interface. Leveraging our expertise with photoswitchable amphiphiles, we induce substantial alterations of the water surface tension through reversible arylazopyrazole isomerization. After studying the binding of charged surfactants to the host, we find that the surface activity differences upon visible light switching for both isomers are comparable to those obtained by UV light excitation. The method is demonstrated on a large concentration range and can be activated using green or red light, depending on the sensitizer chosen. The straightforward implementation of photoswitch sensitization in a complex molecular network showcases how DESC enables the improvement of existing systems and the development of novel applications driven by visible light.

Double-Bridging Increases the Stability of Zinc(II) Metal-Organic Cages

A key feature of coordination cages is the dynamic nature of their coordinative bonds, which facilitates the synthesis of complex polyhedral structures and their post-assembly modification. However, this dynamic nature can limit cage stability. Increasing cage robustness is important for real-world use cases. Here we introduce a double-bridging strategy to increase cage stability, where designed pairs of bifunctional subcomponents combine to generate rectangular tetratopic ligands within pseudo-cubic Zn8L6 cages. These cages withstand transmetalation, the addition of competing ligands, and nucleophilic imines, under conditions where their single-bridged congeners decompose. Our approach not only increases the stability and robustness of the cages while maintaining their polyhedral structure, but also enables the incorporation of additional functional units in proximity to the cavity. The double-bridging strategy also facilitates the synthesis of larger cages, which are inaccessible as single-bridged congeners.

Host-Guest Interactions Induced Enhancement in Oxidase-Like Activity of a Benzothiadiazole Dye Inside an Aqueous Pd8L4 Barrel

The effect of host-guest interactions on the chemistry of encapsulated molecules is a fascinating field of research that has gained momentum in recent years. Much of the work in this field has been focused on the effect of such interactions on catalysis and photoluminescence of encapsulated dyes. However, the effect of such interactions on related photoinduced processes, such as photoregulated oxidase-mimicking activity, has not been explored much. Herein, we report a unique example of enhancement of oxidase-like activity of a benzothiadiazole dye (G1) in water through encapsulation within a M8L4 molecular barrel (1). Favorable host-guest interactions helped the encapsulated guest G1 to have better photoinduced electron transfer to molecular oxygen leading to increased production of superoxide radical anions and oxidase-like activity. Furthermore, encapsulation inside 1 also caused a change in the redox potentials of the guest (G1) which after photoinduced electron transfer produced a better oxidizing agent than free G1. These phenomena combined to enhance the oxidase-like activity of dye G1 upon encapsulation inside cage 1. The present report demonstrates a unique effect of host-guest chemistry on photoregulated processes.

Multiple-stimuli fluorescent responsive metallo-organic helicated cage arising from monomer and excimer emission

Effectively regulating monomer and excimer emission in a singular supramolecular luminous platform is challenging due to high difficulty of precise control over its aggregation and dispersion behavior when subjected to external stimuli. Here, we show a metallo-cage (MTH) featuring a triple helical motif that displays a unique dual emission. It arises from both intramolecular monomer and intermolecular excimer, respectively. The distorted molecular conformation and the staggered stacking mode of MTH excimer are verified through single crystal X-ray diffraction analysis. These structural features facilitate the switch between monomer and excimer emission, which are induced by changes in concentration and temperature. Significantly, adjusting the equilibrium between these two states in MTH enables the production of vibrant white light emission in both solution and solid state. Moreover, when combined with a PMMA (polymethyl methacrylate) substrate, the resulting thin films can serve as straightforward fluorescence thermometer and thermally activated information encryption materials.

Guest Encapsulation Alters the Thermodynamic Landscape of a Coordination Host

The architecture of self-assembled host molecules can profoundly affect the properties of the encapsulated guests. For example, a rigid cage with small windows can efficiently protect its contents from the environment; in contrast, tube-shaped, flexible hosts with large openings and an easily accessible cavity are ideally suited for catalysis. Here, we report a "Janus" nature of a Pd6L4 coordination host previously reported to exist exclusively as a tube isomer (T). We show that upon encapsulating various tetrahedrally shaped guests, T can reconfigure into a cage-shaped host (C) in quantitative yield. Extracting the guest affords empty C, which is metastable and spontaneously relaxes to T, and the T⇄C interconversion can be repeated for multiple cycles. Reversible toggling between two vastly different isomers paves the way toward controlling functional properties of coordination hosts "on demand".

Disequilibrating azobenzenes by visible-light sensitization under confinement

Photoisomerization of azobenzenes from their stable E isomer to the metastable Z state is the basis of numerous applications of these molecules. However, this reaction typically requires ultraviolet light, which limits applicability. In this study, we introduce disequilibration by sensitization under confinement (DESC), a supramolecular approach to induce the E-to-Z isomerization by using light of a desired color, including red. DESC relies on a combination of a macrocyclic host and a photosensitizer, which act together to selectively bind and sensitize E-azobenzenes for isomerization. The Z isomer lacks strong affinity for and is expelled from the host, which can then convert additional E-azobenzenes to the Z state. In this way, the host-photosensitizer complex converts photon energy into chemical energy in the form of out-of-equilibrium photostationary states, including ones that cannot be accessed through direct photoexcitation.

Recent Developments in the Design of New Water-Soluble Boron Dipyrromethenes and Their Applications: An Updated Review

Boron-dipyrromethene (BODIPY) and its derivatives play an important role in the area of organic fluorophore chemistry. Recently, the water-soluble boron-dipyrromethene dyes have increasingly received interest. The structural modification of the BODIPY core by incorporating different neutral and ionic hydrophilic groups makes it water-soluble. The important hydrophilic groups, such as quaternary ammonium, sulfonate, oligoethylene glycol, dicarboxylic acid, and sugar moieties significantly increase the solubility of these dyes in water while preserving their photophysical properties. As a result, these fluorescent dyes are utilized in aqueous systems for applications such as chemosensors, cell imaging, anticancer, biolabeling, biomedicine, metal ion detection, and photodynamic treatment. This review covers the most current developments in the design and synthesis of water-soluble BODIPY derivatives and their wide applications since 2014.

Interaction of Ions in Organic and Aqueous Media with an Ion-Pair Sensor Equipped with a BODIPY Reporter: An ON1-OFF-ON2-ON3 Fluorescent Assay

Here, we present a ditopic ion-pair sensor, B1, containing the BODIPY reporter unit in its structure, which is shown to be able-thanks to the presence of two heterogeneous binding domains-to interact with anions in an enhanced manner in the presence of cations. This enables it to interact with salts even in 99% aqueous solutions, making B1 a good candidate in terms of visual salt detection in the aquatic environment. Receptor B1's ability to extract and release salt was applied in the transport of potassium chloride through a bulk liquid membrane. Working with a concentration of B1 in the organic phase and with the presence of a specific salt in an aqueous solution, an inverted transport experiment was also demonstrated. By varying the type and the amount of the anions added to B1, we were able to develop diverse optical responses, including a unique four-step ON1-OFF-ON2-ON3 output.

BODIPY as a Multifunctional Theranostic Reagent in Biomedicine: Self-Assembly, Properties, and Applications

The use of boron dipyrromethene (BODIPY) in biomedicine is reviewed. To open, its synthesis and regulatory strategies are summarized, and inspiring cutting-edge work in post-functionalization strategies is highlighted. A brief overview of assembly model of BODIPY is then provided: BODIPY is introduced as a promising building block for the formation of single- and multicomponent self-assembled systems, including nanostructures suitable for aqueous environments, thereby showing the great development potential of supramolecular assembly in biomedicine applications. The frontier progress of BODIPY in biomedical application is thereafter described, supported by examples of the frontiers of biomedical applications of BODIPY-containing smart materials: it mainly involves the application of materials based on BODIPY building blocks and their assemblies in fluorescence bioimaging, photoacoustic imaging, disease treatment including photodynamic therapy, photothermal therapy, and immunotherapy. Lastly, not only the current status of the BODIPY family in the biomedical field but also the challenges worth considering are summarized. At the same time, insights into the future development prospects of biomedically applicable BODIPY are provided.

Cell Uptake of Steroid-BODIPY Conjugates and Their Internalization Mechanisms: Cancer Theranostic Dyes

Estradiol-BODIPY linked via an 8-carbon spacer chain and 19-nortestosterone- and testosterone-BODIPY linked via an ethynyl spacer group were evaluated for cell uptake in the breast cancer cell lines MCF-7 and MDA-MB-231 and prostate cancer cell lines PC-3 and LNCaP, as well as in normal dermal fibroblasts, using fluorescence microscopy. The highest level of internalization was observed with 11β-OMe-estradiol-BODIPY 2 and 7α-Me-19-nortestosterone-BODIPY 4 towards cells expressing their specific receptors. Blocking experiments showed changes in non-specific cell uptake in the cancer and normal cells, which likely reflect differences in the lipophilicity of the conjugates. The internalization of the conjugates was shown to be an energy-dependent process that is likely mediated by clathrin- and caveolae-endocytosis. Studies using 2D co-cultures of cancer cells and normal fibroblasts showed that the conjugates are more selective towards cancer cells. Cell viability assays showed that the conjugates are non-toxic for cancer and/or normal cells. Visible light irradiation of cells incubated with estradiol-BODIPYs 1 and 2 and 7α-Me-19-nortestosterone-BODIPY 4 induced cell death, suggesting their potential for use as PDT agents.

Mechanoresponsive Metal-Organic Cage-Crosslinked Polymer Hydrogels

We report the formation of metal-organic cage-crosslinked polymer hydrogels. To enable crosslinking of the cages and subsequent network formation, we used homodifunctionalized poly(ethylene glycol) (PEG) chains terminally substituted with bipyridines as ligands for the Pd₆ L₄ corners. The encapsulation of guest molecules into supramolecular self-assembled metal-organic cage-crosslinked hydrogels, as well as ultrasound-induced disassembly of the cages with release of their cargo, is presented in addition to their characterization by nuclear magnetic resonance (NMR) techniques, rheology, and comprehensive small-angle X-ray scattering (SAXS) experiments. The constrained geometries simulating external force (CoGEF) method and barriers using a force-modified potential energy surface (FMPES) suggest that the cage-opening mechanism starts with the dissociation of one pyridine ligand at around 0.5 nN. We show the efficient sonochemical activation of the hydrogels HG_{3 -6} , increasing the non-covalent guest-loading of completely unmodified drugs available for release by a factor of ten in comparison to non-crosslinked, star-shaped assemblies in solution.

Energy Transport in Dichroic Metallo-organic Crystals: Selective Inclusion of Spatially Resolved Arrays of Donor and Acceptor Dyes in Different Nanochannels

In this study, the precise positioning and alignment of arrays of two different guest molecules in a crystalline host matrix has been engineered and resulted in new optically active materials. Sub-nm differences in the diameters of two types of 1D channels are sufficient for size-selective inclusion of dyes. Energy transport occurs between the arrays of different dyes that are included in parallel-positioned nanochannels by Förster resonance energy transfer (FRET). The color of individual micro-sized crystals are dependent on their relative position under polarized light. This angular-dependent behavior is a result of the geometrically constrained orientation of the dyes by the crystallographic packing of the host matrix and is concentration dependent.

Pseudo-heterolepticity in Low-Symmetry Metal-Organic Cages

Heteroleptic metal-organic cages, formed through integrative self-assembly of ligand mixtures, are highly attractive as reduced symmetry supramolecular hosts. Ensuring high-fidelity, non-statistical self-assembly, however, presents a significant challenge in molecular engineering due to the inherent difficulty in predicting thermodynamic energy landscapes. In this work, two conceptual strategies are described that circumvent this issue, using ligand design strategies to access structurally sophisticated metal-organic hosts. Using these approaches, it was possible to realise cavity environments described by two inequivalent, unsymmetrical ligand frameworks, representing a significant step forward in the construction of highly anisotropic confined spaces.

BODIPY-based metal-organic frameworks as efficient electrochemiluminescence emitters for telomerase detection

A boron dipyrromethene (BODIPY)-based metal-organic framework (MOF) nanoemitter was for the first time designed with enhanced electrochemiluminescence (ECL) intensity due to the suppression of non-radiative dissipation originating from the ordered arrangement of BODIPY molecules in the framework. Thus, an ECL biosensor was developed for telomerase detection with excellent performance in real samples.

Fluorescence enhancement via structural rigidification inside a self-assembled Pd4 molecular vessel

Restriction of intramolecular motion (RIM) is fundamental for the high emission of aggregation-induced emission (AIE)-active molecules in aggregates or the solid-state. However, they are weakly emissive in dilute solution, which limits their application in dilute solutions. A Pd₄ molecular vessel (MP1) was constructed by assembling [cis-(en)Pd(NO₃)₂] (M) with a tetradentate donor (L) in a 2 : 1 molar ratio. The active intramolecular motions of an AIE active molecule SG are restricted in the narrow cavity of MP1 upon encapsulation. As a result, SG displayed significant enhancement in its emission in dilute solution upon addition of MP1. This strategy of achieving high emission of AIE active compounds in dilute solution by confinement driven RIM might have potential in designing materials for high emission in the aggregated state as well as in dilute solution.

Visible-Light-Responsive Self-Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination

A photoswitchable ligand based on azobenzene is self-assembled with palladium(II) ions to form a [Pd2 (E-L)4 ]4+ cage. Irradiation with 470 nm light results in the near-quantitative switching to a monomeric species [Pd(Z-L)2 ]2+ , which can be reversed by irradiation with 405 nm light, or heat. The photoswitching selectivity towards the metastable isomer is significantly improved upon self-assembly, and the thermal half-life is extended from 40 days to 850 days, a promising approach for tuning photoswitching properties.

Ternary host-guest complexes with rapid exchange kinetics and photoswitchable fluorescence

Confinement within molecular cages can dramatically modify the physicochemical properties of the encapsulated guest molecules, but such host-guest complexes have mainly been studied in a static context. Combining confinement effects with fast guest exchange kinetics could pave the way toward stimuli-responsive supramolecular systems-and ultimately materials-whose desired properties could be tailored "on demand" rapidly and reversibly. Here, we demonstrate rapid guest exchange between inclusion complexes of an open-window coordination cage that can simultaneously accommodate two guest molecules. Working with two types of guests, anthracene derivatives and BODIPY dyes, we show that the former can substantially modify the optical properties of the latter upon noncovalent heterodimer formation. We also studied the light-induced covalent dimerization of encapsulated anthracenes and found large effects of confinement on reaction rates. By coupling the photodimerization with the rapid guest exchange, we developed a new way to modulate fluorescence using external irradiation.

Molecular Engineering of Noncovalent Dimerization

Dimers are probably the simplest model to facilitate the understanding of fundamental physical and chemical processes that take place in much-expanded systems like aggregates, crystals, and other solid states. The molecular interplay within a dimer differentiates it from the corresponding monomeric state and determines its features. Molecular engineering of noncovalent dimerization through applied supramolecular restrictions enables additional control over molecular interplay, particularly over its dynamic aspect. This Perspective introduces the recent effort that has been made in the molecular engineering of noncovalent dimerization, including supramolecular dimers, folda-dimers, and macrocyclic dimers. It showcases how the variation in supramolecular restrictions endows molecular-based materials with improved performance and/or functions like enhanced emission, room-temperature phosphorescence, and effective catalysis. We particularly discuss pseudostatic dimers that can sustain molecular interplay for a long period of time, yet are still flexible enough to adapt to variations. The pseudostatic feature allows for active species to decay along an alternate pathway, thereby spinning off emerging features that are not readily accessible from conventional dynamic systems.

Spanning BODIPY fluorescence with self-assembled micellar clusters

BODIPY dyes possess favorable optical properties for a variety of applications including in vivo and in vitro diagnostics. However, their utilization might be limited by their water insolubility and incompatibility with chemical modifications, resulting in low aggregation stability. Here, we outline the route for addressing this issue. We have demonstrated two approaches, based on dye entrapment in micellar coordination clusters (MCCs); this provides a general solution for water solubility as well as aggregation stability of the seven BODIPY derivatives. These derivatives have various bulky aromatic substituents in the 2,3,5,6- and meso-positions and can rotate relative to a dipyrrin core, which also provides molecular rotor properties. The molecular structural features and the presence of aromatic groups allows BODIPY dyes to be used as "supporting molecules", thus promoting micelle-micelle interaction and micellar network stabilization. In the second approach, self-micellization, following BODIPY use, leads to MCC formation without the use of any mediators, including chelators and/or metal ions. In both approaches, BODIPY exhibits an excellent optical response, at a concentration beyond its solubilization limit in aqueous media and without undesired crystallization. The suggested approaches represent systems used to encapsulate BODIPY in a capsule-based surfactant environment, enabling one to track the aggregation of BODIPY; these approaches represent an alternative system to study and apply BODIPY's molecular rotor properties. The stabilized compounds, i.e., the BODIPY-loaded MCCs, provide a unique feature of permeability to hydrophilic ligand-switching proteins such as BSA; they exhibit a bright "turn-on" fluorescence signal within the clusters via macromolecular complexation, thus expanding the possibilities of water-soluble BODIPY-loaded MCCs utilization for functional indicators.

Electronic transitions in noncovalent BODIPY dimers: TD-DFT study

Theoretical computations have been performed on the absorption spectra of (8-R-TMB)₂ dimers with varying character of substituents at the 8 position (meso) at monomers units (R = NH₂, OH, CH₃, H, COH, CF₃, CN). The obtained results (TD-CAM-B3LYP) show that the first four lower transitions of studied dimers (S₀ → S_i, i = 1-4) are intrinsically linked with delocalized HOMO and LUMO orbitals of the two monomers, which constitute a dimer. For all the dimers, S₀ → S₁ and S₀ → S₃ transitions are strongly forbidden, whereas S₀ → S₂ and S₀ → S₄ are allowed. There is a good agreement between the TD-CAM-B3LYP theory and the simple model of exciton coupling for two identical chromophores with the planes of two moieties, which are stacked upon each other. Intensities of the allowed transitions depend strongly on the nature of the substituent at the meso position. For the dimers with monomer units bearing electron-donor groups, S₀ → S₂ transitions are more intense compared with S₀ → S₄ ones. As the donor properties of the meso substituents diminish and electron acceptor properties enlarge, the intensity of these transitions becomes lower, whereas that of S₀ → S₄ transitions becomes higher. For the dimers with NH₂ and CN substituents, the almost inverse ratio of intensities of the transitions discussed is observed (for example, f = 0.80 and 0.06 (in the case of NH₂) and f = 0.09 and 0.72 (CN) for S₀ → S₂ and S₀ → S₄, respectively). Protonation of the tertiary amine function drastically 'switches off' its electron-donating properties. As a result, the red shift of the bands and redistribution of intensities of the allowed S₀ → S₂ and S₀ → S₄ transitions is predicted for protonated forms of the (8-NH₂-TMB)₂ dimer. This peculiarity can be of especial importance for the design of dimers with a sensor function.

Encapsulation within a coordination cage modulates the reactivity of redox-active dyes

Confining molecules within well-defined nanosized spaces can profoundly alter their physicochemical characteristics. For example, the controlled aggregation of chromophores into discrete oligomers has been shown to tune their optical properties whereas encapsulation of reactive species within molecular hosts can increase their stability. The resazurin/resorufin pair has been widely used for detecting redox processes in biological settings; yet, how tight confinement affects the properties of these two dyes remains to be explored. Here, we show that a flexible Pd^II₆L₄ coordination cage can efficiently encapsulate both resorufin and resazurin in the form of dimers, dramatically modulating their optical properties. Furthermore, binding within the cage significantly decreases the reduction rate of resazurin to resorufin, and the rate of the subsequent reduction of resorufin to dihydroresorufin. During our studies, we also found that upon dilution, the Pd^II₆L₄ cage disassembles to afford Pd^II₂L₂ species, which lacks the ability to form inclusion complexes - a process that can be reversed upon the addition of the strongly binding resorufin/resazurin guests. We expect that the herein disclosed ability of a water-soluble cage to reversibly modulate the optical and chemical properties of a molecular redox probe will expand the versatility of synthetic fluorescent probes in biologically relevant environments.

Coexistence of 1 : 1 and 2 : 1 inclusion complexes of indigo carmine

We show that the optical properties of indigo carmine can be modulated by encapsulation within a coordination cage. Depending on the host/guest molar ratio, the cage can predominantly encapsulate either one or two dye molecules. The 1 : 1 complex is fluorescent, unique for an indigo dye in an aqueous solution. We have also found that binding two dye molecules stabilizes a previously unknown conformation of the cage.

We show that the optical properties of indigo carmine can be modulated by encapsulation within a coordination cage.

Guest-Driven Self-Assembly and Chiral Induction of Photofunctional Lanthanide Tetrahedral Cages

Chiral luminescent lanthanide-organic cages have many potential applications in enantioselective recognition, sensing, and asymmetric catalysis. However, due to the paucity of structures and their limited cavities, host-guest chemistry with lanthanide-organic cages has remained elusive so far. Herein, we report a guest-driven self-assembly and chiral induction approach for the construction of otherwise inaccessible Ln₄L₄-type (Ln = lanthanide ions, i.e., Eu^III, Tb^III; L = ligand) tetrahedral hosts. Single crystal analyses on a series of host-guest complexes reveal remarkable guest-adaptive cavity breathing on the tetrahedral cages, reflecting the advantage of the variation tolerance on coordination geometry of the f-elements. Meanwhile, noncovalent confinement of pyrene within the lanthanide cage not only leads to diminishment of its excimer emission but also facilitates guest to host energy transfer, opening up a new sensitization window for the lanthanide luminescence on the cage. Moreover, stereoselective self-assembly of either Λ₄- or Δ₄- type Eu₄L₄ cages has been realized via chiral induction with R/S-BINOL or R/S-SPOL templates, as confirmed by NMR, circular dichroism (CD), and circularly polarized luminescence (CPL) with high dissymmetry factors (g_lum) up to ±0.125.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Realization of Single-Phase White-Light-Emitting Materials with Time-Evolution Ultralong Room-Temperature Phosphorescence by Coordination Assemblies

Two Cd-based supramolecular coordination polymers, [Cd₃(CzIP)₃(DMF)₂] (1) and [Cd₂(CzIP)₂(DMF)₄] (2), were synthesized by using 5-(carbazol-9-yl) isophthalate (CzIP) as ligands. These two compounds exhibit multiple luminescence emissions; apart from fluorescence, time- and temperature-dependent ultralong phosphorescence (RTP) were also achieved under room conditions. Significantly, compound 1 has a long-lived afterglow of 0.93 s at 545 nm under ambient conditions. Compound 2 shows nearly pure white-light emission with CIE coordinates of (0.33, 0.33) via the dual emission of fluorescence and phosphorescence. It has come to our attention that it is the first example of a luminescent coordination polymer with single-phase white-light emission and color-evolution RTP. In addition, the long-lived RTP materials can be used in time-dependent anticounterfeiting and white-light-emitting diodes. Experimental and singlet and triplet state calculations indicate that both C-H···π interaction and inter- and intramolecular charge transfer interactions could be beneficial to the emission of ultralong RTP.

Mechanochemical Release of Non-Covalently Bound Guests from a Polymer-Decorated Supramolecular Cage

Supramolecular coordination cages show a wide range of useful properties including, but not limited to, complex molecular machine-like operations, confined space catalysis, and rich host-guest chemistries. Here we report the uptake and release of non-covalently encapsulated, pharmaceutically-active cargo from an octahedral Pd cage bearing polymer chains on each vertex. Six poly(ethylene glycol)-decorated bipyridine ligands are used to assemble an octahedral PdII6 (TPT)4 cage. The supramolecular container encapsulates progesterone and ibuprofen within its hydrophobic nanocavity and is activated by shear force produced by ultrasonication in aqueous solution entailing complete cargo release upon rupture, as shown by NMR and GPC analyses.

Preparation of a Multicarbazole-Based Nanocapsule Capable of Largely Modulating Guest Spectroscopic Properties in Water

A nanocapsule composed of multiple carbazole panels (ca. 12 panels) was quantitatively generated from bent carbazole-based amphiphiles in water. Unlike previously reported macrocycles and coordination cages bearing several carbazole panels, the resultant nanocapsule displays enhanced emissivity and improved electrochemical stability as compared with the monomeric amphiphile. The spectroscopic properties of substituted coumarin and boron-dipyrromethene dyes can be modulated upon encapsulation by the nanocapsule in water. In the cavity, a highly blue-shifted absorption band is observed from largely twisted coumarin dyes and two absorption bands are found from boron-dipyrromethene dimers stacked in an unusual L-shaped fashion. Moreover, the encapsulated dimers exhibit unique excimer-like emission.

Design of BODIPY dyes as triplet photosensitizers: electronic properties tailored for solar energy conversion, photoredox catalysis and photodynamic therapy

BODIPYs are renowned fluorescent dyes with strong and tunable absorption in the visible region, high thermal and photo-stability and exceptional fluorescence quantum yields. Transition metal complexes are the most commonly used triplet photosensitisers, but, recently, the use of organic dyes has emerged as a viable and more sustainable alternative. By proper design, BODIPY dyes have been turned from highly fluorescent labels into efficient triplet photosensitizers with strong absorption in the visible region (from green to orange). In this perspective, we report three design strategies: (i) halogenation of the dye skeleton, (ii) donor-acceptor dyads and (iii) BODIPY dimers. We compare pros and cons of these approaches in terms of optical and electrochemical properties and synthetic viability. The potential applications of these systems span from energy conversion to medicine and key examples are presented.

BODIPY dimers: structure, interaction, and absorption spectrum

The object of the present study are BODIPY molecules obtained previously by Piskorz et al. (Dyes Pigm. 178:108322, 2020) for their antimicrobial activity. Structural analysis of the BODIPY dimers is presented in context of the aggregation influence on the photophysical properties. The thorough investigation of the nature of intermolecular interaction in the representative BODIPY dimers is provided together with the decomposition of the interaction energy into the components of well-defined origin according to SAPT procedure. For the model BODIPY systems the careful examination of the interaction nature for the dimer structure based on experimental crystal study as well as fully optimized is given. The tendencies observed in the model dimers are further on investigated for two pairs of BODIPY systems designed for biomedical application. The analyzed molecules are shown to maximize the mutual interaction by the optimization of the stacking dispersion contacts between the aromatic rings of the molecules, therefore producing stable dimers. The estimation of SAPT0 interaction energy components confirms the dominating dispersion character arising from mutual BODIPY core contacts. The influence of the dimerization process on the photophysical properties of the systems studied theoretically depends to the high extend on the dimerization mode and is significant for parallel and antiparallel dispersion-governed dimers.

Cyclization-Promoted Ultralong Low-Temperature Phosphorescence via Boosting Intersystem Crossing

Ultralong organic phosphorescence holds great promise as an important approach for optical materials and devices. Most of phosphorescent organic molecules with long lifetimes are substituted with heavy atoms or carbonyl groups to enhance the intersystem crossing (ISC), which requires complicated design and synthesis. Here, we report a cyclization-promoted phosphorescence phenomenon by boosting ISC. N-butyl carbazole exhibits a phosphorescence lifetime (τ_p) of only 1.45 ms and a low phosphorescence efficiency in the solution state at 77 K due to the lack of efficient ISC. In order to promote its phosphorescence behavior, we explored the influence of conjugation. By linear conjugation of four carbazole units, possible ISC channels are increased so that a longer τ_p of 2.24 s is observed. Moreover, by cyclization, the energy gap between the singlet and triplet states is dramatically decreased to 0.04 eV for excellent ISC efficiency accompanied by increased rigidification to synergistically suppress the nonradiative decay, resulting in satisfactory phosphorescence efficiency and a prolonged τ_p to 3.41 s in the absence of any heavy atom or carbonyl group, which may act as a strategy to prepare ultralong phosphorescent organic materials by enhancing the ISC and rigidification.

Visible-Light-Driven Rotation of Molecular Motors in Discrete Supramolecular Metallacycles

The organization of molecular motors in supramolecular assemblies to allow the amplification and transmission of motion and collective action is an important step toward future responsive systems. Metal-coordination-driven directional self-assembly into supramolecular metallacycles provides a powerful strategy to position several motor units in larger structures with well-defined geometries. Herein, we present a pyridyl-modified molecular motor ligand (MPY) which upon coordination with geometrically distinct di-Pt(II) acceptors assembles into discrete metallacycles of different sizes and shapes. This coordination leads to a red-shift of the absorption bands of molecular motors, making these motorized metallacycles responsive to visible light. Photochemical and thermal isomerization experiments demonstrated that the light-driven rotation of the motors in the metallacycles is similar to that in free MPY in solution. CD studies show that the helicity inversions associated with each isomerization step in the rotary cycle are preserved. To explore collective motion, the trimeric motor-containing metallacycle was aggregated with heparin through multiple electrostatic interactions, to construct a multi-component hierarchical system. SEM, TEM, and DLS measurements revealed that the photo- and thermal-responsive molecular motor units enabled selective manipulation of the secondary supramolecular aggregation process without dissociating the primary metallacycle structures. These visible-light-responsive metallacycles, with intrinsic multiple rotary motors, offer prospects for cooperative operations, dynamic hierarchical self-assembled systems, and adaptive materials.

Coordination-driven supramolecular syntheses of new homo- and hetero-polymetallic Cu(i) assemblies: solid-state and solution characterization

New luminescent Cu(i) discrete assemblies D and FM and 1D coordination polymer E are reported. Deep insights of self-assembly processes based on flexible Cu(i) precursors are highlighted together with the preservation in solution of Cu(i) assemblies.

Enhanced photoreduction of water catalyzed by a cucurbit[8]uril-secured platinum dimer

A cucurbit[8]uril (CB[8])-secured platinum terpyridyl chloride dimer was used as a photosensitizer and hydrogen-evolving catalyst for the photoreduction of water. Volumes of produced hydrogen were up to 25 and 6 times larger than those obtained with the corresponding free and cucurbit[7]uril-bound platinum monomer, respectively, at equal Pt concentration. The thermodynamics of the proton-coupled electron transfer from the Pt(ii)–Pt(ii) dimer to the corresponding Pt(ii)–Pt(iii)–H hydride key intermediate, as quantified by density functional theory, suggest that CB[8] secures the Pt(ii)–Pt(ii) dimer in a particularly reactive conformation that promotes hydrogen formation.

The cucurbit[8]uril macrocycle can secure a platinum terpyridyl complex into a particularly reactive dimer that catalyzes the photoreduction of water.