Light- and pH-regulated Water-soluble Pseudorotaxanes Comprising a Cucurbit[7]uril and a Flavylium-based Axle.

A linear double pyridinium-terminated thread comprising a central chalcone moiety is shown to provide two independent binding sites with similar affinity for cucurbit[7]uril (CB7) macrocycles in water as judged from NMR, UV-Visible and fluorescence spectroscopies. Association results in [2] and [3]pseudorotaxanes, which are both pH and photosensitive. Switching from the neutral chalcone to the cationic flavylium form upon irradiation at 365 nm under acidic conditions provided an enhanced CB7 association (K1:1 increases from 1.2×105  M-1 to 1.5×108  M-1 ), limiting spontaneous on-thread cucurbituril shuttling. This co-conformational change in the [2]pseudorotaxane is reversible in the dark with kobs =4.1×10-4  s-1 . Threading the flavylium moiety into CB7 leads to a dramatic increase in the fluorescence quantum yield, from 0.29 in the free axle to 0.97 in the [2]pseudorotaxane and 1.0 in the [3]pseudorotaxane.

Ratiometric Luminescence Detection of Copper(I) by a Resonant System Comprising Two Antenna/Lanthanide Pairs.

Selective and sensitive detection of Cu(I) is an ongoing challenge due to its important role in biological systems, for example. Herein, we describe a photoluminescent molecular chemosensor integrating two lanthanide ions (Tb3+ and Eu3+) and respective tryptophan and naphthalene antennas onto a polypeptide backbone. The latter was structurally inspired from copper-regulating biomacromolecules in Gram-negative bacteria and was found to bind Cu+ effectively under pseudobiological conditions (log KCu+ = 9.7 ± 0.2). Ion regulated modulation of lanthanide luminescence in terms of intensity and long, millisecond lifetime offers perspectives in terms of ratiometric and time-gated detection of Cu+. The role of the bound ion in determining the photophysical properties is discussed with the aid of additional model compounds.

Bioinspired Luminescent Europium-Based Probe Capable of Discrimination between Ag+ and Cu.

Due to their similar coordination properties, discrimination of Cu+ and Ag+ by water-soluble luminescent probes is challenging. We have synthesized LCC4Eu, an 18 amino acid cyclic peptide bearing a europium complex, which is able to bind one Cu+ or Ag+ ion by the side chains of two methionines, a histidine and a 3-(1-naphthyl)-l-alanine. In this system, the naphthyl moiety establishes a cation-π interaction with these cations. It also acts as an antenna for the sensitization of Eu3+ luminescence. Interestingly, when excited at 280 nm, LCC4Eu behaves as a turn-on probe for Ag+ (+150% Eu emission) and as a turn-off probe for Cu+ (-50% Eu3+ emission). Shifting the excitation wavelength to 305 nm makes the probe responsive to Ag+ (+380% Eu3+ emission) but not to Cu+ or other physiological cations. Thus, LCC4Eu is uniquely capable of discriminating Ag+ from Cu+. A detailed spectroscopic characterization based on steady-state and time-resolved measurements clearly demonstrates that Eu3+ sensitization relies on electronic energy transfer from the naphthalene triplet state to the Eu3+ excited states and that the cation-π interaction lowers the energy of this triplet state by 700 and 2400 cm-1 for Ag+ and Cu+, respectively. Spectroscopic data point to a modulation of the efficiency of the electronic energy transfer caused by the differential red shift of the naphthalene triplet, deciphering the differential luminescence response of LCC4Eu toward Ag+ and Cu+.

Alkylation of the α-amino C-H bonds of anilines photocatalyzed by a DMEDA-Cu-benzophenone complex: reaction scope and mechanistic studies.

The Cu(ii) complex 1 incorporating a BP chromophore is a highly active and chemoselective photocatalyst for the alkylation of α-amino C-H bonds of anilines. The reaction was shown to proceed with a broad substrate scope in the absence of additives. Extensive mechanistic studies were performed, in particular using transient absorption spectroscopy, and spectroscopic signatures of key intermediates were identified in the conditions of catalysis. Finally, the ability of 1 to act as a multitask catalyst was showcased by conducting multi-component CuAAC and olefin hydroalkylation reactions in one-pot.

Five-component, one-pot synthesis of an electroactive rotaxane comprising a bisferrocene macrocycle.

The templated clipping of a ferrocene-grafted isophthalic acid derivative to encircle a hydrogen-bonding axle through the reaction with 1,4-bis(aminomethyl)benzene is described. The constituent electroactive macrocycle of the resultant [2]rotaxane is a homologue of the versatile benchmark tetraamide variant developed by Leigh and co-workers. The relative templating effect of different hydrogen-bonding motifs in rotaxane and pseudorotaxane generation is compared, with yields varying from 0 to 41%. The electrochemical properties and single crystal X-ray structure of a doubly ferrocene-decorated [2]rotaxane are further reported.

Photochromic rotaxanes and pseudorotaxanes.

Among stimulus-responsive molecular ring-on-thread rotaxanes and pseudorotaxanes, variants incorporating photochromic sub-units are attracting considerable attention as their properties and structure can be remotely and precisely controlled, additionally without producing chemical waste. The focus herein is on photoswitching-driven assembly/disassembly and modulation of properties resulting from light-activated isomerization or changes in electronic properties.

Photoreversible stretching of a BAPTA chelator marshalling Ca2+-binding in aqueous media.

Free calcium ion concentration is known to govern numerous biological processes and indeed calcium acts as an important biological secondary messenger for muscle contraction, neurotransmitter release, ion-channel gating, and exocytosis. As such, the development of molecules with the ability to instantaneously increase or diminish free calcium concentrations potentially allows greater control over certain biological functions. In order to permit remote regulation of Ca2+, a selective BAPTA-type synthetic receptor / host was integrated with a photoswitchable azobenzene motif, which upon photoirradiation would enhance (or diminish) the capacity to bind calcium upon acting on the conformation of the adjacent binding site, rendering it a stronger or weaker binder. Photoswitching was studied in pseudo-physiological conditions (pH 7.2, [KCl] = 100 mM) and dissociation constants for azobenzene cis- and trans-isomers have been determined (0.230 µM and 0.102 µM, respectively). Reversible photoliberation/uptake leading to a variation of free calcium concentration in solution was detected using a fluorescent Ca2+ chemosensor.

Molecular engineering of logic gate types by module rearrangement in 'Pourbaix Sensors': the effect of excited-state electric fields.

Two types of fluorescent logic gates are accessed from two different arrangements of the same modular components, one as an AND logic gate (1) and the other as a PASS 0 logic gate (2). The logic gates were designed with an 'electron-donor-spacer1-fluorophore-spacer2-receptor' format and demonstrated in 1 : 1 (v/v) methanol/water. The molecules consist of ferrocene as the electron donor, 4-aminonaphthalimide as the fluorophore and a tertiary alkylamine as the receptor. In the presence of high H+ and Fe3+ levels, regioisomers 1a and 1b switch 'on' as AND logic gates with fluorescence enhancement ratios of 16-fold and 10-fold, respectively, while regioisomers 2a and 2b are functionally dormant, exhibiting no fluorescence switching. The PASS 0 logic of 2a and 2b results from the transfer of an electron from the excited state fluorophore to the ferrocenium unit under oxidising conditions as predicted by DFT calculations. Time-resolved fluorescence spectroscopy provided lifetimes of 8.3 ns and 8.1 ns for 1a and 1b, respectively. The transient signal recovery rate of 1b is ∼10 ps while that of 2b is considerably longer on the nanosecond timescale. The divergent logic attributes of 1 and 2 highlight the importance of field effects and opens up a new approach for regulating logic-based molecules.

Designed Long-Lived Emission from CdSe Quantum Dots through Reversible Electronic Energy Transfer with a Surface-Bound Chromophore.

The size-tunable emission of luminescent quantum dots (QDs) makes them highly interesting for applications that range from bioimaging to optoelectronics. For the same applications, engineering their luminescence lifetime, in particular, making it longer, would be as important; however, no rational approach to reach this goal is available to date. We describe a strategy to prolong the emission lifetime of QDs through electronic energy shuttling to the triplet excited state of a surface-bound molecular chromophore. To implement this idea, we made CdSe QDs of different sizes and carried out self-assembly with a pyrene derivative. We observed that the conjugates exhibit delayed luminescence, with emission decays that are prolonged by more than 3 orders of magnitude (lifetimes up to 330 µs) compared to the parent CdSe QDs. The mechanism invokes unprecedented reversible quantum dot to organic chromophore electronic energy transfer.

Synthetic water soluble di-/tritopic molecular receptors exhibiting Ca2+/Mg2+ exchange.

Structural integration of two synthetic water soluble receptors for Ca2+ and Mg2+, namely 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and o-aminophenol-N,N,O-triacetic acid (APTRA), respectively, gave novel di- and tritopic ionophores (1 and 2). As Mg2+ and Ca2+ cannot be simultaneously complexed by the receptors, allosteric control of complexation results. Potentiometric measurements established stepwise protonation constants and showed high affinity for Ca2+ (log K = 6.08 and 8.70 for 1 and 2, respectively) and an excellent selectivity over Mg2+ (log K = 3.70 and 5.60 for 1 and 2, respectively), which is compatible with magnesium-calcium ion exchange. While ion-exchange of a single Mg2+ for a single Ca2+ is possible in both 1 and 2, the simultaneous binding of two Mg2+ by 2 appears prohibitive for replacement of these two ions by a single Ca2+. Ion-binding and exchange was further rationalized by DFT calculations.

Polymersome Popping by Light-Induced Osmotic Shock under Temporal, Spatial, and Spectral Control.

The light-triggered, programmable rupture of cell-sized vesicles is described, with particular emphasis on self-assembled polymersome capsules. The mechanism involves a hypotonic osmotic imbalance created by the accumulation of photogenerated species inside the lumen, which cannot be compensated owing to the low water permeability of the membrane. This simple and versatile mechanism can be adapted to a wealth of hydrosoluble molecules, which are either able to generate reactive oxygen species or undergo photocleavage. Ultimately, in a multi-compartmentalized and cell-like system, the possibility to selectively burst polymersomes with high specificity and temporal precision and to consequently deliver small encapsulated vesicles (both polymersomes and liposomes) is demonstrated.

Photoinduced Electron Transfer and Hole Migration in Nanosized Helical Aromatic Oligoamide Foldamers.

A series of photoactive triads have been synthesized and investigated in order to elucidate photoinduced electron transfer and hole migration mechanism across nanosized, rigid helical foldamers. The triads are comprised of a central helical oligoamide foldamer bridge with 9, 14, 18, 19, or 34 8-amino-2-quinolinecarboxylic acid repeat units, and of two chromophores, an N-terminal oligo(para-phenylenevinylene) electron donor and a C-terminal perylene bis-imide electron acceptor. Time-resolved fluorescence and transient absorption spectroscopic studies showed that, following photoexcitation of the electron acceptor, fast electron transfer occurs initially from the oligoquinoline bridge to the acceptor chromophore on the picosecond time scale. The oligo(para-phenylenevinylene) electron donor is oxidized after a time delay during which the hole migrates across the foldamer from the acceptor to the donor. The charge separated state that is finally generated was found to be remarkably long-lived (>80 µs). While the initial charge injection rate is largely invariant for all foldamer lengths (ca. 60 ps), the subsequent hole transfer to the donor varies from 1 × 109 s-1 for the longest sequence to 17 × 109 s-1 for the shortest. In all cases, charge transfer is very fast considering the foldamer length. Detailed analysis of the process in different media and at varying temperatures is consistent with a hopping mechanism of hole transport through the foldamer helix, with individual hops occurring on the subpicosecond time scale (kET = 2.5 × 1012 s-1 in CH2Cl2). This work demonstrates the possibility of fast long-range hole transfer over 300 Å (through bonds) across a synthetic modular bridge, an achievement that had been previously observed principally with DNA structures.

Harnessing Reversible Electronic Energy Transfer: From Molecular Dyads to Molecular Machines.

Reversible electronic energy transfer (REET) may be instilled in bi-/multichromophoric molecule-based systems, following photoexcitation, upon judicious structural integration of matched chromophores. This leads to a new set of photophysical properties for the ensemble, which can be fully characterized by steady-state and time-resolved spectroscopic methods. Herein, we take a comprehensive look at progress in the development of this type of supermolecule in the last five years, which has seen systems evolve from covalently tethered dyads to synthetic molecular machines, exemplified by two different pseudorotaxanes. Indeed, REET holds promise in the control of movement in molecular machines, their assembly/disassembly, as well as in charge separation.

High performance optical oxygen sensors based on iridium complexes exhibiting interchromophore energy shuttling.

A doubly pyrene-grafted bis-cyclometallated iridium complex with engineered electronically excited states demonstrates reversible electronic energy transfer between adjacent chromophores giving rise to extremely long-lived red luminescence in solution (τ = 480 µs). Time-resolved spectroscopic studies afforded determination of pertinent photophysical parameters including rates of energy transfer and energy distribution between constituent chromophores in the equilibrated excited molecule (ca. 98% on the organic chromophores). Incorporation into a nanostructured metal-oxide matrix (AP200/19) gave highly sensitive O2 sensing films, as the detection sensitivity was 200-300% higher than with the commonly used PtTFPP and approaches the sensitivity of the best O2-sensing dyes reported to date.

Electronic Energy Transfer Modulation in a Dynamic Foldaxane: Proof-of-Principle of a Lifetime-Based Conformation Probe.

Abiotic aromatic oligoamide foldamers are shown to self-assemble in solution to form a double helix, which can accommodate a bichromophoric thread in its central void. While in solution reversible electronic energy transfer is instilled between chromophoric termini of the free, flexible thread as evidenced through delayed luminescence, upon rigidification of the rod the chromophores are mutually distanced and effectively decoupled. Consequently, the chromophores display their individual photophysical characteristics. The observed conformation-dependent changes of dynamic luminescence properties, which are particularly sensitive to distance, offers a new strategy for lifetime-based detection of geometry on the molecular scale as demonstrated through real-time luminescence detection of molecular complexation leading to foldaxane formation.

Reactivity of a Zn(Cys)2(His)2 Zinc Finger with Singlet Oxygen: Oxidation Directed toward Cysteines but not Histidines.

Singlet oxygen ((1)O2) is an important reactive oxygen species in biology that has deleterious effects. Proteins constitute the main target of (1)O2 in cells. Several organisms are able to mount a transcriptional defense against (1)O2. ChrR and MBS are two proteins with Zn(Cys)2(His)2 zinc finger sites that are involved in the regulation of the defense against (1)O2. In this article, we investigate the reactivity of Zn⋅CPF, a Zn(Cys)2(His)2 classical ßßα zinc finger, with (1)O2. We show that Zn⋅CPF interacts with (1)O2 mainly by physical quenching using a combination of (1)O2 luminescence quenching and kinetic competition experiments. The chemical reaction, which accounts for 5% of the interaction, leads to oxidation of cysteines but not histidines. Primary photooxidation products, identified by HPLC and mass spectrometry, are sulfinate (75±5%) and disulfides (25±5%). The peptides that have a single cysteine thiolate oxidized into a sulfinate are still able to bind one equivalent Zn(2+) but with a dramatic reduction of the binding constant compared to Zn⋅CPF despite the preservation of the ßßα fold, as shown by NMR and CD titrations. Finally, Zn⋅CPF is compared to Zn⋅LTC, a treble clef Zn(Cys)4 zinc finger, to gain further insight into the behavior of zinc fingers toward (1)O2.

Reversible photocapture of a [2]rotaxane harnessing a barbiturate template.

Photoirradiation of a hydrogen-bonded molecular complex comprising acyclic components, namely, a stoppered thread (1) with a central barbiturate motif and an optimized doubly anthracene-terminated acyclic Hamilton-like receptor (2b), leads to an interlocked architecture, which was isolated and fully characterized. The sole isolated interlocked photoproduct (Φ = 0.06) is a [2]rotaxane, with the dimerized anthracenes assuming a head-to-tail geometry, as evidenced by NMR spectroscopy and consistent with molecular modeling (PM6). A different behavior was observed on irradiating homologous molecular complexes 1⊂2a, 1⊂2b, and 1⊂2c, where the spacers of 2a, 2b, and 2c incorporated 3, 6, and 9 methylene units, respectively. While no evidence of interlocked structure formation was observed following irradiation of 1⊂2a, a kinetically labile rotaxane was obtained on irradiating the complex 1⊂2c, and ring slippage was revealed. A more stable [2]rotaxane was formed on irradiating 1⊂2b, whose capture is found to be fully reversible upon heating, thereby resetting the system, with some fatigue (38%) after four irradiation­thermal reversion cycles.

Lanthanide Luminescence Modulation by Cation-π Interaction in a Bioinspired Scaffold: Selective Detection of Copper(I).

A prototype luminescent turn-on probe for Cu(+) (and Ag(+)) is described, harnessing a selective binding site (log Kass = 9.4 and 7.3 for Cu(+) and Ag(+), respectively) based on the coordinating environment of the bacterial metallo-chaperone CusF, integrated with a terbium-ion-signaling moiety. Cation-π interactions were shown to enhance tryptophan triplet population, which subsequently sensitized, on the microsecond timescale, the long-lived terbium emission, offering a novel approach in bioinspired chemosensor design.

Terpy(Pt-salphen)2 switchable luminescent molecular tweezers.

The design and synthesis of switchable molecular tweezers based on a luminescent terpy(Pt-salphen)2 (1; terpy=terpyridine) complex is reported. Upon metal coordination, the tweezers can switch from an open "W"-shaped conformation to a closed "U"-shaped form that is adapted for selective recognition of cations. Closing of the tweezers by metal coordination (M=Zn(2+), Cu(2+), Pb(2+), Fe(2+), Hg(2+)) was monitored by (1)H NMR and/or UV/Vis titrations. During the titration, exclusive formation of the 1:1 complex [M(1)] was observed, without appearance of an intermediate 1:2 complex [M(1)2]. The crystallographic structure of the 1:1 complex was obtained with Pb(2+) and showed a distorted helical structure. Selective intercalation of Hg(2+) cations by the closed "U" form was observed. The tweezers were reopened by selective metal decoordination of the terpyridine ligand by using tris(2-aminoethyl)amine (tren) as a competitive ligand without modification of the Pt-salphen complex. Detailed photophysical studies were performed on the open and closed tweezers. Structured emission was observed in the open form from the Pt-salphen moieties, with a high quantum yield and a long lifetime. The emission is slightly modified upon closing with 1 equivalent of Zn(2+) or Hg(2+), whereas a dramatic quenching was obtained upon intercalation of additional Hg(2+).

Sunlight-driven copper-catalyst activation applied to photolatent click chemistry.

The synthesis, full characterization, photoreduction properties, and catalytic activity for the copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reaction of a copper(II)-DMEDA (N,N'-dimethylethylendiamine) complex is reported. Spectroscopic studies (UV/Vis, EPR) demonstrated that under daylight illumination highly effective copper(II) to copper(I) reduction occurs in this complex. These findings are in agreement with a high photoreduction quantum yield value of 0.22 in MeOH, and a value approaching unity as determined in THF. The reduction process, which can also be conducted by irradiation at 365 nm by using a standard TLC (thin layer chromatography) lamp, is ascribed to a highly efficient photoinduced electron transfer (PET) process mediated by the benzophenone photosensitizer present in the carboxylate counterion. Having deaerated the reaction mixture, the photogenerated copper(I) species proved to be highly active for the CuAAC reaction, demonstrated by reactions conducted with low catalyst loading (0.5 mol %) on a range of clickable protected and non-protected mono- and disaccharides. Once initiated, the reaction can be stopped at any time on introducing air into the reaction medium. Deoxygenation followed by irradiation restores the activity, making the copper(II)-DMEDA complex a switchable catalyst of practical value.