A π-Extended Phenanthrene-Fused Aza[7]helicenium as Novel Chiroptically-Active Architecture in Organic and Aqueous Media

The synthesis and characterization of an original π-extended cationic azahelicene is reported. The phenanthrene-fused aza[7]helicene derivative encompasses a total of ten aromatic fused rings leading to a dissymmetric yet helically...

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 K_Cu⁺ = 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.

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 (K_1:1 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 k_obs =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.

Bioinspired Luminescent Europium-Based Probe Capable of Discrimination between Ag+ and Cu. Inorg Chem 2021;60:10791-10798. [PMID: 34236828 DOI: 10.1021/acs.inorgchem.1c01486]

Due to their similar coordination properties, discrimination of Cu⁺ and Ag⁺ by water-soluble luminescent probes is challenging. We have synthesized LCC4^Eu, 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, LCC4^Eu 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, LCC4^Eu 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 LCC4^Eu toward Ag⁺ and Cu⁺.

Damming an electronic energy reservoir: ion-regulated electronic energy shuttling in a [2]rotaxane

We demonstrate the first example of bidirectional reversible electronic energy transfer (REET) between the mechanically bonded components of a rotaxane. Our prototypical system was designed such that photoexcitation of a chromophore in the axle results in temporary storage of electronic energy in a quasi-isoenergetic “reservoir” chromophore in the macrocycle. Over time, the emissive state of the axle is repopulated from this reservoir, resulting in long-lived, delayed luminescence. Importantly, we show that cation binding in the cavity formed by the mechanical bond perturbs the axle chromophore energy levels, modulating the REET process, and ultimately providing a luminescence read-out of cation binding. Modulation of REET processes represents an unexplored mechanism in luminescent molecular sensor development.

Delayed emission due to reversible electronic energy transfer (REET) between chromophores in the axle and macrocycle components of a rotaxane is demonstrated. The REET process can be modulated by metal ion binding in the cavity of the rotaxane.

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.

A fluorous sodium l-prolinate derivative as low molecular weight gelator for perfluorocarbons

We report the first study dealing with the self-assembly of an α-amino acid derivative in perfluorocarbons. Rheology, microscopy, and spectroscopy studies reveal that the fluorous sodium l-prolinate derivative 1 self-assembles in perfluorocarbons to form a three-dimensional network of left-handed nano-helices resulting in solvent gelation. Singlet oxygen lifetime measured in a gel of perfluorodecalin is about 1000 times longer than in pure water.

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.

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.

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.

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.

2D and 3D surface photopatterning via laser-promoted homopolymerization of a perfluorophenyl azide-substituted BODIPY

An innovative photopatterning process is described that allows, in a single laser-promoted operation, the covalent attachment of a molecule on a surface (2D patterning - xy dimensions) and its photopolymerization to grow micro-/nanostructures with spatial control in a third z-dimension. The surface patterning process, based on nitrene reactivity, was harnessed using the highly fluorescent azide-substituted boron difluoride dipyrromethene (BODIPY) 1 that was prepared in a single synthetic step from the parent pentafluorophenyl BODIPY on reacting with NaN₃. Using the laser of a fluorescence microscope (375 nm or 532 nm) 1 could be grafted on adapted surfaces and then homopolymerised. In this study we show that using glass coverslips coated with PEG/high density alkyne groups (density of ∼1 × 10¹⁴ per cm²), the patterning process was much more spatially confined than when using PEG only coating. Varying the irradiation time (1 to 15 s) or laser power (0.14-3.53 μW) allowed variation of the amount of deposited BODIPY to afford, in the extreme case, pillars of a height up to 800 nm. AFM and MS studies revealed that the nano/microstructures were formed of particles of photopolymerized 1 having a mean diameter of ca. 30 nm. The emission spectra and fluorescence lifetimes for the patterned structures were measured, revealing a red-shift (from ∼560 nm up to 620 nm) of the maximum emission and a shortening (from ∼6 ns to 0.8 ns) of the fluorescence lifetimes in areas where the density of BODIPY is high. As an application of the patterning process, a figure formed of 136 dots/pillars was prepared. The confocal hyperspectral fluorescence image revealed that the figure is clearly resolved and constituted by highly photoluminescent red dots whose fluorescence intensities and emission color proved to be highly reproducible. SEM and AFM studies showed that the luminescent dots were pillars with a conical shape, an average height of 710 ± 28 nm and a FWHM of 400 ± 20 nm.

Photosensitizer localization in amphiphilic block copolymers controls photodynamic therapy efficacy

Localization of the photosensitizer conjugation site in amphiphilic block copolymers is shown to have a great impact on photodynamic therapy efficiency. To this end, an asymmetric multifunctional derivative of the azadipyrromethene boron difluoride chelate (aza-BODIPY) was synthesized and inserted at specific locations in polypeptide-based rod-coil amphiphilic block copolymers. A study of the photophysical properties of the vesicle nanocarriers, obtained by self-assembly of these copolymers, as well as in vitro tests on two cancer cell lines were performed. This study aims at providing guidelines for the optimization of the synthetic design of therapeutic nanomedicines with minimal amounts of photosensitive molecules.

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.

A blue 4',7-diaminoflavylium cation showing an extended pH range stability

The introduction of two amine substituents in 4' and 7 positions, leads to the formation of a blue flavylium cation, 7-(N,N'-diethylamino)-2-(9-julolidine)-1-benzopyrilium, which is extremely stable across a wide acidic pH range. The kinetic and thermodynamic constants of the multistate system have been calculated by studying the relaxation kinetics after equilibrium perturbation by addition of base (direct pH jumps) or acid (reverse pH jumps). Except for the cis-chalcone, which is an elusive species, the relative energy levels of the other species could be calculated and a global energy level diagram constructed. The diagram explains that the stability of the diamino compound is due to the high energy level of the hemiketal species, which is difficult to access in acidic medium.

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

Reversible electronic energy transfer is used for sensing oxygen traces and results in very high sensitivity.

Water-soluble naphthalimide-based ‘Pourbaix sensors’: pH and redox-activated fluorescent AND logic gates based on photoinduced electron transfer

Naphthalimide-based ‘Pourbaix sensors’ for redox potential and pH fluoresce with a lifetime of 8.5 ns while photoinduced electron transfer occurs on a time scale of 20 ps.

Switchable platinum-based tweezers with Pt-Pt bonding and selective luminescence quenching

Molecular tweezers incorporating peripheral platinum salphen complexes and a central chelating terpyridine group have been synthesized. The terpyridine can be switched upon metal binding between a free 'W' shaped form and a coordinated 'U' form. The crystallographic structure of the zinc-closed molecular tweezers was obtained and presented a strong π-stacking between the Pt-salphen units associated with a Pt-Pt bond. The luminescence properties, notably in response to selected guest ions (Zn(2+), Pb(2+), Hg(2+)) and the resulting mechanical motion, have been investigated by UV-Vis and emission spectroscopy. While ion coordination to the terpy resulted in no significant changes in the luminescence, a selective intercalation of a second Hg(2+) associated with a large differential quenching was observed.

Photodriven [2]rotaxane–[2]catenane interconversion

The reversible photocyclomerization of terminal anthracene units enables the interconversion between [2]rotaxane and [2]catenane molecular topologies.

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+).

Direct observation of reversible electronic energy transfer involving an iridium center

A cyclometalated iridium complex is reported where the core complex comprises naphthylpyridine as the main ligand and the ancillary 2,2'-bipyridine ligand is attached to a pyrene unit by a short alkyl bridge. To obtain the complex with satisfactory purity, it was necessary to modify the standard synthesis (direct reaction of the ancillary ligand with the chloro-bridged iridium dimer) to a method harnessing an intermediate tetramethylheptanolate-based complex, which was subjected to acid-promoted removal of the ancillary ligand and subsequent complexation. The photophysical behavior of the bichromophoric complex and a model complex without the pendant pyrene were studied using steady-state and time-resolved spectroscopies. Reversible electronic energy transfer (REET) is demonstrated, uniquely with an emissive cyclometalated iridium center and an adjacent organic chromophore. After excited-state equilibration is established (5 ns) as a result of REET, extremely long luminescence lifetimes of up to 225 μs result, compared to 8.3 μs for the model complex, without diminishing the emission quantum yield. As a result, remarkably high oxygen sensitivity is observed in both solution and polymeric matrices.