Unprotected Galactosamine as a Dynamic Key for a Cyclochiral Lock.

The discrimination of d-galactosamine (G), representative of the amino-sugar class of compounds, has been probed through nano-ESI-FT-ICR mass spectrometry by isolating the relevant [C·H·G]+ proton-bound complexes with the enantiomers of the cyclochiral resorcin[4]arene C and allowing them to react toward three primary amines (B = EtNH2, iPrNH2, and (R)- and (S)-sBuNH2). The system under investigation presents several features that help to unveil the behavior of unprotected G in such a supramolecular architecture: (i) the hydrophobic derivatization of the C convex side forces the polar guest G to be coordinated by the cyclochiral concave region; (ii) protonated d-galactosamine exists as an anomeric mixture, dynamically interconverting throughout the experimental time-window; and (iii) different basicities of B allow the experiment to subtly tune the reactivity of the [C·H·G]+ complexes. Three [C·H·G]+ aggregate-types were found to exist, differing in both their origin and reactivity. The most reactive adducts ([C·H·G]ESI+), generated in the electrospray environment, undergo a G-to-B ligand exchange in competition with a partial isomerization to the unreactive [C·H·G]GAS+-type complexes. Finally, the poorly reactive [C·H·G]SOL+ aggregates are formed in solution over an hours-long time scale. A cyclochirality effect on the reactivity was found to depend on the considered [C·H·G]+ aggregate-type.

Reversible photoswitching of the DNA-binding properties of styrylquinolizinium derivatives through photochromic [2 + 2] cycloaddition and cycloreversion.

It was demonstrated that styrylquinolizinium derivatives may be applied as photoswitchable DNA ligands. At lower ligand:DNA ratios (≤1.5), these compounds bind to duplex DNA by intercalation, with binding constants ranging from K b = 4.1 × 104 M to 2.6 × 105 M (four examples), as shown by photometric and fluorimetric titrations as well as by CD and LD spectroscopic analyses. Upon irradiation at 450 nm, the methoxy-substituted styrylquinolizinium derivatives form the corresponding syn head-to-tail cyclobutanes in a selective [2 + 2] photocycloaddition, as revealed by X-ray diffraction analysis of the reaction products. These photodimers bind to DNA only weakly by outside-edge association, but they release the intercalating monomers upon irradiation at 315 nm in the presence of DNA. As a result, it is possible to switch between these two ligands and likewise between two different binding modes by irradiation with different excitation wavelengths.

Photocontrolled DNA minor groove interactions of imidazole/pyrrole polyamides.

Azobenzenes are photoswitchable molecules capable of generating significant structural changes upon E-to-Z photoisomerization in peptides or small molecules, thereby controlling geometry and functionality. E-to-Z photoisomerization usually is achieved upon irradiation at 350 nm (π-π* transition), while the Z-to-E isomerization proceeds photochemically upon irradiation at >400 nm (n-π* transition) or thermally. Photoswitchable compounds have frequently been employed as modules, e.g., to control protein-DNA interactions. However, their use in conjunction with minor groove-binding imidazole/pyrrole (Im/Py) polyamides is yet unprecedented. Dervan-type Im/Py polyamides were equipped with an azobenzene unit, i.e., 3-(3-(aminomethyl)phenyl)azophenylacetic acid, as the linker between two Im/Py polyamide strands. Only the (Z)-azobenzene-containing polyamides bound to the minor groove of double-stranded DNA hairpins. Photoisomerization was exemplarily evaluated by 1H NMR experiments, while minor groove binding of the (Z)-azobenzene derivatives was proven by CD titration experiments. The resulting induced circular dichroism (ICD) bands of the bound ligands, together with the photometric determination of the dsDNA melting temperature, revealed a significant stabilization of the DNA upon association with the ligand. The (Z)-azobenzene acted as a building block inducing a reverse turn, which favored hydrogen bonds between the pyrrole/imidazole amide and the DNA bases. In contrast, the E-configured polyamides did not induce any ICD characteristic for minor groove binding. The incorporation of the photoswitchable azobenzene unit is a promising strategy to obtain photoswitchable Im/Py hairpin polyamides capable of interacting with the dsDNA minor groove only in the Z-configuration.

Exploiting Fast Exciton Diffusion in Dye-Doped Polymer Nanoparticles to Engineer Efficient Photoswitching.

Photoswitching of bright fluorescent nanoparticles opens new possibilities for bioimaging with superior temporal and spatial resolution. However, efficient photoswitching of nanoparticles is hard to achieve using Förster resonance energy transfer (FRET) to a photochromic dye, because the particle size is usually larger than the Förster radius. Here, we propose to exploit the exciton diffusion within the FRET donor dyes to boost photoswitching efficiency in dye-doped polymer nanoparticles. To this end, we utilized bulky hydrophobic counterions that prevent self-quenching and favor communication of octadecyl rhodamine B dyes inside a polymer matrix of poly(D,L-lactide-co-glycolide). Among tested counterions, only perfluorinated tetraphenylborate that favors the exciton diffusion enables high photoswitching efficiency (on/off ratio ∼20). The switching improves with donor dye loading and requires only 0.1-0.3 wt % of a diphenylethene photochromic dye. Our nanoparticles were validated both in solution and at the single-particle level. The proposed concept paves the way to new efficient photoswitchable nanomaterials.

Ultrafast Z → E photoisomerisation of structurally modified furylfulgides.

Femtosecond broadband transient absorption spectroscopy has been used in a comparative study of the ultrafast photo-induced Z → E isomerisation reactions of four photochromic furylfulgides with selected structural motifs in n-hexane as solvent. The results show that all studied Z-fulgides exhibit fast and direct processes along barrierless excited-state pathways involving a conical intersection (CI) between the S1 and S0 electronic states. The excited-state lifetimes range from τ1 = 0.18 ps for the methyl derivative to τ1 = 0.32 ps for the benzofurylfulgide. The impulsive rise of the absorption by vibrationally hot Z- and E-isomers back in the electronic ground state following electronic deactivation and isomerisation indicates that the initially prepared wave packet persists even after passage of the CI. Furthermore, the results provide qualitative evidence for a quickly dephasing vibrational coherence in the electronic ground state. In contrast to the significant changes observed for the corresponding E- and C-isomers [Renth et al., Int. Rev. Phys. Chem., 2013, 32, 1-38], the excited-state dynamics of the Z-isomers is not affected by varied sterical hindrance from methyl and isopropyl substituents at the central hexatriene unit, or by intramolecular bridging, and remains unaltered upon extension of the π-electron system in a benzannulated furyl fulgide.

A study of acridine and acridinium-substituted bis(terpyridine)zinc(II) and ruthenium(II) complexes as photosensitizers for O2 (1Δg) generation.

The homoleptic zinc(II) and ruthenium(II) metal complexes of bis(tridentate) 9-acridine and 10-methyl-9-acridinium-substituted terpyridines were tested for their suitability as triplet photosensitizers (PS) using the photooxidation of 1,5-dihydroxynaphthalene (DHN) to Juglone as a model reaction. Singlet oxygen (O2(1)Δg) generation is superior or comparable to Ru(bpy)3(2+) for the acridine complexes, whereas the acridinium complexes are ineffective. The molecular structure of the bis(9-(5-([2,2':6',2''-terpyridin]-4'-yl)thien-2-yl)-10-methylacridinium)zinc(II) complex ([Zn(MeATT)2][PF6]4) is determined by X-ray structure analysis, whereas for other complexes DFT calculations were performed for structural parameters to obtain insights into their electronic properties.

Photoswitchable DNA-binding properties of a photochromic spirooxazine derivative.

An N-methylphenanthrolinium-annelated spirooxazine derivative 2(SO) whose DNA-intercalating properties are reversibly changed by a photochromic reaction was prepared. Upon irradiation at 350 nm the spirooxazine is transformed to the corresponding photomerocyanine 2(PM) that binds to DNA. After irradiation with visible light the spirooxazine 2(SO), which exhibits no significant DNA-binding properties, is regained. The association of 2(PM) with DNA was examined by CD and absorption spectroscopy, fluorescent intercalator displacement and viscometric titration.

Enantioselective supramolecular devices in the gas phase. Resorcin[4]arene as a model system.

This review describes the state-of-art in the field of the gas-phase reactivity of diastereomeric complexes formed between a chiral artificial receptor and a biologically active molecule. The presented experimental approach is a ligand-displacement reaction carried out in a nano ESI-FT-ICR instrument, supported by a thermodynamic MS-study and molecular-mechanics and molecular-dynamics (MM/MD) computational techniques. The noncovalent ion-molecule complexes are ideal for the study of chiral recognition in the absence of complicating solvent and counterion effects.

Cyclochiral resorcin[4]arenes as effective enantioselectors in the gas phase.

The effect of cyclochirality of rccc-2,8,14,20-tetra-n-decyl-4,10,16,22-tetra-O-methylresorcin[4]arene (C) on the enantiodiscrimination of a number of chiral bidentate and tridentate aromatic and aliphatic biomolecules (G) has been investigated by nano-electrospray ionization (nano-ESI)-Fourier transform ion cyclotron resonance mass spectrometry. The experimental approach is based on the formation of diastereomeric proton-bound [C·H·G](+) complexes by nano-ESI of solutions containing an equimolar amount of quasi-enantiomers (C) together with the chiral guest (G) and the subsequent measurement of the rate of the G substitution by the attack of several achiral and chiral amines. In general, the heterochiral complexes react faster than the homochiral ones, except when G is an aminoalcoholic neurotransmitter whose complexes, beyond that, exhibit the highest enantioselectivity. The kinetic results were further supported by both collision-induced dissociation experiments on some of the relevant [C(2) ·H·G](+) three-body species and Density functional theory (DFT) calculations performed on the most selective systems.

Synthesis and characterization of photoswitchable fluorescent SiO2 nanoparticles.

Switchable fluorescent silica nanoparticles have been prepared by covalently incorporating a fluorophore and a photochromic compound inside the particle core. The fluorescence can be switched reversibly between an on- and off-state via energy transfer. The particles were synthesized using different amounts of the photoswitchable compound (spiropyran) and the fluorophore (rhodamine B) in a size distribution between 98 and 140 nm and were characterized in terms of size, switching properties, and fluorescence efficiency by TEM, and UV\Vis and fluorescence spectroscopy.

Molecular capsules derived from resorcin[4]arenes by metal-coordination.

A short introduction to the fundamental features and recent developments of supramolecular chemistry is presented besides defining scope and limitation of this review article. A brief overview about calix[n]arenes and especially resorcin[4]arenes and their conformationally rigid cavitands is given. Selected examples are presented to demonstrate the dependence of self-assembly of cavitands exhibiting different flexibility either due to their basic macrocycle or due to flexible receptor units commonly located at the o,o'-position of the resorcinarene ring. In addition, the process of self-assembly is also controlled by metal coordination geometry as shown by one example. The receptor units may also be connected at the methylene group of the cavitand as shown by one example. Examples of supramolecular architectures are presented utilizing the special features of 2,2':6',2″-terpyridine (terpy) metal-binding ligand. The synthesis and characterization of a metallo-supramolecular Zn-coordination cage with a diameter of 4-5 nm based on a cavitand-terpy building block is presented in detail.

Functional characterization of a supramolecular affinity switch at the single molecule level.

Surface-immobilized and switchable resorcin[4]arene receptor molecules were quantitatively investigated with atomic force microscopy (AFM) and AFM-single molecule force spectroscopy (AFM-SMFS). The upper rim of the supramolecular receptor cavities was modified with two photodimerizable anthracene moieties. The molecular constitution can be externally switched and controlled by exposure to ultraviolet (UV) light and heat. The topography as well as the complexation affinity against small ammonium guest ions of the two isomers were investigated at the single molecule level. Our results demonstrate the feasibility to externally control the supramolecular receptor's affinity and simultaneously quantify and associate these binding properties with the structural change of the resorcin[4]arenes structure on the basis of the measured molecule corrugation height.

Anthracene-resorcin[4]arene-based capsules: synthesis and photoswitchable features.

Herein we present the synthesis and the photochemical behavior of several new hemicarcerands containing anthracene units as photoactive species. By means of NMR investigations of compounds 9 and 11 the dimerization mode was revealed as a 9,10-9',10'-dimerization, classically known from anthracene. Nevertheless only compound 11 could be converted to the opened form upon irradiation with 300 nm. Reopening of compounds 9 and 10 could not be achieved so far either by heating or by irradiation.

Reversible photoswitching of dye-doped core-shell nanoparticles.

We present a simple and versatile mechanism for the reversible photoswitching of dye-doped core-shell nanoparticles. Photochromic dithienylethenes are incorporated into the outer shell, close enough to the dyes entrapped in the core to efficiently quench them by energy transfer when photoconverted with UV light. The emission can be switched back on by irradiation with λ > 450 nm.

Electronic and steric effects on the photo-induced C→E ring-opening of structurally modified furylfulgides.

The ultrafast C→E ring-opening reactions of four selectively modified furylfulgides have been studied by means of ultrafast broadband transient absorption spectroscopy after femtosecond laser excitation at λ = 500 nm. A large difference in the dynamics was found in the case of benzannulation at the furyl moiety as an example for an electronic effect by extension of the conjugated π-electron system compared to furylfulgides carrying sterically different alkyl substituents at the central cyclohexadiene (CHD) ring. The measured very similar spectro-temporal absorption maps for the furylfulgides with a methyl or isopropyl group at the CHD ring or an intramolecular alkyl bridge from the CHD to the furyl moiety showed two distinctive excited-state absorptions with slightly different decay times. The first time constant (τ(1) = 0.39-0.57 ps) was assigned to the rapid departure of the excited wavepacket from the Franck-Condon region. The slightly longer second decay time of τ(2) = 0.66-0.92 ps, depending on the compound, was attributed to the electronic deactivation and ring-opening through a conical intersection to the S(0) state. In contrast, the benzannulation at the furyl moiety was found to lead to a bi-phasic excited-state decay with τ(2) = 4.7 ps and a much slower additional contribution of τ(3) = 17.4 ps, ≈25 times longer compared to the normal furylfulgides. The drastic change is attributed to a trapping of excited molecules in a local potential energy minimum en route to the conical intersection.

Kinetic studies on visible-light-switchable photochromic fluorophores based on diarylethenes.

We present three recently developed photochromic fluorophores that are based on diarylethenes with elongated conjugated π-systems. The diarylethenes 1 and 3 can be switched from their open to their closed form with visible light. The diarylethenes 1 and 2 are covalently coupled to a standard rhodamine B-based fluorophore and act as photoswitchable resonance energy acceptors. By controlling their switching state, the fluorescence intensity of the dye can be modulated. The third compound 3 is a diarylethene that shows photoswitchable inherent fluorescence due to its stilbazolium-like structure. Ensemble experiments demonstrate that diarylethene-based photoswitches show superior characteristics regarding their switching performance, thermal stability and fatigue resistance. These attributes make them promising candidates for super-resolution imaging methods that are based on the determinate fluorescence switching of fluorophores between an off- and an on-state.

Inherently chiral resorcin[4]arenes: a new concept for improving the functionality.

A new reactive postion at the upper rim of inherently chiral resorcin[4]arenes was introduced through cleavage of an up to now unreactive methoxy group through the demethylating reagent 9-I-9-BBN. Conservation of the inherent chirality was warranted through the use of a protecting group at the free phenol group.

Unprecedented gas-phase chiroselective logic gates.

The gas-phase encounters between 2-aminobutane and proton-bound chiral resorcin[4]arene/nucleoside complexes behave in the gas phase as supramolecular "chiroselective logic gates" by releasing the nucleoside depending on the resorcin[4]arene and the 2-aminobutane configurations.

Tuning of switching properties and excited-state dynamics of fulgides by structural modifications.

The ultrafast photo-induced dynamics of the E-isomers of four selected photochromic fulgides with distinct structural motifs have been elucidated by femtosecond broadband transient absorption spectroscopy in n-hexane as solvent. E→C and E→Z isomerisations, respectively, with time constants of ∼0.12 ± 0.02 ps and ∼0.34 ± 0.03 ps taking place in parallel were found for derivatives with a methyl substituent at the central hexatriene (HT) unit. In contrast, fulgides with increased steric constraints by an iso-propyl substituent or by intramolecular bridging displayed virtually zero E→Z isomerisation, but instead a desired accelerated and more efficient ring closure in a reaction time of only ∼50 ± 10 fs. Both photoisomerisations appear to follow excited-state pathways with distinctive conical intersections. For the ring closure, direct barrierless pathways with steep downhill gradients are likely. Furthermore, the results indicate conformer-specific reactions, with ring closure exclusively by the E(α) conformer and E→Z isomerisation predominantly by the E(ß) conformer, because the E(α)→Z channel is unfavoured by the faster and kinetically more competitive E(α)→C reaction. DFT calculations of the equilibrium structures showed that the sterically demanding groups at the HT unit shift the conformer equilibria towards the E(α) conformers. At the same time, they appear to cause a favourable pre-orientation of the furyl unit that accelerates the conrotatory ring closure in the E(α)→C reaction. Benzo-annulation of the furyl unit has little effect on the observed dynamics. Overall, the results demonstrate how the excited-state dynamics and thereby the photoswitching properties of fulgides can be successfully tuned and improved by structural modifications at the chromophores.