Photophysical and electrochemical properties of π-extended molecular 2,1,3-benzothiadiazoles

The reaction of 4,7-dibromo-2,1,3-benzothiadiazole with arylboronic acids (phenyl, 1-naphthyl, 4-methoxyphenyl, 4-chlorophenyl and 4-trifluoromethylphenyl) in the presence of catalytic amounts of a NCP-pincer palladacycle affords photoluminescent π-extended 4,7-diaryl-2,1,3-benzothiadiazoles 4a-e in high yields. These 4,7-diaryl-2,1,3-benzothiadiazoles exhibit high fluorescent quantum yields, high electron affinities and adequate band gap values for testing as OLEDs. The 4,7-bis-naphthyl-2,1,3-benzothiadiazole 4b presents two different lifetimes (bi-exponential decay) due to the presence of two atropisomers. The Sonogashira coupling reaction of 4,7-diethynyl-2,1,3-benzothiadiazole 6 with the corresponding halo-aryl compounds (iodobenzene, 1-bromonaphthalene, 4-iodoanisole, 4-bromo-N,N-dimethylaniline and 2-bromopyridine) afforded the photoluminescent π-extended 4,7-bis-alkynylaryl-2,1,3-benzothiadiazoles 7a-e, also in high yields. These 4,7-diethynyl-2,1,3-benzothiadiazoles also present high fluorescent quantum yields, high electron affinities and adequate band gap values for testing as OLEDs. The 4,7-disubstituted-2,1,3-benzothiadiazoles 4a-e and 7a-e exhibit different electrochemical behavior. The presence of two ethynyl spacers in 2,1,3-benzothiadiazoles 7a-e shifts the reduction potentials to less cathodic values and also results in two well-defined and distinct reduction processes.     

Z→E Photoisomerization of Homoactivated Carbon Carbon Double Bonds

Z-configured 1,4-diene or β,γ-unsaturated carbonyl systems are readily available by the Wittig reaction. Isomerization required for access into the E-series can easily be accomplished by irradiation using an ordinary tungsten lamp and diphenyl disulphide sensitizer. There is very little formation of conjugated isomers (less than 1%).     

Z → E Photoisomerization of Homoactivated Carbon Carbon Double Bonds

Z-configured 1,4-diene or β, γ-unsaturated carbonyl systems are readily available by the Wittig reaction. Isomerization required for access into the E-series can easily be accomplished by irradiation using an ordinary tungsten lamp and diphenyl disulphide sensitizer. There is very little formatation of conjugated isomers (less than 1%).     

Photocatalyzed E→Z Contra-thermodynamic Isomerization of Vinyl Boronates with Binaphthol

The photocatalytic contra-thermodynamic E→Z isomerization of vinyl boronates by using a binaphthol catalyst is disclosed. The reaction, based on the transient formation of a suitable chromophore with a BINOL derivative as the catalyst, allowed geometrical isomerization in good-to-excellent Z/E ratio and excellent-to-quantitative yields. The mechanism of this E→Z contra-thermodynamic isomerization was studied, and the formation of a transient chromophore species is suggested.     

Ratiometric electrochemical detection of Pd•••π interactions: application towards electrochemical molecular logic gates

Abstract

The widespread and large scale use of platinum group metals, especially palladium, in a wide variety of industrial applications has seen their levels in wastewater streams, roadside dust and even pharmaceuticals significantly rise over recent years. Due to the possible environmental damage and potential health risk this may cause, there is now substantial demand for inexpensive, efficient and robust methods for the detection of palladium. Based upon self-immolative linker technologies, we have designed and synthesised a number of allyl ether-functionalised electrochemical probes to determine the optimum probe structure required to deliver a ratiometric electrochemical detection method capable of achieving a limit of detection of <1 mg/mL within 20 min through the use of disposable screen-printed carbon electrodes. Combined with an enzymatic assay, this method was then used to achieve a proof-of-principle ratiometric electrochemical molecular logic gate.     

Protolytic properties of the structurally rigid analogs of 2,6-distyrylpyridine. Widening the pH sensitivity range by the photochemical E→Z isomerisation and introduction of substituents capable to protolytic interactions

Protolytic interactions in the series of prospective fluorescent ratiometric wide-range pH indicators — structurally rigid analogs of 2,6-distyrylpyridine — (3E,5E)-3,5-dibenzylidene-8-phenyl-1,2,3,5,6,7-hexahydrodicyclopentano[b,e]pyridine — were investigated. The pyridine nitrogen atom basicity in these compounds is significantly lower in comparison with that of unsubstituted heterocycle and 2,6-distyrylpyridine. The photochemical E→Z photoisomerization and the side benzene rings substituents influence on the acid-base equilibria were studied. The complex multi-stage mechanism of the acid-base interactions of the polysubstituted compounds was elucidated. The most significant spectral effects were typical to the N,N-dialkylamino substituted compounds of the investigated series. The widest pH sensitivity interval covering nearly the full range of 0–14 units was demonstrated for compounds with both proton donor and proton acceptor substituents.      

Superior Z→E and E→Z photoswitching dynamics of dihydrodibenzodiazocine, a bridged azobenzene, by S1(nπ*) excitation at λ = 387 and 490 nm

The ultrafast Z→E and E→Z photoisomerisation dynamics of 5,6-dihydrodibenzo[c,g][1,2]diazocine (1), the parent compound of a class of bridged azobenzene-based photochromic molecular switches with a severely constrained eight-membered heterocyclic ring as central unit, have been studied by femtosecond time-resolved spectroscopy in n-hexane as solvent and by quantum chemical calculations. The diazocine contrasts with azobenzene (AB) in that its Z rather than E isomer is the energetically more stable form. Moreover, it stands out compared to AB for the spectrally well separated S(1)(nπ*) absorption bands of its two isomers. The Z isomer absorbs at around λ = 404 nm, the E form has its absorption maximum around λ = 490 nm. The observed transient spectra following S(1)(nπ*) photoexcitation show ultrafast excited-state decays with time constants τ(1) = 70 fs for the Z and <50 fs for the E isomer reflecting very fast departures of the excited wave packets from the S(1) Franck-Condon regions and τ(2) = 270 fs (320 fs) related to the Z→E (resp. E→Z) isomerisations. Slower transient absorption changes on the time scale of τ(3) = 5 ps are due to vibrational cooling of the reaction products. The results show that the unique steric constraints in the diazocine do not hinder, but accelerate the molecular isomerisation dynamics and increase the photoswitching efficiencies, contrary to chemical intuition. The observed isomerisation times and quantum yields are rationalised on the basis of CASPT2//CASSCF calculations by a S(1)/S(0) conical intersection seam at a CNNC dihedral angle of ≈96° involving twisting and torsion of the central CNNC moiety. With improved photochromism, high quantum yields, short reaction times and good photostability, diazocine 1 and its derivatives constitute outstanding candidates for photoswitchable molecular tweezers and other applications.     

Photocatalytic E→Z Contra-Thermodynamic Isomerization of Vinyl Silanes with Lewis Base

Herein, we disclosed the contra-thermodynamic E→Z isomerization of alkenyl silanes, according to the in situ formation of a chromophoric species, in the presence of rac-BINAP as the catalyst. The reaction carried out in DMSO or CH₃CN under irradiation at 405 nm allowed the interconversion of the E-isomers into the Z-congeners in good to excellent yields and outstanding Z/E selectivities, on 18 examples. Finally, the mechanism of this E→Z isomerization was studied to get insight into the reaction mechanism.     

Chiroptical switching of molecular universal joint triggered by complexation/release of a cation: A stepwise synergistic complexation

Pillar[5]arene-based molecular universal joints(MUJs), bearing fused crown ether subring(MUJ1 and MUJ3) or a ring without ether oxygen atom(MUJ2), were synthesized and enantio–differentiated. Significant chiral inversion was observed for the crown ether-fused MUJs upon the addition of equivalent cations Na+, showing an anisotropy(g) factor of 0.014, while alkyl subring-fused MUJ2 showed no CD inversions. Unprecedentedly, sodium ion triggered rolling-in motion of the subring to the pillar[5]arene...     

Synthesis and NMR Characterization of (Z,Z,Z,Z,E,E,ω)-Heptaprenol

We describe a practical, multigram synthesis of (2Z,6Z,10Z,14Z,18E,22E)-3,7,11,15,19,23,27-heptamethyl-2,6,10,14,18,22,26-octacosaheptaen-1-ol [(Z(4),E(2),ω)-heptaprenol, 4] using the nerol-derived sulfone 8 as the key intermediate. Sulfone 8 is prepared by the literature route and is converted in five additional steps (18% yield from 8) to (Z(4),E(2),ω)-heptaprenol 4. The use of Eu(hfc)(3) as an NMR shift reagent not only enabled confirmation of the structure and stereochemistry of 4, but further enabled the structural assignment to a major side product from a failed synthetic connection. The availability by this synthesis of (Z(4),E(2),ω)-heptaprenol 4 in gram quantities will enable preparative access to key reagents for the study of the biosynthesis of the bacterial cell envelope.     

Assessing computationally efficient isomerization dynamics: ΔSCF density-functional theory study of azobenzene molecular switching

We present a detailed comparison of the S0, S1 (n → π?) and S2 (π → π?) potential energy surfaces (PESs) of the prototypical molecular switch azobenzene as obtained by Δ-self-consistent-field (ΔSCF) density-functional theory (DFT), time-dependent DFT (TD-DFT) and approximate coupled cluster singles and doubles (RI-CC2). All three methods unanimously agree in terms of the PES topologies, which are furthermore fully consistent with existing experimental data concerning the photo-isomerization mechanism. In particular, sum-method corrected ΔSCF and TD-DFT yield very similar results for S1 and S2, when based on the same ground-state exchange-correlation (xc) functional. While these techniques yield the correct PES topology already on the level of semi-local xc functionals, reliable absolute excitation energies as compared to RI-CC2 or experiment require an xc treatment on the level of long-range corrected hybrids. Nevertheless, particularly the robustness of ΔSCF with respect to state crossings as well as its numerical efficiency suggest this approach as a promising route to dynamical studies of larger azobenzene-containing systems.     

Singlet 1A″ methylnitrene: A possible intermediate in the photochemical decomposition of methylazide

The interconversion of methylnitrene and methylenimine is studied in the singlet excited state ¹A″ using ab initio methods. At the MP4SDQ/6-31G**//HF/3-21G level plus zero-point energy contributions, the energy barrier for the rearrangement to methylenimine is calculated to be 9.5 kcal/mol. The ¹A″ methylnitrene lies only about 8 kcal/mol above the closed-shell singlet ¹A′ state which is a saddle point on the energy surface. This suggests that the open-shell singlet ¹A″ methylnitrene might constitute a short-lived intermediate during the direct photolytic decomposition of methylazide in the presence of collisions.     

Study of inclusion complex of β-cyclodextrin and diphenylamine: photophysical and electrochemical behaviors

The photophysical, electrochemical and photoprototropic behaviors of diphenylamine (DPA) in aqueous β-cyclodextrin (β-CD) solution have been investigated using absorption spectroscopy and cyclic voltammetric techniques. Absorption of the neutral and cationic form of DPA is enhanced due to the formation of a 1:1 complex with β-CD. The formation of this complex has been confirmed by Benesi-Hildebrand plot and docking studies by RasMol tool methods. The solid complex of β-CD with DPA is investigated by FT-IR, XRD and AFM methods. The thermodynamic parameters (ΔG, ΔH and ΔS) of inclusion process are also determined. The pK(a) values of neutral-monocation equilibria have been determined with absorption (conjugate acid-base) titrations. A mechanism is proposed to explain the inclusion process.     

Inclusion complex of α-cyclodextrin and the extended viologen dication: a model of an insulated molecular wire

An extended viologen dication 1, containing one viologen subunit, was used as a model for the inclusion complex formation between cyclodextrin (CD) molecules and molecular wires comprising several subunits. UV–Vis and fluorescence spectroscopic measurements confirmed the formation of two types of the inclusion complexes 1:1 and 2:1 between αCD and 1 in the aqueous solution containing 20% of ethanol. The complex formation constants were obtained from the fluorescence spectral changes: K _a  = 25 ± 3 mM^?1 for [αCD–1] complex and K _a  = 0.21 ± 0.07 mM^?2 for [(αCD)₂–1] complex, respectively. Cyclodextrins βCD and γCD do not form the inclusion complexes with 1 in these aqueous solutions. The time-dependent differential capacitance measurements confirmed the adsorption of 1 in the form of a complex at the electrode/electrolyte interface. These studies were conducted with the aim to find the most suitable CD cavity that would separate individual molecular wires from each other on the electrode/electrolyte interface.     

A zerovalent nickel-2,2′-bipyridine complex: an efficient catalyst for electrochemical homocoupling of ortho-substituted halides and their heterocoupling with meta- and para-substituted halides

Electroreduction of ortho-substituted halides ArX in a nickel-2,2′-bipyridyl system and N-methylpyrrolidone as solvent gave the homocoupling product ArAr. The nickel-2,2′-bipyridyl system is also a catalyst for the electrochemical heterocoupling of ArX with meta- or para-substituted halides Ar′X′.     

Stochastic kinetic study of protein aggregation and molecular crowding effects of Aβ40 and Aβ42

Two isoforms of β-amyloid peptides, Aβ40 and Aβ42, differ from each other only in the last two amino acids, IA, at the end of Aβ42. They, however, differ significantly in their ability in inducing Alzheimer's disease (AD). The rate curves of fibril growth of Aβ40 and Aβ42 and the effects of molecular crowding have been measured in in vitro experiments. These experimental curves, on the other hand, have been fitted in terms of rate constants for elementary reaction steps using rate equation approaches. Several sets of such rate parameters have been reported in the literature. Employing a recently developed stochastic kinetic method, implemented in a browser-based simulator, popsim, we study to reveal the differences in the kinetic behaviors implied by these sets of rate parameters. In particular, the stochastic method is used to distinguish the kinetic behaviors between Aβ40 and Aβ42 isoforms. As a result, we make general comments on the usefulness of these sets of rate parameters.     

Vibrational analysis of the S—trans conformation of (E,E), (Z,E) and (Z,Z)-2,4-hexadienes in the gaseous and solid states

The infrared (IR) spectra of liquid, gaseous and solid states as well as the liquid and solid phases Raman ones of the pure (Z,Z)-2, 4-hexadiene are recorded for the first time between 3100–50 cm⁻¹. Furthermore, the IR spectra of the gaseous and solid states and the solid Raman ones have been obtained (3100-50 cm⁻¹) for the (E,E) and (Z,E) isomers. A comparison between the present and our previous results for the liquid phase have enabled us to complete and confirm our previous assignment and to conclude that the planar s-trans conformation is prevailing in the three physical states under conditions used in this work.     

A sixfold triplet-sensitized Z/E isomerization of a π-perimeter macrocycle

On irradiation, all-Z [2.2.2.2.2.2]orthoparacyclophene (1) is converted to the all-E isomer (2). The photo-isomerization occurs via the triplet state and proceeds without any groun state intermediates.     

Photochemistry and photophysics of osazones I: photochemical and thermal isomerization of D-“arabino”-hexulose phenylosazone

The photochemical and thermal N-chelate—O-chelate isomerization of D-“arabino”-hexulose phenylosazone (PH) in solution was studied. The rate constants and activation parameters for thermal isomerization were calculated. An obvious wavelength effect on the photoisomerization quantum yield and on the position of the photostationary state was observed. Direct photoisomerization is believed to occur via a common excited singlet state. PH was found to have photochromic properties. The photoisomerization reaction was proved to be a fast efficient method of synthesizing the O-chelate isomer of osazone.     

Hydration of β-cyclodextrin: A molecular dynamics simulation study

We study by molecular dynamics simulations the hydration of -cyclodextrin. Our simulations show that within these barrel-shaped molecules hydrophobicity dominates, while at the top and bottom sides of the barrel interactions with water are mostly hydrophilic in nature. These results agree with crystallographic data at 120 K and, in particular, with the spontaneous hydration process of a cyclodextrin crystal in wet atmosphere. The predicted structure of the hydration shells is discussed and compared with previous molecular mechanics calculations which report an overall hydrophobic behavior. Moreover, the temperature dependence of the hydration process is discussed.