Open Isomer Research Articles

Stable and Unstable 6,6‐ and 5,6‐ Closed and Open Adducts of Fullerene C60

DFT/PBE/TZ2P research of total energy of various adducts at variation of 5,6‐ and 6,6‐ bond was carried out. On the basis of the analysis of local minima, the conclusion on structures stability and instability was drawn. Acyclic and cyclic derivatives containing 4‐6‐membered rings are established to form the closed adducts only. Thus the [6,6]‐closed structures are 15–18 kcal/mole more stable than [5,6]‐closed. The [6,6]‐closed adducts with strained 3‐membered cycles and their [5,6]‐open isomers are found to be stable. The analysis of strain energy is accomplished.

Properties of Stable Organic Bond-Stretched Non-Lewis Molecules

The conditions required for the existence of a stable bond-stretched singlet isomer of hetero derivatives of bicyclo[2.1.0]pentane (which is a cyclopentane-1,3-diyl derivative) are discussed. Such species are non-Lewis systems with a ruptured C-C bond (formally diradicals), in which two electrons occupy the nonbonding orbital. A high-level calculation shows that in contrast with the carbon substituted compounds, in which the open form is a transition state between two classical-bonded closed bicyclic forms, in the heterosubstituted molecules, the open form is calculated to be a stable minimum. The ionization potentials of the open forms are considerably lower than those of their bicyclic isomers and also of regular organic radicals/diradicals. Nitrogen atoms are found to be more effective than oxygen or sulfur in stabilizing the open isomer. In this case, the open isomer is calculated to be a little more stable than the bicyclic compound, and a barrier of approximately 40 kcal/mol is computed for the ring closing reaction. Thus, the open isomer is both thermodynamically and kinetically stable. This result rationalizes some experimental observations that indicated the existence of non-Lewis singlet species.

Switchable Fluorescent and Solvatochromic Molecular Probes Based on 4‐Amino‐N‐methylphthalimide and a Photochromic Diarylethene (Eur. J. Org. Chem. 15/2008)

AbstractThe cover picture is misleading! the light‐induced changes of a newly designed photochromic, fluorescent, and solvatochromic compound. The open isomer (top) is colorless and fluorescent. Its emission changes from blue to yellow‐green with increasing polarity of the solvent (in the picture: cyclohexane, tetrachloromethane, benzene, toluene, xylene, and dioxane, from left to right). The closed isomer (bottom) is blue and nonemissive (for clarity, the picture was taken with some amount of the open isomer remaining). UV and visible (green) light interconverts both isomers. As a result, the dye provides multiple readout signals: color changes, on/off switching of the fluorescence signal, and the color of the emission that can be used to sense changes in the polarity of the microenvironment. Detailed synthesis and properties are reported in the article by M. L. Bossi et al. on p. 2531 f. Irene B?ttcher‐Gajewski (MPI for Biophysical Chemistry) is acknowledged for the photographical content of the cover picture.

Discrimination in Metal‐Ion Binding to RNA Dinucleotides with a Non‐Bridging Oxygen or Sulfur in the Phosphate Diester Link

Replacement of a non-bridging oxygen in the phosphate diester bond by a sulfur has become quite popular in nucleic acid research and is often used as a probe, for example, in ribozymes, where the normally essential Mg(2+) is partly replaced by a thiophilic metal ion to reactivate the system. Despite these widely applied rescue experiments no detailed studies exist quantifying the affinity of metal ions to such terminal sulfur atoms. Therefore, we performed potentiometric pH titrations to determine the binding properties of pUp((S))U(3-) towards Mg(2+), Mn(2+), Zn(2+), Cd(2+), and Pb(2+), and compared these data with those previously obtained for the corresponding pUpU(3-) complexes. The primary binding site in both dinucleotides is the terminal phosphate group. Theoretically, also the formation of 10-membered chelates involving the terminal oxygen or sulfur atoms of the (thio)phosphate bridge is possible with both ligands. The results show that Mg(2+) and Mn(2+) exist as open (op) isomers binding to both dinucleotides only at the terminal phosphate group. Whereas Cd(pUpU)(-) only exists as Cd(pUpU)(-)(op), Cd(pUp((S))U)(-) is present to about 64 % as the S-coordinated macrochelate, Cd(pUp((S))U)(-)(cl/PS). Zn(2+) forms with pUp((S))U(3-) three isomeric species, that is, Zn(pUp((S))U)(-)(op), Zn(pUp((S))U)(-)(cl/PO), and Zn(pUp((S))U)(-)(cl/PS), which occur to about 33, 12 (O-bound), and 55 %, respectively. Pb(2+) forms the 10-membered chelate with both nucleotides involving only the terminal oxygen atoms of the (thio)phosphate bridge, that is, no indication of S binding was discovered in this case. Hence, Zn(2+) and Cd(2+) show pronounced thiophilic properties, whereas Mg(2+), Mn(2+), and Pb(2+) coordinate to the oxygen, macrochelate formation being of relevance with Pb(2+) only.

Proton-acceptor properties of azahomofullerene and fullerenoaziridine containing a cyanuric acid fragment

13C and 1H NMR and UV spectral studies on 1,3-diallyl-5-[4-(azahomo[60]fullereno)butyl]hexahydro-1,3,5-triazine-2,4,6-trione ([5,6]-open isomer) and 1,3-diallyl-5-[4-(aziridino[60]fullereno)butyl]hexahydro-1,3,5-triazine-2,4,6-trione ([6,6]-closed isomer), as well as quantum-chemical calculations, showed that only the latter possesses weakly basic properties.

Unexpected Carbon–Carbon Bond Cleavage in the Reactions of the Pentagonal Bipyramidal Cluster Fe3(CO)6(μ-CO)2[μ3-η4-{RCC(Et)C(Et)CR}] with Styrene. Synthesis and X-ray Structure of Fe3(CO)6[μ3-η4-{EtC2CR}]2 (R = CH3(=CH2))

The thermal reactions of 2-methyl-1-hexen-3-yne [CH3CH2C≡CC(=CH2)CH3, metey] with Fe3(CO)12 have been studied: cluster opening or fragmentation and alkyne dimerization occur. Main products are the open triangular isomers [Fe3(CO)6(μ-CO)2{CH3(=CH2)CC(Et)C(Et)C(=CH2)CH3}] (complex 3a) and [Fe3(CO)6(μ-CO)2{C(Et)CCH3(=CH2)C(Et)CCH3(=CH2)}] (complex 3b). The structure and isomerism of the complexes has been confirmed by X-ray studies. The minor products of the reaction have been characterized by spectroscopic techniques. An attempt at exploiting the reactivity of the “free” C=C bonds of the coordinated ene-yne was made: complex 3a was reacted with styrene under thermal conditions. Unexpectedly considerable yields of the closed triangular cluster [Fe3(CO)6{EtC2C(=CH2)CH3}2] (complex 5) have been obtained. This behaviour had not been previously observed. The unprecedented structure of complex 5 has been confirmed with an X-ray study.

The Effect of Addition of Fluorescent Moieties to Dihydropyrenes: Enhancing Photochromicity and Fluorescence Monitoring

A series of dihydropyrenes with appending fluorescent moieties were synthesized with the objective of increasing the photochromic efficiency for this class of compounds and to establish how suitable fluorescence would be to follow their photochromic behavior. The ring opening quantum yields of dihydropyrenes with aroyl substituents at the 4-position showed increased ring opening quantum yields without a decrease of the half-life for the thermal reversion of the less stable open isomer, the metacyclophanediene to the dihydropyrene. The fluorescence of the appending naphthoyl or pyrenoyl moieties was not suitable to follow the photochromic cycling of the dihydropyrenes. However, the emission detected above 600 nm of the closed isomer of the dihydropyrene moiety was shown to be a good monitoring method for the photochromic cycling.

Distinguishing Different Isomers of the Photochromic CMTE Molecule on the Si(100) Surface Studied by STM

The adsorption of the cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (CMTE) molecule on the clean Si(100)-2 × 1 surface was studied using scanning tunneling microscopy at room temperature. These molecules belong to the class of photochromic dye molecules that undergo reversible photoisomerization between the open and closed isomers when irradiated with light. This is the first observation of individual photochromic molecules on a surface where it is possible to identify the two photoisomers. By changing the ambient lighting conditions prior to deposition, the relative proportion of the two forms of molecules observed on the silicon surface can be changed. The deduction is that the triangular form corresponds to the closed isomer, and the three-lobed form corresponds to the open isomer.

Ring Closure Reaction Dynamics of Diarylethene Derivatives in Solution

Photochromic ring closure reaction dynamics of 1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluoro cyclopentene and its derivatives in solution has been studied by femtosecond time-resolved fluorescence. Time-resolved spontaneous fluorescence of the open isomer reveals a fast component of around 1 ps and a slow component on the order of 100 ps. Fluorescence time profiles, reaction quantum yields, and relative populations of the parallel (C(s) symmetry) and antiparallel (C(2) symmetry) conformations indicate that both time components are attributable mostly to the C(2) conformer that undergoes the ring closure reaction. The fast component is assigned to the direct ring closure reaction, and the slow component is assigned to the reaction through conformation change. Time constants of the slow component for the derivatives are inversely proportional to the reaction quantum yields, suggesting that the rate of the conformational dynamics is comparable to the rate of other population relaxation processes. The relative amplitude and exact time constant of the fast component depend on the detection wavelength displaying a higher relative amplitude with shorter time constant at longer wavelengths. The results allow us to propose a conformational inhomogeneity model, in which a broad distribution of conformations of the open isomers in the ground state is projected into two minima in the excited electronic potential surface to lead to the slow and the fast reaction pathways.

The Protonation of Allene and Some Heteroallenes, a Computational Study

Protonation of allene and seven heteroallenes, X = Y = Z, at the terminal and central positions has been studied computationally at the MP2/6-311+G**, B3LYP/6-31+G**, and G3 levels. In all but one case protonation at a terminal position is preferred thermodynamically. The exception is allene, where protonation at C2 giving allyl cation prevails by about 10 kcal/mol over end-protonation, which gives the 2-propenyl cation. In the heteroallenes, protonation at a terminal carbon is strongly favored, activated by electron donation from the other terminal atom. Transition states for identity proton-transfer reactions were found for 10 of the "end-to-end" proton transfers. When the transfer termini are heteroatoms these processes are barrier free. We found no first-order saddle point structures for "center-to-center" proton transfers. An estimate of DeltaH++ for an identity center-to-center proton transfer could be made only for the reaction between the allyl cation and allene; it is approximately 22 kcal/mol higher than DeltaH++ for the end-to-end proton transfer between the 2-propenyl cation and allene. First-order saddle points for the proton transfer from H3S+ to both C1 and C2 of allene were found. The difference in activation enthalpies is 9.9 kcal/mol favoring protonation at C1 in spite of the thermodynamic disadvantage. We infer that protonation of X = Y = Z at central atoms passes through transition states much like primary carbenium (nitrenium, oxenium) cations, poorly conjugated with the attached vinylic or heterovinylic group. Several other processes following upon center protonation were studied and are discussed in the text, special attention being given to comparison of open and cyclic isomers.

A QM/MM Study of the Bergman Reaction of Dynemicin A in the Minor Groove of DNA

The Bergman cyclization of the natural enediyne dynemicin A in its triggered form (2) bound to the minor groove of DNA is compared with the corresponding reaction of its open isomer (4) utilizing QM/MM methodology. The two isomers are typical representatives of 10-membered cyclic (2) and acyclic (4) enediynes, which possess significantly different barriers for the Bergman reaction in the gas phase (2, 20.4 kcal/mol; 4, 31.3 kcal/mol). In the case of the cyclic enediyne (2) the explicit consideration of environmental factors such as the receptor DNA, the solvent water, and charge neutralization by counterions has only minor effects on the energy profile of the cyclization reaction and the corresponding optimized structures when compared with the gas phase. The energetics of the reaction is predominantly determined by QM (electronic) effects. This makes it possible to replace the explicit description of the environment by an implicit one, thus avoiding costly QM/MM calculations and using instead a decoupled QM+MM approach. A conformationally driven hinge mechanism is identified for 2 that makes it possible for the ligand to adjust to the dimensions of the minor groove without significant energy loss. In the case of the acyclic enediyne 4 a QM/MM treatment is necessary to describe the Bergman cyclization in the minor groove. QM/MM corrects the cyclization barrier from 31.3 to 23.7 kcal/mol, which makes the reaction feasible under physiological conditions. The reduction of the barrier is a result of transition-state stabilization, which is caused by an increased dipole moment and hence stronger electrostatic interactions with the environment. In both cases the anionic charge of dynemicin A is largely shielded by water solvation and ion pair formation so that it does not significantly affect the energetics of the Bergman cyclization.

Langmuir Monolayers of a Photoisomerizable Macrocycle Surfactant

An amphiphilic photoisomerizable macrocycle has been prepared that forms stable Langmuir monolayers at the air-water interface. The hydrophilic core of the molecule switches between closed and open isomers upon irradiation by the appropriate wavelengths of light. Isotherm measurements, Brewster angle microscope images, and atomic force micrographs (of transferred Langmuir-Blodgett films) suggest a phase transition between a face-on to a tilted edge-on molecular orientation as a function of surface concentration. In the face-on phase, in situ photoisomerization results in a reversible increase in surface pressure due to greater molecular crowding in the open configuration.

Synthesis and Tautomeric Transformations of 2‐(tert‐Butyl)‐1,2,4‐Benzotriazine‐3(2H)‐thiones.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Electrochemical Remote Control for Dithienylethene-Ferrocene Switches

The electrochromic properties of dithienylethene derivates is a field of great importance and interest. In this manuscript we describe a potential molecular remote control system, which can be electrochemically triggered. Diferrocenyl compounds containing a diethylenene photoelectrochromic core with perhydro- and perfluorocyclopentene ring were prepared in order to induce a change in the chromic properties via an intramolecular electron-transfer reaction from the redox group to the photochromic core. The electrochemical behavior of open and closed isomers was thoroughly studied using photochemical, electrochemical, and spectroelectrochemical techniques. The first redox couple was typically assigned to the ferrocene for the open isomer. However, for the closed isomer, an electrocatalytic ring-opening process was observed for the perhydrocyclopentene ring, while a slightly different electrochemical process was observed for the perfluorocyclopentene system. Mechanistic investigations revealed that an interna...

Electrochemical Oxidation Mechanism of Photochromic Switches: Electrodimerisation, Ring Closure or Ring Opening?

Simple photochromic dithienylethylenes with either perfluoro or perhydro cyclopentene ring, and a variety of substituents have been prepared and their electrochemical behavior explored by cyclic voltammetry. All present two electron irreversible oxidation waves in their open form, but the radical cation of the open isomers can follow three different reaction pathways: dimerisation, ring closure, or ring reopening. Whereas the chloro derivative follows a dimerisation mechanism (EC 2E mechanism), the phenylthio substituted compound displays an efficient oxidative ring closure (ECE or DISP1 mechanism). Interesting electrochromic behavior is associated with this compound, a redox process occurring in the range 0.5-1.5V is observed by monitoring the absorption species changes (colored species) in function of the applied potential. Furthermore, electrochromic properties are also found in the corresponding ring closed isomers. Depending on the substituents on the thiophene ring and the perfluro or perhydro cyclopentene ring, open isomers can be obtained from oxidation (chemical or electrochemical) of the corresponding ring closed isomers via EC mechanism. These observations should be taken into account for the potential design of three-state conjugated systems and photoelectrical molecular switching.

Preparation and structures of [6,6]-open difluoromethylene[60]fullerenes: C60(CF2) and C60(CF2)2

Difluoromethylenation of C60 with thermally decomposed CF2ClCO2Na provides novel C60(CF2) and C60(CF2)2 compounds with unique [6,6]-open structures. A theoretical survey of CF2 derivatives of C60 demonstrates that carbon cage opening can be controlled via charging of the molecule and that the thermodynamically preferable structures combine the trend to form open isomers with the compactness of the addition motifs, which results in the formation of windows in the fullerene cage.

A density functional study of geometry and electronic structures of [(SiO 4)(M III) 2(OH) 2W 10O 32] 4−, M = Mo, Ru and Rh

The effects of redox-active M-atoms (M = Mo, Ru and Rh) of the di-d -transition-metal-substituted γ-Keggin polyoxometalates [(SiO 4)(M III) 2(OH) 2W 10O 32] 4−, on the geometry and electronic structure of these species were evaluated at the density functional level. It was shown that open isomers (without two bridging OH ligands between the M centers) may coexist with closed isomers (with bridging OHs) only for M = Mo. The ground electronic state of the closed isomer of all the studied species is found to be the singlet 1A 1 state in C 2v symmetry, but some of the high-spin states are very close in energy. The calculated M–M distance in the ground state increases via M = Mo (2.28 Å) < Ru (2.60 Å) < Rh (3.00 Å). Simultaneously the calculated M–M distances in high-spin states are very similar and are within of 3.13–3.25 Å. The M–O Si 1 (or M–O Si 2) bond distances, which vary from 2.00 to 2.17 Å for the ground state of all compounds, elongate upon going to high-spin states. These geometry trends are explained in terms of the frontier orbitals of these species.

Synthesis and tautomeric transformations of 2-(tert-butyl)-1,2,4-benzotriazine-3(2H)-thiones

Treatment of 2-(tert-butyl)-1,2,3,4-benzotetrazinium tetrafluoroborates with sodium thiocyanate afforded 2-(tert-butylazo)phenyl isothiocyanates 3, which exist in equilibrium with 2-(tert-butyl)-1,2,4-benzotriazine-3(2H)-thiones 3′. The equilibrium depends on the substituents R in the benzene ring: the percentage of the open isomer 3 is about 20% for R = H or Me; for R = Cl or Br, the equilibrium is completely shifted to cyclic isomer 3′. The equilibrium is slow on the time scale of the 1H and 13C NMR experiments. For compounds 3a/3′a (R = H), the spectra at 24 °C show two sets of signals, while those at 0 °C contain only signals for isomer 3′a.

Diarylethenes with intramolecular donor–acceptor structures for photo-induced electrochemical change

Abstract Diarylethenes with donor–acceptor groups were synthesized to induce electrochemical switching by light. Photoisomerization was induced by 1,2-bis(2-methyl-1-benzo[ b ]thiophen-3-yl)perfluorocyclopentene (BTF, 1 ) unit, while the 3,4-ethylenedioxythiophene (T) and nitro (N) groups were directly connected to BTF, to extend π-electron delocalization. Spectral change to a longer wavelength through photochromism was significant in the donor–acceptor structures ( 6 ), accompanied by an increase in the molar absorption coefficients, than those of the unsubstituted BTF ( 1 ) or the BTF substituted with only acceptor group ( 2c and 3c ). A significant peak shift toward lower redox energy was observed when the molecules were converted from an open isomer to a closed isomer. The plot of the reduction potentials (( E 1/2 red , V vs Ag/AgCl) vs LUMO energy eV) for the diarylethenes indicates that the reduction potential is strongly dependent on the nature of the substituents around the diarylethene unit. When 6 was applied to a photocell of Au/PC/ITO glass, in which PC is the polystyrene containing 6 , it became possible to switch the conductivity of the cell through the film by UV–vis irradiation, as estimated by the I–V plot on a photocell. The conductivity of the cell exposed to UV light was three times larger than that of the cell exposed to visible light, and 10 times larger than that of the cell containing 3 , indicating the importance of the push–pull structure for π-electron connectivity through the donor–BTF–acceptor.

Molecular AND Logic Gate Based on Electric Dichroism of a Photochromic Dihydroindolizine

Very ANDy: A photochromic molecular switch has been found to act as an AND Boolean logic gate. The switch takes as input both UV light, which causes photoisomerization of a dihydroindolizine to an open, dipolar form, and an electric field, which aligns this isomer in solution (see scheme). The output of the gate is read as the electric linear dichroism response of the open isomer. The gate can be reset with visible light.