Mechanistic insights into the photochromism of trans-10b,10c-dimethyl-10b,10c-dihydropyrene derivatives

A series of dimethyldihydropyrene derivatives was studied to elucidate the photochemical mechanism associated with the switching between the dimethyldihydropyrene (DHP, closed) and metacyclophanediene (CPD, open) forms of the molecule. Quantum yields of ring opening and closure, fluorescence quantum yields and lifetimes, as well as laser flash photolysis studies were performed to establish the effect of substituents on the switching efficiency. Ring opening of the DHPs occurs from the first singlet excited state. The low quantum yields for the ring opening reaction observed (< or =0.042) are a consequence of the low rate constant (< or =1.7 x 10(7) s(-1)) for this process. The quantum yields for ring closure of the CPD were determined for select compounds and were of the order of 0.1-0.4. These results show that the efficiency for ring opening of this class of compounds is intrinsically low, but can be modulated to some extent by the introduction of substituents. These properties should be taken into account when considering what type of photoswitching devices DHPs might be useful for.     

The Radical Anion of 1,2-Diphenylcyclopentene

ESR. and ENDOR. studies are reported for the radical anions of 1,2-diphenylcyclopentene ( 3 ) and its di(pe+deuteriophenyl)-derivative (3-D₁₀). Comparison of the coupling constants of the phenyl protons in 3 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}. with the analogous values for the radical anions of 1,2-diphenyl substituted cyclopropene ( 1 ) and cyclobutene ( 2 ) reveals regular changes in the sequence 1 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}, 2 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}, 3 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}, which are caused by an increasing twist of the phenyl groups about the C(1), C(1′)- and C(2), C(1″)-bonds linking them to the ethylene fragment. Such a twist is shown to be also responsible for the large difference in the coupling constants of the methylene β-protons in 3 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}. (0.659 and 0.293 mT). It is suggested that - in order to minimize the losses caused by this twist in the π-delocalization energy - the 2 p_z-axes at the centres 1 and 2 deviate from a perpendicular orientation to the mean plane of the cyclopentene ring. A deviation by 19° from such an orientation is required to account for the observed β-proton coupling constants in terms of their conventional cos²-dependence on the dihedral angles θ.     

The Complexed Triphosphaallyl Radical,Cation, and Anion Family

Radically complex : The photolytic reaction of [Cp*P{W(CO)₅}₂] (Cp*=C₅Me₅) with a diphosphene produces, via a radical intermediate, an air‐stable complexed triphosphaallyl radical, in which the unpaired electron is evenly distributed over both terminal P atoms. Oxidation of the radical leads to a triphosphaallyl cation, which is only stable at low temperatures in solution, whereas the stable triphosphaallyl anion is formed by reduction (see picture, Mes*=2,4,6‐tri‐tert‐butylphenyl).

     

The Radical Anion and Dianion of Tetraazapentacene

The mono‐ and bis‐reduction of 6,13‐bis((triisopropylsilyl)ethynyl)quinoxalino[2,3‐b]phenazine ( 1 ) with potassium anthracenide in THF is reported. Both the radical anion 1 ^.? and the dianion 1 ^2? were isolated and characterized by optical and structural (single‐crystal X‐ray diffraction) methods. Solutions of the radical anion 1 ^{. ?} were stable in air for several hours and characterized by EPR spectroscopy. Dianion 1 ^2? is highly fluorescent and photostable.     

The Radical Anion,Dianion and Electron Transport Properties of Tetraiodotetraazapentacene

Tetraiodotetraazapentacene I₄TAP , the last missing derivative in the series of halogenated silylated tetraazapentacenes, was synthesized via condensation chemistry from a TIPS-ethynylated diaminobenzothiadiazol in three steps. Single and double reduction furnished its air-stable monoanion and relatively air-stable dianion, both of which were characterized by crystallography. All three species are structurally and spectroscopically compared to non-halogenated TAP and Br₄TAP . I₄TAP is an n-channel material in thin-film transistors with average electron mobilities exceeding 1 cm² (Vs)⁻¹.     

The Radical Anion of Cyclopentasilane‐Fused Hexasilabenzvalene

The radical anion of cyclopentasilane‐fused hexasilabenzvalene was synthesized by the reduction of the corresponding neutral compound. X‐ray crystallographic analysis showed a more trans‐bent structure of the disilene moiety than the neutral compound. Theoretical calculations showed that the highly trans‐bent structure is attributed to the hexasilabenzvalene structure. The EPR spectrum showed that an unpaired electron exists mainly at the disilene moiety. In the UV/Vis spectrum, a large bathochromic shift was observed compared with the neutral compound.     

The ESR. Spectrum of the Diphenylcyclopropenone Radical Anion

The ESR. spectrum of the relatively unstable radical anion of diphenylcyclo-propenone (II) has been observed upon electrolytic reduction of II in N,N-di-methylformamide and 1,2-dimethoxyethane. Simple MO models account well for the π-spin distribution and for the restricted rotation of the phenyl substituents in II\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}. A rather facile loss of a CO molecule by II\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document} results in formation of the radical anion of tolane (diphenylacetylene; III). No ESR. spectra could be obtained for the radical anions of dialkylcyclopropenones which are even shorter-lived than II\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}, although decay by decarbonylation seems to be less favoured with them than with II\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}. In presence of air, electrolytic reduction of either II or its dimethyl and di-t-butyl analogues yields the correspondingly disubstituted semidione anions.     

Radical Anion Intermediates in Organic Chemistry

Radical anions are reactive intermediates in a variety of organic reactions. They make possible several unique synthetic conversions and provide an opportunity for investigating structural relationships. Examples of such reactions are given and current mechanistic views are discussed.     

The Radical Anion of 2,7-Diazapyrene,a Change in Orbital Sequence on Protonation

ESR.-spectra are reported for the radical anion I · Θ of 2,7-diazapyrene (I), along with those for the radical cations I(2H) · ⊕ and I(2 CH₃) · ⊕ of 2,7-dihydro-2,7-diazapyrene and its 2,7-dimethyl-derivative, respectively. In contrast to the analogous radical ions of 4,4′-bipyridyl (II) and other previously studied diazaaromatic compounds, there is a striking change in the ¹⁴N and proton coupling constants on going from the radical anion I · Θ to the radical cations I(2H) · ⊕ and I(2 CH₃) · ⊕. This change can be rationalized in terms of the HMO model of the pyrene π-system. A reversal in the energy sequence of the lowest antibonding orbitals is predicted upon an increase in the absolute value of the Coulomb integral for the azasubstituted π-centres, such an increase simulating the enhanced electronegativity of the azanitrogen atoms 2 and 7 on protonation.     

Biomimetic Ketone Reduction by Disulfide Radical Anion

The conversion of ribonucleosides to 2′-deoxyribonucleosides is catalyzed by ribonucleoside reductase enzymes in nature. One of the key steps in this complex radical mechanism is the reduction of the 3′-ketodeoxynucleotide by a pair of cysteine residues, providing the electrons via a disulfide radical anion (RSSR^•−) in the active site of the enzyme. In the present study, the bioinspired conversion of ketones to corresponding alcohols was achieved by the intermediacy of disulfide radical anion of cysteine (CysSSCys)^•− in water. High concentration of cysteine and pH 10.6 are necessary for high-yielding reactions. The photoinitiated radical chain reaction includes the one-electron reduction of carbonyl moiety by disulfide radical anion, protonation of the resulting ketyl radical anion by water, and H-atom abstraction from CysSH. The (CysSSCys)^•− transient species generated by ionizing radiation in aqueous solutions allowed the measurement of kinetic data with ketones by pulse radiolysis. By measuring the rate of the decay of (CysSSCys)^•− at λ_max = 420 nm at various concentrations of ketones, we found the rate constants of three cyclic ketones to be in the range of 10⁴–10⁵ M⁻¹s⁻¹ at ~22 °C.     

A Stable Crystalline Triarylphosphine Oxide Radical Anion

Triarylphosphine oxides (Ar₃P=O) are being intensely studied as electron‐accepting (n‐type) materials. Despite the widespread application of these compounds as electron conductors, experimental data regarding the structural and electronic properties of their negatively charged states remain scarce owing to their propensity for follow‐up chemistry. Herein, a carefully designed triarylphosphine oxide scaffold is disclosed that comprises sterically demanding spirofluorenyl moieties to shield the central phosphoryl (P=O) moiety. This compound undergoes chemical one‐electron reduction to afford an exceptionally stable radical anion, which was isolated and characterized by X‐ray crystallography for the very first time. The experimental data, corroborated by computational studies, shall allow for the construction of phosphine oxide materials with enhanced stability.     

Implications of Alkyl Nitro Radical Anion Decay Rates

Abstract

Current-reversal chronopotentiometry was used to measure the rates of decomposition of tertiary alkyl radical anions. Radical ions studied were chosen to yield both constrained and unconstrained products (e.g., 1-nitroadamantyl vs. t-nitrobutyl) in an effort to illuminate the effect that such structural differences might have upon the decay rates. Temperature studies were performed to yield thermodynamic details; supporting electrolyte concentration changes and three solvents were employed to explore ion pairing effects.     

Visible Light Photocatalysis of Radical Anion Hetero-Diels-Alder Cycloadditions

We have discovered a photocatalytic intramolecular hetero-Diels-Alder reaction of tethered bis(enones). This transformation involves the intermediacy of an enone radical anion and constitutes the formal coupling of an electron-deficient heterodiene with an electronically mismatched enone dienophile. The diastereoselectivity and regioselectivity of the process are high, and the dihydropyran products are amenable to a variety of synthetically useful transformations.     

氰基苯阴离子与CO2间的内球电子转移

用半经验AM1方法以及从头算方法在3-21G和6－31G*基组水平上研究了从氰基苯阴离子到CO2的电子转移过程．结果表明，对于先驱物(precursor),三种计算方法得出的给体、受体间的距离分别为0.2728nm(AM1)、0.2479nm（UHF/3-21G）和0.2769nm(UHF/6-31G*)．在这样短的距离内给体的HOMO与受体的LUMO轨道具有相当程度的重叠，应产生较强的相互作用，说明此反应是内球电子转移反应，从而解释了此类体系的电子转移反应不符合Marcus理论的原因．计算给出先驱物的束缚能为0.19eV(AM1)和0.26eV(6-31G*)     

α-Silyl Ethers as Hydroxymethyl Anion Equivalents in Photoinduced Radical Electron Transfer Additions

Nucleophilic α -hydroxymethyl radicals can be generated from α-silyl ethers by irradiation in the presence of 9,10-anthracenedicarbonitrile (ADC) and biphenyl (BP). Under these conditions the hydroxymethyl radicals are not oxidized further but add directly to electron-poor alkenes [Eq. (a); EWG=electron-withdrawing group]. The radical adduct undergoes back electron transfer to form the carbanion, which is protonated. R=PhCH₂, tBuMe₂Si, Me; TMS=Me₃Si.