Redox-switching of intramolecular magnetic interaction through π-conjugation mode change of 1,2-bis(4-dianisylamino)-1,2-bis(3-N-oxylamino)-substituted tetraarylethylene

To develop the redox-switching system of intramolecular magnetic interaction, 1,2-bis[3-(N-tert-butyl-N-oxylamino)phenyl]-1,2-bis[4-{N,N-bis(4-methoxyphenyl)amino}phenyl]ethylene, tetraarylethylene with two nitroxide radical groups at the meta-position, was synthesized, and characterized by the electrochemical method and ESR spectroscopy. Cyclic voltammetry showed the tetraarylethylene core has the lower oxidation potential than the substituted nitroxide radical moiety. ESR spectroscopy in frozen solution revealed that the neutral form shows the fine-structured spectrum characteristic of the spin triplet species, while the dicationic form shows the anisotropic hyperfine-structured spectrum characteristic of the randomly-oriented nitroxide radical, indicating the drastic change of intramolecular magnetic interaction.     

One-pot synthesis of 1-aryl-3-methyl-1,3-dienes using methallyl(trimethyl)silane and aldehydes and their low temperature (Z)→(E) isomerization induced by sulfur dioxide

2-Methylprop-2-ene-1-sulfonyl fluorides can be easily prepared via the ene reaction of methallylsilanes and SO₂. In the presence of a base, aldehydes and 2-methylprop-2-ene-1-sulfonyl fluorides give 1,3-(E) and (Z)-dienes. Their (Z)→(E) isomerization by classical means fails or leads to their polymerization. It is shown that SO₂ can isomerize 1-aryl-3-methyl-1,3-dienes at low temperature, without formation of sulfolenes (cheletropic addition/elimination). Preliminary mechanistic studies suggest that SO₂ adds to 1,3-dienes forming 1,4-diradical intermediates that are responsible for the (Z)→(E) isomerizations.     

含绿色荧光蛋白发色团双自由基分子的非线性光学性质

采用密度泛函理论(DFT)方法研究了系列含绿色荧光蛋白发色团双自由基分子光学异构体的几何结构、极化率(αs)和第一超极化率(βtot).结果表明,引入电子给受体取代基使分子的极化率增大,而对第一超极化率有不同影响.对于光照前的反式结构,引入电子受体βtot值增加,且βtot值随取代基吸电子能力的增强而增大;引入电子给体βtot值降低,且βtot值随取代基给电子能力的增强而减小.当分子变成相应的顺式结构时,其βtot值变化趋势与反式结构的结果正好相反.光异构化前后分子的βtot值变化不同,引入电子受体使顺式结构的βtot值比反式结构的小,其中―NO2使顺式结构的βtot值减小为反式结构的1/6;引入电子给体使反式结构的βtot值比顺式结构的小,其中―NH2使反式结构的βtot值减小为顺式结构的1/6.从而,光异构化起到调节非线性光学(NLO)响应的作用.     

Bis-periazulene: a simple Kekulé biradical with a triplet ground state

B3LYP/6-31G* calculations on bis-periazulene (cyclohepta[def]-fluorene) predict a triplet ground state for this molecule. The singlet has an aromatic 14π-electron periphery but is 2 kcal/mol higher in energy. The results agree with earlier predictions by Heilbronner. Received: 19 August 1998 / Accepted: 6 October 1998 / Published online: 23 February 1999     

Theoretical investigations on the conversions of cyclic polysulfides to acyclic polysulfide diradicals and subsequent reactions of biological interest

Sulfur is one of the most necessary biogenic elements in nature that must be assimilated by all organisms; it is an essential macronutrient for living organisms and has multiple roles in plant development. The oxidation of elemental sulfur is a complex process involving the contact of cells with sulfur particles, the oxidation of sulfur to sulfite, and the oxidation of sulfite to sulfate. To provide hypothesis concerning the most probable processes in the early states, we determined, by quantum-chemical calculations, the energies of some allotropic forms containing up to 32–40 sulfur atoms and energetics of their reactions with triplet dioxygen. The most probable reactions occurred with methylpolysulfane anions with an electron transfer to give the superoxide anion radical (and thiyls radicals) and especially the formation of peroxydic polysulfane anions. Calculations confirmed that the triplet diradical is more stable than the singlet one for acyclic polysulfide chains.     

The Fock Matrix Analysis for Atomic Orbitals in Molecular Orbitals I. A New Look on the Covalent Bond in H2?

The character of the molecular orbitals can be better accounted for in terms of molecular adapted atomic orbitals and the Fock matrix expanded in these atomic orbital sets. A clean‐cut and unique criterion for the diradicals and the covalent bonds can be given for the molecular orbitals in both restricted and unrestricted Hartree‐Fock wavefunctions. Instead of the picture that overlap charge migrates into the bonding region, the new analysis displays another picture that the charge densities for the electrons with α and β spins give rise to two opposite spin density shifts. If the α one shifts from atom A toward atom B then it is vice versa for the β one. The spin density shifts proceed until the bonding molecular orbitals form.     

A Highly Stable Organic Luminescent Diradical

It is very challenging to obtain stable room-temperature luminescent open-shell singlet diradicals. Herein we report the first stable Müller's hydrocarbon TTM-PhTTM with luminescent properties. Variable-temperature electron paramagnetic resonance spectroscopy measurements and theoretical calculations show that TTM-PhTTM has an open-shell singlet ground state with a diradical character of 90 %. Because of a small singlet-triplet energy gap, the open-shell singlet ground state can be thermally excited to a triplet state. TTM-PhTTM shows room-temperature deep-red emission in various solutions. Unusually high stability of TTM-PhTTM was also observed owing to effective steric hindrance and spin delocalization. Our results are beneficial to the rational design and discovery of more stable luminescent diradical materials.     

Effects of Electron Donor and Different Solvents on Polarizability and Second Hyperpolarizability of Diradical Complex Involving X(X=B,Al, Ga)

The polarizability(a)and second hyperpolarizability(γ)were systemically investigated for singlet diradical complex involving X(X=B,Al,Ga)atom.The results show that both the a and γ can be effectively t...     

Dimerization of a Reactive Azaacene Diradical: Synthesis of a Covalent Azaacene Cage

Two series of regioisomeric dicyanomethylene substituted dithienodiazatetracenes with formal para- or ortho-quinodimethane subunits were synthesized and characterized. Whereas the para-isomers (p-n, diradical index y₀=0.01) are stable and isolable, the ortho-isomer (y₀=0.98) dimerizes into a covalent azaacene cage. Four elongated σ-CC bonds are formed and the former triisopropylsilyl(TIPS) -ethynylene groups transformed into cumulene units. The azaacene cage dimer (o-1)₂ was characterized by X-ray single crystal structure analysis and temperature-dependent infrared (IR), electron paramagnetic resonance (EPR, solid-state), nuclear magnetic resonance (NMR) and ultraviolet-visible (UV/Vis) spectroscopies (solution) indicating reformation of o-1.     

1,3-Alternate calix[4]arene nitronyl nitroxide tetraradical and diradical: synthesis, X-ray crystallography, paramagnetic NMR spectroscopy, EPR spectroscopy, and magnetic studies

Calix[4]arenes constrained to 1,3-alternate conformation and functionalized at the upper rim with four and two nitronyl nitroxides have been synthesized, and characterized by X-ray crystallography, magnetic resonance (EPR and ¹H NMR) spectroscopy, and magnetic studies. Such calix[4]arene tetraradicals and diradicals provide scaffolds for through-bond and through-space intramolecular exchange couplings.