Reduction of pyracylene by alkali metals: Mechanism and formation of aggregates

Five reaction stages have been identified in the reduction of pyracylene to its dianion. The same stages were also observed in the photochemical oxidation starting with the dianion. The formation of dimers of anions and of mixed valence aggregates is discussed.     

Photoinduced electron transfer between C60 and N,N,N′,N′-tetramethylbenzidine (NTMB). Fourier transform electron paramagnetic resonance study

Fourier transform EPR spectroscopy was employed in studying the electron transfer (ET) reaction and the quenching mechanisms of the photoexcited triplet state of C₆₀ as electron acceptor and N,N,N′,N′-tetramethylbenzidine (NTMB) as electron donor in benzonitrile solution. The ET reaction product, the cation radical NTMB^*+, interacts with ^3*C₆₀, leading to photoinduced electron polarization of NTMB^*+ via triplet-doublet mixing mechanism combined with triplet mechanism. The quenching of ^3*C₆₀ and the polarization behavior of NTMB^*+ are discussed.     

Electron spin polarization of functionalized fullerenes. Reversed quartet mechanism

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to study two functionalized fullerenes consisting of a C60 moiety covalently linked to TEMPO radical via spacers of different length. Photoinduced electron spin polarization (ESP) reflecting a non-Boltzmann population within the energy levels of the spin system was observed in the electronic ground and excited states. Both fullerenes are characterized by a sign inversion of their TREPR spectra. A new mechanism of ESP generation was suggested to explain the experimental results. This mechanism, termed as the reversed quartet mechanism (RQM), includes the intersystem crossing process, which generates ESP in the excited trip-doublet and trip-quartet (2T1 and 4T1) states. This ISC is accompanied by ESP transfer to the ground state (2S0) by either electron-transfer reaction (in our case via charge transfer state, 2CT, i.e., 2T1--> 2CT --> 2S0 or internal conversion, 2T1--> 2S0.     

Exploring the photoexcited triplet states of aluminum and tin corroles by time-resolved Q-band EPR

The photoexcited triplet states of three 5,10, 15-tris(pentafluorophenyl)corroles (tpfc), hosting Sn(IV) and Al(III) in their core, namely, Sn(Cl)(tpfc), Al(pyr)₂(tpfc) and Al(pyr)₂(tpfc-Br₈), were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal E7. Only two of these metallocorroles, namely, Sn(Cl)(tpfc) and Al(pyr)₂(tpfc-Br₈), exhibit TREPR spectra following pulsed laser excitation. This result is rationalized in terms of a very low quantum yield of triplet formation in Al(pyr)₂(tpfc). Analysis of the spin polarized Q-band (34 GHz) EPR spectra of Sn(Cl)(tpfc) and Al(pyr)₂(tpfc-Br₈) provides detailed information on the magnetic and kinetic parameters of the triplet states as well as on the molecular ordering of the complexes in the liquid crystal. With the assignment of the zero-field splitting parameterD<0 for the Sn(Cl)(tpfc) and Al(pyr)₂(tpfc-Br₈), one can evaluate the dominant intersystem crossing path for these metallocorroles. Analysis reveals that in Sn(Cl)(tpfc) the in-plane triplet sublevels are preferentially populated, i.e.,A _X, A_Y?A _Z. This can be rationalized in terms of weak electronic interactions between the Sn(IV) ion and the corrole π-system, consistent with the domed structure of Sn(Cl)(tpfc). In Al(pyr)₂(tpfc-Br₈), however, the out-of-plane triplet sublevel is predominantly populated, i.e.,A _Z>A _X, A_Y, which is attributed to a large increase in the spin-orbit coupling strength arising from the peripheral bromine atoms on the corrole skeleton.     

Photogalvanic cell based on the photolysis of rubidium anions in THF

A photogalvanic cell composed of two half cells of the type [Rb-THF]_I and [Rb-THF, crown ether (or kryptand)]_II is reported. Photocurrents and photovoltages, recorded at low temperatures, in the order of 6 nA and 80 mV, respectively are obtained with light excitation at wavelenghts above 640 nm. The corresponding values with excitation above 320 nm, are 30 nA and 250 mV, respectively.     

PHOTOREDUCTION OF PORPHYRINS TO CHLORINS BY TERTIARY AMINES IN THE VISIBLE SPECTRAL RANGE. OPTICAL AND EPR STUDIES

Abstract— The photoreduction of free base porphyrins by tertiary amines in the visible spectral range leads to the formation of chlorin. The increase of the apparent first order rates to yield chlorin is correlated with the inductive effect on the nitrogen of the amine used. A mechanism involving a charge transfer interaction between the photoexcited singlet of the prophyrin and the amine is proposed. The porphyrin radicals formed recombine to form a light sensitive dimer which disproportionates in the dark to yield chlorin and porphyrin. The mechanism is elucidated by the use of EPR, laser and flash photolysis.     

Location of superconductivity in La2-βSrβCuO4

Evidence is presented that the superconductivity of La_{2 ?β} Sr_β CuO₄ and La₂CuO4_+δ occurs via holes in the LaO planes, rather than in the CuO₂ planes.     

Creep mechanism of gas-pressure-sintered silicon nitride polycrystals II. Deformation mechanism

As reported in the companion paper, part I, the creep behaviour of two silicon nitride ceramics is governed by a stress exponent n?=?0.6?±?0.1. It is known that, under certain conditions, silicon nitride ceramics may undergo a transition n?=?1?→?n<1 (the ‘shear-thickening transition’) at a well-defined stress σ*. This transition has not been observed, which, together with considerations regarding the value of the transition stress σ*, leads us to reject the shear-thickening phenomenon as an explanation for the creep behaviour of the materials investigated. Instead, it may be understood in terms of a particular grain boundary sliding mechanism accommodated by solution–precipitation.     

A mathematical model for weighing investments

This model assumes three separate investors each with a set amount of money. The full amount is to be invested in six investments, namely: long‐term bonds, short‐term bonds, treasury bonds, common stock, treasury bills, and commercial paper. The investment is made according to weights assigned to safety of principal, growth, current income, taxable income and volatility. The weights are derived by obtaining pairwise comparison matrices and then using an averaging process.     

Uniqueness for Boundary Value Problems for Second Order Finite Difference Equations

We establish maximum principles for second order difference equations and apply them to obtain uniqueness for solutions of some boundary value problems.