Spin relaxation in the phosphorescent triplet state of several pyridyl carbonyl compounds

Spin relaxation is observed in benzophenone, isomers of benzoylpyridines and dipyridyl ketones. Models are used to describe optically detected echo shapes observed by transient techniques in the phosphorescent triplet state. Framed after the density matrix approach, the echo shapes were found to adequately fit the analytical expressions. Hahn spin echo, Carr-Purcell multiple echo, and rotary echo pulse sequences were used to study the nuclear contribution to the electron spin dephasing and the proximity effect of the nitrogen nuclei to the localized triplet excitation in the carbonyl portion of the molecule.     

Solvent effects on triplet state quenching by tetramethylpiperidcme-N-oxide

Measurements of the triplet state quenching of a series of aromatic hydrocarbons by tetramethylpiperidine-N-oxide in two solvents, acetonitrile and perfluoro-n-hexane, are reported. In both solvents the previously established trend of quenching rate constants with the triplet energy of the aromatic hydrocarbon being quenched was observed. No correlation of quenching efficiency with the charge transfer properties of the aromatic hydrocarbon—free radical collision complex was found.     

Solvent effects on the singlet - triplet equilibrium and reactivity of a ground triplet state arylalkyl carbene

Results from intramolecular singlet and triplet specific reactivity in solvents of different Polarity suggest that the spin state equilibrium of 1,2-diphenyl-1-butylidene, a triplet ground state carbene. is largely susceptible to solvent polarity. The results are consistent with stabilization of the zwitterionic singlet state in solvents of high polarity.     

Solvent effects in hydrogen abstraction from cholesterol by benzophenone triplet excited state

Hydrogen abstraction from the C-7 position of cholesterol (Ch) by triplet excited benzophenone (BZP) exhibits remarkable solvent-dependence in product studies. Kinetic measurements on the intramolecular version of the process in dyads containing covalently linked Ch and BZP units reveal important solvent effects and significant stereodifferentiation.     

Kinetic energy release and position of transition state during the intramolecular substitution of ionized 2-benzoyl pyridines

The loss of substituents X from molecular ions of ortho substituted 2-benzoyl pyridines has been investigated as a function of the dissociation energy of the C? X bond. Comparison of unimolecular and collisional induced decompositions of the resulting [M ? X]⁺ ions and reference ions arising from 3-hydroxypyrido[1,2-α]indole shows that cyclic fragment ions are formed in every case by an intramolecular substitution reaction with the exception of the parent compound (X = H), which gives rise to a mixture of [M ? H]⁺ ions with different structures. The heat of formation of the cyclic ion has been estimated experimentally and by calculation using thermochemical data, and from this value and the appearance energies, the activation energies of the reverse reactions have been evaluated for the different reaction systems. Measurement of the kinetic energy release during the substitution reactions shows that only part of the reverse activation energy is released as kinetic energy. The energy partitioning quotient varies from 0.37 to 0.08 depending on the dissociation energy of the C? X bond or the reaction enthalpy. A sudden change in the energy partitioning quotient is observed with increasing exothermicity of the reaction, paralleling the behaviour of similar reaction systems. These results are interpreted as a demonstration of the influence of the variation of transition state position on the energy partitioning quotient.     

Solvent effects upon the proton NMR linewidth in dimethylformamide

The NMR linewidth of the formyl proton in dimethylformamide has been studied in the neat liquid and twelve organic solvents. It varies by more than one order of magnitude which is referred to solvent viscosity changes and to the protonation of dimethylformamide. The data do not allow the distinction between N- and O-protonation without further knowledge of the solvent dependence of either the nitrogen-14 spin lattice relaxation time or the spin-spin coupling ²J(NH).     

Solvent effects on optically detected magnetic resonance in triplet spin labels

We have calculated solvent effects on the zero-field splitting (ZFS) constants induced by electron spin–spin coupling (SSC) in the low-lying triplet states of azaaromatic molecules in solutions using multiconfiguration self-consistent-field wave functions and the polarizable continuum model. The second-order spin–orbit coupling (SOC) contribution to the splitting of the ^3* states is found to be almost negligible, and the calculations therefore provide a good estimate of the ZFS parameters and their solvent dependence based only on the electron spin–spin coupling expectation values. The correlation between the shift in the ZFS and the phosphorescence frequency that has been observed in optically detected magnetic resonance experiments in low-temperature glasses is supported by our direct SSC calculations without taking SOC into account. This makes it possible to distinguish between the two theories that earlier were proposed to explain the inhomogeneous broadening of triplet state spectra, and discard the one that is exclusively based on the SOC-induced mixing of the singlet and triplet states.Contribution to the Jacopo Tomasi Honorary IssueAcknowledgments. This work was supported (B. M.) by the Swedish Royal Academy of Science (KVA). This work was also supported by the Norwegian Research Council through a grant of computer time from the Program for Supercomputing. We are grateful to B. Schimmelpfenning for his valuable assistance in the computations.     

Solvent effects on the aggregation state of lithium dialkylaminoborohydrides

DFT calculations were performed to determine the effects of ethereal solvents on the aggregation state of lithium dialkylaminoborohydrides (LABs). The calculations included dimerization energies in the gas phase, with continuum solvation only, microsolvation with coordinating ethereal ligands, and a combination of the microsolvation and continuum models. The continuum model alone overestimates the stability of the dimers, apparently due to the lack of steric effects from the coordinating ethereal ligands. The use of the combined microsolvation and continuum solvation models predicts lithium dimethylaminoborohydride to be a mixture of monomer and dimer in THF, and more sterically hindered lithium aminoborohydrides to exist primarily as monomers. The kinetics of amination of 1-chlorodecane by lithium dimethylaminoborohydride showed no detectable change in reaction rate with time, suggesting that the LAB reagent may exist primarily as a monomer in THF.     

Kinetics of carbonyl triplet reactions in water

The presence of urea during the gelling of AlPO₄–Al₂O₃ catalysts (AlPO₄/Al₂O₃ weight ratio =3) has been studied with respect to its effect on the porous texture and surface acidity of the resulting gels. An increase in pore volume and mean pore radius with no modification in surface acidity is found when the AlPO₄–Al₂O₃ catalyst is modified by the addition of 5 mol% urea in the AlPO₄ solution.

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, AlPO₄/Al₂O₃ ( AlPO₄/Al₂O₃=3) . , AlPO₄/Al₂O₃ 5 .% AlPO₄ - .

     

The phosphorescent state of dibenzothiophene

The ESR study of phosphorescent o,o'-bridged biphenyls has shown that the nature of the heteroatom has little effect on the molecular properties of the lowest triplet state of these molecules. This observation allows us to assign the symmetry of this state and to propose an electronic scheme for the phosphorescence in this family of aromatic compounds.     

Solvent isotope effects upon the kinetics of some simple electrode reactions

The effects of replacing H₂O with D₂O solvent upon the electrochemical kinetics of simple transition-metal redox couples containing aquo, ammine or ethylenediamine ligands have been investigated at mercury electrodes as a means of exploring the possible contribution of ligand-aqueous solvent interactions to the activation barrier to outer-sphere electron transfer. The general interpretation of solvent isotope effects upon electrode kinetics is discussed; it is concluded that double-layer corrected isotopic rate ratios (k^H/k^D)^E determined at a constant electrode potential vs. an aqueous reference electrode, as well as those determined at the respective standard potentials in H₂O and D₂O (k_S^H/k_S^D), have particular significance since the solvent liquid-junction potential can be arranged to be essentially zero. For aquo redox couples, values of (k_S^H/k_S^D) were observed that are substantially greater than unity and appear to be at least partly due to a greater solvent-reorganization barrier in D₂O arising from ligand-solvent hydrogen bonding. For ammine and ethylenediamine complexes values of (k^H/k^D)^E substantially greater than, and smaller than, unity were observed upon the separate deuteration of the ligands and the surrounding solvent respectively. Comparison of isotope rate ratios for corresponding electrochemical and homogeneous outer-sphere reactions involving cationic ammine and aquo complexes yields values of (k^H/k^D) for the former processes that are typically markedly larger than those predicted by the Marcus model from the homogeneous rate ratios. These discrepancies appear to arise from differences in the solvent environments in the transition states for electrochemical and homogeneous reactions.     

Polymerization photoinitiated by carbonyl compounds. VIII. Solvent and photoinitiator concentration effects

The interaction of benzil and thioxanthone triplets with triethylamine leads to the production of free radicals that are able to initiate the polymerization of methylmethacrylate and vinyl acetate. The process is mediated by a charge transfer complex. The initiation efficiency is higher in benzene than in acetonitrile, a result explained in terms of competition between back electron transfer, production of free ions and ketyl radical formation. The initiation efficiency decreases when the carbonyl concentration increases, because of the occurrence of a radical molecule reaction such as The α-hydroxy radicals produced in this reaction are unable to initiate the polymerization chain.     

Temperature effects on the intramolecular decay of the lowest triplet state of benzophenone

By monitoring the emission specturm, yield, and lifetime from 77 to 400°K for benzophenone in poly(chlorotrifluoroethylene), we have been able to establish the thermally activated delayed fluorescence process and have also observed a large temperature effect on the rate constant for nonradiative decay from the lowest triplet state to the ground singlet state.     

Temperature effects on the quenching of the triplet state 3Au of biacetyl

Rate data for the quenching of the triplet state ³A_u of biacetyl were determined for a number of collision partners in the gas phase as a function of temperature. Quenching by oxygen and nitric oxide resulted in negative temperature coefficients and the results were interpreted on the basis of the formation of collision complexes. The Arrhenius parameters determined for the reaction of triplet biacetyl molecules with acetaldehyde are an indication that the n,π* configuration of biacetyl behaves like a biradical in the gas phase. The results are consistent with a hydrogen atom abstraction reaction channel.     

Electron localization function comparative study of ground state,triplet state,radical anion,and cation in model carbonyl and imine compounds

The modifications of bonding in carbonyl and imine compounds upon excitation, electron attachment, and ionization were studied within the framework of the electron localization function (ELF). The topological analysis of ELF allows a partition of the molecular space into regions having a clear chemical meaning: the basins. The electronic populations of the basins associated with bonding and nonbonding character, as well as the basin spin densities, were calculated at the MP2 level of the quantum mechanical calculation. Good agreement was found with the classical view in terms of mesomeric structures corresponding to the dominant localization of electrons contained in frontier molecular orbitals (MOs). The variations of the basins population were compared to the predictions of MO theory. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 897–910, 1999     

Electron paramagnetic resonance measurements and transient effects of the phosphorescent state of 1, 3-diazaazulene

The zero-field splittings and g-tensors of the phosphorescent state of 1,3-diazaazulene imbedded in the naphthalene crystal have been measured at 77 K. The transient effect of the photoexcitation to the metastable triplet state is observed and the dynamics of the photomagnetization is deduced from the study.     

Synthesis and the effects of substitution upon photochromic diarylethenes bearing an isoxazole moiety

A new class of unsymmetrical photochromic diarylethenes bearing an isoxazole moiety was synthesized and the effects of substitution on their optical and electrochemical properties were investigated systematically. Each of the compounds exhibited remarkable photochromism and functioned as a fluorescent photoswitch both in solution and in poly(methyl methacrylate) films. The electron-donating substituents effectively shifted the absorption maximum and the emission peak to a longer wavelength direction, while the electron-withdrawing substituents notably enhanced the fluorescent quantum yields and oxidation onsets of these diarylethene derivatives. As compared to the unsubstituted parent diarylethene, introduction of the electron-donating/withdrawing substituents could efficiently modulate the optical and electrochemical properties of the diarylethenes bearing an isoxazole moiety. All results indicated that the isoxazole moiety and the substitution effects played a very important role during the process of photochromic reaction for these diarylethene derivatives.     

Significant effects of substituents on substituted naphthalenes in the higher triplet excited state

Substituent effect on the lifetimes of a series of substituted naphthalenes (Np) in the higher triplet excited state (Tn) was studied with transient absorption measurements using the two-color two-laser flash photolysis technique. Lifetimes of Np(Tn) in cyclohexane solution were determined from the triplet energy transfer quenching by carbon tetrachloride to be 0.98-63 ps. The different lifetimes of Np(Tn) were explained by the energy gap law for the internal conversion from Np(Tn) to Np(T1), indicating that Np(Tn) quenched by carbon tetrachloride is assigned to Np(T2) with the longest lifetime among Np(Tn). The lifetime of Np(Tn) was correlative with the Hammett sigmap constant. Electronic characters of substituents showed a more significant influence on the energy of the T2 state than that of the T1 state.