The radical cations of several preferably sterically hindered phenols have been observed through the radiationinduced electron transfer from the phenol (ArOH) to the parent cations of nonpolar solvents (n-and iso-butyl chloride, cyclohexane, freon-113). For p-methoxyphenol and its 2,6-di-tert-butyl-substituted derivative, ArOH •+ has been characterized after radiolysis at 77 K by EPR and optical absorption spectroscopies. Their broad optical absorption bands at λ ) 430 and <310 nm agree well with those seen at room temperature after pulse radiolysis of the phenol solutions in n-butyl chloride. Generally, also the other phenols studied show similar absorption spectra, but the bands are not so pronounced as in the case of the p-methoxy-substituted ones. Electron transfer from the phenols to n-butyl chloride radical cation proceeds with a rate constant of (1.5-2.0) × 10 10 dm 3 mol -1 s -1 at 293 K. In the case of phenols containing a further functional group distant from the phenol aromatic ring (phenyl acrylate, amine groups), depending on their ionization potential, after the primary ionization intramolecular charge transfer phenomena from or to the phenol unit are observed. ArOH •+ decay under dissociation to phenoxyl radicals (ArO • ) and protons. In the 266 nm laser flash photolysis under ionizing conditions (absorption of two UV photons) in aqueous solutions of the phenols, phenoxyl radicals and solvated electrons are observed, whereas in n-butyl chloride solution ArOH •+ of almost of the studied phenols were found to exist in the nanosecond time range.

No abstract

Phenol radical cations as well as phenoxyl radicals were observed as direct products of free electron transfer from phenol type solute molecules to solvent parent radical cations generated by ionizing irradiation. It is shown that the finding of the two species in comparable amounts can be explained by a nuclear-structure dependent solute cation dissociation behavior: Quantum-chemical calculations indicate that for phenol as solute primarily its conformers with perpendicular C-OH axis orientation to the aromatic ring tend to prompt deprotonation after ionization. A quite similar behavior could be predicted also for the heteroanalogous thiophenols and selenophenols. Quite generally considerable changes in the electron distribution of the groundstate molecules with the twisting angle of the -OH, -SH and -SeH groups could be calculated, with the greatest differences between "parallel" and "perpendicular" conformations. On the assumption that the fast electron-transfer projects the equilibrium solute conformer distribution onto the solute cation conformer one it is demonstrated that the experimental findings are compatible with a simple solute-cation internal relaxation model. By applying the quantum-chemically calculated conformer interconversion barrier heights it can be understood that radical fraction among the direct products increases if phenols are substituted by thiophenols or those by selenophenols, as observed in experiment.

Electron-transfer reactions from phenols to parent radical cations of solvents were studied using pulse radiolysis. Phenols bearing electron-withdrawing, electron-donating and bulky substituents were investigated in non-polar solvents such as cyclohexane, n-dodecane, n-butyl chloride and 1,2-dichloroethane. The experiments revealed the direct, synchronous formation of phenoxyl radicals and phenol radical cations in all cases and in nearly the same relative amounts. This was explained by two competing electron-transfer channels which depend on the geometry of encounter between the parent solvent radical cations and the solute phenol molecules. The mechanism is analysed at a microscopic level, treating di †usion as a slow process and the local electron transfer as an extremely rapid event. Furthermore, the e †ect of various phenol substituents and solvent types on the electron-transfer mechanism and on the decay kinetics of the solute phenol radical cations was analysed. The results were further substantiated using a quantum chemical approach.

No abstract

The two sp(3) hybridized fluorine atoms of a Bodipy dye have been synthetically replaced with the linear donor ligand 4-ethynylpyridine (-C≡C-Py) to form a rigid and highly symmetrical 109.5° building block in which the fluorophore subunit is vertically aligned to the plane formed by the -C≡C-Py donors. Upon reaction of the above tecton with a 90° organoplatinum acceptor unit, an intensely fluorescent rhomboid cavitand was manifested in solution. In contrast to the vast majority of coordination-driven self-assembled chromophoric systems, the present one fully conserves the excellent photophysical properties of the parent Bodipy dye. These unique features of the present metallosupramolecular entity constitute a fascinating metal-to-ligand self-assembled prototype for building compact and intensely luminescent materials with host-guest capabilities.

Pulse radiolysis of naphthols (NpOH) and hydroxybiphenyls (ByOH) in n-butyl chloride (BuCl) at room temperature exhibits electron transfer at a bimolecular rate constant of (1.0−2.8) × 1010 dm3 mol-1 s-1. The experiments reveal the direct formation of two types of transients:  phenol type radical cations (NpOH•+, ByOH•+) and phenoxyl type radicals (NpO•, ByO•). This is explained by a mechanism involving two different electron-transfer channels. The solute radical cations exhibit two optical absorption bands in the 570−650 and 360−460 nm regions and undergo electron transfer with triethylamine and proton transfer with ethanol with bimolecular rate constants of (4−12) × 109 and (3−6) × 108 dm3 mol-1 s-1, respectively. NpO• and ByO• have relatively long lifetimes and show absorption bands in the 340−400 and 470−540 nm regions. By way of comparison, these phenoxyl type radicals are separately generated by pulse radiolysis in aqueous alkaline solution containing sodium azide, i.e., by oxidation of the solutes with N3 • radicals. Under these conditions, the phenoxyl radicals decay by second-order kinetics with 2k = (1.2−4.5) × 108 dm3 mol-1 s-1. The various modes of formation and decay of the phenolic radical cations are analyzed over a wide range of dose rate and solute concentrations. In comparison to radical cations of one-ring phenols, the increased stability of NpOH•+ and ByOH•+ is explained by the delocalization of the positive charge over the whole aromatic system, a postulate supported by open-shell quantum chemical calulations.

The electron transfer from aniline and its N-methyl as well as N-phenyl substituted derivatives (N-methylaniline, N,N-dimethylaniline, diphenylamine, triphenylamine) to parent solvent radical cations was studied by electron pulse radiolysis in n-butyl chloride solution. The ionization results in the case of aniline (ArNH2) and the secondary aromatic amines (Ar2NH, Ar(Me)NH) in the synchronous and direct formation of amine radical cations, as well as aminyl radicals, in comparable amounts. Subsequently, ArNH2*+ deprotonates in a delayed reaction with the present nucleophile Cl-, and forms further ArNH*. In contrast, tertiary aromatic amines such as triphenylamine and dimethylaniline yield primarily the corresponding amine radical cations Ar3N*+ or Ar(Me2)N*+, only. The persistent Ar3N*+ forms a charge transfer complex (dimer) with the parent amine molecule, whereas Ar(Me2)N*+ deprotonates to carbon-centered radicals Ar(Me)NCH2*.