Ralf Hermann

Free electron transfer from several phenols to radical cations of non-polar solvents

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 diffusion as a slow process and the local electron transfer as an extremely rapid event. Furthermore, the effect 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.

Novel bacteriochlorine for high tissue-penetration: photodynamic properties in human biliary tract cancer cells in vitro and in a mouse tumour model.

Photodynamic therapy of bile duct cancer using hematoporphyrin derivative (HPD) and laser light of 630 nm wavelength is confined to a tumouricidal tissue penetration of 4 mm, which might be doubled with laser light between 700 and 800 nm. Therefore, we investigated the photosensitising properties of a novel bacteriochlorine, tetrakis-pyridyl-tetrahydroporphyrin tosylat (THP) with high absorption at 763 nm. Two biliary cancer cell lines (BDC, GBC) were incubated with HPD or THP to assess cellular uptake kinetics, dark cytotoxicity, and photodynamic cytotoxicity (laser light exposure 1-20 J/cm2). Tumours grown from BDC cells in subcutaneous tissue of severe combined immunodeficient mice were treated with laser light of 30 J/cm2 after injection of THP. The concentrations that killed 50% of cells in the dark were 680 microg/ml of HPD, but > 6400 microg/ml of THP in BDC cells, and 220 microg/ml of HPD, but 6400 microg/ml of THP in GBC cells. Both cell lines exhibited uptake and retention of THP and photodynamic cytotoxicity (up to 86% cells killed). THP induced tumour-selective phototoxicity in the cholangiocarcinoma model. The novel bacteriochlorine THP exhibits photosensitiser properties in biliary tract cancer cells in vitro and in vivo and could achieve deep tumouricidal tissue penetration due to photoactivation at 763 nm.

Thiol radical cations and thiyl radicals as direct products of the free electron transfer from aromatic thiols to n-butyl chloride radical cations

Radical and ionic reactions were observed in the pulse radiolysis of thiophenols (ArSH=thiophenol, o-, m- and p-thiocresol or 2-thionaphthol) in n-butyl chloride solution. The main source of aromatic thiyl radicals is the reaction of butyl radicals with the thiols, which proceeds at 1.0–5.6×108 dm3 mol−1 s−1. This radical generation path is completely quenched in the presence of oxygen. Under these conditions, only the electron transfer reaction between n-butyl chloride parent ions and the thiophenols remains and could be well analyzed. It takes place at a rate constant of about 1.5×1010 dm3 mol−1 s−1 and takes two parallel paths–common electron transfer yielding thiophenol radical cations and a more complex ionic reaction resulting directly in thiyl radicals. The latter is thought to proceed via an encounter complex geometry, ArSH···ClBu•+, in which electron transfer is directly followed by immediate deprotonation. The thiyl radicals and the thiol radical cations are characterized by their optical absorption spectra and their kinetic properties. Quantum chemical calculations underpin our mechanistic interpretation and provide information about the charge distribution and reactivity of the thiol radical cations.

Picosecond fluorescence of nucleic acid bases

Abstract After excitation with UV femtosecond laser pulses, the dynamics of the first excited singlet states of nucleic acid bases in polar solutions were directly determined by fluorescence detection with a sufficient time resolution using a sensitive streak camera. The single-shot detection followed by a numerical synchronization allowed the study of emission lifetimes of singlet states, even with low quantum yields of the order of 10−4. The measured fluorescence decays in a few picoseconds. The typical values for the S1-state lifetimes in water are between 6 and 9 ps for the purine bases and between 1 and 2 ps for the pyrimidines. In the nucleosides and nucleotides of purine bases, the lifetimes decrease towards 1 ps. Studying less polar solvents, such as ethanol and acetonitrile, especially for adenine dissolved in ethanol, a remarkable lifetime of 16 ps was found.

Stability of phenol and thiophenol radical cations – interpretation by comparative quantum chemical approaches

Abstract The deprotonation kinetics of phenol-type radical cations, formed via a very efficient electron transfer in the pulse radiolysis of non-polar solutions, for example n -chlorobutane, is governed mainly by electronic effects due to the nature of the phenol substituents, whereas steric effects are of minor importance; thiophenols, which are sulphur analogues of phenols, exhibit a similar behavior. Comparative quantum chemical calculations show that the calculated spin densities at the hetero atoms correlate well with the experimentally determined radical cation lifetimes. Not only the Density Functional Theory (DTF) B3LYP but also the semiempirical quantum chemical model PM3 can be applied for the open shell systems mentioned.

Diphenol radical cations and semiquinone radicals as direct products of the free electron transfer from catechol, resorcinol and hydroquinone to parent solvent radical cations

In the pulse radiolysis of solutions of catechol, resorcinol and hydroquinone in n-butylchloride, dihydroxybenzene radical cations (40%) as well as semiquinone radicals (60%) are observed as direct synchronously formed products of the electron transfer from the solvent parent ions to the solute. This is explained in terms of free electron transfer succeeding in nearly every encounter of the reactants, which in the case of the studied dihydroxybenzenes involves femtosecond molecular dynamics effects. The rotation of the C–OH bond causes cycling of the molecule through transient conformations also exhibiting different electron distributions. From the more chemical point of view, the diphenol radical cations represent the first and till now unknown intermediates of oxidative semiquinone radical formation.

Nucleophilic effects on the deprotonation of phenol radical cations

Considering dependence on their electronic structure, in non-polar surroundings phenol radical cations exhibit lifetimes between 100 and 500 ns and decay by deprotonation forming phenoxyl radicals. Adding various quenchers of different nucleophilicity, we determined a set of rate constants for the acceleration of deprotonation i.e. the reaction of phenol radical cations with the nucleophiles. The data are basically discussed in terms of Reichardts polarity scale, also considering effects like competing electron transfer aided by comparing the differences of calculated adiabatic ionization potentials as well as spin densities and local charges on heteroatoms for phenols and the quenchers.

Encounter geometry determines product characteristics of electron transfer from 4-hydroxythiophenol to n-butyl chloride radical cations

Abstract The electron transfer reaction between the n -butyl chloride parent ion and 4-thiophenol was studied using pulse radiolysis in solutions of 4-thiophenol in n -butyl chloride. It was found to have a diffusion-controlled rate constant of 1.5×10 10 dm 3 mol −1 s −1 and to involve contributions from all functional groups, i.e. –SH, –OH and the aromatic ring. Consequently, thiyl and phenoxyl radicals and 4-hydroxythiophenol radical cations were observed as direct products of this ion–molecule reaction. This unexpected reaction behavior could be explained by the hypothesis that the encounter geometry of the reaction partners determines the product characteristics.

The photoisomerization of spiro[cyclohexadiene-indoline] via an intramolecular charge transfer state

Abstract The coloration of spiroindolines in solution under UV excitation is an indication of photochromic behavior. The occurrence of dual fluorescence is due to two different photoisomers — the spiro form and the merocyanine form of the molecule. Stokes shifts of the spiroindoline fluorescence up to 8000 cm −1 show the charge transfer character of the excited singlet state, where relaxation leads either to a charge separation between the indoline and the cyclohexadiene moiety (emitting state of the spiro compound, S CT * ) or the C–C bond cleavage in the cyclohexadiene (precursor of the merocyanine formation, X CT * ). Fluorescence and photoisomerization are observed in time-resolved measurements on a picosecond time scale. The charge separation is stabilized in acetonitrile, where the longest lifetime for the S CT * state (580 ps) and the lowest yield of merocyanine was observed. In nonpolar cyclohexane the fluorescence decay and the formation of merocyanine proceed within 10 ps.

Deactivation of the first excited singlet state of thiophenols

On the bases of picosecond and nanosecond laser flash photolysis with detection by emission and absorption spectroscopy, a quantitative description is given of all deactivation channels of the first excited singlet state of thiophenols ArSH(S(1)) such as fluorescence, intersystem crossing (ISC), chemical dissociation into radicals, and radiation-less internal conversion (IC). For this purpose, the photolysis of thiophenol and its methyl-, methoxy-, and chloro-substituted derivatives was studied in solvents of increasing polarity: 1-chlorobutane, ethanol, and acetonitrile. The fluorescence lifetime of the thiophenols was found to range from some hundreds of picoseconds up to a few nanoseconds, correlating with fluorescence quantum yields between 0.001-0.040, at room temperature. Depending on the substitution pattern of the aromatic ring, the quantum yield of the S-H bond dissociation was found to be between 0.3-0.5, irrespective of the solvent polarity. In laser photolysis, no triplet formation of the investigated compounds could be observed neither by the direct way nor by subsequent sensitization with beta-carotene. As a difference to the total, the radiation-less internal conversion (Phi(IC)>or= 0.5) was found to be the dominating process.