No abstract

UV irradiation (266 or 280 nm) of benzhydryl triarylphosphonium salts Ar(2)CH-PAr(3)(+)X(-) yields benzhydryl cations Ar(2)CH(+) and/or benzhydryl radicals Ar(2)CH(•). The efficiency and mechanism of the photo-cleavage were studied by nanosecond laser flash photolysis and by ultrafast spectroscopy with a state-of-the-art femtosecond transient spectrometer. The influences of the photo-electrofuge (Ar(2)CH(+)), the photo-nucleofuge (PPh(3) or P(p-Cl-C(6)H(4))(3)), the counterion (X(-) = BF(4)(-), SbF(6)(-), Cl(-), or Br(-)), and the solvent (CH(2)Cl(2) or CH(3)CN) were investigated. Photogeneration of carbocations from Ar(2)CH-PAr(3)(+)BF(4)(-) or -SbF(6)(-) is considerably more efficient than from typical neutral precursors (e.g., benzhydryl chlorides or bromides). The photochemistry of phosphonium salts is controlled by the degree of ion pairing, which depends on the solvent and the concentration of the phosphonium salts. High yields of carbocations are obtained by photolyses of phosphonium salts with complex counterions (X(-) = BF(4)(-) or SbF(6)(-)), while photolyses of phosphonium halides Ar(2)CH-PPh(3)(+)X(-) (X(-) = Cl(-) or Br(-)) in CH(2)Cl(2) yield benzhydryl radicals Ar(2)CH(•) due to photo-electron transfer in the excited phosphonium halide ion pair. At low concentrations in CH(3)CN, the precursor salts are mostly unpaired, and the photo-cleavage mechanism is independent of the nature of the counter-anions. Dichloromethane is better suited for generating the more reactive benzhydryl cations than the more polar and more nucleophilic solvents CH(3)CN or CF(3)CH(2)OH. Efficient photo-generation of the most reactive benzhydryl cations (3,5-F(2)-C(6)H(3))(2)CH(+) and (4-(CF(3))-C(6)H(4))(2)CH(+) was only achieved using the photo-leaving group P(p-Cl-C(6)H(4))(3) and the counter-anion SbF(6)(-) in CH(2)Cl(2). The lifetimes of the photogenerated benzhydryl cations depend greatly on the decay mechanisms, which can be reactions with the solvent, with the photo-leaving group PAr(3), or with the counter-anion X(-) of the precursor salt. However, the nature of the photo-leaving group and the counterion of the precursor phosphonium salt do not affect the rates of the reactions of the obtained benzhydryl cations toward added nucleophiles. The method presented in this work allows us to generate a wide range of donor- and acceptor-substituted benzhydryl cations Ar(2)CH(+) for the purpose of studying their electrophilic reactivities.

Bond cleavage and bond formation are central to organic chemistry. Carbocations play a key role in our understanding of nucleophilic substitution reactions that involve both processes. The precise understanding of the mechanism and dynamics of the photogeneration of carbocations and carbon radicals is therefore an important quest. In particular, the role of electron transfer for the generation of carbocations from the radical pair is still unclear. A quantitative femtosecond absorption study is presented, with ultrabroad probing on selected donor and acceptor substituted benzhydryl chlorides irradiated with 270 nm (35 fs) pulses. The ultrafast bond cleavage within 300 fs is almost exclusively homolytic, thus leading to a radical pair. The carbocations observable in the nanosecond regime are generated from these radicals by electron transfer from the benzhydryl to the chlorine radical within the first tens of picoseconds. Their concentration is reduced by geminate recombination within hundreds of picoseconds. In moderately polar solvents this depletion almost extinguishes the cation population; in highly polar solvents free ions are still observable on the nanosecond timescale. The explanation of the experimental findings requires the microscopic realm of the intermediates to be accounted for, including their spatial and environmental distributions. The distance dependent electron transfer described by Marcus theory is combined with Smoluchowski diffusion. The depletion of the radical pair distribution at small distances causes a temporal increase of the mean distance and the observed stretched exponential electron transfer. A close accord with experiment can only be reached for a broad distribution of the nascent radical pairs. The increase in the inter-radical and inter-ion pair distance is measured directly as a shift of the UV/Vis absorption of the products. The results demonstrate that, at least for aprotic solvents, traditional descriptions of reaction mechanisms based on the concept of contact and solvent-separated pairs have to be reassessed.

Complex multi-stage relaxation and reaction pathways after the optical excitation of molecules makes the disentanglement of the underlying mechanisms challenging. We present four examples that a new transient spectrometer with excitation fully tunable from the deep UV to the IR and 225 to 1700 nm probing allows for an analysis with greatly reduced ambiguity. The temporal resolution of about 50 fs allows us to resolve all relevant processes. For each example there is a new twist in the sequence of relaxation steps that had previously been overlooked. In malachite green it appears that the importance of the phenyl twisting has been overemphasized and rather a charge transfer state should be considered. In TINUVIN-P the predicted twisting as the driving motion for the ultrafast IC is confirmed and leads to a resolution of the earlier puzzle that the sub-5 ps regime shows kinetics deviating from a pure cooling process despite the sub-ps proton transfer cycle. For the bond cleavage of Ph 2 CH-Cl and Ph 2 CH-Br the degree of electron transfer within the radical pair can now be determined quantitatively and leads to a profound understanding of the long term cation yield. For the first time coherent wavepacket motion in the photoproducts is reported. Last but not least the measurement of the GSB recovery in the deep UV allows for the surprising result, that even after S 2 excitation of cyclopentenones the triplet states are reached with near unity probability within a few picoseconds.

The identification of the transition state or a short-lived intermediate of a chemical reaction is essential for the understanding of the mechanism. For a direct identification typically transient optical spectroscopy is used, preferentially with high temporal resolution. We combine broad-band femtosecond transient absorption measurements and on-the-fly molecular dynamics calculations to decipher the microscopic evolution of the geometry and solvation of photogenerated benzhydryl cations (Ar(2)CH(+), Ar = phenyl, p-tolyl, m-fluorophenyl, or m,m'-difluorophenyl) in bulk solution. From the high level quantum chemical calculations on the microsolvated cation we can deduce a narrowing and blue shift of the cation absorption that is nearly quantitatively equal to the experimental finding. The roughly 300 fs initial increase in the absorption signal found for all investigated combinations of benzhydryl chlorides or phosphonium salts as benzhydryl cation precursors and solvents is therefore assigned to the planarization and solvation of the nascent fragment of the bond cleavage. The actual cleavage time cannot directly be deduced from the rise of the spectroscopic signal. For alcohols as solvent, the cation combines on the picosecond time scale either with one of the solvent molecules to the ether or to a lesser degree geminately with the leaving group. The study shows that the absorption signal attributable to a species like the benzhydryl cation does not mirror the concentration during the first instances of the process. Rather, the signal is determined by the geometrical relaxation of the photoproduct and the response of the solvent.