A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

Abstract

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.

Electronic transient spectroscopy from the deep UV to the NIR: unambiguous disentanglement of complex processes

“…In addition, we determined the position of the maximum of the radical and cation absorption for each delay. 45,46 The resulting temporal dependence of the peak position shows the already mentioned ps relaxation due to solvation and planarization and also the oscillations. This conrms the interpretation as a vibronic wavepacket.…”

“…What our setup also provides is sufficient spectral reproducibility and resolution to allow the observation of shis as small as 0.1 nm or 10 cm À1 . These can be identied with solvation, ion or radical pairing, 46 and cooling. The temporal evolution of the shis gives extra information on the dynamics.…”

Wavepacket Splitting in the First 100 fs Determines the Products from the Bond Cleavage of Diphenylmethylchloride

“…The transmission changes of the probe pulses can monitor the appearance and evolution of the radical and cation product, respectively, since they have their strongest absorption bands at these wavelengths. This has been identified by broadband transient absorption measurements [3,4,5]. To avoid any pulse broadening and coherent artifacts due to cell windows, we perform the pump-probe experiment in a 50 µm free-flowing jet.…”

“…These two changes can be modeled with the identical stretched exponential function with τ SE = 11.1 ps. The changeover is due to electron transfer within the radical pair that leads to the ion pair [5,6]. The electron transfer is fastest for the pairs with the closest distance and eventually stops because the near neighbors are used up and the other pairs diffuse apart.…”

Ground and Excited State Surfaces for the Photochemical Bond Cleavage in Phenylmethylphenylphosphonium Ions

“…The two lowest excited singlet states in the model system correspond to local π−π* transitions. Compared with other well-studied benzhydryl derivatives, 3,8,[10][11][12][13]19,22,24 the phosphonium leaving group offers a second chromophore and enhances the complexity of the excitation mechanism. Our calculations show that the excitation on the PH 2 Ph + moiety leads to the S 1 state, and the excitation on the PhCH 2 moiety leads to the S 2 state.…”