Photogeneration and reactions of benzhydryl cations and radicals: A complex sequence of mechanisms from femtoseconds to microseconds

Abstract

Benzhydryl radicals and cations are reactive intermediates central to the understanding of organic reactivity. They can be generated from benzhydryl halides by UV irradiation. We performed transient absorption (TA) measurements over the range from femtoseconds to microseconds to unravel the complete reaction scheme. The 290-720-nm probe range allows the unambiguous monitoring of all fragments. The appearance of the radical is delayed to the optical excitation, the onset of the cation signal is found even later. Ab initio calculations show that this non-rate behavior in the 100 fs range is due to wavepacket motion from the Franck-Condon region to two distinct conical intersections. The rise of the optical signal with a quasi-exponential time of 300 fs is assigned to the planarization and solvation of the photoproducts. The bond cleavage predominantly generates radical pairs. A subsequent electron transfer (ET) transforms radical pairs into ion pairs. Due to the broad interradical distance distribution and the distance dependence, the ET is strongly non-exponential. Part of the ion pairs recombine geminately. The ET and the recombination are terminated by the depletion of close pairs and diffusional separation. The remaining free radicals and cations undergo further reactions in the nanosecond to microsecond regime.Benzhydryl cation and radical photogeneration 1489 Fig. 1 (a) Transient spectrum at a pump-probe delay of 40 ps and (b) false color representation of the 2D TA data matrix of tol(Ph)CHCl in CH 3 CN after 270 nm UV excitation. The delay time axis is linear between -1 and 1 ps and logarithmic beyond 1 ps. (c) Scheme of the ultrafast bond cleavage.

Tuning the Ground and Excited State Dynamics of Hemithioindigo Molecular Motors by Changing Substituents

“…All measurements were performed in CH 2 Cl 2 solution at 22 °C with 35 fs pump pulses at about 420 nm. The data evaluation was done by kinetic analysis using multi‐exponential fitting in a selected wavelength range or globally and a thorough spectral analysis. For details and a discussion of the setup, experimental conditions, and the evaluation procedures refer to the Supporting Information section 2 or reference .…”

Complete Mechanism of Hemithioindigo Motor Rotation

“…For the slower and/or thermal induced dynamics, the same sample is pumped with the ns-pulse and probed with the same supercontinuum. Delays up to 700 μs with an instrumental response function of 2.5 ns are possible. , This time range is further extended by chopping down the ns-laser to a repetition rate of 50 Hz and then probing the same excited volume multiple times. Delays of up to 18 ms are achieved with this setup.…”