Speaker: Julia Weinstein, The University of Sheffield
Program Description:
Milan Delor,1 Paul Scattergood,1 Tao Cheng,1 Stuart Archer,1 Theo Keane,1 Guanzhi Wu,1
Igor Sazanovich,2 George Farrow,1 Alexander Auty,1 Dimitri Chekulaev,1 Peter Portius,1 Gregory M. Greetham,2 Anthony W. Parker,2 Anthony J. H. M. Meijer,1 Mike Towrie,2 Julia A. Weinstein1
1Department of Chemistry, The University of Sheffield, S3 7HF, U.K.
2Laser for Science Facility, Research Complex at Harwell, STFC, OX11 0QX, U.K.
One of the major challenges of the fascinating field of photoinduced charge separation - a fundamental process which lies at the heart of reactions in natural and artificial systems powered by the energy of light – is how to control reaction pathways, and direct reactivity “at will”.
Nuclear-electronic (vibronic) coupling is of particular interest in this regard since the Born-Oppenheimer approximation is not valid on the ultrafast timescales intrinsic to photo-processes. Perturbing vibronic coupling may thus offer a way to affect photochemical reactions.[1-2] Such perturbation can be achieved by introducing a narrow-band IR pulse after initial population of an excited state to selectively affect vibration(s) that are coupled to electron transfer processes; the overall sequence of ultrafast pulses used is {UVpump- narrowband IR pump - broadband IRprobe}.
Fig 1. Modulation of electron transfer in a linear D-B-A system (Left), and in a “fork” system which has two electronically identical, but vibrationally distinct pathways.
The presentation will focus on recent work on IR-perturbation of photoinduced charge separation in transition metal Donor-Bridge-Acceptor complexes. In the first type of systems, D-B-A, (Fig. 1, left), selective excitation of bridge-localised vibrational modes in the excited state was shown to drastically change the yield of the product states, up to 100%.[3-4] In the second, fork-type, system D-B-A-B-D (Fig. 1, right), which have competing electron transfer pathways differing only by isotopic labelling of the bridge, 13C vs. 12C, selective IR-excitation of either bridge affects the yield of charge-separation along both ‘arms’.[5] This effect potentially offers the means to direct electron flow along a pre-selected reaction pathway. The effect of the lifetime of the branching state, the driving force for various processes involved,[6] and the strong vs. weak- coupling regimes on the IR-control efficiency will be considered. The suggested mechanisms – derived from ultrafast TRIR, TA, Fluorescence upconversion, and FSRS experimental methods, as well as quantum-chemical calculations - and potential applications of the fundamental effects observed will be discussed in the broad context of the state-of-the-art in the field.
Finally, some recent results on ultrafast X-ray studies of copper complexes will also be presented.
[1] Z Lin, et al J. Am. Chem. Soc., 2009, 131, 18060; [2] Y. Yue, et al, Dalton Trans., 2015, 47, 8609.
[3] M. Delor, et al Science, 2014, 346, 1492. [4] M. Delor, et al, Nature Chem., 2015, 7, 689.
[5] M. Delor, et al, Nature Chem., 2017, 9, 1099.
[6] A. Auty, et al, Chemical Science, 2023, 14, 11417.
