Ultrafast dynamics of molecular chromophores in solution Julia Weinstein 1 , T. Cheng 1 , G. Wu 1 , C. Royle 1 , A. Auty 1 , J. Shipp 1 , M.V. Appleby 1 , R. Cowin 1 , I. Ivalo 1 , R. Fernandez-Teran 1 , A.J.H.M. Meijer 1 , I. Sazanovich 2 , M. Towrie 2 , G. Greetham 2 1 University of Sheffield, UK, 2 Rutherford Appleton Laboratory, UK One of the fascinating challenges in the field of photoinduced charge separation 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 {UV pump -“narrow”IR pump-IR probe}, thereafter “IR-control”. Despite a plethora of studies on IR-driven photochemistry in gas phase,[3] it has been assumed that IR- perturbation in solution would not be possible due to ultrafast vibrational energy dissipation. Our work on IR- perturbation of electron transfer explores Pt(II) complexes as chromophores and/or bridges in the Donor-Bridge- Acceptor (DBA) systems.If excited state relaxation occurs via branching charge-transfer excited state, it becomes possible for aselective excitation of bridge vibrations to drasticallychange the yield of the product states, up to 100%.[2,4] Here, recent work on novel DBA complexes, in various solvents, have been characterised by ultrafast transient absorption, time-resolved IR, 2DIR, and fluorescence upconversion[6] spectroscopies. In cases when a branching excited state was detected, IR-control studies were undertaken, to discover the role of several factors in the IR- control effect: (i) Strong- vs. weak coupling regimes between Donor/Bridge and Bridge/Acceptor. (ii) Investigation of the mechanism of the effect using isotopic substitution of the bridge. (iii) Tuning the energetics of the different steps in charge separation/recombination process. Excitingly, the “vibrational control” effect potentially offers the means to direct electron flow along a selected reaction pathway. The potential mechanisms of this effect and the role of the environment will be discussed. References 1. Z. Ma et al, Chem. Sci. 2018, 6395.
2. Delor, M. et al, Science, 2014, 346, 1492. 3. Crim, F. F. Nature Chem., 2011,3, 344. 4. Delor, M. et al, Nature Chem., 2015, 7, 689. 5. Delor, M. et al, Nature Chem., 2017, 9, 1099. 6. Farrow, G. et al, PCCP 2021, 21652.
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