An intriguing problem in photo-induced processes is how to control reaction pathways by external perturbation. Selective IR-excitation of vibrations directly coupled to charge transfer process on the ultrafast timescale can potentially be used as the means to change the rates, and the yields, of individual competing channels in the excited state. Our work on IR-perturbation of photoinduced charge separation explores Pt(II) complexes as bridges or chromophores in molecular cascades for photoinduced electron transfer, Donor-Bridge-Acceptor (DBA). In the linear systems D-B-A, selective excitation of bridge vibrations in the excited state was shown to drastically change the yield of the product states, up to 100%. In the fork-type system D-B-A-B-D, which have competing electron transfer pathways differing only by 13C-labelling of one bridge, selective IR-excitation of either bridge affects the yield of charge-separation along both ‘arms’. The effect of mode-specific IR-excitation of branching state on the outcome of the process in solution means that the mechanism(s) underpinning this effect outcompete various vibrational relaxation pathways. Or, that vibrational relaxation leads to the population of a mode directly coupled to the reaction coordinate. Or perhaps both. In order to understand the mechanism of the effect, we explore: (i) variation of the structure of the bridge to move from strong- to weak-coupling regime. The mode of coupling – unsurprisingly – dramatically changes how the system behaves, and the efficiency of the IR-control effect.; (ii) assymetric isotopic substitution of one of the pathways (-13C≡13C- vs. -12C≡12C); (iii) the relative energetics of competing pathways; (iv) structural reorganisation induced by IR-excitation. To test each of the factors, several model series of D-B-A complexes, all with the Pt-acetylide core but different in one or several of the parameters (i-iv) above were made, and investigated by ultrafast transient absoprtion, transeint infrared, and T-2DIR spectroscopies, and in the UV(pump)-IR(pump)-IR(probe) 3-pulse experiments. The transient IR-excitation effect potentially offers the means to direct electron flow along a pre-selected reaction pathway. The suggested mechanisms and potential applications of the fundamental effect observed will be discussed.

Changing excited state dynamics in donor-bridge-acceptor systems with mode-specific infrared excitation: What's the mechanism?

ACS Fall 2021

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