title: Electron transfer dynamics and excited state branching in a charge-transfer platinum(II) donor-bridge-acceptor assembly.
abstract: A linear asymmetric Pt(ii) trans-acetylide donor-bridge-acceptor triad designed for efficient charge separation, NAP[triple bond, length as m-dash]Pt(PBu3)2[triple bond, length as m-dash]Ph-CH2-PTZ (), containing strong electron acceptor and donor groups, 4-ethynyl-N-octyl-1,8-naphthalimide (NAP) and phenothiazine (PTZ) respectively, has been synthesised and its photoinduced charge transfer processes characterised in detail. Excitation with 400 nm, ∼50 fs laser pulse initially populates a charge transfer manifold stemming from electron transfer from the Pt-acetylide centre to the NAP acceptor and triggers a cascade of charge and energy transfer events. A combination of ultrafast time-resolved infrared (TRIR) and transient absorption (TA) spectroscopies, supported by UV-Vis/IR spectroelectrochemistry, emission spectroscopy and DFT calculations reveals a self-consistent photophysical picture of the excited state evolution from femto- to milliseconds. The characteristic features of the NAP-anion and PTZ-cation are clearly observed in both the TRIR and TA spectra, confirming the occurrence of electron transfer and allowing the rate constants of individual ET-steps to be obtained. Intriguingly, has three separate ultrafast electron transfer pathways from a non-thermalised charge transfer manifold directly observed by TRIR on timescales ranging from 0.2 to 14 ps: charge recombination to form either the intraligand triplet (3)NAP with 57% yield, or the ground state, and forward electron transfer to form the full charge-separated state (3)CSS ((3)[PTZ(+)-NAP(-)]) with 10% yield as determined by target analysis. The (3)CSS decays by charge-recombination to the ground state with ∼1 ns lifetime. The lowest excited state is (3)NAP, which possesses a long lifetime of 190 μs and efficiently sensitises singlet oxygen. Overall, molecular donor-bridge-acceptor triad demonstrates excited state branching over 3 different pathways, including formation of a long-distant (18 Å) full charge-separated excited state from a directly observed vibrationally hot precursor state.
authors.affiliation: Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK. Julia.Weinstein@sheffield.ac.uk A.Meijer@sheffield.ac.uk.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Scattergood
authors.forename: Milan
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authors.lastname: Delor
authors.forename: Igor V
authors.initials: IV
authors.lastname: Sazanovich
authors.forename: Oleg V
authors.initials: OV
authors.lastname: Bouganov
authors.forename: Sergei A
authors.initials: SA
authors.lastname: Tikhomirov
authors.forename: Alexander S
authors.initials: AS
authors.lastname: Stasheuski
authors.forename: Anthony W
authors.initials: AW
authors.lastname: Parker
authors.forename: Gregory M
authors.initials: GM
authors.lastname: Greetham
authors.forename: Michael
authors.initials: M
authors.lastname: Towrie
authors.forename: E Stephen
authors.initials: ES
authors.lastname: Davies
authors.forename: Anthony J H M
authors.initials: AJ
authors.lastname: Meijer
authors.forename: Julia A
authors.initials: JA
authors.lastname: Weinstein