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.
Highlights
Photoinduced electron transfer, an elementary process, underpins a variety of applications, especially in light-harvesting, photocatalysis, and optoelectronics
The initial excitation of 1 with visible light leads to the formation of a manifold of charge-transfer states where electron density shifts from the Pt-acetylide bridge to the NAP acceptor
This charge-transfer manifold undergoes excited state branching on the ultrafast (0.2–14 ps) timescale over three pathways to form a long-lived (190 μs) acceptor-localized triplet state, to recombine back to the ground state, and to engage in forward electron transfer with the formation of the full charge-separated state
Summary
Photoinduced electron transfer, an elementary process, underpins a variety of applications, especially in light-harvesting, photocatalysis, and optoelectronics. The use of Pt(II) containing bridging units is attractive for ultrafast ET investigations owing to the square planar coordination environment around the metal centre which provides synthetic versatility for the specific attachment of ligands, and enables control over the directionality of electron transfer.[14,15,16] The utilisation of donor acetylide ligands in particular facilitates the formation of more stable, longer-lived charge-separated states due to strong-field acetylides raising the energy of a deactivating dd-state, and led to a large family of cis bis-acetylide Pt(II) complexes employing redox-active diimine acceptor ligands.[16,17,18,19,20,21]. The combination of vibrational spectroscopy with timeresolved electronic transient absorption spectroscopy (TA) along with UV/Vis/IR (spectro)electrochemical and computational methods resolves the rich photophysics of the chargetransfer assemblies, involving multiple excited states evolving across the femto-to-millisecond time range
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