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.

Towards Water Soluble Mitochondria-Targeting Theranostic Osmium(II) Triazole-Based Complexes

“…The yield of singlet oxygen generation, φ ( 1 O2), of complex 1 Cl was measured in air-equilibrated acetonitrile against the standard perinaphthenone. A φ ( 1 O2) of 57% was determined by direct measurement of the 1 Δg state emission from 1 O2 in the NIR (λem 1275 nm) under 355 nm irradiation by a pulsed Nd:YAG laser as described previously [50]. It is proposed that apoptotic cell death after light treatment is associated with localisation of photosensitizers to mitochondria [51].…”

Directing the path of light-induced electron transfer at a molecular fork using vibrational excitation

“…One effect is the nondiscriminative overall acceleration of ET that could be similarly achieved using shorter-wavelengths electronic excitation, as observed in other Pt-acetylide complexes. 36 The second outcome is a spatial pathway-specific perturbation that has never been achieved previously. This latter effect causes an efficient and selective deceleration of the pumped pathway: when ( 13 C) is pumped, formation of the 12 CSS becomes the preferred pathway and vice versa.…”

“…Another important component in our system is the metal centre which acts as a vibrational energy transfer bottleneck that contributes to the localization of injected quanta of energy on either arm of the molecule. [38][39][40][41][42] Furthermore, the presence of the metal offering a two-step charge-separation process starting from a metal-to-acceptor charge transfer followed by reductive quenching from the donor is a particularly effective design strategy to achieve IR-control: 15,16,35,36 it allows the rapid population of a far-fromequilibrium gateway state where an IR pump can be introduced, and in this way perturb an otherwise symmetrical system in a spatially selective manner at a crucial crossroad where small structural and energetic fluctuations dictate the chosen electronic pathway. 43 Complementary to such excited state evolution is the ability to delay the control pulse with respect to actinic excitation, thus influencing reactivity at the decisive moments of light-induced molecular function [44][45][46][47] and circumventing the myriad of processes occurring immediately after electronic excitation, which would rapidly scramble the selectivity of mode-specific excitation.…”

“…lifetimes of 3 MLCT* compared to 3 MLCT), similarly to a range of other Pt-acetylide charge transfer systems in solution. 35,36 Taking into account this rate acceleration, we then proceed to fit the off-diagonal kinetic response from Figure 3c. The two free parameters to optimize are the equilibration rate between the two CSSs and the relative rate difference, if any, between the two charge-separation pathways.…”

Ultrafast charge transfer dynamics in supramolecular Pt(ii) donor–bridge–acceptor assemblies: the effect of vibronic coupling

“…The position and profile of the 1895 cm $1 band also closely resemble bridge$to$ acceptor charge transfer in NAP$containing systems, indicating a similar electron density environment experienced by the bridge. 39 We thus assign this signal to a bridge$to$NDI charge transfer (CT), where electron density is shifted from the Ph$C≡C$Pt$C≡C bridge to the NDI acceptor.…”