• 13C or Not 13C: selective synthesis of asymmetric carbon-13-labeled platinum(II) cis-acetylides.

      Archer, Stuart A.; Keane, Theo; Delor, Milan; Meijer, Anthony J. H. M.; Weinstein, Julia A.; University of Sheffield; Department of Chemistry, University of Sheffield, Sheffield S3 7HF, United Kingdom; Department of Chemistry, University of Sheffield, Sheffield S3 7HF, United Kingdom; Department of Chemistry, University of Sheffield, Sheffield S3 7HF, United Kingdom; Department of Chemistry, University of Sheffield, Sheffield S3 7HF, United Kingdom; et al. (American Chemical Society, 2016-08-09)
      Asymmetric isotopic labeling of parallel and identical electron- or energy-transfer pathways in symmetrical molecular assemblies is an extremely challenging task owing to the inherent lack of isotopic selectivity in conventional synthetic methods. Yet, it would be a highly valuable tool in the study and control of complex light-matter interactions in molecular systems by exclusively and nonintrusively labeling one of otherwise identical reaction pathways, potentially directing charge and energy transport along a chosen path. Here we describe the first selective synthetic route to asymmetrically labeled organometallic compounds, on the example of charge-transfer platinum(II) cis-acetylide complexes. We demonstrate the selective 13C labeling of one of two acetylide groups. We further show that such isotopic labeling successfully decouples the two ν(C≡C) in the mid-IR region, permitting independent spectroscopic monitoring of two otherwise identical electron-transfer pathways, along the 12C≡12C and 13C≡13C coordinates. Quantum-mechanical mixing leads to intriguing complex features in the vibrational spectra of such species, which we successfully model by full-dimensional anharmonically corrected DFT calculations, despite the large size of these systems. The synthetic route developed and demonstrated herein should lead to a great diversity of asymmetric organometallic complexes inaccessible otherwise, opening up a plethora of opportunities to advance the fundamental understanding and control of light-matter interactions in molecular systems.
    • Directing the path of light-induced electron transfer at a molecular fork using vibrational excitation.

      Delor, Milan; Archer, Stuart A.; Keane, Theo; Meijer, Anthony J. H. M.; Sazanovich, Igor V.; Greetham, Gregory M.; Towrie, Michael; Weinstein, Julia A.; University of Sheffield; Research Complex at Harwell (Springer, 2017-06-19)
      Ultrafast electron transfer in condensed-phase molecular systems is often strongly coupled to intramolecular vibrations that can promote, suppress and direct electronic processes. Recent experiments exploring this phenomenon proved that light-induced electron transfer can be strongly modulated by vibrational excitation, suggesting a new avenue for active control over molecular function. Here, we achieve the first example of such explicit vibrational control through judicious design of a Pt(II)-acetylide charge-transfer Donor-Bridge-Acceptor-Bridge-Donor “fork” system: asymmetric 13C isotopic labelling of one of the two -C≡C-bridges makes the two parallel and otherwise identical Donor→Acceptor electron-transfer pathways structurally distinct, enabling independent vibrational perturbation of either. Applying an ultrafast UVpump(excitation)-IRpump(perturbation)-IRprobe(monitoring) pulse sequence, we show that the pathway that is vibrationally perturbed during UV-induced electron-transfer is dramatically slowed down compared to its unperturbed counterpart. One can thus choose the dominant electron transfer pathway. The findings deliver a new opportunity for precise perturbative control of electronic energy propagation in molecular devices.
    • Directly coupled versus spectator linkers on diimine ptii acetylides—change the structure, keep the function

      Archer, Stuart A.; Keane, Theo; Delor, Milan; Bevon, Elizabeth; Auty, Alexander J.; Chekulaev, Dimitri; Sazanovich, Igor V.; Towrie, Michael; Meijer, Anthony J. H. M.; Weinstein, Julia A.; et al. (Wiley, 2017-12-27)
      Modification of light‐harvesting units with anchoring groups for surface attachment often compromises light‐harnessing properties. Herein, a series of [donor–acceptor–anchor] platinum(II) diimine (bis‐)acetylides was developed in order to systematically compare the effect of conjugated versus electronically decoupled modes of attachment of protected anchoring groups on the photophysical properties of light‐harvesting units. The first examples of “decoupled” phosphonate diimine PtII complexes are reported, and their properties are compared and contrasted to those of carboxylate analogues studied by a diversity of methods. Ultrafast time‐resolved IR and transient absorption spectroscopy revealed that all complexes have a charge‐transfer (CT) lowest excited state with lifetimes between 2 and 14 ns. Vibrational signatures and dynamics of CT states were identified; the assignment of electronic states and their vibrational origin was aided by TDDFT calculations. Ultrafast energy redistribution accompanied by structural changes was directly captured in the CT states. A significant difference between the structures of the electronic ground and CT excited states, as well as differences in the structural reorganisation in the complexes bearing directly attached or electronically decoupled anchoring groups, was discovered. This work demonstrates that decoupling of the anchoring group from the light‐harvesting core by a saturated spacer is an easy approach to combine surface attachment with high reduction potential and ten times longer lifetime of the CT excited state of the light‐absorbing unit, and retain electron‐transfer photoreactivity essential for light‐harvesting applications.