| Summary: | Organometallic analogues of phenylene-ethynylene (PE) expand upon favorable optical and electronic properties of PE by introducing novel functionalities including mid-gap redox potentials, stable metalloradicals, and long-lived phosphorescent excited states. In particular, tungsten-alkylidyne complexes and tungsten-alkylidyne oligo-phenylene-ethynylene (WC-OPE) complexes possess highly tunable redox potentials and energetic excited states. The oxidation potentials of a series of d2 tungsten-alkylidyne complexes of the general form W(CR)L4X (R = H, alkyl, aryl; L = neutral ligand; X = anionic ligand) are shown to vary by more than 2.0 V as a function of ligand set, and DFT calculations of the dxy HOMO energies of these complexes provide a simple predictive descriptor for their redox potentials. Relative to PEs, replacement of an ethynyl-carbon with tungsten in WC-OPEs results in destabilization of the pi(WCAr) HOMO, but minimal perturbation of the pi*(WCAr) LUMO. The effective-conjugation lengths of these compounds are identical to their PE analogues, but their oxidation potentials are more moderate. Ground- and excited-state resonance Raman spectroscopies of these complexes reveal that the W≡C oscillator is heavily coupled to several characteristic phenylene vibrational modes, but uncoupled to the equatorial ligand oscillations. DFT calculations of the luminescent 3[(dxy) 1(pi*(WCAr))1] excited state reproduce the observed excited-state resonance-Raman spectra; this benchmark validates the calculated excited-state molecular structures, which show that the excited-state distortions of the WC-OPE unit decrease with increasing length of the OPE group. Incorporation of variable length WC-OPEs into tungsten-alkylidyne/zinc-porphyrin donor-bridge-acceptor charge-transfer assemblies allows for control of the electronic donor-acceptor coupling, which in turn increases the lifetime of the ion-pair state to 24.1 ns, long enough to undergo bimolecular electron transfer to a solution-phase CO2 reduction catalyst.
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