Electroswitchable Photoluminescence Activity : Synthesis, Spectroscopy, Electrochemistry, Photophysics, and X-ray Crystal and Electronic Structures of [Re(bpy)(CO)3(C≡C-C6H4-C≡ C)Fe(C5Me5)(dppe)][PF6]n (n = 0, 1)

A novel heterobimetallic alkynyl-bridged complex, [Re(bpy)(CO) ₃(C≡C-C₆H₄-C≡C)Fe(C ₅Me₅)(dppe)], 1, and its oxidized species, [Re(bpy)(CO)₃(C≡C-C₆H₄-C≡ C)Fe(C₅Me₅)(dppe)][PF₆], 2, have been synthesized and their X-ray crystal structures determined. A related vinylidene complex, [Re(bpy)(CO)₃(C≡C-C₆H ₄-(H)C=C)Fe-(C₅Me₅)(dppe)][PF₆], 3, has also been synthesized and characterized. The cyclic voltammogram of 1 shows a quasireversible reduction couple at -1.49 V (vs SCE), a fully reversible oxidation at -0.19 V, and a quasireversible oxidation at +0.88 V. In accord with the electrochemical results, density-functional theory calculations on the hydrogen-substituted model complex Re(bpy)(CO)₃(C≡C-C ₆H₄-C≡C)Fe(C₅H₅)(dHpe) (Cp = C₅H₅, dHpe = H₂P-(CH₂) ₂-PH₂) (1-H) show that the LUMO is mainly bipyridine ligand π* in character while the HOMO is largely iron(II) d orbital in character. The electronic absorption spectrum of 1 shows low-energy absorption at 390 nm with a 420 nm shoulder in CH₂Cl₂, while that of 2 exhibits less intense low-energy bands at 432 and 474 nm and additional low-energy bands in the NIR at ca. 830, 1389, and 1773 nm. Unlike the related luminescent rhenium(I)-alkynyl complex [Re(bpy)(CO)₃(C≡C-C ₆H₄-C≡C-H)], 4, complex 1 is found to be nonemissive, and such a phenomenon is attributed to an intramolecular quenching of the emissive dπ(Re)→π*(bpy) ³MLCT state by the low-lying MLCT and LF excited states of the iron moiety. Interestingly, switching on of the luminescence property derived from the dπ(Re)→π *(bpy) ³MLCT state can be demonstrated in the oxidized species 2 and the related vinylidene analogue 3 due to the absence of the quenching pathway.