Syntheses, Electrochemical, Linear Optical, and Cubic Nonlinear Optical Properties of Ruthenium-Alkynyl-Functionalized Oligo(phenylenevinylene) Stars

The syntheses of trans-[Ru(C≡CC₆H₄-4-CHO)(C≡CC₆H₄-4-R)(dppe)₂] (R=H (9a), NO₂ (9b), CHO (9c), C≡CC₆H₃-3,5-Et₂ (9d), (E)-CHCHC₆H₄-4-tBu (9e); dppe=1,2-bis(diphenylphosphino)ethane), trans-[Ru(C≡CC₆H₄-4-R)Cl(dppe)₂] (R=C≡CC₆H₃-3,5-Et₂ (11a), (E)-CHCHC₆H₄-4-tBu (11b), (E)-CHCHC₆H₄-4-NO₂ (11c)), 1,2,4,5-{trans-[(dppe)₂(RC₆H₄C≡C)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₄C₆H₂ (R=H (14a), C≡CC₆H₃-3,5-Et₂ (14b), (E)-CHCHC₆H₄-4-tBu (14c)), 1-I-3,5-{trans-[(L₂)₂(R)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₂C₆H₃ (L₂=1,1-bis(diphenylphosphino)methane (dppm)), R=Cl (15a); L₂=dppe, R=C≡CPh (15b), R=C≡CC₆H₄-4-NO₂ (15c)), 1-Me₃SiC≡C-3,5-{trans-[(L₂)₂(R)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₂C₆H₃ (L₂=dppm, R=Cl (16a); L₂=dppe, R=C≡CPh (16b)), 1-HC≡C-3,5-{trans-[(dppe)₂(R)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₂C₆H₃ (R=Cl (17a), R=C≡CPh (17b)), and 1,3,5-{trans-[(dppe)₂(3,5-R₂-C₆H₃C≡C)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₃C₆H₃ (R=(E)-CHCHC₆H₄-4-C≡C-trans-[Ru(C≡CPh)(dppe)₂] (18)) are reported together with those of the precursor alkynes 1-RC≡C-3,5-Et₂C₆H₃ (R=SiMe₃ (2), H (3), C₆H₄-4-C≡CSiMe₃ (5), C₆H₄-4-C≡CH (6)). The identities of 9c, 9d, 9e, 11a, and trans-[Ru{C≡CC₆H₄-4-(E)-CHCHC₆H₄-4-tBu}₂(dppe)₂] (12 and 12′) were confirmed by single-crystal X-ray diffraction studies. The electrochemical properties of 9a-e, 11a-b, 14a-c, 15a-c, 16b, 17a, 17b, and 18 were assessed by cyclic voltammetry; the studies reveal that potentials for the fully/quasi-reversible metal-centered oxidation processes decrease upon introduction of solubilizing alkyl substituents and increase upon increasing acceptor substituent strength; other structural variations have little impact. UV/Vis-NIR spectroscopic studies on these complexes reveal lowest-energy metal-ligand charge transfer (MLCT) bands that redshift upon increasing the acceptor substituent strength, blueshift on alkyl incorporation, and gain in intensity on progression from linear to star complexes. Low-temperature UV/Vis-NIR spectroelectrochemical studies of 14a-c show the appearance of an intense low-energy band at 7400-7900cm^-1 that is redshifted upon π-system lengthening and alkyl substituent incorporation. The cubic nonlinear optical properties of 9d, 9e, 14a-c, 15a-c, 16b, 17a,b, and 18 were assayed by femtosecond Z-scan studies at benchmark wavelengths (750 and 800nm) in the near-IR region, with nonlinearity increasing upon nitro incorporation; the values for the E-ene-linked dendrimers in these studies are much larger than yne-linked analogues. Compounds 9d, 9e, 14a-c, and 18 were further examined by broad-spectral-range femtosecond Z-scan studies; the cruciform complexes have appreciable multiphoton absorption cross-sections, with maximal values close to two and three times the wavelength of the linear optical absorption maxima. Super stars: (4-Formylphenylethynyl)ruthenium complexes (see figure) are shown to undergo "chemistry-on-complex" Horner-Wadsworth-Emmons coupling to afford a range of tri- and tetraruthenium-functionalized star molecules and a nonaruthenium dendrimer. The products are nonlinear optical (NLO)-active, with linear optical properties that are redox-switchable.