Quadratic and cubic hyperpolarizabilities of nitro-phenyl/-naphthalenyl/-anthracenyl alkynyl complexes

Abstract

1-Nitronaphthalenyl-4-alkynyl and 9-nitroanthracenyl-10-alkynyl complexes [M](CC-4-C₁₀H₆-1-NO₂) ([M] = trans-[RuCl(dppe)₂] (6b), trans-[RuCl(dppm)₂] (7b), Ru(PPh₃)₂(η⁵-C₅H₅) (8b), Ni(PPh₃)(η⁵-C₅H₅) (9b), Au(PPh₃) (10b)) and [M](CC-10-C₁₄H₈-9-NO₂) ([M] = trans-[RuCl(dppe)₂] (6c), trans-[RuCl(dppm)₂] (7c), Ru(PPh₃)₂(η⁵-C₅H₅) (8c), Ni(PPh₃)(η⁵-C₅H₅) (9c), Au(PPh₃) (10c)) were synthesized and their identities were confirmed by single-crystal X-ray diffraction studies. Electrochemical studies and a comparison to the 1-nitrophenyl-4-alkynyl analogues [M](CC-4-C₆H₄-1-NO₂) ([M] = trans-[RuCl(dppe)₂] (6a), trans-[RuCl(dppm)₂] (7a), Ru(PPh₃)₂(η⁵-C₅H₅) (8a), Ni(PPh₃)(η⁵-C₅H₅) (9a), Au(PPh₃) (10a)) reveal a decrease in oxidation potential for ruthenium and nickel complexes on proceeding from the phenyl- to naphthalenyl- and then anthracenyl-containing bridge. HOMO → LUMO transitions characteristic of MCC-1-C₆H₄ to 4-C₆H₄-1-NO₂ charge transfer red-shift and gain in intensity on proceeding to the ruthenium complexes; the low-energy transitions have increasing ILCT character on proceeding from the phenyl- to naphthalenyl- and then anthracenyl-containing bridge. Spectroelectrochemical studies of the Ru-containing complexes reveal the appearance of low-energy bands corresponding to chloro-to-Ru^III charge transfer that red-shift on proceeding from the phenyl- to naphthalenyl- and then anthracenyl-containing bridge. Second-order nonlinear optical (NLO) studies at 1064 nm employing ns pulses and the hyper-Rayleigh scattering technique reveal an increase in quadratic optical nonlinearity upon introduction of metal to the precursor alkyne to afford alkynyl complexes and on proceeding from ligated-gold to -nickel and then to -ruthenium for a fixed alkynyl ligand. Quadratic NLO data of the gold complexes optically transparent at the second-harmonic wavelength reveal an increase in β_HRS on proceeding from the phenyl- to the naphthalenyl-containing complex. Broad spectral range third-order nonlinear optical studies employing fs pulses and the Z-scan technique reveal an increase in two-photon absorption cross-section on replacing ligated-gold by -nickel and then -ruthenium for a fixed alkynyl ligand. Computational studies undertaken using time-dependent density functional theory have been employed to assign the nature of the key optical transitions and suggest that the significant optical nonlinearities observed for the ruthenium-containing complexes correlate with the low-energy formally Ru → NO₂ band which possesses strong MLCT character, while the more moderate nonlinearities of the gold complexes correlate with a band higher in energy that is primarily ILCT in character.