Organometallic complexes for nonlinear optics

Molecular engineering for optical properties of 5-substituted-1,10-phenanthroline-based Ru(II) complexes

A series of homo- and heteroleptic Ru(ii) complexes [Ru(phen)3-n(phen-X)n](PF6)2 (n = 0-3, X = CN, epoxy, H, NH2) were prepared and characterized. The influence of electron-withdrawing or electron-releasing substituents of the 1,10-phenanthroline ligands on the photo-physical properties was evaluated. It reveals fundamental interests in the fine tuning of redox potentials and photo-physical characteristics, depending both on the nature of the substitution of the ligand, and on the symmetry of the related homo- or heteroleptic complex. These complexes exhibit linear absorption and two-photon absorption (2PA) cross-sections over a broad range of wavelength (700-900 nm) due to absorption in the intra-ligand charge transfer (ILCT) and the metal-to-ligand charge transfer (MLCT) bands. These 2PA properties were more particularly investigated in the 700-1000 spectral range for a family of complexes bearing electro-donating ligands (phen-NH2).

Spontaneous Symmetry Breaking Facilitates Metal-to-Ligand Charge Transfer: A Quantitative Two-Photon Absorption Study of Ferrocene-phenyleneethynylene Oligomers

Change of the permanent molecular electric dipole moment, Δμ, in a series of nominally centrosymmetric and noncentrosymmteric ferrocene-phenyleneethynylene oligomers was estimated by measuring the two-photon absorption cross-section spectra of the lower energy metal-to-ligand charge-transfer transitions using femtosecond nonlinear transmission method and was found to vary in the range up to 12 D, with the highest value corresponding to the most nonsymmetric system. Calculations of the Δμ performed by the TD-DFT method show quantitative agreement with the experimental values and reveal that facile rotation of the ferrocene moieties relative to the organic ligand breaks the ground-state inversion symmetry in the nominally symmetric structures.

Gas phase anion photoelectron spectroscopy and theoretical investigation of gold acetylide species

We conducted gas phase anion photoelectron spectroscopy and density functional theory studies on a number of gold acetylide species, such as AuC₂H, AuC₂Au, and Au₂C₂H. Based on the photoelectron spectra, the electron affinities of AuC₂H, AuC₂Au, and Au₂C₂H are measured to be 1.54(±0.04), 1.60(±0.08), and 4.23(±0.08) eV, respectively. The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of AuC₂H and AuC₂Au are measured to be about 2.62 and 2.48 eV, respectively. It is interesting that photoelectron spectra of AuC₂H^- and AuC₂Au^- display similar spectral features. The comparison of experimental and theoretical results confirms that the ground-state structures of AuC₂H^-, AuC₂Au^-, and their neutrals are all linear with Au-C≡C-H and Au-C≡C-Au configurations. The similar geometric structures, spectral features, HOMO-LUMO gaps, and chemical bonding between AuC₂H^-/0 and AuC₂Au^-/0 demonstrate that Au atom behaves like H atom in these species. The photoelectron spectrum of Au₂C₂H^- shows that Au₂C₂H has a high electron affinity of 4.23(±0.08) eV, indicating Au₂C₂H is a superhalogen. Further, we found an unusual similarity between the terminal Au atom of Au₂C₂H^- and the iodine atom of IAuC₂H^-.

Organometallic Complexes for Non-Linear Optics. 59. Syntheses and Optical Properties of Some Octupolar (N-Heterocyclic Carbene)gold Complexes

The syntheses of octupolar alkynes 1,3,5-{4-(4-HC≡CC6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (4) and 1,3,5-{4-(4-HC≡CC6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (6), diphenylamino-substituted 1,3,5-(4-Ph2NC6H4-1-C≡C)3C6H3 (7), 1,3,5-(4-Ph2NC6H4-1-C≡C-4-C6H4-1-C≡C)3C6H3 (8), 1,3,5-{4-(4-Ph2NC6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (9), and 1,3,5-{4-(4-Ph2NC6H4-1-C≡C-4-C6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (10), and (N-heterocyclic carbene)gold-appended 1,3,5-{[(NHC-iPr)Au]C≡C}3C6H3 (11), 1,3,5-{[(NHC-iPr)Au]C≡C-4-C6H4-1-C≡C}3C6H3 (12), 1,3,5-{4-([(NHC-iPr)Au]C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (13), and 1,3,5-{4-([(NHC-iPr)Au]C≡C-4-C6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (14) [NHC-iPr = κC-cyclo-CN(2,6-C6H3iPr2)CH=CHN(2,6-C6H3iPr2)] are reported. The low-energy bands in the linear optical absorption spectra of all three sets of compounds are red-shifted and increase in intensity upon π-delocalizable ‘arm’ lengthening. The diphenylamino- and (NHC-iPr)gold-terminated compounds do not exhibit measurable second-harmonic generation as assessed by hyper-Rayleigh scattering at 1064 nm using nanosecond pulses. Computational studies have been employed to rationalize the optical properties of the new compounds. Calculations on 7–10 reveal that the lowest-energy transitions with large oscillator strengths are predominantly [Ph2NC6H4] (π) → [arms + core] (π*) in character, whereas calculations on 11–14 suggest that the low-energy transitions relate to the transfer of electron density from the Au-alkynyl core group to the terminal NHC groups.

Nonlinear optical properties of intriguing Ru σ-acetylide complexes and the use of a photocrosslinked polymer as a springboard to obtain SHG active thin films

The nonlinear optics of Ru σ-acetylides have been reported.

Syntheses, Spectroscopic, Electrochemical, and Third-Order Nonlinear Optical Studies of a Hybrid Tris{ruthenium(alkynyl)/(2-phenylpyridine)}iridium Complex

The synthesis of fac-[Ir{N,C1′-(2,2′-NC5H4C6H3-5′-C≡C-1-C6H2-3,5-Et2-4-C≡CC6H4-4-C≡CH)}3] (10), which bears pendant ethynyl groups, and its reaction with [RuCl(dppe)2]PF6 to afford the heterobimetallic complex fac-[Ir{N,C1′-(2,2′-NC5H4C6H3-5′-C≡C-1-C6H2-3,5-Et2-4-C≡CC6H4-4-C≡C-trans-[RuCl(dppe)2])}3] (11) is described. Complex 10 is available from the two-step formation of iodo-functionalized fac-tris[2-(4-iodophenyl)pyridine]iridium(III) (6), followed by ligand-centered palladium-catalyzed coupling and desilylation reactions. Structural studies of tetrakis[2-(4-iodophenyl)pyridine-N,C1′](μ-dichloro)diiridium 5, 6, fac-[Ir{N,C1′-(2,2′-NC5H4C6H3-5′-C≡C-1-C6H2-3,5-Et2-4-C≡CH)}3] (8), and 10 confirm ligand-centered derivatization of the tris(2-phenylpyridine)iridium unit. Electrochemical studies reveal two (5) or one (6–10) Ir-centered oxidations for which the potential is sensitive to functionalization at the phenylpyridine groups but relatively insensitive to more remote derivatization. Compound 11 undergoes sequential Ru-centered and Ir-centered oxidation, with the potential of the latter significantly more positive than that of Ir(N,C′-NC5H4-2-C6H4-2)3. Ligand-centered π–π* transitions characteristic of the Ir(N,C′-NC5H4-2-C6H4-2)3 unit red-shift and gain in intensity following the iodo and alkynyl incorporation. Spectroelectrochemical studies of 6, 7, 9, and 11 reveal the appearance in each case of new low-energy LMCT bands following formal IrIII/IV oxidation preceded, in the case of 11, by the appearance of a low-energy LMCT band associated with the formal RuII/III oxidation process. Emission maxima of 6–10 reveal a red-shift upon alkynyl group introduction and arylalkynyl π-system lengthening; this process is quenched upon incorporation of the ligated ruthenium moiety on proceeding to 11. Third-order nonlinear optical studies of 11 were undertaken at the benchmark wavelengths of 800 nm (fs pulses) and 532 nm (ns pulses), the results from the former suggesting a dominant contribution from two-photon absorption, and results from the latter being consistent with primarily excited-state absorption.

Crystal structure of ({4-[(4-bromo-phen-yl)ethyn-yl]-3,5-di-ethyl-phen-yl}ethyn-yl)triiso-propyl-silane

The title compound, C29H37BrSi, was synthesized by the Sonogashira coupling of [(3,5-diethyl-4-ethynylphen-yl)ethyn-yl]triiso-propyl-silane with 4-bromo-1-iodo-benzene. In the structure, the two phenyl rings are nearly parallel to each other with a dihedral angle of 4.27 (4)°. In the crystal, π-π inter-actions between the terminal and central phenyl rings of adjacent mol-ecules link them in the a-axis direction [perpendicular distance = 3.5135 (14); centroid-centroid distance = 3.7393 (11) Å]. In addition, there are weak C-H⋯π inter-actions between the isopropyl H atoms and the phenyl rings of adjacent mol-ecules.

Length-dependent convergence and saturation behavior of electrochemical, linear optical, quadratic nonlinear optical, and cubic nonlinear optical properties of dipolar alkynylruthenium complexes with oligo(phenyleneethynylene) bridges

The syntheses of trans-[Ru{4,4'-C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (19), trans-[Ru{4,4',4''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (20), trans-[Ru{4,4',4'',4'''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppe)(2)] (21), trans-[Ru{4,4',4'',4'''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (22), trans-[Ru{4,4',4'',4'''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (23), and trans-[Ru{4,4',4'',4''',4''''-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(2)[2,5-(OEt)(2)]C[triple bond]CC(6)H(4)NO(2)}Cl(dppm)(2)] (24) are reported, together with those of precursor alkynes, complexes with the donor-pi-bridge-acceptor formulation that affords efficient quadratic and cubic NLO compounds; the identity of 19 was confirmed by a structural study. The electrochemical properties of 19-24 and related complexes with shorter pi-bridge ligands were assessed by cyclic voltammetry, and the linear optical, quadratic nonlinear optical, and cubic nonlinear optical properties were assayed by UV-vis-NIR spectroscopy, hyper-Rayleigh scattering studies at 1064 and 1300 nm, and broad spectral range femtosecond Z-scan studies, respectively. The Ru(II/III) oxidation potentials and wavelengths of the optical absorption maxima decrease on pi-bridge lengthening, until the tri(phenyleneethynylene) complex is reached, further chain lengthening leaving these parameters invariant; theoretical studies employing time-dependent density functional theory have shed light on this behavior. The quadratic nonlinearity beta(1064) and two-photon absorption cross-section reach maximal values at this same pi-bridge length, a similar saturation behavior that may reflect a common importance of ruthenium-to-alkynyl ligand charge transfer in electronic and optical behavior in these molecules.

Theoretical Study of Two-Photon Absorption Properties of a Series of Ferrocene-Based Chromophores

The electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) properties of a series of ferrocene-based chromophores with TCF-type acceptors (TCF = 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran) have been studied by using the ZINDO-SOS method. The results have revealed that OPA and TPA of ferrocenyl derivatives are affected by the strength of the acceptor, especially the pi-bridge conjugation length. The TPA cross section increases with increasing acceptor strength and pi-bridge conjugation length. The TCF-type acceptor with a phenyl group can lead to a larger TPA cross section. Quadrupole molecules have the largest TPA cross sections (2000-3000 GM), which are about 4 times that of the corresponding dipolar molecules, indicating larger interactions between the top and bottom branches. Finally, the origins of the two-photon excitations for ferrocenyl derivatives are analyzed. The calculations show that ferrocenyl derivatives with TCF-type acceptors (especially quadrupole molecules) are promising candidates for TPA materials.