Heptametallic, Octupolar Nonlinear Optical Chromophores with Six Ferrocenyl Substituents

New complexes with six ferrocenyl (Fc) groups connected to ZnII or Cd^(II) tris(2,2′-bipyridyl) cores are described. A thorough characterisation of their BPh_(4)− salts includes two single-crystal X-ray structures, highly unusual for such species with multiple, extended substituents. Intense, visible d(Fe^(II))→π* metal-to-ligand charge-transfer (MLCT) bands accompany the π→π* intraligand charge-transfer absorptions in the near UV region. Each complex shows a single, fully reversible Fe^(III/II) wave when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined by using hyper-Rayleigh scattering and Stark spectroscopy. The latter gives static first hyperpolarisabilities β_0 reaching as high as approximately 10^(−27) esu and generally increasing with π-conjugation extension. Z-scan cubic NLO measurements reveal high two-photon absorption cross-sections σ2 of up to 5400 GM in one case. DFT calculations reproduce the π-conjugation dependence of β_0, and TD-DFT predicts three transitions close in energy contributing to the MLCT bands. The lowest energy transition has octupolar character, whereas the other two are degenerate and dipolar in nature

Synthesis, Structures, and Optical Properties of Ruthenium(II) Complexes of the Tris(1-pyrazolyl)methane Ligand

Four new complex salts [Ru^(II)Cl(Tpm)(L^A)_2][PF_6]_n [Tpm = tris(1-pyrazolyl)methane; n = 1, L^A = pyridine (py) 1 or ethyl isonicotinate (EIN) 2; n = 3, L^A = N-methyl-4,4′-bipyridinium (MeQ^+) 3 or N-phenyl-4,4′-bipyridinium (PhQ^+) 4] have been prepared and characterized. Electronic absorption spectra show intense d → π^* metal-to-ligand charge-transfer (MLCT) absorption bands, while cyclic voltammetry reveals a reversible Ru^(III/II) wave, accompanied by quasireversible or irreversible L^A-based reductions for all except 1. Single crystal X-ray structures have been obtained for 1•Me_2CO, 2, and 3•Me_2CO. For 2–4, molecular first hyperpolarizabilities β have been measured in acetonitrile solutions via the hyper-Rayleigh scattering (HRS) technique at 800 nm. Stark (electroabsorption) spectroscopic studies on the MLCT bands in frozen butyronitrile allow the indirect estimation of static first hyperpolarizabilities β_0. The various physical data obtained for 3 and 4 are compared with those reported previously for related cis-{Ru^(II)(NH_3)_4}^(2+) species [Coe, B. J. et al. J. Am. Chem. Soc. 2005, 127, 4845]. TD-DFT calculations on the complexes in 1–4 confirm that their lowest energy absorption bands are primarily Ru^(II) → L^A MLCT in character, while Ru^(II) → Tpm MLCT transitions are predicted at higher energies. DFT agrees with the Stark, but not the HRS measurements, in showing that β_0 increases with the electron-accepting strength of L^A. The 2D nature of the chromophores is evidenced by dominant β_(xxy) tensor components

Nonlinear Optical Chromophores with Two Ferrocenyl, Octamethylferrocenyl, or 4‑(Diphenylamino)phenyl Groups Attached to Rhenium(I) or Zinc(II) Centers

The compounds 4,4′-bis[(E)-2-R-vinyl]-2,2′-bipyridyl {R = ferrocenyl [(Fcv)_2bpy], octamethylferrocenyl [(Me_8Fcv)_2bpy] or 4-(diphenylamino)phenyl [(Dapv)_2bpy]} are used to prepare eight new complexes with Zn^(II)Cl_2, Zn^(II)(OAc)_2, or fac-Re^ICl(CO)_3 centers. The recently reported complex fac-Re^ICl(CO)_3[(Dapv)_2bpy] (Horvath, R. et al. Inorg. Chem. 2013, 52, 1304) is also studied. Electronic absorption spectra show intense d → π^* metal-to-ligand charge-transfer (MLCT) and π → π^* intraligand charge-transfer (ILCT) absorption bands, the relative energies of which correlate logically with the molecular structure. Cyclic voltammetry reveals a reversible oxidation wave for the Fc/Me_8Fc complexes, accompanied by quasireversible or irreversible ligand-based reductions. The Re complexes also show irreversible Re^(II/I) waves. Single-crystal X-ray structures are reported for (Me_8Fcv)_2bpy, Zn^(II)Cl_2[(Me_8Fcv)_2bpy], Zn^(II)(OAc)_2[(Fcv)_2bpy]·CHCl_3, and fac-Re^ICl(CO)_3[(Me_8Fcv)_2bpy]·0.5CHCl_3. Molecular first hyperpolarizabilities β are measured in DCM solutions via the hyper-Rayleigh scattering (HRS) technique at 1300 nm. Stark (electroabsorption) spectroscopic studies on only the MLCT bands in frozen butyronitrile allow the indirect estimation of lower limits for the overall static first hyperpolarizabilities β_0. Time-dependent density functional theory (TD-DFT) calculations on selected complexes confirm the expected assignments of their low energy absorption bands, with the best results obtained by using the M06 functional and Def2-TZVP/SVP/TZVPP mixed basis set. DFT predicts that the total static first hyperpolarizability β_(tot) increases in the Zn^(II)Cl_2 series in the order R = Fc < Me_8Fc < Dap, consistent with the HRS and Stark data. The computed β values increase substantially on moving from the gas phase to a DCM or MeCN solvent medium, and the essentially 2D nature of the chromophores leads to dominant β_(xxy) tensor components

Tunable Chiral Second-Order Nonlinear Optical Chromophores Based on Helquat Dications

Fourteen new dipolar cations have been synthesized, containing methoxy or tertiary amino electron donor groups attached to helquat (Hq) acceptors. These Hq derivatives have been characterized as their TfO^– salts by using various techniques including NMR and electronic absorption spectroscopies. UV–vis spectra show intense, relatively low energy absorptions with λ_(max) ≈ 400–600 nm, attributable to intramolecular charge-transfer (ICT) excitations. Single-crystal X-ray structures have been solved for two of the chromophores, one as its PF_6^– salt, revealing centrosymmetric packing arrangements (space groups Pbca and P1̅). Molecular quadratic nonlinear optical (NLO) responses have been determined directly by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and indirectly via Stark (electroabsorption) spectroscopy for the low energy absorption bands. The obtained static first hyperpolarizabilities β_0 range from moderate to large: (9–140) × 10^(–30) esu from HRS in MeCN and (44–580) × 10^(–30) esu from the Stark data in PrCN. The magnitude of β_0 increases upon either extending the π-conjugation length or replacing a methoxy with a tertiary amino electron donor substituent. Density functional theory (DFT) and time-dependent DFT calculations on selected tertiary amino chromophores confirm that the low energy absorptions have ICT character. Relatively good agreement between the simulated and experimental UV–vis absorption spectra is achieved by using the CAM-B3LYP functional with the 6-311G(d) basis set. The β_(tot) values predicted by using DFT at the same level of theory are large ((472–1443) × 10^(–30) esu in MeCN). Both the theoretical and experimental results show that para-conjugation between Hq and electron donor fragments is optimal, and enlarging the Hq unit is inconsequential with respect to the molecular quadratic NLO response

Helquat Dyes: Helicene-like Push–Pull Systems with Large Second-Order Nonlinear Optical Responses

Helquat dyes combine a cationic hemicyanine with a helicene-like motif to form a new blueprint for chiral systems with large and tunable nonlinear optical (NLO) properties. We report a series of such species with characterization, including determination of static first hyperpolarizabilities β_0 via hyper-Rayleigh scattering and Stark spectroscopy. The measured β_0 values are similar to or substantially larger than that of the commercial chromophore E-4′-(dimethylamino)-N-methyl-4-stilbazolium. Density functional theory (DFT) and time-dependent DFT calculations on two of the new cations are used to probe their molecular electronic structures and optical properties. Related molecules are expected to show bulk second-order NLO effects in even nonpolar media, overcoming a key challenge in developing useful materials

Synthesis, Structures, and Optical Properties of Ruthenium(II) Complexes of the Tris(1-pyrazolyl)methane Ligand

Four new complex salts [Ru^(II)Cl(Tpm)(L^A)_2][PF_6]_n [Tpm = tris(1-pyrazolyl)methane; n = 1, L^A = pyridine (py) 1 or ethyl isonicotinate (EIN) 2; n = 3, L^A = N-methyl-4,4′-bipyridinium (MeQ^+) 3 or N-phenyl-4,4′-bipyridinium (PhQ^+) 4] have been prepared and characterized. Electronic absorption spectra show intense d → π^* metal-to-ligand charge-transfer (MLCT) absorption bands, while cyclic voltammetry reveals a reversible Ru^(III/II) wave, accompanied by quasireversible or irreversible L^A-based reductions for all except 1. Single crystal X-ray structures have been obtained for 1•Me_2CO, 2, and 3•Me_2CO. For 2–4, molecular first hyperpolarizabilities β have been measured in acetonitrile solutions via the hyper-Rayleigh scattering (HRS) technique at 800 nm. Stark (electroabsorption) spectroscopic studies on the MLCT bands in frozen butyronitrile allow the indirect estimation of static first hyperpolarizabilities β_0. The various physical data obtained for 3 and 4 are compared with those reported previously for related cis-{Ru^(II)(NH_3)_4}^(2+) species [Coe, B. J. et al. J. Am. Chem. Soc. 2005, 127, 4845]. TD-DFT calculations on the complexes in 1–4 confirm that their lowest energy absorption bands are primarily Ru^(II) → L^A MLCT in character, while Ru^(II) → Tpm MLCT transitions are predicted at higher energies. DFT agrees with the Stark, but not the HRS measurements, in showing that β_0 increases with the electron-accepting strength of L^A. The 2D nature of the chromophores is evidenced by dominant β_(xxy) tensor components

Molecular nonlinear optics: advanced chromophore characterization for target-specific (supra)molecular designs

The coherent, monochromatic and intense light of a laser opened the way to research in nonlinear optics, the field in which this doctoral research is situated. Nonlinear optical (NLO) materials are able to alter the color of light, switch it or change its transmission characteristics, depending on its intensity. The technological applications of such nonlinear optical interactions are myriad. After introducing the general aspects of (non)linear optics in the first chapter, the experimental details of linear absorption, hyper-Rayleigh scattering (HRS) and depolarization ratio measurements are discussed in Chapter 2. The different approaches to deal with the competing process of multi-photon fluorescence in the HRS measurements are discussed in detail. In Chapter 3, a study on the second-order nonlinear optical properties of organometallic molecular structures with increasing dimensionality is elaborated. The flexible nature of the substitution and oxidation states of transition metal complexes enables their use as donor or acceptor moieties in chromophores. The presence of a metal center allows for other geometries, as a palette of coordination patterns is available for metals. Intrinsically connected to the presence of a transition metal center, these compounds possess reversible redox switching behavior. The NLO properties of one-dimensional, thermally stable ferrocenyl "push-pull" chromophores with an isophorone derived acceptor, ferrocene-diketopyrrolopyrroles and ferrocene-α-cyanostilbenes are discussed, as well as two-ferrocenyl V-shaped chromophores with a rhenium or zinc center and heptametallic octopolar compounds. We have established clear structure-property relations towards molecular optical switches with varying polarization sensitivity. Chapter 4 deals with the development of porphyrin-based chromophores for bio-medical imaging, more specifically the development of voltage-sensitive dyes to track brain activity when intercalated in the membranes of neurons. In the development of these dyes, function as well as form requirements, i.e. the appropriateness of the compounds for the experimental question and biological target respectively must be taken into account. As an example for nonlinear optical imaging of a cell membrane, a dipolar compound should have amphiphilic properties. Porphyrins can make excellent second-harmonic generation (SHG) dyes to probe voltage changes over neuronal membranes. The finding that the free-base porphyrin core is sufficiently electron-deficient that the hyperpolarizability does not increase on addition of a pyridinium electron-acceptor creates opportunities in the design of new SHG probes. Besides, experimental results show the possibility of elaborating the porphyrin bridge by adding another porphyrin building block to optimize the voltage sensitivity of these extraordinary membrane probes. Dispersion of first and second hyperpolarizability values within the bio-optical transparency window should be mapped and exploited to optimize the possibilities of the SHG probe. Helically wrapped single-walled carbon nanotubes (SWNTs) are studied in chapter 5. We report for the first time experimental measurements of the first hyperpolarizability of individualized, length-sorted (700 ± 50 nm long) single chirality (6,5) SWNTs. The chiral, opto-electronically active polymers wrap the nanotubes in an exclusively left-handed, single-chain helical fashion and feature PZn2, PZn3 and PZnRuPZn octopolar chromophores as integral parts of the polymer backbone. The non-covalent functionalization of these electronically homogeneous SWNTs by NLO chromophores demonstrates an appealing strategy to design new classes of electro-optic materials that feature enhanced hyperpolarizabilities over the telecommunication-relevant spectral domain. The importance of target-oriented development, taking into account both form and function requirements, is clear from the very different approaches of chromophore molecular design in chapters 3, 4 and 5. We explored higher dimensionality as well as extension along one dimension and supramolecular chirality in the domain of molecular optical switches, biological imaging as well as optical telecommunication.nrpages: 162status: publishe

Thermally stable ferrocene-alpha-cyanostilbenes as efficient materials for second order nonlinear optical polarizability

© 2016 The Royal Society of Chemistry. A set of new ferrocene-α-cyanostilbene (Fc-α-CNS) conjugated dyads was synthesized and their optical, nonlinear optical (NLO) and electrochemical properties were investigated. The second-order nonlinear polarizabilities were determined using hyper-Rayleigh scattering (HRS) with femtosecond pulsed laser light at 840 nm. The dyads exhibit structure-dependent NLO properties, which could be rationalized by correlating optical with electrochemical data. The dyad with the strongest (formyl) acceptor group in combination with the longer conjugation path between this acceptor and the Fc (ferrocene) donor, showed the largest static hyperpolarizability value, as compared to dyads with shorter conjugation and/or weaker acceptors. A partially symmetrical and longer triad showed the longest absorption wavelength, yet the smallest dipolar properties (dipole moment and first hyperpolarizability), are due to partial cancellation, in agreement with symmetry correlations.status: publishe

Synthesis, linear and nonlinear optical properties of thermally stable ferrocene-diketopyrrolopyrrole dyads

© 2015 The Royal Society of Chemistry. A set of new ferrocene-diketopyrrolopyrrole (Fc-DPP) conjugated dyads was synthesized and their optical, nonlinear optical (NLO) and electrochemical properties were investigated. The second-order nonlinear polarizabilities were determined using hyper-Rayleigh scattering with femtosecond pulsed laser light at 840 nm. The dyads exhibited structure dependent NLO response, which could be explained by correlating optical as well as electrochemical data. In the latter case, it is shown that the amplitude of the Fc based one-electron redox process of D-π-A type dyads is doubled in the dyads of the type D-π-A-π-D, where the acceptor (DPP) is flanked by two Fc donors.status: publishe

Nonlinear Optical Chromophores with Two Ferrocenyl, Octamethylferrocenyl, or 4-(Diphenylamino)phenyl Groups Attached to Rhenium(I) or Zinc(II) Centers

© 2015 American Chemical Society. The compounds 4,4′-bis[(E)-2-R-vinyl]-2,2′-bipyridyl {R = ferrocenyl [(Fcv)2bpy], octamethylferrocenyl [(Me8Fcv)2bpy] or 4-(diphenylamino)phenyl [(Dapv)2bpy]} are used to prepare eight new complexes with ZnIICl2, ZnII(OAc)2, or fac-ReICl(CO)3 centers. The recently reported complex fac-ReICl(CO)3[(Dapv)2bpy] (Horvath, R. et al. Inorg. Chem. 2013, 52, 1304) is also studied. Electronic absorption spectra show intense d → π∗ metal-to-ligand charge-transfer (MLCT) and π → π∗ intraligand charge-transfer (ILCT) absorption bands, the relative energies of which correlate logically with the molecular structure. Cyclic voltammetry reveals a reversible oxidation wave for the Fc/Me8Fc complexes, accompanied by quasireversible or irreversible ligand-based reductions. The Re complexes also show irreversible ReII/I waves. Single-crystal X-ray structures are reported for (Me8Fcv)2bpy, ZnIICl2[(Me8Fcv)2bpy], ZnII(OAc)2[(Fcv)2bpy]·CHCl3, and fac-ReICl(CO)3[(Me8Fcv)2bpy]·0.5CHCl3. Molecular first hyperpolarizabilities β are measured in DCM solutions via the hyper-Rayleigh scattering (HRS) technique at 1300 nm. Stark (electroabsorption) spectroscopic studies on only the MLCT bands in frozen butyronitrile allow the indirect estimation of lower limits for the overall static first hyperpolarizabilities β0. Time-dependent density functional theory (TD-DFT) calculations on selected complexes confirm the expected assignments of their low energy absorption bands, with the best results obtained by using the M06 functional and Def2-TZVP/SVP/TZVPP mixed basis set. DFT predicts that the total static first hyperpolarizability βtot increases in the ZnIICl2 series in the order R = Fc 8Fc xxy tensor components.status: publishe