Exceptional three- to six-photon absorption at organometallic dendrimers.

The light-intensity dependence of multi-photon absorption (MPA) affords outstanding spatial control. Furthermore, compared to the higher-energy photons needed for analogous linear absorption, the lower-energy photons involved in MPA often correspond to important wavelengths, such as those of the biological and telecommunications "windows". It is therefore of crucial importance to develop molecules that exhibit outstanding MPA cross-sections. However, although progress has been made with two-photon absorption, there is currently a dearth of efficient instantaneous n-photon absorbers (n > 2), a key reason being the scarcity of structure-property studies required to understand higher-order MPA. We herein report systematically-varied metallodendrimers up to third-generation in size, together with their nonlinear absorptive responses over the spectral range 600-2520 nm. We show that the dendrimers exhibit exceptional instantaneous three- to six-photon absorption cross-sections, with maximal values increasing with dendrimer generation and installation of solubilizing group, and we report that changing the groups at the dendrimer periphery can shift the wavelengths of the nPA maxima. We also describe time-dependent DFT studies that have facilitated assignment of the key linear and nonlinear transitions and disclosed the crucial role of the metal in the outstanding MPA performance.

Outstanding Quadratic to Septic Optical Nonlinearity at Dipolar Alkynylmetal-Porphyrin Hybrids.

Porphyrins are important macrocycles with applications in several areas including therapy, catalysis, and sensing. Strong nonlinear optical (NLO) responses are the key to fully exploiting the potential of these biocompatible molecules. We herein report that certain metal-alkynyl donor/nitro acceptor-functionalized porphyrins are attractive candidates for NLO applications. We show that specific examples exhibit record quadratic optical nonlinearity, exceptional two-photon absorption, and outstanding three-photon absorption, and we report the first porphyrins that exhibit four-photon absorption. The two-, three-, and four-photon absorption maxima are found at the corresponding multiples of linear absorption bands that time-dependent density functional theory assigns as admixtures of porphyrin-localized π*←π and donor-porphyrin to porphyrin-acceptor charge-transfer transitions.

Giant Multi-Photon Absorption by Heptazine Organometalation.

Multi-photon absorption (MPA) is of increasing interest for applications in technologically important "windows" of the electromagnetic spectrum (near-infrared III, NIR-III, 1550-1870 nm; and the new 2080-2340 nm region); however, few molecules exist that display strong MPA at these long wavelengths. We herein report the syntheses of the first 2,5,8-s-heptazine-cored organometallic complexes, together with organic analogues. The complexes exhibit outstanding 3PA cross-sections in the NIR-III and exceptional 4PA cross-sections in the new 2080-2340 nm window. We demonstrate that replacing organic donor groups by organometallic units results in an order of magnitude increase in 3PA, the "switching on" of 4PA, and a dramatic improvement in photo-stability. Our results highlight the impressive outcomes possible with an "organometalation" approach to NLO materials design.

Outstanding Multi-Photon Absorption at π-Delocalizable Metallodendrimers.

Multi-photon absorption (MPA) has attracted interest for applications exploiting the tight spatial control of interaction volume and long wavelength excitation. However, a deficiency of molecules exhibiting higher-order nPA (n-photon absorption, n>2) and a lack of structure-property studies to define the key structural characteristics needed to optimize higher-order MPA performance have hindered practical development. We herein report the syntheses of second- and third-generation metallodendrimers and assessment of their nonlinear absorption, together with those of zero- and first-generation analogues. We report the first 5PA and 6PA data for an organometallic. The largest dendrimer exhibits exceptional three-, four-, five- and six-photon absorption to femtosecond-pulsed light. The systematically varied compounds highlight the crucial role of metal-to-oligo(phenyleneethynylene) charge transfer in promoting outstanding MPA activity.

Room Temperature Hydrolysis of Benzamidines and Benzamidiniums in Weakly Basic Water.

Benzamidinium compounds have found widespread use in both medicinal and supramolecular chemistry. In this work, we show that benzamidiniums hydrolyze at room temperature in aqueous base to give the corresponding primary amide. This reaction has a half-life of 300 days for unsubstituted benzamidinium at pH 9, but is relatively rapid at higher pH's (e.g., t1/2 = 6 days at pH 11 and 15 h at pH 13). Quantum chemistry combined with first-principles kinetic modeling can reproduce these trends and explain them in terms of the dominant pathway being initiated by attack of HO- on benzamidine. Incorporation of the amidinium motif into a hydrogen bonded framework offers a substantial protective effect against hydrolysis.

Syntheses and quadratic nonlinear optical properties of 2,7-fluorenylene- and 1,4-phenylene-functionalized o-carboranes.

o-Carboranes C-functionalized by (4-substituted-phen-1-yl)ethynyl-1,4-phenyl groups or (2-substituted-fluoren-7-yl)ethynyl-2,7-fluorenyl groups, in which the pendant functionalization is electron-withdrawing nitro or electron-donating diphenylamino groups, have been synthesized and in many cases structurally characterized. Diphenylamino-containing examples coupled via the two π-delocalizable bridges to the electron-accepting o-carborane unit exhibit the greater quadratic optical nonlinearities at 1064 nm (hyper-Rayleigh scattering, ns pulses), the nonlinearities also increasing on proceeding from 1,4-phenylene- to 2,7-fluorenylene-containing bridge. The most NLO-efficient example 2-(n-butyl)-1-(2-((9,9-di(n-butyl)-2-(N,N-diphenylamino)-9H-fluoren-7-yl)ethynyl)-9,9-di(n-butyl)-9H-fluoren-7-yl)-1,2-ortho-carborane, consisting of diphenylamino donor, fluorenyl-containing bridge, o-carborane acceptor, and solubilizing n-butyl units, exhibits large 〈ß〉HRS (230 × 10-30 esu) and frequency-independent (two-level model) 〈ß0〉 (96 × 10-30 esu) values. Coupling two (2-((9,9-di(n-butyl)-2-(N,N-diphenylamino)-9H-fluoren-7-yl)ethynyl)-9,9-di(n-butyl)-9H-fluoren-7-yl) units to the 1,2-ortho-carborane core affords a di-C-functionalized compound with enhanced nonlinearities (309 × 10-30 esu and 129 × 10-30 esu, respectively).

Molecular Tectonics: A Node-and-Linker Building Block Approach to a Family of Hydrogen-Bonded Frameworks.

While numerous hydrogen-bonded organic frameworks (HOFs) have been reported, typically these cannot be prepared predictably or in a modular fashion. In this work, we report a family of nine diamondoid crystalline porous frameworks assembled via hydrogen bonding between poly-amidinium and poly-carboxylate tectons. The frameworks are prepared at room temperature in either water or water/alcohol mixtures. Importantly, both the cationic and anionic components can be varied and additional functionality can be incorporated into the frameworks, which show good stability including to prolonged heating in DMSO or water.

Easily-prepared Hydroxy-containing Receptors Recognize Anions in Aqueous Media.

Despite their ready availability, O-H groups have received relatively little attention as anion recognition motifs. Here, we report two simple hydroxy-containing anion receptors that are prepared in two facile steps followed by anion exchange, without the need for chromatographic purification at any stage. These receptors contain a pyridinium bis(amide) motif as well as hydroxyphenyl groups, and bind mono- and divalent anions in 9:1 CD3 CN:D2 O, showing a selectivity preference for sulfate. Notably, a "model" receptor that does not contain hydroxy groups shows only very weak sulfate binding in this competitive solvent mixture. In the solid state, X-ray crystallographic studies show that the receptors tend to form extended assemblies with anions; however, 1 H and DOSY NMR studies as well as molecular dynamics simulations show that only 1:1 complexes are present in solution. Molecular dynamics simulations suggest that one of the receptors suffers from competing intramolecular hydrogen bonding, while another binds partially-hydrated anions, with the receptor's O-H groups forming hydrogen bonds to water molecules within the anion's coordination sphere.

Exceptional Two-Photon Absorption in Alkynylruthenium-Gold Nanoparticle Hybrids.

Ruthenium alkynyl "star" complexes with tri(2-thienyl)-, tris(1,2,3-triazolyl)-, or triphenyl-benzene cores stabilize gold nanoparticles (AuNPs). Cyclic voltammetry, transmission electron microscopy, molecular modeling, dynamic light scattering, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy studies are consistent with ca. 5 trithienyl- or triazolyl-benzene-cored star complexes decorating the exterior of each AuNP. The ca. 2.5 nm diameter (by transmission electron microscopy) trithienylbenzene-cored gold nanoparticle hybrids are significantly less absorbent than classical Brust nanoparticles stabilized by 1-dodecanethiol; with femtosecond pulsed radiation, they exhibit exceptionally strong saturable absorption and two-photon absorption across the visible range and into the near-infrared region (3 000 000 GM at 500 nm and 46 000 GM at 750 nm; 1 GM is equal to 10-50 cm4 s photon-1).

Quadratic and Cubic Optical Nonlinearities of Y-Shaped and Distorted-H-Shaped Arylalkynylruthenium Complexes.

Straightforward syntheses of bis[bis{1,2-bis(diphenylphosphino)ethane}ruthenium]-functionalized 1,3,5-triethynylbenzene-cored complexes via a methodology employing "steric control" permit facile formation of Y-shaped Sonogashira coupling products and distorted-H-shaped homo-coupled quadrupolar products. Cyclic voltammetric data from these products reveal two reversible metal alkynyl-localized oxidation processes for all complexes. The wavelengths of the linear optical absorption maxima are dominated by the nature of the peripheral alkynyl ligand rather than the substituent at the unique arm of the "Y" or at the quadrupolar complex "core". The quadratic optical nonlinearities of the Y-shaped complexes were assessed by the hyper-Rayleigh scattering technique at 800 nm and employing 100 fs light pulses; introduction of donor NEt2 and/or acceptor NO2 to the wedge periphery resulted in non-zero nonlinearities, with the largest ßHRS,800 values being observed for the complexes containing the 4-nitrophenylalkynyl ligands. Depolarization ratios are consistent with substantial off-diagonal first hyperpolarizability tensor components and 2D nonlinear character. Computational studies employing time-dependent density functional theory have been employed to assign the key low-energy transitions in the linear optical spectra and to compute the quadratic nonlinear optical tensorial components. Cubic optical nonlinearities of the quadrupolar complexes were assessed by the Z-scan technique over the range 500-1600 nm and employing 130 fs light pulses; two-photon absorption cross-sections for these distorted-H-shaped complexes are moderate to large in value (up to 5500 GM at 880 nm), while one example displays significant three-photon absorption (1300×10-80  cm6 s2 at 1200 nm).

Synthesis, characterization and third-order nonlinear optical properties of a dodecaruthenium organometallic dendrimer with a zinc(ii) tetraphenylporphyrin core.

A new Zn(ii) porphyrin-based dendrimer (52) containing twelve Ru(ii) alkynyl fragments, has been prepared following a convergent approach in two steps from 5,10,15,20-tetra(4-ethynylphenyl)porphyrinatozinc(ii) (6). The cubic nonlinear optical (NLO) properties of 52 and other derivatives of 6 have been measured by third-harmonic generation (THG) at 1907 nm and by Z-scan over the spectral range 500-1700 nm, revealing the remarkable NLO response of 52 in the near-IR range. These results highlight the beneficial role of the extended "cross fourchée"-like polymetallic structure of 52 on its third-order NLO properties.

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

1-Nitronaphthalenyl-4-alkynyl and 9-nitroanthracenyl-10-alkynyl complexes [M](C[triple bond, length as m-dash]C-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](C[triple bond, length as m-dash]C-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](C[triple bond, length as m-dash]C-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 MC[triple bond, length as m-dash]C-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-RuIII 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 ßHRS 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.

Linear Optical, Quadratic and Cubic Nonlinear Optical, Electrochemical, and Theoretical Studies of "Rigid-Rod" Bis-Alkynyl Ruthenium Complexes.

The syntheses of oligo(p-phenylene ethynylene)s (OPEs) end-functionalized by a nitro acceptor group and with a ligated ruthenium unit at varying locations in the OPE chain, namely, trans-[Ru{(C≡C-1,4-C6 H4 )n NO2 }(C≡CR)(dppe)2 ] (dppe=1,2-bis(diphenylphosphino)ethane; n=1, R=1,4-C6 H4 C≡C-1,4-C6 H4 C≡CPh, 1,4-C6 H4 NEt2 ; n=2, R=Ph, 1,4-C6 H4 C≡CPh, 1,4-C6 H4 C≡C-1,4-C6 H4 C≡CPh, 1,4-C6 H4 NO2 , 1,4-C6 H4 NEt2 ; n=3, R=Ph, 1,4-C6 H4 C≡CPh), are reported. Their electrochemical properties were assessed by cyclic voltammetry, their linear optical properties and quadratic and cubic nonlinear optical properties were assayed by UV/Vis/NIR spectroscopy, hyper-Rayleigh scattering studies employing nanosecond pulses at 1064 nm, and broad spectral range Z-scan studies employing femtosecond pulses, respectively, and their linear optical properties and vibrational spectroscopic behavior in the formally RuIII state was examined by UV/Vis/NIR and IR spectroelectrochemistry, respectively. The potentials of the metal-localized oxidation processes are sensitive to alkynyl-ligand modification, but this effect is attenuated on π-bridge lengthening. Computational studies employing time-dependent density functional theory were undertaken on model complexes, with a 2D scan revealing a soft potential-energy surface for intra-alkynyl-ligand aryl-ring rotation; this is consistent with the experimentally observed blueshift in optical absorption maxima. Quadratic optical nonlinearities are significant and cubic NLO coefficients for these small complexes are small. The optimum length of the alkynyl ligands and the ideal metal location in the OPE to maximize the key coefficients have been defined.

Supramolecular frameworks based on 5,10,15,20-tetra(4-carboxyphenyl)porphyrins.

We have investigated the hydrogen bond-driven assembly of nickel and freebase tetra(carboxyphenyl) and tetra(carboxylatophenyl) porphyrins. When the tetracarboxylates were crystallized with bis(amidinium) species, their crystal structures contained a range of hydrogen bond geometries, and we did not obtain well-ordered networks. Use of a tetrahedral tetra(amidinium) building block yielded a 3D framework material with a PtS topology, which contains only a "paired" hydrogen bonding arrangement. This framework is highly porous, with ∼3/4 of the unit cell volume occupied by disordered solvent molecules, although it loses crystallinity upon removal from solvent. Favourable interactions between porphyrin carboxylic acid hydrogen bond donors and bipyridine nitrogen atoms were then used to prepare a stable 2D porphyrin grid-like network.

Hydrogen bond-Driven Self-Assembly between Amidinium Cations and Carboxylate Anions: A Combined Molecular Dynamics, NMR Spectroscopy, and Single Crystal X-ray Diffraction Study.

A combination of molecular dynamics (MD), NMR spectroscopy, and single crystal X-ray diffraction (SCXRD) techniques was used to probe the self-assembly of para- and meta-bis(amidinium) compounds with para-, meta-, and ortho-dicarboxylates. Good concordance was observed between the MD and experimental results. In DMSO solution, the systems form several rapidly exchanging assemblies, in part because a range of hydrogen bonding interactions is possible between the amidinium and carboxylate moieties. Upon crystallization, the majority of the systems form 1D supramolecular polymers, which are held together by short N-H⋅⋅⋅O hydrogen bonds.

Supramolecular anion recognition in water: synthesis of hydrogen-bonded supramolecular frameworks.

The interaction of tetratopic amidinium-containing receptors with terephthalate anions leads to porous framework materials assembled through charge-assisted hydrogen bonds. The frameworks form in good yield within minutes in water at room temperature, but no framework material is obtained if other anions (Cl-, Br-, NO3-, SO42- or isophthalate2-) are used in place of terephthalate. Two forms of the framework can be prepared: one with a connected pore network, and a more dense phase with discrete voids. We demonstrate that these are the kinetic and thermodynamic products, respectively. Either framework can be prepared independently and can be converted to the other form in response to stimuli. Furthermore, the frameworks can be controllably disassembled and reassembled in response to acid/base triggers suggesting that this new class of materials may have applications in the selective encapsulation and release of guests.

Exceptionally large two- and three-photon absorption cross-sections by OPV organometalation.

Oligo(p-phenylenevinylene)s (OPVs) containing up to 8 PV units and end-functionalized by ruthenium alkynyl groups have been prepared and their nonlinear absorption properties assessed using the Z-scan technique and employing low repetition rate femtosecond pulses. Exceptionally large two-photon absorption (ca. 12 500 GM at 725 nm) and three-photon absorption cross sections (ca. 1.6 × 10(-76) cm(6) s(2) at 1100 nm) are found for the 8PV-containing example, highlighting the potential of an "organometalation" approach to NLO-efficient organic materials.

Dynamic Permutational Isomerism in a closo-Cluster.

Permutational isomers of trigonal bipyramidal [W2RhIr2(CO)9(η(5)-C5H5)2(η(5)-C5HMe4)] result from competitive capping of either a W2Ir or a WIr2 face of the tetrahedral cluster [W2Ir2(CO)10(η(5)-C5 H5)2] from its reaction with [Rh(CO)2(η(5)-C5HMe4)]. The permutational isomers slowly interconvert in solution by a cluster metal vertex exchange that is proposed to proceed by Rh-Ir and Rh-W bond cleavage and reformation, and via the intermediacy of an edge-bridged tetrahedral transition state. The permutational isomers display differing chemical and physical properties: replacement of CO by PPh3 occurs at one permutational isomer only, while the isomers display distinct optical power limiting behavior.

Synthesis, Optical, Electrochemical, and Theoretical Studies of Dipolar Ruthenium Alkynyl Complexes with Oligo(phenylenevinylene) Bridges.

The syntheses of oligo(p-phenylenevinylene)s (OPVs) end-functionalized with a ligated ruthenium alkynyl unit as a donor and a nitro as acceptor, namely trans-[Ru{C≡C-1-C6 H4 -4-(E)-CH=CH-1-C6 H2 -2,5-Et2 -4-(E)-CH=CH-1-C6 H2 -2,5-Et2 -4-(E)-CH=CH-1-C6 H4 -4-NO2 }Cl(dppe)2 ] (Ru4), trans-[Ru{C≡C-1-C6 H4 -4-(E)-CH=CH-1-C6 H2 -2,5-Et2 -4-(E)-CH=CH-1-C6 H2 -2,5-Et2 -4-(E)-CH=CH-1-C6 H2 -2,5-(n-hexyl)2 -4-(E)-CH=CH-1-C6 H2 -2,5-(n-hexyl)2 -4-(E)-CH=CH-1-C6 H4 -4-NO2 }Cl(dppe)2 ] (Ru6), and trans-[Ru{C≡C-1-C6 H4 -4-(E)-CH=CH-1-C6 H2 -2,5-Et2 -4-(E)-CH=CH-1-C6 H2 -2,5-Et2 -4-(E)-CH=CH-1-C6 H2 -2,5-(n-hexyl)2 -4-(E)-CH=CH-1-C6 H2 -2,5-(n-hexyl)2 -4-(E)-CH=CH-1-C6 H2 -2,5-(2-ethyl-n-hexyl)2 -4-(E)-CH=CH-1-C6 H2 -2,5-(2-ethyl-n-hexyl)2 -4-(E)-CH=CH-1-C6 H4 -4-NO2 }Cl(dppe)2 ] (Ru8), are reported, together with those of precursor alkynes. Their electrochemical properties were assessed by cyclic voltammetry (CV), their linear optical and quadratic nonlinear optical (NLO) properties assayed by UV/Vis-NIR spectroscopy and hyper-Rayleigh scattering studies at 1064 nm, respectively, and their linear optical properties in the formally RuIII state examined by UV/Vis-NIR spectroelectrochemistry. Computational studies employing time-dependent density functional theory were undertaken on model complexes to rationalize the optical observations.

Syntheses and Optical Properties of Azo-Functionalized Ruthenium Alkynyl Complexes.

The syntheses of trans-[Ru(C≡C-1-C6 H4 -4-N=N-1-C6 H4 -4-C≡C-1-C6 H4 -4-NO2 )Cl(L2 )2 ] (L2 =dppm (Ru1), dppe) (Ru2)), trans-[Ru(C≡C-1-C6 H4 -4-N=N-1-C6 H4 -4-(E)-CH=CH-1-C6 H4 -4-NO2 )Cl(dppe)2 ] (Ru3), and trans-[Ru(C≡C-1-C6 H4 -4-(E)-CH=CH-1-C6 H2 -2,6-Et2 -4-N=N-1-C6 H4 -4-NO2 )Cl(dppe)2 ] (Ru4) are reported, together with those of precursor alkynes. Their electrochemical properties were assessed by cyclic voltammetry (CV), linear optical and quadratic nonlinear optical (NLO) properties assayed by UV/Vis-NIR spectroscopy and hyper-Rayleigh scattering studies at 1064 nm, respectively, and their linear optical properties in the formally RuIII state examined by UV/Vis-NIR spectroelectrochemistry. These data were compared to those of analogues with E-ene and yne linkages in place of the azo groups. Computational studies using time-dependent density functional theory were undertaken on model compounds (Ru2'-Ru4') to rationalize the optical behaviour of the experimental complexes.