Investigation of the second hyperpolarizability of Ru-alkynyl complexes by z-scan and nonlinear scattering

Present-day methods for determining the performance of third-order nonlinear optical materials include Z-scan, degenerate four-wave mixing and third-harmonic generation (THG). All these techniques possess severe drawbacks; for example, in THG, since all media (air and glass walls of the cell) present a third-order effect, eliminating these contributions requires careful, complex analysis or use of vacuum chambers. We have developed nonlinear scattering as a sensitive, straightforward technique for determining the second hyperpolarizability of samples in solution. Herein, we will for the first time show the applicability of the technique to measure organometallic Ru-complexes, optimized for high nonlinear responses. The investigated compounds showed a significant second hyperpolarizability |γ|, ranging from 1.1 for the least efficient to 2.8 ∙ 10-33 esu for the most efficient molecule, and comparable to fullerene C60 in thin films. It was deemed infeasible to extract hyperpolarizabilities using a high-frequency femtosecond laser source by a modified z-scan setup, which, in contrast to nonlinear scattering, could not account for the high degree of thermal lensing present in the investigated compoundsWe thank the Fund for Scientific Research-Flanders (PhD fellowship N.V.S) and the Australian Research Council for support. N.A.A.S thanks the Ministry of Higher Education of Malaysia and the National Defence University of Malaysia for a PhD scholarship

Fine-tuning polyoxometalate non-linear optical chromophores: a molecular electronic “Goldilocks” effect

A new aryl-imido polyoxometalate non-linear optical chromophore (POMophore) with a diphenylamino donor group attains the highest βzzz, 0 value (196 × 10−30 esu by Hyper-Rayleigh Scattering, HRS), and best transparency/non-linearity trade off yet for such materials. Stark spectroscopic and DFT investigation of this compound, plus NMe2 and carbazole analogues, show that its high performance results from a combination of strongly dipolar electronic transitions, and strong electronic communication across the π-system

Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study

Ten organoimido polyoxometalate (POM) based chromophores have been synthesized and studied by hyperRayleigh Scattering (HRS), Stark and Resonance Raman spectroscopies and DFT calculations. HRS β0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ca. 5 × that of the DAS+ cation), but systems with no donor, or the –NO2 acceptor show no activity, in some cases despite large DFT-predicted β-values. In active systems with short (phenyl) π-bridges, β0-values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β0/N3/2 (N = number of bridge π-electrons) thus appear to break empirical performance limits (β0/N3/2 vs λmax) for planar organic systems. However, β0-values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms involvement of the POM in the electronic transitions whether donor groups are present or not, but Stark spectroscopy indicates that in their absence the transitions have little dipolar character (hence NLO inactive), consistent with DFT-calculated frontier orbitals which extend over both POM and organic group. Stark and DFT also suggest that β is enhanced in the short compounds because extension of charge transfer (CT) onto the POM increases excited state dipole moment changes. With extended π-systems this effect does not increase CT distances relative to a –NO2 acceptor, so β0-values do not exceed that of DMNPEA. Overall, our results show that: (i) the organoimido–POM unit is an efficient acceptor for 2nd order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation

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‐C6H4)nNO2}(C≡CR)(dppe)2] (dppe=1,2‐bis(diphenylphosphino)ethane; n=1, R=1,4‐C6H4C≡C‐1,4‐C6H4C≡CPh, 1,4‐C6H4NEt2; n=2, R=Ph, 1,4‐C6H4C≡CPh, 1,4‐C6H4C≡C‐1,4‐C6H4C≡CPh, 1,4‐C6H4NO2, 1,4‐C6H4NEt2; n=3, R=Ph, 1,4‐C6H4C≡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.We thank the Australian Research Council, the Fund for Scientific Research–Flanders (FWO-Vlaanderen; FWO G.0312.08), and the Katholieke Universiteit Leuven (GOA/2006/03) for financial support, and Yuwen Wu (ANU) for providing a crystal of 14 suitable for the X-ray diffraction study

Rhenium(I) Tricarbonyl Complexes with Peripheral N-Coordination Sites: A Foundation for Heterotrimetallic Nonlinear Optical Chromophores

The compounds 4,4′-dicyano-2,2′-bipyridyl (dcbpy), 4,4′-bis(4-cyanophenyl)-2,2′-bipyridyl (bcpbpy), 4,4′-bis[(E)-2-(4-cyanophenyl)vinyl]-2,2′-bipyridyl (bcpvbpy), and 4,4′-bis[(E)-2-(4-pyridyl)vinyl]-2,2′-bipyridyl (bpvbpy) are used to prepare nine new complexes with fac-{Re^I(CO)_3(L)}^(n+) (L = Cl^–, MeCN, pyridine (py), or 4-cyanopyridine (cpy)) centers. The cationic complexes are isolated as their OTf^– salts. These new monometallic species are studied together with the known fac-Re^ICl(CO)_3(qpy) (qpy = 4,4′:2′,2″:4″,4‴-quaterpyridyl) (de Wolf, P.; et al. Inorg. Chim. Acta 2003, 355, 280–285). Coordination of {Ru^(II)(NH_3)_5}^(2+) centers to the peripheral N atoms of fac-Re^ICl(CO)_3(qpy) and fac-[Re^I(CO)_3(bcpbpy)(py)]^+ affords heterotrimetallic complexes that are isolated as their PF_6^– salts. Single-crystal X-ray structures are reported for the solvates fac-Re^ICl(CO)_3(dcbpy)·Me_2CO, fac-[Re^I(CO)_3(dcbpy)(MeCN)]OTf·PhMe, fac-[Re^I(CO)_3(dcbpy)(py)]OTf·0.5PhMe, fac-[ReI(CO)3(dcbpy)(cpy)]OTf·0.5Me2CO, fac-Re^ICl(CO)_3(bcpbpy)·0.5PhMe, and fac-[Re^I(CO)_3(bcpbpy)(MeCN)]OTf·MeCN. Cyclic voltammetry reveals irreversible ReII/I processes in all cases, while the trimetallic compounds show a reversible two-electron RuIII/II oxidation wave. Ligand-based reductions are generally irreversible, but reversible waves are observed for the dcbpy complexes. Electronic absorption spectra show strong d → π^* metal-to-ligand charge-transfer (MLCT) and intraligand π → π^* absorptions. The Ru^(II)-based MLCT bands have lower energies and higher intensities than the ReI-based ones. The monometallic species emit weakly in the red/NIR region (λ_(em) = 594–732 nm; ϕ = 0.6–8.1%) when excited at 350–520 nm. Molecular first hyperpolarizabilities β for the trimetallic complex salts are measured in MeCN solutions via the hyper-Rayleigh scattering technique at 1064 nm. Stark (electroabsorption) spectroscopic studies on the low-energy MLCT bands in frozen PrCN allow the indirect estimation of static first hyperpolarizabilities β_0. The trimetallic complexes have relatively large β_0 values, ca. 500 × 10^(–30) esu or more, which are substantially greater than those of the monometallic species

Molecular design in nonlinear optics: an experimental and computational perspective

Optics, the science of the interaction of light with matter, has become ubiquitous in our society. Light is propagating in the form of photons over optical fibers, very much as electricity in the form of electrons, is transported over electrical conductors. Indeed, photonics is the technology platform that relies on the manipulation of photons, rather than electrons in electronics. Nonlinear optics (NLO) is a sub-field describing all optical processes that depend in a nonlinear manner on the applied optical field. In this project we will focus on second-order effects, meaning they are quadratically dependent on the strength of the field. This interaction is only present in materials where loosely bound electrons are available that can feel the detailed electromagnetic properties of light. Organic molecules with lots of such electrons have been synthesized and tested for their NLO properties. More elaborated molecules, such as those with a transition metal ion incorporated, exhibit different electronic properties based on the oxidation state of the metal. Because these molecules can also be reversibly switched between these two forms, their optical properties can be altered. We envisage the detailed, both experimental and theoretical, study of such molecular switches for a better fundamental understanding of their electronic and optical phenomena. This involves the determination of their second-order NLO properties both with highly accurate quantum chemical methods as well as measurements with a unique laser setup.status: publishe

Unexpected High Second-Order Nonlinear Optical Activity of Metal Complexes with Three-Branched Hexadentate 2,2 '-Bipyridine Ligands

Two hexadentate bipyridine ligands and their RuII and NiII complexes were prepared. The helical alignment of the three electron-donor-π-bridge-electron-acceptor (d-π-A) single-strands with bundle architecture in cooperation with the metal center can strongly enhance the nonlinear optical (NLO) properties. The complexation of the novel cage-type hexadentate ligands with a paramagnetic NiII -core almost doubles the βHRS values compared with the corresponding diamagnetic RuII complexes. The hyper-Rayleigh scattering (HRS) was performed with a highly sensitive setup for simultaneous discrimination between multi-photon fluorescence and the molecular first hyperpolarizability.status: publishe

Third-Harmonic Scattering for Fast and Sensitive Screening of the Second Hyperpolarizability in Solution

Organic materials are promising candidates for integration in optical network components allowing fast communication. Ultimate speeds can be obtained by exploiting third-order nonlinear optical light–matter interactions that ultimately rely on the molecular second hyperpolarizability (γ). The exploration of molecular structure–property relations is crucial to optimize γ but requires state of the art measurement techniques which are both sensitive and efficient. Unfortunately, present-day methods for probing the performance of third-order nonlinear optical (NLO) materials fail to meet at least one of those requirements. We have developed third-harmonic scattering (THS) as an alternative method to measure γ in solution, featuring a simple experimental setup and straightforward data analysis. Since the signal strength relies on |γ|², the method proves to be very sensitive and allows rapid screening of organic molecules in dilute solutions for potential use in third-order NLO applications. In this manuscript, we demonstrate the experimental procedure and calibration of THS and have determined the second hyperpolarizability |γ| of commonly used solvents, which can be used as an internal calibration standard. As a proof of concept we determined γ of <i>trans</i>-stilbene and found it to be in excellent agreement with values obtained by other techniques

Synthesis, characterization and second-order nonlinear optical behaviour of ferrocene-diketopyrrolopyrrole dyads: the effect of alkene vs. alkyne linkers

© The Royal Society of Chemistry 2016. New, thermally stable dipolar ferrocene-diketopyrrolopyrrole (Fc-DPP) dyads with alkene as a linker exhibited structure dependent first hyperpolarizabilities, βHRS, recorded by a femtosecond HRS technique using a femtosecond (120 fs) pulsed laser light system at 900 nm at ambient temperature. On the basis of linear optical, electrochemical and TD-DFT studies, a good structure-polarization relationship has been established to account for the observed trends in first hyperpolarizabilities. The dyads exhibited fluctuating but matching solvatochromism. Nonlinear optical properties are modulated both by the strength of the acceptor as well as the length and nature of the π-conjugation bridge. βHRS of the dyads were compared with structurally related dyads in which the DPP core and the Fc donor are linked via an alkyne bridge. It is interesting to observe that the replacement of an alkyne link with alkene in these D-π-A chromophores does not necessarily furnish enhanced βHRS.crosscheck: This document is CrossCheck deposited related_data: Supplementary Information identifier: Kamaljit Singh (ORCID) copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal history: Received 4 August 2016; Accepted 27 September 2016; Accepted Manuscript published 27 September 2016; Advance Article published 4 October 2016; Version of Record published 20 October 2016status: publishe