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

Controlled partial interpenetration in metal–organic frameworks

International audienceInterpenetration, the entwining of multiple lattices, is a common phenomenon in metal–organic frameworks (MOFs). Typically, in interpenetrated MOFs the sub-lattices are fully occupied. Here we report a family of MOFs in which one sub-lattice is fully occupied and the occupancy level of the other can be controlled during synthesis to produce frameworks with variable levels of partial interpenetration. We also report an ‘autocatenation’ process, a transformation of non-interpenetrated lattices into doubly interpenetrated frameworks via progressively higher degrees of interpenetration that involves no external reagents. Autocatenation maintains crystallinity and can be triggered either thermally or by shear forces. The ligand used to construct these MOFs is chiral, and both racemic and enantiopure partially interpenetrated frameworks can be accessed. X-ray diffraction, nonlinear optical microscopy and theoretical calculations offer insights into the structures and dynamic behaviour of these materials and the growth mechanisms of interpenetrated MOFs

Unveiling the nonlinear optical response of Trictenotoma childreni longhorn beetle

The wings of some insect species are known to fluoresce under illumination by ultraviolet light. Their fluorescence properties are however, not comprehensively documented. In this article, the optical properties of one specific insect, the Trictenotoma childreni yellow longhorn beetle, were investigated using both linear and nonlinear optical (NLO) methods, including one- and two-photon fluorescence and second harmonic generation (SHG). These three distinct optical signals discovered in this beetle are attributed to the presence of fluorophores embedded within the scales covering their elytra. Experimental evidence collected in this study indicates that the fluorophores are non-centrosymmetric, a fundamental requirement for SHG. This study is the first reported optical behavior of this type in insects. We described how NLO techniques can complement other more convenient approaches to achieve a more comprehensive understanding of insect scales and integument properties

Octopolar materials for second-order nonlinear optics.

In this work, the molecular and supramolecular second-order nonlinear optical (SONLO) properties of octopolar chromophores are studied. Molecules of octopolar symmetry are noncentrosymmetric, a fundamental requirement for efficient SONLO structures, and nonpolar. The main research interest in these systems lies in their ability to spontaneously adopt noncentrosymmetric crystal structures, as opposed to traditionally studied dipolar chromophores. A typical recurring motif are extended C3-symmetric pi-systems, substituted with electron donating and electron accepting groups. In this work, we have studied several series of octopolar chromophores, derived from this motif. The main goal was to gain insight in the structure-function relationship of the studied systems, related to their molecular SONLO properties. In other words: how can we tune the molecular SONLO response by chemical synthesis, i.e. the introduction of various molecular functionalities? Since technological applications ultimately rely on bulk materials, we also studied the bulk properties of a series of compounds, and explored how we can control the supramolecular organization, determining the SONLO response, by molecular engineering. After introducing the field of linear and nonlinear optics, and the employed characterization methods, we discuss a very broad interdisciplinary study of the electronic structure and corresponding molecular properties of a series of thienylethynyl octopolar compounds. A variety of spectroscopic techniques is used to study the structure-function relationship of these compounds. These results are backed-up by theoretical calculations. The SONLO response of the compounds shows a subtle modulation depending on the nature of the substituents and is shown to depend on the charge-transfer character from the core to the periphery, as effected by the electron donating/accepting character from the side groups. The experimental SONLO results are reproduced well by theoretical calculations and allow for a qualitative comparison between various models. In the next chapter we present a study on the linear and nonlinear optical properties of a series of hexaazatriphenylene (HAT) molecules. The electron deficient HAT core is extended with peripheral electron donating groups of different strength. Furthermore the effect of the spacer (double or triple carbon-carbon bonds) connecting the core to its periphery is investigated. Linear and nonlinear optical spectroscopy, electrochemistry and theoretical modeling provide insight in the electronic properties of these molecules, revealing a distinct influence of the different spacers and the mesomeric effect of the donor groups on the charge-transfer properties and ultimately the SONLO response. In a fifth chapter, an extensive series of triindoles with different conjugation length and peripheral substituents is investigated by linear and nonlinear optical spectroscopy, in combination with electrochemistry. A study of the first hyperpolarizability of these compounds reveals an unusually strong wavelength dependence that cannot be explained by the conventional three-level model for octopoles. To explain for this behaviour, we carried out an extensive study at multiple wavelengths and were able to model the full wavelength dependence of the first hyperpolarizability by a theoretical analysis using Thomas-Kuhn sum rules. We show that the unusually strong dispersion can be understood in terms of the mixing of higher excited states through the nonlinear optical interaction. This study reveals that dispersive effects can have a much greater influence on the SONLO response than was assumed so far. In chapter 6, a series of donor-acceptor substituted 1,3,5 trisalkynylbenzenes is studied. The molecular properties are explored by linear and nonlinear spectroscopy. The charge-transfer character of the chromophores is reflected in the linear regime. The introduction of strong electron accepting groups results in an increased SONLO response. In a second part we studied the bulk properties of a series of nitrated analogues, that display discotic liquid crystalline mesophases at higher temperatures. The bulk SONLO response, depends on the length and nature of the alkoxy side chains, driving the supramolecular organization. The introduction of chiral side groups, in an effort to promote a chiral crystal packing, is investigated and shown to lead to a considerable SONLO response in the crystalline state. The crystal structure of these compounds is determined by polarized SHG microscopy. For a series with achiral side chains of different length, longer side chains lead to centrosymmetric structures. The crystalline state of an analogue with shorter side chains exhibits a large SONLO response. The symmetry is determined by polarized SHG microscopy. Surprisingly, a chiral point group was found for the achiral analogues as well, critically confirmed by circular dichroism spectroscopy. Finally, we present an image analysis formalism, based on straightforward polarized second-harmonic generation microscopy tests. This convenient formalism allows assessing the local organization and homogeneity of crystalline structures, of crucial importance for technological applications, with high resolution and sensitivity.Dankwoord i Samenvatting v Abstract vii List of abbreviations ix Chapter 1 Nonlinear optics 1-1 1.1 General aspects of nonlinear optics 1-3 1.1.1 Linear interaction of light and matter 1-3 1.1.2 Nonlinear interaction of light and matter 1-5 1.1.3 Tensors and symmetries in second-order nonlinear optics 1-7 1.2 Relevant nonlinear optical processes 1-13 1.2.1 Second-harmonic generation 1-13 1.2.2 Hyper-Rayleigh scattering 1-15 1.3 Materials for NLO 1-22 1.3.1 Traditional D--A molecules 1-22 1.3.2 Octopolar molecules 1-27 1.4 References 1-34 Chapter 2 Experimental techniques and methods 2-41 2.1 UV-VIS absorption spectroscopy 2-43 2.2 Circular dichroism spectroscopy 2-45 2.3 Hyper-rayleigh scattering 2-48 2.3.1 Set-up 2-48 2.3.2 Data analysis 2-49 2.3.3 Correcting for multi photon fluorescence with demodulation and phase data 2-52 2.3.4 Depolarization ratio 2-54 2.4 Second-harmonic generation 2-56 2.4.1 Set-up 2-56 2.4.2 Continuous polarization measurements 2-57 2.5 Second-harmonic generation microscopy 2-61 2.5.1 Set-up 2-61 2.5.2 Point group symmetry determination with polarized second-harmonic microscopy via symmetry traces 2-63 2.6 References 2-67 Chapter 3 Spectroscopy, nonlinear optics and theoretical investigations of thienylethynyl octopoles with a tunable core 3-71 3.1 Introduction 3-73 3.2 Materials 3-75 3.3 Experimental and theoretical details 3-76 3.3.1 Spectroscopic measurements 3-76 3.3.2 Nonlinear optical characterization 3-76 3.3.3 Quantum-Chemical Calculations 3-76 3.3.4 Nonlinear Optics Calculations 3-77 3.4 Results and discussion 3-78 3.4.1 Electronic structure 3-78 3.4.2 Nonlinear spectroscopy 3-82 3.5 Summary and conclusions 3-86 3.6 References 3-87 Chapter 4 Hexaazatriphenylene derivatives 4-91 4.1 Introduction 4-93 4.2 Materials 4-95 4.3 Experimental and theorethical methodologies 4-96 4.3.1 Spectroscopic and Electrochemical Measurements 4-96 4.3.2 Nonlinear Optical Spectroscopy 4-96 4.3.3 Theoretical Methods 4-97 4.4 Results and discussion 4-98 4.4.1 Electronic structure 4-98 4.4.2 Nonlinear spectroscopy 4-103 4.5 Conclusions 4-107 4.6 References 4-108 Chapter 5 Dispersion overwhelms charge-transfer in determining the magnitude of the first hyperpolarizability in triindole octopoles 5-113 5.1 Introduction 5-115 5.2 Materials 5-117 5.3 Experimental methods 5-118 5.3.1 Linear spectroscopy 5-118 5.3.2 Hyper-rayleigh scattering 5-118 5.3.3 Electrochemistry 5-119 5.3.4 Theoretical analysis: Sum over states calculations 5-119 5.4 Results 5-122 5.4.1 Linear spectroscopy 5-122 5.4.2 Hyper-rayleigh scattering 5-124 5.4.3 Theoretical analysis: Sum over states calculations 5-131 5.5 Discussion 5-136 5.6 Conclusion and perspective 5-137 5.7 References 5-139 Chapter 6 Donor-acceptor substituted 1,3,5 trisalkynylbenzenes: Octopolar mesogens for nonlinear optics. 6-143 6.1 Introduction 6-145 6.2 Materials 6-148 6.3 Linear optical spectroscopy 6-150 6.3.1 Experimental 6-150 6.3.2 Results and discussion 6-150 6.4 Hyper-Rayleigh scattering 6-153 6.4.1 Experimental 6-153 6.4.2 Results and discussion 6-154 6.5 Mesomorphic properties 6-172 6.6 Circular dichroism spectroscopy 6-176 6.6.1 Experimental 6-176 6.6.2 Results and discussion 6-176 6.7 Second-harmonic generation measurements 6-181 6.7.1 Experimental 6-181 6.7.2 Continuous polarization measurements 6-181 6.7.3 Temperature dependent SHG measurements 6-185 6.7.4 Relative SHG measurements 6-187 6.8 Second-harmonic generation microscopy 6-188 6.8.1 Chiral, octopolar crystals from a chiral, octopolar molecule 6-188 6.8.2 Chiral, polar crystals from an achiral, octopolar molecule 6-193 6.9 Conclusions 6-208 6.10 References 6-210 Chapter 7 Conclusions and perspectives 7-215 List of publications 7-223 Articles in internationally reviewed scientific journals 7-223 Articles in international conferences proceedings 7-223 Submitted articles 7-224 Environmental, Health and Safety Protocols 7-225nrpages: 225status: publishe

Direct fabrication of complex 3D hierarchical nanostructures by reactive ion etching of hollow sphere colloidal crystals

Direct reactive ion etching (RIE) of hollow SiO2 sphere colloidal crystals (HSCCs) is employed as a facile, low-cost method to fabricate complex three-dimensional (3D) hierarchical nanostructures. These multilayered structures are gradually transformed into nanostructures of increasing complexity by controlling the etching time, without complicated procedures (no mask needed). The resulting 3D topologies are unique, and cannot be obtained through traditional approaches. The formation mechanism of these structures is explained in detail by geometrical modeling during the different etching stages, through shadow effects of the higher layers. SEM images confirm the modeled morphological changes. The nanostructures obtained by our approach show very fine features as small as ∼30 nm. Our approach opens new avenues to directly obtain complex 3D nanostructures from colloidal crystals and can find applications in sensing, templating, and catalysis where fine tuning the specific surface might be critical.crosscheck: This document is CrossCheck deposited related_data: Supplementary Information copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal copyright_licence: This article is freely available. This article is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported Licence (CC BY-NC 3.0) history: Received 6 July 2016; Accepted 8 August 2016; Accepted Manuscript published 9 August 2016; Advance Article published 22 August 2016; Version of Record published 1 September 2016status: publishe

Nucleation and particle growth of poly(3-alkylthiophenes)

Nucleation – the initial step of the crystallization – is still rather unexplored, while this step has a major impact on the outcome [1]. One of the possible systems for studying nucleation are organic polymers, namely polythiophenes, where nucleation and particle growth has recently been examined by non-linear optics (NLO) [2]. In this research, dynamic small-angle X-ray scattering (SAXS) experiments are used for nanoscale information about size, structure and morphology of monomers/aggregates in the initial stages of the particle growth. In total, 3 different poly(3-alkylthiophenes) were studied by sealing various concentrations of the polymer in a mixture of a THF solvent and MeOH anti-solvent. The polymer mixtures were dissolved by heating, and the aggregation with subsequent particle formation was observed during the cooling period. In-house trials were succeeded by synchrotron experiments at the ESRF BM26 beamline. Results are showing differences in particle formation between the types of the polymers, based on the concentration, side-chain, presence of defects in the backbone of the polymer or the ratio of solvent/anti-solvent in the mixture. Aggregation of polymers is also observed prior to the precipitation of particles. In conclusion, these experiments, together with work in the field of NLO, are providing deeper insight in particle formation of organic materials. [1] Davey, Roger J. et al. Crystal engineering – nucleation, the key step. CrystEngComm. 4, 257 – 264 (2002). [2] Moris, Michèle et al. Harmonic light scattering study reveals structured clusters upon the supramolecular aggregation of regioregular poly(3-alkylthiophene). Communications Chemistry. 2:130 (2019)

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

Advent of Plasmonic Behavior: Dynamically Tracking the Formation of Gold Nanoparticles through Nonlinear Spectroscopy

The preparation of gold nanoparticles through reduction of chloroauric acid by trisodium citrate, also known as the Turkevich synthesis, was analyzed both ex situ and in situ. In situ experiments consist of dynamically tracking second and third harmonic light scattering and multiphoton luminescence. By complementing in situ data with ex situ quenching experiments, which enabled further UV–vis–NIR spectroscopy, as well as transmission electron microscopy (TEM) and dynamic light scattering (DLS) characterization, we obtained new insight into the mechanistic process of gold nanoparticle growth. Our results reveal that the growth proceeds through a metastable state of aggregation and offer additional evidence for a sharp transition from metallic molecular cluster to plasmonic nanoparticle behavior in the initial stage of the process. While multiphoton luminescence can be used as a marker for this transition, second and third harmonic scatterings reveal surface and bulk information such as size, shape, and the presence of aggregates.status: publishe