Search

Metal-organic frameworks (MOFs) are crystalline porous molecular materials, which consist of organic molecules (linkers) and metal centers, for example lanthanide (Ln) ions. In recent years, these materials have emerged as a new class of materials for photonics applications such as solid-state lighting, lasing and nonlinear optics.[1,2] In this work, we present a series of LnMOFs doped with terbium (Tb3+) and europium (Eu3+) ions with 1,3,5-benzenetricarboxylic acid (H3BTC) as linker. The crystallization process was monitored in situ using nonlinear scattering techniques and showed that the Ln-BTC MOFs crystallize in non-centrosymmetric space groups. The compounds crystallize in tetragonal space groups P4122 (Tb-BTC) and P4322 (Eu-BTC), as well as the mixed doped Tb100-xEuxBTC compounds with different molar ratios (x = 0.01, 0.05 and 0.1). The structures were characterized with single-crystal X-ray diffraction (XRD), powder XRD, FTIR and CHN analysis. The crystallization process was monitored in situ using nonlinear scattering techniques and showed that the Ln-BTC MOFs crystallize in non-centrosymmetric space groups. Additionally, the optical response of the LnBTC MOFs could be revealed by the combined power of linear and nonlinear spectroscopy (two-photon, SHG). The results obtained from the nonlinear optical study will be used as an input for the quantum spectroscopy investigation that is in progress, and it’s based on the interaction of MOFs with entangled photons.

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).

In situexperimental studies have been key in uncovering the often elusive pathways of nucleation and crystal growth. In the field of material science and medicine this offers the prospect of controlling crystallization processes to fight disease or tailor materials towards specific applications. To further advance this quest there is a need for flexible techniques mapping the different stages of crystallization with maximal sensitivity. This article reviews the benefits of nonlinear optical techniques to take on this challenge. We provide a perspective on various nucleation and crystal growth studies that were carried out by nonlinear optical probing techniques. A theoretical background is established, different relevant nonlinear optical phenomena are defined, and optical setups that have been used by various authors are summarized. A primary focus is demonstrating the benefits of nonlinear optical techniques for thein situstudy of crystallization. These benefits include low detection limits, complementary information by combining second- and third-order techniques, as well as relatively simple bench-top setups. Through microscopy, high contrast imaging of concomitant formations can moreover be achieved. The discussion outlines several studies involving ionic compounds, noble metal nanoparticles, polymers, metal organic frameworks and pharmaceutical compounds. Finally, we discuss future evolutions in nonlinear optical probing that are expected to further advance the field.

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 7-225 nrpages: 225 status: published