Stijn Van Cleuvenbergen

Controlled partial interpenetration in metal–organic frameworks

Interpenetration, 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. Interpenetration of metal–organic frameworks (MOFs) is a common phenomenon, in which a structure consists of two or more identical, entangled sub-lattices. Now, MOFs with variable, fractional degrees of occupancy of one of two sub-lattices have been prepared. The extent of interpenetration can be controlled either during synthesis or by autocatenation, a framework rearrangement process.

Molecular engineering of chromophores for combined second-harmonic and two-photon fluorescence in cellular imaging

A series of chromophores with enhanced second- and third-order nonlinear optical properties were engineered for use in combined second-harmonic and two-photon fluorescence microscopy. Electron-accepting moieties imparted nonlinear optical properties to the chromophores. The electron-rich carbazole core served as a template towards one- or two-dimensional chromophores. More efficient acceptor groups (pyridinium, benzazolium, benzothiazolium) on the carbazole donor core resulted in improved second- and third-order nonlinear optical properties. A selection of these chromophores was tested in a cellular environment with a multimodal multiphoton microscope. The structural differences of the chromophores resulted in high selectivity for mitochondria or the nucleus in two-photon fluorescence and ranging from no signal to high selectivity for mitochondria in the SHG channel.

Thermally stable ferrocenyl “push–pull” chromophores with tailorable and switchable second-order non-linear optical response: synthesis and structure–property relationship

A combined UV-visible, cyclic voltammetric theoretical and hyper-Rayleigh scattering study has been carried out on a new series of ferrocenyl chromophores (2a–2d and 7–11) to develop a structure–property relationship, by varying the acceptor strength and the conjugation path length between the donor and the acceptor and by disubstitution of the ferrocene moiety. This allowed systematic establishment of the contribution of the conjugation bridge, acceptor strength and substitution on both rings of ferrocene towards non-linear optical response as well as its redox switchability with one of the chromophores (8) having an on/off ratio of 25, the highest for ferrocenyl chromophores, and also thermal stability up to 300 °C. To deduce the participation of the available polarizable electrons towards the overall non-linear optical properties (with electron contribution from both the rings as observed from average bond length calculation from crystal data of 2c and 7 and the absorption studies), respective intrinsic hyperpolarizability (β0int) of the dipolar chromophores was calculated. Interestingly, the shorter chromophore 2c showed exceptionally high non-linear optical response and intrinsic hyperpolarizability compared to its longer counterparts.

Synthesis, Spectroscopy, Nonlinear Optics, and Theoretical Investigations of Thienylethynyl Octopoles with a Tunable Core

We have synthesized three new molecules that have three thienylethynyl arms substituting a central benzene core and different electron donor/acceptor groups in the three remaining phenyl positions. The absorption, fluorescence, phosphorescence, and transient triplet-triplet spectra are analyzed in the light of the electronic structure of the ground and excited states obtained from quantum-chemical calculations. From the above, the relevant photophysical data (including quantum yields, lifetimes, and rate constants) could be derived. It was found that the major deactivation pathway is internal conversion, which competes with the fluorescence and intersystem crossing processes. For the three investigated compounds, we provide convincing theoretical support corroborating these findings and further conclusions based on the theoretical information obtained. These molecules are one of the very few cases in which the depolarization ratios, obtained from the NLO optical measurements, clearly reflect the octopolar configuration. Molecular hyperpolarizabilities have been measured and display a typical dependence on the donor-acceptor substitution pattern.

Nonlinear Optical Thin Film Device from a Chiral Octopolar Phenylacetylene Liquid Crystal

A set of chiral discotic phenylacetylenes have been synthesized by 3-fold Sonogashira coupling between different ethynylbenzenes and triiodobenzenes. The resultant bulk materials are fully characterized by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction. The octopolar nature of the target compounds is studied by UV-vis absorption spectroscopy and hyper-Raleigh scattering in solution. Optimization of the donor-acceptor substitution yields both high hyperpolarizability values and appreciable mesomorphic properties. A simple thin film device for second harmonic generation has been prepared from the nitro-substituted liquid crystalline derivative.

Real-Time Fluorescence Detection in Aqueous Systems by Combined and Enhanced Photonic and Surface Effects in Patterned Hollow Sphere Colloidal Photonic Crystals

Hollow sphere colloidal photonic crystals (HSCPCs) exhibit the ability to maintain a high refractive index contrast after infiltration of water, leading to extremely high-quality photonic band gap effects, even in an aqueous (physiological) environment. Superhydrophilic pinning centers in a superhydrophobic environment can be used to strongly confine and concentrate water-soluble analytes. We report a strategy to realize real-time ultrasensitive fluorescence detection in patterned HSCPCs based on strongly enhanced fluorescence due to the photonic band-edge effect combined with wettability differentiation in the superhydrophobic/superhydrophilic pattern. The orthogonal nature of the two strategies allows for a multiplicative effect, resulting in an increase of two orders of magnitude in fluorescence.

Fabrication of optomicrofluidics for real-time bioassays based on hollow sphere colloidal photonic crystals with wettability patterns

An optomicrofluidic device was developed by introducing 3D wettability patterns into hollow SiO2 sphere colloidal photonic crystals. Aqueous liquids flow through the superhydrophilic channel due to the surface tension confinement effect. Based on the significant fluorescence enhancement from photonic band gap (PBG) effects in these channels, real-time specific bioassays with high sensitivity were realized. To demonstrate this strategy, with two complementary single stranded DNA molecules acting as a target (fluorophore labeled) and a probe respectively, a 150-fold enhancement of fluorescence was observed compared with a similar device on a standard glass plate. This enhancement results from the strong PBG effect in an aqueous environment for these structures. While the PBG effect diminishes from refractive index matching in conventional solid sphere colloidal photonic crystals with water infiltrated, it is effectively enhanced in hollow sphere colloidal photonic crystals. This is because the dense shell of the hollow spheres prevents water from infiltrating into the inner air cavity of the hollow spheres, while water fills the voids between spheres. This creates a larger refractive index contrast, resulting in a pronounced PBG effect and strong fluorescence enhancement.

Sandwich Approach toward Inverse Opals with Linear and Nonlinear Optical Functionalities

Three-dimensionally ordered macroporous materials have unique structural and optical properties, making them useful for numerous applications in catalysis, membrane science, and optics. Accessible and economic fabrication of these materials is essential to fully explore the many possibilities that these materials present. A new templating method to fabricate three-dimensionally ordered macroporous materials without overlayers is presented. The resulting structures are freestanding inverse opals with large-area uniformity. The versatility and power of our fabrication method is demonstrated by synthesizing inverse opals displaying fluorescence, chirality, upconversion, second harmonic generation, and third harmonic generation. This economical and versatile fabrication method will facilitate research on inverse opals in general and on linear and nonlinear optical effects in 3D photonic crystals specifically. The relative ease of synthesis and wide variety of resulting materials will help the characterization and improvement of existing anomalous dispersion effects in these structures, while providing a platform for the discovery and demonstration of novel effects.

Record-high hyperpolarizabilities in conjugated polymers

Disubstituted poly(phenanthrene), a conjugated polymer, has been studied by hyper-Rayleigh scattering. Although this compound lacks the donor–acceptor motif that is typically associated with a strong second-order nonlinear optical response, the (intrinsic) hyperpolarizability ranks among the highest ever measured, breaking the longstanding apparent limit. The linear and nonlinear optical properties of the polymer depend strongly on the solvent conditions, affecting the macromolecular organization. An explanation for these unexpected results is postulated and is based on modulation of conjugation along the polymer backbone. As the molecular structure of the compound does not at all fit into the classical paradigms, our observations put these theories into perspective.

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 trans-stilbene and found it to be in excellent agreement with values obtained by other techniques.