Molecular nonlinear optics: advanced chromophore characterization for target-specific (supra)molecular designs

Abstract

The coherent, monochromatic and intense light of a laser opened the way to research in nonlinear optics, the field in which this doctoral research is situated. Nonlinear optical (NLO) materials are able to alter the color of light, switch it or change its transmission characteristics, depending on its intensity. The technological applications of such nonlinear optical interactions are myriad. After introducing the general aspects of (non)linear optics in the first chapter, the experimental details of linear absorption, hyper-Rayleigh scattering (HRS) and depolarization ratio measurements are discussed in Chapter 2. The different approaches to deal with the competing process of multi-photon fluorescence in the HRS measurements are discussed in detail. In Chapter 3, a study on the second-order nonlinear optical properties of organometallic molecular structures with increasing dimensionality is elaborated. The flexible nature of the substitution and oxidation states of transition metal complexes enables their use as donor or acceptor moieties in chromophores. The presence of a metal center allows for other geometries, as a palette of coordination patterns is available for metals. Intrinsically connected to the presence of a transition metal center, these compounds possess reversible redox switching behavior. The NLO properties of one-dimensional, thermally stable ferrocenyl "push-pull" chromophores with an isophorone derived acceptor, ferrocene-diketopyrrolopyrroles and ferrocene-α-cyanostilbenes are discussed, as well as two-ferrocenyl V-shaped chromophores with a rhenium or zinc center and heptametallic octopolar compounds. We have established clear structure-property relations towards molecular optical switches with varying polarization sensitivity. Chapter 4 deals with the development of porphyrin-based chromophores for bio-medical imaging, more specifically the development of voltage-sensitive dyes to track brain activity when intercalated in the membranes of neurons. In the development of these dyes, function as well as form requirements, i.e. the appropriateness of the compounds for the experimental question and biological target respectively must be taken into account. As an example for nonlinear optical imaging of a cell membrane, a dipolar compound should have amphiphilic properties. Porphyrins can make excellent second-harmonic generation (SHG) dyes to probe voltage changes over neuronal membranes. The finding that the free-base porphyrin core is sufficiently electron-deficient that the hyperpolarizability does not increase on addition of a pyridinium electron-acceptor creates opportunities in the design of new SHG probes. Besides, experimental results show the possibility of elaborating the porphyrin bridge by adding another porphyrin building block to optimize the voltage sensitivity of these extraordinary membrane probes. Dispersion of first and second hyperpolarizability values within the bio-optical transparency window should be mapped and exploited to optimize the possibilities of the SHG probe. Helically wrapped single-walled carbon nanotubes (SWNTs) are studied in chapter 5. We report for the first time experimental measurements of the first hyperpolarizability of individualized, length-sorted (700 ± 50 nm long) single chirality (6,5) SWNTs. The chiral, opto-electronically active polymers wrap the nanotubes in an exclusively left-handed, single-chain helical fashion and feature PZn2, PZn3 and PZnRuPZn octopolar chromophores as integral parts of the polymer backbone. The non-covalent functionalization of these electronically homogeneous SWNTs by NLO chromophores demonstrates an appealing strategy to design new classes of electro-optic materials that feature enhanced hyperpolarizabilities over the telecommunication-relevant spectral domain. The importance of target-oriented development, taking into account both form and function requirements, is clear from the very different approaches of chromophore molecular design in chapters 3, 4 and 5. We explored higher dimensionality as well as extension along one dimension and supramolecular chirality in the domain of molecular optical switches, biological imaging as well as optical telecommunication.nrpages: 162status: publishe