The Photochemistry of Organic Materials for Photonic Devices

<p>Optically active organic chromophores have attracted much interest in recent years for their potential for use in photonic devices. Chromophores such as compound (1) have been found to have a very high second order nonlinear susceptibility ( β ) value of 650 × 10⁻³⁰esu in dimethyl formamide.¹ The performance of 1 in a polymer film is much lower than this due to the formation of aggregates which hinder the poling process necessary to ensure a noncentrosymmetric arrangement of the molecules in order to display second order nonlinear behaviour. The molecular aggregation behaviour of a set of second order nonlinear compounds based on compound 1 have been studied in this thesis. These compounds share the backbone shown in figure 1 with pendant groups added to the R₁ R₂ and R₃ positions, with the aim of finding substituent groups that can be added to the optically active merocyanine backbone that reduce the aggregation and increase the solubility of the compounds. This in turn will make them more suitable for use in photonics devices. It was found that a C₁₁H₂₃ alkyl chain added to the R₃ position made the largest contribution to decreasing aggregation. Bulky groups on the R₁ and R₂ positions also reduced aggregation. As a result compounds 5 and 8, with R₃ = C₁₁H₂₃ and bulky groups attached displayed the least aggregation of the compounds studied. ¹ See Figure 1 (pg. i): Merocyanine backbone with substituent positions marked.</p>

The Photochemistry of Organic Materials for Photonic Devices

<p>Optically active organic chromophores have attracted much interest in recent years for their potential for use in photonic devices. Chromophores such as compound (1) have been found to have a very high second order nonlinear susceptibility ( β ) value of 650 × 10⁻³⁰esu in dimethyl formamide.¹ The performance of 1 in a polymer film is much lower than this due to the formation of aggregates which hinder the poling process necessary to ensure a noncentrosymmetric arrangement of the molecules in order to display second order nonlinear behaviour. The molecular aggregation behaviour of a set of second order nonlinear compounds based on compound 1 have been studied in this thesis. These compounds share the backbone shown in figure 1 with pendant groups added to the R₁ R₂ and R₃ positions, with the aim of finding substituent groups that can be added to the optically active merocyanine backbone that reduce the aggregation and increase the solubility of the compounds. This in turn will make them more suitable for use in photonics devices. It was found that a C₁₁H₂₃ alkyl chain added to the R₃ position made the largest contribution to decreasing aggregation. Bulky groups on the R₁ and R₂ positions also reduced aggregation. As a result compounds 5 and 8, with R₃ = C₁₁H₂₃ and bulky groups attached displayed the least aggregation of the compounds studied. ¹ See Figure 1 (pg. i): Merocyanine backbone with substituent positions marked.</p>

Paper-Based Analytical Devices for Colorimetric and Luminescent Detection of Mercury in Waters: An Overview

Lab-on-paper technologies, also known as paper-based analytical devices (PADs), have received increasing attention in the last years, and nowadays, their use has spread to virtually every application area, i.e., medical diagnostic, food safety, environmental monitoring, etc. Advantages inherent to on-field detection, which include avoiding sampling, sample preparation and conventional instrumentation in central labs, are undoubtedly driving many developments in this area. Heavy metals represent an important group of environmental pollutants that require strict controls due to the threat they pose to ecosystems and human health. In this overview, the development of PADs for Hg monitoring, which is considered the most toxic metal in the environment, is addressed. The main emphasis is placed on recognition elements (i.e., organic chromophores/fluorophores, plasmonic nanoparticles, inorganic quantum dots, carbon quantum dots, metal nanoclusters, etc.) employed to provide suitable selectivity and sensitivity. The performance of both microfluidic paper-based analytical devices and paper-based sensors using signal readout by colorimetry and luminescence will be discussed.

X-ray transient absorption reveals the 1Au (nπ*) state of pyrazine in electronic relaxation

Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized 1B2u (ππ*) (S2) and 1B3u (nπ*) (S1) states, the participation of the optically dark 1Au (nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computations. Despite 1Au (nπ*) and 1B3u (nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The 1Au (nπ*) state is populated in 200 ± 50 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.