Supramolecular Photochemistry Studies of New Light-Harvesting Materials based on Host-Guest Systems

Abstract

The research presented in this PhD thesis describes the design, the synthesis and the optical and structural properties of new light harvesting materials, prepared using host-guest mesoporous silica systems. The use of host–guest chemistry strategies aims to achieve precise control over the position and functionality of luminescent molecules within the silica pores. The introduction Chapter 1 provides insight into energy conversion systems and their applications, while Chapter 2 describes and explains the techniques used throughout the research project. Chapter 3 covers the design of the new light-harvesting antennae based on host-guest materials and the study of the energy transfer mechanisms within them. The materials are based on periodic mesoporous organosilicas (PMOs), in which a donor dye is embedded within the framework. In this work, a Zinc(II) quinolinolate complex (ZnPMO) will play the role of energy donor, whereas luminescent compounds such as Rose Bengal and Eosin Y act as acceptors. Subsequently, a helically chiral N,N,O,O BODIPY was synthesised, through a route that involves four reaction steps, starting with a Suzuki-Miyaura reaction between Boc protected pyrroleboronic acid and 2-bromophenol, followed by condensation and chelation to yield the ester-substituted N,N,O,O-BODIPY. The latter was then hydrolysed to form a carboxylic acid, followed by an amide coupling reaction using 3-aminopropyl triethoxysilane (APTES), in order to form a silylated BODIPY and use this as a third acceptor dye. We focused on controlling the donor:acceptor (D:A) ratio and then studied the interaction between them, which is interpreted in terms of Förster Resonance Energy Transfer (FRET). Then, the effect of the D:A ratio on the efficiency of FRET has been investigated. In Chapter 4, a full photophysical spectroscopy study was carried out to analyse the homoconjugative interactions between the aryl rings in a series of new 7,7-diarylnorbornane (DPN) derivatives, as well as some other non-homoconjugated molecules, known as 7,7 diphenylmethane (DPM) derivatives for comparison. Furthermore, we incorporated the DPM molecules into silica particles, in order to investigate the effect of incorporation into host guest materials. The experiments resulted in conformational restriction of the aromatic rings and, therefore, both absorption and emission properties of DPMs and DPNs become more similar in some cases, indicating the possibility of induced homoconjugation by spatial restriction. Chapter 5 presented a systematic photophysical investigation of three series of new mono substituted diketopyrrolopyrroles (DPPs), focusing on the effects of N-alkylation and core substitution on the optical properties. The impact of varying the para-substituent (F, Cl, Br, I, MeO) on one of the two aryl rings attached to the DPP core was studied, along with varying the N-alkyl substituent (benzyl, benzoyl, 3,5-bis(trifluoromethyl)benzyl). All DPPs exhibit strong absorption and high molar absorptivity, and also show intense green emission, high quantum yields and large radiative rates, making them suitable for applications requiring bright emission, such as light-emitting devices and imaging probes, so as for light-harvesting materials. Finally, a new research line was explored in Chapter 6, which laid the foundations for a study to understand the interaction between gold nanoparticles (AuNPs) and luminescent dye molecules within hybrid nanomaterials. Core–shell nanostructures were prepared, consisting of a gold core and a mesoporous silica shell (Au@SiO₂), with a BODIPY dye attached to the silica outer surface. The reason for such design is to control and optimise the plasmonic interactions, by varying the distance between the dye and the gold core via silica shell thickness. Although the formation of uniform and stable Au@SiO₂ nanostructures remain a challenge, this chapter establishes the synthesis of the host matrix and gold nanospheres with a diameter of 30 nm, optimises conditions for Au@SiO₂ systems, and outlines the alternative surface modification methods to enhance silica nucleation on the gold surface as a future work. In parallel, a BODIPY chromophore was synthesised and functionalised with a triethoxysilane group, enabling attachment to silica surfaces.