Design and synthesis of novel porphyrin and carbonyl-bridged triarylamine for self-assembly and solar cell applications
Creators: name: ALJABRI, Mahmood
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)
classificationCode: 340304
subject: Optical properties of materials
classificationCode: 340503
subject: Organic chemical synthesis

description: Supramolecular chemistry is currently gaining substantial interest because it is no longer seen as promising in certain fields; but is shifting towards becoming applicable in a wide range of applications. Therefore, a current goal of supramolecular chemistry is to undertake research that moves this branch of chemistry towards existing or potential applications. One of the most important goals is to design molecules that can selectively and specifically interact to provide well-defined nanostructures and to understand the mechanism behind the interactions that lead to specific morphologies. Based on an understanding of the state of the art of porphyrin-based nanomaterials, in this thesis we have rationally designed some new porphyrin-based molecules to study their self-assembly and explore possible applications in both photocatalysis and solar cells.

An octaphosphonate porphyrin (porphyrin 1) was nominated as a candidate to study the potential of controlling self-assembly with nucleobases, due to the prospect of interaction between phosphonate groups on the porphyrin and amine moieties on nucleobases, resulting in the preparation of well-order materials. A fluorinated porphyrin (TPFPP) was designed due to the ability of the electronegative fluorine groups to affect properties such as solubility in mixed solvents, allowing the porphyrin to self-assemble in well-defined structures due to its hydrophobicity. We have also designed a protoporphyrin IX (PPIX), which through having two carboxylic acid groups has a tendency to co-assembly with the amine groups in arginine. These porphyrin derivatives led to the formation of desirable morphologies such as nano-flower morphologies, which affect their activity in applications such as photocatalysis. The final part of this thesis describes the development of new organic materials with favourable properties for use in organic photovoltaic devices leading to improvements in a solar cell device. In particular, the design and synthesis of new organic molecules functionalized with electron acceptor moieties, based on techniques learnt in the first part of the thesis, enabled a better understanding of solar energy conversion with functional organic materials.
 
Firstly, supramolecular self-assembly of an octaphosphonate tetraphenyl porphyrin (porphyrin 1) with three different nucleobases (adenine, cytosine, and thymine) was studied. Porphyrin 1 with 8 and 10 equiv. of cytosine produced light-harvesting ring-like structures, that is, architectures similar to those observed in natural light-harvesting antenna. However, the porphyrin assembled with adenine or thymine resulted in prisms and microrods, respectively. UV-Vis absorption, fluorescence, and dynamic light scattering were used to determine the mode of aggregation in solution. Scanning electron microscopy and X-ray diffraction spectroscopy was used to visualize the self-assembled nanostructures and their behaviour in the solid state, respectively. Thus, we believe that this study has helped us develop a deeper understanding on how one needs to manipulate donor/acceptor subunits in supramolecular assemblies to construct artificial antenna architectures. 
Porphyrin aggregates formed via self-assembly have shown promising photocatalytic activity due to the combination of their optical and morphological properties. The structural and physical properties of a porphyrin affect its self-assembly and the properties of the resulting aggregates. The hydrophobicity/hydrophilicity of the porphyrins in solvent mixtures, which affect self-assembly and can be altered by the introduction of fluorine groups. We report the synthesis of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP). The differing solubilities of TPFPP in organic and aqueous solutions were exploited to promote the self-assembly of monomeric TPFPP in THF/H2O solvent mixtures. The effect of the H2O fraction on the assembly process and resulting morphologies was probed using UV-Vis spectroscopy, photoluminescence spectroscopy, and scanning electron microscopy (SEM). It was observed that well-defined TPFPP microrods with a diameter of 1-3 micrometre and length of 20-100 micrometre, as well as octahedral crystals 30 micrometre in size were produced with H2O fractions of 70 and 80%, respectively. These TPFPP aggregates with controlled morphologies exhibited high photocatalytic activity, evident in photocatalytic degradation studies with rhodamine B (RhB) which degraded under visible light irradiation with rate constants of 3.76×10-3 (with microrods) and 2.93×10-3 min-1 (with octahedral crystals). A possible mechanism for the photocatalytic activity of the TPFPP aggregates for RhB degradation was proposed.
 
Furthermore, in our continuous investigation of discovering new porphyrin-based nanomaterials and probe their effectiveness as photocatalysts, the fabrication of controlled supramolecular nanostructures via self-assembly of protoporphyrin IX (PPIX) was studied with enantiomerically pure L-arginine and D-arginine, and we have shown that stoichiometry controlled the morphology formed. The nanostructure morphology was mainly influenced by the delicate balance of pi-stacking interactions between PPIX cores, as well as H-bonding between the deprotonated acidic head group of PPIX with the guanidine head group of arginine. PPIX self-assembled with L-/D-arginine to create rose-like nanoflower structures for four equivalents of arginine that were 5-10 micrometre in length and 1-4 micrometre diameter. We employed UV-vis, fluorescence spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), dynamic light scattering (DLS) and Fourier transform infrared spectroscopy (FTIR) techniques to characterize the resulting self-assembled nanostructures. Furthermore, we investigated the catalytic activity of PPIX and arginine co-assembled materials. The fabricated PPIX-arginine nanostructure displayed enhancement of photocatalytic activity through degradation of RhB with a decrease in dye concentration of around 78-80% under simulated visible radiation.
 
Finally, in a collaborative project to extend our understanding of organic functional materials in the field of organic solar cell devices, we report for the first time the use of a carbonyl-bridged triarylamine (CBT) core with diketopyrrolopyrrole (DPP) terminal units to generate a three-dimensional, non-planar non-fullerene electron acceptor with favourable properties for use in organic photovoltaic devices. The carbonyl-bridged triarylamine-functionalized, small molecule non-fullerene electron acceptor, 2,6,10-tris(5-(2,5-bis(2-ethylhexyl)-3,6-dioxo-4-(thiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)thiophen-2-yl)-4H-benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-4,8,12-trione (coded as R1), was synthesized via the industrially viable Suzuki cross-coupling reaction using the commercially and cheaply available substrates. Using PTB7 as a donor, a power conversion efficiency of 9.33% was achieved in simple, solution-processable bulk-heterojunction devices, a result that is amongst the best in the literature for three-dimensional non-fullerene acceptors.

This thesis thus provides series of novel molecules of porphyrin-based nanomaterials that were designed and fabricated in following various self-assembly methodologies, leading to a better understanding of the morphologies formed and enhancement of photocatalytic performance. Also, the synthesis and characterization of novel non-fullerene acceptor molecules based on a newly discovered core has been presented to increase our knowledge in the area of photovoltaic and reinforce our understanding of organic photovoltaic devices.