Tetrapyrrolic Materials for Solar Energy Production

The increasing global demand for energy, largely satisfied through fossil fuel consumption, represents one of the most critical challenges of the 21st century due to its strong contribution to greenhouse gas emissions and climate change. In this context, the development of sustainable and low-impact energy conversion technologies has become essential. Among renewable energy sources, photovoltaics plays a central role, and particular interest has recently been directed toward organic photovoltaic (OPV) systems due to their potential advantages, including low-cost fabrication, mechanical flexibility, lightweight properties, and suitability for large-area and building-integrated applications. However, despite significant progress, OPV technologies still suffer from limitations in terms of power conversion efficiency and long-term stability. Therefore, the design and development of novel photoactive materials with improved optoelectronic properties remain a key objective. In this framework, the present PhD work focuses on the synthesis, characterization, and application of tetrapyrrolic macrocycles, specifically thioalkyl-substituted porphyrazines, as promising materials for solar energy conversion. Tetrapyrroles are particularly appealing as photoactive systems due to their structural similarity to natural light-harvesting molecules such as chlorophyll, as well as their strong absorption in the visible region, high chemical stability, and tunable electronic properties. Among them, porphyrazines have been relatively less explored in comparison to porphyrins and phthalocyanines, despite offering unique opportunities for molecular engineering. Thioalkyl-substituted porphyrazines exhibit distinctive optical and electronic features due to the presence of sulfur atoms, which introduce additional electronic transitions and broaden the absorption spectrum, enabling panchromatic light harvesting across the visible range. The main objective of this work was the synthesis of novel asymmetrically substituted thioalkyl porphyrazines and their metal complexes, along with the investigation of their structural, spectroscopic, and electrochemical properties, and their implementation in different types of organic photovoltaic devices, including dye-sensitized solar cells (DSSCs) and bulk heterojunction (BHJ) solar cells. Several synthetic strategies were developed to introduce functional groups at the β-position of the porphyrazine macrocycle. New thiophene-based substituents bearing ester functionalities were successfully introduced through Suzuki–Miyaura and Liebeskind–Srogl cross-coupling reactions. These modifications were designed to promote anchoring to TiO₂ surfaces in DSSCs and to enhance electronic interactions within the macrocycle. However, the inability to efficiently convert ester groups into the corresponding carboxylic acids limited the applicability of these systems as DSSC sensitizers. In parallel, a series of porphyrazines functionalized with extended aromatic moieties, such as pyrene and helicene units, were synthesized to improve π–π interactions with carbon-based electron acceptors. These systems were investigated for application in BHJ solar cells and in supramolecular nanohybrids with nanocarbon materials. In addition, a novel β-η¹-palladium(II) thioethyl porphyrazine complex was isolated as catalytic intermediate during the Suzuki cross-coupling reactions. This compound represents the first example of a isolable -ring metalated tetrapyrrole. This organometallic compound was fully characterized by spectroscopic methods and single-crystal X-ray diffraction. Moreover, detailed experimental and computational studies (TDDFT) revealed peculiar electronic features, including charge-transfer transitions involving the metal fragment, suggesting its potential relevance for optoelectronic applications . The interaction between β-η¹-palladium(II) thioethyl porphyrazine and graphene nanoflakes was also investigated through the preparation of non-covalent nanohybrids. Spectroscopic and microscopy techniques confirmed the formation of these hybrid systems, highlighting changes in optical properties and significant fluorescence quenching. Despite these promising characteristics, preliminary photodetectors based on these materials did not exhibit measurable photocurrent, indicating that further optimization is required. In the last stage of the thesis work, the first bulk heterojunction solar cells incorporating thioalkyl porphyrazines as donor materials were fabricated and evaluated. Although the achieved power conversion efficiencies were relatively low (up to ~0.1%), these results represent a significant proof of concept, demonstrating, for the first time, the feasibility of employing porphyrazine-based systems in BHJ architectures. Among the investigated compounds, the best performance was obtained with symmetric porphyrazines and helicene-substituted derivatives. Overall, this work provides new insights into the design, synthesis, and application of porphyrazinebased materials for organic photovoltaics. Although the performance of the fabricated devices is still limited, the results highlight the strong potential of thioalkyl porphyrazines as tunable chromophores for solar energy conversion. Future improvements will require the optimization of molecular structure, device architecture, and processing conditions, as well as the exploration of alternative acceptor materials and hybrid systems.