BRIDGING EXPERIMENT AND THEORY IN PORPHYRIN CHEMISTRY: DEVELOPING MATERIALS FOR SENSING, OPTICAL, AND CATALYTIC TECHNOLOGIES

Throughout my three-year PhD, I conducted an in-depth exploration of porphyrin chemistry with a particular focus on the systematic design, synthesis, and characterization of innovative porphyrin-based materials. Working within the SunLab group (renowned for its expertise in this field) provided an ideal environment to investigate new molecular architectures for applications in catalysis, sensing, solar energy conversion, and nonlinear optics (NLO). In all these fields, enhanced charge-transfer characteristics play a key role, and this requirement has guided the rational design of the systems developed during my doctoral project. The first part of my PhD (Chapter 2) centered on catalysis, specifically on the synthesis and evaluation of four RhIII porphyrins for the hydrolysis of ammonia borane (AB) aimed at hydrogen production. After optimizing the synthetic routes for all Rh–porphyrin complexes, we performed extensive spectroscopic characterization and assessed their catalytic performance in collaboration with Prof. Villa at the University of Milan. While ionic, water-soluble RhIII porphyrins exhibited high activity, their progressive deactivation over successive runs highlighted an inherent limitation of homogeneous catalysis. In contrast, the neutral and water-insoluble TAP_Rh complex demonstrated remarkable stability and reusability, underscoring the advantages of catalyst heterogenization. These results motivated the development of Rh–porphyrin-based covalent organic frameworks (COFs) as robust heterogeneous catalytic architectures featuring ordered porosity and accessible active centers. By incorporating Rh–porphyrin units into imine- and triazole-linked COFs, we created modular catalytic systems where linker chemistry and the electronic nature of fluorinated versus non-fluorinated monomers finely tuned hydrogen-release efficiency, structural robustness, and interaction with the aqueous reaction environment. As discussed in detail in Chapter 2, the catalytic activity of these COFs was investigated under two complementary design strategies: (i) systems in which the RhIII center is directly coordinated to the porphyrin core during the COF synthesis, and (ii) post-metallated frameworks, where RhIII is introduced within the porous architecture after COF formation but without direct coordination to the porphyrin core. Despite the fact that Rh uptake in the post-metallated materials is approximately half of that incorporated through direct metallation, these post-synthetic metalated systems consistently displayed superior catalytic performance. Among all the COFs synthesized (ICOF, TCOF, and FTCOF), those featuring imine linkages exhibited the highest activity, highlighting the crucial role of linker flexibility and framework connectivity in enabling efficient mass transport and effective exposure of catalytic sites. Parallel to the catalytic work, the second major research line (Chapter 3) focused on the development of porphyrin–semiconductor hybrid materials for volatile organic compound (VOC) sensing. Building on previous collaborative studies with Prof. Cappelletti’s group (University of Milan), in which ZnTPP and its perfluorinated analogue ZnTPPF20 had been compared, we broadened this effort to evaluate our newly synthesized β-substituted porphyrins in chemoresistive SnO₂-based sensors. After optimizing the synthesis of the target porphyrins, we assessed their behavior toward acetone detection under UV illumination and in dark conditions. This work also included a detailed investigation of how structural features of the porphyrin influence its interaction with the SnO₂ surface. In particular, we examined a β-substituted derivative incorporating an ethynyl–benzothiadiazole spacer terminated with a cyanoacrylic anchoring group (ZnTPPF20CN), which may exist in two configurational isomers (E and Z) due to restricted rotation around the C=C bond of the pendant group. Although theoretical and literature evidence indicates that one isomer is favoured under thermodynamic control, we considered both to account for potential kinetic trapping or configurational rearrangements during surface adsorption. To investigate how the pendant group directs SnO₂ interaction, we hypothesized and modelled different adsorption modes for both isomers. Particular attention was devoted to chemisorption pathways involving the cyanoacrylic moiety, by analogy with known adsorption behaviour of carboxylic groups on metal-oxide surfaces. Two primary modes were considered: a monodentate configuration featuring coordination through a single oxygen atom, and a bidentate mode involving deprotonation and chelation to multiple Sn centers. DFT calculations (performed using the SIESTA code), supported by experimental measurements, demonstrated how binding motif and isomeric configuration significantly influence adsorption strength, charge-transfer efficiency, and interfacial electronic coupling. Together, these results confirmed that tailored functionalization can effectively tune the porphyrin–semiconductor interaction, ultimately improving acetone-sensing performance while preserving the mesoporous architecture of SnO₂. The third major axis of my PhD research (Chapter 4) focused on nonlinear optics (NLO). Here, I developed two distinct families of perfluorinated ZnII porphyrins: a β-series in which an A4-perfluorinated core bears ethynylphenyl substituents with NMe₂ donor or NO₂ acceptor groups at the β-pyrrolic positions, and a click-series in which the same donor/acceptor patterns were introduced via a triazole–phenyl linker on an A3B-type platform. Although perfluorinated porphyrins are well known in the context of Dye-Sensitized Photoelectrosynthetic Cells and catalytic processes, they had never been investigated from an NLO perspective. The comparison between our newly synthesized betaNMe₂/betaNO₂ systems and previously reported analogues (BP1 and BP2) provided insightful information on how peripheral fluorination modulates charge-transfer properties and can potentially enhance second-order NLO response. This study relied on complete spectroscopic and electrochemical characterization carried out at SunLab. During my second year, I strengthened this NLO-focused work through a six-month research stay at the University of Copenhagen under the supervision of Prof. K. M. Mikkelsen. There, I acquired extensive expertise in computational chemistry, complementing experimental findings with DFT and TD-DFT calculations using Gaussian16 and ORCA. This combined computational–experimental approach allowed us to deepen our understanding of the electronic structures and charge-transfer mechanisms of the porphyrin systems. The expertise developed in Copenhagen is now being applied to ongoing catalytic studies, particularly to elucidate the mechanistic steps of AB dehydrogenation. Overall, this thesis follows a coherent trajectory from molecular porphyrins to extended hybrid materials across three distinct application areas: catalysis, sensing, and nonlinear optics. The evolution from homogeneous Rh–porphyrin catalysts to COF-based heterogeneous systems, the development of functionalized porphyrins for semiconductor interfaces, and the design of perfluorinated push–pull architectures for NLO collectively illustrate how rational molecular design, supported by experimental and computational approaches, enables the development of advanced functional materials. • Rhodium Porphyrin-based Materials for Ammonia–Borane Hydrolytic Dehydrogenation: This section explores the field of catalysis, analysing the design strategies and structural optimization approaches that enable the development of efficient catalysts for hydrogen evolution from AB. It includes the optimization of synthetic routes used to prepare the desired catalysts and the rational progression from homogeneous and heterogeneous RhIII–porphyrin catalysts to RhIII–porphyrin-based covalent organic frameworks (COFs), designed as heterogeneous and recoverable catalytic systems. Their characterization and application in AB dehydrogenation are also discussed in detail. • Porphyrins for VOC Sensing: The subsequent section focuses on the use of β-functionalized porphyrins (conceived as an evolution of our previously published work) in sensor technologies, clarifying their role in enhancing sensor response under mild operating conditions. A sensing mechanism supported by theoretical calculations is also proposed for the novel β-substituted porphyrin employed in this work, shedding light on the improved sensing performance compared with the earlier ZnTPPF20 system. • Perfluorinated Porphyrins for Nonlinear Optics (NLO): The final section of the thesis investigates two new perfluorinated ZnII porphyrins in the context of their nonlinear optical properties. While similar systems have been previously explored in Dye-Sensitized Photo-Electrochemical Cells (DSPECs), no data are available regarding the effect of perfluorination on push–pull architectures for NLO applications. This part of the work relies on both the synthesis of these new push–pull systems and the integration of experimental results with DFT calculations, with the aim of unveiling the fundamental characteristics of these prospective nonlinear materials.