Amphiphilic Porphyrin-Perylenebisimide Architectures – Synthesis and Self-Assembly

Self-assembled architectures, built from single organic molecules, possess huge potential for the development of next-generation functional materials or drug delivery systems. To this day, however, predicting and controlling the structure of the assemblies by sole consideration of molecular-level attributes is a highly challenging task. Therefore, understanding the impact of basic molecular characteristics, like shape, size, or charge, on the large-scale properties of assembled nanostructures is indispensable when seeking to purposefully introduce desired functionalities. To deepen the knowledge of these fundamental structure-to-function relations, synthesizing tailored sets of molecules that allow to methodically probe the influence of molecular-level variables is a key step in the advancement of this active field of research. In this context, this thesis concerns the development of a modular synthetic approach towards new amphiphilic porphyrin-perylenebisimide (PBI) conjugates. For the first time, the influences of shape, sterically demanding substituents, and polar head-groups on their self‑assembly and opto-electronic capabilities were systematically evaluated. Further, the implementation of the chromophores into a supra-amphiphilic structure was demonstrated. The primary focus of this thesis was on developing a straightforward synthetic route towards bola-amphiphilic bis(tetra-aryl-porphyrin)-PBI triads, where oligo-carboxylic acid NEWKOME dendrons were attached to the porphyrins as polar head-groups. Here, a key achievement was realizing the SONOGASHIRA cross-couplings of dendronized phenyl-acetylene tetra-aryl porphyrins with suitable bis-halogenated PBIs, allowing for a seamless modular synthesis. Using this methodology, a set of three bola-amphiphiles was synthesized, bearing differently sized dendron generations, as well as varying sterically demanding substituents at the PBI bay-position. Investigation of the self-assembly behavior in aqueous media via steady-state spectroscopy and cryo-transmission electron microscopy (cryo-TEM) revealed highly stable aggregates with a similar close-range structure – PBI-centered stacks with the porphyrins spaced apart – yet distinctly different large-scale morphologies. Subsequently, these results sparked interest in studying the influence of the meso‑substituents at the porphyrin on the supramolecular organization, for which a conventional amphiphilic dyad architecture was chosen. In this project, a set of three porphyrin-PBI dyads was synthesized, especially including one with only a bis-meso substituted porphyrin. Based on this scope, we demonstrated that the amphiphilic dyads possess a highly complex self-assembly behavior in terms of both their molecular-level structure and their large-scale morphology, as revealed by solvent-, time-, and temperature-dependent spectroscopy, and corroborated by scanning transmission electron microscopy (STEM). Most notably, however, the anticipated trend towards stronger porphyrin-porphyrin interactions was clearly observed for the bis-meso substituted derivative. In parallel, we set out to expand the scope of the underlying porphyrin-PBI system. To facilitate this, the inclusion of π-extended porphyrins into the architecture was probed synthetically in collaboration with the JUX group. Here, cross-coupling of halogenated rylenebisimides with suitable porphyrin building blocks was effective in creating 12 new panchromatic conjugates. These synthetic foundations open the door to studying these chromophores regarding their photophysical and self-assembly capabilities. Next, the lateral expansion of the original calamitic scaffold, resulting in a discotic amphiphilic pentad, was accomplished. This was achieved by a four-fold SUZUKI cross‑coupling using a tetra-iodo-PBI derivative and the corresponding dendronized boronic ester porphyrin, resulting in a to-date unrealized porphyrin-PBI conjugate. Interestingly, these conjugates show a quenched fluorescence, in contrast to their imide-connected analogues. It was observed that these amphiphiles assemble into thermodynamically highly stable aggregates, which STEM imaging revealed to consist of highly regular 2-dimensional (2D) sheets that coil onto each other, resulting in larger, irregular superstructures. Lastly, the possibility of non-covalently assembling the bola-amphiphilic structure itself was demonstrated. Representing a novel concept, the chromophores were connected by a H‑bonding motif, thereby creating a bola-supra-amphiphile. Notably, the H-bonds were self‑stabilizing through the assembly of an amphiphilic cyanuric-acid porphyrin and a hydrophobic HAMILTON-PBI into larger spherical aggregates, as evidenced by cryo-TEM. Transient absorption spectroscopy revealed multidirectional charge-transfer processes, depending on the chromophore excited, which are absent when the aggregates are dissolved. Overall, this thesis illustrates a trend that higher structural preorganization results in a less complex assembly landscape of the covalent amphiphiles and gives precedence for by‑design control over the light-matter interactions in the assembly. Moreover, it establishes an unprecedented method of supra-amphiphile assembly. In the future, the conceptual roadmap developed in this work enables further fundamental research to profoundly understand, expand, and apply amphiphilic photoactive architectures.