Multi-component porphyrin self-assembly

F.A. Helmich

Research output: ThesisPhd Thesis 1 (Research TU/e / Graduation TU/e)

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The self-assembly of organic molecules offers an attractive bottom-up approach to create nano-meter sized objects. By usage of chromophoric building blocks in a multi-component environment, supramolecular assemblies become highly interesting for numerous applications in the fields of sensing, catalysis and light harvesting. Regarding the development of functional supramolecular materials and mimicking natural systems, it is highly important to understand self-assembly processes in detail and the ability to control them; particularly the control over the arrangement and number of molecules in an aggregate. However, due to the presence of multiple interacting components in a dynamic assembly, these systems are inherently complex, which requires in-depth analyses in multi-component self-assembly. Rather than the description of their functional properties, the focus in this thesis is on insights into multi-component self-assembly of porphyrin monomers into non-discrete helical architectures. By the employment of modeling tools, the thermodynamic aspects of different interacting moieties via orthogonal supramolecular interactions are addressed, which allows one to predict the behavior of several systems comprising of Zn-porphyrins and different axial ligands. In these analyses, the role of cooperativity is closely examined, which strongly enhances the stimuli-responsiveness of the system. The supramolecular chirality has been applied as an additional probe to study the multi-component porphyrin self-assembly. In addition, chiral amplification experiments reveal that the supramolecular chirality provides a level of control over the mixing of different porphyrin monomers in an aggregate. Lastly, besides thermodynamic assessments, the kinetic properties of multi-component porphyrin assemblies have been studied. Here, different dynamic processes such as the exchange of monomers in an aggregate, aggregate interconversions and the imprint of helical conformations have been investigated. In Chapter 1, a literature overview is presented on the formation of porphyrin-based assemblies, in which different non-covalent interactions are used to construct well-defined architectures. The versatility of the porphyrin building block allows for a diversity of supramolecular motifs, which self-assemble by different mechanisms. Functional properties arise in multi-component mixtures, in which strategies are presented to control the mutual chromophore arrangement in discrete and non-discrete assemblies. Besides stoichiometric and positional control, a special focus in on cooperativity and the stimuli-responsiveness of porphyrin-based aggregates in multi-component systems. A library of metallo-porphyrin derivatives is developed in Chapter 2, in which the effect of metal center, amide linker and side-chain chirality on the one-component self-assembly is investigated. A hydrogen bond-assisted and highly cooperative self-assembly process is deduced for all amide-functionalized metallo-porphyrins. Their cofacial arrangement results into extended, helical, 1-dimensional H-type aggregates, which are fully analyzed for self-assembled S-chiral zinc-porphyrin "S-Zn" in methylcyclohexane (MCH). Temperature- and concentration-dependent UV-vis and CD measurements have been performed to obtain a thermodynamic description for the cooperative self-assembly. The resulting thermodynamic parameters for S-Zn have been deduced and applied in multiple equilibrium models that describe the self-assembly of S-Zn in the presence of different axial ligands in Chapter 3/4/5. In Chapter 3, the self-assembly of S-Zn is studied in the presence of pyridine, which depolymerizes the porphyrin stacks in a bimodal fashion into hydrogen-bonded, pyridine-capped dimers. As a result of cooperativity, a monomer-driven depolymerization mechanism is validated by fitting spectroscopy data of the pyridine titration to a corresponding thermodynamic model. Simulations on this depolymerization model are used to assess the competition between hydrogen bonding and metal-ligand association in a coupled system, which reveals a dilution-induced self-assembly of the porphyrin stacks. A slow interconversion between dimers and stacks is observed upon the addition of MCH and this kinetic property is used to control the distribution between both aggregate types by diffusive mass transfer in a microfluidic H-cell. Using the design rules for a strong pyridine-responsiveness between stacks and dimers established in Chapter 3, the photo-induced alteration of the binding constant of phenylazopyridines is explored to photo-regulate the self-assembly of S-Zn in Chapter 4. In the absence of hydrogen bonding, the thermodynamic properties of the photo-induced (de)complexation have been deduced by titration and irradiation studies, which have subsequently been introduced in a modified depolymerization model. High conversion ratios and a large difference in binding constant between both isomers induce a strong photo-switch-ability, which is predicted between 2% and 90% of stacked S-Zn monomers. Regardless the experimental deviations from the model, a reversible photo-induced (de)polymerization of the porphyrin stacks between 1% and 81% is achieved based on CD spectroscopy. Corroborating the spectroscopic measurements, the irradiation of the auxiliary causes a change in solution viscosity. In Chapter 5, the self-assembly of S-Zn is studied in the presence of the bidentate axial ligand DABCO. In the absence of hydrogen bonding, the thermodynamic properties of the Zn-porphyrin:DABCO 2:1 sandwich complexation have been deduced by UV-vis and 1H-NMR titrations, which reveal that coordination of the second nitrogen of DABCO is less favorable than the first. This negative cooperativity has subsequently been introduced in a modified depolymerization model, which describes the DABCO-induced formation of an alternating supramolecular block copolymer comprising of DABCO units and hydrogen bonded, Zn-porphyrin dimers. Using multiple analytical techniques, DABCO titration experiments reveal the formation of chiral, elongated structures at a 2:1 stoichiometry of S-Zn and DABCO, respectively. The unexpected stability of the alternating copolymer towards excessive amounts of DABCO is analyzed by the model, which demonstrates that stability is provided by positive cooperativity. Unlike their stability towards excessive amounts of DABCO, preliminary chain-stopping experiments indicate that the alternating copolymers readily depolymerize upon the addition of monotopic Mn(III)-porphyrins, which reveal energy transfer upon chain stopping. In Chapter 6, the coaggregation of porphyrin monomers with different chirality is studied by chiral amplification experiments. Efficient coaggregation has been observed between achiral and chiral porphyrins, as evidenced by a strong Sergeant-and-Soldiers effect. On the other hand, no chiral amplification is observed in the Majority-Rules experiment as a consequence of narcissistic self-sorting. The distinctive behavior in chiral amplification is quantified by modeling studies that reveal a combination of high helix reversal and high mismatch penalties. The latter penalty is also operative at the end of the stack as evidenced by the induction of the supramolecular chirality upon the addition of a chiral chain stopper. For mixed-metal Sergeants-and-Soldiers and Majority-Rules studies, the same trends in chiral amplification are also observed by fluorescence quenching between Zn and Cu-porphyrins. Within the same library, the limits of coaggregation are explored by a Diluted-Majority-Rules experiment, which demonstrate that opposite enantiomers only coaggregate when achiral comonomers are added to the system. The dynamic properties of mixed-metal porphyrin assemblies are investigated by the selective removal of Zn-porphyrin comonomers by axial ligation with quinuclidine. The extraction process proceeds at different time scales depending on the coaggregated state; slow extraction kinetics are found for the Sergeant-and-Soldiers and Diluted-Majority-Rules systems, while an instant extraction process has been deduced for the self-sorted Majority-Rules system. By simultaneously monitoring the supramolecular chirality during extraction, a chiral memory effect is observed for both systems that showed slow extraction kinetics. For the Sergeant-and-Soldiers system, the remaining supramolecular backbone comprises of achiral Cu-porphyrins only, which give rise to a long-lasting chiral memory with slow, entropy-driven atropisomerization. The stability of the chiral memory is analyzed by time- and temperature-dependent CD studies; for the latter, the memory can be erased and partially restored upon heating and cooling of the solutions. In case of the Diluted-Majority-Rules experiment, the remaining supramolecular backbone comprises of a mixture of achiral and chiral Cu-porphyrins. Being present in an aggregate with the unpreferred helicity, the remaining chiral Cu-porphyrins induce a short chiral memory with enthalpy-driven atropisomerization. The final Chapter is an outlook towards porphyrin-based polymeric systems, in which non-symmetrical porphyrins are covalently linked to polymeric backbones. Preliminary feasibility studies have been performed to investigate if porphyrin-functionalized polymers offer a suitable platform to convey the stimuli-responsiveness deduced in solution to the macroscopic level.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Chemical Engineering and Chemistry
  • Meijer, E.W. (Bert), Promotor
  • Schenning, Albert P.H.J., Copromotor
Award date7 Feb 2012
Place of PublicationEindhoven
Print ISBNs978-90-386-3075-5
Publication statusPublished - 2012


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