Sensors and sensibility: Tailoring the mechano-responsivity of macromolecules & polymer materials

This thesis describes our explorations in the development of new molecular tools to probe complex soft material systems at the nanoscale. In large part, our efforts have been dedicated to conjugated polymer-based molecular sensors for applications in complex fluids. We have contributed both to developing new chemical strategies for incorporating new function in these macromolecular sensors, but also to develop a deeper understanding of how they work and what their intrinsic limitations are.

In Chapters 2, 3, 5 we explore new ways to introduce mechanoresponsive moieties into the backbone of semiconducting sensor polymers. In Chapter 2 we introduce a spiropyran moiety into a polyfluorene backbone. Spiropyran functions as a molecular switch whose two states exhibit unique photochromic properties. The molecular switch can be activated by a variety of triggers including but not limited to mechanical-, pH-, light- and temperature-stimuli, and is thermally reversible in the small molecule state. Incorporated into the extended π-conjugated system of a semiconducting polymer backbone, activation of these molecular switches results in spectroscopic shifts of the complete semi-conducting system and a loss of the thermal reversibility. We explore the influence of different chain architectures of spiropyran-co-fluorene polymers and the consequences of expanding the conjugation length of the spiropyran monomer on its switching properties.

In Chapter 5 we continue along this concept, by introducing a different type of molecular switch into polyfluorene backbones. Here we focus our attention to anthracene monomers, which remain susceptible to modification via a Diels-Alder reaction also post-polymerisation. The Diels-Alder adduct occurs anti-parallel on the central benzene ring of the anthracene causing splitting of conjugation into two isolated systems, thereby interrupting the flow of electrons along the semiconducting polymer backbone and straining the macromolecular conformation. Retro-Diels-Alder reactions, which restore the extended π-conjugation is shown to be achievable both thermally and mechanically. The Diels-Alder adduct of these poly(fluorene-co-anthracene) polymers results in polymers with switchable conjugation. Moreover, we show how tuning of the composition of the polymers enables the tuning of the pre-strain existing on the Diels-Alder adduct and thereby tailoring of the easy of activation and the rate at which this occurs.

In Chapter 3 we investigate a different route to create mechano-responsive semiconducting polymers, by the introduction of a molecular rotor into the backbone of a semiconducting chain. These small molecule molecular rotors have been widely applied as rigidochromic probes, e.g. as free volume probes in a variety of complex experimental systems. We incorporate the rotating phenyl group of a BODIPY rotor into the π-extended system of a polyfluorene polymer. We show that the energetic separation of the rotating moiety and the BODIPY core remains proficient to retain its rigidochromic properties. Furthermore, the spectral separation between the non-responsive blue backbone emission and the responsive green bodipy emission creates an internal fluorescence emission standard. The energy transfer from the donor backbone to the acceptor rotor ensures this can be achieved using a single channel blue excitation source. This allows for the polymer to be used as a ratiometric free volume sensor in fluorescence imaging while maintaining high spectral and temporal resolution.

Previous work from our group showed how acceptor-doped conjugated polymers are very well suited for use as molecular mechanosensors. Yet various effects remained poorly explored. In Chapter 4, we investigate chain-length effects on the energy transfer within acceptor-doped conjugated polymer sensors. We perform an exhaustive fractionation of a polymer batch into different fractions of higher monodispersity and different chain lengths. Using a combination of extensive spectroscopic study and Monte Carlo simulations, we show that the interplay between both chain-length and acceptor doping degree play a fundamental role in the usability of an acceptor based conjugated polymer as a strain sensor. We attribute this to a probabilistic process which occurs during the growth of the polymer chain in its synthesis. Although the law of large numbers averages these effects out for a homogeneous batch of sufficiently long polymers this is often ignored for shorter polymers. These results provide fundamental insights into the function of macromolecular strain sensors and guidelines how their use can be improved.

In addition to main theme of this thesis, as summarized above, a side-project on the assembly and phase behavior of soft colloids resulted in two published chapters. Both chapters aimed to understand how the details of the softness of small colloidal particles changes their properties in large ensembles in dense suspensions. In Chapter 6 we use experiments and theory to explain how the softness of particles changes the apparent way in which the liquid suspension turns solid and how these apparent changes vanish when correcting for the size changes inherent to soft particles. This resolves an open issue in the field of colloidal glasses. In Chapter 7 we continue this exploration by studying how softness, and in particular details on the shape of the soft potential, alters the formation of ordered phases. We show how small changes in the interaction potential can sort large changes in the crystal structures formed. These in-silico results provide guides to the preparation of non-close-packed and non-hexagonal crystal symmetries using colloidal particles as the building blocks.