New Synthetic Methods for π-Conjugated Systems

The field of conjugated polymer synthesis is dominated by the use of cross coupling reactions and oxidative coupling (“intermolecular Scholl” reactions).  Similarly, we have used these established polymerizations to produce complex polymer structures.  This approach is really required when integrating complex functionality and receptors into a polymer.   However, there are places wherein new tools are needed.  Robust chain growth syntheses that can be initiated from surfaces and combined with different classes of polymerizations will enable new compositions and structures.  In one such case we developed cationic chain growth syntheses of polythiophenes (Bonillo, B.; Swager, T. M. “Chain-Growth Polymerization of 2-Chlorothiophenes Promoted by Lewis Acids” J. Am. Chem. Soc. 2012, 134, 18916-18919).

A novel chain growth polymerization of thiophenes and elastomeric block polymers that look like stretchable gold foil.

More recently we have developed methods making use of dehydrative reactions to produce extended chromophores (Voll, C. C. A.; Swager, T. M. “Extended π-conjugated Structures via Dehydrative C-C Coupling” J. Am. Chem. Soc. 2018, 140, 17962-17967).  We are in the process of extending these methods to polymer synthesis.

Transition metal free coupling reactions by simple dehydrative coupling.

We also have an enduring interest in ladder polymer structures and introduced one of the first syntheses of graphene ribbons (Goldfinger, M. B.; Swager, T. M. “Fused Polycyclic Aromatics via Electrophile-Induced Cyclization Reactions:  Application to the Synthesis of Graphite Ribbons” J. Am. Chem. Soc. 1994, 116, 7895-6).  Note from the title of this paper we called them graphite ribbons rather than graphene.  This is because this was nearly 20 years before the term graphene had been accepted.

One of the earliest syntheses of a graphene ribbon.  This annulation reaction has been now widely used to make complex polycyclic aromatic materials.

Our key novel annulation step in this sequence wherein a pendant alkyne is cyclized to create a new aromatic ring has since been used by many groups (including ours) with modifications in conditions and electrophiles to make complex materials.  It remains as one of the more reliable methods to produce complex graphene ribbon structures. Most recently in collaboration with a former group member, Tomoyuki Ikai (now a Professor at Nagoya), this method has been extended to create chiral ribbon polymers.   We have abundant new targets for this method, so stay tuned.

A helical tube created by our annulation reaction applied to a chiral triptycene.

Our love of π-systems is also founded on their outstanding properties, including optical processes.  A current topic in the group is developing materials that optically respond to magnetic fields by a mechanism called the Faraday effect.  As illustrated, this effect involves the rotation of a plane of polarized light by passing through a material along the direction of an applied magnetic field. Magnetic sensors using this effect can be very sensitive and can even measure brain activity (moving ions in brain function create magnetic fields).  The transducers in these sensors have traditionally been transition metals, but recently it has been found that π-conjugated polymers can have sensitivities (determined by the Verdet constant) that are more than 100X those of the inorganics. We think this is only the beginning and are striving to develop detailed structure property relationships that will help to generate materials with even higher sensitivity.  Imagine what we can do with a material that is 104 times more sensitive.  In this case we can use thin films of organics in sensors rather than large and expensive oriented inorganic crystals.

Schematic for the observation of the Faraday response wherein plane polarized light is rotated by interacting with a materials under the influence of a magnetic field.

Our research in Faraday rotations is evolving rapidly.  We have built a custom spectrometer to study this effect and measure the rotations over a wide range of wavelengths.  It is clear that there is going to be a special place for π-conjugated molecules and polymers and we have recently found that helical polythiophenes display large Verdet constants (Wang, P.; Jeon, I.; Lin, Z.; Peeks, M.; Savagatrup, S.; Kooi, S.; Van Voorhis, T.; Swager, T. M. “Insights into Magneto-Optics of Helical Conjugated Polymers” J. Am. Chem. Soc.  2018, 140, 6501-6508).   

Chiral helical polythiophenes that display large Faraday rotations.