59th Annual Report on Research 2014

Interest in semiconducting polymers and fused-oligomeric aromatic molecules for charge transport and storage/conversion of energy has exploded because of their potential to be low-cost alternatives to inorganic materials, easy processibility, and tunable synthesis.  For the past funding period, my group has pursued the synthesis of compounds that contain a thieno-[3,2-b]pyrrole structural motif because we anticipated them to exhibit promising and tunable electronic properties.  Our syntheses were designed to leverage our C–H bond amination methods that our laboratory has developed.  To achieve our goals, the following Specific Aims were proposed: Specific Aim #1.  Synthesize a range of polypyrrolothiophenes from monomers constructed via a Rh₂(II)-catalyzed C–N bond formation and study the relationship between their molecular structure and bulk electronic properties. Specific Aim #2.  Synthesize novel fused oligo-N-heterocycles and compare their bulk properties to acenes. Over the next few paragraphs, I detail our pursuit of these two Aims: Vinyl azides were transformed into soluble bispyrrolothiophenes through a series of C–h bond functionalization reactions by Crystalann Jones, an African-American graduate student who graduated this year. The performance of her bipyrrolothiophenes were optimized in thin film field effect transistors to obtain a mobility of ~ 0.3 cm²/(Vás) and an I_on/I_off of >10⁶ (Figure 1).  Despite exhibiting promising preliminary activity, AFM revealed that these films were amorphous solids.  Second generation bispyrrolothiophenes will be designed to maximize crystal packing in thin films. This project was published in Chemistry — A European Journal (2014, 20, 5938) and was highlighted on the back cover.

Figure 1. Crystal structure of thin film of bispyrrolothiophene.

The synthesis of N-heterocyclic heptacenes that contain a repeating fused-array of indoles and thiophenes was further explored (Scheme 1).  Our original synthesis was modified by: (1) placing removable SEM groups on the indolo-nitrogen instead of alkyl- or aryl groups; and (2) bromination of the resulting heteroacene to 4,9-dibromo acene.  The SEM-protecting groups could be removed iteratively from product N-heterocyclic acene to enable individual modification of each indolo-nitrogen to explore the effect of the N-substituent on the crystal packing. Thus far we have found that the use of N-aryl substituent induces ¹-stacking between layers, which we will leverage to achieve "bricklayer" crystal packing, which has been identified as the ideal isoform for electron-movement through an organic field effect transistor.

Scheme 2. Synthesis of N-heterocyclic acenes.

The third project that we have continued to pursue was the synthesis of cruciform molecules using pyrrolothiophenes on the electron-donating axis (Figure 3).  Using the synthesis first developed by a former undergraduate in our laboratory, Michelle Lee, several new bispyrrolothiophene cruciforms were synthesized.  Ab initio calculations showed that when the aryl acetylene moieties were pyridine that significant separation of the HOMO and LUMO occurred.  We anticipated that this separation could be exploited in sensing applications, and found both the absorbance- and fluorescence spectra changed dramatically upon addition of Zn(II) salts.  This cruciform is selective to Zn(II) salts: no change in the optical spectra was observed on the addition of other metal salts.  By changing the identity of the aryl groups, we will build on these results to synthesize cruciforms that are sensitive to toxic metal salts to be used as a colorimetric assays.

Figure 3. Electronic and optical properties of bispyrrolothiophene cruciform, a zinc sensor.

In conclusion, we have made significant progress to synthesize a diverse array of novel N-heterocycles that exhibit a range of promising electronic properties.  The data obtained from these studies formed the basis of an NSF grant proposal, which was funded.  We will build on these results to design and synthesize new organic materials that exhibit better charge mobilities in transistors or function as fluorescent probes.