Indophenine Reaction as a Method for Synthesizing Conjugated Polymers with Sub-1 eV Optical Bandgaps

Abstract

Traditionally, conjugated polymer syntheses have relied heavily on metal-catalyzed cross-coupling reactions. These cross-couplings require appropriately functionalized monomers, increasing the length and complexity of the overall synthesis; in addition, some of the commonly used reagents (e.g., trialkyltin chlorides) are highly toxic. Even after a polymer has been successfully prepared, residual metal impurities can also compromise its optoelectronic properties and performance in devices. As such, there is a demand for metal-free approaches to conjugated polymer synthesis. In this work, the indophenine reaction was used to polymerize thiophene and bifunctional isatin comonomers, with only sulfuric acid used as the catalyst. The structural influence of the comonomers was evaluated using different thiophenes and bifunctional isatins. The resulting polymers have very wide absorption bands that span the UV/vis/NIR regions, with optical bandgaps as low as 0.76 eV. The polymers show electrochromic behavior, as determined through spectroelectrochemical measurements.

Synthesis of Conjugated Polymers: Comparing the Indophenine Reaction with Traditional Methods

“…34−37 In a recent paper from our group, we showed that the indophenine reaction, long known to produce colorful compounds for dyes and pigments, can be used as a metalfree polymerization reaction for the synthesis of conjugated polymers. 38 The reaction produced polymers in good yield and with reasonable molecular weights, demonstrating the utility of the methodology. However, because these particular polymers could only be prepared by way of the indophenine reaction, it was impossible to directly compare the indophenine reaction to more traditional methods of conjugated polymer synthesis.…”

“…41,47 Previous work has shown that this theoretical treatment and number of repeat units is able to provide an accurate estimation of the electronic transitions of indophenine-based polymers. 38 We calculated the electronic transitions associated with four distinct systems: a defect-free structure; a structure with a single monothiophene defect; a structure with two defects separated by a defect-free repeat unit; and a structure with two defects where the defects are on adjacent repeat units. In the defect-free structure, the results show that the S 1 ← S 0 transition is by far the most intense, with the S 2 ← S 0 having less than half of the oscillator strength and the higher-energy transitions being weaker still.…”