Thiazole vs. Thiophene: Heterocycle Effects on the Properties of Fused-Ring Conjugated Materials

Abstract

Conjugated polymers and related molecular materials comprise a field of materials chemistry focused on the development of semiconducting organic plastics that find use in applications such as organic solar cells and organic light-emitting diodes. The optical and electronic properties of these molecules, such as absorption and emission of light, can be tuned through engineering at the molecular level. However, many of the current molecules of choice suffer from high-lying frontier orbitals, which results in a mismatch of energy levels to common components of electronic devices along with potential oxidative instability, constraining device performance in real environments. To rectify these issues, the electron-deficient thiazole heterocycle has been incorporated into fused-ring conjugated motifs of both organic and inorganic nature. The new thiazole materials all exhibited the expected stabilization of their frontier orbitals compared to the thiophene analogues. The absorption profiles of the thiazole materials are similar to the thiophene analogues, but with reduced molar absorptivity as a general trend, potentially limiting the efficiency of thiazole derived materials as components of photovoltaic devices. Through experimentation and development of multiple new classes of organic and inorganic thiazole materials, it was found that a larger proportion of thiazole content correlates to a larger decrease in molar absorptivity, but also a larger relative stabilization of the frontier orbitals. The limitations in molar absorptivity can thus be mitigated to an extent by increasing the molecule’s effective conjugation path through functionalization with additional conjugated units, but with the countereffect of less-stabilized frontier orbitals.