Extended spiro-germole systems as novel charge transport and charge generation materials

Rylan Wolfe1,  Heibbe de Oliveira1,  Veaceslav Coropceanu1,  Jean-Luc Brédas2,  John Reynolds1
1Georgia Institute of Technology, 2King Abdullah University of Science and Technology


Abstract

Materials for charge transport and charge generation have historically fallen into two categories: polymers or planar small molecules. Here, we highlight an as-yet-unexploited class of materials – cruciform molecules with non-planar conjugation through a spiro-metallole center. Spiro-type compounds have long been implemented in OLED devices as charge transport layers due to their film forming properties and elevated glass transition temperatures. Additionally, dithienogermole has proven itself as an important building block in both polymeric and small molecule systems, with polymer OPVs reporting efficiencies up to 8.5%. We combine these concepts to create molecules with the following in mind: discrete molecules with reproducible syntheses, extended conjugation for the broad absorption of the visible spectrum, non-planar structures for multi-dimensional transport and polycrystalline domains, and spiro-germole centered compounds for enhanced intramolecular conjugation and charge transfer. We report the efficient synthesis of spiro-dithienogermole cores with bromo and stannyl reactive groups available for further functionalization. All thiophene donor systems are realized via a four-fold Stille cross coupling reaction. The resultant family will study the effect of conjugation length on crystallinity, intermolecular packing, and ultimately charge transport. While the role of the germanium center is uncertain, experimental evidence supports the two halves are electronically linked, despite being perpendicular. FET and SCLC measurements in comparison to the planar analogues will be presented. Theoretical evidence for intra- and intermolecular charge transfer rates will be provided in support of the experimental results.