Accessing Multiple Phases via Thermodynamic or Kinetic Pathways: The Impact of Bivalent Ferrocene Spacers on 2D Hybrid Perovskite Formation

Many semiconductor technologies require interfacing materials with different properties. 2D hybrid perovskites are one of the most promising candidates, combining the advantages of organic and inorganic layers. The networks of linked metal-halide octahedra with voids filled by organic counterions have proven high variability and can be tailored to specific applications. The geometric and electronic setup of the organic linker molecule between inorganic layers affects the crystal structure and the overall optoelectronic properties. Monoamines typically form bilayers in so-called Ruddlesden-Popper phases (RPs), while bisamines allow for making Dion-Jacobson phases (DJs), with only a monolayer directly bridging the inorganic layers. Therefore, it would be highly interesting if one could compare RPs to DJs directly to each other, meaning that they have been prepared using exactly the same organic linker molecule, which is the aim of the study presented here. Because of the potential interaction of π-conjugated compounds with the electronic system of the semiconductor, we have selected a special linker here: a divalent ferrocene derivative containing one primary amine attached to each of the cyclopentadienyl rings. These linkers form novel quasi-DJs, and their structure was determined by 3D electron diffraction and density functional theory. We found that by different crystallization kinetics, two quasi-DJ variants and even RPs can be obtained from the same spacer molecule. It takes time for the ferrocene-based linker to adjust to a particular conformation, giving the system also time to form octahedral connections other than the classic DJ/RP corner-sharing. The different octahedral linkages, ranging from face- to corner-sharing, have a significant influence on the optoelectronic properties. Thus, with a change of the synthesis conditions, it is possible to change the structure and the properties of the same educts. Our study also enables the first direct comparison of quasi-DJ and RP phases by achieving both with the same spacer molecule.