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

Many technologies require interfacing materials with different properties, which becomes more difficult if the two compounds have diverging structures or compositions. In semiconductor science, for instance, one frequently needs compounds with different band gaps, but typically they belong to different material classes. Not so for hybrid perovskites. The networks of linked metal- (e.g. lead) halide octahedra with voids filled by organic counterions have proven high variability. In particular, impressive achievements were made in tailoring the properties of layered materials. The geometric and electronic setup of the organic linker molecule between inorganic layers impacts the crystal structure and the overall optoelectronic properties. Monoamines typically form bilayers in the so-called Ruddlesden-Popper phases (RPs), while bis-amines allow for making Dion-Jacobsen phases (DJs) with only a monolayer directly bridging the inorganic sheets. 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 ferrocene derivative containing one primary amine attached to each of the cyclopentadienyl rings. As wanted, these linkers form novel DJs; their structure was resolved by 3D electron diffraction. We found that by different crystallization kinetics, one can obtain two DJ variants and even RPs because it takes time for the ferrocene-based linker to adjust to a particular conformation. Because the linkage of the octahedra is also different for the three cases, we could observe notable changes in optoelectronic properties.