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.