Two-Dimensional Lead Halide Perovskites Templated by a Conjugated Asymmetric Diammonium

We report novel two-dimensional lead halide perovskite structures templated by a unique conjugated aromatic dication, N,N-dimethylphenylene-p-diammonium (DPDA). The asymmetrically substituted primary and tertiary ammoniums in DPDA facilitate the formation of two-dimensional network (2DN) perovskite structures incorporating a conjugated dication between the PbX42- (X = Br, I) layers. These 2DN structures of (DPDA)PbI4 and,(DPDA)-PbBr4 were characterized by single-crystal X-ray diffraction) showing uniquely low distortions in the Pb-X-Pb bond angle for 2D perovskites. The Pb-I-Pb bond angle is very close to ideal (180 degrees) for a 2DN lead iodide perovskite, which can be attributed to the ability of the rigid diammonium DPDA to insert into the PbX62- octahedral pockets. Optical characterization of (DPDA)PbI4 shows an excitonic absorption peak at 2.29 eV (541 nm), which is red-shifted in comparison to similar 2DN lead iodide structures. Temperature-dependent photoluminescence of both compounds reveals both a self-trapped exciton and free exciton emission feature. The reduced exciton absorption energy and emission properties are attributed to the dication-induced structural order of the inorganic PbX42- layers. DFT calculation results suggest mixing of the conjugated organic orbital component in the valence band of these 2DN perovskites. These results demonstrate a rational new strategy to incorporate conjugated organic dications into hybrid perovskites and will spur spectroscopic investigations of these compounds as well as optoelectronic applications.