Visualizing the impact of chloride addition on the microscopic carrier dynamics of MAPbI3 thin films using femtosecond transient absorption microscopy.

The spatial heterogeneity of carrier dynamics in mixed halide perovskite CH3NH3PbI3-xClx thin films with a range of different chloride additions is mapped using femtosecond transient absorption microscopy (TAM). The comparison of TAM images of fibrous and granular polycrystalline CH3NH3PbI3-xClx films indicates that the impact of chloride addition on the local heterogeneity of carrier dynamics is highly dependent on the film preparation method and the resulting morphology. In addition to signals of pristine CH3NH3PbI3, CH3NH3PbI3-xClx films with a fibrous structure show long-lived excited state absorption (ESA) signals in localized, microscopic regions. The ESA signal exhibits transient absorption with a rise time of about 5 ps after the excitation pulse, indicating that these distinct micrograins have preferential carrier trapping properties. The chemical composition of these micrograins does not differ detectably from their surroundings. In contrast, in CH3NH3PbI3-xClx films with a granular structure, Cl addition does not seem to affect the charge carrier dynamics. These results provide insight into the localized effects of halide mixing and on the resulting photophysical properties of mixed halide perovskite materials on the micrometer length scale.

Tuning two-electron transfer in terpyridine-based platinum(ii) pincer complexes.

An important factor in obtaining reversible multi-electron transfer is overcoming large changes in coordination geometry. One strategy is to use ligands that can support the geometries favored before and after the electron transfer. Pip2NCN- pincer and terpyridine ligands are used to support square planar Pt(ii) and octahedral Pt(iv). For the Pt(ii) complexes, [Pt(Z-pip2NCN)(R-tpy)]+ (Z = NO2, MeO, H; R = H, tertyl butyl, tolyl), 1H NMR spectroscopy shows that the Z-pip2NCN- ligand is monodentate whereas the R-terpyridyl ligand is tridentate. The availability of flanking piperidyl groups of the monodentate pincer ligand is essential for the stabilization of the metal center upon oxidation. Pt(Z-pip2NCN)(R-tpy)+ complexes undergo two-electron platinum centered oxidation near 0.4 V and two Pt(tpy) centered reductions near -1.0 V and -1.5 V. An estimate of n ox/n red = 1.8 is consistent with an oxidation that involves two-electron transfer per Pt center. Variation in the pincer-(Z) and terpyridine-(R) substituents allows for tuning of the oxidation process over a 260 mV range and the two reduction processes over ranges of 230 mV (first reduction) and 290 mV (second reduction step).

Halide exchange studies of novel Pd(ii) NNN-pincer complexes.

Palladium(ii) complexes with an NNN type pincer ligand (pip2NNN = 2,6-bis(piperdyl-methyl)pyridine) are synthesized and characterized. Electronic and 1H NMR spectra point to decreasing filled/filled repulsions between the dπ(Pd) orbitals and the halide lone pair orbitals along the Cl < Br < I series. For all complexes, the most downfield α-piperidyl resonance of the pip2NNN ligand is sensitive to changes in the coordinated halide while the meta-pyridyl and benzylic resonances are sensitive to changes in the counter anion. This sensitivity is utilized to study halide association and exchange at the fourth coordination site. Conductivity and 1H NMR spectroscopy confirm the interaction between the exogenous anion (Cl-, Br-, BF4 -) and Pd(pip2NNN)X+ (X = Cl, Br).