Abstract:The introduction of a mobile and polarised organic moiety as a cation in three-dimensional lead-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the timescales of the orientational mobility of the organic cation and the molecular mechanism behind its motion remain unclear, with different experimental and computational approaches providing very different qualitative and quantitative description of the molecular dynamics. Here we use ultrafast two-dimensional vibrational spectroscopy of methylammonium (MA) lead iodide, to directly resolve the rotation of the organic cations within the MAPbI3 lattice. Our results reveal two characteristic time constants of motion. Using ab-initio molecular dynamics simulations, we identify these as a fast (~300 fs) 'wobbling-ina-cone' motion around the crystal axis, and a relatively slow (~3 ps) jump-like reorientation of the molecular dipole with respect to the iodide lattice. The observed dynamics are essential for understanding the electronic properties of perovskite materials. TOC figure:

We have used femtosecond midinfrared spectroscopy to study the orientational mobility of water molecules in the hydration shells of hydrophobic groups. Our results show that hydrophobic groups are surrounded by a number of water molecules that display much slower orientational dynamics than the bulk liquid and that are therefore effectively immobilized. It turns out that each methyl group is surrounded by four immobilized water OH groups.

We use femtosecond mid-infrared pump-probe spectroscopy to study the orientational relaxation of HDO molecules dissolved in H2O. In order to obtain a reliable anisotropy decay we model the effects of heating and correct for these effects. We have measured the reorientation time constant of the OD vibration from 2430 to 2600 cm(-1), and observe a value of 2.5 ps that shows no variation over this frequency interval. Our results are discussed in the context of previous experiments that have been performed on the complementary system of HDO dissolved in D2O.

We use polarization-resolved mid-infrared pump-probe spectroscopy to study the effect of urea on the structure and dynamics of water. Surprisingly, we find that, even at high concentrations of urea (8 M), the orientational dynamics of most water molecules are the same as in pure liquid water, showing that urea has a negligible effect on the hydrogen-bond dynamics of these molecules. However, a small fraction of the water molecules (approximately one water molecule per urea molecule) turns out to be strongly immobilized by urea, displaying orientational dynamics that are more than six times slower than in bulk water. A likely explanation is that these water molecules are tightly associated with urea, forming specific urea-water complexes. We discuss these results in light of the protein denaturing ability of aqueous urea.hydrogen bonding ͉ infrared pump-probe spectroscopy ͉ orientational dynamics ͉ protein denaturation S olutions of urea in water display a number of interesting properties: hydrocarbons dissolve more readily in them than in pure water, and concentrated solutions can be used to denature proteins in a reversible way. The desire to understand these properties has triggered a great deal of research regarding the structure of aqueous solutions of urea (1-13). An important question that is encountered throughout the literature is to what extent the hydrogen-bond network of water is perturbed by the incorporation of a urea molecule, as one of the models explaining protein denaturation by urea is built on the assumption that urea strongly alters the hydrogen-bond structure of water (14). The urea-water system has been studied by using a variety of experimental and theoretical techniques, all of which shed light on a different aspect of the system. Linear infrared and Raman spectroscopies can provide structural information about the hydrogen-bond network, but they depend on simulations to unravel the effect of intermolecular interactions on spectral band shapes (15). Neutron diffraction experiments produce atom-atom radial distribution functions, and, as such, form more direct methods to obtain structural information (7). A different class of experimental techniques probes the hydrogen-bond network by observing the dynamics of water molecules. A stiffening of the network is revealed by the slowing down of the water dynamics, whereas faster dynamics are indicative of the weakening of the network. Among these methods are dielectric relaxation (5), NMR (6), and optical Kerr effect (OKE) spectroscopy (8). Dielectric relaxation and OKE provide dynamical information down to the picosecond time scale but probe the response of the solution as a whole, making it difficult to separate the water response from the urea response. NMR experiments selectively probe the dynamics of water, but determine a timeaveraged response, so that water molecules in the urea solvation shell cannot be distinguished from molecules in the bulk. As a result, it is not yet clear to what extent the hydrogen-bond structure of water is change...

Three-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic methylammonium (MA) cation orientation in a range of pure and mixed trihalide perovskite materials. For pure MAPbX (X = I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, but can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilization of the organic cation, are observed in the mixed MAPb(ClBr) and MAPb(BrI) alloys, which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems.