We demonstrate that metal carboxylate complexes (L–M(O2CR)2, R = oleyl, tetradecyl, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, tetrahydrofuran, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N',N'-tetramethylbutylene-1,4-diamine, pyridine, N,N,N',N'-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by 1H NMR spectroscopy and depends most strongly on geometric factors like sterics and chelation, though also on the hard/soft match with the cadmium ion. The results suggest that ligands displace L–M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface bound Cd(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd:Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1Se-2S3/2h absorption and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) and 20% to <1% (CdS). These changes are partially reversed upon rebinding of M(O2CR)2 at room temperature (~60 %) and fully reversed at elevated temperature. A model is proposed where electron accepting M(O2CR)2 complexes (Z-type ligands) reversibly bind to nanocrystals leading to a range of stoichiometries for a given core size. The results demonstrate that nanocrystals lack a single chemical formula, but are instead dynamic structures with concentration-dependent compositions. The importance of these findings to the synthesis and purification of nanocrystals as well as ligand exchange reactions is discussed.

Controlling the size of colloidal nanocrystals is essential to optimizing their performance in optoelectronic devices, catalysis, and imaging applications. Traditional synthetic methods control size by terminating the growth, an approach that limits the reaction yield and causes batch-to-batch variability. Herein we report a library of thioureas whose substitution pattern tunes their conversion reactivity over more than five orders of magnitude and demonstrate that faster thiourea conversion kinetics increases the extent of crystal nucleation. Tunable kinetics thereby allows the nanocrystal concentration to be adjusted and a desired crystal size to be prepared at full conversion. Controlled precursor reactivity and quantitative conversion improve the batch-to-batch consistency of the final nanocrystal size at industrially relevant reaction scales.

Layered perovskites with the formula (R-NH)PbI have excellent environmental stability but poor photovoltaic function due to the preferential orientation of the semiconducting layer parallel to the substrate and the typically insulating nature of the R-NH cation. Here, we report a series of these n = 1 layered perovskites with the form (aromatic- O-linker-NH)PbI where the aromatic moiety is naphthalene, pyrene, or perylene and the linker is ethyl, propyl, or butyl. These materials achieve enhanced conductivity perpendicular to the inorganic layers due to better energy level matching between the inorganic layers and organic galleries. The enhanced conductivity and visible absorption of these materials led to a champion power conversion efficiency of 1.38%, which is the highest value reported for any n = 1 layered perovskite, and it is an order of magnitude higher efficiency than any other n = 1 layered perovskite oriented with layers parallel to the substrate. These findings demonstrate the importance of leveraging the electronic character of the organic cation to improve optoelectronic properties and thus the photovoltaic performance of these chemically stable low n layered perovskites.

ConspectusPeptide amphiphiles (PAs) are small molecules that contain hydrophobic components covalently conjugated to peptides. In this Account, we describe recent advances involving PAs that consist of a short peptide sequence linked to an aliphatic tail. The peptide sequence can be designed to form β-sheets among the amino acids near the alkyl tail, while the residues farthest from the tail are charged to promote solubility and in some cases contain a bioactive sequence. In water, β-sheet formation and hydrophobic collapse of the aliphatic tails induce assembly of the molecules into supramolecular one-dimensional nanostructures, commonly high-aspect-ratio cylindrical or ribbonlike nanofibers. These nanostructures hold significant promise for biomedical functions due to their ability to display a high density of biological signals on their surface for targeting or to activate pathways, as well as for biocompatibility and biodegradable nature.Recent studies have shown that supramolecular systems, such as PAs, often become kinetically trapped in local minima along their self-assembly reaction coordinate, not unlike the pathways associated with protein folding. Furthermore, the assembly pathway can influence the shape, internal structure, and dimension of nanostructures and thereby affect their bioactivity. We discuss methods to map the energy landscape of a PA structure as a function of thermal energy and ionic strength and vary these parameters to convert between kinetically trapped and thermodynamically favorable states. We also demonstrate that the pathway-dependent morphology of the PA assembly can determine biological cell adhesion and survival rates.The dynamics associated with the nanostructures are also critical to their function, and techniques are now available to probe the internal dynamics of these nanostructures. For example, by conjugating radical electron spin labels to PAs, electron paramagnetic resonance spectroscopy can be used to study the rotational diffusion rates within the fiber, showing a liquidlike to solidlike transition through the cross section of the nanofiber. PAs can also be labeled with fluorescent dyes, allowing the use of super-resolution microscopy techniques to study the molecular exchange dynamics between PA fibers. For a weak hydrogen-bonding PA, individual PA molecules or clusters exchange between fibers in time scales as short as minutes. The amount of hydrogen bonding within PAs that dictates the dynamics also plays an important role in biological function. In one case, weak hydrogen bonding within a PA resulted in cell death through disruption of lipid membranes, while in another example reduced hydrogen bonding enhanced growth factor signaling by increasing lipid raft mobility.PAs are a promising platform for designing advanced hybrid materials. We discuss a covalent polymer with a rigid aromatic imine backbone and alkylated peptide side chains that simultaneously polymerizes and interacts with a supramolecular PA structure with identical chemistry to that of the side chai...

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