Hybrid organic/lead halide perovskites are promising materials for solar cell fabrication, resulting in efficiencies up to 18%. The most commonly studied perovskites are CH3NH3PbI3 and CH3NH3PbI3-xClx where x is small. Importantly, in the latter system, the presence of chloride ion source in the starting solutions used for the perovskite deposition results in a strong increase in the overall charge diffusion length. In this work we investigate the crystallization parameters relevant to fabrication of perovskite materials based on CH3NH3PbI3 and CH3NH3PbBr3. We find that the addition of PbCl2 to the solutions used in the perovskite synthesis has a remarkable effect on the end product, because PbCl2 nanocrystals are present during the fabrication process, acting as heterogeneous nucleation sites for the formation of perovskite crystals in solution. We base this conclusion on SEM studies, synthesis of perovskite single crystals, and on cryo-TEM imaging of the frozen mother liquid. Our studies also included the effect of different substrates and substrate temperatures on the perovskite nucleation efficiency. In view of our findings, we optimized the procedures for solar cells based on lead bromide perovskite, resulting in 5.4% efficiency and Voc of 1.24 V, improving the performance in this class of devices. Insights gained from understanding the hybrid perovskite crystallization process can aid in rational design of the polycrystalline absorber films, leading to their enhanced performance.

Most practical materials are held together by covalent bonds, which are irreversible. Materials based on noncovalent interactions can undergo reversible self-assembly, which offers advantages in terms of fabrication, processing and recyclability, but the majority of noncovalent systems are too fragile to be competitive with covalent materials for practical applications, despite significant attempts to develop robust noncovalent arrays. Here, we report nanostructured supramolecular membranes prepared from fibrous assemblies in water. The membranes are robust due to strong hydrophobic interactions, allowing their application in the size-selective separation of both metal and semiconductor nanoparticles. A thin (12 µm) membrane is used for filtration (∼5 nm cutoff), and a thicker (45 µm) membrane allows for size-selective chromatography in the sub-5 nm domain. Unlike conventional membranes, our supramolecular membranes can be disassembled using organic solvent, cleaned, reassembled and reused multiple times.

Design of an extensive supramolecular three-dimensional network that is both robust and adaptive represents a significant challenge. The molecular system PP2b based on a perylene diimide chromophore (PDI) decorated with polyethylene glycol groups self-assembles in aqueous media into extended supramolecular fibers that form a robust three-dimensional network resulting in gelation. The self-assembled systems were characterized by cryo-TEM, cryo-SEM, and rheological measurements. The gel possesses exceptional robustness and multiple stimuli-responsiveness. Reversible charging of PP2b allows for switching between the gel state and fluid solution that is accompanied by switching on and off the material's birefringence. Temperature triggered deswelling of the gel leads to the (reversible) expulsion of a large fraction of the aqueous solvent. The dual sensibility toward chemical reduction and temperature with a distinct and interrelated response to each of these stimuli is pertinent to applications in the area of adaptive functional materials. The gel also shows strong absorption of visible light and good exciton mobility (elucidated using femtosecond transient absorption), representing an advantageous light harvesting system.

Most molecular self-assembly strategies involve equilibrium systems, leading to a single thermodynamic product as a result of weak, reversible non-covalent interactions. Yet, strong non-covalent interactions may result in non-equilibrium self-assembly, in which structural diversity is achieved by forming several kinetic products based on a single covalent building block. We demonstrate that well-defined amphiphilic molecular systems based on perylene diimide/peptide conjugates exhibit kinetically controlled self-assembly in aqueous medium, enabling pathway-dependent assembly sequences, in which different organic nanostructures are evolved in a stepwise manner. The self-assembly process was characterized using UV/Vis circular dichroism (CD) spectroscopy, and cryogenic transmission electron microscopy (cryo-TEM). Our findings show that pathway-controlled self-assembly may significantly broaden the methodology of non-covalent synthesis.

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Perylene diimide (PDI) bearing polyethylene glycol substituents at the imide positions was reduced in water with sodium dithionite to produce an aromatic dianion. The latter is stable for months in deoxygenated aqueous solutions, in contrast to all known aromatic dianions which readily react with water. Such stability is due to extensive electron delocalization and the aromatic character of the dianion, as evidenced by spectroscopic and theoretical studies. The dianion reacts with oxygen to restore the parent neutral compound, which can be reduced again in an inert atmosphere with sodium dithionite to give the dianion. Such reversible charging renders PDIs useful for controlled electron storage and release in aqueous media. Simple preparation of the dianion, reversible charging, high photoredox power, and stability in water can lead to development of new photofunctional and electron transfer systems in the aqueous phase.

Organic crystals are of primary importance in pharmaceuticals, functional materials, and biological systems; however, organic crystallization mechanisms are not well-understood. It has been recognized that “nonclassical” organic crystallization from solution involving transient amorphous precursors is ubiquitous. Understanding how these precursors evolve into crystals is a key challenge. Here, we uncover the crystallization mechanisms of two simple aromatic compounds (perylene diimides), employing direct structural imaging by cryogenic electron microscopy. We reveal the continuous evolution of density, morphology, and order during the crystallization of very different amorphous precursors (well-defined aggregates and diffuse dense liquid phase). Crystallization starts from initial densification of the precursors. Subsequent evolution of crystalline order is gradual, involving further densification concurrent with optimization of molecular ordering and morphology. These findings may have implications for the rational design of organic crystals.

Self-assembling systems, whose structure and function can be reversibly controlled in situ are of primary importance for creating multifunctional supramolecular arrays and mimicking the complexity of natural systems. Herein we report on photofunctional fibers self-assembled from perylene diimide cromophores, in which interactions between aromatic monomers can be attenuated through their reduction to anionic species that causes fiber fission. Oxidation with air restores the fibers. The sequence represents reversible supramolecular depolymerization-polymerization in situ and is accompanied by a reversible switching of photofunction.