Control over size, shape, and photonics of self-assembled organic nanocrystals.

The facile fabrication of free-floating organic nanocrystals (ONCs) was achieved via the kinetically controlled self-assembly of simple perylene diimide building blocks in aqueous medium. The ONCs have a thin rectangular shape, with an aspect ratio that is controlled by the content of the organic cosolvent (THF). The nanocrystals were characterized in solution by cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering. The ONCs retain their structure upon drying, as was evidenced by TEM and atom force microscopy. Photophysical studies, including femtosecond transient absorption spectroscopy, revealed a distinct influence of the ONC morphology on their photonic properties (excitation energy transfer was observed only in the high-aspect ONCs). Convenient control over the structure and function of organic nanocrystals can enhance their utility in new and developed technologies.

Crystallization of methyl ammonium lead halide perovskites: implications for photovoltaic applications.

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.

Mechanism of crystalline self-assembly in aqueous medium: a combined cryo-TEM/kinetic study.

Understanding the crystallization of organic molecules is a long-standing challenge. Herein, a mechanistic study on the self-assembly of crystalline arrays in aqueous solution is presented. The crystalline arrays are assembled from perylene diimide (PDI) amphiphiles bearing a chiral N-acetyltyrosine side group connected to the PDI aromatic core. A kinetic study of the crystallization process was performed using circular dichroism spectroscopy combined with time-resolved cryogenic transmission electron microscopy (cryo-TEM) imaging of key points along the reaction coordinate, and molecular dynamics simulation of the initial stages of the assembly. The study reveals a complex self-assembly process starting from the formation of amorphous aggregates that are transformed into crystalline material through a nucleation-growth process. Activation parameters indicate the key role of desolvation along the assembly pathway. The insights from the kinetic study correlate well with the structural data from cryo-TEM imaging. Overall, the study reveals four stages of crystalline self-assembly: 1) collapse into amorphous aggregates; 2) nucleation as partial ordering; 3) crystal growth; and 4) fusion of smaller crystalline aggregates into large crystals. These studies indicate that the assembly process proceeds according to a two-step crystallization model, whereby initially formed amorphous material is reorganized into an ordered system. This process follows Ostwald's rule of stages, evolving through a series of intermediate phases prior to forming the final structure, thus providing an insight into the crystalline self-assembly process in aqueous medium.

Hydrophobic self-assembly affords robust noncovalent polymer isomers.

In covalent polymerization, a single monomer can result in different polymer structures due to positional, geometric, or stereoisomerism. We demonstrate that strong hydrophobic interactions result in stable noncovalent polymer isomers that are based on the same covalent unit (amphiphilic perylene diimide). These isomers have different structures and electronic/photonic properties, and are stable in water, even upon prolonged heating at 100 °C. Such combination of covalent-like stability together with structural/functional variation is unique for noncovalent polymers, substantially advancing their potential as functional materials.

Pathway-dependent self-assembly of perylene diimide/peptide conjugates in aqueous medium.

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.

Thermodynamic factors impacting the peptide-driven self-assembly of perylene diimide nanofibers.

Synthetic peptides offer enormous potential to encode the assembly of molecular electronic components, provided that the complex range of interactions is distilled into simple design rules. Here, we report a spectroscopic investigation of aggregation in an extensive series of peptide-perylene diiimide conjugates designed to interrogate the effect of structural variations. By fitting different contributions to temperature dependent optical absorption spectra, we quantify both the thermodynamics and the nature of aggregation for peptides by incrementally varying hydrophobicity, charge density, length, as well as asymmetric substitution with a hexyl chain, and stereocenter inversion. We find that coarse effects like hydrophobicity and hexyl substitution have the greatest impact on aggregation thermodynamics, which are separated into enthalpic and entropic contributions. Moreover, significant peptide packing effects are resolved via stereocenter inversion studies, particularly when examining the nature of aggregates formed and the coupling between π electronic orbitals. Our results develop a quantitative framework for establishing structure-function relationships that will underpin the design of self-assembling peptide electronic materials.

Self-assembly of light-harvesting crystalline nanosheets in aqueous media.

A methodology leading to facile self-assembly of crystalline aromatic arrays in dilute aqueous solutions would enable efficient fabrication and processing of organic photonic and electronic materials in water. In particular, soluble 2D crystalline nanosheets may mimic the properties of photoactive thin films and self-assembled monolayers, covering large areas with ordered nanometer-thick material. We designed such solution-phase arrays using hierarchical self-assembly of amphiphilic perylene diimides in aqueous media. The assemblies were characterized by cryogenic transmission electron microscopy (cryo-TEM), revealing crystalline order and 2D morphology (confirmed by AFM studies). The order and morphology are preserved upon drying as evidenced by TEM and AFM. The 2D crystalline-like structures exhibit broadening and red-shifted absorption bands in UV-vis spectra, typical for PDI crystals and liquid crystals. Photophysical studies including femtosecond transient absorption spectroscopy reveal that two of the assemblies are superior light-harvesters due to excellent solar spectrum coverage and fast exciton transfer, in one case showing exciton diffusion comparable to solid-state crystalline systems based on perylene tetracarboxylic dianhidride (PTCDA).