Laura Zoppi

Effect of Molecular Packing on Corannulene-Based Materials Electroluminescence

The present investigation reports for the first time a detailed theoretical analysis of the optical absorption spectra of corannulene-based materials using state-of-the-art first-principles many-body GW-BSE theory. The study specifically addresses the nature of optical excitations for predictions regarding suitability for device fabrication. The well-defined structure-correlation relationship in functionalized corannulenes is used in a focused investigation of the predicted optoelectronic properties in both the isolated state and bulk crystals. The findings suggest that the excitonic properties are strongly dependent on the specific substituent group as well as the crystalline arrangement. Arylethynyl-substituted corannulene derivatives are shown to be the most suitable for device purposes.

Reversible phase transitions in a buckybowl monolayer.

Like penguins on ice, buckybowl molecules move closer together when cooled on a copper surface (see model of a corannulene molecule adsorbed on Cu(111)). Upon heating, the molecules spread out into the original crystal phase again. The lower density at room temperature can be explained by the increase in entropy owing to the excitation of bowl vibrations at the surface.

Corannulene derivatives as non-fullerene acceptors in solution-processed bulk heterojunction solar cells

Corannulene derivatives were used in organic solar cells for the first time. Using Cor-PI and Cor-NI as acceptors, we achieved power conversion efficiencies up to 0.32% and 1.03%, suggesting potential applications of these fullerene segments as non-fullerene acceptors.

Electron transport and optical properties of curved aromatics

The extended family of curved carbon π‐systems offers a unique possibility for building up structures with a tunable spectrum of structural and electronic properties. Such a structure–property profile motivates the creative use of these materials as active components in molecular devices. Key to these functional building blocks is the curvature, which confines the electronic states in one or more directions (nanoscale directions) imparting remarkable physical phenomena to a material. In this respect, the formation of electronic excitations in form of excitons has a fundamental role in determining the optical and transport properties of this class of materials. The role of the curvature on electronics properties of curved aromatics is discussed for systems of varying dimensionalities, ranging from 0D (fullerenes, molecular bowls) to 1D (carbon nanotubes) and 3D (bulk crystals). Recent progress in the area of optical and transport properties of the largest classes of curved aromatic systems is discussed, and focus is given to molecules in isolation, molecules on surfaces, crystalline systems, and molecular nanojunctions.

Quadruple Anionic Buckybowls by Solid-State Chemistry of Corannulene and Cesium

The buckybowl corannulene is known to be an excellent electron acceptor. UV photoelectron spectroscopy studies were performed with thin-film systems containing corannulene and cesium. Adsorption of submonolayer quantities of corannulene in ultrahigh vacuum onto thick Cs films, deposited at 100 K on a copper(111) substrate, induces a transfer of four electrons per molecule into the two lowest unoccupied orbitals. Annealing of thick corannulene layers on top of the cesium film leads to the formation of a stable film composed of C20H104– ions coordinated to four Cs+ ions. First-principles calculations reveal, as the most stable configuration, four Cs+ ions sandwiched between two corannulene bowls.

Extended Corannulenes: Aromatic Bowl/Sheet Hybridization.

Among sheet/sheet polynuclear aromatic hydrocarbon (PAH) hybrids, a buckybowl-graphene hybrid has been used as a model to explore the effects of physical properties of PAHs with distinct planar and bowl regions. Activation of a C(Ar)-F bond was used to synthesize this corannulene/graphenic hybrid. Photophysical and voltammetric studies together with high-level computations revealed curvature and extended π-effects on the properties of these materials.

Structure-property relationships of curved aromatic materials from first principles.

CONSPECTUS: Considerable effort in the past decade has been extended toward achieving computationally affordable theoretical methods for accurate prediction of the structure and properties of materials. Theoretical predictions of solids began decades ago, but only recently have solid-state quantum techniques become sufficiently reliable to be routinely chosen for investigation of solids as quantum chemistry techniques are for isolated molecules. Of great interest are ab initio predictive theories for solids that can provide atomic scale insights into properties of bulk materials, interfaces, and nanostructures. Adaption of the quantum chemical framework is challenging in that no single theory exists that provides prediction of all observables for every material type. However, through a combination of interdisciplinary efforts, a richly textured and substantive portfolio of methods is developing, which promise quantitative predictions of materials and device properties as well as associated performance analysis. Particularly relevant for device applications are organic semiconductors (OSC), with electrical conductivity between that of insulators and that of metals. Semiconducting small molecules, such as aromatic hydrocarbons, tend to have high polarizabilities, small band-gaps, and delocalized π electrons that support mobile charge carriers. Most importantly, the special nature of optical excitations in the form of a bound electron-hole pairs (excitons) holds significant promise for use in devices, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), and molecular nanojunctions. Added morphological features, such as curvature in aromatic hydrocarbon structure, can further confine the electronic states in one or more directions leading to additional physical phenomena in materials. Such structures offer exploration of a wealth of phenomenology as a function of their environment, particularly due to the ability to tune their electronic character through functionalization. This Account offers discussion of current state-of-the-art electronic structure approaches for prediction of structural, electronic, optical, and transport properties of materials, with illustration of these capabilities from a series of investigations involving curved aromatic materials. The class of curved aromatic materials offers the ability to investigate methodology across a wide range of materials complexity, including (a) molecules, (b) molecular crystals, (c) molecular adsorbates on metal surfaces, and (d) molecular nanojunctions. A reliable pallet of theoretical tools for such a wide array relies on expertise spanning multiple fields. Working together with experimental experts, advancements in the fundamental understanding of structural and dynamical properties are enabling focused design of functional materials. Most importantly, these studies provide an opportunity to compare experimental and theoretical capabilities and open the way for continual improvement of these capabilities.

Theoretical investigation of the binding process of corannulene on a Cu(111) surface.

DFT-GGA calculations, enhanced to include effects of dispersion, are used to investigate the adsorption process of corannulene on a Cu(111) surface. In accord with experiments, we consider the dynamics of corannulene approaching the surface in a tilted fashion, concave side-up, enabling interactions between one of the six-membered rings and the surface over a 3-fold hollow site. Electronic structure analyses, including projected density of states and detection of work function modification, are used to aid in the understanding of the specific nature of the interaction between the corannulene and the metal surface in the complex system. Results show substantial charge rearrangement at the interface, the net effect being a large interface dipole that, added to the intrinsic molecular dipole, causes a significant decrease of the surface work function. Despite the charge rearrangement, no appreciable charge transfer occurs, and the general orbital structure of the individual components is retained. The analysis suggests that the adsorption of corannulene on Cu(111) is not a chemisorption process. Increased packing of corannulene on the surface leads to progressively smaller adsorption-induced interface dipoles, due to the depolarizing field created by the molecules.

Pasteur's Experiment Performed at the Nanoscale: Manual Separation of Chiral Molecules, One by One.

Understanding the principles of molecular recognition is a difficult task and calls for investigation of appropriate model systems. Using the manipulation capabilities of scanning tunneling microscopy (STM) we analyzed the chiral recognition in self-assembled dimers of helical hydrocarbons at the single molecule level. After manual separation of the two molecules of a dimer with a molecule-terminated STM tip on a Cu(111) surface, their handedness was subsequently determined with a metal atom-terminated tip. We find that these molecules strongly prefer to form heterochiral pairs. Our study shows that single molecule manipulation is a valuable tool to understand intermolecular recognition at surfaces.

Static and Field-Oriented Properties of Bowl-Shaped Polynuclear Aromatic Hydrocarbon Fragments

First principles techniques are used to investigate the structure, linear polarizability, and field-oriented property trends of the series of bowl shaped polynuclear aromatic hydrocarbon fragments, C20H10, C30H10, C40H10, and C50H10. Such structures represent a sequence of minimalistic, capped bucky tube units based on the corannulene molecule, with interesting technological promise imparted by their curvature. Specific issues associated with how the intrinsic dipole and static linear polarizability influences the orientation of these structures in the presence of an external electric field are addressed and shown to correlate well with a simple analytical model. At moderate electric fields, the induced dipoles become comparable and even larger than the intrinsic dipoles due to the large in-plane polarizabilities in these systems. This generates a nontrivial and field dependent orientation of the molecule that can be exploited, for example, to induce switching behavior within molecular nanojunctions.