One- to Two-Exciton Transitions in Perylene Bisimide Dimer Revealed by Two-Dimensional Electronic Spectroscopy

The excited-state energy levels of molecular dimers and aggregates play a critical role in their photophysical behavior and an understanding of the photodynamics in such structures is important for developing applications such as photovoltaics and optoelectronic devices. Here, exciton transitions in two different covalently bound PBI dimers are studied by two-dimensional electronic spectroscopy (2DES), a powerful spectroscopic method, providing the most complete picture of vibronic transitions in molecular systems. The data are accurately reproduced using the equation of motion-phase matching approach. The unambiguous presence of one-exciton to two-exciton transitions are captured in our results and described in terms of a molecular exciton energy level scheme based on the Kasha model. Furthermore, the results are supported by comparative measurements with the PBI monomer and another dimer in which the interchromophore distance is increased

Conditional quantum nonlocality in dimeric and trimeric arrays of organic molecules

Arrays of covalently bound organic molecules possess potential for light-harvesting and energy transfer applications due to the strong coherent dipole-dipole coupling between the transition dipole moments of the molecules involved. Here, we show that such molecular systems, based on perylene-molecules, can be considered as arrays of qubits that are amenable for laser-driven quantum coherent control. The perylene monomers exhibit dephasing times longer than four orders of magnitude a typical gating time, thus allowing for the execution of a large number of gate operations on the sub-picosecond timescale. Specifically, we demonstrate quantum logic gates and entanglement in bipartite (dimer) and tripartite (trimer) systems of perylene-based arrays. In dimers, naturally entangled states with a tailored degree of entanglement can be produced. The nonlocality of the molecular trimer entanglement is demonstrated by testing Mermin's (Bell-like) inequality violation.Comment: 14 pages, 8 figures, comments are welcom

Excitonic coupling in covalently-bound Perylene Bisimide dimers revealed by two-dimensional electronic spectroscopy

Supramolecular structures based on Perylene Bisimides (PBIs) have been extensively studied because of their fundamental photophysical properties and for their application in a range of different optoelectronic devices. Two-dimensional electronic spectroscopy (2D-ES) is the most complete third order (χ(3)) technique, it has been shown that it is particularly useful to disentangle close-lying energy levels and to reveal dynamics in coupled molecular systems. Two different PBI covalently “head-to-tail” bound dimers (D0 and D1) with increasing interchromophoric separation, and a reference monomer (M) were synthesised and studied by means of 2D-ES in order to characterise the 1 to 2-exciton state transition and how its behaviour changes in relation to the PBI-PBI distance

Modeling charge and energy transfer in organic molecular materials

The understanding of nanoscale physics, chemistry and biology still poses unanswered questions such as how the optical and electrical properties of materials evolve from those of individual molecules, and organic semiconductors fall in this class of materials. The main processes occurring in such systems are both charge and energy transfer, responsible for the practical operation of electronic devices. Therefore, an understanding at a fundamental level of the electronic properties of the involved molecules can help the optimization of each process, for a better global performance of the material. My three years PhD activity was developed along two major lines of research: charge and energy transport, both based on the computational investigation of intramolecular properties and intermolecular interactions. Strictly related to energy transport are the optical properties of condensed phase materials and how they evolve from those of isolated molecular components. The charge transport properties were investigated for several organic molecular crystals showing semiconducting behavior, whose experimental crystal structure and charge mobilities are available. As the same interactions that drive the transport of charge play also a role in determining the optical properties and the energy transport in molecular aggregates, in my research activity I investigated such processes as well. In this regard, I took into account a dimer of perylene-bisimide, with the aim of elucidating the role of charge transfer states and their effect on optoelectronic properties. Additionally, to assess the propagation of excited states in a molecular material a kinetic constant is required, similarly to charge transport, but the expression in this case includes the overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor. To this end I also developed a code devoted to the simulation of linear absorption and emission spectra of an isolated molecule, starting from computed quantum mechanical properties

GNA as a scaffold for chromophores aggregates and design of silicon-based DNA binders

My work mainly includes two parts, one is GNA as a scaffold for molecular aggregates of chromophores, and the other one is the design of octahedral silicon complexes as DNA binders. In the first part of the thesis, simplified glycol nucleic acid (GNA) was used as the template for the helical assembly of covalently bound homochromophores and heterochromophores: perylene bisimide (B), phthalocyanine (Y), and porphyrin (P), and different numbers of chromophores could be stacking inside a duplex which led to functional artificial double-helical structures. In the second part of the thesis, the first examples of biologically active complexes based on octahedral silicon were designed, and silicon complexes with simple arenediol ligands and phenazinediol ligands were successfully synthesized. These kinds of siliconcomplexes could be used to intercalate with DNA duplexes, detect mismatched DNA base pairs, and stabilize the formation of the G- quadruplex

Air-Processed Polymer Photovoltaics: Advanced Morphological Characterization and Synthesis of New Materials for Increased Light Absorption

Organic photovoltaics (OPVs) have undergone intense development in recent years due to their potential to produce inexpensive, flexible, lightweight, and portable solar cells utilizing organic photoactive materials. Polymer-based OPVs have recently surpassed the 10 % in laboratory-scale devices, but require strict processing conditions in inert environments. Materials and processing methods that are workable in air are therefore desired. In addition to the light-absorbing and charge transport properties of the active layer materials, the morphology, crystallinity and orientation of the components of the phase-separated blend are paramount in determining OPV device performance This dissertation explores the processing of the model polymer OPV system under environmental conditions, and how that processing affects the morphology of the active layer at the molecular scale. Advanced x-ray scattering was utilized along with conventional laboratory characterization to mechanistically determine how the processing steps involved affected the performance of the final devices. Additionally, a number of new compounds with hypothesized air stability were synthesized and characterized. Light-absorbing polyhedral oligomeric silsesquioxane (POSS) molecules were synthesized and incorporated into the active layer of a model polymer OPV to act as conductive nanostructuring agents. The air-stable thieno[3,4-b]pyrazine subunit was copolymerized into a donor-acceptor copolymer and used as a broadly-absorbing donor polymer in the conventional polymer:fullerene OPV architecture. Finally, a series of non-fullerene acceptors was synthesized based on perylene bisimide, an air-stable dye that has shown wide use in organic electronics due to its broad absorption and good electron transport capability, and utilized in polymer OPV devices. Alternate, higher-yielding pathways for the synthesis of intermediate compounds required for the target compounds were found, which expands available architectures available for perylene bisimide-based materials

Controlled Aggregation of Peptide Substituted Perylene-Bisimides

In recent years there has been an intersection of supramolecular chemistry and materials science, with a particular focus on the controlled self-assembly of functional building blocks. The impetus for assembly of organised architectures is a requirement due to organic electronic device performance being sensitive to the geometric configuration of adjacent molecular semiconductors, interacting by means of overlapping π-orbitals to create electronic conduction. Inspired by the formation of elegant supramolecular structures in nature, this work employs perylene bisimides coupled to synthetic peptides which are able to control the assembly of chromophores in solution. Through examining the perturbations of optical absorption and fluorescence spectroscopic signatures, the presence of aggregates, and also the geometric configurations of adjacent chromophores are determined. By exploring these features as a function of peptide design, pH, solvent composition, and ionic strength, it is demonstrated that aggregation is strongly induced by the peptide and the aromatic core, with significant dependence on the electrostatic repulsion between peptide segments. By manipulating solvent compositions, we demonstrate the ability to induce controlled reorganisation of aggregates through the introduction of charge onto the peptide sequence in high water concentration solution. Furthermore, application of the exciton model to absorption spectra establishes the tuneability of aggregates by specific ion binding between neighbouring peptides. Our results demonstrate the capability of peptide sequences to drive aggregation of molecular semiconductor building blocks; moreover, the peptides allow fine tuning of the electronic overlap between neighbouring building blocks. The proof of concept paves the way for further investigation into utilising this assembly control for device fabrication, in particular, we see this work being applicable to biosensor devices