On exciton-vibration and exciton-photon interactions in organic semiconductors

Organic semiconductors are materials that are promising for novel optoelectronic applications, such as more efficient solar cells and LEDs. The optoelectronic response of these materials is dominated by bound electron-hole pairs called excitons, which are often strongly affected by hundreds of possible molecular vibrations. Although quantum theory contains all the ingredients to describe these complex phenomena, in practice it is only possible to solve the corresponding equations in small systems with few vibrations. As a result, it has been common to assume weak exciton-vibration interactions and to employ perturbative approaches. Similarly, exciton-photon interactions have almost universally been treated in the so-called weak coupling regime. However, in recent years it has become increasingly clear that these approximations can break down in organic semiconductors, placing an important roadblock towards the novel energy-harvesting technologies that could be based on these materials. In this thesis we address this issue by developing methods to treat exciton-photon and exciton-vibration interactions, without relying on any approximation regarding their magnitude. We propose a first principles description of hybrid exciton-light (polariton) states that result from strong exciton-photon interactions. We discuss a method to treat strong exciton-vibration interactions, showing that the spatial extent of exciton states controls their magnitude. Subsequently, we present a beyond Born-Oppenheimer method based on tensor networks to study real-time exciton dynamics. By using these methods, we show how selective excitation of vibrational modes can enhance charge transfer. Moreover, through rigorous comparison to experiments, we highlight that tensor network methods are highly accurate, and we generate a `movie' of the photophysical process of singlet fission, which occurs during early light-harvesting by organic molecules and has the potential to increase solar cell efficiencies. Finally, we construct a singlet fission model including the effects of excess energy, vibrations and the solvent of molecules concurrently, demonstrating that the fission mechanism can be qualitatively changed in a controlled manner, allowing for its acceleration by an order of magnitude.Winton Programme for the Physics of Sustainabilit

Investigating the role of curvature on the formation and thermal transformations of soot

In this work, the role of curved polycyclic aromatic hydrocarbons (cPAH) on the initial formation mechanism and thermal transformations of soot was explored. Experimental and computational techniques were used to probe the integration, presence and impact of internal pentagonal rings on the nucleation mechanism of these particulates. A significant charge polarisation was found to occur when an internal pentagonal ring pyramidalises the aromatic network. Phase contrast transmission electron microscopy allowed for the extent of conjugation and degree of curvature to be determined in early soot nanoparticulates with 15 aromatic rings and two pentagons being the median species. The dipole moment of such a species was calculated to be 5.32 debye. The polarity was found to be persistent at flame temperatures with inversion and fluctuations being minimal. Homogeneous nucleation was considered with homodimerisation energies with one or two internal pentagonal rings within cPAH found to be comparable in energy to flat PAH (fPAH) homodimers of similar weight, with more pentagons reducing the binding energy. Ion-induced nucleation was considered with binding energies calculated between chemi-ions and cPAH suggesting small stable clusters at flame temperatures. However, physical and ion-induced nucleation of cPAH were found to be insufficient alone to explain the formation of soot. The impact of curvature on the reactivity of PAH were then studied. Strong crosslinks between σ-radicals and cPAH were found to form at their rim due to decreased aromaticity. Partially saturated rim-based pentagonal rings were also found to form localised π-radicals that allow stacked and bonded complexes to form, suggesting a covalently stabilised soot nucleation. Finally, the curved geometry of highly annealed soot, otherwise known as non-graphitising carbon, was explored using annealed molecular dynamics simulations and a discrete mesh analysis method. Analysis of the angular defect of the meshes revealed an excess of negative curvature. The coexistence of curved and layered ribbon-like structures was found to be possible due to the presence of a small number of non-sp² defects such as screw dislocations and free edges, which will impact the synthesis of novel carbon materials and the oxidation of thermally annealed soot. The incorporation of curvature and pentagonal rings is therefore considered critical for understanding the properties, formation and destruction of combustion generated carbonaceous particles and other carbon materials.This project is supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme

Modelling the photochemical properties of conjugated oligomers; understanding their application as photocatalysts

In this Ph.D. project, I introduce a new computational methodology, based on (timedependent) Density Functional Theory ((TD-)DFT), in order to determine if a molecule, here a conjugated oligomer, has the required photochemical properties to drive thermodynamically one or both water splitting half-reactions. This new approach takes electronic excitations into account rather than only relying on a static HOMO-LUMO description of the electronic structure, and therefore provides a more rigorous prediction of relevant thermodynamic potentials than ground-state DFT alone; it offers a relatively quick way of consistently screening for new photocatalysts for solar-driven water splitting. Using this computational framework, I investigate the optical properties of oligo(pphenylene), one of the simplest conjugated oligomers imaginable, as well as its thermodynamic potentials, relevant to the splitting of water into molecular hydrogen and oxygen. I then validate the methodology by confronting it to experimental data, before applying it to a wide range of conjugated oligomers, to determine whether or not they could be promising photocatalysts for water splitting, be it for the production of molecular hydrogen, oxygen gas, or both. In particular, I expose the reasons for the experimental lack of overall water splitting usually observed, and more particularly, the inability of many materials to oxidise water. Aside from purely photocatalytic considerations, I also discuss the optical properties of those oligomers and polymers, as they are tightly linked to their photocatalytic performance, with a particular emphasis on p-phenylene. I consistently study its three main isomers in order to shed some light into the relationship between their molecular structures and absorption/fluorescence spectra, and find the origin of the dramatic difference in the features exhibited by the latter, using a single computational approach, which, to the best of my knowledge, has never been done before

Design Rules for Solid State Fluorescence Exploiting Excited State Intramolecular Proton Transfer

Aggregation-induced emission (AIE) offers a route for the development of solid state organic luminescent technologies, overcoming the common and undesirable phenomenon of aggregation caused quenching. Excited-state intramolecular proton transfer (ESIPT) is an attractive feature to incorporate into the an AIE-active material, which results in red-shifted fluorescence and reduced self-absorption. ESIPT coupled to AIE can produce materials with emission across the visible spectrum, with applications in imaging, detection, optoelectronics, and solid state organic lasers. However, maximising fluorescence is a formidable challenge in attaining first-principles materials design, due to the interplay between the electronic structure of the chromophore and the crystalline environment. In this work, computational methods are used to investigate how the molecular properties and the environment mediate fluorescence for ESIPT systems. We concentrate on a family of systems, 2’-hydroxychalcones (HCs), with substituent- and morphology-dependent fluorescence. The aim of this thesis is to uncover the mechanism behind why some of these compounds undergo AIE, whilst some remain dark in both solution and the solid state. By initially isolating molecular properties, we find the systems are non-fluorescent in vacuum due to nonradiative decay via conical intersections. Using cluster models, we then probe the potential energy surfaces in the solid state, assessing how intra- and intermolecular processes dictate fluorescence. Based on our calculations, we establish guiding principles which mediate fluorescence in these materials. The scope is then extended to a related set of molecules, 2-hydroxyphenylpropenones, whose AIE behaviour is even more pronounced. We account for their remarkable photochemical properties through the design rules established for the 2’-hydroxychalcones. We systematically investigate competing excited state decay channels in a total of eleven systems to evaluate the factors needed for efficient ESIPT fluorophores, accounting for the crystal morphology, exciton coupling, and exciton hopping rates. This study of structure-property relationships for luminophores based on the ESIPT mechanism bridges the understanding of molecular photochemistry with crystal structure, aiding the development of highly efficient solid state emitters

Synthetic and computational studies of conjugated acetylenic systems

Conjugated molecular materials offer a range of useful physical properties from highly efficient luminescent behaviour to wire like conduction. This thesis describes the synthesis, molecular and electronic structures of a range of unusually conjugated organic and organometallic compounds. A combination of synthetic and absorption spectroscopic studies, as well as calculations (DFT) were used to probe the electronic structure of the 1, 4-bis(phenylethynyl)benzene and 1,2,4,5-tetraethynyl benzene framework. These revealed that the ground state of l,4-bis(phenylethynyl)benzene type molecules can be described as thermally populated distributions of conformers, while the exited state is best described in terms of planar conformations with considerable acetylenic character. A novel synthesis of 1,1,2,2-tetraethynylethenes has been discovered involving cross-coupling reactions of tetrachloroethene with terminal acetylenes. Reactions of these tetraethynylethenes with [Co(_2)(CO)(_6)(L(_2))][L(_2)= (CO)(_2), dppm] give complexes with one or two (trans) cobalt fragment coordinated to the acetylenic moieties. A theoretical study of the electronic structure of [Ru{(C=C)(_n)R}(L)(_2)Cp] (n = 1-6; L = CO, PH(_3); R = CH(_3), H, C(_6)H(_4)NH(_2)-p, C(_6)H(_5), C(_6)H(_4)NO(_2)-P, CN) has been carried out. The redox potentials of these species may be tuned by the length of the polyynyl ligand, the nature of the supporting ligands, as well the electronic properties of the non-metal end-cap. Electrochemical and spectroelectrochemical analysis of [Ru(_4)(CO)(_11)](Ų4-(RC(_2)C≡CR)(_2)](^0/2-) have revealed an unusual reversible conversion of 62/64 CVE cluster frameworks. This switching behaviour has been modelled using [Ru(_4)(C0)(_11)(Ų4-HC(_2)H)(_2)](^0/2)- via DFT methods

Theoretical modelling of ultrafast photodynamics

This thesis presents detailed electronic structure calculations and mixed quantum-classical dynamics simulations of the photodynamics of two small polyatomic molecules using "on-the-fly" surface-hopping. Most of the emphasis in this work is on CS2, which upon absorption of a UV photon undergoes a complex photodissociation process propagating across the potential energy surfaces of multiple singlet and triplet electronic states, under the influence of both nonadiabatic and spin-orbit coupling. Backed by extensive CASSCF and post-CASSCF electronic structure calculations, excitation to the 11B2 state is considered as a first exploration of the dynamics over the first picosecond, accounting for the lowest-lying four singlet and four triplet states. Following this, dynamics occurring after excitation to the 21B2 state, which is the state typically excited in time-resolved experimental studies of this system, are simulated. The additional computational complexity (with dynamics evolving on 19 interacting singlet and triplet states) and the limitations of "on-the-fly" techniques for a simulation of this size is discussed. This motivates initial steps towards generating full-dimensional grid-based surfaces for CS2 on which dynamics could later be simulated. These studies reinforce the importance of spin-orbit coupling in the dynamics and shine a light on the competitive nature of the singlet and triplet dissociation channels. Secondly, the short-time dynamics of trimethylamine are simulated, also using surface-hopping. Two sets of simulations are compared with regard to their description of the main dynamical features of the system, including dissociation of a methyl fragment and the extensive interplay between the low-lying 3pxyz and 3s Rydberg states, behaviour characteristic of tertiary substituted aliphatic amine systems. It is concluded that the sixth singlet state (3d) plays a significant role in the dissociation mechanism. The calculations and simulations here demonstrate the increasing utility of the conceptually intuitive surface-hopping approach in studying two contrasting classes of photochemical reactions, namely over-the-barrier photodissociation in CS2 and the photodynamics of low-lying Rydberg states in trimethylamine. In both cases, a comparison is made with complementary time-resolved experimental work by collaborators, articulating the need for experiment and theory to work together to provide a complete description of these fundamental chemical processes

Photodetectors for graphene-based integrated photonics

The development of integrated optical circuits has enabled a diverse portfolio of chipscale photonic applications—ranging from data communication over sensing to imaging—that is set to grow further as new device concepts in, for example, quantum information processing and optical neural networks mature. While silicon photonics has emerged as a viable candidate to translate proof-of-principle demonstrations to mass-manufacturing, the fabrication of photonic integrated circuits (PICs) and their subcomponents remains highly heterogeneous, which drives up cost and slows down progress towards faster and more power-efficient performance. In this dissertation, I demonstrate how single-layer graphene (SLG) can bring active functionality to arbitrary passive waveguide platforms, offering a more universal approach to developing integrated photonic components. SLG can be transferred to PICs without the limitations or complexity of traditional deposition or bonding processes. It also stands to outperform conventional semiconductors in terms of speed and spectral operating range due to its ultra-fast carrier dynamics, high carrier mobility, and gapless bandstructure. Using the key component for optical-to-electrical signal conversion—the photodetector—as an example, here I demonstrate graphene-based components on three different PIC platforms. First, I show plasmonic enhanced graphene photodetectors (GPDs) on silicon nitride waveguides, which generate a voltage from optically generated hot-carrier distributions in SLG via the photo-thermoelectric (PTE) effect. Then, I demonstrate PTE-GPDs—fabricated from layered material heterostructures—on a silicon-on-insulator microring resonator. Both detectors operate at Telecom wavelengths (λ = 1.55 μm), are compatible with high-speed (> 10 GHz) operations, and hold the current records in voltage responsivity—R ∼ 12 V/W and R ∼ 90 V/W—for waveguide-integrated GPDs fabricated from chemical vapour deposited and mechanically exfoliated SLG, respectively. I then go on to show how established GPD concepts can be translated to an integrated mid-infrared (λ = 3.8 μm) platform based on sub-wavelength grating waveguides in silicon and study how light-graphene interaction under in-plane incidence can be further optimised to improve GPD performance. Finally, I develop new approaches towards high-quality, scalable SLG on PICs, critically required to advance graphene-based integrated photonics towards industrial production

Application of atomistic modelling to molecular solids containing hydrogen bonds

The work presented in this thesis is mainly concerned with crystal structures containing hydrogen bonds. Chapter 1 and 2 mainly discuss the background and basic concepts used in this study such as the importance of hydrogen bond in crystal engineering, co-crystals and polymorphism, and recent studies of urea co-crystals. Chapter 3 is a study about urea/oc,co-dihydroxyalkanes co-crystal structures. It begins with parameterising DMAREL to obtain lattice energy from a set of homologous co- crystals where DMA multipoles were generated from different method, GDMA and MOLPRO. The simulated lattice energy, structures and interaction energy were discussed and compared whether there is possibilities for the co-crystals to appear in different urea ribbon structures (parallel and anti-parallel) as these could not be crystallised experimentally. The energy data shows that urea/a,co-dihydroxyalkanes co-crystal of anti-parallel ribbon type structure are more thermodynamically favoured compared to the parallel structure. In latter part of this chapter, attempts to construct and simulate the anti-parallel urea ribbon co-crystal type structures from initial experimental structures were discussed. In Chapter 4, sulfur pair potential was modelled to fit for use in DMAREL for TTCA structure simulation. The original potentials were taken from Lennard Jones potential of a-S8 crystal structure. Initially, the potentials were modelled against a-Sg and thiourea crystal. Improved potentials were applied to a set of S-contained structure, specifically with similar environment to TTCA to validate the reliability of this potential against other molecules. Potentials works fairly well for 5 out of 10 molecules simulated, where TTCA shows poorest performance against the potential even though it has improved from the original sulfur potential. Contrasting crystal structure between TTCA and CA when substituted between each other is discussed. The final chapter, Chapter 5 is the continuation from work in Chapter 3. In urea co-crystal, it was found that urea structure was not exactly planar. We then continue on the search of different conformation of urea molecule in as and solid structure. First, the conformation of urea monomer were discussed and followed by calculation of larger planar urea clusters where one of the urea is substituted with either Cj or Cs conformers. Urea clusters were build systematically mimicking dense urea crystal structure. Planar structure was finally obtained by using 5 urea molecules involving four hydrogen bonds.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Application of atomistic modelling to molecular solids containing hydrogen bonds

The work presented in this thesis is mainly concerned with crystal structures containing hydrogen bonds. Chapter 1 and 2 mainly discuss the background and basic concepts used in this study such as the importance of hydrogen bond in crystal engineering, co-crystals and polymorphism, and recent studies of urea co-crystals. Chapter 3 is a study about urea/oc,co-dihydroxyalkanes co-crystal structures. It begins with parameterising DMAREL to obtain lattice energy from a set of homologous co- crystals where DMA multipoles were generated from different method, GDMA and MOLPRO. The simulated lattice energy, structures and interaction energy were discussed and compared whether there is possibilities for the co-crystals to appear in different urea ribbon structures (parallel and anti-parallel) as these could not be crystallised experimentally. The energy data shows that urea/a,co-dihydroxyalkanes co-crystal of anti-parallel ribbon type structure are more thermodynamically favoured compared to the parallel structure. In latter part of this chapter, attempts to construct and simulate the anti-parallel urea ribbon co-crystal type structures from initial experimental structures were discussed. In Chapter 4, sulfur pair potential was modelled to fit for use in DMAREL for TTCA structure simulation. The original potentials were taken from Lennard Jones potential of a-S8 crystal structure. Initially, the potentials were modelled against a-Sg and thiourea crystal. Improved potentials were applied to a set of S-contained structure, specifically with similar environment to TTCA to validate the reliability of this potential against other molecules. Potentials works fairly well for 5 out of 10 molecules simulated, where TTCA shows poorest performance against the potential even though it has improved from the original sulfur potential. Contrasting crystal structure between TTCA and CA when substituted between each other is discussed. The final chapter, Chapter 5 is the continuation from work in Chapter 3. In urea co-crystal, it was found that urea structure was not exactly planar. We then continue on the search of different conformation of urea molecule in as and solid structure. First, the conformation of urea monomer were discussed and followed by calculation of larger planar urea clusters where one of the urea is substituted with either Cj or Cs conformers. Urea clusters were build systematically mimicking dense urea crystal structure. Planar structure was finally obtained by using 5 urea molecules involving four hydrogen bonds.EThOS - Electronic Theses Online ServiceGBUnited Kingdo