Charge separation: From the topology of molecular electronic transitions to the dye/semiconductor interfacial energetics and kinetics

Charge separation properties, that is the ability of a chromophore, or a chromophore/semiconductor interface, to separate charges upon light absorption, are crucial characteristics for an efficient photovoltaic device. Starting from this concept, we devote the first part of this book chapter to the topological analysis of molecular electronic transitions induced by photon capture. Such analysis can be either qualitative or quantitative, and is presented here in the framework of the reduced density matrix theory applied to single-reference, multiconfigurational excited states. The qualitative strategies are separated into density-based and wave function-based approaches, while the quantitative methods reported here for analysing the photoinduced charge transfer nature are either fragment-based, global or statistical. In the second part of this chapter we extend the analysis to dye-sensitized metal oxide surface models, discussing interfacial charge separation, energetics and electron injection kinetics from the dye excited state to the semiconductor conduction band states

Development and Application of Efficient Methods for the Computation of Electronic Spectra of Large Systems

In this thesis, an efficient procedure to compute electronic excitation spectra of molecular systems is presented, focusing particularly on the computation of electronic circular dichroism (ECD) spectra. ECD spectroscopy is commonly used to distinguish between the two enantiomers of a chiral compound. Due to a strong sensitivity to the three-dimensional structure, reliable simulation of ECD spectra of solvated molecules by quantum chemical methods requires the knowledge of the relevant conformers along with the corresponding ECD signals (i.e., the individual transition intensities and energies) and Boltzmann populations. The latter point can be addressed by an established thermochemical protocol. It combines electronic energies computed in gas phase by dispersion-corrected density functional theory (DFT-D) with nuclear ro-vibrational and solvation contributions to yield the free energies in solution. This model is applied to study the association of two intermolecular frustrated Lewis pairs (FLPs). Though this case study does not aim at computing an ECD spectrum, it provides insight on whether such a scheme could also be suited to rank conformers in solution. Comparison to high-level reference methods and partially available experimental data suggests that the largest uncertainty can be attributed to the implicit solvation model. The errors for different dimer arrangements, however, appear to be within the order of 1 kcal mol-1, which is encouraging for the pursued computation of conformer free energies. In combination with a quadruple-ζ basis set, hybrid DFT-D methods like the PW6B95-D3 are almost converged with respect to a complete basis and provide satisfactory results for the electronic energy contribution. Hence, they are recommended choices for the final electronic structure level to rank different conformers in routine calculations. The major part of this thesis deals with the development and application of cost-efficient excited state methods. The current state-of-the-art to compute ECD spectra for systems with roughly 100 atoms is the time-dependent density functional theory (TD-DFT) approach. Based on the latter, the simplified TD-DFT (sTD-DFT) method is developed. The excited state treatment is accelerated by at least three orders of magnitude, resulting from semiempirically approximated two-electron integrals and a significant reduction of the involved matrix dimensions. The introduced approximations are in line with the ones in the previously presented simplified Tamm-Dancoff approximated TD-DFT (sTDA-DFT). It is shown that the sTD-DFT and the sTDA-DFT approaches provide roughly the same accuracy for vertical excitation energies, as well as absorption and ECD spectra, as their parental schemes, i.e., TD-DFT and Tamm-Dancoff approximated TD-DFT (TDA-DFT), respectively. Thus, sTD-DFT is an efficient approach that is suitable for the computation of ECD spectra. Furthermore, sTD-DFT calculations conducted on "snapshots" from molecular dynamics (MD) simulations offer an appealing way to effectively incorporate vibronic effects without a quantum mechanical (QM) treatment of the nuclei. Such a treatment is exemplified for [16]helicene (102 atoms) and a di-substituted derivative (164 atoms). While the feasibility of applying sTDA-DFT to very large systems is demonstrated for two palladium(II) metallosupramolecular spheres (822 and 1644 atoms, respectively), it is also shown that this method produces ECD spectra of incorrect sign in the origin-independent dipole velocity formalism for extended π-systems. This behavior is due to the Tamm-Dancoff approximation (TDA) and, therefore, it is also present in TDA-DFT and the related configuration interaction singles (CIS) approach. Based on the insights obtained from this study, the A+B/2 correction is developed, which corrects the (simplified) TDA eigenvectors affording origin-independent dipole velocity ECD spectra of roughly (s)TD-DFT quality, while retaining the lower computational cost of the (s)TDA excited state treatment. Combination with a newly developed, purpose-specific extended tight-binding procedure for the ground state yields the ultra-fast sTDA-xTB approach. Due to different adjustments of the atomic orbital basis and the tight-binding Hamiltonian, the method is on a par with TDA-PBE0/def2-SV(P) for vertical excitation energies. The entire computation of an ECD spectrum ( The last part of this thesis reports on another purpose-specific extended tight-binding scheme, GFN-xTB, which provides molecular geometries, harmonic vibrational frequencies, and non-covalent interaction energies with comparable or better accuracy than existing semiempirical methods. Since parameters are available for all elements with Z ≤ 86, the method offers great potential to sample the conformational space of almost arbitrary molecules with up to a few hundred atoms. In combination with the ultra-fast sTDA-xTB approach, ECD spectra can be computed in an almost "black box" manner, e.g., by computing spectra on MD snapshots. Together with the established thermochemistry protocol mentioned above, the newly developed architecture sets the stage for a fully automatic multi-level ECD procedure to be developed in the near future.Diese Dissertation stellt einen effizienten Ansatz zur Berechnung von elektronischen Anregungsspektren molekularer Systeme vor, wobei der besondere Fokus auf der Berechnung von elektronischen Circulardichroismus-(ECD-)Spektren liegt. Die ECD-Spektroskopie wird typischerweise verwendet, um zwischen den beiden Enantiomeren einer chiralen Verbindung zu unterscheiden. Aufgrund der hohen Sensibilität für die räumliche Struktur des Moleküls wird zur zuverlässigen Simulation von ECD-Spektren die Kenntnis der relevanten Konformere inklusive ihrer Boltzmann-Populationen und der jeweiligen ECD-Signale (d.h. deren energetische Lage und Intensitäten) benötigt. Die Populationen können mithilfe eines literaturbekannten Thermochemieprotokolls unter Verwendung der dispersionskorrigierten Dichtefunktionaltheorie (DFT-D) näherungsweise berechnet werden. In der vorliegenden Arbeit wird dieses Modell verwendet, um die Komplexbildung von zwei intermolekularen frustrierten Lewispaaren (FLPs) zu untersuchen. Obwohl diese Fallstudie keine Berechnung eines ECD-Spektrums zum Ziel hat, geben die gewonnenen Erkenntnisse durchaus Aufschluss darüber, ob sich der gewählte Ansatz auch dazu eignet, die Populationen verschiedener Konformere zu bestimmen. Der Vergleich mit hochwertigen Vergleichsrechnungen auf der einen und mit zum Teil verfügbaren experimentellen Daten auf der anderen Seite legt nahe, dass der größte Unsicherheitsfaktor in den Solvatationsbeiträgen vorliegt, welche mithilfe eines impliziten Lösungsmittelmodells bestimmt werden. Allerdings liegen deren geschätzte Fehler für unterschiedliche räumliche Anordnungen des Komplexes, d.h. bei einer gleichbleibenden Systemgröße von ca. 50-100 Atomen, lediglich bei etwa 1 kcal mol-1. Für die Berechnung von freien konformellen Enthalpien ist mit ähnlich großen Fehlern zu rechnen. Kombiniert mit Quadruple-ζ-Basissätzen weisen Hybrid-DFT-Methoden bereits nahezu konvergierte elektronische Energien auf und können bei gleichzeitiger Verwendung einer Dispersionskorrektur relativ genaue Gasphasenenergiebeiträge (so z.B. PW6B95-D3) zu den freien Enthalpien in Lösung beitragen. Der Großteil dieser Dissertation beschäftigt sich mit der Entwicklung und Anwendung von kosteneffizienten Methoden zur Berechnung angeregter Zustände. Die gegenwärtig am häufigsten verwendete Methode zur Berechnung von ECD-Spektren ist die zeitabhängige Dichtefunktionaltheorie (TD-DFT). Von dieser ausgehend wird die vereinfachte TD-DFT Methode (sTD-DFT) entwickelt. Aufgrund der semiempirischen Näherung der Zweielektronenintegrale und der deutlichen Reduzierung der relevanten Matrixdimensionen wird die Berechnung der angeregten Zustände um mindestens drei Größenordnungen beschleunigt. Diese Näherungen sind konsistent zu jenen, die bereits in dem vereinfachten Tamm-Dancoff-genäherten TD-DFT (sTDA-DFT) Ansatz eigeführt wurden. Im Vergleich zu den Ausgangsmethoden, also TD-DFT und seiner Tamm-Dancoff-Näherung (TDA-DFT), ist weder eine signifikante Beeinträchtigung der senkrechten Anregungsenergien noch eine Verschlechterung der Absorptions- und ECD-Intensitäten bemerkbar. Insbesondere die sTD-DFT Methode eignet sich zur effizienten und zuverlässigen Berechnung von ECD-Spektren. Die Effizienz der sTD-DFT Methode ermöglicht unter anderem die Berechnung von Spektren auf Nichtminimumsstrukturen, die aus einer Molekulardynamik-(MD)-Simulation stammen. Somit können vibronische Effekte näherungsweise erfasst werden, ohne dass ein quantenmechanischer (QM) Ansatz für die Kerne verwendet werden muss. Exemplarisch wird dieses Verfahren für das [16]Helicen (102 Atome) und einem disubstituierten Derivat (164 Atome) angewandt. Die Anwendbarkeit der sTDA-DFT Methode auf sehr große Systeme wird am Beispiel von zwei Palladium(II)-metallosupramolekularen Komplexen (822 und 1644 Atome) verdeutlicht, doch zeigt eine weitere Studie, dass Tamm-Dancoff-genäherte (TDA) Methoden für die ECD Spektren von ausgedehnten, delokalisierten π-Systemen im Impulsformalismus das falsche Vorzeichen liefern. Gleiches gilt für den verwandten Konfigurationswechselwirkungs-Ansatz mit Einfachanregungen (CIS). Basierend auf den Erkenntnissen dieser Studie ist es gelungen, die sogenannte A+B/2-Näherung zu entwickeln, welche die entsprechenden Fehler in den TDA Eigenvektoren behebt, ohne die Kosten der Methode sichtlich zu erhöhen. Durch die Kombination des so korrigierten vereinfachten TDA-Ansatzes mit einer speziell optimierten semiempirischen Tight-Binding-Methode für den Grundzustand wird die äußerst schnelle sTDA-xTB-Methode erhalten. Aufgrund verschiedener Modifikationen der Atomorbitalbasis und des Tight-Binding-Potentials erreicht diese Methode eine ähnliche Genauigkeit für senkrechte Anregungsenergien wie z.B. eine DFT-basierende Rechnung auf TDA-PBE0/def2-SV(P) Niveau. Die beachtliche Effizienz der Methode wird im Vergleich zum bereits effizienten sTD-BHLYP/def2-SV(P) Ansatz für das [16]Helicen (alle Anregungen bis 9 eV) deutlich: Während letzterer Ansatz etwas mehr als eine Stunde Rechenzeit benötigt, ist das ECD-Spektrum mit sTDA-xTB bereits nach 10 s verfügbar. Da die Parametrisierung nahezu das gesamte Periodensystem abdeckt, werden Standardrechnungen von Spektren großer Systeme (mit ca. 1000 Atomen) ermöglicht, selbst wenn mehrere Konformere berücksichtigt werden. Im letzten Teil der Arbeit wird eine weitere spezialisierte Tight-Binding-Methode vorgestellt (GFN-xTB), die wiederum auf die Berechnung von Geometrien, harmonischen Frequenzen und nichtkovalenten Wechselwirkungen ausgelegt ist und hierfür bessere Ergebnisse liefert als vergleichbare semiempirische Methoden. Die Verfügbarkeit von Parametern für alle Elemente mit Z ≤ 86 ermöglicht das Absuchen des konformellen Raums für unterschiedliche Systeme mit wenigen hundert Atomen. Zusammen mit sTDA-xTB sind in kürzester Zeit Berechnungen von Sprektren z.B. entlang von MD-Trajektorien möglich. Vereint mit den bereits existierenden Thermochemieprotokollen sind somit die ersten Voraussetzungen für eine völlig automatische Prozedur zur Berechnung von ECD-Spektren geschaffen worden

Taking control of charge transfer : strategic design for solar cells

The thesis is focused on the investigation of the electron transfer mechanisms leading to solar fuel production and to the identification of engineering principles that can be used to design materials able to improve charge separation. Molecular systems composed of three or more subunits arranged in a Donor-Antenna-Acceptor design are required to achieve efficient photoinduced charge separation. It is shown how structural changes in the systems design can be used to systematically optimize the energy gradients and electronic coupling between the molecular subunits, necessary to achieve controlled unidirectional charge transfer. To gain insight into the mechanisms governing the charge transfer processes within a molecular system, the process of photoinduced heterogeneous electron injection is investigated through nonadiabatic dynamics simulations. Coherent electron-nuclear vibrational effects are found to drive the electron transfer process by promoting the coherent superposition of the exciton and the charge transfer quantum state. A photoanode for solar water splitting comprising the functions of light-harvesting, charge separation and catalysis is also investigated. It is observed that, following a fast heterogeneous electron injection, the system catalytic activity is driven by a proton-coupled electron transfer mechanism in which the role of the solvent is crucial.UBL - phd migration 201

Studies of Adsorption of Ringed Carboxylic Acids on the Rutile (110) Surface through Density Functional Theory (DFT)

Contemporary applications of titanium dioxide (TiO2) can be found in many areas, most notably those in development of dye-sensitized solar cells (DSSCs), spurring extensive research on both its bulk and surface properties, as well as adsorption of countless substances onto the latter. Of these studies, theoretical studies have been done through ab initio computational methods based on the framework of DFT. In our studies, we have performed density functional theoretic (DFT) ab initio simulations of TiO2 rutile and anatase bulk crystalline structures, the (110) surface, as well as the adsorption of benzoic acid, phenylalanine and simple zinc porphyrin (ZnPP), consisting of just a simple porphyrin ring and a carboxylic acid group linked to the meso-side, onto the rutile (110) surface, examining all our result in comparison with experimental observations. It is hoped that through these studies, involving increasingly complicated adsorbates, we can either account for experimental results for studies on these same systems, through theoretical and computational models and explanations, or refine the latter to achieve these goals. We also seek to find the relationship between optimized physical structures of these adsorbate-rutile substrate complexes, the energetics and increasingly importantly towards the end, how these influence the complexes’ electronic structures, especially in the case of ZnPP, whose derivatives have been studied as potential dye candidates in DSSCs. In carrying out our ab initio simulations, we have used generalized gradient approximation (GGA) based on the Perdue-Wang 91 (PW91) functionals and DFTD2 (dispersive) methods for optimizations of physical structures of the adsorbatesurface complexes, before performing calculations of the electronic DOS structures for the resultant adsorbate-TiO2 complexes. Through our studies with benzoic acid, we have found that contrary to proposed adsorption configurations from experimental findings through STM, which suggest alignment of the benzene rings along the [1¯10] directions when adsorbed at saturation coverage on the unconstructed (110) surface, the other proposed possible model involving alignment of the rings along the [001] direction has been found to be more energetically stable. Conversely, our DFT calculations have indicated for adsorption on the (1 × 2)-reconstructed rutile (110) surface, the [001]-aligned mode has been found to be more energetically stable, contrary to the proposed [1¯10] alignment of the benzene rings. However, as increasing the tunneling current in our simulated STM has resulted in the carboxylate groups instead of the benzene rings being imaged, this can also possibly indicate need to carry out STM studies at different tunneling current settings to further refine our accounts for how benzoic acid adsorbs on the surfaces of rutile. We have also noted shifts in adsorbate HOMO positions with respect to the valence band maxima, as the benzene ring is rotated by 90◦ Our studies on phenylalanine’s adsorption on the rutile (110) surface have been carried out, in close comparison with result from X-ray photoelectron spectroscopy (XPS) studies. The most stable mode of adsorption has been once more that of the bidentate dissociative (BD) mode, with an adsorption energy value of nearly 2.0 eV/adsorbate. However, in terms of coverage and adsorption patterns, the saturation coverage as produced experimentally was not the most energetically stable, with the patterns at half or even a quarter of the saturation coverage being more so when Van der Waals’ forces have been taken into account. In terms of the orientation of the benzene ring, BD adsorption resulted in an inclination of 35◦ to the (110) normal, as opposed to the 25◦ reported experimentally. Further contractions in the TiO2 valence-conduction bands have been observed, as we study BD adsorption of phenylalanine, at less saturated coverages. Finally, we have carried out DFT studies on the adsorption of the simple ZnPP on the rutile (110) surface, and it has been found that changes in the alignment of the porphyrin ring, in addition to having effects on adsorption energetics, also alter the electronic band structures and the associated energy levels, both before and after excitation. It has been found at the most saturated coverage corresponding to one ZnPP per 8 Ti5c sites, where a porphyrin ring can still undergo a complete rotation about the [110] axis unhindered, adsorption energy is maximized at around 2.4 eV/ZnPP when the porphyrin ring is at about 40-50◦ from either the [001] and the [1¯10] directions, in comparison to 2.2-2.3 eV/ZnPP when the porphyrin rings are nearly aligned with the these directions. Furthermore and much more significantly, we have only found the [001]-aligned configuration to be producing a ZnPP-TiO2 electronic DOS structure that makes the system suitable for DSSC applications

From Dye Sensitized Solar Cells to Organic Field Effect Transistors: A Computational Investigation into the Structural and Electronic Properties of Novel Phthalocyanines

Phthalocyanines (Pc) have gained intense research attention in many diverse application areas due to their highly tunable electronic and structural properties through modification of the molecular periphery and metal center. Throughout this work a series of novel perfluoro-isopropyl substituted MPc have been investigated through theoretical methods. First, the synthetic mechanisms of these Pcs will be explored to gain insight into the experimentally observed Pc product distribution. By examining the electronic structure and formation energies of the various Pc precursors, we explain the product distribution as well as propose the formation of additional Pcs, which were not currently believed to form. The effect of metal center and peripheral modification on the Pc structural and electronic properties is also determined through a systematic investigation of several Pcs with varying degree of peripheral modification as well as several different metal centers. Increased modification of the Pc periphery with strongly electron withdrawing groups lowers the energy of the molecular frontier orbitals; increasing the chemical stability of the Pc. Open d-shell metal centers also introduce several partially occupied states near the top of the Pc valence band, which have electron density localized on the metal center. The bulky groups on the periphery of the Pc also act to mitigate molecular aggregation. To access the degree of aggregation as a function of peripheral modification, a molecular dynamics forcefield within the CHARMM parameterization model was developed specific to these Pcs. This also allows for the simulation of bulk and thin film properties important to various application areas. Finally, we propose a completely solid state dye sensitized solar cell (DSSC) design in which these chemically robust modified Pcs are sandwiched between n-TiO2 and p-NiO, acting as both photosensitizer and electron shuttle. Through analysis of the electronic structure of the Pc|semiconductor systems, the free energy associated with hole injection into the valence band of NiO upon photoexcitation of the sensitizer and electron injection into the conduction band of TiO2 from the reduced form of the Pc are calculated. Significant molecular orbital coupling between the Pc and semiconductors results in estimated charge transfer lifetimes on the femtosecond time scale on both NiO and TiO2. Additionally, the calculated excited state lifetimes of the Pc is found to be on the nanoseconds time scale, allowing ample time for charge transfer prior to the spontaneous relaxation of the Pc excited state. In the absence of a liquid electrolyte solution, the Pc molecule will need to also act as electron shuttle in our cell design. The charge transfer properties within the Marcus-Hush electron transfer theoretical framework are calculated. Results indicate that intermediate modification of the Pc periphery leads to high hole and electron mobilities. This is a promising result for our proposed DSSC design, but also makes these Pcs a viable semiconducting material in other application areas, such as light emitting diodes (LEDs) or organic field effect transistors (OFETs)

From Dye Sensitized Solar Cells to Organic Field Effect Transistors: A Computational Investigation into the Structural and Electronic Properties of Novel Phthalocyanines

Phthalocyanines (Pc) have gained intense research attention in many diverse application areas due to their highly tunable electronic and structural properties through modification of the molecular periphery and metal center. Throughout this work a series of novel perfluoro-isopropyl substituted MPc have been investigated through theoretical methods. First, the synthetic mechanisms of these Pcs will be explored to gain insight into the experimentally observed Pc product distribution. By examining the electronic structure and formation energies of the various Pc precursors, we explain the product distribution as well as propose the formation of additional Pcs, which were not currently believed to form. The effect of metal center and peripheral modification on the Pc structural and electronic properties is also determined through a systematic investigation of several Pcs with varying degree of peripheral modification as well as several different metal centers. Increased modification of the Pc periphery with strongly electron withdrawing groups lowers the energy of the molecular frontier orbitals; increasing the chemical stability of the Pc. Open d-shell metal centers also introduce several partially occupied states near the top of the Pc valence band, which have electron density localized on the metal center. The bulky groups on the periphery of the Pc also act to mitigate molecular aggregation. To access the degree of aggregation as a function of peripheral modification, a molecular dynamics forcefield within the CHARMM parameterization model was developed specific to these Pcs. This also allows for the simulation of bulk and thin film properties important to various application areas. Finally, we propose a completely solid state dye sensitized solar cell (DSSC) design in which these chemically robust modified Pcs are sandwiched between n-TiO2 and p-NiO, acting as both photosensitizer and electron shuttle. Through analysis of the electronic structure of the Pc|semiconductor systems, the free energy associated with hole injection into the valence band of NiO upon photoexcitation of the sensitizer and electron injection into the conduction band of TiO2 from the reduced form of the Pc are calculated. Significant molecular orbital coupling between the Pc and semiconductors results in estimated charge transfer lifetimes on the femtosecond time scale on both NiO and TiO2. Additionally, the calculated excited state lifetimes of the Pc is found to be on the nanoseconds time scale, allowing ample time for charge transfer prior to the spontaneous relaxation of the Pc excited state. In the absence of a liquid electrolyte solution, the Pc molecule will need to also act as electron shuttle in our cell design. The charge transfer properties within the Marcus-Hush electron transfer theoretical framework are calculated. Results indicate that intermediate modification of the Pc periphery leads to high hole and electron mobilities. This is a promising result for our proposed DSSC design, but also makes these Pcs a viable semiconducting material in other application areas, such as light emitting diodes (LEDs) or organic field effect transistors (OFETs)