Quantifying Electronic Phenomena in Organic Chromophores

Challenging ground and excited state problems in the chemistry of common organic chromophores are investigated with state-of-the-art quantum chemical methods. We present a comprehensive excited state molecular dynamics analysis of (a) fundamental building blocks in organic electronics (thiophene and its derivatives), (b) aggregation-induced emission systems (tetraphenylethylene), and (c) organic fluorophores used for imaging and sensing applications (BODIPY and its derivatives). We identify the efficient excited state deactivation pathways which are essential to understanding the photochemical stability and emissive properties of these compounds. The internal conversion mechanisms of theoretically challenging thiophene and bithiophene molecules are investigated with a trajectory surface hopping approach utilizing reliable electronic structure methods. We gain new insights into the photochemistry and photophysics of these systems, including a new mechanism in thiophene excited state decay and the increased photostability of bithiophene, thereby complementing earlier theoretical and experimental literature. The origin of the non-emissive behavior of tetraphenylethylene in the gas phase is explained by identifying energetically accessible conical intersections which promote radiationless decay. It is implied that restricted access to the conical intersection induces strong emission upon aggregation - a phenomenon that attracted significant research attention recently. Finally, the concept of conical intersection accessibility is utilized to explain the fluorescence quenching in certain meso-substituted BODIPY derivatives. We deliver a full mechanistic picture of the nonradiative decay of these molecules, invoking the role of excited state charge transfer and weak intramolecular interactions. Understanding the failures of quantum chemical electronic structure methods is crucial for subsequent improvements of commonly applied theoretical approximations. To elucidate the origins behind the unbalanced description of the low-lying pipi* excited states of heteroaromatic molecules (including thiophene and its derivatives, or in general fused heteroaromatics), we employ a range of quantum chemical approaches, from more approximate time-dependent density functional theory (TDDFT) to highly accurate wavefunction-based methods. The drawbacks of standard TDDFT were ascribed to the ubiquitous adiabatic approximation, rather than different functional approximations. On the other hand, the performance of wavefunction-based methods is found to be largely dependent on the treatment of electron correlation, which is key for a balanced description of excited states with disparate electronic character. Non-covalent molecular interactions are at the origin of many chemical and physical phenomena. While quantifying intermolecular interactions has become a routine task, intramolecular interactions are still considered particularly difficult to treat by theoretical methods. Here, we develop an original wavefuction-based method for quantifying intermolecular and intramolecular interactions on equal footing. The method, intra-SAPT, makes use of a single Slater determinant wavefunction, subject to perturbational corrections. The scheme decomposes interaction energies into physically meaningful components: electrostatics-exchange, induction and dispersion.

Photophysics and Photochemistry from First Principles

Matteo Guglielmi

Quantum chemical methods are important tools for the predictions of electronic structure and energetics of molecules providing the interpretation of spectroscopic experiments and reaction mechanisms. In this thesis, density functional theory (DFT) and wavefunction-based methods are used in order to evaluate their ability in predicting experimental data with a particular attention to photoprocesses occurring in both gas and condensed phase systems. The cases studied in this work are inspired by problems both from biology and material science. In the first study, the photoexcitation properties of the molecule 7-hydroxyquinoline·(NH3)3, a prototypical molecular wire, are investigated. A concerted mechanism according to which all protons are transferred simultaneously in a fast process (∼ 100 fs) that amounts to the net transport of one proton from the oxygen to the nitrogen of 7-hydroxyquinoline is observed. In addition, the proton transfer pathway involves all three ammonia molecules and not only two as previously proposed. The presence of potential energy surface crossings with a dark-state is responsible for the low experimentally observed quantum-yield. To better understand the complex photophysics of the amino acid tryptophan, which is widely used as a probe of protein structure, we performed DFT and TDDFT calculations of gas phase tryptophan solvated with a controlled number of water molecules. The addition of two water molecules sufficiently lengthens the excited state lifetime enabling vibrationally resolved spectra. Quantum chemical calculations at the RI-CC2/aug-cc-pVDZ level, together with TDDFT/pw based first-principles MD simulations of the excited state dynamics, clearly demonstrate how interactions with water destabilize the photodissociative states and increase the excited state lifetime. Due to the availability of high-resolution experimental data, this system is ideally suited as benchmark model for the evaluation of the performance of different quantum chemical methods. An extensive low-energy isomer search was carried out for [TrpH·(H2O)n=0,1,2]+ using a hierarchy of theoretical methods ranging from classical force fields to a variety of DFT methods (BLYP, B3LYP, M05, M05-2X, M06, M06-2X and M06-HF with different basis sets) up to the CBS-C level. For the low-energy structures, the harmonic vibrational frequencies are calculated and compared with high resolution experimental IR spectra. It turns out that in all three cases, CBS-C is able to predict the lowest energy isomers that are in agreement with the experimentally observed vibrational spectra. In most cases, M06 provides energetics in close agreement with the CBS-C results. On the other hand, M05-2X is the only functional that yields highly-reliable predictions of the vibrational spectra. A natural extension of the tryptophan model to condensed phase was performed by an investigation of the [Trp-Lys]+ motif studied in the protein Human Serum Albumin via QM/MM simulations. Like in the case of gas phase [TrpH]+, solvation effects lead to an increase of the σN-H orbital energies. As a result, the first photoaccessible state of sW K+ is primarily a photostable π orbital, while in dW K+, photoexcitation leads to a dissociative pathway. The last application of this work concerns the optical properties of a novel dye for the sensitization of titanium dioxyde nanoparticles in Grätzel solar cells. The presence of a new absorption band in the visible region opens the way for a rational design of new dyes with improved properties.

EPFL2010

Computational Quantum Chemical Studies on Radicals

Peter Rudolf Tentscher

Radicals play an important role in many areas of chemistry, such as atmospheric, aquatic, polymer, and biological, to name a few. Radicals are often highly reactive species which are short-lived and therefore harder to study by experimental techniques. In principle, computational quantum chemistry enables the study of arbitrary chemical species, including elusive radicals. However, the approximate quantum chemical methods which are used for production simulations need to be validated for the type of problem in question. First, it is necessary to validate that highly accurate reference methods commonly applied to closed-shell systems are equally applicable to radicals. I show that the “gold standard” for closed-shell systems (coupled cluster with singles, doubles, and perturbative triples excitations (CCSD(T))) can produce molecular geometries and vibrational frequencies suitable for (sub-)kJ/mol thermochemistry for radical species as well. However, additional diagnosis is necessary, especially for vibrational frequency calculations. Coupled cluster methods are then used to compute benchmark binding energies between small radicals and water or hydrogen fluoride as a proxy to the aqueous condensed phase. I find that many common density functional theory (DFT) methods cannot reproduce these binding energies to within chemical accuracy (1 kcal/mol). However, some functionals, for example MPW1K and BHandHLYP, perform better than others. If more accurate ab initio methods are computationally too demanding for the system of interest, these DFT methods should be used for (micro-)solvated radicals. Complexes of radicals with polar solvent are found to be more strongly bound than expected. This can not be explained by dispersive and electrostatic interactions alone. Using natural bond orbitals, I show that additional donor-acceptor interactions may be responsible for the strong intermolecular binding observed. One of the most fundamental processes involving aqueous organic radicals is the one-electron oxidation of closed-shell organics. I study the vertical ionization of neutral organics. That is, the ionization is faster than the reorganization of the solvent (and solute), and the conformation is assumed to be unchanged during the ionization process. I find that the difference between the gas-phase and aqueous-phase vertical ionization energy, ∆VIE = VIEaq − VIEgas, ranges from ≈ 0 to -1.0 eV for small aromatic compounds. The differences in ∆VIE between different compounds seem to be caused by the solvent structures of the ≈ 70 water molecules closest to the solute. In natural surface waters, oxidized organics arise through the reaction of photochemically created radicals with organic molecules already present in the water column. One example for such a reaction is the oxidation of sulfonamide antibiotics. I show that upon oxidation of sulfadiazine and related compounds, an aniline radical cation is formed. The reactivity of this oxidized species is strikingly different from its reduced counterpart: the barrier for a nucleophilic attack on the aniline ring is drastically lowered, and a fast rearrangement reaction can take place. I demonstrate that current quantum chemical protocols are suitable for mechanistic studies of solvated radicals, and I successfully apply them to such problems. To achieve chemical accuracy for quantum computational protocols, further developments are necessary. To validate novel approaches, the benchmark data presented in this thesis may be used.

EPFL2013

Quantum chemical studies of reactive species in water

Daniela Trogolo

In this thesis, quantum chemical methods have been applied to elucidate the thermodynamics and the kinetics of reactions involving reactive species in water. Due to their high reactivity in water, many transient species are difficult to study by experimental means only. Here, quantum chemical models are used to provide a deeper insight into the chemical nature and aqueous behaviors of such species. In the first chapter, I investigate the gas phase electronic structure and the thermodynamics of inorganic chloramines, bromamines, and bromochloramines, collectively termed halamines. The halamines are halogen oxidants that arise from reactions between ammonia and hypohalous acids during water disinfection processes, and these reactive species are implicated in the formation of disinfection byproducts that are harmful to human health. Despite their relevance in both drinking water chemistry and in biochemistry, the stabilities and speciation of these molecules are difficult to investigate by experimental means. To accurately predict the electronic structures and gas phase thermodynamic properties of halamines, I design a computational protocol, TA14, based on the high-quality Weizmann and Feller-Peterson-Dixon composite methods. TA14 combines a systematic sequence of wave function theory calculations, including the evaluations of dynamical and static electron correlation (CCSDTQ), core/valence electron correlation contributions, scalar and spin-orbit relativistic contributions, and VPT2 anharmonic vibrations. Using TA14, I successfully assess the gas phase total atomization energies, free enthalpies of formation, and Gibbs free energies of formation of halamines within uncertainty bounds of 1-3 kJ/mol. Analysis of the energy components contributing to total atomization energies of halamines reveals that N-Cl and N-Br bonds are held together mostly or entirely by electron correlation forces, with small or even negative Hartree Fock contributions. For example, the Hartree Fock component of the total atomization energy is negative for both NBr3 and NBr2Cl, implying that these molecules would be predicted as unstable without accounting for dynamical electron correlation. Reported thermochemical data enable the determination of equilibrium constants for reactions involving halamines, opening possibilities for more quantitative studies of the chemistry of these poorly understood compounds. In the second chapter, I evaluate the aqueous equilibria and speciation of halamines. I combine theoretical benchmark-quality gas phase Gibbs free energies of formation (chapter 2) with the computed Gibbs free energies of solvation, thereby obtaining aqueous phase Gibbs free energies of formation for halamines. The 'half-and-half' solvation approach, based on averaging the estimates of SMD implicit solvent model and the cluster-continuum solvent model, produces an average error of 3.3 kJ/mol in the free energies of solvation for a set of structurally related molecules containing H, N, O, and Cl. Taking into consideration the combined uncertainties of the computed gas free energy of formation values and the computed free energy of solvation values, we assign an uncertainty of 6-7 kJ/mol to the theoretical standard Gibbs free energies of formation in aqueous phase of halamines. Aqueous Gibbs free energies of formation values are key thermodynamic properties for investigating chemical processes involving halamines during drinking water treatment. The newly reported thermodynamic data can be used to determine the stabilities (reaction equilibria) of halamines in water. Based on our estimated uncertainties of 6-7 kJ/mol in the computed aqueous free energy of formation values of halamines, we expect roughly 1 order-of-magnitude uncertainty in the aqueous equilibrium constants for the reactions leading to the production of halamines in water. ...

EPFL2016