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Abstract

Functional molecules are species that hold properties which enable them to perform a specific function. In their targeted design, quantum mechanical investigations play a central role, as they complement experimental data and enable the prediction of molecular properties. Functional organic molecules are the focus of research efforts in various fields, where organic electronics is one of the most prominent. Especially acenes and their derivatives are promising candidates for various applications in organic electronics. In this regard, their absorption spectra present a feature of particular interest and are well-characterized experimentally and theoretically. In Chapter 3, the excited states of acene cations are examined using algebraic diagrammatic construction (ADC) methods and time- dependent density functional theory (TD-DFT). Benchmark studies are performed to assess the accuracy of excitation energies obtained at different levels of ADC and different exchange-correlation kernels for TD-DFT against experimental values. It is shown that excited states observed in neutral acenes are retained in their cationic counterparts by analyzing the main orbital contributions of the excitations. Furthermore, new excited states that appear in the spectra of the cations are characterized. The behavior of the excitation energies of both neutral and cationic acenes with increasing length is examined using TD-DFT, which shows that excitation energies decrease with increasing acene length and converge towards a certain value. The increasingly poor description of longer acenes using DFT, due to the single-reference ground state description, is discussed. Furthermore, the exciton properties, i.e. exciton size, hole and electron size and correlation coefficient are computed for the excited states of neutral and cationic acenes using TD-DFT. Their behavior with increasing acene length is explored, where different trends can be observed for the excited states of neutral and cationic acenes. For excited states of the same character, the excited state of the neutral acene shows a larger exciton size, than that of the corresponding cationic acene. It is also observed that with increasing acene length, the hole size grows faster than the electron size for cationic excited states, while for neutral excited states they increase equally fast. Furthermore, the correlation coefficient is larger and grows faster for excited states that are described by at least two orbital transitions, while it is smaller and grows slower for excited states that are mainly described by one orbital transition. Finally, previously established criteria are employed to show that the excited states 1Bb and 2Bb have plasmonic character. Organic molecules with high degrees of diradical character are another class of molecules which show favorable properties for the application in organic electronics. As open-shell systems they are generally unstable so that the design of stable organic diradicals is an active field of research. The theoretical description of most species of interest is challenging, since the use of multi-reference methods would be required, but is not computationally feasible. The open-shell nature of several organic diradicals is investigated in Chapter 4 by computing and analyzing the diradical character, multiplicity of the ground state, singlet-triplet gap and other properties using DFT, TD-DFT and spin-flip TD-DFT (SF-TD-DFT). The first project investigates two regioisomeric benzodithiophenes, where the ortho-isomer shows dimerization to form a dimer cage. The results show a closed-shell ground state for the para-isomer and a diradical ground state for the ortho- isomer. Furthermore, the reaction mechanism of the dimerization is proposed to proceed via the consecutive formation of two sets of two σ-bonds. In the second project, the open-shell nature of an indeno-diazatetracene-σ-dimer, built from two radical monomer units, is investigated and indicates a high diradical character of the ground state. Derived from this system, two diradical model dimers are designed and examined in the third project to explore the relation between their molecular geometry, ground state description and open-shell character. Both dimers show the expected interdependence of the angle between the monomers, the ground state employed for optimization and the diradical character. Furthermore, the development of the excitation energies of the first excited singlet and triplet states with varying diradical character is analyzed and the expected mirrored behavior of the singlet-triplet gap and the diradical character is observed. The possibility of inverted energy gaps between the first excited singlet and triplet states present an interesting opportunity with regard to fluorescence emitters in organic light-emitting diode (OLED) applications. While this singlet-triplet gap is positive for most systems, the inversion could increase the population of the emissive singlet state and thereby increase OLED efficiency. It was observed in a previous study that negative singlet-triplet gaps become smaller and eventually positive as the accuracy of the theoretical treatment is increased. In Chapter 5, the influence of the employed level of theory on the singlet-triplet gaps is investigated by computing them at increasing ADC orders and increasing basis set size. Initial computations show that inverted singlet-triplet gaps are obtained for the investigated molecules only at highly symmetric geometries. The singlet-triplet gaps obtained at ADC(1) are positive and then decrease going to ADC(2), where only negative values are observed. The trend shows another increase for ADC(3), however only resulting in a positive value for one of the investigated systems. The singlet-triplet gap then decreases again for ADC(4) and is negative for all investigated molecules. Furthermore, the effect of the basis set size on the singlet-triplet gap is examined and shows an increase with increasing basis set size, which is however small in comparison to the effect of the employed level of ADC. Label molecules enable the performance of fluorescence and electron paramagnetic resonance (EPR) spectroscopy on biomolecules, which support the elucidation of their structure. In previous work, a mechanism for quenching of fluorescence in an established spin label via internal conversion (IC) to a dark low-lying excited state was presented. Based on this, the design of a combined fluorescence and spin label is explored in Chapter 6 using TD-DFT. Excitation energies, oscillator strengths and transition densities are computed for the excited states of the existing spin label and derivatives. A dark first excited doublet state and bright second excited doublet state are observed for all investigated molecules. Furthermore, the equilibrium geometries of both excited states and the minimum energy crossing point of the conical intersection (CI) seam space between them is computed. The efficiency of the proposed fluorescence quenching pathway is analyzed using the computed points on the potential energy surfaces. Based on this investigation, a decreased rate of IC is expected for one of the derivatives.