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Abstract

Ab initio quantum chemical methods are useful tools and widely employed for the study of excited states, which are the central quantities in photochemistry. A robust understanding of photochemical processes following electronic excitation of molecules does not only allow for the characterization of the physical and chemical properties, but also offers a solid foundation for the successful design and development of novel light-responsive molecular devices. In light of the importance of excited states in photochemical processes, appropriate quantum chemical methods for a reliable and accurate description of the electronic structure need to be chosen carefully. In practical calculations, density functional theory (DFT) and time-dependent DFT (TDDFT) can provide a good compromise between accuracy and computational effort for the treatment of ground and excited states. The algebraic diagrammatic construction (ADC) scheme for the polarization propagator, which is known to deliver accurate excited states information, is often used to benchmark TDDFT results. The basic background of quantum chemistry and several popular quantum chemical methods for the study of excited states are introduced in chapter 2. To obtain insights into novel multiphotochromic properties and the unique isomerization behavior of multiple azobenzenes, a series of linear- and non-linear multiazobenzenes are investigated with selected quantum chemical methods in chapter 3 and chapter 4. Moreover, an evaluation of the restricted-virtual-space (RVS) approximation within the algebraic diagrammatic construction (ADC) scheme for the polarization propagator for the purpose of speeding up excited state calculations of medium-sized and large molecular systems is presented in chapter 5. In more detail, chapter 3 presents investigations of the absorption spectra and the isomerization mechanism of the parent azobenzene (AB) and linear coupled azobenzenes ((AB-(n)). The relaxed potential energy surfaces along the CNNC rotation, CNN inversion and the concerted-inversion pathways were calculated with TDDFT to explore the isomerization mechanism of linear multiazobenzenes. The results show that the order of electronically excited states changes with increasing chain length because the excitation energies of the low-lying excited states display different levels of decline. A significant dual band appears in the π-π* absorption band of cis-AB-(n)s due to strong excitonic coupling between two connected sub-azo moieties. The S1 potential energy surface of AB and the linear AB-(n)s is essentially barrierless along the rotation pathway and a conical intersection is most likely to appear in all cases of the AB-(n). Thus, it is concluded that the isomerization in the n-π* state favours the rotation mechanism. Although a large barrier was found in the S1 potential surface along the concerted-inversion pathway, the concerted-inversion path is considered to be an energetically favorable mechanism when excitation to the S2 state or even higher excited states occurs. Because the energy gap between the S2 and S1 states becomes quite small, rapid relaxation from higher excited states to the S1 state occurs more easily, which is beneficial to overcome the potential energy barrier on the S1 surface. In the subsequent chapter 4, several non-linear azobenzene systems consisting of two or three azo subunits are studied. The absorption spectra of o-, m- and p-bisazobenzenes were primarily calculated using time-dependent density functional theory together with and without conductor-like polarizable continuum models (C-PCM) modelling solvation. The results show that intramolecular excitonic interaction between the azo subunits occurs in the case of the o-bisazobenzenes, which accounts for the two significant excitonic bands in the absorption spectrum. Strong π-conjugation extending over the two azo subunits was observed for p-bisazobenzene, leading to planarity of the molecule as well as low quantum yield for switching. In contrast, m-bisazobenzene exhibits a very similar spectral feature compared to the monomeric azobenzene and the azo subunits operate nearly independently from each other. The following investigation of meta-tris-azobenzene (MTA) is based on the intriguing meta- connection pattern. TDDFT simulations fully characterize spectral differences in the absorption spectra of MTA and various substituted MTA derivatives. It is found that the distribution of the main π-π* band of MTA is also quite similar to the monomer azobenzene, only differing in intensity, which reveals decoupling of the three sub-azo units. The auxochromic shift in the absorption spectrum can be modulated by a series of introduced functional groups. This study shows that each individual sub-azo unit of substituted MTA can be selectively and reversibly switched by specific wavelengths of light. In addition, the photophysical and photochemical properties of cyclotrisazobenzene (CTA), which shows a high stability due to its constrained ring system, were addressed. The PES along the isomerization pathway of the azobenzene units in CTA show that isomerization is essentially impossible even though the CTA molecule is excited to higher excited states. It indicates that relaxation of excited CTA does not lead to photoisomerization. This study can be extended to relevant CTA derivatives, thereby probably revealing unexpected multiphotochromic behavior. Overall, quantum chemical investigations of coupled multiazobenzenes not only provide deeper insight into multiphotochromic properties and theirs unique isomerization behavior, but also pave the way for the design and development of novel photoresponsive applications, based on azo subunits, with different connection patterns. In Chapter 5, the applicability and limitations of the restricted virtual space (RVS) approximation for use within the algebraic diagrammatic construction (ADC) scheme for the polarization propagator up to third order is evaluated. In RVS-ADC, not only the core but also a substantial amount of energetically high-lying virtual orbitals is disgarded in excitation energy calculations of low-lying excited states. RVS-ADC calculations are performed for octatetraene, indole, and pyridine using different standard basis sets of triple-zeta quality, i.e. 6-311G*, cc-pVTZ and def2-TZVP. The results show that freezing core and less than 30% percent of the high-lying virtual orbitals has a negligible effect on ππ* excited states within RVS-ADC(2). However, for nπ* or πσ* states, the RVS approximation is generally less reliable, whereas its accuracy is greatly improved by using the third-order ADC level. Furthermore, a unified and basis-set independent normalized virtual orbital threshold (NVT) is introduced, making the RVS approximation controllable and applicable.