Intramolecular Charge Transfer in Solvated Organic Molecules: A Quantum Chemical Study

Photochemical reactions are ubiquitous in nature. Furthermore, they play an important role in organic and inorganic synthesis. In such reactions, light energy is used to induce chemical transformations. Two prime examples of such reactions are photosynthesis – the conversion of light energy to chemical energy, and the cis-trans isomerization reaction in the retina which enables vision. Upon absorption of light, the electrons in a molecule are promoted to higher energy levels; the molecule is elevated from its electronic ground state to an electronically excited state. Once in the excited state, different processes may take place such as the emission of light, interactions with the environment, or chemical reactions. One class of processes often involved in photochemical reactions and of particular importance are charge-transfer processes. A charge transfer can occur intermolecularly i.e., between different molecules or within a molecular complex from a ligand to the center and vice versa. These intermolecular charge- transfer processes, for example, occur in organometal complexes and play a crucial role in numerous catalytic reactions. In intramolecular charge-transfer processes, charge is transferred from one part of a molecule, the donor moiety, to a different part of the molecule, the acceptor moiety. Among those, so-called twisted intramolecular charge-transfer (TICT) processes and the corresponding TICT states are of particular interest. TICT molecules are characterized by the fact that they undergo a rotation about a single bond connecting the donor and acceptor moiety in the excited state. This twisting is accompanied by a charge transfer from the donor to the acceptor, resulting in a highly polar CT state exhibiting a mutually perpendicular orientation of donor and acceptor subsystems. Due to this process, TICT compounds show a very interesting unusual fluorescence behavior, named dual fluorescence, in medium polar and polar solvents. Dual fluorescence means that there are two fluorescence bands observable: one corresponding to the transition from the TICT state and the other from the transition from a planar excited state. In this dissertation, photochemical reactions of different small- and medium-sized organic molecules have been investigated with a particular focus on intramolecular charge-transfer. The well-known and experimentally well-investigated small donor-acceptor TICT compound N- pyrrolobenzonitrile (PBN) and its thiophene derivative 5-(1H-pyrrole-1-yl)thiophenecarbonitrile (TCN) are studied, among others. For this purpose, different quantum chemical electronic structure methods are employed. Especially when used along with spectroscopy, these ab initio methods are powerful tools to study the molecular structure and photochemical reactions. Quantum chemical methods can provide reliable excitation energies, excited state properties, and absorption strengths and allow for the computation of deactivation pathways. In this work, it is shown that the chosen quantum chemical methodology is well suited to describe the photochemical behavior of the considered organic compounds and new insights into the photochemistry of these systems are provided.