Einflüsse auf die photochemisch induzierte Cycloreversion an Dimeren des Cumarins und davon abgeleiteter Verbindungen

The presented dissertation aims to broaden the scientific horizon in the field of the photo-induced [2+2]-cycloaddition and -reversion. This should result in a deeper understanding of structural influences on these photochemical processes. Within the scope of this work, reactions are presented that result from the direct interaction between photons and molecules, as well as those in which the absorption of light only serves as a trigger for more complex, parallel chemical transformations. The direct interaction between light and organic matter is investigated in this thesis using the photochemically induced [2+2]-cycloaddition and -reversion of coumarin and structures derived from it. These reactions are already utilized in a variety of applications, for example during crosslinking of offset-printing plates. Irradiation with light of low-energy in the UV-A region leads to the formation of new bonds (cycloaddition), whereas the impact of high-energy light of the UV-B/C region leads to bond-cleavage (cycloreversion). This work focuses on the cycloreversion processes as these are less studied and understood compared to cycloadditions. Influences of the molecular structure on the efficiency of the photochemical reactions are described, and explained in this work. It is demonstrated that the introduction of electron-donating groups leads to an increased reaction rate of cycloreversion. This observation is explained by the better stabilization of the radical intermediate through the additionally introduced electron density. The same applies to the finding that the derivatives in head-to-head configuration (hh) undergo the cycloreversion faster than those in head-to-tail configuration (ht). However, high reaction rates do not automatically go hand in hand with higher efficiency of the photoreaction, expressed as the quantum yield, which is largely influenced by the internal ring tension of the system and the absorption coefficient at the corresponding wavelength. As a reaction of light in the UV-C range may not be possible due to the requirements of the field of application, the influence of substituents on the two-photon induced cycloreversion is investigated. An enhanced reaction rate and improved absorption behavior of the system, due to the substitution of the coumarin’s lactone by a lactam group, are demonstrated. Since the probability of multiphotonic processes is strongly influenced by the substrate’s symmetry, the asymmetrical hybrid dimer of lactone and lactam undergoes the cycloreversion less efficiently than the two corresponding homo dimers. The indirect interaction between light in the UV-A spectral range and organic matter is investigated within experiments in an aerobic environment. As the absorption coefficients of the dimers above 300 nm seem to be negligible, the cycloreversion initiated by light within this spectral region is not expected. However, in practice, such reactions are demonstrated using the example of quinolinone. Investigating the dimerization of the monomer in a photo-flow reactor irradiated with light of 345 nm, a dynamic equilibrium between cycloaddition and -reversion is identified, shifting towards the monomeric form with ongoing reaction time. This behavior is accompanied by the formation of a side product, which is made up of two monomers that are coupled via a single bond and presents a dead-end of the reaction, hence showing a negative influence on the reaction. Further experiments show that the absorption of light and a subsequent energy transfer from monomeric and dimeric species towards oxygen dissolved in the reaction medium leads to the formation of the reactive species singlet oxygen. This compound serves as a potent oxidizing agent of the dimer in the further course of the reaction and is thus able to initiate a self-accelerating oxidative cyclobutane cleavage. Carrying out the reactions either under inert conditions or the addition of singlet oxygen scavengers and quenchers leads to suppression of this undesired reaction pathway. The general validity of this mechanism is demonstrated by experiments on dimers of different substitutions under aerobic reaction conditions. It is demonstrated that the redox potential and the configuration of the dimer have a decisive influence on whether a reaction with singlet oxygen occurs. Due to the different degrees of stabilization of the intermediate radical cation, the derivatives in the hh-configuration are prone to oxidative cyclobutane cleavage whereas those in ht-configuration prove being inert. The introduction of electron-donating or withdrawing groups allows the manipulation of the dimer’s oxidation potential, which enables the prediction of the reaction’s behavior.