Catalysis of Quantum Chain Photoisomerizations: Calculations of Triplet Energy Surfaces and Measurements of Quantum Yields

Doctoral thesis, 2001

Triplet state cis-trans photoisomerizations of olefins is the theme of this thesis, and the goal is to make the isomerizations as efficient as possible. The shape of the triplet potential energy surface (T1 PES) is essential for this, and the possibility of calculating these T1 PESs for two smaller olefins, using the computationally inexpensive density functional theory (DFT) methods, was investigated. Both pure gradient corrected and hybrid DFT methods were found to give a good agreement with experimental values and results from high-quality ab initio methods. Further investigations showed that DFT methods could also be applied to larger olefins with equally good results. Furthermore, these calculations of triplet energies, T1 PESs, geometries and spin densities of olefins with different substituents give valuable information about the electronic aspects that are important for isomerization. The calculations also showed that there is a relationship between the aromaticity/anti-aromaticity of the olefinic substituent and the shape of the T1 PES. The experimental work was focused on catalysis of cis-trans isomerizations. First, the cis-trans isomerization of 8-(3,5-di-tert-butylstyryl)fluoranthene was catalyzed using camphorquinone as a sensitizer and acridine as the catalyst. The quantum yields were found to increase as the concentration of catalyst rose, until a maximum was reached, after which the isomerization rate declined. When acridine is used, a more efficient quantum chain process is achieved: the quantum efficiency increases 5.5 times. The isomerizations of stilbene derivatives in the presence of zinc porphyrins were also investigated. Catalytic behavior similar to that noted above was found, but in this example a complex is involved; reverse triplet energy transfer within the complex, as well as triplet energy transfer between complexes, can account for this type of catalysis. A quantum yield of about 85 can be reached in this case.