Experimental and Theoretical Studies of Cis-Trans Photoisomerizations and Conditions for Effective Catalysis

Doctoral thesis, 2001

Triplet-sensitized cis-trans photoisomerizations have been investigated both experimentally and theoretically. Mapping of the reaction path for isomerization of 1,4-bis(1-propenyl)benzene (1) on the triplet state energy surface has been done by quantum yield measurements and laser flash photolysis. The triplet lifetime and triplet energies of the three isomers cis,cis (ZZ-1), cis,trans (ZE-1), and trans,trans (EE-1) were measured. The reaction pathway is mainly diabatic, since the decay from the triplet excited state surface predominantly occurs from the twisted intermediate ( 3E,p*) between ZE-1 and EE-1. In a parallel theoretical investigation we studied the computationally inexpensive density functional theory (DFT) method for calculations of triplet state potential energy surfaces (T1-PES) of polyenes. Different DFT methods were first used for the calculations of the cis-trans isomerizations of 1,3-butadiene and 1,3,5-hexatriene. Both pure gradient-corrected DFT and hybrid DFT methods show good agreement when compared to experimental results and to ab initio methods such as CASSCF, CASPT2, and MP4. Then we used DFT calculations to study whether cis-trans isomerizations followed an adiabatic or a diabatic pathway. Triplet state energies, PESs, geometries, and spin densities of seven olefins were investigated. The DFT calculations of the PESs show good agreement with the experimental findings. One important factor that dictates the character of the process undergoing isomerization is the relative stability of the planar structure. The adiabatic character of the isomerization process increases with delocalization of the spin density away from the carbon-carbon double bond in the planar olefin structure. Photocatalysis of triplet state cis-trans isomerization following a quantum chain process has been investigated. The efficiency and the limitations are studied both experimentally and theoretically. The quantum yield increases and reaches a maximum with the concentration of the catalyst, and then decreases due to self-quenching of the catalyst in its triplet state at high concentrations. Important factors when optimizing the catalytic reaction are the triplet lifetime and triplet energy of the catalyst.