Tether-assisted [2+2] photocycloaddition of dicyclopentadiene derivatives and carbon cored tripodal sensitizers for semiconductor nanoparticles

This thesis consists of three chapters related to two projects. Chapter I describes the Cu(I)-catalyzed intramolecular [2+2] photocycloaddition of diesters made of endo-dicyclopentadiene derivatives linked by the ester bonds with alkyl or adamantyl based tethers. These reactions are highly regio- and stereoselective and the tethers can be easily cleaved afterward upon reduction with LiAlH₄. Photolysis of the diesters afforded a 1:1 mixture of the heretofore unknown exo-cis-exo dimer, originating from the (R,S/S,R) diastereomer of the diester and the exo-trans-exo, deriving from the (R,R/S,S) diastereomer. The role of the tethers length and structure on the course of the photocycloadditions was investigated, and it was observed that adamantyl-based tethers provided the shortest reaction times and highest yields. X-ray diffraction analysis and NMR techniques were applied to characterize the syn and anti photocycloaddition products.; Chapter II describes the synthesis of two carbon cored tripod sensitizers: Ru(bpy)₂(C-tripod-phen)2+, Ru(bpy)₂(C-tripod-bpy) 2+ and the study of the electron transfer processes occurred at the tripod-sensitizer/semiconductor interfaces. IR spectroscopy showed that single carboxylic acid binding mode could be obtained by pretreating the TiO ₂ films with an acidic solution. Tripod bound to the TiO ₂ surface displayed a weak electronic coupling between the ruthenium sensitizer and the semiconductor. Nanosecond transient absorption spectroscopy showed that electron recombination could be fit to a sum of two second order processes. The rate of charge separation for tripod was not detected by femtosecond transient absorption spectroscopy indicating an injection faster than 110 fs (the length of the laser pulse) was occurred. The dipole moment changes from ground to excited state of tripods and other ruthenium model complexes were measured by Stark spectroscopy and it was found that the transferred electron at the excited state does not extend out further than the bipyridine or phenanthroline ring.