| Carotenoids play important roles in photosynthesis. They act as light-harvesting components by transferring their excitation energy to nearby chlorophylls. Also, carotenoids function as photoprotective agents by quenching the triplet states of chlorophylls, thereby preventing the sensitization of singlet oxygen. In this dissertation, artificial supermolecular systems that mimic these energy transfer processes are described. They consist of macrocyclic compounds—benzochlorin, purpurin, texaphyrin, and a Si-phthalocyanine—covalently linked to carotenoids. In all cases, except for the Si-phthalocyanine, the carotenoids are attached to the periphery of the macrocycles. In the Si-phthalocyanine, two carotenoids are attached axially to the central Si atom, one extending above and one below the plane of the tetrapyrrole. It was observed that singlet-singlet energy transfer from the carotenoids to the macrocycles occurred with efficiencies that varied from 28% for carotenopurpurin to 96% for dicaroteno-Si-phthalocyanine. Triplet-triplet energy transfer, from the macrocycles to the carotenoids was also observed. For example, the rate constant for triplet-triplet energy transfer in the carotenotexaphyrin was 6.67 × 107 s−1 . Both the singlet and triplet energy transfer parameters correlated with structural features of the systems such as the site and type of linkage used to attach the carotenoids. These correlations indicate that the chromophores need to be in van der Waals contact or have π-orbital interactions in order to show high energy transfer efficiencies and rates.; Carotenoids are prototypes of molecular conductors because of their relatively small HOMO-LUMO gaps and their delocalized π-electron systems. This dissertation also describes the studies of molecular conductivity in self-assembled monolayers. A method to form these monolayers using thiolated molecules (alkanethiols, alkanedithiols, carotenethiol and carotenedithiol) over gold substrates is described in detail. A conducting atomic force microscope was used to make electrical contacts to these monolayers in order to characterize their structure and obtain current-voltage characteristics. This method allows a reproducible and unambiguous way to determine single-molecule conductivity. |
