Conjugated organic molecules as models for potential sensors
This dissertation focuses on water-soluble phenylenevinylene (PV) and sensor-capable PV systems. All oligo PVs and poly(phenylenevinylenes) (PPVs) described below were synthesized using standard Heck coupling methodology.
New water-soluble PV systems have been made by two different strategies. As ionic side chain substituted water-soluble PVs, two oligo-PV systems with pyridinium groups were made and shown to be more water-soluble than analogous systems with triethylammonium groups, even though two out of three systems of the triethylammonium-based systems were polymeric and so had numerous ionic side chains. The triethylammonium-bearing systems showed blue fluorescence in solution but emission of the pyridinium-based systems was not visible due to photoinduced electron transfer quenching. Another type of water-soluble, segmented copolymer based on PPV was made that incorporated a nonionic but hydrophilic poly(ethylene glycol) (PEG). This PEGylated PPV was readily soluble in water, and exhibited strong blue fluorescence.
Cleavable amide bonds were tested as a basis for producing model systems for making sensors. A functionalizable diamino side-chain 2.5-oligo PV was used as a basic core synthetic unit. This system is described as “sticky molecule” since the amino groups can be easily functionalized. Two types of electron transfer quenchers, phthalimide and pyridinium, were attached to the sticky molecule by amide linkages. The luminescence of both systems was drastically quenched, but was shown to increase greatly when the amide bonds were cleaved by bases. These systems can be considered as "turn-on" sensor systems.
Two pyrene units were also linked to the sticky molecule. This multichromophoric system displayed energy transfer from the pyrene to the PV core, such that the emission spectrum only shows the PV core peak, even when excited at wavelengths where the pyrene is the absorber. The energy transfer efficiency was calculated to be ∼60%. After the amide bonds were cleaved by acids, the pyrene emission was observed upon excitation at pyrene absorption wavelengths, which showed disruption of the energy transfer by allowing the pyrene the "escape" the core PV group. This can be considered either a "turn-off" process for the core PV, or a "turn-on" process for the pendant pyrene group.