Energy transfer in multichromophoric, segmented PPV copolymers and oligomers.

摘要

This work has involved the syntheses, characterization and electronic spectroscopic study of PPV-related segmented copolymers and oligomers with pendant hydroxy groups ("sticky" systems) and pyrene chromophores (multichromophoric systems). The multi-step synthesis used a Heck coupling methodology as the olefination procedure to ensure an all-trans conformation phenylenevinylene (PV) product. The all-trans conformation on the PV core chromophore was confirmed by FTIR and 1H NMR.; Steady-state spectroscopy for the "sticky" systems showed a significant improvement on the photoluminescence response in comparison with structurally similar PV systems. X-ray analysis showed that the structural architecture of the "sticky" PV systems exhibits a stabilizing interaction between a = CH group in the conjugated PV unit and the pendant hydroxyethoxy groups. This interaction helps to planarize the PV unit, and may be responsible for the improved photophysical behavior. Similar conformational considerations may influence the structurally similar multichromophoric systems.; Steady-state electronic spectroscopy for the multichromophoric systems showed a very efficient energy transfer (ET) process from the pyrene pendants to the oligo-PV core chromophore. The influence of the nature and the length of the tether on the ET efficiency and mechanism was studied by synthesizing multichromophoric systems with 5, 8 and 11 tethering atoms with either ester or ether link functionalities. The results showed that multichromophoric systems with 8 tethering atoms and ester functionalities gave the highest ET efficiency.; Time-resolved spectroscopy on the multichromophoric systems with 8 tethering atoms suggested a pure Forster ET mechanism with close interchromophoric distances (≅3.1 A). A similar distance was also found at the energetically preferred conformation by computational modeling. This short distance requires a folding tether conformation, allowing close approach of the pendant pyrenes to the core phenylenevinylene chromophores. This could allow as interchromophoric, electronic orbital interaction as described by the Dexter ET model. The folding conformation was investigated by 2D NOESY NMR and by steady-state spectroscopy adding pi-stacking impurities. However, the results showed no conclusive experimental data to support a significant equilibrium amount of folded conformation.