Excitation delocalization in porphyrin nanorings

Thesis

This thesis is concerned with the photophysical properties of porphyrin nanorings. In particular, the impact of symmetry, conformation and structure on electronic delocalization, radiative rate and emission polarization memory loss are investigated and exciton migration is probed on a number of porphyrin nanorings. Ultrafast time-resolved spectroscopy techniques are used to study the fluorescence dynamics of these newly synthesized compounds.

First, photophysical implications of lifting the rotational symmetry are investigated using a number of small porphyrin rings specially modified for this purpose. For more severe symmetry distortions, increased optical transitions to the lowest excited state have been observed, which is dipole-forbidden due to symmetry reason. Also, the degeneracy in the allowed first excited state is lifted for broken symmetry, leading to a polarization switching effect.

The effect of conformation on emission depolarization memory loss is studied using a series of similarly sized porphyrin rings with rigid structures exhibiting various degrees of out-of-plane distortion. Excitations can access any segment on the nanoring and this is not affected by the conformation. However, severe out-of-plane distortions lead to lowered emission anisotropy; this conclusion is further supported by molecular dynamics simulation.

Finally, exciton migration within porphyrin ring complex is investigated. Russian doll complex which consists of two concentric porphyrin rings acts as a single emitter, even though spectral features of both ring components are found in the absorption spectrum. Lifetimes and radiative rates of the complex and the individual rings components further confirmed that excitons migrate from the outer 12-ring to the inner 6-ring within 40 ps. A nanotube consisting of two conjugated 6 porphyrin rings linked by dimers acting as staves also exhibits energy migration. Emission anisotropy measurements suggest that excitation is transferred from the staves of the nanotube to the ring plane effectively within 280 fs.

Authors: Gong, J

• DOI: 10.5287/ora-jbkz9kdpd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford