Twelve porphyrin dyads linked by an ethene bridge were synthesised as model systems for conjugated polymers. The extent of interporphyrin interaction was investigated for meso-meso and meso-β linked homo- and heterobimetallo-porphyrin dyads. To complement these dyads, model monomers with alkenyl substituents were also studied. Once the synthesis of these compounds was achieved, the extent of interaction was studied using UV-visible and fluorescence spectroscopy and molecular modelling. In order to gain a true indication of the extent of interaction in a dyad, the effect of the bridge as a substituent must be accounted for. This was achieved by studying the series of monomers by UV-visible and fluorescence spectroscopy. The increased conjugation resulting from mono- and bis-alkenyl substituents results in a red shift of the origin of transition energies in the absorption spectrum which is accompanied by a broadened and less intense Soret band and an increase in the intensity of the Q bands. The emission of these compounds also displays an increase in Stokes shift and a loss of vibronic coupling due to the increased conjugation. The serendipitous synthesis of three asymmetric meso-β ethene-linked porphyrin dyads was achieved by the use of palladium-catalysed Heck coupling of mesoethenyl- with meso-bromoporphyrins. A possible mechanism for this meso to β rearrangement was proposed. A series of nine meso-meso ethene-linked dyads was synthesised by palladium-catalysed Suzuki coupling of meso-(2-iodoethenyl)- with meso-borolanylporphyrins. All of these dyads were characterised by 1D and 2D NMR as well as MS analysis. The absorption spectra of ethene-linked dyads exhibit a split Soret band and a red-shifted and intensified HOMO-LUMO band. In the meso-β dyads, the degree of splitting in the Soret band is sufficient only to generate a shoulder on the red edge, whereas in the meso-meso dyads two separate bands appear. The extent of splitting is believed to be an indication of the amount of porphyrin-porphyrin interaction. The fluorescence profiles of the dyads change dramatically depending upon the central substituents in the porphyrins and the wavelength used for irradiation, which suggests that different conformations of these compounds give rise to different parts of their absorption and emission profiles. The fluorescence profiles of the dyads also do not reflect their absorption profiles, and therefore the excitation of the dyad is believed to be accompanied also by a change in geometry. All ethene-linked dyads exhibited an anti-Stokes shift, and the excitation spectra of the different parts of the fluorescence envelope also support the possibility of different conformers contributing to the fluorescence spectra. Molecular mechanics and time-dependent quantum mechanical calculations were performed on seven ethene-linked porphyrin dyads. These calculations further support the proposal of different conformations contributing to the physical properties of ethene-linked dyads. Electronic structure calculations also show considerable electron density on the alkene for the meso-meso ethene-linked dyads, which highlights the important influence of this bridge upon the electronic nature of these conjugated diporphyrins.
| Identifer | oai:union.ndltd.org:ADTP/265401 |
| Date | January 2006 |
| Creators | Locos, Oliver Brett |
| Publisher | Queensland University of Technology |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | Copyright Oliver Brett Locos |
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