Synthesis and investigations of novel alkenylporphyrins and bis(porphyrins)

Abstract

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.