A series of compounds which utilise the macrocycle, trans-6, 13-dimethyl- 1,4,8,11-tetraazacyclotetradecane-6,13-diamine, L1 as a linker between a photoactive chromophore and an appropriate energy or electron donor have been synthesised and their properties explored by a combination of both steady state and time resolved photophysical measurements. Corresponding model compounds of the parent macrocycle with a single photoactive component were also prepared. In their free base forms, these ligands display pronounced photoactivity, undergoing intramolecular photoinduced electron transfer reactions which efficiently quench the emission. Upon complexation with Zn(II), the characteristic fluorescence of the chromophore is revived to a varying extent, dependant on the oxidising potential of the excited state.
For the purpose of studying intramolecular electronic energy migration, the symmetrically disubstituted compounds of L1 with the naphthalene and anthracene chromophores were prepared. In their free base forms, the photophysical behaviour of these ligands was analogous to the model compounds. As their Zn(II) complexes, emission from the chromophore is revived, although the electronic coupling between chromophores was ineffective in promoting energy migration.
The asymmetrically disubstituted compounds of L1 with the naphthalene, anthracene and pyrene chromophores were prepared in order to study intramolecular electronic energy transfer (EET). In complex with Zn(II), steady state measurements have established the electronic coupling between chromophores in this case was sufficient to promote EET. Time resolved measurements, together with molecular mechanics calculations, have allowed an identification of this coupling as primarily Coulombic, and the kinetics of the energy transfer process are well approximated by the Forster mechanism for dipole-dipole interactions.
For the purpose of studying intramolecular photoinduced electron transfer (PET), ligands incorporating the naphthalene or anthracene chromophore in combination with the organic or organometallic electron donors, dimethylaniline and ferrocene, were prepared. For Zn(II) complexes of the former ligands, only limited evidence of PET activity was observed, manifest as a moderate reduction in their excited state lifetimes compared to model complexes. This lack of PET activity was attributed to inefficient electronic coupling induced by the remoteness of the donor acceptor pair, and the absence of an operant through-bond pathway. For the latter complexes, with ferrocene as the electron donor, efficient excited state deactivation was observed, but the mechanism in this case was attributed to energy transfer.