Dr Andrew Benniston (Senior Lecturer)
As co-founder of the Molecular Photonics Laboratory (MPL) at Newcastle in 2001, Dr Benniston's role is primarily concerned with the design and synthesis of multi-component supermolecules for fundamental studies in energy and/or electron transfer. A more recent aspect is applying the developed systems for photonic applications such as real-time monitoring of physical processes in nano-environments, solid-state luminescence pressure-sensitive materials, photochromic materials for data recording, and fluorescence sensors for biological imaging. The complexity of the molecular systems produced depends greatly on the problem to be addressed, but high purity and full structural characterisation are paramount. The donor-acceptor dyad, Mes-Acr+, for example, though structurally simple had to be prepared ultra pure for in depth measurement of its fluorescence properties and triplet formation.
In comparison, in order to work out the full 'angle effect' on electronic communication along a molecular axis required careful design and synthesis of a range of molecules. In one case, several molecular donor-acceptor assemblies incorporating a biphenylene-based bridge were produced. The dependence of the dihedral angle on both hole and energy transfer through the bridge was determined. This theme was extended to examine electron delocalisation in strapped viologens, using spectroelectrochemistry, EPR and optical spectroscopy. Additional studies have focused on donor-acceptor pairs separated by short bridges intended to promote strong electronic coupling between the terminals. Here, the topology of the bridge is of paramount importance in controlling the dynamics of light-induced electron transfer and it has been shown that seemingly rigid connectors are in fact quite flexible. The consequence of this internal flexibility is that high-energy conformers can promote rapid electron transfer. As such, the activation energy for electron transfer comprises numerous terms.
Promoting very long range energy transfer has been a major challenge, and dinuclear Ru/Os poly(pyridine) complexes bridged by multiple ethynylene-naphthalene (EN) units have been specifically synthesized to study such a phenomenon. The EN units are especially effective at sustaining electron exchange over distances approaching 80Å. This can be in part attributed to the basic properties of the bridge units at promoting triplet-triplet energy transfer.
Other projects undertaken by the Benniston group have seen the development of multi-component spiropyran-based systems as photochromic switches that are opened via an exciplex intermediate. This has one advantage that direct illumination of the spiropyran is not necessary, and second-generation systems are in the pipeline based on selenospiropyrans. Further multi-component systems based on hindered di-pyrene moieties have been designed, for molecular imaging via delayed fluorescence created by triplet-triplet annihilation. Here, the aim is to circumvent the problem of autofluorescence and use time-gated techniques for imaging biological samples. On-going research is directed at building opto-electronic devices for imaging science and sensor technology.
A recurring theme running throughout much of Benniston's research is the development of donor-spacer-acceptor assemblies containing several chromophores. This strategy enriches the photophysical behaviour by providing for several competing pathways and allows for fine-tuning of the various energy levels. A variety of dyads formed from porphyrin - metal bis-terpyridine complexes (M = Ru(II) or Os(II)) has been synthesized and studied. More recently, attention has turned to organic-based materials such as the boron dipyrromethene - oligothiophene dyad shown opposite. This particular dyad displays singlet-singlet, singlet-triplet and triplet-triplet energy transfer, involving both Förster and Dexter mechanisms, and also undergoes light-induced electron transfer according to which terminal is excited. Remarkably, fast electron exchange takes place within the crystal lattice.