Prof Anthony Harriman (Professor of Physical Chemistry)
Professor Harriman's interdisciplinary research programme addresses a wide range of fundamental problems in chemistry and biophysics. Light-induced energy- and/or electron-transfer chemistry is a unifying theme for much of this research.
Several sets of "strapped phenylene bridges" have been investigated as a means by which to examine the precise relationship between the extent of electronic coupling along the molecular axis and the dihedral angle of the connecting linkage. These molecular systems are built around a biphenyl molecule equipped with a dialkoxy tether attached at the 2,2'-positions. The length of the tether controls the central dihedral angle. Individual systems have been used to examine the angle effect for nonradiative decay, electron and spin delocalisation, and electron exchange. An unexpected result to emerge from this work is that the angle effect on the rate of through-bond electron exchange depends on the strength of electronic coupling between the terminal donor-acceptor pair. Thus, strongly coupled systems exhibit a weak sensitivity towards the central dihedral angle but weakly coupled systems show a significant angle effect. This finding is now being developed as a means by which to stabilise charge-transfer states against fast charge recombination.
Tremendous progress has been made in understanding how covalent bridges mediate long-range electron-transfer and electron-exchange reactions. A fundamental question remains, however, regarding how to construct molecular systems capable of effecting unusually (i.e., >100 Â) long-range transfer. Current wisdom, inspired by natural photosynthesis, demands the use of a cascade of redox co-factors that span a wide range of reduction potentials and has each relay carefully positioned along the array. Harriman's recent research has challenged this long-established view. Here, the dynamics of electron exchange have been probed for a series of Ru/Os bis(2,2':6',2"-terpyridine) binuclear complexes separated by ethynylene-naphthalene repeat units. The triplet energies of donor, acceptor and spacer are remarkably close and, somewhat surprisingly, adjacent ethynylene-naphthalene units are but weakly coupled. Fast photon injection occurs from donor to proximal spacer unit. This step is eversible. Photon migration occurs between adjacent spacer units until irreversible trapping by the acceptor closes the circuit. The overall rate of end-to-end electron exchange is relatively insensitive to changes in the molecular length, at least up to 60 Â. The net result is a highly effective molecular-scale wire for long-range electron exchange.
Over the past five years the MPL has been measuring how the dynamics of long-range energy and/or electron transfer depend on molecular topology. The conclusions drawn from countless investigations are being used to design new molecular-scale switches, gates and circuits. Current research is aimed at developing linear arrays in which the direction of electron transfer can be switched in a rational manner. Extending the circuit should result in the first molecular system able to direct an electron around a complete revolution in either direction.
Light harvesting is a major part of the natural photosynthetic apparatus and has been subjected to detailed examination by theoretical and experimental studies. The Harriman group has developed many artificial models, most recently being built around boron dipyrromethene (Bodipy) dyes. These latter materials include a range of dyes bearing multiple photon collectors covalently linked at the meso or boron sites. Electronic energy transfer from the substituent to the Bodipy core is extremely fast and is followed by efficient radiative decay. The Bodipy dye can also be equipped with a redox-active unit such that rapid intramolecular energy transfer is used to sensitize an electron-transfer reaction. This is a highly active research area, the systems having numerous potential applications. One example concerns the development of new fluorescent probes for monitoring local changes in the fluidity of the medium. A quite disparate application is related to the design of fluorescent crystals for use in organic light-emitting diodes.