Professor John McGarvey

Research

Laser Photochemistry and Raman Spectroscopy

Processes driven by light play a key role in several recent developments in chemistry and analytical science. Of central importance to this role is a firmly based understanding of the photophysics, dynamics and spectroscopy of the species involved. One of our primary interests concerns the excited states of molecular and supramolecular systems containing one or more metal centres. Time-resolved spectroscopic methods (resonance Raman, absorption, luminescence) are used to interrogate the excited states of mono- and polynuclear systems. Recent examples of this work, carried out in collaboration with the JG Vos group at Dublin City University are the investigation of the binuclear compounds [(Ru(H₈-bpy)₂)₂((Metr)₂Pz)](PF₆)₂ (1) and [(Ru(D₈-bpy)₂)₂((Metr)₂Pz)](PF₆)₂ (2), where bpy is 2,2' -bipyridine and H₂(Metr)₂Pz is the planar ( 'ditopic' ) ligand 2,5-bis(5' -methyl-4' H-[1,2,4]triaz-3' -yl)-pyrazine.

 

 

 

Photo-driven devices

Ruthenium and other metal polypyridyl complexes are widely recognised for their potential in the development of useful photo-driven devices. One area of study, in conjunction with Dublin City University (Prof. J.G. Vos), concerns the influence of the chemical environment (e.g. pH) on photophysical behaviour and energy order in the excited state manifold. Of particular interest are bi- and polynuclear species with asymmetric bridging ligands and the consequent possibilities for vectorial energy or electron transfer within assemblies incorporating such units.

 

 

Metal Complex Photophysics

One more specific application of metal polypyridyl complexes concerns their application as probes for DNA. Vibrational spectra are of particular value in providing direct structural information on the reactive species involved in interactions such as intercalation and photo-induced cleavage. The nature of the resonance Raman technique, based upon the excitation and probing of vibronic transitions, makes it particularly effective for investigating vibrational structure in both the ground and electronically-excited states of metal complexes. The resonance Raman (RR) spectra reveal changes in the vibrational modes of even the ground electronic states of the complexes when in the presence of DNA. Changes in the vibrational spectra of the excited states are of special interest in relation to photocleavage of DNA, mediated by the lowest metal-ligand charge-transfer (MLCT) states. Among the types of information we seek is evidence that the intensity of RR bands may differ for the D- and L-enantiomers of chiral metal complexes, hopefully providing convenient marker bands for the distinctive interaction of the enantiomers with the chiral binding sites of the DNA.

 

 

Spin-State Crossover

Spin-state crossover in transition metal complexes is a long standing phenomenon but one which has acquired a new lease of life, with considerable potential application in molecular switching and display, through the thermo-, photo- or piezo-chromism of the species and the use of ligand design to control spin state lifetimes,. Using pulsed lasers to perturb the spin equilibria and to monitor the ensuing relaxation processes spectroscopically as a function of pressure and temperature, the Belfast group investigates thermal and optical switching of molecular spin states which have considerable potential for use in storage and display media. We have close collaboration with European groups in this area, including H. Toftlund, University of Southern Denmark, Odense and A. Bousseksou at CNRS Laboratory for Coordination Chemistry, Toulouse. Molecular modelling and computation provide a useful adjunct to the spectroscopic and kinetic studies..

 

 

Surface-Enhanced Raman Spectroscopy

Surfaces for enhancing Raman scattering and other optical effects have been the subject of extensive recent study, not only for their inherent interest but in relation to phenomena such as spontaneous molecular assembly at surfaces. When silver or gold colloids are mixed with a range of organic and inorganic stabilisers lustrous, mobile, metal-like films form at the aqueous:non-aqueous interface. When thiols are used as the stabilisers, their propensity to form self-assembled monolayers persists in the films. When deposited, the films display a high degree of order which is being investigated by SERS and scanning probe microscopy. The deposited films also act as convenient substrates for spectroscopic detection of low levels of other adsorbed species. The potential for analytical application is being explored. 

 

 

A new area of interest concerns biomedical applications of Raman microscopy

Raman spectroscopists in the School of Chemistry have combined with a number of groups from the Schools of Medicine ( Respiratory Medicine, Ophthalmology, Pathology) and Pharmacy to investigate the use of Raman microscopy for improving diagnosis of complex diseases. The group is focusing on a number of areas related to the diagnosis and behaviours of cancers, factors affecting sight in diabetes and the formulation of new drug delivery systems.