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1. Photochemistry, Photophysics and Spectroscopy of Molecules with Fluorescent Highly Excited States

2. Effects of Molecular Solvation on Excited State Proton Transfer Reactions

3. Electronic Energy Pooling by Excited State Annihilation

4. Applied Photochemistry and Photophysics

 

 

 

1. Photochemistry, Photophysics and Spectroscopy of Molecules with Fluorescent Highly Excited States

The Steer group has been responsible for many of the advances in our understanding of the physical photochemistry, photophysics, microsolvation and spectroscopy of molecules which fluoresce strongly from highly excited valence states. Our past research has focussed on two classes of such compounds, thiones and non-alternant aromatic hydrocarbons, and is now being extended to pophyrins and metalloporphyrins. We were the first to discover the anomalous fluorescence of the thiocarbonyls, and we have extensively characterized them. Our work has been widely cited (especially Chem. Rev. 93, 67 (1993)) and has been responsible for alerting researchers worldwide (USA, Japan, Germany, France, UK) to the utility of the thiocarbonyls and non-alternant aromatics as models for investigating and utilizing photon-initiated processes in highly excited states. The importance of this research has most recently been noted by R.D. Levine (J. Phys. Chem. B, 105, 5589 (2001)) who, in describing the use of molecules with fluorescent S2 excited states in the design of molecular logic gates, states "We draw attention to a relative paucity of photophysical information on...S2 levels of potential donors and acceptors. ...it would be useful to have more." 

2. Effects of Molecular Solvation on Excited State Proton Transfer Reactions

Through the use of supersonic expansion and fast pulsed laser techniques, the Steer group has made significant progress in understanding the effects of solvation, at the molecular level, on inter- and intramolecular proton transfer reactions. We have sorted out existing discrepancies in the photophysics of the important model compound, tropolone, in condensed media and have provided the first evidence of a dynamic state-reordering process responsible for a >100-fold increase in its rate of radiationless decay with increasing temperature spanning the solvent's glass transition temperature.

3. Electronic Energy Pooling by Excited State Annihilation

In 1980 in collaboration with researchers at the NRC, we were the first to demonstrate the process of electronic energy pooling via molecular excited singlet-excited singlet annihilation in fluid media. In accord with our predictions, we have now found its triplet counterpart (triplet-triplet annihilation giving a highly excited singlet product) in an aromatic thione in solution at room temperature. These observations provide proof-of-concept for multiple photon energy capture and (temporary) storage in a highly excited molecule, and constitute both an opportunity for and a hitherto unrecognized efficiency limit on methods of solar energy conversion and storage, including those based on porhyrins and metalloporphyrins.

4. Applied photochemistry and photophysics

The laser chemistry group, in collaborations with colleagues, continues to seek practical applications of the results of its fundamental photophysical and spectroscopic investigations. To this end we have systematically investigated the spectroscopy and dynamics of pyrene-labeled synthetic polypeptides. Pyrene is frequently used as a fluorescent probe in biophysical studies of the binding of small peptides, but little attention has been paid to the effect of the tether used for attaching the pyrene label to the binding molecule. We have shown that the nature of the tether determines the excited state lifetimes and relaxation dynamics, and have discovered a useful empirical correlation between readily measurable absorption parameters of these pyrene-tethered compounds and their excited state lifetimes. 

We have also undertaken a study of the photochemisty and photophysics of RAFT (Reversible Addition-Fragmentation chain Transfer) polymerization agents used in the synthesis of light-harvesting (and other) polymers. Some of the most frequently used RAFT agents are molecules containing the dithioester or related functional groups; a majority of the resulting macromolecules therefore have a thiocarbonyl-containing residue at a chain terminus that influences the polymer's properties.  The RAFT residue quenches electronically excited light-harvesting chromophores by resonant electronic energy transfer.