TIME-RESOLVED PHOTOACOUSTIC-SPECTROSCOPY - NEW DEVELOPMENTS OF AN OLD IDEA

Acoustic waves generated by heat emission in radiationless transitions from photoexcited molecules can be detected by suitable transducers. Their study allows the investigation of thermal relaxations, thus providing thermodynamic and kinetic data on short-lived species produced by the absorption of pulses of light. In this field of research the best technique has proved to be the so-called pulsed-laser, time-resolved photoacoustic spectroscopy, which is based on piezoelectric detection of pressure waves in the time domain. Deconvolution processing of the transient signals gives both the lifetimes of excited states and the energy content of the transitions, provided that decay times are in the range 5 ns-5 mu s Moreover, when compared with proper theoretical models emphasizing the energy balance, the photoacoustic results can help to build a complete picture of the deactivation pathways, including photochemical events. The biophysical applications, although numerous and widespread both in basic and applied research, offer the real possibility of giving information on photobiological processes in conditions very close to the living state. Among the more significant contributions obtained in this area, the results on photosynthesis and photosensitivity of plants and photosynthetic micro-organisms, structural and functional dynamics of respiratory proteins, photocycles of rhodopsin and bacteriorhodopsin and photophysical properties of several natural pigments are particularly relevant, together with some medical and biotechnological applications. Another promising field of application of photoacoustics concerns photoactive drugs and,the photophysics of fluorescent probes for conformational studies of proteins, nucleic acids and membranes. In general terms, time-resolved photoacoustic spectroscopy promises to become one of the most powerful techniques in photobiophysics, provided that some limitations in data analysis and time resolution are removed by technical improvements.