The research described in this proposal is a continuation and extension of experiments carried out under our 1985-1988 MBRS grant. It is a study of biologically important molecules in the solid state with respect to their electronic excited state properties using a combination of optical hole-burning and the Stark effect. When an absorbing molecule is dissolved in a matrix, its electronic spectrum is inhomogeneously broadened due to variations in the local environments. If a laser which has a bandwidth much less than the inhomogeneous bandwidth is used to excite these absorbers, it is possible to burn an optical hole in the spectrum. Holes result from photochemistry, transient storage or molecular reorientation. Since the hole widths are often measured in MHz, when coupled with the Stark effect, this becomes a very high resolution probe of molecular properties. The primary long-term objective of this program is to use these techniques to extract detailed excited state information (e.g. dipole moments, polarizabilities, geometries) from simple free base and metallo- porphyrins. Specifically, we will continue to study the free base isobacteriochlorin tautomers and begin to examine its zinc complex. We also expect to finish our comparative study of free base tetraazaporphin and porphin. A secondary and new direction will be the investigation bacteriorhodopsin and its chromophore retinal. This work will be based on the hypothesis that the optical hole burning of these moieties may provide a means of obtaining their high resolution spectra. The methodology involves growth of single mixed (porphyrin/n-alkane) crystals and making low temperature glassy solutions. The sample is placed between electrodes and immersed in liquid N2 or He and an absorption or emission spectrum obtained. Optical holes are burned and scanned with a narrow band laser; the Stark field is applied either DC or pulsed depending on the need. The subsequent electric field effects are then related to molecular excited state properties and dynamics. This program will involve three MBRS students. Each one will be responsible for a separate molecule and will carry out its preparation, purification and run its low and medium resolution spectra. The laser and Stark experiments will be carried out with the PI. Most of the spectroscopic data now available on biological molecules and systems is low resolution because of substantial inhomogeneous broadening of electronic bands. The Stark hole-burning technique has the potential of being a very sensitive probe of even very complex physiological samples.
