The research described in this proposal is a continuation and extension of work being carried out under our current MBRS grant. It is a study of porphyrins and bacteriorhodopsin in the solid state with respect to their electronic ground and excited state properties using single site excitation, optical hole-burning and the Stark effect. When a molecule is dissolved in a matrix its electronic spectrum is inhomogeneously broadened due to variations between the local environments. If a narrow bandwidth laser is used to excite these absorbers, it is sometimes possible to burn an optical hole in the spectrum. Holes results 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. vibrational energies, dipole moments, pipi* and npi* origin energies and coupling, etc.) from biomedically important chromophores. Specifically, in the porphyrins we will study free base and metal complexes of isobacteriochlorin, chlorin, tetraazaporphin and related molecules. (The study of tetraazaporphin has recently taken on added significance since this class of compounds is actively being examined as potential sensitizers in photodynamic therapy.) A secondary area of research is bacteriorhodopsin and its chromophore, retinal. Our overall methodology involves the growth of single mixed crystals (porphyrin/n- alkane) or 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 can be applied either DC or pulsed depending on the need. This research will involve three MBRS students. Each one will be responsible for a separate chromophore and will carry out its preparation, purification and run low and medium resolution spectra. Laser and Stark experiments will be done with the PI. Most of spectroscopic data now available on biomedically important chromophores is low resolution because of substantial inhomogeneous broadening of the electronic bands. Optical hole-burning, single site excitation and the Stark effect have the potential of being very sensitive probes of even very complex samples.
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