PHOTOCHEMISTRY OF ORGANIC MOLECULES:
ELUCIDATION OF THE MECHANISMS OF SELECTED PHOTOCHEMICAL REACTIONS BY CHEMICAL AND SPECTROSCOPIC MEANS
We aim to determine the role, multiplicity, and geometry of key electronically excited and ground-state intermediates, applying both chemical and spectroscopic means. We are guided by the premise that a detailed understanding of simple photochemical reactions establishes the proper foundation for the study of more complex photochemical and photobiological systems. Our methods include quantitative measurements of product quantum yields, determination of rate constants by employing steady-state and transient emission and absorption spectroscopy, use of triplet excitation transfer to sensitize or quench triplet reaction pathways, and studies of the effects of medium and temperature on the photochemical and photophysical events. An important goal is the integration of all the information into a self-consistent kinetics model for each reaction. Much of this work depends on the use of computers.
A connecting thread that runs through much of our research is the elucidation of the various pathways that lead to the cis-trans photoisomerization of olefins. We continue to extend our well-recognized studies on stilbenes and 1,3-dienes to the diphenylpolyenes and the styrylarenes, molecules that combine properties of both systems. We have developed the method of principal component analysis with self-modeling (PCA-SM) to resolve the spectra of such systems into the separate contributions from distinct conformers. The ability to define conformer-specific photochemistry and photophysics has led to several unambiguous validations of Havinga’s (nonequilibrating excited rotamers) NEER principle. For instance, the pronounced excitation wavelength dependence of the cis-trans photoisomerization and photocyclization of cis-1-(2-naphthyl)-2-phenylethene ( c-NPE, Scheme 1) is analogous to that in previtamin D and is quantitatively explained in terms of selective conformer excitation. This is strong evidence that recent challenges to Havinga’s NEER principle explanation of vitamin D related photochemistry are incorrect.
We are applying the methods developed in our conformer work (PCA-SM and the related singular value decomposition - SM) to the resolution of spectral matrices describing the time evolution of the intermediates in the photocycles of bacteriorhodopsin (BR) and selected site-specific mutants of BR. Our approach has the advantage in that it yields extremely accurate intermediate spectra and their time evolution as an initial step and relegates selection of a kinetics model for their interconversion to the final stage of the analysis. Fruitful research collaborations are in progress with research groups worldwide.
Dr. Saltiel was a recipient of an NSF Postdoctoral Fellowship at the University of California at Berkeley, an R. A. Welch Foundation Lectureship, and the 1998 Florida Award from the Florida sections of the American Chemical Society; he is also an Alfred P. Sloan Foundation Fellow. Dr. Saltiel was honored for his work in photochemistry in a symposium at the 1998 National ACS meeting in Boston and in a special issue of the Journal of Physical Chemistry [102, 5320–5321 (1998)].
