Monitoring the formation and decay of transient photosensitized intermediates using pump-probe UV resonance Raman spectroscopy. I: Self-modeling curve resolution.
Resolution of transient excited-state Raman scattering from ground-state and solvent bands is a challenging spectroscopic measurement since excited-state spectral features are often of low intensity, overlapping the dominant ground-state and solvent bands. The Raman spectra of these intermediates can be resolved, however, by acquiring time-resolved data and using multidimensional data analysis methods. In the absence of a physical model describing the kinetic behavior of a reaction, resolution of the pure-component spectra from these data can be accomplished using self-modeling curve resolution, a factor analysis technique that relies on the correlation in the data along a changing composition dimension to resolve the component spectra. A two-laser UV pump-probe resonance-enhanced Raman instrument was utilized to monitor the kinetics of amine quenching of excited-triplet states of benzophenone. The formation and decay of transient intermediates were monitored over time, from 15 ns to 100 micros. Factor analysis of the time-resolved spectral data identified three significant components in the data. The time-resolved intensities at each Raman wavenumber shift were projected onto the three significant eigenvectors, and least-squares criteria were developed to find the common plane in the space of the eigenvectors that includes the observed data. Within that plane, the three pure-component spectra were resolved using geometric criteria of convex hull analysis. The resolved spectra were found to arise from benzophenone excited-triplet states, diphenylketyl radicals, and the solvent and ground-state benzophenone.
Kleimeyer
James A
JA
Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850, USA.
Harris
Joel M
JM
D000465:Algorithms; D000588:Amines / Q000737:chemistry / Q000528:radiation effects; D001577:Benzophenones / Q000737:chemistry* / Q000528:radiation effects; D008956:Models, Chemical; D008968:Molecular Conformation; D010777:Photochemistry / Q000379:methods*; D017319:Photosensitizing Agents / Q000737:chemistry* / Q000528:radiation effects; D013056:Spectrophotometry, Ultraviolet / Q000379:methods*; D013059:Spectrum Analysis, Raman / Q000379:methods*
D000588:Amines; D001577:Benzophenones; D017319:Photosensitizing Agents; C047723:benzophenone