Multidimensional analysis techniques for monitoring excited-state photophysics and photochemistry

New approaches for multidimensional analysis of excited state photophysics and photochemistry in condensed media have been developed. Excited states are important in diverse areas such as free-radical polymerization reactions and as probes of inhomogeneous media. Traditional one-dimensional kinetic or spectroscopic studies, however, lack the informing power to resolve highly overlapped kinetic signatures or component spectra from complex systems. Combined kinetic and spectral measurements make use of both correlated behavior along the kinetic axis and band shape variations along the spectral axis to guide component resolution along both dimensions. A multidimensional least-squares analysis of transient Raman data acquired as a function of 355 nm laser intensity was used to resolve the spectra of triplet-state benzophenone from the spectra of its ground-state and the solvent. Nonlinear least-squares regression analysis with a model describing the kinetics of triplet state dissociation allow the excited state spectrum and dissociation quantum yield to be resolved at wavelengths deeper in the UV. In the absence of a physical model, factor analysis techniques allow the kinetic signatures and spectra to be resolved. Nonlinear least-squares analysis of transient resonance Raman spectra of a triplet-state photosensitizer (benzophenone) acquired as a function of quencher concentration and laser intensity allows the quenching rate constants and Raman spectra of the sensitizer excited state and intermediate photoproducts to be resolved from the data. Long-lived excited triplet states of acridine yellow dissolved in a rigid saccharide glass are proposed as an extremely sensitive sensor material for optical thermometry. Activated reverse-intersystem crossing from the triplet to the singlet excited state provides a temperature-dependent decay pathway that competes with phosphorescence to depopulate the triplet state. The origins of nonexponential decay kinetics observed in the rigid media are investigated by time and wavelength resolved phosphorescence measurements. This study is the first example of multidimensional least-squares analysis in which the kinetics driving the evolution of component concentrations in time have been related to the wavelength dependence and shape of the resolved spectra.