Time-resolved chemically induced dynamic electron polarization

Time-Resolved Chemically Induced Dynamic Electron Polarization and Optical Emission Studies... 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

Time-resolved laser flash ESR spectroscopy generates radicals with nonequilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963, are known as chemically induced dynamic electron polarization (CIDEP). Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively. ... 

Appropriate modifications of the ESR spectrometer and generation of free radicals by flash photolysis allow time-resolved (TR) ESR spectroscopy [71]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They may be enhanced as much as one hundredfold and may appear in absorption, emission, or in a combination of both modes. These spectra indicate the intermediacy of radicals with substantial deviations from equilibrium populations. Significantly, the splitting pattern characteristic for the spin density distribution of the intermediate remains unaffected thus, the CIDEP (chemically induced dynamic electron polarization) enhancement facilitates the detection of short-lived radicals at low concentrations. 

In order to study the viscosity effect on the quenching of triplet excited state of (53) by TEMPO, chemically induced dynamic electron polarization and transient absorption spectra have been measured in ethylene glycol, 1,2-propanol and their mixtures. The results indicate that the quenching rate constant is viscosity-dependent and decreases linearly with the increase in solvent viscosity. The spectroscopy and dynamics of near-threshold excited states of the isolated chloranil radical anion have been studied using photoelectron imaging taken at 480 nm, which clearly indicates resonance-enhanced photodetachment via a bound electronic excited state. Time-resolved photoelectron imaging reveals that the excited state rapidly decays on a timescale of 130 fs via internal conversion. ... 

The seminal work of Marcus and Hush has had a significant impact on the development of PET. Pioneering efforts by Sutin, Hopfield, Jortner, and others established the connection between thermal electron transfer and photoelectron transfer [6]. This work set the stage for a notable series of experiments where laser flash spectroscopy [7], chemically induced nuclear polarization [8], resonance Raman spectroscopy [9], time-resolved microwave conductivity [10], and time-resolved photoacoustic calorimetry [11], to site only a few examples, have been successfully employed to chart the dynamics of PET in homogeneous solution, the solid-state, and organized assemblies. 

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