HYSCORE applications

Almost naturally, CW EPR spectroscopy can also contribute to rmderstanding the electronic properties of polymers that are envisioned for application in the fields of polymer electronics (e.g., [98,99]) and photovoltaics (e.g., [100-102]). Unlike in the smdies highlighted before, in these cases aU materials are EPR active and paramagnetic probes do not have to be added. Going beyond the conventional use of simple CW EPR spectroscopy to study electronic defects, Van Doorslaer, Goovaerts, Groenen, and coworkers have used multifrequency (X-, Q-, and W-band) EPR techniques such as HYSCORE and pulse ENDOR (see Sect. 2.1) to elucidate the extension of polarons in films of electro-active polymers [103]. 

Abstract Multi-resonance involves ENDOR, TRIPLE and ELDOR in continuous-wave (CW) and pulsed modes. ENDOR is mainly used to increase the spectral resolution of weak hyperfine couplings (hfc). TRIPLE provides a method to determine the signs of the hfc. The ELDOR method uses two microwave (MW) frequencies to obtain distances between specific spin-labeled sites in pulsed experiments, PELDOR or DEER. The electron-spin-echo (ESE) technique involves radiation with two or more MW pulses. The electron-spin-echo-envelope-modulation (ESEEM) method is particularly used to resolve weak anisotropic hfc in disordered solids. HYSCORE (Hyperfine Sublevel Correlation Spectroscopy) is the most common two-dimensional ESEEM method to measure weak hfc after Fourier transformation of the echo decay signal. The ESEEM and HYSCORE methods are not applicable to liquid samples, in which case the FID (free induction decay) method finds some use. Pulsed ESR is also used to measure magnetic relaxation in a more direct way than with CW ESR. 

HYSCORE has a higher sensitivity in the region of low nuclear frequencies than that of ENDOR spectroscopy. Single crystal measurements have therefore been applied to studies of nitrogen-containing paramagnetic species (nuclear Zeeman frequency v( " N) = 1 MHz at X-band) of interest in biochemical and fundamental applications. The local structure around the species may be obtained as discussed below for the radical H3CCHCOO in irradiated /-alanine. 

HYSCORE spectra take longer to record than 2- and 3-pulse ESEEM. As mentioned above, the technique is therefore more commonly applied to orientationally disordered systems, e.g. frozen solutions in chemical and biochemical applications, heterogeneous samples in applications to catalysis and environmental sciences. 

In addition to CW ESR, ENDOR, and pulsed techniques (HYSCORE spectra, 2D-ESTN, and ESEEM) were also used for dendrimer analysis. The ESR techniques have provided useful information on the dendrimers structure, properties, and applications, and their interactions with selected species. 

The stated aim of this review is to demonstrate that elassical analyses of physieal organie ehemistry are feasible with respect to complex systems such as supported metal catalysts through the application of advanced EMR spectroscopic techniques and determining the relevant spin Hamiltonian parameters via the Zeeman-dependent hyperfine spectrum. The principles of analysis were outlined in the preceding section and entail replicate collection of ESEEM or ENDOR spectra by incremental steps and mapping the trajectory of peak positions. Deconvolution of peaks may be made either by traditional tau-suppression in the stimulated echo pulse sequence or via advanced pulse sequences such as HYSCORE (2-D ESEEM, Hofer, 1994). Mapping of spectral peak position as it varies depending on the Zeeman field is very important to the accurate determination of hyperfine terms. 

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