TREPR

The following diseussion of the time dependenee of the EPR response in a TREPR experiment is based on the assumption that the transient paramagnetie speeies is long lived with respeet to the spin relaxation parameters. 

M continually decreases under the influence of spin-spin relaxation which destroys the initial phase coherence of the spin motion within they z-plane. In solid-state TREPR, where large inliomogeneous EPR linewidths due to anisotropic magnetic interactions persist, the long-time behaviour of the spectrometer output, S(t), is given by... 

As a sununary it may be of interest to point out why TREPR spectroscopy and related methods remain important in the EPR regime, even though pulsed EPR methods are becoming more and more widespread. (1) For the case of an inliomogeneously broadened EPR line the time resolution of TREPR compares favourably with pulsed teclmiques. 

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

Figure Bl.16.2. X-band TREPR spectra obtained at 0.1 ps after 308 mn photolysis of a fliiorinated peroxide dimer in Freon 113 at room temperature. Part A is the A/E RPM spectrum obtained upon direct photolysis part B is the E/A RPM spectrum obtained upon triplet sensitization of this reaction using benzophenone.

Figure Bl.16.15. TREPR spectmm after laser flash photolysis of 0.005 M DMPA (5) in toluene, (a) 0.7 ps, 203 K, RE power 10 mW O, lines CH (8), spacing 22.8 G , benzoyl (6), remaining lines due to (7). (b) 2.54 ps, 298 K, RE power 2 mW to avoid nutations, lines of 7 only. Reprinted from [42].

Contradictory evidence regarding the reaction to fonn 8 and 9 from 7 led the researchers to use TREPR to investigate the photochemistry of DMPA. Figure B1.16.15A shows the TREPR spectrum ofthis system at 0.7 ps after the laser flash. Radicals 6, 7 and 8 are all present. At 2.54 ps, only 7 can be seen, as shown in figure B1.16.15B. All radicals in this system exliibit an emissive triplet mechanism. After completing a laser flash intensity sPidy, the researchers concluded that production of 8 from 7 occurs upon absorption of a second photon and not tiiemially as some had previously believed. 

Figure Bl.16.16. TREPR spectrum of TEMPO radicals in 1,2-epoxypropane solution with benzophenone, 1 ps after 308 inn laser flash. Reprinted from [45],...

Figure BL16.18 shows TREPR speetra of this system in SDS mieelles at various delay times. The A/E/A/E pattern observed at early delay times is indieative of a singlet-bom SCRP. Over time, a net absorptive...

Figure Bl.16.18. TREPR spectra observed after laser excitation of tetraphenylhydrazine in an SDS micelle at room temperature. Reprinted from [61].

SEEPR detection of the transient radical during electrochemical preparation in solution TREPR measurements... 

Many new physical methods were developed in response to needs of spin chemists. In particular, the time-resolved EPR (TREPR) ° and time-resolved NMR (CIDNP) techniques were found to be of unparalleled utility in terms of mechanistic understanding of radical chemistry. Theoretical work to explain CIDNP and CIDEP phenomena was able to link, for the first time, the spin physics of radical pairs to their diffusion, molecular tumbling, confinement (solvent cages versus supramo-lecular environments ), and the effects of externally applied magnetic fields. ... 

The next three polymers in this series are aU ethyl acrylates, meaning that while the backbone (a) substituent is different in all three structures, the ester side-chain group (P) is the same for aU of them ( CH2CH3). Polymer 3 is poly(ethyl methacrylate) (PEMA), and 4 is poly(ethyl cyanoacrylate) (PECA), which may be recognizable as a primary component of the so-called superglues. Polymer 5 is poly(ethyl acrylate) (PEA), with H on the backbone a-position. Erom structure 3 to 4 to 5, the a-substituent becomes simpler in structure and this will be reflected in the observed TREPR spectra below in terms of the number of observed transitions, and in some cases the linewidths as well. 

The second mode is to run the experiment at a fixed magnetic field and sweep the second boxcar gate over time to collect kinetic information. There are two problems with this approach. Eirst, the experiment must be repeated several times with a slow scan rate in order to get satisfactory S/N. To extract the EPR kinetic curve, the experiment is repeated off resonance and the two curves subtracted. Kinetics are more easily obtained using a high-bandwidth transient digitizer instead of a boxcar, and many researchers perform TREPR in this fashion." ° It is important to note here two... 

A detailed description of CIDEP mechanisms is outside the scope of this chapter. Several monographs and reviews are available that describe the spin physics and chemistry. Briefly, the radical pair mechanism (RPM) arises from singlet-triplet electron spin wave function evolution during the first few nanoseconds of the diffusive radical pair lifetime. For excited-state triplet precursors, the phase of the resulting TREPR spectrum is low-field E, high-field A. The triplet mechanism (TM) is a net polarization arising from anisotropic intersystem crossing in the molecular excited states. For the polymers under study here, the TM is net E in all cases, which is unusual for aliphatic carbonyls and will be discussed in more detail in a later section. Other CIDEP mechanisms, such as the radical-triplet pair mechanism and spin-correlated radical pair mechanism, are excluded from this discussion, as they do not appear in any of the systems presented here. 

FIGURE 14.2 X-band TREPR spectra of main-chain polymer radical la produced from 248 nm laser flash photolysis of atactic, isotactic, and syndiotactic PMMA in propylene carbonate at 0.8 ps delay time. The temperature for each spectrum is shown in °C, and the magnetic held sweep width is 150 G for aU spectra, which exhibit net E CIDEP in aU cases. Simulations of each fast motion spectrum (highest temperature) are shown at the bottom of each data set. Hyperflne values for each simulation are 3 anCCHs) = 22.9 G, 2aH(CH2)= 16.4 G, 2aH(CH2) = 11.7G for isotactic PMMA 3ah(CH3) = 22.9G, 2aH(CH2>= 16.2G,... 

The left side of Fig. 14.4 shows TREPR spectra obtained 1.0 ps after 248 nm laser flash photolysis of eight acrylic polymers, with computer simulations on the right side. 

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