Half-field resonance

Figure 7.7. Triplet spin energy levels vs magnetic field H, for (a) H x. (b) H y and (c) H z. Note the zero-field spitting the half-field resonance at C due to the Am = 2 transitions, and the full resonances which occur at different fields A and B for the Anis= -E 1 and —1 transitions [61].

Figure 7.42 displays the full- and half-field triplet ODMR powder pattern in P3HTE [52]. As is clearly seen, the full-field pattern is relatively structureless but asymmetric, and the half-field resonance contains two components clearly due to distinct triplet states. The lower-field component appears to be more closely related to the half-field resonance of P3HT (Figure 7.39), and is probably due to triplet excitons localized... 

The full- and half-field resonances of a typical DHO-PPV film prepared by Wudl and co-workers [24] are shown in Figure 7.43. The patterns were initially discussed in terms of two triplet states. Indeed, as Figure 7.43(b) shows, the observed half-field pattern is in good agreement with a simulated pattern assuming... 

The temperature dependence of the EL- and cr-detected half-field resonances are shown in Figure 7.48. As clearly seen, their amplitude rapidly decreases with increasing T the EL-detected resonances and the low-field cr-detected line are not observable at T > 50 K, and the high-field cr-detected pattern vanishes at r > 100 K. This behaviour is believed to result from the diffusivities of the triplets. 

Figure 7.48. The temperature dependence of the amplitudes of the higher- (triangles) and lower-field (circles) EL- and a-detected half-field resonances shown in Figure 7.47 (25).

Studies of the TMM biradical itself began in the late sixties when Dowd observed an ESR spectrum interpretable as that of the TMM triplet species from a sample of 4-methylenepyrazoline that was photolyzed in a glassy matrix at — 196°C. Further, all six hydrogens were equivalent from the splitting of the half-field resonance line (Am = 2 transitions) thus indicating that the TMM triplet had symmetry Similar observations were made upon photolysis of 3-methylenecyclobutanone. Both reactions were claimed to produce MCP as the sole product. However, photolysis of the ketone at —ITC in butadiene gives not only MCP but... 

For half-field resonances, the powder pattern of thermalized spins consists of A divergence at... 

The PIA band at 1.35 eV in the pure MEH-PPV shows a strong response at the (forbidden) half field resonance of spin = 1, indicating triplet character for this PIA band and dominance of the neutral photoexcitations in MEH-PPV (Figure 8.13). There is a small, residual spin = 1/2 response even within the pure... 

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