Pentacene doped naphthalene

A number of attempts to transfer the spin polarization directly from the triplet electrons to the 13C spins that are present in the sample have been unsuccessful. Nevertheless, it is possible to polarize the 13C spins indirectly, by polarizing the 1H spins first, and then transporting the enhanced 1H polarization to the I3C spins by the conventional crosspolarization technique. Figure 18B shows the enhanced 13C spectra obtained in this way using the same crystal of pentacene-doped naphthalene. 

However, the recent experiments demonstrating (MI)ONP in polycrystalline samples were actually achieved by selective implementation of the ISE. In this work, an enhancement factor of 3160 was achieved in a poly crystalline (ground and pressed) sample of pentacene-doped naphthalene at 100 K in a field of 0.32 T (Fig. 8). However, instead of sweeping over the entire triplet ESR line (as is normally done with the ISE), the field sweep was designed to cover only a partial area of the ESR spectrum. With this approach (as implemented), only those crystallites with the long axis of the pentacene chromophores... 

Fig. 8. MIONP enhancement of nuclear spin polarization in a polycrystalline sample of pentacene doped naphthalene/ (a) Build-up curve for the enhanced spin polarization, with an MIONP repetition rate of 50 Hz (determined by the laser) the time constant for the build-up was 3730 s. TCP is an acronym for integrated cross-polarization - the authors term for their variant of the ISE. (b) 13.6 MHz single-shot H NMR spectra, taken with (top) and without bottom) MIONP enhancement (respectively, acquired at 100 K and room temperature). The top spectrum was obtained with a 10,000 s MIONP build-up time. The enormous sensitivity enhancement obtained with ONP is clear from comparing the two spectra. (Figures courtesy of K. Takeda and T. Terao, reprinted from Ref. [71], copyright 2001, with permission from Elsevier.)...

To date, the organic substances studied via ONP have typically been planar, aromatic molecules in pure or mixed crystalline form. While ONP was first demonstrated in pure anthracene, the most commonly-studied systems for ONP (and related OEP) experiments over the years have been fluorene doped with acridine," " phenanthrene, ° phenazine, ° and anthracene," ° as well as naphthalene and p-terphenyl doped with pentacene. ° ° Other investigations of ONP have included studies of anthracene doped withphenazine, tetracyanobenzene," and tetracene, as well as acridine, ° dibromodiphenylether with benzophenone" and dichlorobenzophenone, and various halogenated benzenes. ... 

---