Dynamic Nuclear Polarization with Photoexcited Triplet Electrons in a Glassy Matrix
Dynamic Nuclear Polarization with Photoexcited Triplet Electrons in a Glassy Matrix
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玻璃基体中光激发三重态电子的动态核极化
DOI：
10.1002/anie.201305674
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
and Masahiro Kitagawa
中科院分区：
文献类型：
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作者：
Kenichiro Tateishi;Makoto Negoro;Akinori Kagawa;and Masahiro Kitagawa
核磁共振波谱和核磁共振成像是对大块材料和人体内部微观结构进行无损分析的有力方法。作为一种提高其灵敏度的方法，动态核极化（DNP）引起了人们的广泛关注。核自旋信号的强度与自旋极化成正比。在通常用于核磁共振波谱和核磁共振成像的磁场中，核自旋在室温下的热极化为10À5或更小的数量级。DNP是一种将自旋极化从电子转移到原子核的方法电子自旋的热极化比1H自旋的热极化大660倍，因此DNP最多能将1H自旋极化（即灵敏度）提高660倍。在较低的温度下，电子自旋在热平衡中极化程度更高，因此DNP可以实现更高的核自旋极化。（注意，即使在较低的温度下，加DNP后的极化增强因子的上限，即相同温度下DNP后/ DNP前的极化之比，并没有增加。）例如，在4.2 K、2.5 T下，体核极化率可达10%数量级。通过开发特殊的外围设备，如样品转移系统[2,3]、温度跳跃系统[2 - 4]和低温魔角旋转系统[5,6]，我们能够将液氦温度附近低温下的超极化与高分辨率核磁共振波谱或MRI相结合。到目前为止，DNP已经成功地对各种材料进行了超极化，如跨膜蛋白、淀粉样蛋白纳米晶体[7]、[8]和多孔介质界面[9]以及MRI可注射分子[3]使用玻璃基质的样品制备方法是影响dnp .[5]多功能性的最重要因素之一在上述研究中，[2-9]所关注的材料与自由基一起共掺杂到玻璃基体中。如果能在液氮温度以上实现超极化，将简化外围设备和实验，拓宽应用领域。有许多令人感兴趣的样品需要较高的温度。克服传统DNP极化增强因子上限（660）的一种解决方案是使用非平衡电子自旋作为极化剂。许多分子，如并五苯，具有光激发的三重态，其中三重态自旋亚能级上的居群分布由于系统间交叉而高度偏置，无论温度和磁场强度如何在掺杂了并五苯的单晶样品中，DNP利用光激发的三重态电子作为极化剂[11-13]，在105 K[14]下获得了70%的体1H自旋极化，在室温下获得了34%的自旋极化在此，我们报告了在玻璃矩阵中使用光激发三重态电子的DNP的首次演示。我们已经成功地在掺并五苯的玻璃基质中极化了共掺杂剂，这是增加样品种类和拓宽应用领域的第一步。在本研究中，我们使用了两种宿主分子。一种是极性分子，二苯甲酮（以下简称BZP）。另一种是非极性分子，邻terphenyl（以下简称OTP）。OTP玻璃也被用于传统的DNP图1总结了这些分子的玻璃化转变温度Tg和熔点Tm，以及通常用于传统DNP的甘油/水和乙醇/水的混合物。在目前的情况下，我们可以……
NMR spectroscopy and MRI are powerful methods for the non-destructive analysis of microscopic structures inside bulk materials and human bodies. As a method to enhance their sensitivities, dynamic nuclear polarization (DNP) has attracted great attention. The intensity of a signal from nuclear spins is proportional to the spin polarization. In magnetic fields conventionally used for NMR spectroscopy and MRI, thermal polarization of nuclear spins at room temperature is in the order of 10À5 or less. DNP is a means of transferring spin polarization from electrons to nuclei.[1] The thermal polarization of electron spins is 660 times larger than that of 1H spins, and therefore, DNP can enhance the 1H spin polarization (hence the sensitivity) by a factor of at most 660. With a lower temperature, electron spins are more highly polarized in thermal equilibrium and therefore a higher nuclear spin polarization can be achieved with DNP.(Note that, even at lower temperature, the upper limit of the polarization enhancement factor with DNP, which is defined as the ratio of the polarization after/before DNP at the same temperature herein, is not increased.) For example, at 4.2 K in 2.5 T, the bulk nuclear polarization can reach the order of 10%. Developing special peripheral equipment, such as a sample transfer system,[2, 3] a temperature jump system,[2–4] and a cryogenic magic angle spinning system,[5, 6] we are able to combine hyperpolarization at cryogenic temperatures around liquid helium temperature with high-resolution NMR spectroscopy or MRI. Until now, DNP has succeeded in hyperpolarizing various materials, such as trans-membrane proteins,[7] nanocrystals of amyloid,[8] and interfaces of porous media,[9] as well as injectable molecules for MRI.[3] The sample preparation method using a glassy matrix is one of the most important factors with regards to the versatility ofDNP.[5] In the above-mentioned study,[2–9] materials of interest are codoped into a glassy matrix together with free radicals. If hyperpolarization can be achieved above liquid nitrogen temperature, the peripheral equipment and the experiments will be simplified and the application field will be broadened. There are many samples of interest for which a higher temperature is preferable. One solution for overcoming the upper limit (660) of the polarization enhancement factor achieved by conventional DNP is to use non-equilibrated electron spins as polarizing agents. A number of molecules, such as pentacene, have photoexcited triplet states where the population distribution over the triplet spin sublevels is highly biased owing to intersystem crossing regardless of temperature and magnetic field strength.[10] With single crystal samples doped with pentacene, DNP utilizing the photoexcited triplet electrons as polarizing agents [11–13] has achieved bulk 1H spin polarizations of 70% at 105 K [14] and 34% at room temperature.[15] Herein, we report the first demonstration of DNP using photoexcited triplet electrons in a glassy matrix. We have succeeded in polarizing codopants in a glassy matrix doped with pentacene, as a first step to increase the variety of samples and to broaden the application field. In the present work, we use two kinds of host molecule. One is a polar molecule, benzophenone (henceforth referred to as BZP). Another is a non-polar molecule, o-terphenyl (henceforth referred to as OTP). OTP glass has also been used in conventional DNP.[16] The glass transition temperatures Tg and the melting points Tm of these molecules are summarized in Figure 1, alongside mixtures of glycerol/water and ethanol/water, which are often used in conventional DNP. In the present hosts, we can …