We performed time-resolved transient absorption and fluorescence anisotropy measurements in order to study tautomerization of porphycene in rigid polymer matrices at cryogenic temperatures. Studies were carried out in poly(methyl methacrylate) (PMMA), poly(vinyl butyral) (PVB), and poly(vinyl alcohol) (PVA). The results prove that in all studied media hydrogen tunnelling plays a significant role in the double hydrogen transfer which becomes very sensitive to properties of the environment below approx. 150 K. We also demonstrate that there exist two populations of porphycene molecules in rigid media: “hydrogen-transferring” molecules, in which tautomerization occurs on time scales below 1 ns and “frozen” molecules in which double hydrogen transfer is too slow to be monitored with nanosecond techniques. The number of “frozen” molecules increases when the sample is cooled. We explain this effect by interactions of guest molecules with a rigid host matrix which disturbs symmetry of porphycene and hinders tunnelling. Temperature dependence of the number of hydrogen-transferring molecules suggests that the factor which restores the symmetry of the double-minimum potential well in porphycene are intermolecular vibrations localized in separated regions of the amorphous polymer.

中文翻译：

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局部微环境对卟啉中双氢转移的影响。

我们进行了时间分辨的瞬态吸收和荧光各向异性测量，以研究在低温下刚性聚合物基质中卟啉的互变异构。在聚甲基丙烯酸甲酯（PMMA），聚乙烯醇缩丁醛（PVB）和聚乙烯醇（PVA）中进行了研究。结果证明，在所有研究的介质中，氢隧穿在氢的双重转移中起着重要作用，而氢的转移对约25℃以下的环境特性变得非常敏感。150K。我们还证明，在刚性介质中存在两个卟啉分子种群：“氢转移”分子，其中互变异构发生在小于1 ns的时间尺度上；“冻结”分子，其双氢转移太慢而无法用纳秒技术监控。样品冷却后，“冻结”分子的数量增加。我们通过来宾分子与刚性主体基质的相互作用来解释这种效应，该刚性基质干扰了卟啉的对称性并阻碍了隧穿。氢转移分子数量的温度依赖性表明，恢复卟啉中双最小势阱对称性的因素是位于非晶态聚合物分离区域中的分子间振动。