University of Texas at Austin , Austin, Texas 78712, United States.
ACS Nano. 2016 Dec 27;10(12):11058-11065. doi: 10.1021/acsnano.6b05924. Epub 2016 Nov 21.
We present a detailed study of the tautomerization, that is, the switching of hydrogen protons, between different sites in the molecular frame of phthalocyanine (HPc) on a Ag(111) substrate by means of scanning tunneling microscopy (STM) and STM-based pump-and-sample techniques. Our data reveal that the symmetry mismatch between the substrate and the molecular frame lifts the energetic degeneracy of the two HPc tautomers. Their energy difference is so large that only one tautomer can be found in the ground state. Tip-induced tautomerization was triggered at sufficiently high bias voltages. The excited metastable HPc tautomer was found to exhibit a lifetime of at least several days, as derived from the fact that the molecule did not change back to the ground state within experimentally accessible time scales as long as noninvasive tunneling parameters were used to probe the state of the molecule. By the controlled removal of a hydrogen proton from the molecule, a four-level system was created. Pump-and-sample experiments reveal that the lifetime of the metastable positions amounts to seconds only. Current- and bias-dependent studies indicate that the presence of the STM tip modifies the potential barrier, thereby allowing for a controlled tuning of the metastable tautomer's lifetime.
我们通过扫描隧道显微镜（STM）和基于 STM 的泵浦-取样技术，对酞菁（HPc）分子在 Ag（111）衬底上不同位置之间的互变异构（即氢质子的转换）进行了详细研究。我们的数据表明，衬底和分子框架之间的对称性失配消除了两种 HPc 互变异构体的能量简并性。它们的能量差非常大，以至于在基态下只能找到一种互变异构体。在足够高的偏压下，尖端诱导的互变异构被触发。发现激发的亚稳态 HPc 互变异构体的寿命至少为数天，这是因为只要使用非侵入性隧道参数来探测分子的状态，分子在实验可及的时间范围内不会恢复到基态。通过从分子中去除一个氢质子，创建了一个四能级系统。泵浦-取样实验表明，亚稳态位置的寿命仅为数秒。电流和偏压相关研究表明，STM 尖端的存在改变了势垒，从而允许对亚稳态互变异构体的寿命进行控制调节。
