单分子瞬时带电态中电子-振动耦合特性的亚纳米荧光成像研究

Probing vibronic coupling of a transiently charged state of a single molecule through subnanometer resolved electroluminescence imaging

分子内的电子-振动耦合特性对电子跃迁和分子光谱特征有重要影响,是分子光谱学研究的中心课题之一.本文利用具有亚纳米分辨的扫描隧道显微镜诱导发光成像技术,通过高度局域的隧穿电子来充电激发单个苝四甲酸二酐分子,研究该分子的瞬时带电态(-2价态)的电致发光特性以及相应的电子-振动跃迁的实空间成像特征.具有亚分子分辨的光谱成像结果表明,0-0纯电子跃迁的光子图“两点”亮斑特征是沿分子短轴的,而某些电子振动峰的光子图“两点”亮斑方向却沿着分子长轴,相对于0-0跃迁光子图的图案旋转了90°.这表明这些振动态所对应的跃迁偶极取向是沿着分子长轴的,相对于0-0的纯电子跃迁的偶极取向发生了明显的变化,说明这些分子振动模式在电子的跃迁过程中对电子态空间分布产生了重要扰动.理论计算表明,这种跃迁偶极的变化源自于与Herzberg-Teller贡献相关的电子-振动耦合.反对称的振动模式对纯电子跃迁的跃迁密度、特别是具有较大跃迁密度的原子产生了强烈的动态扰动,或者说对分子波函数进行了“整形手术”,从而诱导出沿分子长轴方向的跃迁电荷振荡,引发沿分子长轴的跃迁偶极.本文结果为从实空间的视角来直观理解分子瞬时带电态中的电子-振动耦合的微观图像提供了新的途径.

The intramolecular vibronic coupling has a great effect on molecular electronic transitions and associated spectral characteristics,which is a central topic in the study of molecular spectroscopy.In this paper,we investigate the vibronic coupling of a transiently charged state within a single 3,4,9,10-perylenetetracarboxylicdianhydride(PTCDA)molecule in real space by imaging the spatial distribution of single-molecule electroluminescence via highly localized excitation of tunneling electrons in a plasmonic nanocavity.The electron injections from a scanning tunneling microscope tip into a PTCDA molecule on a silver-supported ultrathin salt layer produce a transient doubly charged molecular anion that emits vibrationally resolved fluorescence.The sub-molecular resolved spectroscopic imaging for the–2 valence transiently charged state shows a two-spot pattern along the molecular short axis for the purely electronic 0-0 transition.However,the observed two-spot orientation for certain anti-symmetric vibronic-state imaging is found to be evidently different from the purely electronic 0-0 transition,rotating 90°,which reflects the change in the transition dipole orientation from along the molecular short axis to the long axis.Such a change directly reveals the occurrence of strong vibronic coupling associated with a large Herzberg-Teller(HT)contribution,which goes beyond the conventional Franck-Condon(FC)picture.Combined with theoretical calculations,the anti-symmetric vibration is found to have a strong dynamic disturbance to the transition density of purely electronic transitions,especially those atoms with large transition densities,which induces a strong transition charge oscillation along the long axis of the molecule and thus leads to a transition dipole along the long axis of the molecule.On the other hand,for vibronic emissions associated with the totally symmetric molecular vibration(such as the v1(Ag)mode described above),the observed two-spot orientation in the vibronic-state imaging pattern is found to be the same as the purely electronic 0-0 transition,which directly reveals its FC-dominated nature.Notably,the vibration-induced emission associated with HT-dominated contributions(such as the v2(B3g)mode)is often discussed in the literature by using an intensity borrowing mechanism via the state mixing with other high-lying eigenstates.In the present work,the v2-vibration with B3g symmetry is likely to modulate the zero-order electronic wavefunction of the S1 state in a way to best resemble that of the S2 state(i.e.,induce efficient mixing of the electronic excited state S1 with the electronic excited state S2),so that the v2-vibration induced emission seems to borrow intensities from neighboring S2→S0 transitions.Our results provide a new route for the real-space understanding of the microscopic picture for the vibronic coupling within a single molecule in a transiently charged state.