Diffusion, Rotation, and Surface Chemical Bond of Individual 2H-Tetraphenylporphyrin Molecules on Cu(111)

Abstract

We address the dynamic behavior and the surface chemical bond of 2H-tetraphenylporphyrin (2HTPP) on Cu(111) around room temperature by variable-temperature scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) in ultrahigh vacuum. Between 280 and 345 K, the molecules predominantly display unidirectional diffusion along one of the three densely packed substrate AE110ae directions, which is attributed to a high site selectivity of the adsorbateÀsubstrate bond. Above 305 K, the diffusion direction is found to change occasionally by (120°. The activation barriers for the unidirectional diffusion and for rotation of the diffusion direction are determined to 0.71 ( 0.08 and 1.28 ( 0.12 eV, respectively. XPS shows that the iminic nitrogen atoms of 2HTPP interact strongly with the Cu surface. It is postulated that the local bonding situation is similar as in the initial complex (sitting-atop complex), which has previously been observed during the surface-confined in situ metalation of porphyrins.

Studying the dynamic behaviour of porphyrins as prototype functional molecules by scanning tunnelling microscopy close to room temperature

“…These can also be transformed into the corresponding entropy values, as shown by Eichberger et al, when investigating an unexpectedly fast surface diffusion of 2HTPyP dimers on Cu(111). 33 As an outlook, we want to stress the importance of the results by Buchner et al, 34 Eichberger et al 33 and Ditze et al 37 achieved by UHV STM, which demonstrate that entropic effects have to be generally considered for large organic molecules on surfaces at and close to RT. The general character of these investigations is underlined by recent STM experiments at the liquid/ solid interface.…”

“…21,101,102,106,107 On the other hand, the uncoordinated iminic nitrogen atoms of free base porphyrins like 2HTTBPP are expected to form a strong attractive interaction with the Cu substrate; 33,80,81 this type of interaction was already discussed in the previous sections for the smaller 2HTPP. [34][35][36]58,79,108 The investigation by Ditze et al indeed reveals that 2HTTBPP exhibits a very unusual and interesting adsorption behaviour. Fig.…”

“…Note that diffusion measurements by STM found in literature are typically carried out at tunnelling resistances in the range of 1-10 GO. 33,[47][48][49]64 Since the tunnel resistance in the experiments by Buchner et al 34 is even higher, i.e., the tip-substrate interaction is lower, a significant influence of the STM measurement on the thermally induced diffusion can be practically ruled out. In addition, no influence upon changing image size, scanning speed and the fast scan direction (vertical/horizontal) was reported.…”

“…34). These movies comprise roughly 50 images with an acquisition time of 20-40 s per image, allowing for the analysis of roughly 2500 diffusion events.…”

“…4,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] In this contribution, we review recent advances in the understanding of the dynamic behaviour of porphyrins, as representatives of organic molecules, adsorbed on a Cu(111) single crystal surface in ultra-high vacuum (UHV). Based on temperaturedependent measurements of individual molecules, the kinetics and energetics of diffusion 33,34 and rotation, 34 of the so-called self-metalation reaction 35,36 and finally of conformational switching of particular free base porphyrins 37 can be determined. Even though these processes appear to be very different at first glance, they can all be regarded as activated processes, as schematically sketched in Fig.…”