Stability and Exchange Processes in Ionic Liquid/Porphyrin Composite Films on Metal Surfaces

Abstract

In light of increasing interest in the development of organic–organic multicomponent heterostructures on metals, this molecular-scale study investigates prototypical composite systems of ultrathin porphyrin and ionic liquid (IL) films on metallic supports under well-defined ultrahigh vacuum conditions. By means of angle-resolved X-ray photoelectron spectroscopy, we investigated the adsorption, stability, and thermal exchange of the resulting films after sequential physical vapor deposition of the free-base porphyrin 5,10,15,20-tetraphenylporphyrin, 2H-TPP, and the IL 1-methyl-3-octylimidazolium hexafluorophosphate, [C8C1Im][PF6], on Ag(111) and Au(111). 2H-TPP shows two-dimensional growth of up to two closed molecular layers on Ag(111) and Au(111) and three-dimensional island growth for thicker films. IL films on top of a monolayer of 2H-TPP exhibit Stranski–Krastanov-like growth and are stable up to 385 K. The 2H-TPP layer leads to destabilization of the IL films, compared to the IL in direct contact with the bare metals, by inhibiting the specific adsorption of the ions on the metal surfaces. When the porphyrin is deposited on top of [C8C1Im][PF6] at low temperature, the 2H-TPP molecules adsorb on top of the IL film at first but replace the IL at the IL/metal interfaces upon heating above 240 K. This exchange process is most likely driven by the higher adsorption energy of 2H-TPP on Ag(111) and Au(111) surfaces, as compared to the IL. The behavior observed on Ag(111) and Au(111) is identical. The results are highly relevant for the stability of porphyrin/IL-based thin film catalyst systems and molecular devices, and more generally, stacked organic multilayer architectures.

Ultrathin ionic liquid films on metal surfaces: adsorption, growth, stability and exchange phenomena

“…[92] Au ( 149(2) [88] 380 [140] TP-XPS; WL desorption shifts to < 370 K after replacement by [C 8 [140] TP-XPS; WL desorption shifts to < 420 K on 2H-TPPcovered Ag (111). [81][82][83] Au ( 157 [221] 162(3) [88] 12 [140] TPD; same temperature for multilayer and WL desorption. [229,230] Cu ( (2) [212] 152(3) [212] 195 (19) [231] -Anions in contact with Cu surface decompose below 300 K. [75] a: Redhead analysis [232] with frequency factor…”

“…The flux was checked using a quartz crystal microbalance to ensure stable and reproducible evaporation rates. To remove volatile impurities, the compounds were thoroughly degassed in UHV for more than 24 hours at elevated temperatures close to the respective evaporation conditions [71,78,[81][82][83]86,87]. During deposition, the chamber background pressure was typically around 1 × 10 −9 mbar.…”

“…How do the ions arrange at the interfaces? How do the initial stages of thin film growth depend on the chemical nature of IL and support, and how stable are the ultrathin IL films at elevated temperatures [71,74,76,78,[80][81][82][83]. We further extend the scope by looking at mixtures of ILs on metal surfaces on the nanoscale.…”

“…1-methyl-3-octylimidazolium hexafluorophosphate 91 [2] 194 [151] 202 [205] 203 [150] 32.5 f, [8] 36.5 f, [2] 1.19 f, [216] 1.22 f, [2,212,219] 1.235 f, [8] 1.237 f, [205] 0.458 f, [8] 0.46 [6] 0.461 f, [219] 0.77 [81][82][83] […”

“…N-butyl-N-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate 157 [224] 277 [224] 38 g, [224] 1.59 [212] 0.613 [212] 0.85 [225,226] 0.34 c, [83] a: Can depend strongly on heating rate, thermal history (e.g. cooling rate) or water content [2,146,148,149,208,215,223].…”