Hungry Porphyrins: Protonation and Self‐Metalation of Tetraphenylporphyrin on TiO₂(110) ‐ 1 × 1

Abstract

We have studied the adsorption and self‐metalation of tetraphenylporphyrin (2HTPP) on TiO2(110) ‐ 1 × 1 with X‐ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). Upon adsorption at room temperature the first monolayer of 2HTPP is protonated by protons from the surface, forming porphyrin diacid (4HTPP2+). In STM, single molecules with low mobility and no long‐range order were observed. As the surface is heated to 400 K, any unprotonated 2HTPP molecules in the second layer metalate, forming titanyl tetraphenylporphyrin (TiOTPP), but the protonated molecules in the first layer remain. As the surface is heated further to 550 K even the protonated 4HTPP2+ molecules metalate and only TiOTPP remain on the surface.

Interfacial Reactions of Tetraphenylporphyrin with Cobalt‐Oxide Thin Films

“…The diacid speciesi sa lso very temperature-stable on TiO 2 (110) and metalates only at significantly highertemperatures than free-base 2HTPP. [12] On CoO(111)inc ontrast we only observe about 50 %protonation of the unmetalated2 HTPP molecules, see Figure 3. In addition, heatingt o2 75 Kc onverts the protonated species completely into free-base 2HTPP without changing the degree of self-metalation, even thoughs urface-OH groups are stable up to more than 450 Ko nC oO(111).…”

“…This feature has previously been observedf or 2HTPP on TiO 2 (110) and was assigned to a protonation of the iminic nitrogen atoms of the porphyrin macrocycle, presumably with surfaceh ydroxyl groups, forming ap orphyrin diacid molecule. [11,12] Theb ehavior of the protonated porphyrin molecules on CoO(111)i sv ery different from that on TiO 2 (110). On TiO 2 (110) at submonolayer coverages 2HTPP is fully protonated into the diacid form.…”

“…Recent research effort was devoted to gain understanding of the porphyrin/oxide interface. [11][12][13][14][15][16][17][18][19] Adsorption of free-base porphyrin molecules on both oxide [12][13][14]18] and metal [20][21][22][23][24] surfaces can result in the incorporation of substrate metal atoms into the center of the macrocycle,t hat is, in self-metalation. For metal surfaces self-metalation proceeds via ar edox process in which am etal atom is oxidized and incorporated into the macrocycle ring and the Ha toms attached to the aminic nitrogen atoms in the molecule are reduced to H 2 .…”

“…This protone xchange reaction resultsi nt he formation of at emperature-stable doubly protonated porphyrin-diacid species on TiO 2 (110) surfaces [11] which hinders selfmetalation. [12] Therefore, the accessibility of surfacem etal cations and surface hydroxyl groups should be ak ey factor controlling the self-metalation and proton-exchange reactions. In this context, studying the interaction of free-base porphyrin molecules with cobalt oxide thin films is particularly attractive due to the possibility of tuning the chemical nature of the substrate topmost layer with ease.…”

“…This is probably because the abstracted hydrogen atoms can be stabilized by the oxygen and form hydroxyl groups . Self‐metalation on oxide substrates, on the other hand, is proposed to take place via an ion‐exchange reaction, in which a metal cation from the oxide surface is incorporated into the center of the macrocycle whilst two protons are released from the aminic nitrogen atoms. Porphyrin molecules deposited onto oxide substrates can also undergo a protonation reaction in which the iminic nitrogen atoms at the macrocycle core capture protons presumably from surface hydroxyl groups.…”