  The temperature dependent stepwise deprotonation of
5,10,15,20-tetra(p-hydroxyphenyl)porphyrin is investigated using photoelectron
spectroscopy. An abundance of pyrrolic relative to iminic nitrogen and a
decrease in the ratio of the amount of -NH- to -N= with increasing annealing
temperature is found. In contrast to the molecules adsorbed on Au(111), on the
more reactive Ag(110) surface, partial dissociation of the hydroxyl groups and
subsequent diffusion and rebonding of hydrogen to the central nitrogen atoms
resulting in a zwitterionic molecule was clearly observed. Moreover, partial
C-H bond cleavage and the formation of new covalent bonds with adjacent
molecules or the surface starts at a relatively high annealing temperature of
300{\deg}C. This reaction is identified to occur at the carbon atoms of the
pyrrole rings, which leads also to a shift in the N 1s signal and changes in
the valence band of the molecules. Our results show that annealing can
significantly alter the molecules which were deposited depending on the maximum
temperature and the catalytic properties of the specific substrate. The thermal
stability should be considered if a molecular monolayer is prepared from a
multilayer by desorption, or if annealing is applied to enhance the
self-assembly of molecular structures.
