Porphyrin derivatives are promising candidates for the generation of functional molecular architectures on surfaces. The unambiguous identification of individual porphyrin species is a prerequisite in this regard. We investigated a layer of intermixed tetraphenylporphyrins (TPP), namely 2HTPP, FeTPP and CoTPP at room temperature. Voltage-dependent STM imaging was successfully applied to discriminate the very similar porphyrins. The characteristic appearance of CoTPP at low negative voltages (>-0.4 V) can be traced back to a specific interaction of the Co dz2 orbital with the underlying Ag substrate. The dumbbell appearance observed for FeTPP over a wide bias range and for CoTPP at lower negative voltages (<-0.4 V) is in line with the well known saddle shape of the porphyrin macrocycle in the adsorbed state.
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