In situ observation of a deprotonation driven phase transformation-4,4'-Biphenyldicarboxylic acide on Au(111)

We have studied the growth of 4,4′-biphenyldicarboxylic acid (BDA) domains on Au(111) by means of low-energy electron microscopy (LEEM) and selective area low-energy electron diffraction (μLEED). Between 300 and 400 K, BDA forms three different 2-D crystalline phases, which are attributed to different degrees of deprotonation. At room temperature, BDA condenses in needle-shaped domains (α-phase) with the carboxylic acid–dimer as the main molecular bonding motif. Upon annealing from room temperature to above about 330 K, the needle-shaped domains change irreversibly into more compact ones. This remarkable feature is attributed to a partial deprotonation of the carboxylic end groups and thus breaking of the dimers, which allows the molecules to establish hydrogen bonds with adjacent molecules by a slight rotation (β-phase). A third phase is formed upon direct adsorption of BDA at substrate temperatures above 330 K. In this γ-phase the deprotonated BDA molecules form an open 2-D metal–organic coordination network, probably by incorporation of thermally excited Au adatoms.