In recent years particular attention has been devoted to the investigation of 2D phase transitions of electroinactive organic compounds at single-crystal electrodes from a kinetic point of view, using electrochemical techniques such as cyclic voltammetry, chronoamperometry, chronocoulometry, and the phase-sensitive lock-in technique. Among the systems so far investigated we can mention coumarin on Au(lll) and Au(100)-(hex) [14], uridine on Au(lll) [15, 16], uracil on Au(100), Au(100)-(hex) [17], Ag(lll) [18, 19], and Ag(100) [18], thymine on Ag(lll) and Ag(100) [20], and cytosine on Au(lll) [21]. Ordered adlayer structures have also been reported for thymine, cytosine, guanine and adenine on Au(l 11) employing in-situ STM [22].

Practically all these organic compounds are purine and pyrimidine derivatives which are present as components of DNA and RNA, where they encode genetic information; their molecules have both proton-donor and proton-acceptor groups, and are therefore likely to form intermolecular hydrogen bonds in the adsorbed state [23], Many of these systems [18, 21] exhibit both an ordered, liquid-like physisorbed state which shows the typical behavior of 2D condensed films at the mercury/water interface and, at more positive potentials, a further ordered, chemisorbed state which shows an extreme stability even at relatively high temperatures and is relatively insensitive towards the surface crystallographic orientation. While STM images of chemisorbed adlayers of uracil [17] and of cytosine [21] on Au(lll) have been obtained, no STM images of physisorbed adlayers have so far been reported. This is quite probably due to the high mobility of the physisorbed molecules and to their inability to alter the electron density profile of the metal substrate to a detectable extent.

2D condensation of uracil on Ag(l 11) in the chemisorbed state shows both a short-and a long-time behavior [19], STM images of chemisorbed uracil on Ag(l 11) are now under investigation in our laboratory in an attempt to monitor their evolution in time.

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