Steric and electronic effects of electrochemically generated aryl radicals on grafting of the graphite surface

Grafting of aryl radicals generated by electrochemical reduction of aryldiazonium salts has been extensively studied on various surfaces. However, there exists two unclear aspects; the first one is the generality of the blocking ability of simple functional groups toward multilayer growth, and the second one is the electronic impact of substituent groups of aryl radicals on grafting efficiency. To address these aspects, we have studied the electrochemical functionalization of graphite using aryldiazonium salts having electron-donating or electron-withdrawing groups at the 3,4,5-positions. Atomic force microscopy investigation of the functionalized surfaces revealed the formation of monolayers for all aryldiazonium salts, and thus, nitro, carboxy, ester, methyl, and methoxy groups at the 3,4,5-positions of the benzene ring suppress polyaryl growth. The degree of grafting estimated by scanning tunneling microscopy imaging and Raman spectroscopy of the functionalized surfaces depends on the electronic state of the aryl radicals, in which the radicals with electron-donating groups show a high degree of functionalization, whereas those with electron-withdrawing groups exhibit a low degree of functionalization. We discuss several possibilities that affect grafting density. Though there are several factors, we hypothesize that one factor to explain the observed reactivity trend is the electronic property of the aryl radicals, namely, the relative position of the singly occupied molecular orbital energy levels of the aryl radicals with respect to the graphite Fermi energy level.