STRUCTURES OF SELF-ASSEMBLED MONOLAYERS OF AROMATIC-DERIVATIZED THIOLS ON EVAPORATED GOLD AND SILVER SURFACES - IMPLICATION ON PACKING MECHANISM

Structures of self-assembled monolayers of 4'-alkoxybiphenyl-4-methanethiol, 4'-CH3(CH2)(m)OC6H4C6H4-4-CH2SH (I), (6-alkoxynaphth-2-yl)methanethiol, 6-CH3(CH2)(m)OC10H6-2-CH2SH (II), and 4'-alkoxybiphenyl-4-thiol, 4'-CH3(CH2)(m)OC6H4C6H4-4-SH (III) (m = 15, 16) on surfaces of evaporated Au and Ag were characterized and compared with the corresponding structures of n-alkanethiol, CH3(CH2)(m)SH (TV) (m = 15, 16). The results revealed that compounds I, II, and III form well-ordered monolayers on Au and Ag, with nearly identical structure of the hydrocarbon tail, presumably because of the same herringbone packing of the aromatic chromophores on nearly identical lattices of Au and Ag substrates. The herringbone packing of the aromatic chromophores, supported by molecular mechanics calculations, in turn determines the lattice and the spacing of the hydrocarbon chains. That compounds I and III appear to have the same structure indicates that the binding geometry of sulfur can be either sp(3) (surface-S-C bond angle of similar to 109 degrees) or sp hybridized (surface-S-C bond angle of similar to 180 degrees), depending on packing interactions in other parts of the molecules. The least sterically demanding n-alkanethiol (IV) allows a greater density of packing on Ag but not on Au, causing the binding geometry of sulfur to the surface to differ on the two metals. The tighter packing of IV on Ag makes monolayers of thiols with short chains more ordered conformationally than those of Au.