Side chain position, length and odd/even effects on the 2D self-assembly of mono-substituted anthraquinone derivatives at the liquid/solid interface

Abstract

The formation of self-assembled adlayers of 1-hydroxyanthraquinone (1-HA) and 2-hydroxyanthraquinone (2-HA) derivatives with various side chain length were investigated using scanning tunneling microscopy for the purpose of determining the influence of chemical structure on 2D molecular arrangement in a self-assembly process. Different structures labeled as Linear I, Linear II, Linear III, Linear IV and Z-like were presented based on their packing modes. Weak O⋯H–C hydrogen bonds existing between adjacent anthraquinone moieties are the key forces driving the formation of ribbon A, A′, B and C, which are the basic rows of the self-assembled structures. The emergence of odd or even numbers of carbon atoms in the alkyl chain inducing structural diversity is an indication that one of the driving forces for 1-HA-OC_n (n = 15, 16) and 2-HA-OC_n (n = 12, 14–16) molecules to assemble into ordered 2D nanostructures is the van der Waals interactions between interdigitated alkyl chains. 1-HA-OC₁₆ and 1-HA-OC₁₅ exhibited lamellar structures packed in Linear I and Linear II fashions. 2-HA-OC₁₅ and 2-HA-OC₁₆ adopted Linear III structures and Z-like packing modes. Moreover, when the number of carbon atoms in the side chain of 2-HA-OC_n molecules was decreased to 12, the self-assembled pattern could present a Linear IV phase. Notably, 2-HA-OC₁₄ showed the coexistence of Z-like and Linear IV phases. Systematic experiments revealed that a better understanding of the alkyl chain position, length and odd/even effects on 2D self-assembly would shed light on better control of assembly patterns and the design of new molecular materials.