Solvent Co-adsorption Changing the Type of Halogen-Based Bonds Associated with the Position of Bromine Substituents Revealed by Scanning Tunneling Microscopy

A systematic investigation of the solvent co-adsorption effect on the self-assembly of 3,8-dibromo-6-(hexadecyloxy)­phenanthridine (3,8-BHP) was performed at the liquid/graphite interface by scanning tunneling microscopy. 1-Octanoic acid, n-tetradecane, n-pentadecane, n-hexadecane, and n-heptadecane were chosen as the solvents. 3,8-BHP molecules formed the same twist-shaped nanopattern by forming intermolecular Br···N and Br···H bonds in all the solvents at high solution concentrations. The co-adsorption of 1-octanoic acid solvents at low concentrations changed the molecular arrangement by forming the stronger molecule–solvent COOH···N and C–H···O–H­(COOH) hydrogen bonds. n-Pentadecane, n-hexadecane, and n-heptadecane also co-adsorbed in the adlayers at low concentrations, which resulted in the change in intermolecular interactions. In n-pentadecane and n-hexadecane solvents, the molecular packing and intermolecular interactions in the two-row lamella of 3,8-BHP were the same, in which the intermolecular C–H···Br hydrogen bonds and Br···Br interactions were the main driving forces. The difference among the nanostructures in n-pentadecane and n-hexadecane solvents depended on the number and length of the alkyl chain of the co-adsorbed solvents, which determined the probability of single-row lamellae. The results demonstrate that the molecule–solvent van der Waals forces are strong enough to transform the intermolecular forces involving bromine groups. Our work will help to understand the importance of the solvent co-adsorption effect on the change of halogen-based bonds associated with the position of the bromine substituent for the construction of polymorphic self-assembled structures.