STM investigation of structural isomers: alkyl chain position induced self-assembly at the liquid/solid interface

Abstract

Investigating and regulating the self-assembly structure is of great importance in 2D crystal engineering and it is also gaining significant interest in surface studies. In this work, we systematically explored the variation of self-assembled patterns induced by the changeable side chain position. Moreover, molecules with different alkyl chain lengths (n = 15, 16) were also synthesized and probed for the purpose of understanding how an odd/even number of carbon atoms in the peripheral chains can affect the molecular adlayers. Structural isomers of bis-substituted anthraquinone derivatives 1,8-A-2OC_n, 2,6-A-2OC_n, 1,4-A-2OC_n and 1,5-A-2OC_n (n = 15, 16) were used and investigated by STM. 1,8-A-2OC₁₆ and 1,8-A-2OC₁₅ molecules adopted Z-like I and Linear I structures, respectively. 2,6-A-2OC₁₆ and 2,6-A-2OC₁₅ molecules were severally arranged in Linear II and Linear III configurations. 1,4-A-2OC_n (n = 15, 16) molecules were staggered in a Z-like II fashion and 1,5-A-2OC_n (n = 15, 16) molecules displayed a Linear IV nanostructure. Therefore, we arrive at a conclusion that self-assembly structures of anthraquinone isomers are chain-position-dependent, and designing isomeric compounds can be taken into consideration in regulating assembled structures. Besides, 2D nanopatterns of 1,8-A-2OC_n and 2,6-A-2OC_n can be regulated by the odd/even property of the side chains, but this is not the case for 1,4-A-2OC_n and 1,5-A-2OC_n, ascribed to the difference in driving forces for them. It is believed that the results are of significance to the alkyl chain position induced assembly configurations and surface research studies of structural isomers.