Molecular flexibility as a factor affecting the surface ordering of organic adsorbates on metal substrates

The effect of molecular flexibility on the surface ordering of complex organic adsorbates is explored, using alpha,omega-dihexylquaterthiophene (DH4T) and mixed DH4T|tetracene phases on Ag(111) as model systems. The structure of DH4T/Ag(111) interfaces is determined by the flexibility of the hexyl chains at either end of the quaterthiophene backbone: Above 273 K, DH4T forms a nematic liquid crystalline phase with a director close to the [112] direction of the silver substrate. At 273 K, a reversible phase transition to a long-range ordered, point-on-line coincident phase is observed. However, this ordered state is still affected substantially by the flexible nature of DH4T, which materializes in a large number of local structural defects. If traces of DH4T are coevaporated with tetracene, inclusions of a 1:1 stoichiometric DH4T|tetracene phase are found in a tetracene/Ag(111) matrix (alpha-phase). In this mixed phase, the two surface enantiomers of pro-chiral DH4T on one hand and tetracene on the other form a complex stripe structure. The mixed phase shows a higher degree of order than present at the pure DH4T/Ag(111) interface, which also lacks chiral organization. The addition of tetracene molecules as structural templates stabilizes certain conformations of DH4T and thus, by balancing its structural flexibility, allows the surface-induced chirality of DH4T to become a decisive factor in determining the structure of the mixed phase.