Spontaneous Change in Molecular Orientation at Order–Disorder Transition of Tetracene on Ag(111)

Abstract

We investigate the molecular orientation of tetracene in ordered and disordered layers on Ag(111) using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Quantitative analysis of NEXAFS intensities reveals that the compact monolayer α-phase at 150 K consists of essentially flat-lying molecules with an average tilt angle α < 9° with respect to the Ag(111) surface. At room temperature the tetracene monolayer is dynamically disordered with α = 15 ± 4° (α′-phase). The increase in the average tetracene tilt angle indicates that orientational degrees of freedom perpendicular to the surface drive the order–disorder transition in the tetracene monolayer on Ag(111). We argue that the phase transition can be explained by a delicate balance between the interfacial enthalpy and the orientational entropy. Furthermore, we suggest that tetracene/Ag(111) may be a good example of Onsager’s system of nonattracting rigid anisotropic objects (so-called “hard rods”) in two dimensions. On the other hand, the complex bilayer β-phase at 150 K includes tilted molecules with α ≈ 36°, in good agreement with the structure model introduced by Soubatch et al. [Phys. Rev. B 2011, 84, 195440]. This demonstrates that, under the influence of a second layer, π-conjugated molecules in contact with a metal surface can tilt out of the surface plane.