We studied the molecular orientation of pentacene monolayer phases on the Au(110) surface by means of near-edge X-ray absorption spectroscopy at the carbon K-shell and scanning tunneling microscopy. The highest coverage phase, displaying a (6 x 8) symmetry, is found to be formed by two types of differently oriented molecules mimicking regular arrays of nanorails. Flat-lying molecules, aligned side-by-side with the long molecular axis along the [001] direction, form long crosstie chains extending in the [1 (1) over bar0] direction. In between the adjacent flat chains, additional molecules, tilted by 90 degrees around their molecular axis, line up head-to-tail into rails extending along [1<(1)over bar0]. These molecules are very weakly hybridized with the substrate, as indicated by their lowest unoccupied molecular orbitals, which closely resemble those of the free molecule. The nanorail structure is found to be stable up to 420 K in vacuum and to also remain in place after exposure to air, thus being a template well suited for further self-assembly of organic heterostructures. The tilted quasi-free molecules open the possibility for an optimal lateral pi-coupling to other molecules or molecular assemblies.
