Structure of a monolayer of molecular rotors on aqueous subphase from grazing-incidence X-ray diffraction

Abstract

In situ grazing-incidence X-ray scattering shows that a monolayer of artificial rod-shaped dipolar molecular rotors produced on the surface of an aqueous subphase in a Langmuir trough has a structure conducive to a 2D ferroelectric phase. The axes of the rotors stand an average of 0.83 nm apart in a triangular grid, perpendicular to the surface within experimental error. They carry 2,3-dichlorophenylene rotators near rod centers, between two decks of interlocked triptycenes installed axially on the rotor axle. The analysis is based first on simultaneous fitting of observed Bragg rods and second on fitting the reflectivity curve with only three adjustable parameters and the calculated rotor electron density, which also revealed the presence of about seven molecules of water near each rotator. Dependent on preparation conditions, a minor and variable amount of a different crystal phase may also be present in the monolayer.grazing-incidence X-ray scattering | X-ray reflectivity | molecular rotors | aqueous-surface monolayer | synchrotron radiation W e have been examining 2D assemblies of dipolar molecular rotors in an effort to detect collective behavior (1). Ultimately, we hope to produce an artificial 2D ferroelectric phase of dipolar azimuthal molecular rotors located on a flat electrical insulator, both for fundamental investigations and for its possible applications in nanoscience.To meet this goal, theory (2) suggests that the rotors should be assembled in a trigonal lattice. The surface assembly is expected to be ferroelectric between the Debye temperature T D , below which rotational barriers prevent the rotors from turning, and the Curie temperature T C , above which thermal disorder dominates. The former condition calls for small rotational barriers, no higher than 1-2 kcal/mol [in 3D assemblies, rotational barriers as low as 0.7 kcal/mol have been achieved (3)]. The latter condition requires large rotatable dipoles μ spaced a small distance a apart, since T C is expected to be proportional to μ 2 /a 3 . After dealing with surface inclusions, in which molecular rotors were contained on the surface of a host crystal, and detecting ferroelectric interactions but no ferroelectric phase in bulk inclusions (4), we are now also exploring monolayers produced on aqueous surfaces using a Langmuir-Blodgett (LB) trough (5) and molecular rotors designed to assemble into a trigonal lattice (6). The hope is that the rotor axes will be perpendicular to the surface and separated by a small lattice constant (a < 1 nm) and that large monocrystalline domains can be produced. Ultimately, a monolayer of a suitable structure is to be transferred from the aqueous surface to a solid substrate for further study, possibly only after cross-linking for increased sturdiness.Many organic materials pack closely at the air-water interface. Their unit cells tend to be tilted and distorted (7-19), but some nearly perfectly straight structures have also been observed (20). We were inspired by close-packed LB films of fatty acids, which hav...

Triptycene‐Based Molecular Rods for Langmuir‐Blodgett Monolayers

“… [13,14] . They have been thoroughly investigated not only i) in solution [3–5,13–15] and ii) in solid‐state as ordered 3‐D materials, [16–22] but also on iii) interfaces (solid‐liquid, solid‐gas, liquid‐gas), mostly in the form of regular 2‐D arrays [6,10,23–29] . In particular, using interfaces is highly attractive as it combines the ability of molecular machines to freely operate, which is characteristic for systems in solution, with a high order typical for crystalline materials.…”

Synthesis of dipolar molecular rotors as linkers for metal-organic frameworks

“…Equally interesting is the collective behaviour of ensembles of molecular rotors in two and three dimensions, i.e., on surfaces and in materials. Different strategies have been employed to prepare ordered arrays of rotors in two dimensions, such as the inclusion of rotors in channels on the surface of particular crystals [3] on metal surfaces [4–6] or in Langmuir–Blodgett films [7]. Several strategies have also been pursued to implement molecular rotors in the solid state.…”

Molecular Rods: Facile Desymmetrization of 1,4‐Diethynylbicyclo[2.2.2]octane

“…They are also essential motifs in systems designed for studying various physical phenomena such as singlet energy or electron, transfer. Efforts to construct regular two‐dimensional and three‐dimensional arrays of rotors frequently rely on the use of bridgehead substituted bicyclo[1.1.1]pentanes (BCP),, , , bicyclo[2.2.2]octanes (BCO),, and triptycenes as key building blocks. A few of the BCO‐based systems already showed ultra‐fast rotation, in the solid state and interesting phenomena such as correlated motion in pairs of neighboring rotators, as well as second harmonic generation responses due to local space‐inversion symmetry breaking in centrosymmetric crystals , …”