Arrays of Dipolar Molecular Rotors in Tris(o-phenylenedioxy)cyclotriphosphazene

Abstract

Regular two-dimensional or three-dimensional arrays of mutually interacting dipolar molecular rotors represent a worthy synthetic objective. Their dielectric properties, including possible collective behavior, will be a sensitive function of the location of the rotors, the orientation of their axes, and the size of their dipoles. Host-guest chemistry is one possible approach to gaining fine control over these factors. We describe the progress that has been achieved in recent years using tris (o-phenylenedioxy)cyclotriphosphazene as a host and a series of rod-shaped dipolar molecular rotors as guests. Structures of both surface and bulk inclusion compounds have been established primarily by solid-state nuclear magnetic resonance (NMR) and powder X-ray diffraction (XRD) techniques. Low-temperature dielectric spectroscopy revealed rotational barriers as low as 1.5 kcal/mol, but no definitive evidence for collective behavior has been obtained so far.

Time-Resolved Fluorescence Anisotropy of Bicyclo[1.1.1]pentane/Tolane-Based Molecular Rods Included in Tris(o-phenylenedioxy)cyclotriphosphazene (TPP)

“…Unfortunately, the signals of the TMS carbon atoms do not provide much additional information. Their broad signal is shifted downfield to 1.4 ppm, which is compatible with the notion that the TMS groups are all located outside the channel, but we know from other work 21 that the TMS group is not a good marker for detecting insertion inside a channel. Even if it is included, the chemical shifts of its methyl carbons sometimes undergo hardly any change relative to solution.…”

“… 18 An examination of the requirements for proper stopper action in TPP channels is continuing. 19 − 21 …”

Bulk Inclusions of Double Pyridazine Molecular Rotors in Hexagonal Tris(o-phenylene)cyclotriphosphazene

“…Preparation and study of regular two- and three-dimensional (2D and 3D) assemblies of molecular-level devices are attracting considerable attention as documented by many original articles, reviews, and book chapters published in recent years. − In general, 2D arrays of molecular devices are accessible using the Langmuir–Blodgett technique, − self-assembly on suitable (mostly metallic) substrates, − or formation of surface inclusions by host–guest interactions − where anchoring parts of molecular devices are inserted inside pores in the host lattice, while their functional parts protrude outside the surface. The 3D materials are usually made by (co-)crystallization of individual components − or by sorption of guest molecules into a well-organized porous host matrix, such as a MOF − or hexagonal tris­( o -phenylene)­cyclotriphosphazene (TPP, Figure ).…”

“…The internal diameter is 4.5–5 Å. Since the crystal is molecular, this diameter and the channel-to-channel spacing are capable of expanding by as much as 10% when the channels are filled with guest molecules. , …”

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 , …”