Synthesis, Rotational Dynamics, and Photophysical Characterization of a Crystalline Linearly Conjugated Phenyleneethynylene Molecular Dirotor

Abstract

We report the synthesis, crystal structure, solid-state dynamics, and photophysical properties of 6,13-bis((4-(3-(3-methoxyphenyl)-3,3-diphenylprop-1-yn-1-yl)phenyl)ethynyl)-5,7,12,14-tetrahydro-5,14:7,12-bis([1,2]benzeno)pentacene (1), a molecular dirotor with a 1,4-bis((4-ethynylphenyl)ethynyl)benzene (BEPEB) chromophore. The incorporation of a pentiptycene into the molecular dirotor provides a central stator and a fixed phenylene ring relative to which the two flanking ethynylphenylene rotators can explore various torsion angles; this allows the BEPEB fluorophore dynamics to persist in the solid state. X-ray diffraction studies have shown that molecular dirotor 1 is packed so that all the BEPEB fluorophores adopt a parallel alignment, this is ideal for the development of functional materials. Variable temperature, quadrupolar echo (2)H NMR studies have shown that phenylene rotator flipping has an activation energy of 9.0 kcal/mol and a room temperature flipping frequency of ∼2.6 MHz. Lastly, with measurements in solution, glasses, and crystals, we obtained evidence that the fluorescence excitation and emission spectra of the phenyleneethynylene chromophores is dependent on the extent of conjugation between the phenylene rings, as determined by their relative dihedral angles. This work provides a promising starting point for the development of molecular dirotors with polar groups whose amphidynamic nature will allow for the rapid shifting of solid-state absorption, fluorescence, and birefringence, in response to external electric fields.

Artificial Molecular Machines

“…(b) X-ray structure of the dirotor 147 . X-ray crystal structure reprinted with permission from ref ( 1462 ). Copyright 2013 American Chemical Society.…”

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

“…Much progress has been made in this direction [28][29][30][31][32][33][34][35], and rotational barriers close to 1 kcal/mol have been achieved by choosing rotor molecules with small intrinsic barriers that pack in ways that minimize the contribution of intermolecular interactions to the resulting overall barriers. Only a few of these rotors were dipolar.…”

Anisotropic Thermal Expansion as the Source of Macroscopic and Molecular Scale Motion in Phosphorescent Amphidynamic Crystals

“…We and others have addressed it by exploring the emergent properties of amphidynamic crystals, which are built with a combination of lattice forming elements and moving parts . To date, examples have been reported of amphidynamic crystals with the potential of controlling gas adsorption and desorption, electric, optic, shape memory, and luminescence, among other properties …”