Synthesis, Rotational Dynamics, and Photophysical
Characterization of a Crystalline Linearly Conjugated Phenyleneethynylene
Molecular Dirotor
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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
(<b>1</b>), 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 <b>1</b> 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 <sup>2</sup>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