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Version 1 2019-07-19, 23:03Version 1 2019-07-19, 23:03
Posted on 2019-07-25 - 19:16
In this work, the synthesis, X-ray
crystal structure analysis, studies on mesomorphism, and luminescence
spectroscopy of 2,7-diphenyl-4,5-diazafluorenones carrying 2, 4, or
6 peripheral alkoxy side chains of different lengths in the mesogenic
unit are reported. The latter was attached to starting 2,7-dibromo-4,5-diazafluoren-9-one
via Suzuki cross coupling of the respective 2-(alkoxyphenyl)-tetramethyl-1,2,3-borolane.
The liquid crystalline properties were found to depend on both the
number of side chains and their lengths. Derivatives with two short
alkoxy chains (C6–C8) in the periphery formed nematic (N) and
smectic C (SmC) mesophases, while for the higher homologues only SmC
mesophases were observed. Compounds carrying six alkoxy side chains
with a minimum length of C8 assembled into columnar hexagonal phases.
Homologues with C14 and C16 chain length displayed additionally two
columnar rectangular mesophases. Four peripheral alkoxy chains, however,
led to the loss of mesomorphism. Crystal structure data of a series
of rod-shaped diazafluorenones helped to rationalize their liquid
crystalline self-assembly and to understand geometry and thermal stability.
Temperature-dependent luminescence spectra of diazafluorenone with
two C12 chains revealed aggregation-induced emission (AIE) depending
on the supramolecular order of the bulk phase. These studies suggested
that AIE requires a lower degree of aggregation as compared to mesophase
formation.
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