A series of MIDA (N-methylimino diacetic acid) boronates carrying 4-alkoxy, 3,4-bisalkoxy, or 3,4,5-trisalkoxyphenyl substituents were synthesized and their mesomorphic properties characterized by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) techniques such as small- and wide-angle X-ray scattering (SAXS and WAXS, respectively). Most derivatives were liquid crystalline. In the case of mono- and bisalkoxy-substituted derivatives, C chains already induced smectic A (SmA) mesophases despite the bulky MIDA head group. With increasing chain length, columnar hexagonal (Col ) phases replaced SmA phases in the disubstituted series. Quantum chemical calculations on a series of MIDA boronates show that the B-N bond is a dative bond with a positive charge on the boron atom and negative charges on the nitrogen and oxygen atoms. In addition, no π-interaction between the aryl moiety and B-N bond was found, thus the mesogenic unit is electronically decoupled from the MIDA head group. These theoretical findings were supported by IR and Raman spectra as well as by asingle crystal structure analysis of 4-ethoxyphenyl MIDA boronate. Calculations of the electrostatic potential of the MIDA boronate reveal a special polarity pattern that can support the formation of a two-dimensional network and is likely to explain the liquid crystalline self-assembly. The absence of any electronic cross-talk between the MIDA head group and B-aryl or B-alkyl substituents allows the efficient tailoring of the mesophase type through variation of the substituents.
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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