Imidazolium based ionic liquid crystals: structure, photophysical and thermal behaviour of [Cnmim]Br·xH2O (n = 12, 14; x=0, 1)

✍ Scribed by A. Getsis; A.-V. Mudring

• Publisher: John Wiley and Sons
• Year: 2008
• Tongue: English
• Weight: 779 KB
• Volume: 43
• Category: Article
• ISSN: 0232-1300
• DOI: 10.1002/crat.200800345

✦ Synopsis

The long chain imidazolium halides [C n mim]Br•xH 2 O (n = 10, 12; x = 0, 1) have been synthesized and their structural and thermal behaviour together with their photophysical properties characterized. X-ray structure analyses of the monohydrates ([C 12 mim]Br•H 2 O: triclinic, P1, no. 2, Z = 2, Pearson code aP112, a = 550.0(5) pm, b = 779.4(5) pm, c = 2296.1(5) pm, α = 81.89(5)°, β = 83.76(5)°, γ = 78.102(5)°, 3523 unique reflections with I o > 2σ(I o ), R 1 = 0.0263, wR 2 = 0.0652, GooF = 1.037, T = 263(2) K; [C 14 mim]Br•H 2 O: triclinic, P1, no. 2, Z = 12, Pearson code aP11, a = 549.86(8) pm, 782.09(13) pm, c = 2511.3(4) pm, α = 94.86(2)°, β = 94.39(2)°, γ = 101.83(2)°, 2063 unique reflections with I o > 2σ(I o ), R 1 = 0.0429, wR 2 = 0.0690, GooF = 0.770, T = 293( 2) K) show for both compounds similar bilayered structures. Sheets composed of hydrophilic structure regions constituted by positively charged imidazolium head groups, bromide anions and hydrogen bonded water alternate with hydrophobic areas formed by interdigitated long alkyl chains belonging to imidazolium cations with different orientation. Combined differential scanning calorimetry and polarizing optical microscopy shows that the monohydrates as well as the anhydrous imidazolium salts are thermotropic liquid crystals which adopt smectic mesophases. The mesophase region is larger in case of the monohydrates when compared to the anhydrous compounds indicating that water obviously stabilizes the mesophase. All compounds show an intense whitish photoluminescence with short lived ( 1 π← 1 π*) and long lived ( 1 π← 3 π*) transitions.

Dedicated to Prof. Ladislav Bohatý on the occasion of his 60 th birthday