Nanostructured Liquid Crystals Combining Ionic and Electronic Functions

Abstract

New molecular materials combining ionic and electronic functions have been prepared by using liquid crystals consisting of terthiophene-based mesogens and terminal imidazolium groups. These liquid crystals show thermotropic smectic A phases. Nanosegregation of the pi-conjugated mesogens and the ionic imidazolium moieties leads to the formation of layered liquid-crystalline (LC) structures consisting of 2D alternating pathways for electronic charges and ionic species. These nanostructured materials act as efficient electrochromic redox systems that exhibit coupled electrochemical reduction and oxidation in the ordered bulk states. For example, compound 1 having the terthienylphenylcyanoethylene mesogen and the imidazolium triflate moiety forms the smectic LC nanostructure. Distinct reversible electrochromic responses are observed for compound 1 without additional electrolyte solution on the application of double-potential steps between 0 and 2.5 V in the smectic A phase at 160 degrees C. In contrast, compound 2 having a tetrafluorophenylterthiophene moiety and compound 3 having a phenylterthiophene moiety exhibit irreversible cathodic reduction and reversible anodic oxidation in the smectic A phases. The use of poly(3,4-ethylenedioxythiophene)-poly(4-styrene sulfonate) (PEDOT-PSS) as an electron-accepting layer on the cathode leads to the distinct electrochromic responses for 2 and 3. These results show that new self-organized molecular redox systems can be built by nanosegregated pi-conjugated liquid crystals containing imidazolium moieties with and without electroactive thin layers on the electrodes.

Electrofluorochromism in π-conjugated ionic liquid crystals

“…Here an electrochromic response in the bulk LC phases is observed in electron acceptor p-conjugated ionic liquid crystals (ILCs), while a similar behaviour has been reported only by Kato and colleagues for hole-conducting ILCs self-segregating in SmA phases 26,27 .…”

“…As previously suggested by Kato and colleagues, electrochromism in p-conjugated ILCs is possible because of the coupling of electronic and ionic functions in the nanosegregated structure of ILCs 26,27 . Electron injection at the cathode causes the reduction of the thienoviologen dication and the reduced species is transferred to the anode by electron hopping via p-p intermolecular interaction through the film.…”

“…Supplementary Figure 3 displays a representative impedance spectrum of 1 at 120°C where a high frequency relaxation process ( relaxation frequency B400 Hz) and an incomplete low frequency one can be seen. The first process is attributed to the bulk resistance of the material (R b ) while the latter is associated with the charge transfer resistance of the interface between sample and electrode [26][27][28][29] . Ionic conductivity (s) was calculated using the equation (1):…”

“…where K cell is the cell constant given by the ratio between cell gap and electrode area [26][27][28][29] . R b was obtained from the semicircle diameter on the real axis (Z 0 ) ( Supplementary Fig.…”

Integration of electro-active π-conjugated units in nanosegregated liquid-crystalline phases

“…Phenylterthiophene derivatives bearing an imidazolium unit exhibit nanosegregated smectic phases in which hole-transporting redox-active layers and ion-conductive layers are integrated separately. 25,26 As shown in Figure 1, the DC bias application induces the formation of an electrical double layer that promotes hole injection from the anode to the redox active layers. This coupling of the electronic function of the π-conjugated units with the ionic conductivity causes anodic oxidation, resulting in electrochromism without an electrolyte solution.…”