Sante Cospito

Electrofluorochromism in π-conjugated ionic liquid crystals

Materials in which photoluminescence is modulated by redox processes are known as electrofluorochromic. Intrinsically switchable fluorophores, incorporating both redox and fluorescent moieties, could be ideal electrofluorochromic materials if they possess high fluorescence quantum yields in at least one of their redox states. Fluorescent liquid crystals with redox active centres could combine the above requirements with the advantage to work in bulk anisotropic phases. However, electrofluorochromic liquid crystals have not been reported yet because their synthesis is challenging due to aggregation-caused fluorescent quenching. Here we show the first examples of electrofluorochromic π-conjugated ionic liquid crystals based on thienoviologens. These ordered materials, combining ionic and electronic functions, are highly fluorescence in the bulk state (quantum yield>60%). Their direct electrochemical reduction leads to fast and reversible bulk electrofluorochromic response in both columnar and smectic phases allowing for fluorescence intensity modulation and colour tuning.

Electro-optical Properties of Neutral and Radical Ion Thienosquaraines.

Thienosquaraines are an interesting class of electroactive dyes that are useful for applications in organic electronics. Herein, the redox chemistry and electrochromic response of a few newly synthesized thienosquaraines are presented. These properties are compared to those of the commercial 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine. The stability of the radical ions formed in electrochemical processes strongly affects these properties, as shown by cyclic voltammetry, in situ spectroelectrochemistry, and quantum chemical calculations. Furthermore, all of the dyes show aggregation tendency resulting in panchromatic absorption covering the whole UV/Vis spectral range.

Switching from columnar to calamitic mesophases in a new class of rod-like thienoviologens

Here we report the synthesis and the liquid crystalline properties of a new class of thienoviologen salts of the dication series 4,4′-(2,2′-bithiophene-5,5′-diyl)bis(1-alkylpyridinium)++ (mV++s) having different alkyl-chain lengths (m = 9–11). As determined by DSC, polarized optical microscopy and X-ray diffraction analyses, the anion of the salt plays a crucial role in disclosing the liquid crystalline nature of the mV++s. It is shown that the bis(triflimide) salts, mV(NTf2)2s, exhibit a unique bimesomorphic behaviour, self-segregating into columnar rectangular and/or smectic phases. Despite the rod-like shape of these π-extended viologen dications, columnar phases are obtained by the self-assembly of disk-like structures formed by the association of dications into dimers through bridging interacting anions. Interestingly, the bimesomorphism allows “switching” from a columnar to a calamitic phase and vice versa by changing the temperature and/or the alkyl-chain length. In particular, as the length of the alkyl chains increases from m = 9, where a stable rectangular columnar mesophase is observed, to m = 10, an incipient calamitic mesomorphism takes place which manifests itself with the formation of a lamello-columnar mesophase. This ultimately evolves toward a smectic A phase for m = 11.

n-Type Columnar Liquid Crystal Combining Ionic and Electronic Functions

The research in the field of electronic and photonic devices based on ordered organic semiconducting materials has been boosted by the development of new multifunctional materials. The order characterizing liquid crystals has been exploited for improving charge mobility performances. However, while in the last decade much effort has been addressed to the synthesis of hole-conducting materials (p-type), less attention has been paid on the development of electron-conducting ones (n-type). Here we show the bulk multifunctional properties of a π-conjugated thienoviologen ionic liquid crystal which exhibits strong electron acceptor character. Notably, the combination of its optical properties with high ionic conductivities and fast intermolecular electron transport, leads to unique bulk electrochromic functions.

2D/3D switchable displays through PDLC reverse mode parallax barrier

ABSTRACT In this paper, we present a new type of parallax barrier, for switchable 2D/3D display vision, obtained by using a Reverse Mode Polymer Dispersed Liquid Crystal display (RV-PDLC). The parallax barrier was prepared by sandwiching the RV-PDLC film between two ITO conductive glass supports. Strips of ITO were removed from one of the supports, in order to alternate conductive to not conductive strips. In this way the RV-PDLC films could be electrically switched in a parallax barrier able to turn a 2D image in a 3D one. The RV-PDLC barrier was obtained in tree steps: (1) a nematic diacrylate monomer (NDM) was dissolved in a nematic liquid crystal (NLC) in the presence of a small quantity of radical polymerisation initiator; (2) the mixture was homeotropically aligned between the glass conductive supports, previously treated with an aligning chemical; (3) the liquid crystal monomer was polymerised by UV curing. A 2D/3D switchable device was finally obtained by coupling the parallax barrier with a 2D tablet display. GRAPHICAL ABSTRACT