Going through a phase: ICs of azobenzene derivatives show a photoinduced IC–IL phase transition (photoliquefaction) upon UV irradiation, and the resulting cis-azobenzene ILs are reversibly photocrystallized by illumination with visible light. The photoliquefaction of ICs is accompanied by a significant increase in ionic conductivity at ambient temperature and holds potential as energy storage materials.

Ionic Liquids ▣ ‘‘designer solvents’’ ▣ liquids that consist entirely of ions Commonly used criterion melt below 100 oC become popular Easier to handle than salts that melt at elevated temperatures Many ionic liquids even melt below room temperature. ▣ ‘‘designer solvents’’ Finely-tuned by the independent selection of cation and anion to be the optimum solvent for a given application Energy-related applications, self assembly, interfacial phenomena Energy Environ. Sci., 2014, 7, 232–250

Photoisomerizable ILs No IC-IL phase transition L. C. Branco and F. Pina, Chem. Commun., 2009, 6204 A. Kawai et al, Chem. Lett. 2010, 39, 230 nematic ionic LC S. Zhang et al, Chem. Commun., 2011, 47, 6641 Non-ionic photoinduced solid-liquid phase transition; not related to photon energy storage - Akiyama et al, Adv. Mater. 24, 2353 - Norikane et al, Nat. Commun., 2015, DOI: 10.1038/ncomms8310 L. Wu et al, Chem. Commun., 2009, 5269

Recent expectations for ILs to serve as heat storage materials - Controlling their physical properties and functions based on the phase-transition phenomena between the ionic crystal (IC) phase and the IL phase. - The photoisomerization of azobenzenes to a high-energy metastable cis isomer -> allows storage of photon energy in the form of molecular strain energy (ca. 50 kJ/mol) -> released as heat in the course of the cis–trans thermal isomerization. ‘closed-cycle solar fuels’ ; absorbing photons during charging and releasing only heat during discharge.

Converting solar energy into chemical potentials for use in solar thermal batteries the stored energy is released as heat when the strained isomer reverts to its stable analog in response to an external trigger. major problems: -> exothermicity of 50 kJ/mol for cis-azobenzene is comparable or even smaller than the minimum gravimetric energy density desired for thermal storage materials (ca. 100 J/g) -> trans–cis photoisomerization of azobenzene chromophores is significantly suppressed within crystals

-> only studied in solutions; photoisomerization in solution inevitably results in significant decrease in the volumetric energy density. Energy Environ. Sci., 2011, 4, 4449 Bull. Chem. Soc. Jpn., 1976, 49, 1381 -> hybrid solar thermal fuel using azobenzene-functionalized carbon nanotubes, Timothy J. Kucharski et al, Nature Chem 2014 Energy storage as a function of template-packing parameters on composite bundling, the amount of energy stored per azobenzene more than doubles from 58 to 120 kJ/mol, molecule–nanostructure hybrids can exhibit significantly enhanced energy-storage capabilities through the generation of template-enforced steric strain

first example of photoresponsive ICs which show reversible photoliquefaction into ILs Oligoethylene oxide units introduced in the ammonium group Enhanced delocalization of anionic charge on the Tf2N ion -> Weakened electrostatic interaction Lowered the MP; IL at ambient temp Weakening van der Waals interaction Pi-pi stacking

Synthetic scheme

Photo-induced liquefaction to the isotropic IL Mol. Ratio of cis isomer; ~ 80 %

Multilamellar nanostructure with a long period of 44.5~45.3 Å; bilayer membrane as unit structure 26 Å; tilted and interdigitated orientation Spacer/tail length depedant

Photon energy storage capacity from photo-induced IC-IL phase transition system at 46 oC; crystallization of trans IL At ~49 oC, exo. - Tf2N Pure cis isomer; ΔH of 51.8 KJ/mol-> consistent with the reported energy stored in a cis-azo chromophore (A. M. Kolpak, J. C. Grossman, Nano Lett. 2011, 11, 3156) - Specific energy required for thermophysical storage (specific energy requirement of 100 J/g); 75.7 kJ/mol

(b) Growth of crystalline phase-> completion of thermal isomerization

AC impedance measurement- photoinduced phase transition on ionic conductivity

2.1x10-5 S/cm 2.0x10-5 S/cm 1.2x10-7 S/cm 4.2x10-10 S/cm Two orders lower than the typical IL 1-methyl-4-butyl imidazolium Tf2N due to large viscosity 1.2x10-7 S/cm 280 times Enhanced mobility 4.2x10-10 S/cm Suppressed migration of ionic species Br-; lower self diffusion coefficient of bromide ion

conclusion - ICs formed by azobenzene derivatives showed photoliquefaction into ILs. The photoliquefaction approach significantly enhances the photon energy storage capacity of azobenzene chromophores. Ionic conductivity was prominently increased concurrently with the photoliquefaction. To the best of their knowledge, this is the first example of reversible photoliquefaction and crystallization of ICs.