Photo-electrochromic materials: From Ionic Liquids Polymeric Ionic Liquids and Eutectic Mixtures to ionogel based devices.

Hugo CRUZ¹, Sara SANTIAGO², Noemi JORDÃO¹, Karolina ZALEWSKA¹, Sandra GAGO¹, Jordi HERNANDO², Gonzalo GUIRADO², Luis C. BRANCO^1
1LAQV-REQUIMTE-FCT-UNL, Lisbon, Portugal
²Departament de Química, Universitat Autònoma de Barcelona, , Bellaterra (Barcelona), Spain

Photo and electrochromic molecules are examples of stimuli-responsive materials very useful for applications in smart windows and functional systems [1]. First, we studied the photoelectrochromic properties of four diarylethene ionic liquids (DAE-ILs) composed of a diarylethene dicarboxylate (DAE) moiety, as counter-anion, and different organic cations ([C10mim]+, [C16Pyr] +, [N8,8,8,1]+, [P6,6,6,14]+)[2]. The synthetized ILs as viscous fluids maintain the photochemical and electrochemical properties of this well-known family of photoswitches, as well as the typical features of ILs (e.g. high thermal and electrochemical stability)[3] . In parallel, some polymeric photochromic Ionic Liquids (DAE-PILs) have been developed and studied by the suitable combination between diarylethene dicarboxylate (DAE) anion and different polymeric organic cations such as (poly)pyrolidinium and (poly)imidazolium). Finally, those DAE-ILs are used to formulate smart ionogels (IGs), a type of solid materials that, by showing good conductive properties and retaining the stimuli-responsive behavior of the embedded DAE molecules, allow the fabrication of photoelectrochemical devices. 
All prepared ionic materials and ionogels were characterized by spectroscopic techniques and further photo and electrochemical studies were also evaluated. Photochemical determination of the quantum yields in solution for the ring-opening and closure of [(C10)2Bpy][DAE-COO] ionic liquid have been determined. The electrochemical and electrochromism performance have been studied. [4] Besides, two photochromic deep eutectic solvents systems (DES) based on acidic diarylethene derivative (DAE), as hydrogen-bond donor (HBD) coupled with amines have been also prepared. Thermal, dielectric, photo, electroelectrochemical properties of these new DES have been also investigated. Both compounds were studied in their pure forms and in solution, dissolved in different DES (LiOTf:10Q, LiOTf:10TEA and LiOTf:10EG), depending on the case. The photo stationary state of closed and open rings as well as the electrochromic performance have been established. Thus, this work can open attractive perspectives for responsive photo and electrochromic materials and device applications.
Acknowledgments 
The authors are grateful for financial support from project PID2019- 106171RB-I00 from the Ministerio de Ciencia e Innovación (MICINN) of Spain. This work was also supported by the Associate Laboratory for Green Chemistry-LAQV (FCT/MCTES (UID/QUI/50006/2019)), Portuguese Foundation for Science and Technology/MCTES for the projects MIT-EXPL/CS/0055/2021 and PEst- C/LA0006/2013. The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project N° 022161 (co-financed by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC). Finally, H.C. thanks financial suport in the scope of the framework contract foreseen in the numbers 4, 5, and 6 of article 23 of the Decree-Law 57/2016 of 29 August, changed by Law 57/2017 of 19 July. 
REFERENCES
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[2] N. Jordão et al. ChemPhotoChem 2019, 3, 525 
[3] S. Santiago et al. Microchemical Journal, 2022, 182, 107870  
[4] H. Cruz et al. Electrochem. Sci. Adv. 2022,  e21002