Materials Chemistry Frontiers Blog

Photochromic dyes are a specific class of molecules that can undergo reversible transformations under exposure to light between isomers that possess different optical properties. This peculiar feature makes them appealing for a myriad of applications, ranging from bioimaging, data storage, sensing or optical lenses.

Very recently, they have been employed in photovoltaics. Few photochromic dyes have been investigated as photosensitizers in dye-sensitized solar cells (DSSCs) and the only photochromes that have demonstrated a fully reversible photochromic process with polychromatic light once embedded in solar cells are diphenyl-naphthopyran derivatives, as reported by the group of Renaud Demadrille of CEA Grenoble in France and collaborators of the University Pablo de Olavide in Spain. Using this class of dyes, they demonstrated the fabrication of semi-transparent smart photovoltaic devices capable to self-adapt their transparency to the ambient light. Recently, they have designed and synthesized a series of spiro-indoline naphthoxazine (SINO) and spiro-indoline naphthopyran (NIPS) with a donor-photochrome-acceptor (D-p-A) chemical structure for their use in DSSCs.

Figure 1. General structure of the new dyes and the interconversion reaction between their close (CF), merocyanine (MC) and protonated merocyanine forms (MCH).

The interconversion process and optical properties of the two SINO and two NIPS dyes synthesized in this work have been proven more complex to unravel than the ones of diphenyl-naphthopyrans, as they are not only photochromic but also acidochromic. The interactions of the molecules with both stimuli, i.e. H⁺ and light, have been studied separately or simultaneously, in solution or after grafting onto the surface of TiO₂. A positive photochromism in solution characterized by extremely fast thermal discoloration kinetics has been observed for some of the compounds. All the dyes demonstrated acidochromic properties, but in the presence of acid, only NIPS derivatives showed a negative photochromism, i.e. a fast bleaching under illumination.

Figure 2. (a) Normalized discoloration curves of SINO-1, SINO-2 and NIPS-1 and (b) coloration curve of NIPS-1 after the addition of HCl and bleaching with light (2 x 10^-5 solutions at 25 °C, irradiation source: 200-600 nm/200 W xenon lamp).

After unraveling the photochromic and acidochromic properties of the new molecules, they were employed in the fabrication of dye-sensitized solar cells. The choice of the electrolyte was found to be critical, due to the pH-driven optical behavior of the dyes. If a low-pH electrolyte is used, a non-photochromic protonated open form was mainly produced, which was mostly avoided in the case of a neutral electrolyte. This work highlights that the photochromic properties of the dyes can be conserved when going from the solution to the devices for some of the dyes.

Figure 3.(a) J-V curves (dashed lines recorded in the dark, solid lines recorded under illumination) of opaque NIPS-2-based DSSCs using our acidic homemade electrolyte (black) and Iodolyte (red) and (b) transmittance spectrum of the transparent device together with a picture of the NIPS-2/Iodolyte device (standard irradiation conditions AM 1.5 G, 1000 W m⁻²; 25 °C; active area= 0.36 cm²).

This study is the first to investigate in detail the complex relationships between photochromic, acidochromic and photovoltaic properties for these classes of dyes. The structure-property relationships established will undoubtedly be useful for the development of new photochromic compounds with optimized optoelectronic properties for applications in various fields, including photovoltaics.

Corresponding authors:

Dr. José María Andrés Castán (Interdisciplinary Research Institute of Grenoble, CEA-Grenoble)

José María Andrés Castán is a postdoctoral fellow in the Molecular Systems and nanoMaterials for Energy and Health (SyMMES) at the Atomic and Alternative Energies Commission (CEA) in Grenoble, France. He received his PhD in Materials Science in 2018 at the Université d’Angers in France. His research is focused on the synthesis of photochromic dyes for their use in Dye-Sensitized Solar Cells. He is the author of more than 20 indexed publications.

Dr. Renaud Demadrille (Interdisciplinary Research Institute of Grenoble, CEA-Grenoble)

R. Demadrille is a team leader at the Atomic and Alternative Energies Commission (CEA) in France. He received his PhD in organic chemistry in 2000 from the University of Aix-Marseille with a grant from PPG Industries and Essilor International before to join the R&D department of an international chemical company to work on functional polymer materials. Then he moved to CEA as a postdoctoral fellow to develop semiconducting polymers for organic photovoltaics before being appointed in 2005 as a permanent researcher. His research focuses on the synthesis and the characterization of new pi-conjugated molecules and polymers for organic and hybrid photovoltaics and electronics. In 2018, he was recipient of the “Chemistry Energy” prize of the French Society of Chemistry, and in 2019, he was awarded of an ERC Advanced Grant to develop photochromic solar cells. Since 2020, he is Associate Editor of Journal of Materials Chemistry C and Materials Advances. He is the author of 8 patents, 2 book chapters and more than 90 articles indexed by SCI.

Controllable defects and interface engineering are perceived as promising routes to develop efficient noble-metal-free electrocatalysts for oxygen evolution reaction (OER), the bottleneck of overall water splitting. Recently Metal-organic frameworks (MOFs) have received particular attention for their remarkable OER performance because of their vast surface area, ability to tune porosity, and functionalization with mixed metals/ligands. Among various MOFs, Prussian blue analogs (PBAs) have been extensively studied for their potential in catalyzing the OER process. However, pristine PBA cubes suffer from low conductivity and exhibit insufficient OER activity, resulting in high overpotential and limiting their OER electrocatalytic applicability.

PBAs’ components, topologies, and surface engineering can be modified to improve their catalytic properties to address these limitations. One common structural alteration is the introduction of vacancies (defects) within the PBA cubes to facilitate ion diffusion, often accomplished through chemical etching that can change the local electronic configuration to boost the OER kinetics. There are only a few works on the selective etching of PBAs to enhance their performance; therefore, modification of the PBAs by the chemical etching process is a trending topic in OER electrocatalyst designing.

Recently, the Zidki’s research group at Ariel University highlighted the significance of combining the PBA etching effect with the decoration of molybdenum disulfide (MoS₂) on the edges and surfaces of Co-Fe PBA for catalyzing the OER kinetics. They proposed using an ethanol-water mixture as a mild etchant, eliminating the need for a capping or stabilizing agent.

Figure 1. (i) Synthetic scheme of Etched-PBA-MoS2 nanoframes. (ii) HR-SEM images of PBA nanocubes (a), Etched-PBA nanocages (c), and Etched-PBA-MoS2 nanoframes (e), and their corresponding TEM images (b,d, and f).

The Etched-PBA-MoS₂ nanoframes presented superior OER performance, requiring an overpotential as low as 260 mV on carbon cloth substrate to obtain the current density of 10 mA cm⁻² with a corresponding Tafel slope of 55 mV dec^-1 (Fig. 2a and b). The Etched-PBA-MoS₂ nanoframes also outperform with a lower charge transfer resistance (R_ct) value (smallest semicircle), indicating its faster charge-transfer kinetics (Fig. 2c). The catalyst retains its catalytic activity for a long-term stability test, proving its OER in a real-time application (Fig. 2d).

Figure 2. (a) LSV curves; (b) Tafel plots; (c) EIS-Nyquist plots measured at 1.5 V vs. RHE in 1.0 M KOH; (d) OER stability test of Etched-PBA-MoS2/CC.

This work demonstrates that the excellent electrocatalytic activity arises from two primary factors: (1) hollowing PBA nanocubes by the etching process increases the density of active sites to promote mass transport; (2) binding MoS₂ on the surface of PBA nanocages induces a synergistic effect – the electronic interactions among the active components tune the electronic structures of Co, Fe, and Mo sites. Their work renders a feasible pathway to optimize the etching effect and fasten different metal sulfide heterostructures on PBAs to achieve an excellent OER performance.

Corresponding author:

Dr. Tomer Zidki received his Ph.D. degree in 2010 from the Chemistry Department, Ben-Gurion University, Beer Sheva, Israel, in the field of radical reactions with nanoparticle catalysts. He pursued postdoctoral research at the Brookhaven National Laboratory, NY, USA, where he gained experience in redox catalytic processes. Dr. Zidki is an Assistant Professor in the Chemical Sciences Department, Ariel University, Israel, where he leads the Nanoparticle Catalysts group. He is also the Head of the Linear Electron Accelerator Facility for fast chemical reactions. Dr. Zidki has guided ten Ph.D. and three M.Sc. students. His research focuses on redox catalyzed reactions by nanoparticles and photo- & electrocatalytic water splitting reactions using non-precious catalysts. In addition, Dr. Zidki’s group studies the kinetic mechanisms of redox reactions and radical reactions using the electron accelerator. Another field of interest of Dr. Zidki is Environmental Chemistry, in which he wrote two patents on nitrogen and sulfur oxides removal from flue gases.