Water-soluble Naphthalene Diimide-based electrochromic films

Murray, Nicholas Richard (2025) Water-soluble Naphthalene Diimide-based electrochromic films. PhD thesis, University of Glasgow.

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Abstract

Naphthalene diimides (NDIs) are a class of small organic molecules that exhibit electrochromic (EC) behaviour, undergoing strong, reversible colour changes in response to an applied electrical potential due to the formation of a radical anion. By modifying the NDI structure with different substituents, the optical and physical properties of the material can be tuned. Incorporating amino acid groups yields water-soluble NDIs with favourable electronic and EC properties, enabling device fabrication without the use of harmful organic solvents. However, their application has largely been limited to solution-based devices, which restricts broader implementation, making the development of solid-state NDI films a desirable goal.

We synthesised a series of water-soluble NDI derivatives with different amino acid substituents and studied how the side chains influenced solubility, self-assembly, and radical anion formation. As pH-dependent self-assembly is known to affect the performance of our materials, cyclic voltammetry (CV) and UV-vis absorption spectroscopy were used to assess the electronic and EC properties of the materials in solution and to identify the optimal pH for device construction. The NDIs were then combined with hyaluronic acid and processed into EC films using doctor blade coating, which were evaluated using CV, chronoamperometry, and absorption spectroscopy. Two of the films showed particularly strong performance, undergoing reversible colour changes upon reduction, and were selected for further investigation. To overcome the limitations of conventional spectroscopic methods, computer vision analysis software was used to monitor the EC response of the films in situ, offering a valuable advancement in the study of EC materials.

Next, we optimised the NDI films by altering the parameters of film construction. Four variables were investigated: solution formulation, film thickness, annealing time, and substrate resistivity. Each parameter was systematically adjusted and the behaviour of the film was then evaluated using CV, chronoamperometry, and absorption spectroscopy to assess redox behaviour. Nanoindentation was used to measure mechanical properties. The optimised films were used to construct flexible EC devices, which were tested following bending. Following this approach, we successfully produced films that underwent a reversible transparent-to-black colour change and remained mechanically robust after multiple bending cycles. Notably, these improvements were achieved without additional synthesis or film additives, offering a cost-effective and streamlined approach to film optimisation.

Finally, we explored the effect of different alkali metal counterions on the EC and mechanical properties of the NDI films. NDI solutions were prepared using various alkali hydroxides, and their self-assembly was investigated using small-angle neutron scattering. The choice of counterion was found to directly influence aggregation, with absorbance measurements indicating that these aggregates persisted into the solid state. Using CV, chronoamperometry, and absorption spectroscopy, we demonstrated that counterion selection significantly impacted the electronic and EC properties of the films, with larger counterions generally yielding improved performance, such as faster redox rates and greater colour changes. Mechanical properties were again evaluated via nanoindentation, where the choice of counterion was found to impact elasticity. These findings further highlight the tunability of amino acid-functionalised NDIs and support the continued development of high-performance, flexible EC devices.

Overall, this work demonstrates the potential of amino acid-appended NDI films for use in EC devices. We emphasise the tunability of our materials, with small alterations to film parameters allowing us to achieve films with properties similar to those of existing EC materials, while also maintaining processability in water. The work described herein presents an environmentally friendly and commercially viable approach to film construction, which we hope will guide future EC device development.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: Q Science > QD Chemistry
Colleges/Schools: College of Science and Engineering > School of Chemistry
Funder's Name: Engineering and Physical Sciences Research Council (EPSRC)
Supervisor's Name: Draper, Professor Emily
Date of Award: 2025
Depositing User: Theses Team
Unique ID: glathesis:2025-85387
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 13 Aug 2025 09:08
Last Modified: 13 Aug 2025 11:53
Thesis DOI: 10.5525/gla.thesis.85387
URI: https://theses.gla.ac.uk/id/eprint/85387
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