Red-light photoswitching of indigos in polymer thin films

Through simple synthetic derivatisation, the parent indigo dye becomes a red-light E-Z photoswitch exhibiting negative photochromism and tuneable thermal isomerisation kinetics. These attributes make indigo derivatives extremely attractive for applications related to materials and living systems. However, there is a lack of knowledge in translating indigo photoswitching dynamics from solution to solid state - the environment crucial for most applications. Herein, we study the photoswitching performance of six structurally distinct indigo derivatives in five polymers of varying rigidity. Three key strategies are identified to enable efficient photoswitching under red (660 nm) light: (i) choosing a soft polymer matrix to minimise its resistance toward the isomerisation, (ii) creating free volume around the indigo molecules through synthetic modifications, and (iii) applying low dye loading (<1% w/w) to inhibit aggregation. These strategies are shown to improve both photostationary state distributions and the thermal stability of the Z isomer. When all three strategies are implemented, the isomerisation performance (>80% Z form in the photostationary state) is nearly identical to that in solution. These findings thus pave the way for designing new red-light photochromic materials based on indigos.

Automated summary

By simple synthetic derivatisation, indigo dye can be converted into a red-light E-Z photoswitch, which exhibits negative photochromism and tuneable thermal isomerisation kinetics. This makes indigo derivatives highly attractive for materials and living systems applications. However, the translation of indigo photoswitching dynamics from solution to solid state, which is crucial for most applications, is not well understood. In this study, the photoswitching performance of six structurally distinct indigo derivatives in five polymers of varying rigidity is investigated. Three key strategies are identified to enable efficient photoswitching under red light: choosing a soft polymer matrix, creating free volume around the indigo molecules, and applying low dye loading to inhibit aggregation. Implementation of these strategies leads to isomerisation performance nearly identical to that in solution, paving the way for designing new red-light photochromic materials based on indigos.