A Remarkable Multitasking Double Spiropyran; Bidirectional Visible Light Switching of Polymer Coated Surfaces with dual Redox and Proton Gating

Smart or functional surfaces that exhibit complex multi-modal responsivity, e.g., to light, heat, pH etc., although highly desirable, require a combination of distinct functional units to achieve each type of response, and present a challenge in achieving combinations that can avoid cross-talk between the units, such as excited state quenching. Compounds that exhibit multiple switching modalities help overcome this challenge and drastically reduce the synthetic cost and complexity. Here, we show that a bis-spiropyran photochrome, which is formed through coupling at the indoline 5-position using redox chemistry, exhibits pH gated photochromism, with opening of the spiro moiety by irradiation with UV light and the expected reversion both by heating and irradiation with visible light gated by protonation/deprotonation. Remarkably, when oxidized to its dicationic form, bis-spiropyran2+, visible light can be used instead of UV light to switch between the spiro and merocyanine forms, with locking and unlocking of each state achieved by protonation/deprotonation. The formation of the bis-spiropyran unit by electrochemical coupling is exploited to generate 'smart surfaces', i.e. polymer modified electrodes, avoiding the need to introduce an ancillary functional group for polymerization and the concomitant potential for cross-talk. The approach taken means that the multi-repsonsive properties of the bis-spiropyran are not only retained upon immobilization, but in fact that the effective switching rate can be enhanced dramatically.