Spiropyrans have played a pivotal role in the emergence of the field of chromism following their discovery in the early 20th century, with almost ubiquitous use in materials applications especially since their photochromism was discovered in 1952.
Spiropyrans
undergo Cspiro–O bond breaking to
their ring-open protonated E-merocyanine form upon
protonation and irradiation via an intermediate protonated Z-merocyanine isomer. We show that the extent of acid-induced
ring opening is controlled by matching both the concentration and
strength of the acid used and with strong acids full ring opening
to the Z-merocyanine isomer occurs spontaneously
allowing its characterization by 1H NMR spectroscopy as
well as UV/vis spectroscopy, and reversible switching between Z/E-isomerization by irradiation with UV
and visible light. Under sufficiently acidic conditions, both E- and Z-isomers are thermally stable.
Judicious choice of acid such that its pKa lies between that of the E- and Z-merocyanine forms enables thermally stable switching between spiropyran
and E-merocyanine forms and hence pH gating between
thermally irreversible and reversible photochromic switching.
Adhesion
is one of the most ubiquitous practical applications at
surfaces. With today’s society calling increasingly for more
reusable and “green” alternatives, the demand for readily
reversible adhesives has triggered many studies into this field, in
particular by incorporating molecular photoswitches into composite
materials. Responsive polymers can act as reversible adhesives, but
their employment brings about synthetic drawbacks and challenges in
reproducibility and reusability. Here, our results demonstrate that
even a low molecular weight photoswitch can serve as an on-demand
adhesive when the intermolecular interactions are sufficiently strong.
We show that readily accessible arylazoisoxazoles display a fast photoreversible
solid-to-liquid phase transition and perform as excellent photoreversible
adhesives, with a remarkable durability over 10 immediate reuse cycles
without a loss in adhesive strength or an increase in the unprecedented
response time. Furthermore, the versatility of photoreversible adhesion
is shown at various surfaces ranging from polymeric materials to metals,
demonstrating a wide field of potential application.
Smart or functional surfaces that exhibit complex multimodal 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 by either heating or irradiation with visible light gated by protonation/deprotonation. Remarkably, when the photochrome is oxidized to its dicationic form, bis-spiropyran(2+), 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 not only that the multiresponsive properties of the bis-spiropyran are retained upon immobilization but also that the effective switching rate can be enhanced dramatically.
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