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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. [ABSTRACT FROM AUTHOR]

Azobenzenes are among the best-studied molecular photoswitches and play a key role in the search for red-shifted photoresponsive materials for extended applications. Currently, most approaches deal with aromatic substitution patterns to achieve visible light application, on occasion paired with protonation to yield red-shifted absorption of the azonium species. Appropriate substitution patterns are essential to stabilize the latter approach, as conventional acids are known to induce a fast Z - to E -conversion. Here, we show that steady-state protonation of the azo-bridge instead is possible in simple azobenzenes when the p K a of the acid is low enough, yielding both the Z - and E -azonium as supported by UV-vis- and 1 H NMR spectroscopy as well as density functional theory calculations. Moreover, the steady-state protonation of para -methoxyazobenzene, specifically, yields photoisomerizable azonium ions in which the direction of switching is essentially reversed, that is, visible light produces the out-of-equilibrium Z -azonium. Although the current conditions render the visible light photoswitch unsuitable for in vivo and material application, the demonstrated understanding of simple azobenzenes paves the way for a great range of further work on this already widely studied photoswitch.

The bistability of molecular switches is an essential characteristic in their use as functional components in molecular-based devices and machines. For photoswitches, light-driven switching between two stable states proceeds via short-lived changes of the bond order in electronically excited states. Here, bistable switching of a ditertbutyl-substituted spiropyran photoswitch is instead demonstrated by oxidation and subsequent reduction in an overall four-state cycle. The spiropyran structure chosen has reduced sensitivity to the effect of secondary electrochemical processes such as H + production and provides transient access to a decreased thermal Z - E isomerization barrier in the one electron oxidized state, akin to that achieved in the corresponding photochemical path. Thus, we show that the energy needed for switching spiropyrans to the merocyanine form on demand, typically delivered by a photon, can instead be provided electrochemically. This opens up further opportunities for the utilization of spiropyrans in electrically controlled applications and devices.

The spiropyran family of photochromes are key components in molecular-based responsive materials and devices, e.g., as multiphotochromes, covalently coupled dyads, triads, etc. This attention is in no small part due to the change in properties that accompany the switch between spiropyran and merocyanine forms. Although the spiropyran is a single structural isomer, the merocyanine form represents a family of isomers ( TTT , TTC , CCT , etc.) and protonation states. Combining two spiropyrans into one compound increases the number of possible structures dramatically and the interaction between the units determines, which are impeded due to intramolecular quenching of excited states. Here, we show that the coupling of two spiropyran photochromes through their phenol units yields favorable interactions (crosstalk) between the components that provides access to species inaccessible with the component monospiropyran alone. Specifically, the ring opening of one spiropyran unit, which is thermally stable at -30 °C, prevents ring opening of the second spiropyran unit. Furthermore, whereas protonated E - and Z -monomerocyanines were previously shown to undergo thermal- and photo-equilibration, the corresponding protonated E - and Z - bimerocyanines are thermally stable and show one-way photoisomerization from the Z , Z - to an emissive E , E -bimerocyanine form. Subsequent deprotonation at room temperature resets the system to the bispiro ring-closed form, but deprotonation at -30 °C yields the otherwise inaccessible bimerocyanine form. This form is photochemically inert but undergoes a two-step thermal relaxation via the merocyanine-spiropyran form, showing that the connection at the phenol units provides sufficient intramolecular interaction to fine-tune the complex isomerization pathways of spiropyrans and demonstrating noncommutability in photo- and pH-regulated multistep isomerization pathways.

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.

The coupling of substituted carbazole compounds through carbon-carbon bond formation upon one-electron oxidation is shown to be a highly versatile approach to the formation of redox polymer films. Although the polymerization of single carbazole units has been proposed earlier, we show that by tethering pairs of carbazoles double sequential dimerization allows for facile formation of redox polymer films with fine control over film thickness. We show that the design of the monomers and in particular the bridging units is key to polymer formation, with the diaminobenzene motif proving advantageous, in terms of the matching to the redox potentials of the monomer and polymer film and thereby avoiding limitations in film thickness (autoinsulation), but introduces unacceptable instability due to the intrinsic redox activity of this moiety. The use of a diimide protecting group both avoids complications due to p -diamino-benzene redox chemistry and provides for a redox polymer in which the photoluminescence of the bis-carbazole moiety can be switched reversibly (on/off) with redox control. The monomer design approach is versatile enabling facile incorporation of additional functional units, such as naphthalene. Here we show that a multicomponent carbazole/naphthalene containing monomer ( APCNDI ) can form redox polymer films showing both p- and n- conductivity under ambient conditions and allows access to five distinct redox states, and a complex electrochromic response covering the whole of the UV/vis-NIR spectral region. The highly effective quenching of the photoluminescence of both components in poly- APCNDI enables detailed characterization of the redox polymer films. The poly- APCNDI films show extensive charge trapping, which can be read out spectroscopically in the case of films and is characterized as kinetic rather than chemical in origin on the basis of UV/vis-NIR absorption and resonance Raman spectroscopic analyses. The strong resonantly enhanced Raman scattering for the various oxidized and reduced states of APCNDI enables nondestructive "read-out" of the state of the polymer, including that in which charges are trapped kinetically at the surface, making poly- APCNDI highly suitable for application as a component in organic nonvolatile memory devices.

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.