Visible-Light-Induced Reversible Photochemical Crystal–Liquid Transitions of Azo-Switches for Smart and Robust Adhesives

Abstract

Photochemical crystal ↔ liquid transitions (PCLTs) are interesting phenomena that couple reversible photochemical transformations with thermophysical phase transitions. A potential application of PCLTs is the development of photoresponsive smart materials capable of exerting reversible adhesion capacities on specific surfaces at a desired timing, which are unattainable for conventional adhesives. However, PCLT-based adhesives generally use UV light as the stimulus, which could lead to degradation of materials and health problems. Here, visible-light-controlled smart and robust adhesives are developed using small-molecule azo photoswitches. These azo molecules can undergo very efficient trans-crystal → cis-liquid and cis-liquid → trans-crystal transitions under 405 and 532 nm light irradiations, respectively. Their trans-crystal state displays strong adhesion strengths on various substrates, e.g., 1.13 MPa on quartz/quartz and 1.58 MPa on wood/wood, and very fast light-induced separation of glued substrates can be accomplished within 1 s along with the loss of adhesion strength in the cis-liquid state. Robust switching of the adhesion strength is demonstrated in multiple cycles, and these adhesives can also work well in underwater environments. Visible-light-controlled reversible PCLTs can be a very promising strategy in the pursuit of high-performance photoresponsive adhesives.

Light-melting Diketone Crystal with ON-OFF-ON Luminescence Evolution

“…The proposed PCMT mechanism is based on a conformational change around the single bonds. This is distinct from the reported mechanisms based on the double-bond E/Z isomerization [7][8][9][10][11][12][13][14][15][16][17][18] or the cleavage/formation of the -bond, 19 which has a high energy barrier for the backward reaction in the ground state. The corresponding barrier for SO is not high (calculated to be 6.86 kcal/mol in vacuum, Figure S11).…”

“…Phase transitions, such as melting and crystallization, can induce drastic changes in the material properties. [1][2][3][4][5][6] In particular, the phenomenon of photoinduced crystal-melt transition (PCMT) [7][8][9][10][11][12][13][14][15][16][17][18][19] is extremely useful for applications such as photolithography, 11 thermal energy storage, [15][16][17][18] and light-melt adhesion. 12,13,20,21 However, the process, mechanism, and molecular design principles of PCMT remain largely unknown (Figure 1A).…”

“…[1][2][3][4][5][6] In particular, the phenomenon of photoinduced crystal-melt transition (PCMT) [7][8][9][10][11][12][13][14][15][16][17][18][19] is extremely useful for applications such as photolithography, 11 thermal energy storage, [15][16][17][18] and light-melt adhesion. 12,13,20,21 However, the process, mechanism, and molecular design principles of PCMT remain largely unknown (Figure 1A). Using synchrotron radiation, Hoshino et al elucidated the structural features of an azobenzene derivative that shows PCMT.…”

“…Using synchrotron radiation, Hoshino et al elucidated the structural features of an azobenzene derivative that shows PCMT. [8][9][10] However, PCMT has been observed only in three photochromic motifs: azobenzene, [7][8][9][10][11][12][13][14][15][16][17] hydrazone, 18 and spiropyran. 19 Moreover, owing to a lack of suitable methods/materials, there have been no studies on the real-time and molecular-level observation of the disordering PCMT process.…”

Photoswitchable Adhesives of Spiropyran Polymers

“…Meanwhile, as represented by marine plastic pollution, global environmental problems are worsening, and material recycling is strongly required. − In this context, switching of adhesives between strong bonding during use and on-demand debonding by external stimuli is a key technology to achieve both development of complex architectures and material recycling. , In particular, light is an ideal stimulus that can minimize effects on adherends due to the high spatiotemporal resolution and easy, precise control of the intensity and wavelength . To date, photoswitchable adhesives have been developed by utilizing a variety of photoresponsive systems including large structural changes of azobenzenes, − conjugation changes of diarylethenes, , dimerization of anthracenes, − photothermal effects of supramolecular motifs and dyes, − and other reactions. − Reversibly switchable polymer adhesives are particularly attractive because of their strong adhesion and recyclability. Similarly, large polarity changes in the photoisomerization between noncharged spiropyran (SP) and zwitterionic merocyanine (MC) seem highly suitable for photocontrol of adhesion (Figure a) , because molecular polarity and related interactions between adhesives and substrate surfaces are dominant factors to generate adhesion force. − The dipole moment (μ) drastically changes between 4–6 D for SP and 14–18 D for MC. , However, to the best of our knowledge, SP has been only used as additives in polymer adhesives ,, or as low-molecular-weight adhesives .…”