Self-assembled solvato-morphologically controlled photochromic crystals

Abstract

Here we describe, for the first time, intriguing solvato-morphological control of spiropyran-based microcrystalline structures. These microstructures exhibit reversible photoisomerization upon light irradiation (UV/Vis) in the solid-state. Finally, light-guided aggregation of these microstructures at the liquid/air interface is also demonstrated.Self-assembly is omnipresent in nature as components of any size (from molecules to galaxies) may self-assemble under favorable conditions. 1,2 From the assembly of an ant colony to the folding of a polypeptide chain into a protein, self-assembly is inspiring scientists (chemists, physicists, biologists, and materials scientists) to employ this approach as a ''bottom-up'' tool to create intriguing structures for different applications. [3][4][5] Until now, most research in self-assembly has been focused on molecular components, but more recently, the development of supramolecular chemistry and the direction of technology toward micrometer-and nanometer-scale structures have broadened this focus to include assembly of larger structures. [2][3][4] In principle, self-assembly can provide a highly innovative solution to the challenges brought by the continued drive to reduce device feature scale 6 while also providing great potential for building complex 3D microstructures in a practical way. 3Besides molecular aggregation in solution, interfacial self-assembly has attracted great interest in recent years mainly because of the creative and unique principles of this method for forming complex structures, such as self-assembled monolayers, Langmuir-Blodgett films, nanocrystals, clusters, and capsules.7 Among the self-assembly architectures, a nano-or microstructures with photo-responsive properties is of particular interest due to important potential applications in optical memories, optical switches and displays. 8,9 It is becoming increasingly obvious that the next generation of these materials will incorporate intelligent design of externally addressable photo-responsive units with self-assembly capabilities, as these can produce highly pre-organized nano-or microstructures in a spontaneous manner under the right conditions.Among the various classes of photochromic compounds, spirobenzopyrans are one of the most studied, and have been successfully used in applications such as non-linear optics, data recording, optical and electrical switching, and chemical sensing, among others. 10 The photochromic behaviour of spiropyrans is due to the reversible heterolytic cleavage and rebinding of the C spiro -O bond upon irradiation with UV light and white light, respectively (Scheme 1). These characteristics offer the possibility to modulate or switch various functions at the molecular or supramolecular level using light as the trigger. Several assemblies of spiropyran (SP) derivatives and related photomodulation have been previously reported, indicating the great potential of merocyanine (MC) to be used as a self-assembly unit. Krongauz and coworkers have described the formation of M...

The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome

“…Switching functions have been shown to be restricted in thin films, monolayers and microstructures incorporating the spiropyran photochrome, and often an additional stimulus e.g., pH or temperature change, is needed. 31,105,160,161 When moving toward closely packed ensembles such as crystals and thick layers, however, the changes that need to occur collectively upon ring-opening are strongly hindered both spatially and ionically. By default, the solid state is not an ideal environment in which to undergo changes in their charge and spatial properties, let alone the drastic changes in shape (elongation and planarization) and charge (densely packed zwitterionic charges without a solvation sphere) the spiropyran-merocyanine system undergoes.…”

Nanoporous frameworks exhibiting multiple stimuli responsiveness

“…35). Such efficient photoisomerization at room temperature and under solvent-free conditions is, to our knowledge, unprecedented: although solid-state isomerization of other molecular photoswitches has been reported [36][37][38] , the significant conformational difference between the closed-and the open-ring isomers of spiropyran has effectively prohibited the long-sought [39][40][41] photoisomerization of this class of molecules in the solid state. The MC peak was clearly visible at B590 nm in the optical spectra recorded on suspensions of different PhotoPAFs; in addition, as Fig.…”

Porous self-protonating spiropyran-based NIPAAm gels with improved reswelling kinetics

“…Amplitude sweep measurements in Figure 2a show that the blank gel has a higher viscoelastic modulus (15)(16)(17)(18)(19)(20) kPa) compared to the 20k porous gel with a modulus of 6 -7 kPa and the least stiff 2k porous gel …”

“…The closed, colourless, hydrophobic form referred often as benzospiropyran and the open, coloured, charged merocyanine form. The molecules can be switched between these two forms by light irradiation (photochromism) [15][16][17], electrochemical stimulation (electrochromism) [17][18][19][20], or changing the solvent environment (solvatochromism). [17,21] As a result of these unique properties, spiropyrans have been used, in combination with many different materials, for the fabrication of both sensors, actuators and stimulus responsive materials.…”