Orthogonally Incorporating Dual‐Fluorescence Control into Gated Photochromism for Multifunctional Molecular Switching

Ion valence-gated photochromism of an aza-crowned diarylethene

A non-photochromic diarylethene 2o with an N-phenylaza-15-crown-5 was synthesized. When the nitrogen atom in the aza-crown ring was protonated, it became photochromic due to the prevention of a twisted intramolecular charge transfer (TICT). Although addition of a monovalent metal cation (Li+, Na+, K+, Rb+, Cs+, Cu+, Ag+) in acetonitrile could not stop the TICT so that it was not photochromic, the addition of a multivalent metal cation (Mg2+, Ca2+, Sr2+, Ba2+, Fe2+, Ni2+, Al3+, Sb5+) changed 2o to be photochromic due to the strong attraction of the lone pair on the nitrogen atom. In the presence of excess Cu2+, 2o was oxidized to be EPR-detectable 2o·+, which was thermally unstable as well as inert towards visible-light irradiation. However, 2o·+ was further oxidized to be fairly stable 2o2+ by the irradiation of 365-nm light in the presence of Cu2+. ESI-MS measurements strongly suggested the generation of 2o·+ by mixing 2o with Cu(ClO4)2 in acetonitrile, and the transformation of 2o·+ to 2o2+ by successive 365-nm light irradiation. Fe3+ similarly worked as the oxidant, but the two-step oxidation of 2o to 2o2+ occurred more easily.

A Highly Selective Hg2+ Fluorescent Chemosensor Based On Photochromic Diarylethene With Quinoline Unit

A novel diarylethene-based fluorescent chemosensor containing a quinoline unit (1o) had been designed and synthesized. 1o showed good photochromic ability and fluorescence switching properties by alternating UV/vis light irradiation. The chemosensor showed high "Turn-off" fluorescent selectivity for Hg²⁺ by competitive tests of the fluorescence reaction in the presence other ions in acetonitrile solution. The stoichiometry between the compound 1o and Hg²⁺ was 1:1 by Job's plot curve and HRMS analysis. In addition, the LOD for Hg²⁺ was calculated as 60 nM. The fluorescence emission can be back to the "Turn-on" state by adding EDTA. Based on these facts, a molecular logic gate that including four input signals (UV/vis and Hg²⁺/EDTA) and one output signal (fluorescent intensity at 491 nm) was designed.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

Extending the Legible Time of Light-Responsive Rewritable Papers with a Tunable Photochromic Diarylethene Molecule

Inkless printing based on rewritable papers has recently made great progress because it can improve the utilization rate of papers, which is of great significance for saving resources and protecting the environment. Among them, light-responsive rewritable papers (LRPs) are a hot research topic because light is clean, easily available, wavelength and intensity adjustable, and noncontacting. However, the photochromic material, as the imaging substance of LRPs, is easily affected by environmental conditions, resulting in insufficient time to read the information. In view of this, we designed and constructed an acid/base tunable diarylethene molecular system that can effectively adjust the photochromic properties by reversibly changing the electron density of the diarylethene photoreaction center through protonation and demonstrated its potential as an imaging material with a longer legible time. What makes us more satisfied is that the acidification can not only extend the legible time of carrying information but also bring a clear and stable absorption/fluorescence imaging dual mode, which can better reflect details and improve contrast. Therefore, we believe that this tunable photochromic diarylethene molecule is a potential imaging material for the development of new LRPs.

Enhancing the Operability of Photoexcitation-Controlled Aggregation-Induced Emissive Molecules in the Organic Phase

Controllable aggregation-induced emission luminogens (AIEgens) by photoexcitation can be conducted within a single solvent, thus opening new opportunities for preparing and processing smart materials. However, undesired side-reactions like photooxidation that can easily occur in the organic phase remain, limiting their applications. To enhance the operability of photoexcitation-controlled AIEgens (to specifically produce a phosphorescence characteristic) in the organic phase, in this work, we employ a typical prototype, hexathiobenzene, usually as the specific phosphorescent group, and investigate a series of physical and chemical factors, such as light intensity, dissolved oxygen content, and solvent polarity, to explore ways to control the photoexcitation-controllable AIEgens against the impurities from side-reactions. An organogel strategy was also developed to minimize interference factors and improve the practical application ability. We believe that the presented results provide new insights into the further development of the photoexcitation-based functional materials and the promotion of their practical usage.

Photoswitchable Fluorescent Self-Assembled Metallacycles with High Photostability

In this study, photoswitchable fluorescent supramolecular metallacycles with high fatigue-resistance have been constructed by coordination-driven self-assembly by using bithienylethene with dipyridyl units (BTE) as a coordination donor and a fluorescent di-platinum(II) (Pt-F) as a coordination acceptor. The photo-triggered reversible transformation between the ring-open and ring-closed form of the metallacycles was confirmed by ¹ H NMR, ³¹ P NMR, and UV/Vis spectroscopy. This unique property enabled a reversible noninvasive "off-on" switching of fluorescence through efficient Förster resonance energy transfer (FRET). Importantly, the metallacycles remained structurally intact after up to 10 photoswitching cycles. The photoresponsive property and exceptional photostability of the metallacycles posit their potential promising application in optical switching, image storage, and super-resolution microscopy.

A monomolecular platform with varying gated photochromism

In the development of modern high-performance photoelectric materials, the gated photochromic materials have attracted wide attention. However, the integration of varying signal regulations into gated photochromism to construct efficient photochromic materials is still an urgent necessity. Herein, we designed and synthesized a new gated photoswitching DTEP based on a Schiff base with a diarylethene core. The photochromic properties of compound DTEP can be regulated to different degrees by multiple stimuli, including UV/visible light, Cu²⁺ and Ni²⁺. The compound DTEP showed different response abilities to Cu²⁺ and Ni²⁺, due to the diverse complexation modes between DTEP and Cu²⁺ as well as Ni²⁺. The photochromic properties of compound DTEP could be inhibited completely by the introduction of Cu²⁺ to form a 1 : 1 complexation, while the weak gated photochromism could be found from the DTEP–Ni²⁺ complex in a 1 : 2 stoichiometry. Relying on such varying degrees of gated photochromic properties, a new molecular logic circuit was constructed to undertake complicated logical operations.

A strategy to realize varying degrees of gated photochromic properties by coordinating with different metal ions within one unimolecular system was devised to achieve the construction of a logic circuit for multi-functional molecular switching.

Proton-Gated Ring-Closure of a Negative Photochromic Azulene-Based Diarylethene

Proton-responsive photochromic molecules are attractive for their ability to react on non-invasive rapid optical stimuli and the importance of protonation/deprotonation processes in various fields. Conventionally, their acidic/basic sites are on hetero-atoms, which are orthogonal to the photo-active π-center. Here, we incorporate azulene, an acid-sensitive pure hydrocarbon, into the skeleton of a diarylethene-type photoswitch. The latter exhibits a novel proton-gated negative photochromic ring-closure and its optical response upon protonation in both open and closed forms is much more pronounced than those of diarylethene photoswitches with hetero-atom based acidic/basic moieties. The unique behavior of the new photoswitch can be attributed to direct protonation on its π-system, supported by 1H NMR and theoretical calculations. Our results demonstrate the great potential of integrating non-alternant hydrocarbons into photochromic systems for the development of multi-responsive molecular switches.