Studies on the azobenzene derivative based phase transitions mostly rely on photoisomerization, which require a long time to spontaneously revert back. Here we show a photothermal-driven solid-to-liquid transition and fast reversion of azobenzene derivatives. Owing to the aggregation of suitably substituted azobenzenes, solid-to-liquid transitions can be induced by photothermal effects under irradiation with green light. The liquid-state azobenzene derivatives spontaneously solidify again within 2 min due to heat release in a purely physical fashion. One thus obtains a perfectly reversible adhesion with a strength as high as that of commercial materials. Our work affords a novel concept to construct reversible adhesives via phase transitions of organic compounds induced by light.

Gaseous formaldehyde (FA), a common indoor pollutant, presents a serious threat to human health. As an efficient tool for FA detection, fluorescent probes exhibit the advantages of low cost, ease of use, facile operation, etc. However, previously developed FA fluorescent probes are mostly based on fluorophores with aggregation-caused quenching features and thus require dispersion in solvent to detect FA. In this study, a fluorescent probe (TPE-FA) based on an aggregation-induced emission (AIE) fluorophore (tetraphenylethylene) has been developed for facile detection of gaseous FA through a fluorescence "turn-on" response. TPE-FA reacts with FA through 2-aza-Cope sigmatropic rearrangement. Based on the AIE features of TPE-FA, we fabricated a portable solid sensor, FA test plate, by directly loading TPE-FA on high performance thin-layer chromatography silica gel plate. The FA test plate achieved sensitive, selective, and quantitative detection of gaseous FA. The detection limit (0.036 mg/m 3 ) of the FA test plate is lower than the air quality guideline value of gaseous FA (0.1 mg/m 3 ) recommended by WHO. As a solid sensor for gaseous FA, the FA test plate based on AIE molecule is portable, which enables safer and more convenient use and transport compared to solution-based sensors.

Solid-state photochromic materials with reversible and adjustable optical properties are very appealing because of their wide prospects in advanced functional materials. Yet, it remains a significant challenge to develop such materials in the solid state. In this study, a tetraphenylethene derivative (SP-TPE-SP)-based solid-state photoswitch, which exhibits reversible photochromism in the solid state, was constructed. Efficient photoswitching between SP-TPE-SP and its photoisomer MC-TPE-MC is assisted by the large free volumes caused by the nonplanar molecular structures of the TPE1 moieties and the intramolecular π-π stackings between the two MC moieties. The free volumes are large enough to allow for the transport of HCl gas molecules for an acidochromic response. Furthermore, the morphology of the SP-TPE-SP solid surface can be regulated by ultraviolet light irradiation. The contact angles of the SP-TPE-SP solid surface can be decreased, changing from 97 to 82°. Therefore, SP-TPE-SP with a rather simple molecular structure is appealing for advanced multifunctional materials.

Formaldehyde (FA), as a reactive carbonyl species, is extremely hazardous to human health if its concentration is above normal level. In live cells, lysosome is a main organelle to generate endogenous FA. Thus, the design of facile, stable, and sensitive probes for the detection of FA in lysosome is essential. Herein, a self-assembled fluorescent nanoprobe based on homoallylamino substituted perylene (P-FA) has been developed for FA detection in lysosome. P-FA can react with FA along with emission color change from blue to green. P-FA exhibited high sensitivity and selectivity to FA in DMSO solution. In aqueous solution, P-FA self-assembled into uniform sphere-like nanoparticle as a fluorescent nanoprobe. Furthermore, the reaction between the nanoprobe and FA was greatly facilitated at pH 4–5, which led to a lower detection limit (0.96 μM at pH 5) than that in DMSO. In live cells, P-FA nanoprobe achieved long-term tracking of lysosome (over 12 h). The fluorescent nanoprobe was then used for both exogenous and endogenous FA detection. Our work provides a facile and effective strategy for the detection of FA in lysosome.