Abnormal fast dehydration and rehydration of light- and thermo-dual-responsive copolymer films triggered by UV radiation

Abnormal fast dehydration and rehydration of light- and thermo-dual-responsive copolymer films of poly(oligo(ethylene glycol) methyl ether methacrylate-co-6-(4-phenylazophenoxy)hexyl acrylate), abbreviated as P(OEGMA300-co-PAHA), are triggered by UV radiation. Both rapid kinetic processes are probed by in situ neutron reflectivity (NR). The transition temperatures (TTs) of P(OEGMA300-co-PAHA) are 53.0 (ambient conditions) and 52.5 °C (UV radiation, λ = 365 nm). Thin P(OEGMA300-co-PAHA) films show a random distribution of OEGMA300 and PAHA segments. They swell in a D2O vapor atmosphere at 23 °C (below TT) to a swelling ratio d/das-prep of 1.61 ± 0.01 and exhibit a D2O volume fraction φ(D2O) of 39.3 ± 0.5%. After being exposed to UV radiation for only 60 s, d/das-prep and φ(D2O) significantly decrease to 1.00 ± 0.01 and 13.4 ± 0.5%, respectively. Although the UV-induced trans-cis isomerization of the azobenzene in PAHA induces increased hydrophilicity, the configuration change causes a breaking of the intermolecular hydrogen bonds between OEGMA300 and D2O molecules and unexpected film shrinkage. As compared to thermal stimulus-induced dehydration, the present dehydration rate is 100 times faster. Removal of the UV radiation causes immediate rehydration. After 200 s, d/das-prep and φ(D2O) recover to their hydrated states, which is also 30 times faster than the initial hydration. At 60 °C (above TT), thin P(OEGMA300-co-PAHA) films switch to their collapsed state and are insensitive to UV radiation. Thus, the UV-induced fast dehydration and rehydration depend on the existence of hydrogen bonds.

A simplicity-guided cocktail approach toward multicolor fluorescent systems

A molecular cocktail containing two photochromic diarylethene derivatives that displays multicolor emission spanning blue-green to orange in a color-correlated fashion has been devised. The function does not rely on excited state communication such as energy transfer reactions, which is the typical case for similar systems. Instead, harnessing the intrinsic fluorescent and photochromic properties of the two individual diarylethene derivatives run in parallel is enough to realize the color changes. This offers an extremely flexible situation as for the choice of the fluorophores and their respective concentrations. The function is conveniently demonstrated in bulk solution at μM concentrations, where a single light source serves as the color changing stimulus.