A thermoresponsive structural change based on a disilane-bridged bis(pyridine) ligand and CuI is reported. Single-crystal X-ray analysis revealed that there are two polymorphs in the Cu(I) complex: octanuclear copper(I) complex at 20 °C and 1D staircase copper(I) polymer complex at À 173 °C. The formation of these polymorphs is due to the flexibility of the ligand. CuÀ I bond formation is observed upon cooling the sample from À 10 °C to À 170 °C. The temperature-induced phase transition progression was clarified by DSC, VT-PXRD, and VT-photoluminescence measurements and indicated a reversible temperature-controlled crystal-to-crystal phase transition. Observation on a VT-stage using a high-speed camera showed crystal cracking during single-crystal to single-crystal transitions between these polymorphic forms.
Stimuli-responsive aggregation of polymer chains in water has found a variety of applications in polymer science, biology, and chemical engineering. To date, the majority of the phase transitions between the aggregated and dissolved forms has been observed by changing the solution temperature, and an active and precise control on the phase transition with a high time resolution has been challenging. Herein, a reversible phase transition of poly(allylamine-co-allylurea) (PAU) in an aqueous electrolyte is achieved by electrochemical redox cycling of hexacyanoferrate(II/III) ([Fe(CN) 6 ] 4−/3− ) ion pair. The aggregation and dissolution cycle can be completed in a high-resolution time frame of as short as 5 s. The strong electrostatic interaction between the protonated primary amino group of PAU and the tetravalent [Fe(CN) 6 ] 4− anion induces the aggregation, while the oxidation to the trivalent [Fe(CN) 6 ] 3− anion reduces the attractive force, and the polymer chain redissolves in solution. The ureido group of PAU helps the chain-folding process through the formation of inter/intrachain hydrogen-bonding networks, resulting in the sharp phase transition. By using [Fe(CN) 6 ] 4−/3− as the electron mediator, the electrochemical control on the large transparency change of polymer aqueous solution is realized for the first time.
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