Photoactuation of the Molecular Crystals and the Composite Polymer Films Triggered by the Photochemical Topo-[2 + 2] Cycloaddition of Indole-Substituted Diarylpropenones

The photoinduced [2 + 2] photocycloaddition in solid states offers an important pathway for converting light to mechanical energy, and it has potential applications in wireless, remote-controlled actuators and soft robotics. In this work, the topo-[2 + 2] cycloaddition took place when the synthesized halogenindole-containing diarylpropenone derivatives were irradiated with 365 nm light. Notably, the quantitative conversion from 1-(5-fluoro-1H-indol-1-yl)-3-phenylprop-2-en-1-one (5FPEO) to the only single α-type photodimer was achieved, while the cycloaddition of 1-(5-chloro-1H-indol-1-yl)-3-phenylprop-2-en-1-one (5ClPEO), 1-(5-bromo-1H-indol-1-yl)-3-phenylprop-2-en-1-one (5BrPEO), 1-(6-chloro-1H-indol-1-yl)-3-phenylprop-2-en-1-one (6ClPEO), and 1-(5,6-dichloro-1H-indol-1-yl)-3-phenylprop-2-en-1-one (DiClPEO) yielded β-type cyclobutane derivatives due to the different molecular preorganization within the crystals. The photoinduced bending, twisting, and creaking of the molecular crystals were observed, and the cycloaddition was proposed as the driving force for the photomechanical effects. Significantly, when the microcrystals of the photoreactive indole-substituted diarylpropenones were composited in polymer films, we observed more pronounced and robust photomechanically responsive behaviors in air and in the water phase compared with the molecular crystals. It would broaden their application scenarios as adaptive and multifunctional photoactuators.