Design and fabrication of organic and polymeric soft materials exhibiting solvent-free isothermal changes in mechanical properties triggered by photostimulation are challenging. Here, we report dramatic changes in viscoelasticity of coumarin end-functionalized poly(dimethyl siloxane) with a branched macromolecular architecture under a solvent-free isothermal condition upon alternating irradiation of UV at the wavelength of 365 and 254 nm (UV355 and UV254). Significantly, it is disclosed that storage modulus (G') was increased by a factor of more than 30000. Moreover, the subsequent UV254 irradiation caused sharp decrease in G' and it became comparable to loss modulus. Furthermore, alternating irradiation of UV365 and UV254 caused repetitive increase and decrease in G' of the material. (C) 2018 Elsevier Ltd. All rights reserved.