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Solar energy is a renewable and natural alternative to fossil fuels. In order to efficiently use solar energy, photothermal conversion techniques and materials have been intensively investigated, which are gaining enormous interest in the area of energy conversion and storage. In solar light, 51% of the solar energy concentrates in the visible region and 43% in the infrared (IR) region. Broadening the absorption of photothermal materials is considered as an effective strategy to improve the light utilization efficiency. In this regard, many types of materials have been successfully exploited to achieve broad absorption both in visible and IR regions. However, few photothermal polymers are non-conjugated, as the conjugated polymers would have a low band gap for increasing non-irradiated emission to generate thermal energy. In this paper, we formulated an iodine doping process to endow a non-conjugated polymer complex with outstanding light-absorbing ability. This polymer solution was crosslinked with iodine into an organogel and well developed in the casting and coating areas. Several characterization techniques were carried out to explore the mechanism of the crosslinking process. The resulting polymeric non-conjugated photothermal materials showed pronounced photothermal conversion ability in both visible and IR regions and high stability up to 381 °C.

Great attention is being increasingly paid to photothermal conversion in the near-infrared (NIR)-II window (1000–1350 nm), where deeper tissue penetration is favored. To date, only a limited number of organic photothermal polymers and relevant theory have been exploited to direct the molecular design of polymers with highly efficient photothermal conversion, specifically in the NIR-II window. This work proposes a fused backbone structure locked via an intramolecular hydrogen bonding interaction and double bond, which favors molecular planarity and rigidity in the ground state and molecular flexibility in the excited state. Following this proposal, a particular class of NIR-II photothermal polymers are prepared. Their remarkable photothermal conversion efficiency is in good agreement with our strategy of coupling polymeric rigidity and flexibility, which accounts for the improved light absorption on going from the ground state to the excited state and nonradiative emission on going from the excited state to the ground state. It is envisioned that such a concept of coupling polymeric rigidity and flexibility will offer great inspiration for developing NIR-II photothermal polymers with the use of other chromophores., Low bandgap and large deformation generally conflict each other. This work couples molecular rigidity and flexibility by intramolecular hydrogen bonds and double bonds to achieve NIR-II light absorption and reinforced internal conversion at the same time.

Summary Effective thermal regulation has shown great impacts on tremendous aspects of our life and manufacture. However, the invisible nature of thermal field brings us inconvenience or even security hazard. Herein, we present a method to visualize thermal distribution with the aid of a thermally active material. An ionic liquid with lower critical solution temperature is mixed within hydrogel to demonstrate a hydrogel confined ionic system (HCIS). This particular system turns turbid as the temperature exceeds an established temperature threshold, which is adjustable through applying different concentrations of HCl or NaCl. The system offers straightforward images of the spatial thermal distribution whether simple or sophisticated, which is fully in line with computational simulation. The system is further demonstrated with great promise for the application in fire warning to lower the threat induced by electrical failure. The HCIS opens a practical avenue to visualize thermal distribution and improve our thermal regulation efficiency., Graphical abstract, Highlights • Thermal field distribution can be visualized without professional apparatus • The LCST of ionic liquid can be finely tuned • The mechanism of the LCST adjustment is studied • A fire warning system is established to reduce the threat of fire hazard, Energy Systems; Thermal Engineering; Thermal Property