Flexible and Long-Lived Naphthalene Diimide-Based Photothermal Films with Rapid and Highly Cyclable Heat Conversion

Flexible photothermal materials have attracted considerable attention for applications in biomedicine, energy conversion, and smart devices. While state-of-the-art inorganic nanomaterials and conductive polymers exhibit excellent photothermal properties, simultaneously achieving efficient photothermal performance, environmental stability, and mechanical flexibility remains challenging. Naphthalene diimide (NDI)-based materials are well-known for their strong photothermal response resulting from the formation of radical anions. In this work, we employ glycerol as a plasticizer and humectant to improve film uniformity in NDI-based films, while the incorporation of PVA enhances long-term stability by slowing NDI oxidation and enabling mechanical tunability. Photothermal performance was evaluated using direct thermal imaging and Peltier voltage measurements. Under 617 nm irradiation at 16 mW cm^-2, the NDI-I films achieved a maximum temperature increase of 7.6 °C (pH 5) and a peak Peltier output voltage of 0.0052 V (pH 6). The addition of glycerol and PVA significantly improved film homogeneity and mechanical durability. The resulting PVA/NDI films show remarkable oxidative stability, retaining their reduced state for over 7 days compared to only about 3 h for pristine NDI films. Furthermore, the films demonstrate excellent photothermal cycling stability, with no observable performance degradation after more than 500 irradiation cycles. These results highlight the potential of tailored PVA/NDI composite films for long-lasting, flexible photothermal applications.