Bidirectional π − π stacking for near-infrared photothermal effects and photo-thermo-electric conversion in a semiconductive hydroxamate coordination polymer

Photo-thermo-electric conversion is an emerging technology for utilizing solar energy. As the main component, photothermal materials play a significant role in achieving high conversion efficiency from sunlight to electrical energy. Organic photothermal materials have attracted intense attention recently due to their excellent photothermal efficiency and recycling capabilities. However, broad sunlight absorption in organics heavily relies on the realization of intramolecular or intermolecular charge transfer. Exploring more versatile and efficient methods to construct materials with broad sunlight absorption remains a great challenge. Herein, we report a novel photothermal compound from the perspective of modulating molecular assembly structure. By combining two distinct metal ions, Ba2+ and Mn2+, with the planar ligand H2ONDI, a new coordination polymer, {[BaMn(ONDI)2(H2O)3]⋅H2O}n (1), can be obtained. This coordination polymer exhibits rare bidirectional π − π stacking columns along both the a and b axis. The tightly bidirectional arrangement of ligands in 1 results in a broad optical absorption from the ultraviolet region to the near-infrared area, leading to intriguing near-infrared photothermal effects under irradiation of a laser of 808 nm with a efficiency of 42.3 %. By integrating with thermoelectric devices, 1 shows good application potential in the field of photo-thermo-electric conversion, with the maximum open circuit voltage reaching 313 mV under the irradiation of 1 sun. Furthermore, 1 displays semiconducting behavior because the presence of bidirectional π − π stacking columns facilitates charge transport along two directions, including hopping and band-like charge transport.