Enhancing Near-Infrared Photothermal Performance by Molecular Aggregation Optimization in Semiconductive Coordination Polymers

Near-infrared (NIR) photothermal conversion materials have recently received widespread attention due to their potential in diverse applications. However, highly efficient organic-based NIR photothermal agents remain limited. Developing strategies to enhance the efficiency of NIR photothermal materials and elucidating the relationship between the NIR photothermal performance and molecular aggregation are highly desired. Herein, we report two coordination polymers {[Cd₂(ONDI)(ox)]·2/3(H₂O)}_<i>n</i> (<b>1</b>) and [Ba(ONDI)(H₂O)₂]_<i>n</i> (<b>2</b>), in which the ONDI^2- ligands assemble into different π-π stacking arrangements. Compound <b>1</b> exhibits H-aggregation, while compound <b>2</b> displays X-aggregation. The X-aggregation in compound <b>2</b> extends the optical absorption into the NIR region and enhances the absorption intensity. Consequently, compound <b>2</b> demonstrates a 1.8-fold increase in NIR photothermal efficiency (68.6%) compared to compound <b>1</b> (38.8%), attributed to more effective π-π interactions in X-aggregation. In addition, both compounds show semiconductive properties, with conductivities of 2.1 × 10^-7 S/cm for compound <b>1</b> and 3.0 × 10^-7 S/cm for compound <b>2</b> at 30 °C in a nitrogen atmosphere. These properties arise from the synergistic effects of "band-like" charge transport within crystals and "hopping" charge transport across grain boundaries. By integration of their NIR photothermal effects and semiconductive properties, compounds <b>1</b> and <b>2</b> show interesting NIR photoelectrical responses.