Three azulene-containing bis(squaraine) dyes were synthesized, among which BSQ1 showed distinct aggregation behaviors from the other two dimers and demonstrated excellent photothermal conversion efficiency. In nanoparticles, BSQ1 formed J-aggregates while BSQ2 and BSQ3 displayed H-aggregates. In films, BSQ1 presented the most significant rearrangement of aggregate structures upon thermal annealing with improved charge transport capabilities.

Abstract

The relationship among chemical structure, physicochemical property and aggregation behavior of organic functional material is an important research topic. Here, we designed and synthesized three bis(squaraine) dyes BSQ1, BSQ2 and BSQ3 through the combination of two kinds of unsymmetrical azulenyl squaraine monomers. Their physicochemical properties were investigated in both molecular and aggregate states. Generally, BSQ1 displayed different assembly behaviors from BSQ2 and BSQ3. Upon fabrication into nanoparticles, BSQ1 tend to form J-aggregates while BSQ2 and BSQ3 tend to form H-aggregates in aqueous medium. When in the form of thin films, three bis(squaraine) dyes all adopted J-aggregation packing modes while only BSQ1 presented the most significant rearrangement of aggregate structures as well as the improvement in the carrier mobilities upon thermal annealing. Our research highlights the discrepancy of aggregation behaviors originating from the molecular structure and surrounding circumstances, providing guidance for the molecular design and functional applications of squaraines.