N-Pechmann aza-BODIPYs: enhanced near-infrared photothermal conversion by the cross-conjugated structure

Abstract

Near-infrared (NIR) photothermal conversion was achieved with the dimeric aza-BODIPY analogue called NPAB-55, which features a cross-conjugated, exocyclic double bond to enhance thermal decay from the excited states. NPAB-55 was synthesized according to the Schiff base-forming synthetic protocol using a 5,5-membered N-Pechmann dye. Due to the thermal isomerization to a 6,6-membered N-Pechmann structure during the reaction, NPAB-66, exhibiting intense fluorescence in the visible region in contrast to the non-fluorescent NPAB-55, was also obtained. The photothermal conversion capabilities of NPAB-55 were investigated as nanoparticles fabricated with PEG-PLGA polymers. Due to their high photothermal conversion efficiency of 47% and photothermal stability, the NPAB-55 nanoparticles exhibited reasonable photoacoustic response and photothermal cytotoxicity against cancer cells upon NIR illumination.