NIR-II-absorbing conjugated polymers based on tetra-fused isoindigo ribbons for photothermal conversion and photoacoustic imaging

Abstract

Conjugated polymers (CPs) that have both a high extinction coefficient (ε) and a high photothermal conversion efficiency (η) in the second near-infrared (NIR-II) region (1,000–1,700 nm) are highly desirable for photoacoustic imaging (PAI) and photothermal therapy (PTT). Here, we report that introducing polycyclic, auxochromic, and coplanar effects together into conjugated backbones affords isoindigo-based CPs with strong absorption and high η in the NIR-II region. Water-dispersed nanoparticles (NPs) of the typical polymer P4TTD-OT, with an ε in mass of 37.1 g −1 cm −1 and an η of 72.9% at 1,064 nm, show superior PAI properties. PAI of 40 mm depth at 1,064 nm is achieved at a lower laser fluence of 7.0 mJ cm −2 with chicken breast muscle as a model tissue, while a signal-to-background ratio of up to 42 dB is realized in the in vivo NIR-II PAI using the 4T1 tumor-bearing mouse model. The NPs can also serve as an effective NIR-II nanoagent for PAI-guided PTT. • A strategy for the synthesis of NIR-II-absorbing CPs is proposed • High η up to 72.9% at 1,064 nm is achieved • Tissue penetration depth of PAI signals up to 40 mm is achieved Jiang et al. report that the combination of polycyclic, auxochromic, and coplanar effects can afford conjugated polymers with high NIR-II molar extinction coefficient and photothermal conversion efficiency.