Thienoisoindigo-based semiconducting polymer with a strong near-infrared absorbance is synthesized and its water-dispersed nanoparticles (TSPNs) are investigated as a contrast agent for photoacoustic (PA) imaging in the second near-infrared (NIR-II) window (1000-1350 nm). The TSPNs generate a strong PA signal in the NIR-II optical window, where background signals from endogenous contrast agents, including blood and lipid, are at the local minima. By embedding a TSPN-containing tube in chicken-breast tissue, an imaging depth of more than 5 cm at 1064 nm excitation is achieved with a contrast-agent concentration as low as 40 µg mL . The TSPNs under the skin or in the tumor are clearly visualized at 1100 and 1300 nm, with negligible interference from the tissue background. TSPN as a PA contrast in the NIR-II window opens new opportunities for biomedical imaging of deep tissues with improved contrast.

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Photoacoustic (PA) probes absorbing in the second near-infrared (NIR-II: 1000–1700 nm) window hold great promise for deep-tissue diagnosis and treatment. Currently, NIR-II PA probes typically involve complex synthesis and surfactant adjuvant for processing and delivery. Furthermore, these NIR-II PA probes are “always-on,” leading to inadequate signal-to-background ratio and low specificity. To address these challenges, this study reports a pH-activatable and aggregation-enhanced NIR-II PA probe. Without using any toxic or exotic oxidants, the selected polymer (PPE) is readily doped by oxygen in an ambient environment and simultaneously red-shifts its absorption profile from visible to NIR-II region. By virtue of the carboxyl groups on the side chains, oxygen-doped PPE is readily water-soluble at a physiological pH but tends to aggregate in an acidic environment. The pH-induced aggregation results in a significant PA enhancement and thus allows specific PA imaging of acidic tumor microenvironment in vivo. Our study provides a facile and surfactant-free strategy for achieving water-soluble and pH-responsive NIR-II PA probes, which could be applied for diagnoses of cancer and other diseases associated with changes in pH. It paves the way for the development of new activatable NIR-II imaging probes and also could facilitate the investigation of biological and pathological processes in deep tissue.

While it is known that the chain length strongly affects the properties of π-conjugated polymers, the effects of chain length on the molecular structure, chain conformation, and oxidation state properties in donor−acceptor-type conjugated structures remain unclear. This limits our understanding of how the polymer molecular weight impacts material properties. Here, a discrete and monodisperse oligomer series (n = 3, 5, 7, 9, 15, and 21) and polymers (nPB), composed of the donor 3,4propylenedioxythiophene (ProDOT), the acceptor benzothiadiazole (BTD), and methylthio end-capping groups, are synthesized by C−H-activated cross-coupling. The molecular structure, molecular weight, and dispersity of each oligomer/polymer are thoroughly characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, and gel permeation chromatography. This series reveals a rod-to-coil transition at n = 15 and coil formation at polymer length scales of ∼28 units via solution small-angle neutron scattering characterization. The oxidation states are deciphered via cyclic voltammetry, differential pulse voltammetry, spectroelectrochemistry, and density functional theory calculations. Oligomers 3−9 undergo successive one-electron oxidation steps, while 15 and higher undergo multielectron oxidations per step in CH 2 Cl 2 −TBAPF 6 at a Pt ultramicroelectrode. The electronic transition of each oxidation state (1+, 2+, 3+, etc.) is tracked by absorption spectroscopy, revealing a "bipolaron to di-polaron" transition at n = 7 at which shorter oligomers prefer bipolaron formation and longer oligomers prefer di-polaron formation in their dication states. Furthermore, oxidized 15 has a lower electronic transition energy compared to its polymer homologue, as shown by spectroelectrochemistry, revealing a synergy between the chain length and the oxidation state properties. This study proves that the convergence limit between small molecule and polymer behavior occurs at approximately 15 units and highlights the property transitions that occur as a function of chain length for a donor−acceptor class of conjugated organic materials.

Recently, direct arylation polymerization (DArP) has emerged as a greener polymerization method for donor–acceptor (D–A) type conjugated polymers.