Newly emerging photoacoustic imaging (PAI) technology that couples the advantages of optical imaging and ultrasound imaging allows a fascinating non-invasive imaging paradigm with higher spatial resolution and deeper imaging deepness when compared with traditional optical imaging techniques (e.g., fluorescence). Typically, PA contrast agents, which convert the absorbed photon energy into ultrasonic emission, are essential for a successful PA imaging. Although naturally occurring absorbers such as melanin and hemoglobin can serve as endogenous PA contrast agent to monitor anatomical and physiological variations in diseases, only small fractions of such endogenous contrast agents have been reported. Therefore, in order to fully explore the potential of PAI, exogenous PA contrast agents based on near-infrared (NIR, 650~900 nm) absorption materials are urgently demanded. This article reviews the recent advances of organic optoelectronic materials in PAI such as small organic dye-based nanoparticles, polymer-based nanoparticles in biological imaging field. Then, we systematically summarize the structure properties relationship between contrast agents and the application of PAI to guide the design of new PA contrast agents. Finally, the future opportunities and challenges of PA contrast agents are discussed.

Contents
1 Introduction
2 Small dye nanoparticles
2.1 Perylene-based derivatives
2.2 Cyanine-based derivatives
2.3 BODIPY-based derivatives
2.4 Porphyrin-based derivatives
3 Conjugated polymer-based nanoparticles
4 Conclusions