Perylene diimide and its derivatives (PDIs) have been widely used as organic field-effect transistor (OFET), organic photovoltaic cell (OPV), dye laser and organic light emitting diode (OLED) in the field of optoelectronic materials due to their photo, thermal, chemical stability and high fluorescence quantum yields. However, because of their inherent structure, PDIs have poor water-solubility and easily form aggregates, which has limited their applications in biological fields. So, it's essential to synthesize water-soluble PDIs. This paper systematically presents the synthetic methods for obtaining water-soluble PDIs by introducing anionic substituent, cationic substituent or non-ionic substituent into the imide-position or bay-region of PDIs. Some of these substituents are water-soluble, and the others will achieve the water-solubility of PDIs through electrostatic repulsion or steric hindrance. In addition, several novel biological applications have been listed, such as chemotherapy,photodynamic therapy and fluorescence imaging.PDIs can also be used as a promising photo-acoustic contrast agents due to their good light absorption and photostability.

Contents
1 Introduction
2 Modification of PDIs
3 Modification of water-soluble PDIs
3.1 Anionic substituent
3.2 Cationic substituent
3.3 Non-ionic substituent
4 Biological application of water-soluble PDIs
4.1 Fluorescence probe
4.2 Contrast agent of photo-acoustic imaging
4.3 Chemotherapy
4.4 Photo-sensitizer of photodynamic therapy
5 Conclusion