P680, or Photosystem II primary donor, (where P stands for pigment) refers to any of the 2 special chlorophyll dimers (also named special pairs), PD1 or PD2[1]. These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited as if they were a single molecule. The 680 number is its absorption maximum in the red part of the visible spectrum (680 nm). The primary donor receives excitation energy either by absorbing a photon of suitable frequency (colour) or by excitation energy transfer from other chlorophylls within photosystem II. During excitation, an electron is excited to a higher energy level. This electron is subsequently captured by the primary electron acceptor, a pheophytin molecule located within photosystem II near P680. The oxidized P680 (P680+) is subsequently reduced by an electron originating from water (via Oxygen evolving complex).
Oxidized P680 (P680+) is the strongest biological oxidizing agent known. It has an estimated redox potential of ~1.3 V[2]. This makes it possible to oxidize water during oxygenic photosynthesis.