P680, or Photosystem II primary donor, (where P stands for pigment) refers to any of the 2 special chlorophyll dimers (also named special pairs), P_D1 or P_D2. 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).