text: It is common to describe where electrons reside as electron bands in bulk materials and electron orbitals in molecules. For the sake of expedience the following description will be described in molecular terms.  When a photon excites a molecule, an electron in a ground state orbital can be excited to a higher energy orbital.  This excited state leaves a vacancy in a ground state orbital that can be filled by an electron donor.  It produces an electron in a high energy orbital which can be donated to an electron acceptor.  In these respects a photoexcited molecule can act as a good oxidizing agent or a good reducing agent.
  :Photoinduced oxidation
  :[ML] + hν → [ML]*
  :[ML]* + donor → [ML] + donor
  :Photoinduced reduction
  :[ML] + hν → [ML]*
  :[ML]* + acceptor → [ML] + acceptor
  The end result of both reactions is that an electron is delivered to an orbital that is higher in energy than where it previously resided. This is often described as a charge separated electron-hole pair when working with semiconductors.
  In the absence of a proper electron donor or acceptor it is possible for such molecules to undergo ordinary fluorescence emission. The electron transfer is one form of photoquenching.