text: Common pigments used in phosphorescent materials include zinc sulfide and strontium aluminate. Use of zinc sulfide for safety related products dates back to the 1930s.
  The development of strontium aluminate pigments in 1993 was spurred on by the need to find a substitute for glow-in-the-dark materials with high luminance and long phosphorescence, especially those that used promethium. This led to the discovery by Yasumitsu Aoki (Nemoto & Co.) of materials with luminance approximately 10 times greater than zinc sulfide and phosphorescence approximately 10 times longer. This has relegated most zinc sulfide based products to the novelty category. Strontium aluminate based pigments are now used in exit signs, pathway marking, and other safety related signage.
  Since both phosphorescence (transition from T to S) and the generation of T from an excited singlet state (e.g., S) via intersystem crossing (ISC) are spin-forbidden processes, most organic materials exhibit insignificant phosphorescence as they mostly fail to populate the excited triplet state, and, even if T is formed, phosphorescence is most frequently outcompeted by non-radiative pathways. One strategy to enhance the ISC and phosphorescence is the incorporation of heavy atoms, which increase spin-orbit coupling (SOC). Additionally, the SOC (and therefore the ISC) can be promoted by coupling n-π* and π-π* transitions with different angular momenta, also known as Mostafa El-Sayed's rule. Such transitions are typically exhibited by carbonyl or triazine derivatives, and most organic room-temperature phosphorescent (ORTP) materials incorporate such moieties.   In turn, to inhibit competitive non-radiative deactivation pathways, including vibrational relaxation and oxygen quenching and triplet-triplet annihilations, organic phosphors have to be embedded in rigid matrices such as polymers, and molecular solids (crystals, covalent organic frameworks, and others).