Phosphorescence
In 1677, Hennig Brand isolated a new element that glowed due to a chemiluminescent reaction when exposed to air, and named it "phosphorus".
Whereas the term "fluorescence" tended to refer to luminescence that ceased immediately (by human-eye standards) when removed from excitation, "phosphorescence" referred to virtually any substance that glowed for appreciable periods in darkness, sometimes to include even chemiluminescence (which occasionally produced substantial amounts of heat).
Only after the 1950s and 1960s did advances in quantum electronics, spectroscopy, and lasers provide a measure to distinguish between the various processes that emit the light, although in common speech the distinctions are still often rather vague.
[10] In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light.
[further explanation needed][11] When the stored energy becomes locked in by the spin of the atomic electrons, a triplet state can occur, slowing the emission of light, sometimes by several orders of magnitude.
To escape, the electron needs a boost of thermal energy to help spring it out of the trap and back into orbit around the atom.
[12] Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, in the order of 10 nanoseconds.
These transitions, although "forbidden", will still occur in quantum mechanics but are kinetically unfavored and thus progress at significantly slower time scales.
Common examples include the phosphor coatings used in fluorescent lamps, where phosphorescence on the order of milliseconds or longer is useful for filling in the "off-time" between AC current cycles, helping to reduce "flicker".
In contrast, amorphous materials have no "long-range order" (beyond the space of a few atoms in any direction), thus by definition are filled with defects.
For example, a missing oxygen atom from a zinc oxide compound creates a hole in the lattice, surrounded by unbound zinc-atoms.
Once in orbit, the electron's energy can drop back to normal (ground state) resulting in the release of a photon.
[16] The release of energy in this way is a completely random process, governed mostly by the average temperature of the material versus the "depth" of the trap, or how many electron-volts it exerts.
[citation needed] A trap that has a depth of 2.0 electron-volts would require a great amount of thermal energy (very high temperature) to overcome the attraction, while at a depth of 0.1 electron-volts very little heat (very cold temperature) is needed for the trap to even hold an electron.
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.
Strontium aluminate based pigments are now used in exit signs, pathway marking, and other safety related signage.
One strategy to enhance the ISC and phosphorescence is the incorporation of heavy atoms, which increase spin-orbit coupling (SOC).
[23] 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.
[24][25] 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,[26] covalent organic frameworks,[27] and others).
Stars made of glow-in-the-dark plastic are placed on walls, ceilings, or hanging from strings make a room look like the night sky.
[32] Using blacklights makes these things glow brightly, common at raves, bedrooms, theme parks, and festivals.
