Phosphorescent processes

Strip lighting in a classroom, hospital, business hall or kitchen is often called fluorescent lighting, although in fact it is a phosphorescent process, as above. Each bulb consists of a thin, hollow glass tube that is sealed at both ends. It contains gas such as helium, argon or krypton, and a drop of liquid mercury (about 0.5 mg of mercury per kilogram of lamp, or 0.5 parts per million). Like the neon and sodium lamps above, the pressure inside the tube is about 30 Pa, so the mercury evaporates to become a vapour. It is the mercury that yields the light, albeit indirectly. 

This complex is luminescent in the solid state, exhibiting an emission of 525 nm (exc. 390 nm) at room temperature that shifts slightly to 529 nm (exc. 371 nm) at 77 K. The lifetime of 10 ps suggests a phosphorescent process. In solution, it displays very interesting solvent dependence. It emits at 355 (exc. 281 nm) in MeOH, 365 (exc. 

These methods, used in conjunction with suitable phosphoroscopes, can also be used to measure quantum yields of phosphorescence (process 14) (32). Data are very scanty, due to experimental difficulties, so that estimates of the relative importance of processes 2, 3, 4, 14, and 15 remain very imperfect (48,64). Emission from fluid solutions is only by process 2, although with thorough de-oxygenation to eliminate process 11 process 14 might be detectable (34). Otherwise, process 14 is observed only in rigid glassy solvents, as with naphthalene, phenanthrene, or coronene in boric acid glass at room temperatures. 

Furthermore, the electron can go from S, to a triplet state through an intersystem crossing. From the triplet state the electron can in principle only decay by a non-radiative process back to the ground state S0. Nevertheless, spin-orbit coupling of the electrons and magnetic interactions can allow a weak phosphorescence process from Tj to S0. 

In the phosphorescence process the spin-orbit interaction between the singlet-triplet manifolds is strongest for 1,3(ti, x") <- 3,1 (x, 7r ). This can be understood by the rotation of the lone-pair electron into the x-electron system by the spin-orbit operator. This type of atomic interaction motivates the approximations of one-electron, one-center spin-orbit operators cherished in semi-empirical work. The benzene-type of strong... 

Figure 3-14 Luminescence energy-level diagram of typical organic molecule. So is the ground level singlet state S is the first excited singlet state A is the absorption process T is the first excited triplet state and RVD is the radiationless vibrational deactivation. Q is quenching of the excited singlet or triplet state. F is the fluorescence process from the first excited singlet state. P is the phosphorescence process from the first excited triplet state. RC is the radiationless crossover from the first excited singlet state to the first excited triplet state.

The long lifetime of phosphorescence is also one of its drawbacks. Because of this long lifetime, nonradiational processes can compete with phosphorescence to deactivate the excited state. Thus, the efficiency of the phosphorescence process, and the corresponding phosphorescence intensity, is relatively low. To increase this efficiency, phosphorescence is commonly observed at low temperatures in rigid media, such as glasses. In recent years, room temperature phosphorescence has... 

An implicit reason that these phenomena are usable in electric and electro-optical devices is the brief interval during which these luminescent processes occur, as electronics require such rapid reactions for high operating frequencies to ensure, for example, constant communication and screen updating. Most fluorescent and phosphorescent processes occur on a time scale of ns to ps or ms to /xs, respectively. 

The rigidity of the ring is also important. From previous discussions we have shown that in rigid molecules int — 0. This is quite important if we are to build up a large, long-lived triplet population. If Omt 0 the processes So Si and So Ti would gain in importance, and the population of Si and Ti would then be dissipated by radiationless as well as fluorescent and phosphorescent processes. This would be deleterious to the production of a large triplet population. 

We also compared chemiluminescence and phosphorescence processes and found that the energy spectra of chemiluminescence and of phosphorescence reactions are strikingly similar. Phosphorescence reactions, however, decay completely within 20 to 30 minutes, and thus present no serious problem as a disturbing factor in liquid scintillation counting. It should be stressed, however, that samples for radioactivity measurements should be kept in the dark for about 30 minutes before the start of counting. This may be of decisive importance for scimples with... 

Would the light from fireflies be considered an example of a fluorescence or a phosphorescence process ... 

Fig. 1.25. Typical transitions for fluorescence and phosphorescence processes. After Stuart [505]. Reprinted from B. Stuart, Polymer Analysis, Copyright 2002 John Wiley Sons, Ltd. Reproduced with permission.

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