Phosphorescence spin-forbidden

Figure 10. Electron excitations in radicals (a) Collective representation of one-electron transitions of the A, B, and C types if denotes MO (b) LCI energy-level scheme (Jablonski diagram) for doublet and quartet states indicating why with radicals fluorescence (- - -) but not phosphorescence is observed. Spin-forbidden transitions are represented by dashed lines.

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

Phosphorescence Photon emission. Phosphorescence involves a spin-forbidden radiative transition between states of different multiplicity, usually from the lowest vibrational level of the lowest excited triplet state, Tt. Ti(v = 0) - S0 + hv... 

Phosphorescence arises as the result of a radiative transition between states of different multiplicity, Ti —> So. Since the process is spin-forbidden, phosphorescence has a much smaller rate constant, kp, than that for fluorescence, kf ... 

Phosphorescence is spin-forbidden and thus phosphorescence emission is less intense (Figure 4.10) and less rapid than fluorescence. 

A distinction is made between fluorescence and phosphorescence according to the change in the spin quantum number between the initial and final states. When these quantum numbers are the same AS = 0 and the transition is spin-allowed (but there may be other factors to be taken into account as well, as we shall see) this emission is then defined as a fluorescence. If there is any change in the spin quantum number, AS = 0, the transition is spin forbidden and is defined as a phosphorescence. The important difference between these two forms of luminescence resides in their kinetics a fluorescence is shortlived, with emission lifetimes in the range 1 ns to 1 ts, while phosphorescence lifetimes go from 1 ms to many seconds or even minutes. 

On the other hand, most chemists and many physicists leading with polyatomic organic molecules currently employ the mechanistic definitions advanced by G. N. Lewis and shown in Figure 1. Thus, fluorescence is defined as a radiative transition between states of like multiplicity, e.g., 5 x - So + hv. Phosphorescence is a radiative transition between states of different multiplicity. In organic molecules the process is usually associated with spin-forbidden transitions such as Ti - S0 + hv". 

Several authors have reported that in polar solvents the overall phosphorescence decay of some phenyl alkyl ketones has a long- and a short-lived component they attribute this to simultaneous emission from 3(77,77 ) and 3(77,77 ) states that are not in equilibrium with each other. This interpretation assumes that phosphorescence, a spin-forbidden process, occurs more rapidly than internal conversion from T2 to 7 and therefore seems improbable. It is more likely that one of the phosphorescent species is a photochemical product of the original ketone.13,14... 

In addition to causing fine-structure splitting, magnetic interactions may couple states of different spin multiplicities. As a consequence, so-called spin-forbidden transitions yield some intensity. Well-known examples for this phenomenon are phosphorescence and nonradiative transitions at intersystem crossings. 

An Example The Phosphorescence of Dithiosuccinimide Many thio-carbonyls have photostable excited (n > ji ) and (ti —> ti ) states that tend to relax by photophysical rather than photochemical processes.177,178 Recently, the electronic spectra of dithioimides have been under experimental and theoretical investigation.179-181 The spin-forbidden radiative decay of the lowest-lying triplet state of dithiosuccinimide may serve as an example to illustrate the results of the previous sections. Experimentally a lifetime of 0.10 0.01 ms was determined for the Ti state.179 This value has been corrected for solvent effects, but the transition may include radiative as well as nonradiative depletion mechanisms. 

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