Lifetime of triplet state

Since the first two processes are spin-forbidden, it can clearly be seen that in the absence of triplet quenchers (e.g., oxygen) the triplet will be long lived. Consequently the experimental determination of the lifetime of triplet states... 

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). 

The effect of temperature on the PL spectra is important as regards the internal conversion between the lowest excited triplet state and the ground state, because of the longer lifetimes of triplet states (Anpo and Che, 1999). 

As a result of the longer lifetimes of triplet states of electronically excited organic molecules as compared with their lowest excited singlet... 

We notice that singlet-triplet transitions in molecules are forbidden by spin selection rules and the lifetime of triplet states in crystals exceeds by many orders... 

PHOSPHORESCENCE AND THE TRUE LIFETIME OF TRIPLET STATES IN FLUID SOLUTIONS. 

Together with molecular structure, environmental parameters such as temperature, solvent type, viscosity, pH, and dissolved oxygen content can also affect luminescence. As the solution temperature rises, it follows that the number of collisions between the excited state molecule and the solvent molecules will increase, thus greatly improving the likelihood of radiationless deexcitation to the ground state. Therefore, 4>i for most compounds decreases with increasing temperature. As mentioned earlier, the effect of temperature upon 4>p is even more dramatic due to the vastly greater lifetime of triplet states. 

The relatively long lifetimes of the lowest triplet states of molecules make them important photochemical intermediates (especially in bi-molecular reactions). Phosphorescence spectra are usually recorded from molecules in glassy media at low temperatures (usually 77 K). Under such conditions lifetimes of triplet states as high as several seconds have been measured. 

---