Quantum yield of phosphorescence

As discussed earlier by Senthilnathan and Hurtubise (4), before a saturated ethanol solution of sodium acetate is formed, the solid-surface RTF is less than the RTF from samples prepared with solutions that are saturated with sodium acetate. It was shown by Ramasamy and Hurtubise (12) that both the RTF and RTF quantum yields of the p-aminobenzoic acid anion increased as the amount of sodium acetate increased in the solid mixtures. Figure 4 shows the quantum yield of fluorescence and the quantum yield of phosphorescence )... 

The net result is that for molecules in which Si is (jt,Jt )> both fluorescence and phosphorescence emission will be observed provided (j)f < 1, but if Si is (n,7t ) then the quantum yield of phosphorescence will very likely be much greater than (f f. 

When quantum yield of phosphorescence, intersystem crossing, must also be included ... 

The radiative transition Tj- .S0, from the lowest triplet state Tt gives rise to phosphorescence emission. The quantum yield of phosphorescence emission... 

The quantum yield for decomposition of acetone from the triplet state is about 0.40 at 40°, and the quantum yield of phosphorescence is about 0.02.308 At least 407o of the initially excited molecules are not accounted for by either emission or chemical reaction, and thus must undergo some kind of radiationless decay to the ground state, presumably mostly from the triplet state. Recent studies of hexafluoro-acetone317 indicate that approximately half the triplet molecules formed by intersystem crossing undergo radiationless decay. 

As discovered in ref. 63, the addition of tetranitromethane, C(N02)4, electron acceptor to a solution of CuP in ethanol causes a decrease in the quantum yield of phosphorescence of CuP at 77 K and the appearance in the optical and the EPR spectra of signals which are characteristic of CuP+, NOo, and C(N02)3" particles. The formation of these particles points to electron phototransfer from CuP to C(N02)4. The decay curves of CuP phosphorescence in vitreous solutions containing C(N02)4 in low concentrations are of an exponential character. At sufficiently high concentrations of C(N02)4 (0.3-0.5M), however, these curves deviate from the simple exponential form. The appearance of non-exponential parts on the decay curves has been accounted for by electron tunneling from the triplet excited state of CuP particles to molecules of C(N02)4. 

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. 

Because of the forbiddeness of the transition, Tx —> S0 + ho, the natural phosphorescent lifetime, t°, of a triplet state is long—from approximately 10"3 sec for an n,it triplet to 30 sec for a rr,n aromatic triplet. At room temperature in solution, phosphorescence is often not observed because ISC of Tx to S0 and quenching of Tx by impurities and molecular 02 (see below) competes effectively with phosphorescence. Therefore most phosphorescence studies must be carried out at low temperatures in carefully purified, outgassed, rigid media. Under these conditions the quantum yield of phosphorescence, 9P, defined by Equation 13.10, is often high and approaches 1.0 for some aromatic carbonyls. 

The formation of the triplet state can be directly followed in time through the observation of the transient triplet-triplet (T-T) absorption in flash or modulated photolysis or by the observation of the phosphorescence emission. A typical radiative lifetime of phosphorescence for simple carbonyls is 10 3 s. Therefore, it is extremely difficult to observe the "unrelaxed" phosphorescence emission without collisional relaxation, unless the triplet lifetime is significantly shortened by a competing radiationless process. Under these conditions, the correspondingly low quantum yield of phosphorescence makes such measurement rather difficult. Usually, "relaxed" phosphorescence from a molecule such as biacetyl is observed. Therefore, only the transient T-T absorption can provide useful data in the gas phase, although a determination of the absolute yield is rather involved and difficult. 

This can be compared with the quantum yield of phosphorescence upon direct excitation only into the state 2 (so that is unity) ... 

Quantum yields of phosphorescence and rate constants 302 for radiative and non-radiative T, decay processes Picosecond time-resolved study of ISC 303... 

Spectral analysis of the emission, however, showed, that it was actually delayed phosphorescence, i.e. (PhjNH) produced in the ion-electron recombination must be in a triplet state °. esr studies on transients produced in the photolysis of di-diphenylamine in alcohols at low temperatures also confirmed the existence of (PhjNH) and demonstrated an overall 2-photon absorption mechanism involving a triplet-triplet transition The quantum yield of phosphorescence is 0.05. ... 

If the T, state is deactivated only by first-order processes such as phosphorescence and intersystem crossing with rate constants kf and kjs, the quantum yield of phosphorescence is given according to Equation (5.8) by... 

The study of the triplet dynamics in fully conjugated polymer hosts turned out to be difficult, because the intrinsic phosphorescence is barely observed from these polymers. The main reason is the lack of spin-orbit coupling in these carbon-based conjugated polymers, resulting in low intersystem-crossing (ISC) rates. Consequently, the polymer triplet state is only weakly populated after photoexcitation and the quantum yield of phosphorescence is low. 

Data from reference 69. Op is the quantum yield of phosphorescence, and Xp is the triplet state lifetime. Source Reference 46. 

Donor-acceptor separation Forster distance Rate of reaction at z Dipole orientation parameter Quantum yield of process x Quantum yield of donor emission Quantum yield of emission Quantum yield of fluorescence Quantum yield of luminescence Quantum yield of internal conversion Quantum yield of intersystem crossing Quantum yield of phosphorescence Quantum yield of triplet state formation Quantum yield of vibrational relaxation Quantum yield of singlet oxygen production Lifetime... 

As is shown in Eq. (5) this quantity is defined as the ratio of the quantum yields of phosphorescence to fluorescence (glassy media at 77°K or lower). This ratio is most easily interpreted for molecules for which the sum of the quantum yields of fluorescence and phosphorescence approaches unity. For such systems Eq. (6) holds and the variation of x can be attributed to change in the relative rates of fluorescence, kp, and intersystem crossing, kis, from S. ... 

In Eq. (1.14), Iq,, and / are as defined in Beer and Lambert s Law (Eq. 1.6), 0sr is the quantum yield of intersystem crossing, and OP is the quantum yield of phosphorescence. The former represents the fraction of excited molecules that undergo intersystem crossing from the lowest excited singlet state to the lowest triplet state, and the latter is the fraction of excited molecules having undergone intersystem crossing that are actually deactivated by phosphorescence. 

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