Phosphorescence yield

In addition, the pure solid (N(C2Hs)4)2MnBr4 is recommended as standard. Its phosphorescence yield is high, , = 0.8, the material has only medium absorption despite its high concentration, and there is no long-distance impurity quenching because the luminescence centers are electronically isolated from one another by the large counter ions. 

The long lifetime has important consequences on the decay rates. First, we consider what affects the nonradiative rates (knr) which change the yields of fluorescence and phosphorescence. The nonradiative decay rate is often enhanced in molecules which have flexible constituents (the so-called loose-bolt effect). Therefore, both fluorescence and phosphorescence yields are generally larger for rigid molecules than flexible molecules. For the same reason, a rigid environment will increase the emission yields hence both fluorescence and phosphorescence yields often increase with increasing viscosity. 

The intersystem crossing process has opposite effects on the yields of fluorescence and phosphorescence since it depletes the singlet state and populates the triplet state. It is commonly known that heavy ions, such as iodide and bromide, increase intersystem crossing by spin-orbit coupling.(1617) For proteins, fluorescence can be quenched as phosphorescence yield is enhanced. 8,19) However, although the phosphorescence yield is increased, the lifetime is decreased. This effect arises because spin-orbit coupling, which increases the intersystem crossing rate from 5, to Tt, also increases the conversion rate from T, to S0. 

Tryptophan at 77 K in rigid solution has a phosphorescence quantum yield of 0.17(20) and a lifetime of 6 s. These values at 77 K are relatively invariant from protein to protein and do not vary significantly between buried and exposed tryptophans.(21,22) If one assumes that the intersystem crossing yield is a constant, a calculation of the quantum yield of indole phosphorescence can be roughly estimated from the lifetimes. The phosphorescence yield is related to lifetime by... 

Vanderkooi et al.(m) examined the phosphorescence from tryptophan in sarcoplasmic reticulum vesicles and the purified Ca transport ATPase at room temperature in deoxygenated solutions. The phosphorescence decay is multiexponential the lifetime of the long-lived component of phosphorescence is 22 ms. Addition of ATP or vanadate decreased the phosphorescence yield. The Ca2+-ATPase of the sarcoplasmic reticulum alternates between two conformations, called Ei and E2, during Ca2+ transport. The observations were interpreted to indicate that either the binding of vanadate or phosphate to the phosphorylation site of the ATPase or the induced shift in the conformation from the i to the E2 state produced the phosphorescence quenching. 

The a-diketones show both phosphorescence and fluorescence emission, not only in a glass at 77°K but also in fluid solutions at room temperature, a property which has made these compounds useful in energy transfer studies.25-28 Fluorescence, however, is quite weak with intersystem crossing and decay through the triplet state being the principle mode of decay. The absolute fluorescence yields of biacetyl and benzil in solution are reported to be 0.22% and 0.27%, respectively, while the measured phosphorescence yield... 

Transition metal complexes absorb radiation and exhibit both fluorescence and phosphorescence. In certain cases the fluorescent and phosphorescent yields have been related70 to zero-order crossings in... 

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

The lowest excited triplet states of a-dicarbonyl compounds are considerably less energetic than those of simple carbonyls. For instance the energy of the vibrationally relaxed triplet of glyoxal is 55 kcal,366 as compared to 72 kcal for formaldehyde. Irradiation of glyoxal at 4358 A populates the lowest vibrational levels of the first excited singlet, 30% of which fluoresce and 70% of which cross over to the triplet manifold.388 Almost all of the triplet molecules then decompose to formaldehyde and carbon monoxide, the phosphorescence yield being only 0.1%. 

Of particular interest are the last two examples. Apparently, intramolecular hydrogen abstraction leads quantitatively to cyclobutanols in a-diketones, with no elimination being observed. The phosphorescence yields are, of course, cut drastically by this efficient chemical reaction.396... 

In summary the qualitative data on fluorescence and phosphorescence cannot explain satisfactorily the behavior of excited azines. More quantitative work is needed. In many cases, the sum of fluorescence and phosphorescence yields is not unity and processes of radiationless deactivation must be considered. 

The fluorescence and phosphorescence yields of various /8-diketones are found to be enhanced by coordination to Zn2+, Al3+ and Be2+ in an increasing order. This is ascribed to a ligand-ligand interaction in the excited and ground state, in the case of Zn, most probably arising through both Coulomb exchange and dipole-dipole interaction.748... 

The phosphorescence yield is defined as the ratio between the number of photons emitted in the triplet state and the number of photons absorbed (values between 0 and 1). 

Here %T is the quantum yield of triplets produced by the geminate recombination, while r is the quantum yield of their phosphorescence. The latter is quenched by the subsequent bulk ionization, in encounters of triplets with neutral quenchers. The relative phosphorescence yield obeys the usual Stern-Volmer law, whose constant is identical to that of Eq. (3.364)... 

Kumar and Huber (129) have studied the phosphorescence, fluorescence, and photodecomposition of propynal as a function of excitation energy up to an excess vibrational energy of 6000 cm-- -. They find that the phosphorescence yield is constant until an excess energy of 2952 cm- - is reached, after which a sharp decrease in 4>p with increasing energy is noted. With the pressures used (0.6 torr propynal), any triplets formed will be... 

The phosphorescence of porphin and its deuterated derivatives was studied in Xe matrices at 10 K (62). This significantly enhances intersystem crossing rates and increases the phosphorescence yield. It was possible to identify the 0-0 band of the phosphorescence at 790-794 nm (36 kcal/mol = 1.57 eV) in Xe matrix and the corresponding band of the fluorescence at 613 nm (46.6 kcal/mol = 2.02 eV) in Ne matrix. The same Si-Ti energy splitting for porphin and H2TPP would imply that the latter would have a triplet state energy of 33.5 kcal/mol. 

Figure 19.4C presents data on the phosphorescence decay dynamics on two of the samples in Figure 19.4B, the one without silver and the one having optimum enhancement. These exhibit the characteristic signature of plasmon-enhanced emission, namely that increases in luminescence are accompanied by decreases in excited state lifetime. Ordinarily, radiative rates are fixed by quantum mechanical matrix elements and variation in excited state lifetime is due to changes in nonradiative rates so that increases in lifetime correspond to lower non-radiative rates and increases in luminescence yield. Here, the lifetime is reduced by about a factor of 3 due to increased emissive rate even as the luminescence increases 215 times. These large enhancements cannot be accounted for simply by increase in phosphorescence yield since the yield is greater than 10 % in the absence of silver. It is evident that a substantial fraction of the increase must be accounted for by absorption enhancement. 

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