Benzene phosphorescence lifetimes

Table 19 Phosphorescence lifetimes r (s) for different k spin-sublevels of the 3B U state of benzene and of the 3i 2u state of naphthalene. The benzene values refer to the four mechanisms depicted in Fig. 10 o and i denote the out-of-plane and in-plane polarization, respectively. rav denotes the average lifetime at the high temperature limit. Excitation energies given in eV.

In the following we discuss the phosphorescence lifetimes of 3(n, x ) and 3(x,x ) states of the azabenzenes and the azanaphthalenes, which can differ by several orders of magnitude. The symmetry axes and geometries of the discussed compounds are reproduced in Fig. 14. Since benzene and naphthalene lack (n,x ) transitions their triplet state radiative lifetimes are, as for other hydrocarbons, considerably longer. 

The UV absorption spectra of tr3q)tophan, tyrosine and phenylalanine are virtually identical to those of indole 35), phenol 86) and benzene 27) respectively. Close similarities also exist between the fluorescence and phosphorescence spectra of each pair, as well as between the phosphorescence lifetimes 8.28,38). This and the electron spin resonance observations to be discussed in a later section indicate that the addition of a methyl group or other substituents of the type —CH2R does not have a large effect on the measurable properties of the phosphorescent triplet state of the compounds being discussed here. 

The mean lifetime of triplet benzene in the glassy matrix is not certain, but all authors agree that it is large and it is probably about 20 sec113. And yet the available data on decay of the triplet state in the gas phase (there is no phosphorescence) indicate a mean lifetime of the order of magnitude of 10 4 to 10"5... 

The ground state force field, vibrational normal modes and frequencies have been obtained with MCSCF analytic gradient and hessian calculations [176]. Frequencies computed with the DZ basis set are compared with experimental ones in Table 16. The T - So transition moments were obtained using distorted benzene geometries with atomic displacements along the normal modes, and with the derivatives in Eq. 97 obtained by numerical differentiation. The normal modes active for phosphorescence in benzene are depicted in Fig. 12. The final formula for the radiative lifetime of the k spin sublevel produced by radiation in all (i/f) bands is (ZFS representation x,y,z is used [49]) ... 

Burland and Schmidt [190] performed microwave induced delayed phosphorescence (MIDP) experiments using a sample preparation with a benzene host crystal for both pyrazine and pyrimidine. With this technique they found that the relative radiative rates closely resembled the relative total rates for the different spin sublevels, indicating a high quantum yield and low non-radiative rate. The spin sub-level rates of pyrazine for Tr and rv were given as 284 and 6.3 ms, respectively, to be compared with the best response theory result of 282 and 12 ms. The z (out-of-plane) spin component is symmetry forbidden, and its lifetime (400 ms) derives from the lattice perturbation. 

Considerable data are available for triplet yields of benzene in dilute solutions of different solvents (see Table 13). In the main, two techniques have been used sensitized phosphorescence of biacetyl, sensitized cis-trans isomerization of butene-2, octene-2, and stllbene. All yield comparable results. In saturated hydrocarbon solvents at room temperature, the triplet yield for CgHg is found to be about 0.24 0.01. There is a solvent dependence of this quantity, the yield dropping to 0.15 in ethanol, 0.13 in methanol, and 0.09 in acetonitrile (91). In determining the effect of environment on the rate constant controlling intersystem crossing, values for emission lifetimes in the various systems are needed. These are, as mentioned previously, often unreliable. Cundall and Pereira (91) have reported... 

All of the above data refer to dilute solutions the effect of solute concentration on the phosphorescence emission yield and lifetime is very marked. At concentrations of IM and above, a decrease in Xp and the x ratio has been reported for EPA (214), EOA (144,152), methylcyclohexane (219), and cyclohexane (207). Phosphorescence is negligible in both pure benzene crystals (225) and polycrystalline powder (144,214). This has been attributed to a rapid triplet-triplet annihilation process, an explanation apparently confirmed by Cundall and Pereira (219), who detected solution some slight rather diffuse phosphorescence together with delayed fluorescence from pure benzene. 

The benzene triplet has not been observed to phosphoresce in either the gas or liquid phase, as noted previously but in view of the lifetime measured In glassy media at low temperature this is not surprising, since very small amounts of quenching impurities would reduce emission to undetectable levels. Some detailed investigations have shown that there is an intramolecular process which Induces radiationless decay of the benzene triplet state, other than impurity quenching. 

By competitive methods Ishikawa and Noyes (226) (sensitized biacetyl phosphorescence) and Cundall and Davies (159) (butene-2 isomerization) both estimated the triplet lifetime to be of the order of 10 ys. In a reexamination of the butene-2 system, Lee (227) estimated a value of 100 ys, a finding confirmed by Cundall and Dunnicliff (105). An examination of the kinetics of the benzene photosensitized composition of cyclopentanone decomposition allowed a value of longer than 3 ys to be deduced. This type of experiment is far from satisfactory since photochemical processes can Intervene at low pressures, and impurities and the quenching effects of photoproducts can affect the results. These problems can only be overcome by some form of direct measurement. Parmenter and Ring (228) used a flash method in which 20 torrs of benzene and 0.01 torr of biacetyl were submitted to a 20 J,... 

Simple benzenoid systems have not been as extensively reported as in previous years. West and Miller have studied the fluorescence from dilute solutions of benzene in cyclohexane induced by protons and alpha particles. A model for interpretation of the data involving intratrack quenching by products of the irradiation is not fully adequate. Gibson and Rest have measured quantum yields of fluorescence and phosphorescence in various frozen gas matrices and from these yields and lifetime data have calculated the photophysical rate constants for and at 12 K, which are given in Tables 1, 2, and 3. The... 

A second direct optical-detection method for selective population and depopulation is microwave-induced delayed phosphorescence in zero field (Bq = 0) [25]. Figure 7.26 shows the phosphorescence intensity from quinoline in a durene (tet-ramethyl benzene) host crystal at T= 1.35 K as a function of the time after the end of the UV excitation. The phosphorescing zero-field component here is Tz). Its lifetime is considerably shorter than those of the other two zero-field components, from which furthermore no phosphorescence is emitted. If the zero-field transition... 

CF,)jC,H4, 0-, m-, and p-FC,H4Me, o- and m-FC,HiCF PhF, - CeFe, and C,F6H has been studied, and trends in the radiative and non-radiative decay rates of the excited molecules are discussed. - c/s-But-2-ene is effective in increasing the fluorescent and triplet yields of pentafluorobenzene. The decay rate of triplet benzophenone, produced by laser flash photolysis, is ca. ten times faster in hexafluorobenzene than in benzene, and reversible addition of the triplet species to the aromatic ring may play an important role. The phosphorescence spectrum of decafluoro-benzophenone has been studied, and the triplet state of this ketone (see also p. 380) in inert solvents e.g. perfluoromethylcyclohexane) has a lifetime of 20 ps. U.v. irradiation of decafluorobenzophenone in isopropyl alcohol... 

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