Benzene emission lifetimes

Some fluorescence lifetimes are observed in ps times, although these are unusual cases. In organic molecules the Sj—S0 fluorescence has natural lifetimes of the order of ns but the observed lifetimes can be much shorter if there is some competitive non-radiative deactivation (as seen above for the case of cyanine dyes). A few organic molecules show fluorescence from an upper singlet state (e.g. azulene) and here the emission lifetimes come within the ps time-scale because internal conversion to S and intersystem crossing compete with the radiative process. To take one example, the S2-S0 fluorescence lifetime of xanthione is 18 ps in benzene, 43 ps in iso-octane. 

TABLE 9. solution. Monomer fluorescence emission lifetimes for benzene in... 

TABLE 11. Fluorescence emission lifetimes for benzene solid phase. in the... 

Emission lifetimes for benzene with vapor phase are subject to similar variations with pressure and excitation wavelength as are fluorescence yields. Data are collected in Table 6 and can be seen to show considerable variation with source and experimental technique. Recent measurements, using a single-proton counting technique (114) have shown the emission lifetimes of high pressure CgHg and CgDg to be 77 and 92 ns, respectively, at 25°C. The temperature dependence of the fluorescence lifetime is also shown in Fig. 7. 

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... 

Although Sandman s example does not perfectly represent the whole of the issue of the voluntariness of the risk, it conveys a good sense of what makes it important. It is not surprising, then, that when they learn about benzene emissions to the air they breathe, people living near a petroleum refinery are not going to be easily satisfied simply by an explanation that the lifetime risks of cancer associated with those emissions are no greater than one in 100000, even though these... 

Forster (1968) points out that R0 is independent of donor radiative lifetime it only depends on the quantum efficiency of its emission. Thus, transfer from the donor triplet state is not forbidden. The slow rate of transfer is partially offset by its long lifetime. The importance of Eq. (4.4) is that it allows calculation in terms of experimentally measured quantities. For a large class of donor-acceptor pairs in inert solvents, Forster reports Rg values in the range 50-100 A. On the other hand, for scintillators such as PPO (diphenyl-2,5-oxazole), pT (p-terphenyl), and DPH (diphenyl hexatriene) in the solvents benzene, toluene, and p-xylene, Voltz et al. (1966) have reported Rg values in the range 15-20 A. Whatever the value of R0 is, it is clear that a moderate red shift of the acceptor spectrum with respect to that of the donor is favorable for resonant energy transfer. 

Weller24 has estimated enthalpies of exciplex formation from the energy separation vg, — i>5 ax of the molecular 0"-0 and exciplex fluorescence maximum using the appropriate form of Eq. (27) with ER assumed to have the value found for pyrene despite the doubtful validity of this approximation the values listed for AHa in Table VI are sufficiently low to permit exciplex dissociation during its radiative lifetime and the total emission spectrum of these systems may be expected to vary with temperature in the manner described above for one-component systems. This has recently been confirmed by Knibbe, Rehm, and Weller30 who obtain the enthalpies and entropies of photoassociation of the donor-acceptor pairs listed in Table XI. From a detailed analysis of the fluorescence decay curves for the perylene-diethyl-aniline system in benzene, Ware and Richter34 find that... 

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

We can exclude a predissociation process [39] responsible for the decrease of the lifetime for three reasons, (i) Dispersed emission spectra did not show any indication of emission from the fragment monomer [40]. Thus no dissociation occurs on the time scale of the fluorescence emission, (ii) The additional excitation of the van der Waals stretching vibration in benzene-Ar does not lead to a further decrease of the lifetime, (iii) The stronger decrease of the lifetime of the 61 state in benzene-Kr would not be expeced for a predissociation process since the benzene-Kr complex is more strongly bound and has only a slightly higher density of states since the frequencies of the three van der Waals modes do not differ very much from that of benzene-Ar [41]. 

The calculated radiative lifetime of benzene vapor based on absorption coefficients111 is in excellent agreement with the mean lifetime measured by Donovan and Duncan112 and is about 5 x 10-7 sec. And yet this agreement must be partly fortuitous, since the emission yield is only about 0.2 and hence there must be processes which compete with radiative emission100 101. It is true that, if twenty per cent of the molecules emit, these competitive processes must have rate coefficients not greatly different from the rate coefficient of the process100... 

In solution, the quantum yields and lifetimes for fluorescence emission for benzene are dependent upon solvent (see Tables 8 and 9, and more detailed data in reference 70) and temperature (140), but yield has been shown (141-145) to be Independent of excitation wavelength within the first absorption band (X exc >... 

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. 

The lowest singlet excited state (iT- Tr ) of, for example, ethyl benzene, shows absorption and emission maxima at, respectively (1), n-260 and 280 nm. The temperature dependence of the emission in dilute polar organic solution has been investigated (lb), and it was found that there is a temperature dependent non-radiative rate component that follows an Arrhenius law Ae" / T with an activation energy AE v2300 cm-1 and AMO S-l, The ratio of the emission quantum yield ( e) and lifetime(T) is temperature independent, consistent with a temperature independent radiative rate constant k, and limiting low temperature values of and t, achieved by v250K, are 0.12 and 21 ns, respectively, in dichloroethane solution. 

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