Rate constant fluorescence

From these two experimentally measurable quantities, f and 17, one can obtain the fluorescence rate constant. For many compounds klc = 0 then with the knowledge of kf> klsc can be determined. 

Once the fluorescence quantum yield has been determined, all that is required to calculate the fluorescence rate constant kf is the fluorescence lifetime rf. Direct measurement of this quantity, like the measurement of the fluorescence quantum yield, is difficult, in this case because of the short lifetime of the fluorescent state (shorter than the normal flash from a flash lamp ). There are, however, several methods which have been developed to determine fluorescence lifetimes and these will be the subject of this section. 

The average fluorescence rate constants and kFr are assumed to be the same for all donors and traps, respectively. The integrated fluorescence FD(t) and FT(t) are equal to the probability that the electronic excitation has left the system by donor and trap fluorescence, respectively, at time t after irradiation. The parameter FD(t) is thus equal to the probability that electronic excitation is neither on a site i nor on a site J. 

Little is known about the fluorescence of the chla spectral forms. It was recently suggested, on the basis of gaussian curve analysis combined with band calculations, that each of the spectral forms of PSII antenna has a separate emission, with Stokes shifts between 2nm and 3nm [133]. These values are much smaller than those for chla in non-polar solvents (6-8 nm). This is due to the narrow band widths of the spectral forms, as the shift is determined by the absorption band width for thermally relaxed excited states [157]. The fluorescence rate constants are expected to be rather similar for the different forms as their gaussian band widths are similar [71], It is thought that the fluorescence yields are also probably rather similar as the emission of the sj tral forms is closely approximated by a Boltzmann distribution at room temperature for both LHCII and total PSII antenna [71, 133]. 

Stevens401 irradiated anthracene, 9-phenylanthracene, and 9,10-diphenylanthracene at 3600 A and 280-300°C. He found that both 02 and NO quenched the fluorescence with similar efficiencies. For anthracene at 280°C and 9-phenylanthracene at 300°C, the ratios of the quenching rate constant for NO to the fluorescence rate constant are, respectively, 1120 and 1380 M 1. Ware and Cunningham4384 found the rate constant to be 1.97 x 1011 M 1 sec-1 for the quenching of anthracene vapor by NO at 280°C. They also found the anthracene-fluorescence constant to be 3.51 x 107 sec-1. The ratio of their two rate constants is 5500 M-1, about a factor of five larger than that found by Stevens. 

Fig. 1 A schematic view of the donor-acceptor photophysics. D/A and D /A correspond to the ground and excited donor/acceptor, respectively. It is assumed that only the donor is photoexcited at the rate of k X. Icet is the donor-to-acceptor energy transfer rate constant, and ko/kA are the free donor/acceptor fluorescence rate constants.

The fluorescence rate constant kp is equal to that of acetaldehyde and somewhat larger than that of other simple aldehydes, but is smaller than that of the simple ketones (see Table 2). 

Radiative Processes. As was stated previously, the fluorescence rate constants of formaldehyde and acetaldehyde are larger than those of the higher aldehydes. When the radiative lifetime, TQ(expt), deduced from observed values of tr and by eq. 8 is compared to the values of tr(SB) calculated from the Strickler-Berg equation,... 

Thayer, et al. (242a) have measured Tp values for the discrete vibrational levels of propynal. While the fluorescence rate constant (or tr) was directly determined for only the vibrationless level of the excited state, their analysis of the higher vibrational data is consistent with the assumption that tr is invariant with the vibrational level excited. Since this observation shows a distinct contrast to the case of formaldehyde, it would be quite interesting to study propynal theoretically and compare the result to formaldehyde. This will require knowledge of the transition moment matrix elements a., which are at present unknown. 

The small S-T splitting in Cm (A s,t kcal/mol) is probably a result of the large diameter of the molecule and the resulting small electron-electron repulsion energy." This small splitting, the very low value of the fluorescence rate constant, and the expected large spin-orbital interaction in the spherical Cm explain why intersystem crossing (ISC) is a dominant process. 

Calculation of Fluorescence Rate Constants from Absorption Spectra... 

Equation 2.10, which permit the determination of fluorescence rate constants kf from the broad absorption spectra of molecules, have been derived. The best known is that of Strickler and Berg.31 A simpler version that is easier to use (Equation 2.11)32 is sufficient for most practical purposes ... 

Equation 2.11 Determination of fluorescence rate constants from absorption spectra... 

Many linear polyenes exhibit anomalous fluorescence behaviour in the sense that the fluorescence rate constant kf calculated from the absorption spectrum (Equation 2.11) is much smaller than that determined by lifetime and quantum yield measurements (Equation 3.33). In 1972, Hudson and Kohler reported high-resolution absorption and emission spectra of all-trart.v-l, 8-diphenylocta-l, 3,5,7-tetraene at low temperature, which proved that the lowest excited singlet state Si was not that reached by the strongly allowed 7t,Jt transition (/ 1.5) that is predicted by MO theory and observed at 410nm.3" Rather, very weak (f 0.06), structured absorption that had been hidden under the tail of the jt,jt -absorption in solution spectra was detected at slightly longer wavelengths. 

The fluorescence rate constants of aromatic compounds predicted by Equation 2.11 differ by about two orders of magnitude depending on the nature of the lowest singlet state the smaller representatives with S i 1 Lb have iffs2x 106 s 1 and the larger ones with... 

We consider three decay channels for D in addition to injection Fluorescence (rate constant k ), intramolecular radiationless decay (rate constant k ), and energy transfer quenching within the adsorbed layer (rate constant kg) ... 

Samanta et al. calculated a fluorescence rate constant of 2.4 x 10 s for the triphenylmethyl cation, an order of magnitude larger than for the xanthyl or thioxanthyl cations [15]. The fluorescence quantum yield for the triphenylmethyl cation is < 10, suggesting that its nonradiative rate constant is > 10 ... 

It would then appear that the flash photolysis-resonance fluorescence rate constant data of Atkinson and Pitts (169, 189) and Atkinson, Perry, and Fitts (184, 187) [which are generally In good agreement with the relative-rate data (128, 142, 143,... 

For details, see Michl, J. Bonacic-Koutecky, V. Electronic Aspects of Organic Photochemistry Wiley-Interscience New York, 1990 p. 74. In general, weak absorptions predict long inherent lifetimes, while strong absorptions are associated with high fluorescence rate constants and short inherent fluorescence lifetimes. 

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