Delayed fluorescence recombination

To these three types may possibly be added a fourth, namely, Recombination Delayed Fluorescence which has been reported by other workers in rigid media. It requires ejection of an electron as a first step and would therefore be expected to occur preferentially by excitation with high-energy quanta. A fifth source of delayed fluorescence— Triplet Excitation —might also conceivably operate in fluid solution. Since... 

The photooxidation of p-phenylenediamine to the Wurster s Blue radical cation apparently proceeds by photoionization of the excited triplet state of the neutral molecule,219 and it has been suggested that the delayed fluorescence of perylene may be partly due to photoionization of its triplet state and slow subsequent recombination of... 

Chlorophyll itself shows a short-lived red fluorescence in solution. The green plants show a delayed fluorescence of approximately the same spectrum under specific conditions in which the electron transport chain is blocked. This delayed fluorescence results from the recombination of the charges (a process well known in electroluminescence), and its kinetics are complex and the decay quite long (several seconds). 

Figure 3.53. The quenching kinetics at long times with and without bulk recombination of ions (solid and long dashed lines, respectively). The false IET asymptote p- 5 2) is indicated by a dotted line, while the true asymptotic behavior of delayed fluorescence (t 2) is shown by a short dashed line. All the parameters are the same as for Figure 3.52. (From Ref. 189.)...

Johnson and Willson interpreted the main feature of the observations on solid polyethylene doped with aromatic solutes in terms of an ionic mechanism it was analogous to that proposed for irradiated frozen glassy-alkane-systems in which ionization occurred with G = 3 — 4 [96], The produced charged species, electron and positive hole, were both mobile as indicated by the radiation-induced conductivity. The production of excited states of aromatic solutes was caused mainly by ion-electron neutralization. The ion-ion recombination was relatively slow but it might contribute to the delayed fluorescence observed. On the basis of Debye-Simoluchovski equation, they evaluated the diffusion coefficients of the radical anion of naphthalene and pyrene as approximately 4 x 10 12 and 1 x 10 12 m2 s 1 respectively the values were about three orders of magnitude less than those found in typical liquid systems. 

In the oxidized sample, a prompt Chl-a fluorescence with a 0.42 ns lifetime is seen [Fig. 3 (A), trace (a)]. In the sample maintained at -450 mV and illuminated to photoaccumulate O ", the prompt fluorescence lifetime is 0.18 ns [trace (b)]. In the sample maintained at -450 mV but kept in the dark, the signal shows a prompt and a delayed fluorescence (or delayed light emission) with lifetimes of 1.06 ns and 4.3 ns, respectively [trace (c)]. Note that in the extended time region [panel (B)], only trace (c) still has a measurable emission tail. These results show that neither the sample in the oxidized state nor that with O" photoaccumulated show any delayed light emission. Only the sample in which Qa is pre-reduced shows the A3-ns luminescence. These results support the notion that the delayed emission is a recombination luminescence originating from the [P680 <1)"] state. 

Delayed fluorescence in a rigid matrix proves triplet excitons if other delayed S- —+So emissions, as high temperature phosphorescence or ionisation followed by ion-electron recombination can be excluded (15). The delayed fluorescence observed in solid solutions of Polyriboadenylic acid (45) seems to be due to this latter process (78). 

The fluorescence emission discussed so far is produced by direct excitation of a molecule M to one of its excited singlet states that, after IC to Sx when an upper singlet state was initially populated, emits prompt fluorescence from Si with a lifetime on the order of nanoseconds. In addition, several processes can be envisaged that permit repopulation of Sx following ISC to Tx, which can give rise to emission that has the spectral characteristics of fluorescence, but a lifetime much longer than that of prompt fluorescence, that is, to delayed fluorescence. These processes can be sub-classified as P-type and 4E-type delayed fluorescence,32 We do not consider artificial delayed fluorescence due to impurities, or due to ionization followed by recombination with the formation of 1M. ... 

In isolated chloroplasts fluorescence emitted seconds after excitation has been assigned to recombination between positive and negative charges stored on the donor and acceptor sides, respectively, of photosystem (PS) II (1-3). This suggests that backreactions bring a PS II chlorophyll molecule to an excited state, supposedly the emitter of delayed fluorescence (4). However, while the spectral composition of prompt fluorescence has been extensively investigated, there have been only a few studies of the emission spectra of microsecond delayed fluorescence and almost none at longer delay times. 

Our results show that long delayed fluorescence in functioning chloroplasts at room temperature is emitted at 685 nm supporting earlier assumptions that delayed fluorescence originates in PS II. The 685 nm emission maximum suggests that emission takes place in the PS II core complex following reexcitation by charge recombination. 

It is accepted that there are two kinds of luminescence in ps-ns time domains related to the prompt fluorescence from primary excited state of P and delayed fluorescence from P which is secondary excited by the recombination of P+ and HL- [1,10]. 

A second key assumption which was discussed in the original paper is that the steady-state fluorescence is not dominated by delayed fluorescence. The precise origins of delayed fluorescence, especially components with lifetimes shorter than tens of ns and are not dependent on magnetic fields, is not yet clear [20,211. It is generally believed that all components involve recombination from ion-pair states, P X. Because the delayed, recombination fluorescence involves an activated process, it is reasonable to expect that the field dependence should be very strong, in contrast to the approximately quadratic dependence which is observed 5. Furthermore, the contribution in... 

In Table 8.10, reaction (1) describes the solvent ionisation event, producing S+ / e pairs, whose spin multiplicity is determined by the gemmate pair wavefunction. The intensity of delayed fluorescence of the solution is determined by the rate of radiative deactivation of D (reaction 10). For this reaction scheme, D states are produced via reaction (4) (recombination of D+ /e pairs [32, 37, 38]), from S to luminophore molecules as given in reaction (6) and via triplet-triplet annihilation of... 

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