Lifetime of the triplet state

It will be recalled from previous discussions that the lifetime of the triplet state is given by... 

A linear plot indicates that the luminescence decay is exponential. The slope of the line gives kt, and rt can be calculated as above. The lifetime obtained by measuring the decay of P-type delayed fluorescence is equal to one-half the lifetime of the triplet state (see Section 5.2). Since in fluid solution at room temperature phosphorescence is generally much weaker than delayed fluorescence, the measurement of delayed fluorescence decay offers a convenient method for determining the lifetime of triplets at room temperature. 

On the contrary, at low temperatures and/or in a rigid medium, phosphorescence can be observed. The lifetime of the triplet state may, under these conditions, be long enough to observe phosphorescence on a time-scale up to seconds, even minutes or more. 

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

Following a b-pulsc excitation, a fraction of excited molecules can reach the triplet state, from which they return to the ground state either radiatively or non-radiatively. The concentration of molecules in the triplet state decays exponentially with a time constant tt representing the lifetime of the triplet state... 

Equation (4) describes a partition between two forms of the complex one, with fraction f, that exhibits quenching and one that does not. Equation (5) corresponds to a distribution of forms with a range of quenching rates whose breadth varies inversely with n the occurrence of distributions is well-established for proteins at low temperatures [29]. In either case, the non-exponential kinbtics necessitate that conformational interconversion in the transition range is slow compared to the lifetime of the triplet state and the gating limit is applicable. 

Using laser flash photolysis with a frequency-quadrupled neodynium laser, Stevens and al 161b) measured the lifetime of the triplet state of fluoro- and pentafluoro-benzene in the gas phase along with the energy transfer efficiencies to cis-2-butane and oxygen. The triplet transient absorption decay was found to be predominantly first order with a... 

Until recently the lifetimes of the triplet states of aromatic hydrocarbons in fluid solution at room temperature had been investigated exclusively by the technique of flash absorption spectroscopy. The lifetimes reported for many hydrocarbons, e.g., anthracene or phenan-threne, had been below 1 msec, and it had been assumed that radiationless conversion processes were so rapid under these conditions that the competing radiative triplet-singlet transition would be too slow to per-... 

The lifetime of the triplet state (probably 3B2u) of benzene has recently been determined in two different laboratories26,28 with agreement within a factor of about two. Thus, barring improbable effects of impurity, the mean lifetime of the triplet state of benzene in the gaseous... 

Chemical reaction may occur - the longer lifetime of the triplet state compared to the S i state means that chemical reaction is a much more important feature, and is of prime importance in synthetic photochemistry. 

The emission step (117) does not involve a change in multiplicity whereas (122) does. The lifetime of the triplet state of acetone should be longer than that of the singlet state and indeed it is 2 x 10 4 sec as determined by Duncan and Kaskan60. The lifetime of the triplet state of biacetyl (126) is even longer, about 2x 10 3 sec60,62,63. However in both cases as well as for (117) the molecules in question take part in several concurrent reactions and the experimentally determined lifetimes must be shorter than the true radiative lifetimes. 

In the absence of quencher, the guaiacoxy group reduces the lifetime of the triplet state from > 15 ps in acetoveratrone to -500 ns in the case of a-guaiacoxyacetoveratrone thus, at least 97% of the triplets must decay by processes (deactivation or cleavage) that involve the guaiacoxy group. 

Owing to the longer lifetime of the triplet state it is expected that the protolytic reaction will usually reach equilibrium within the lifetime of the state. Unlike the fluorescence titration method for pAr(S1) described above, the triplet-triplet absorption technique leads directly to pA"(T ]) without the necessity for a knowledge of lifetimes. Phosphorescence titration studies, on the other hand, will involve the lifetime term log t0/To just as for fluorescence. 

The MP-CfioC i) state formed via the intramolecular singlet-energy transfer in the OPVn Cgo dyads is expected to decay predominantly via intersys-tem crossing to the MP-Cfjo(Ti) state, apart from some radiative decay. Consistent with this expectation, the PIA spectrum recorded for all four dyads in toluene solution shows the characteristic MP Cf,o T <— Tn absorption at 1.78 eV with a shoulder at 1.54 eV (Fig. 1.28a). The PIA bands increase in a near-linear fashion with the excitation intensity (—AT oc Ip, p = 0.80-1.00) consistent with a monomolecular decay mechanism. The lifetime of the triplet state lies in the range 140-280 ps. 

Two of the four investigated naphthyridines (1,8- and 2,7-naphthyridine with C%v symmetry) have one spin sublevel that is radiatively forbidden, the other two have all spin sub-levels active. Phosphorescent emission have been observed for all naphthyridines, but the radiative lifetimes are not available. The measured lifetime of 1,5-naphthyridine (0.02 s) is much shorter, than our radiative value. This again indicates that vibronic coupling (leading to non-radiative or radiative decay) is a strong contributor to the lifetimes of the triplet states of azanaphthalenes, in a perturbative sense thus stronger than the action of dipole and spin-orbit coupling. 

It should be noted that in a magnetic field the m=0 spin triplet component and the spin singlet component of positronium mix. The annihilation rate of the triplet increases with increasing field. In the present apparatus with a magnetic field of about 0.02 Tesla this lowers the vacuum lifetime of the triplet state from 142 ns to 140.8 ns. This is well within experimental uncertainties and will be ignored [2,17],... 

Since so (trans) < 0.005], it is possible to rule out a photosensitized isomerization mechanism involving the establishment of thermal equilibrium between 11-cis and all-trans forms during the lifetime of the triplet state. Also excluded is the quantitative population of a common minimum along the C q-C 2 torsional coordinate. This conclusion is consistent with recent studies (169,175) which, in variance with the previous investigation (171), have reported different T-T spectra upon excitation of all-trans and 11-cis retinal. 

The very broad absorption between 300 and 400 nm is comparable to the absorption spectrum of the triplet state of polystyrene (22). The lifetime of this intermediate is essentially independent of the chloromethylation ratio and is comparable to the lifetime of the triplet state of polystyrene (110 ns). 

Analogous considerations have been invoked [23] in order to explain the shorter lifetime of the triplet state in poly(VBP) against free benzophenone in the presence of THF and the higher efficiency of the polymeric system in the photoinduced hydrogen abstraction from the above solvent (Table 3) as well as in... 

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