Triplet of 2, lifetime

Since the first two processes are spin-forbidden, it can clearly be seen that in the absence of triplet quenchers (e.g., oxygen) the triplet will be long lived. Consequently the experimental determination of the lifetime of triplet states... 

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. 

Flash photolysis has been used to study the triplet-triplet absorption spectra (T1 -> Tq) of a number of aromatic molecules both in solution and in the gas phase.<34) A disadvantage associated with the flash technique for obtaining triplet-triplet absorption spectra is that the transient absorption can occur for only a short time after the initial flash (determined by the lifetime of triplet... 

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

If quenching is a diffusion-controlled process (k 3xl09L mol 1 s ), the lifetime t 3x 10-7 s coincides with the lifetime of triplet acetophenone (product of peroxyl radical disproportionation in oxidized ethylbenzene). 

The thermodynamic stabilities of phenonium ions relative to the parent have been determined in the gas phase by measuring the position of the equilibrium between (46) and (47)7 The results followed a Yukawa-Tsuno relationship with a p value of -12.6 and an r+ value of 0.62, the general behaviour being more like benzenium ions than benzyl cations, with tt-delocalization less effective than in benzyl cations. A theoretical study of the elimination of molecular H2 from the benzenium ion C6H7+ shows that the barrier to this process appears to be very small.The gas-phase Friedel-Crafts alkylation reaction of CF3C6L6+ (L = H or D) with C2L4 is accompanied by isotopic scrambling, which has been used to elucidate the mechanism of this process. A theoretical calculation shows that the lifetime of triplet phenyl cation must be very short. ... 

Deuteration has been previously shown to cause an increase in the lifetime of triplet free-base porphyrins ( 7). This has been attributed to the strong coupling of N-H tautomerism with nonradiative decay. In the case of mesoporphyrin IX the increase upon deuteration is approximately two-fold ( ) As indicated in Table III deuteration of the picket fence porphyrin results in little change in the photostationary state composition but an almost twofold increase in the quantum yield of 4,0 -> 3>1. As stated above there is no measurable deuterium isotope effect on the thermal reaction the proportionate increase in quantum yield and triplet lifetime upon deuteration of the picket fence porphyrin is thus completely consistent with the adiabatic mechanism described above. Although the evidence amassed does not completely rule out other possibilities, it seems that the photoatropisomerization is to date best described by the adiabatic pathway in which the porphyrin ground and excited state potential surfaces are modified much as illustrated in Figure 3. 

Radiative lifetime of triplets. Biochemical effects Neutralization time for media at very high viscosity. Time for mitosis. Biochemical effects of metastables. DNA synthesis time Biological effects Late biological effects... 

To estimate the lifetime of triplet carbene 1061 under our standard condition, that is, in degassed benzene at 20 °C, LFP of 1051 was carried out. However, the lifetime of 1061 was too long to be monitored by the LFP system, and a conventional UV-vis spectroscopic method was more conveniently employed in this case. The transient signals due to 1061 did not disappear completely even after 1 h under these conditions. The decay was analyzed as a sum of two exponential decays... 

Concentration of pyrene, c Rate of light absorption,11 einstein liter- sec.-1, Ia Efficiency of delayed fluorescence of monomer, 0M Lifetime of triplet,11 sec., rt ... 

The intrinsic lifetime of triplet Ti state tp°, is the reciprocal of the rate constant kp, for phosphorescence emission and the actual lifetime if, is the reciprocal of the sum of all the steps which deactivate tne triplet. 

If rate constants are competitive, we expect the emission of prompt fluorescence, phosphorescence and delayed fluorescence of energy quanta /7v/, h jp and //vn) respectively. Although //v/ is equal to ftvED, the lifetime of delayed fluorescence will match the lifetime of triplet decay. The rate constant ArEn for E-type delayed fluorescence is temperature dependent and can be expressed as... 

The lifetime of triplet acetone at 25° in the vapor phase, as measured from the rate of decay of phosphorescence, is 0.0002 sec,318 so that the rate of decay is 5 x 103 sec-1. This figure represents the sum of the rates of all decay processes. Since the data at 40° 308 indicate that decomposition and internal conversion of triplet acetone occur approximately 40 times as fast as emission, the radiative lifetime must be on the order of 0.01 sec. Measurements of the rate of phosphorescence decay from solid acetone at 77°K, where all activated fragmentation and most radiationless decay normally disappear, have actually yielded values approximately one-tenth as large as that obtained in the gas phase at room temperature.319 The most recent measurements of the lifetime of triplet acetone at 77°K in frozen glasses does indeed yield an estimate of 0.01 sec for the radiative lifetime of triplet acetone.318... 

Photochemistry of g-Phenylproplophenone. The short lifetime of triplet 6-phenylproplophenone in solution has been recognized for a number of years (16-19) this efficient deactivation makes this ketone particularly photostable (21). In earlier studies we have shown that the efficiency of intramolecular quenching is controlled by the ability of the ketone to achieve conformation (16). 

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 optical and PL spectroscopies have been undertaken to understand the structure-property correlations of this important family of triplet-emitting polymers. The red shift in the absorption features upon coordination of the metal groups is consistent with there being an increase in conjugation length over the molecule through the metal center. The trade-olf relationship between the phosphorescence parameters (such as emission wavelength, quantum yield, rates of radiative and nonradiative decay) and the optical gap will be formulated. For systems with third-row transition metal chromophores in which the ISC efficiency is close to 100%,76-78 the phosphorescence radiative (kr)y, and nonradiative (/cm)p decay rates are related to the measured lifetime of triplet emission (tp) and the phosphorescence quantum yield ([Pg.300]

The phosphorescence decay kinetics of the triplet excited states of CuP molecules (Fig. 14) is adequately described by Eq. (16). Using this equation one can obtain the values of the parameter p = (Tra /2) In2 veT from the initial non-exponential part of the phosphorescence decay curves and the values of t = l/ k, i.e. the characteristic time of phosphorescence decay, from the final exponential part. Then the data on the dependence of the quantum yield of CuP phosphorescence on the concentration of C(N02)4 have been used to estimate the effective radii of electron tunneling from triplet excited copper porphyrins to C(N02)4 within the time x R, = (ac/2) In vet (Table 3). In doing so, the quenching of CuP luminescence by electron abstraction was assumed to be the only process leading to a decrease in the quantum yield of CuP phosphorescence in the presence of C(N02)4. From Table 3 an electron is seen to tunnel, within the lifetime of triplet excited states x at 10-4s, from CuP particles to C(N02)4 molecules over the distance R, 11 A. Further, the parameter vc and ae for different porphyrins were estimated from the values of (3, Rt, and x. These values are also cited in Table 3. 

The effect of temperature on the PL spectra is important as regards the internal conversion between the lowest excited triplet state and the ground state, because of the longer lifetimes of triplet states (Anpo and Che, 1999). 

Figure 38 Relative increase in the monomolecular decay rate constant (A/ // t) (decrease in the lifetime) of triplet excitons in three different anthracene crystals under the positive voltage applied to two different hole injecting electrodes Cul (a) and anthracene positive ions (A+) in nitromethane (b). jSt = t 1 is the triplet decay rate constant with no voltage jh = 239 s-1 for the d = 350 pm-thick crystal, / t = 175s 1 for anthracene with d = 625 pm (from Ref. 243) / t = 200s 1 for the d = 320 pm-thick crystal, A+ injecting contact (see Ref. 238). In the right-top scale in part (b) the Aji/U vs. j/U2 is presented (points) to be compared with Eq. (115) (solid line).

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