Lifetime singlet

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

In the flash photolysis technique a large population of ground state molecules are raised to an excited singlet state by the initial photolysis flash. In a time r (singlet lifetime) after the photolysis flash a certain proportion of... 

The rate constant for the singlet di-7r-methane photorearrangement can be determined from the quantum yield for the reaction and the experimentally measured singlet lifetime ... 

In this we have assumed that the back reaction, fc isc,is negligible. Because the absorption of light is so rapid compared with the decay, it is further assumed that absorption is instantaneous. The observed singlet lifetime is... 

A description of a fast laser photolysis experimental arrangement has been given by Porter and Topp who used a 1.5 Joule, 20nsec ruby giant pulse, frequency doubled in ADP, to measure singlet lifetimes in phenantrene, pyrene and other organic molecules. 

Heavy atom enhancement of intersystem crossing has been used to determine the mechanism of acridine photoreduction in ethanol.115 It was found that addition of sodium iodide decreased the fluorescence intensity and the rate of disappearance of acridine to the same extent, confirming that the singlet state is responsible for photoreduction. From the increase in triplet state absorption upon addition of iodide it was found that Of for acridine was 0.76. Thus the short singlet lifetime (0.8 nsec) of acridine is due to rapid inter-system crossing to unreactive triplet states. 

Singlet lifetimes, fluorescence quantum yields, and singlet oxygen quantum yields are compared for Rose Bengal, Erythrosin, Eosin, and fluorescein in a number of different solvents in Table 11. In virtually every case water differs from the other solvents studied. 

TABLE 11 Singlet Lifetimes, Fluorescence Quantum Yields, and Relative Singlet Oxygen Yield as a Function of Solvent... 

The fluorescence Intensity of substituted stilbenes and stilbene analogues provides a useful indicator of photochemical reactivity. Virtually all of the reported bimolecular photochemical reactions of electronically excited stilbenes involve stilbenes which are fluorescent at room temperature in solution. The absence of fluorescence is indicative of a singlet lifetime too short (< 100 ps) to allow for efficient bimolecular quenching. 

This is illustrated in Fig. 7. This implies motion during the pyrene singlet lifetime, which is of the order of several hundred nanoseconds. Further evidence Is seen in the quenching experiment to be described below. 

A plot of the left-hand side of Eq. (6) against VJVm could allow the evaluation of K and k°m. However, due to limitations in the range of VsIVm experimentally available, the error in the evaluation of °xp is too high and, at most, only the product Kk°m can be approximately obtained. This makes the use of this approach for the evaluation of the partition K and/or the singlet lifetime in the dispersed pseudophase difficult. 

Figure 42 Stern-Volmer plots for fluorescence quenching of PBAC by Co(phen)3+ in the presence of 0.008% BAZrP. Using 200 nsec as the singlet lifetime of PBAC, the rate constant for quenching is calculated to be 3 X 1012 M-1sec. This is much too fast for a dynamic process and may involve long-range electron transfer. (From Ref. 17. Copyright 1995 Overseas Publishers Association.)...

Unlike the reversed shift of the emission band compared to the dihydrogen addend type, the singlet lifetime in the bis- and tris(bis-(ethoxycarbonyl)-methylene) derivatives is increased comparable to the former multiple adducts. The values range from 1.7 to 3.1 ns (tris-adduct), depending on the distorted T7-electron system of the fullerene core [67,108], In comparison to C6o, the fluorescence quantum yield for the malonic ester hexaadduct is increased by the factor 10 (30 X 10 4) [67,111], In the case of both pyrrolidino hexa-adducts (Th 14 and D3 15, Fig. 13), the effect is remarkably higher. The fluorescence quantum yields are increased about 100-fold (-0.02) compared to C6o. On the other hand, the singlet lifetime is only partly increased with -3.5 ns [111,112],... 

In principal, electron transfer reactions with fullerenes could occur via both the singlet- and triplet-excited state. However, due to the short singlet lifetime and the efficient intersystem crossing, intermolecular electron transfer reactions usually occur with the much longer lived triplet-excited state. The result of the electron transfer is a radical ion pair of fullerene and electron donor or acceptor. 

The singlet lifetime is somewhat longer than that of 1DCA. Additionally, the reduced form of the oxidant is a neutral radical exerting less coulombic at-... 

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