Singlet state lifetime

The excited singlet-state lifetime, is the time taken for the concentration of Si to decrease to 1/e of its initial value. 

The technique of time-correlated single-photon counting (Figure 3.4) is used to measure an excited singlet-state lifetime, x. The sample is irradiated with a very short-duration light pulse ( lns) to ensure any given molecule will only be excited once during the pulse. As soon as... 

Since is greater than kf, the observed excited singlet-state lifetime is less than the excited singlet-state radiative lifetime. h only approaches ho as intersystem crossing and internal conversion from Si become much slower processes than fluorescence. 

In summary, the merocyanine lifetime depends on its particular isomeric form and this, itself, is affected by the solvent and substituents. The longest merocyanine isomer singlet-state lifetimes reported are 4 nsec and the shorter ones are tens of picoseconds. After excitation of the merocyanine, a bleached state can form, which leads to the closed form in polar solvents, but the full ring closure is blocked in nonpolar media and the bleached state recovers to its original state. The bleached state is possibly an isomer cis about the central p-methine bond. Scheme 17 describes the transient bleaching of the merocyanine state. 

The direct irradiation of the parent coumarin in the presence of alkenes results only in an inefficient photodimerization and [2 + 2]-photocycloaddition. Lewis acid coordination appears to increase the singlet state lifetime, and leads to improved yields in the stereospecific [2 + 2]-photocycloaddition [95]. Alternatively, triplet sensitization can be employed to facilitate a [2 + 2]-photocycloaddition. Yields of intramolecular [2 + 2]-photocycloadditions remain, however, even with electron-rich alkenes in the medium range at best. The preference for HT addition and for formation of the exo-product is in line with mechanistic considerations discussed earlier for other triplet [2 + 2]-photocycloadditions [96, 97]. Substituted coumarins were found to react more efficiently than the parent compound, even under conditions of direct irradiation. 3-Substituted coumarins, for example, 3-methoxy-carbonylcoumarin [98], are most useful and have been exploited extensively. The reaction of 3-ethoxycarbonylcoumarin (100) with 3-methyl-l-butene yielded cleanly the cyclobutane 101 (Scheme 6.36) with a pronounced preference for the exo-product (d.r. = 91/9). Product 101 underwent a ring-opening/ring-closure sequence upon treatment with dimethylsulfoxonium methylide to generate a tetrahydrodibenzofur-an, which was further converted into the natural product ( )-linderol A (102) [99]. 

Finally, it should be noted that singlet-state lifetimes in [Re(L)(CO)3(bpy)], are long enough to allow for ultrafast electron or energy transfer in supramolecular assemblies, at surfaces or molecule/nanoparticle interfaces, see Sect. 7.3. Indeed, a hot electron injection has been seen with Ti02 nanoparticles [42] or in Re-labeled redox proteins [43],... 

Another concern specific to the use of porphyrin-based photosensitizers is the fact that singlet state lifetimes are usually less than 10 ns and photosensitization occurs mostly from the triplet state. This requires the selection of systems that efficiently generate triplet states, which is not the case of paramagnetic metalloporphyrins. 

The macrocycles of conformationally strained porphyrins, and, in particular, 2,3,7,8,12,13,17,18-octaalkyl-5,10,15,20-tetraarylporphrins, tend to adopt nonplanar conformations. This has dramatic consequences in singlet and triplet lifetimes. Increasing the macrocycle distortion with the introduction of four or more ethyl groups in adjacent P positions decreases triplet lifetimes to submicrosecond values and compromises This factor adds to the decrease of due to decrease of the singlet state lifetime, and intersystems crossing and relatively short-lived triplet states (69), which are inappropriate for PDT. 

Triad 39 was reported by Borovkov and coworkers [82]. Although a model for the porphyrin moiety of these compounds has an excited singlet state lifetime of... 

There have been studies in which pyrene alone has been used as the fluorescent probe for in situ polymerization (Pekcan et al, 1997, 2001, Pekcan and Kaya, 2001). It is assumed that the observed enhancement of fluorescence intensity or increase in singlet-state lifetime of the monomeric pyrene as the network forms arises Irom a decrease in bimolecular quenching events as the viscosity increases and Atq decreases (Equations (3.28) and (3.29)). It is noted that the spectra showed no evidence of pyrene excimer formation, suggesting that the pyrene is isolated in the network at the concentrations employed. 

There are only a limited number of intermolecular [2 + 2] photocycloaddition reactions known to occur in solution because alkene excited singlet state lifetimes are very short (on the order of 10 ns).708 Direct irradiation of neat but-2-ene, for example, yields tetramethylcyclobutane with stereochemistry suggesting the concerted mechanism (Scheme 6.45).709 Inefficient dimerization (

with efficient E Z isomerization (

[Pg.257]

Sulfoxides Ketosulfoxide derivatives (10.19), where R is an alkyl or a phenyl group, exhibit two cleavage processes [34] one in the triplet state (lifetime 7.5 ns for R = methyl) and another in the singlet state (lifetime 3 ns). 

Tsukida K, Saiki K, Takii T and Koyama Y (1982) Separation and determination of cis/lrans-P-caTOtenes by high-performance liquid chromatography. J Chromatogr 245 359-364 Wasielewski MR, Johnson DG, Bradford EG and Kispert LD (1989) Temperature dependence ofthe lowest excited singlet-state lifetime ofall-fi o/w-/8-carotene and fully deuterated all-trans-P-carotene. J Chem Phys 91 6691-6697 Watanabe J, Takahashi H, Nakahara J and Kushida T (1993) Subpicosecond dynamic Stokes shift in /3-carotene solution probed by excitation energy dependence of fluorescence spectrum. Chem Phys Lett 213 351-355 Zechmeister L (1962) Cis-trans isomeric carotenoids vitamins A and arylpolyenes. Academic Press, New York... 

Truscott TG (1990) The photophysics and photochemistry of the carotenoids. Photochem Photobiol B Biol 6 359-371 Wasielewski MA and Kispert LD (1986) Direct measurement of the lowest excited singlet state lifetime of all-trans-)3-carotene and related carotenoids. Chem Phys Lett 128 238-243 Yamamoto HY and Bassi R (1995) Carotenoids localization and function. In Oxygenic Photocynthesis The Light Reactions Ort DR and Yocum CF (eds) Advances in Photosynthesis, Kluwer Academic Publishers, Dordrecht Yamamoto HY (1979) Biochemistry of the violaxanthin cycle. Pure Appl Chem 51 639-64... 

Aggregation of the pyrochlorophyll a dimer molecules could also be responsible for the large variation of lifetime with solvent and temperature. The degree of aggregation of chlorophyll a is sensitive to concentration. The excited singlet-state lifetime of folded pyrochlorophyll a dimer in each chlorocarbon solvent used in this study does not depend on concentration in the range 10 to 10 M. 

Unlike the bis[4-(dimethylamino)phenyl]squaraine derivatives, the pyrrole derivative 25 (Structure 7) is relatively nonfluorescent (cjif = 0.08) with an extremely short excited singlet state lifetime (tf = 222 ps) [70]. The intersystem... 

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