Excited 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. 

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 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]

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... 

A novel face-to-face dimeric porphyrin system was reported recently by Lehn et al. [5,6] in which the two porphyrin rings are held apart by [18]-N204-aza-oxamacrocycles (1). Spectroscopic studies showed that the two porphyrin rings interact together via exciton coupling [7]. This effect serves to shorten the excited singlet state lifetime (Xg = 1.22 ns) and to reduce the fluorescence quantum yield with respect to the corresponding monomeric porphyrin. For 1 in acetonitrile, the quantum yield for formation of the porphyrin excited triplet state was only 0.18, compared to a value of 0.67... 

The above results have to be compared to earlier studies [19] carried-out with the quinone-bearing porphyrin monomer 8. Here, it was established by transient absorption spectroscopy that the excited singlet state lifetime of the porphyrin subunit was reduced to <350 fs, compared to 11.9 ns observed for the quinone-free monomer 7. The transient absorption spectral features observed after decay of the excited singlet state of 8 could be assigned to the porphyrin n-radical cation, as formed by electron transfer... 

The electronic absorption spectrum recorded for 16 in N,N-dimethylformamide (DMF) is similar to that of the corresponding monomeric porphyrin and there was no indication of exciton interaction between the two porphyrin rings [16]. Fluorescence was not observed, presumably because the excited singlet state lifetime is shortened due to the internal heavy-atom effect, but weak phosphorescence was detected in a methanol glass at 77K with a maximum at 690 nm. Excitation of 16 with a... 

Wasielewski, M.R., Kispert, ED. (1986). Direct measurement of the lowest excited singlet state lifetime of all-frans-P-carotene and related carotenoids. Chemkal Physics Letters, Vol. 128, No. 3, pp. 238-243, ISSN 0009-2614. 

The behaviour of a molecule immediately after absorption of a photon is worth considering in detail (6). M( t organic molecules are in the ground singlet state. The absorption of an energy quantum raises the molecule to an excited singlet state (lifetime not exceeding 10 sec). 

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