Excitation quenching

The photochemical process built into 7 was encountered previously with regard to 2, Le, the capability of 02 in quenching excited states of sufficiently long lifetimes. In the case of 7, the process is so efficient that ambient levels of 02 completely kill off phosphorescence, even if the phosphor is enveloped by p-cyclodextrin. 

A two-photon process at 435 nm to generate O( D) has been observed in laboratory studies (Crowley and Carl, 1997). However, the combination of relatively low light intensities and high pressures which quench excited NOz make this unimportant in the atmosphere. 

Backstrom and Sandros pointed out that significant differences in the diameters of donor and acceptor should increase kdlC[ by a factor equal to %[2 + (dA/dD) + (dD/dA)].151 As long as donor and acceptor do not differ in size by a factor of more than two, as is usually the case when polyatomic organic compounds are being studied, such a correction amounts to less than 10%. However, when a difference as large as fivefold exists, as, for example, when oxygen quenches excited polynuclear aromatics, the correction factor becomes 100% or more. 

A study has been made of the relative efficiencies with which various transition metal chelates quench triplet benzophenone.194 The chelates vary widely in efficiency, and no generalizations can be drawn except that in some cases triplet energy transfer to a coupled metal-ligand triplet energy level probably accounts for at least part of the quenching. Rare earth ions can quench excited triplets by energy transfer, since, as discussed earlier, sensitized fluorescence of the metal ion results. 

Figure 3.42 Kinetics of dynamic (diffusional), D, and static, S, quenching. In dynamic quenching the excited state lifetime gets shorter with increasing quencher concentration, from to with no quencher to t1 t2 with added quencher. In static quenching excited state lifetime remains unchanged but the initial concentration of excited states is reduced...

K,MP to CC14, just as in the case of quenching excited states of simpler organic molecules in the presence of CC14 (see Chap. 7, Sect. 4). 

Oxygen also efficiently quenches excited triplet states of other molecules ( iA ) and, in accepting triplet energy, is itself promoted to an excited singlet state. Notice that the total spin orientation is conserved ... 

P. L. Lijnse, Review of Literature on Quenching, Excitation and Mixing Cross Sections for the First Resonance Doublets of the Alkalies Report i-398, Fysish Laboratorium, Utrecht University, 1972. 

The fact that one molecule can quench the excited state of another by energy transfer (or by other means—see below) enables us to calculate the lifetime the quenched excited state has in the absence of quencher. 

Physically, Perrin s equation is valid for static quenching excited molecules and quenchers don t move in and out the quenching volume within the lifetime of the excited state. 

A significant drawback of metals for photoelectrochemical applications lies in their ability to efficiently quench excited states via energy transfer processes, as discussed below. Direct detection of photosensitized electron transfer to or from a metal electrode surface has been observed [30]. However, unlike dye-sensitized semiconductor systems, little examination of the kinetics of such systems has yet been undertaken. 

Before the concerted vs. two-step question was further elucidated, another basic mechanistic puzzle was raised. One research group found that type II cleavage of 2-pentanone was quenched by biacetyl [6], which was known to quench excited triplets rapidly. Another group found that the reaction of 2-hexanone was not quenched under the same conditions [7]. The two groups obviously differed as to which excited state undergoes the reaction. The apparent conflict was neatly solved by the revelation that each of the two ketones reacts from both states, with 2-hexanone undergoing more unquenchable singlet reaction than 2-pentanone [8,9]. 

Many bioactive phytochemicals were systematically studied after their fractionation and characterization from many vegetables and fruits. New biological effects such as the differentiation-inducing activity of carotenoids— particularly vitamin A—were described in the 1930s. In addition, several experimental results suggested that various carotenoids might serve as dietary chemopreventive agents [6] because of their ability to quench excited molecules and excess radicals. 

The fluorescence emission of chlorophylls and bacteriochlorophylls is invariably emitted from the lowest energy excited sate, with typical Stokes shifts of 10-15 run from the Qy absorbance maximum. Fluorescence lifetimes of 2-5 ns are usually observed for monomeric pigments in organic solvents. Fluorescence of chlorophyll and bacteriochlorophyll in vivo is very strongly quenched excited-state lifetimes are typically a few tens of picoseconds and fluorescence quantum yields are at most a few percent. The... 

Upon selective excitation at 598 nm of the Zn porphyrin component of 19, a weak fluorescence, with a quantum yield of 0.0012 and a lifetime of 55 ps, is observed, which corresponds to ca. 97 % fluorescence quenching. Excitation of 19 with a 30-ps... 

Since molecular oxygen is an ubiquitous impurity, the processes by whfch it may quench excited states have been widely studied. Quenching of fluorescent sing)et states may occur through (35) or (36), with very high efficiencies. 

Specific solute-sorbent interactions related to steric effects. Sorption of solutes to NOM has been explained in terms of association or trapping of the solutes in hydrophobic cavities embedded within NOM aggregates [discussed by Schlautman and Morgan (34)]. The NOM is a mixture of chromophores that are capable of quenching excited states or photosensitizing reactions. The net effect may depend on the position of the sorbed solute within hydrophobic cavities of this mixture. The position may, in turn, depend on the size and shape of the solute. 

The relevance of the above kind of electronic shape resonance to chemistry Is twofold. First, In environments such as plasmas, electrochemical cells, and the ionosphere, where free electrons are prevalent, the formation of such temporary anions can provide avenues for the free electrons to "cool down by transferring kinetic energy to the Internal (vibrational and/or electronic) degrees of freedom of the fragment. (6-14) Second, metastable states may play Important roles in quenching excited electronic... 

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