Dark excited states

Non-fluorescing excited states of alkanes. Several relatively long-lived non-fluorescing ("dark") excited states of hydrocarbons capable of sensitizing singlet solute luminescence have been proposed in recent years. 

An additional reason for excited state broadening may be lifetime broadening. As discussed below, most of the nn excited state returns to the ground state on a picosecond timescale by internal conversion. However, there is a small quantum yield for a process leading to a dark excited state of jt character or a triplet state or Kunitski et al. have characterized this dark state for... 

A suitable functional should obviously also provide reliable results, namely (i) accurate ground-state geometries/vibrational frequencies, (ii) balanced description of valence and Rydberg states, (iii) balanced description of bright and dark excited states, (iv) good excited-state geometries/vibrational frequencies, and (v) a correct description of charge transfer states. 

A simple aliphatic ketone such as acetone, when promoted to its n,n excited state, undergoes a single unimolecular photochemical reaction in high quantum yield namely a-cleavage giving a methyl and acetyl radical which react further in secondary dark processes. In general, competition... 

In 1977, Koo and Schuster studied the CL emission produced when diphe-noyl peroxide was decomposed at 24°C in dichloromethane in the dark producing benzocoumarin and polymeric peroxide [111, 112]. No CL emission was observed directly as benzocoumarin is nonfluorescent however, in the presence of aromatic hydrocarbons light was produced because of the fluorescence of these hydrocarbons. The explanation of this phenomenon was based on the above-mentioned CIEEL the aromatic hydrocarbons, which have a low oxidation potential, transfer one electron to diphenoyl peroxide to form a charge-transfer complex, from which benzocoumarin and the corresponding hydrocarbon in the excited state are produced (Fig. 13). 

Oxygen-free reactions of psoralens, when in close proximity to the target, proceed via the first excited states in which the 3,4-and the 4, 5 7r-bonds of the pyrone and furan moieties, respectively, can undergo C4-cyclization reactions with, e.g., unsaturated bonds of lipids, or the C5=C6 double bonds of thymine in DNA. In reactions with DNA the psoralen is believed to intercalate with DNA in the dark. Subsequent irradiation at 400 nm usually leads to furan-side 4, 5 -monoadduct formation, whereas irradiation at 350 nm increases the formation of crosslinks in which the furan and pyrone rings form C4 cycloadducts to thymines on opposite strands [95], Subsequent irradiation of the 4, 5 -monoadducts at 350 nm leads to formation of crosslinks and conversion into pyrone-side 3,4-monoadducts. Shorter wave-... 

The sample is continuously irradiated and the fluctuations in the fluorescence intensity arise due to any event which makes the fluorophore unavailable to be excited to the emissive singlet excited state, such as diffusion of the fluorophore out of the detection volume, formation of a dark state, such as a triplet excited state, or photoreaction. The concentration of fluorophore in the detection volume has to be low (10 13—10 8M) so that the fluctuation in the intensity for one molecule is observable over any background emission. The high concentration limit is a consequence of the fact that the correlated photons from single molecules scale with the number of molecules in the detection volume, while the contribution from uncorrelated photons, arising from the emission from different molecules, scales with the square of the number of molecules. The lowest concentration is determined by the probability of finding a molecule in the detection volume.58... 

Pig. 10-18. (a) PolarizatioD curves of anodic dissolution and (b) Mott-Schottky plots of an n-type semiconductor electrode of molybdenum selenide in the dark and in a photo-excited state in an acidic solution C = electrode capacity (iph) = anodic dissolution current immediately after photoexdtation (dashed curve) ipb = anodic dissolution current in a photostationary state (solid curve) luph) = flat band potential in a photostationary state. [From McEv( -Etman-Memming, 1985.]... 

To secure the efficiency of the one electron transfer, electron acceptors like 9,10-dicyanoanthracene (DCA) or quinones in conjunction with UV or visible light irradiation are commonly employed. Although photosensitized reactions are usually associated with a singlet excited state (equations 4 and 5), some processes of this type can occur thermally in the dark. 

An intriguing possibility, which appears close to realization, is that the excited state reagent may be generated by methods other than irradiation. For example, diphenylcarbene reacts with oxygen (in the dark) to yield ultimately the benzophenone triplet.26 The excited state thus prepared should be fully capable of participating in the cycloaddition reaction. 

The above discussion of the Q-switching process immediately gives some requirements for a good Q-switching dye. First, this dye must exhibit a saturable absorption at the laser wavelength. Second, the residual losses by excited state absorption should be as low as possible. Third, the photochemical stability (and, of course, also the chemical stability in the dark) should be as high as possible. Fourth, the... 

Like other peroxides, also dioxetanes are sensitive to the presence of metal ions and their complexes, which catalyze the decomposition of the dioxetane molecule. In most cases, this decomposition is dark, i.e. no chemiluminesce is generated in such a catalytic cleavage42. An informative exception, for instance, constitutes the chemiluminescent decomposition of the dioxetane 19 in Scheme 13, initiated by the ruthenium complex Ru(bipy)3Cl243. It has been shown that this chemiexcitation derives from the valence change of the ruthenium ion in the process Ru3+ I e — Ru2+, for which the efficiency of the excited-state generation may be as much as 40%44. Hence, when the radical anion of the carbonyl cleavage fragment from the dioxetane and the Ru3+ ion are formed in... 

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