Triplet state in solution

Widengren J., Mets 0. and Rigler R. (1995) Fluorescence Correlation Spectroscopy of Triplet States in Solution A Theoretical and Experimental Study, J. Phys. Chem. 99, 13368-79. 

Rare earth ions, especially Eu+3, Sm+3, Gd+3, Tb+3, and. Dy+3 with electronic configuration (/s through / ), emit characteristic line spectra from their 4/energy states. The energies of the emitting levels are fairly low so that the rare earth ions can be vety useful species as acceptors to detect triplet states in solution by energy transfer. Such quenching processes... 

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

The first direct time-resolved evidence for energy transfer from an upper excited triplet state in solution at room temperature was published in 1987 [50]. This study made use of the two-color technique to photoexcite the 7, state of benzophenone, 70, in benzene solvent. As the extensive (almost quantitative) triplet depletion was not accompanied by any product formation, it was concluded that the excitation energy was transferred to the triplet manifold of the benzene solvent. The energetics of this donor-acceptor system are certainly conductive to this process. The benzophenone 7, and Tn energies (69 kcal/mol and ca. 120 kcal/ mol, respectively—the second photon in the two-color excitation provides roughly 50 additional kcal/mol to the 71 state) bracket the benzene 7j energy (85 kcal/mol) and therefore benzene acts in the same way toward benzophenone as 1,3-cyclohexadiene acts toward anthracene, i.e., as an exclusive upper triplet energy accepter. 

It has been known for some time that bimolecular collisions between triplet states in solution leads to quenching of the triplet state by a diffusion controlled process.71 Recently Parker and Hatchard have shown that delayed fluorescence from solutions of compounds such as pyrene, naphthalene, and anthracene, is due to triplet-triplet annihilation, i.e.,... 

Hammond and co-workers have obtained evidence for chemical reactions of several triplet states in solution using triplet energy transfer. They have shown (a) triplet ethylenes and stilbenes undergo cis-trans isomerizations,41 42 (b) triplet ethyl pyruvate decomposes to give acetaldehyde and carbon monoxide,40 and that (c) triplet azomethane decomposes to give nitrogen and methylene (probably in a triplet state).47... 

Widengren J, Mets U, Rigler R. Fluorescence correlation spectroscopy of triplet states in solution A theoretical and experimental study. J. Phys. Chem. 1995 99 13368-13379. 

A polymer containing pe,ndant benzophenone groups (XV) is found to photocrosslink. The mechanism is inter.molecular coupling of radicals formed via photoreduction in the triplet state. In solution, crosslinking is concentration dependent as intramolecular cyclization competes (21). 

Pyridine-A -oxides display varied photochemical behaviour. As with all heterocyclic A-oxides, photolysis in the gas-phase leads to deoxygenation to the heteroarene via a triplet state. In solution photolysis, the oxygen can be transferred to a solvent molecule undergoing C-H insertion or addition to a double bond, e.g. ... 

A considerable amount of information has been accumulated on the spectroscopic details of carbenes. Often, experiments are carried out in low-temperature matrices, as in the direct observation of di- -butylcarbene produced from the corresponding diazo compound by irradiation at 254 nm at 14 However, solution-phase studies are also important and a study of diadamantyldiazomethane has shown that the derived carbene behaves as a triplet state in solution. This behaviour is reported to be most unusual for a dialkyl carbene. ... 

Quenching of excited singlet or triplet states in solution is often caused by electron transfer. The efficiency of electron transfer depends on the oxidation potential of the electron donor and the reduction potential of the electron acceptor [24, 170]. In contrary to energy transfer (see below), the acceptor is not excited, and the efficiency is independent of the spectral overlap. As a result of electron transfer, radicals of both the donor and the acceptor molecules are produced. Because the radicals are highly reactive electron transfer is of great importance in photochemistry. 

The first condition requires, therefore, a triplet state in solution. During these last few years, several molecules, with a Cj or a higher-symmetry axis, have been synthesized [68]. In a few cases, evidence for a ground-state triplet has been obtained by low-temperature ESR experiments [74]. However, in most cases a Jahn-Teller distortion occurs, that lowers the molecular symmetry and splits the degenerate state [77,78]. 

J. Widengren, 0. Mets, R. Rigler, Fluorescence correlation spectroscopy of triplet states in solution. J. Chem. Phys. 99, 13368 (1995)... 

Figure 2.14 Autocorrelation curves calculated for the fluorescent dye rhodamine 6G in water at different concentrations of potassium iodide. Reprinted from Widengren, etal., Fluorescence correlation spectroscopy of triplet states in solution—a theoretical and experimental stud /. Journal of Physical Chemistry99 (1995) 13368-13379 with permission from the American Chemical Society.)...

Porter, G., and M. R. Wright Energy transfer from the triplet state in solution. J. Chem. Phys. 55, 705 (1958). 

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