Phosphorescence protonation

Lastly, photochemically unstable ligands should be avoided. Re(bpy)(CO)3Cl shows a moderately efficient MLCT emission at room temperature (R. M. Ballew, unpublished results from our laboratory). However, the apparently closely related Re(dpk)(CO)3Cl (dpk = 2,2 -dipyridyl ketone) shows a benzophenone like phosphorescence at 77K indicating that the n-n excited state of the ketone in complex is the lowest state of the complex. No luminescence is seen at room temperature, and even at 77K the dpk triplet state is such a powerful hydrogen atom extractor that it removes protons from alcohol glasses as seen by the formation of the intense blue color of the keto free radical. The absence of an MLCT emission is caused by the greater difficulty of reducing dpk relative to bpy, which pushes the MLCT states above the dpk ligand states. 

In testing the possibility of proton transfer as a quenching mechanism of tyrosine in oligopeptide/polynucleotide complexes, Brun et a/.(102) compared the fluorescence emission spectra of the tyrosine and O-methyltyrosine tripeptides. They noted that, in the complex, the O-methyltyrosine tripeptide had a unique secondary emission near 410 nm. Whether this emission is related to that observed by Libertini and Small(94) is an important question. While one must consider the possibility that two tyrosine side chains could be converted to dityrosine, (96) which has a fluorescence at 400 nm, another intriguing possibility is ambient temperature tyrosine phosphorescence. This could happen if the tyrosine side chain is in a rigid, protective environment, very effectively shielded from collisions with quenchers, particularly oxygen. 

Protonation of (n, 7t )-nitrobenzene had been suggested earlier and later questioned I8) on account of an estimated extremely weak basicity of 3(n, tt )-nitrobenzene. Enhanced basicity of the lowest excited singlet state compared to ground and lowest excited triplet state has been derived from shifts in the phosphorescence and absorption spectra of nitrophenols ). On this basis, the increased rate of nitrobenzene photoreduction in acidic solution is formd to be thermodynamically unfeasible in the lowest excited triplet state >. Although it might be thermodynamically feasible in the excited singlet state, the short lifetime of the latter state may make this possibility unlikely. 

The emission from [Ru(bpz)3] is quenched by carboxylic acids the observed rate constants for the process can be rationalized in terms of the protonation of the non-coordinated N atoms on the bpz ligands. The effects of concentration of carboxylate ion on the absorption and emission intensity of [Ru(bpz)3] have been examined. The absorption spectrum of [Ru(bpz)(bpy)2] " shows a strong dependence on [H+] because of protonation of the free N sites the protonated species exhibits no emission. Phosphorescence is partly quenched by HsO" " even in solutions where [H+] is so low that protonation is not evidenced from the absorption spectrum. The lifetime of the excited state of the nonemissive [Ru(Hbpz)(bpy)2] " is 1.1ns, much shorter than that of [Ru(bpz)(bpy)2] (88 nm). The effects of complex formation between [Ru(bpz)(bpy)2] and Ag on electronic spectroscopic properties have also been studied. Like bpz, coordinated 2,2 -bipyrimidine and 2-(2 -pyridyl)pyrimidine also have the... 

Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)g in AOT/heptane-RMs was studied and the Hfe time of the redox products in reverse micellar system was found to increase by about 300-fold compared to conventional photosystem [335]. The authors have presented a kinetic model for overall photochemical process. Kang et al. [336] reported photoinduced electron transfer from (alkoxyphenyl) triphenylporphyrines to water pool in RMs. Sarkar et al. [337] demonstrated the intramolecular excited state proton transfer and dual luminescence behavior of 3-hydroxyflavone in RMs. In combination with chemiluminescence, RMs were employed to determine gold in aqueous solutions of industrial samples containing silver alloy [338, 339]. Xie et al. [340] studied the a-naphthyl acetic acid sensitized room temperature phosphorescence of biacetyl in AOT-RMs. The intensity of phosphorescence was observed to be about 13 times higher than that seen in aqueous SDS micelles. 

Thymidine phosphate and cytidine phosphate do not phosphoresce in a rigid ethylene glycol-water glass at 77°K109 when directly excited, but thymidine which has lost its proton at Nx does have a triplet which phosphoresces with a decay time of 0.50 sec at high pH uridine also fluoresces with a similar decay time. The quenching of purine fluorescence and appearance of T" fluorescence in UV-irradiated DNA and poly dAT (as well as U" fluorescence in poly rAU) was attributed by Rahn et al.109 to proton transfer from thymine to adenine. This quenches adenine fluorescence and enhances thymine fluorescence. 

C = AcC = G>A>U>T> BrC. Proton transfer is excluded as the explanation of the sensitized thymine fluorescence because the phosphorescences of 1-methylthymidine and 1,3-dimethylthymine (which cannot lose a proton) are both enhanced in frozen alkaline solutions as is the very weak phosphorescence of thymidine in frozen aqueous solution. 

Totally deuterated aromatic hydrocarbons yield measured phosphorescence lifetimes greater than their protonated analogs.182 This behavior is ascribed to the closer spacing of vibrational levels in deuterated compounds with a consequent decrease in probability for nonradiative T -> S0 transitions. Quantum mechanical tunnelling may also contribute to the rate of the radiationless process with the normal compounds. 

Fluorescein and related quinonelike dyes such as eosin have found extensive use as low-energy sensitizers, especially in photooxidation studies. The efficiency of intersystem crossing in fluorescein varies markedly with pH. For the neutral molecule, the quantum yield of fluorescence is only 0.31, while it is 0.91 for the dianion.449 In strong acid solutions, however, protonated fluorescein is strongly phosphorescent. In this state (dissolved in boric acid) fluorescein was the first compound for which T-T absorption in an excited triplet state was observed.7... 

Where chemical processes such as protonation are concerned, it is the general rule that only the first excited singlet state (Sj) and the first excited triplet state (T are involved. This is closely related to Kasha s rule for radiation emission (Kasha, 1950) fluorescence always occurs from the lowest excited singlet state and phosphorescence from the lowest triplet. Since experimental conditions are often arranged so that protonation is in competition with emission, i.e. so that their rates are similar, these rules are easily understood in terms of the much shorter time ( 10-1 2 s) required for the Sj state to be reached from the higher states produced immediately on absorption... 

In the case of xanthone at least, this order is not only shown up in the Forster cycle estimates, but has been confirmed by observing the variation with pH of the optical densities of the triplet states of B and BH+ and comparing it with the fluorescence intensity behaviour (see Fig. 6). Confirmation that the pK order obtained using the Forster cycle is reliable in such cases is also found in a direct determination of p/ (Tj) of benzophenone by a laser technique the value derived is consistent with earlier phosphorescence observations (Rayner and Wyatt, 1974). Ledger and Porter (1972) observed a marked decrease in the phosphorescence intensity of benzophenone near pH 5, and the apparent discrepancy between this result and the p/ (Tj )-value of 1-5 is due to the very large difference in lifetimes of BH+(T,) and B(Tj). Since unprotonated benzophenone has a very shortlived St state [1/kj for the intersystem crossing alone in ethanol is 16 5 ps (Hochstrasser et al., 1974)], protonation in this state is unlikely. However, Forster cycle calculations indicate that the singlet state would be a weaker base than the triplet state. The realization that unprotonated benzaldehyde and acetophenone had Tj states of the... 

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