Phosphorescence proton acidity

A large shift in the high-frequency phosphorescence maximum (Aoo, Table V) resulting from the transition from a fow-lying efectronic state to the ground vibrational level can be observed. Table V shows that the shift increases in the series MgO, 7-AI2O3, and SiOa with a maximum shift magnitude of 10 nm (800 cm ), which correlates with the increases in proton acidity for this series (276). 

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. 

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... 

Upon adsorption of benzophenone on oxides with strongly acidic properties, the phosphorescence spectrum exhibits a structureless band with a Atnax at about 490 nm in addition to the normal phosphorescence of benzophenone. The A max of the excitation spectrum of this band was observed at approximately 380 nm, and its intensity increased in the order of the aluminosilicate, H-mordenite, and HY zeolite. In the spectrum of HY zeolite containing benzophenone, only one structureless phosphorescence band could be observed. A similar phosphorescence band could be observed for benzophenone dissolved in CHCI3, which also involves dry HCl. We can therefore assign phosphorescence at about 490 nm to the protonated form ofbenzophenone. These findings correspond with studies of the photoluminescence of benzophenone dissolved in various concentrated acidic solutions (277). Consequently, since the presence of a phosphorescence spectrum at about 490 nm with benzophenone adsorbed on the aluminosilicate, H-mordenite, or HY zeolite is associated with the presence of the protonated form of benzophenone, the data indicate the existence of proton-donor centers on these oxides with acid strengths < for benzophenone (about 5.6) (216). On HY zeolite, almost all the adsorbed benzophenone changes into protonated benzophenone. On aluminosilicate surfaces, the relative intensities of the phosphorescence spectra attributed to the protonated and unprotonated forms are approximately the same. 

The effects of solution acidity and basicity on luminescence spectra result from the dissociation of acidic functional groups or protonation of basic functional groups associated with the aromatic portions of fluorescing and phosphorescing molecules. Protonation or dissociation can alter the natures and rates of non-radiative processes competing with luminescence and, thereby, affect the quantum yields of emission. For example, the antimalarial mefloquine fluoresces very weakly but phosphoresces well in neutral aqueous media. However, at pH < 1, its protonated form fluoresces intensely, and its phosphorescence is very weak. 

The phosphorescence spectra of HB and derivatives thereof have been measured in the protonated and deprotonated forms (Table 8) (86). The spectral shifts are used below to calculate the acidities in the first triplet state. 

The fluorescence quantum yields for all NABs are very low in aqueous solutions at room temperature, the excitation energy being dissipated through the nonradiative decay channels. On the other hand protonated purines show fluoresc ice at room temperature as well as after being absorbed on a chromatographic p >er. To obtain insight into the excited state properties of the neutral bases and nucleotides in polar solvents, different low temperature experiments were performed. The first low temperature work on nucleic acids was reported in I960, while the phosphorescence of nucleic acids was first reported for adenine derivatives in 1957. The first... 

Azizova et al. 7 > showed that the production of free radicals involved a mechcmism in which the triplet level of the aromatic chromophore appears as an intermediate state. The mechanism proposed electron photoejection after absorption of a second photon by an aromatic molecule already excited to its phosphorescent triplet state 77,78). In acidic media the photoejected electron can be trapped by a proton and the EPR signal of hydrogen atoms is observed. In basic media the EPR signal from 0(—) ions is observed 79). In some instances secondary excitation with visible light induces radical recombination by freeing the trapped electrons 79). 

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