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The complexation behavior of pyropheophorbide-a methyl ester (PPME) with transition metal ions as well as other biologically relevant metal ions has been investigated in water–DMF (2 : 1 v/v) solution. PPME was found to selectively complex Cu2+ions, which leads to a distinct change in its absorption spectrum as well as efficient fluorescence quenching. The degree of fluorescence quenching by Cu2+depended on concentration and time. Upon addition of Cu2+, the fluorescence showed a time-resolved decay on the time scale of minutes to hours, with the decay rate being dependent on the cation concentration. Fitting according to a bimolecular reaction rate law provided a rate constant of 650 ± 90 M−1s−1at 298 K for metallochlorin formation. The potential implications of Cu2+binding for the use of PPME in photodynamic therapy are discussed, along with its use as a fluorescent sensor for detection of micromolar concentrations of Cu2+. [ABSTRACT FROM AUTHOR]

Azoalkanes of the 2,3-diazabicyclo[2.2.2]-oct-2-ene type have been introduced as probes for antioxidants in homogeneous solution as well as in liposomes and micelles. The bimolecular fluorescence quenching of the bridgehead dichloro-substituted 1,4-dichloro-2,3-diazabicyclo[2.2.2]-oct-2-ene (3) was compared with that of the parent compound 2,3-diazabicyclo[2.2.2]-oct-2-ene (1) and the bridgehead-dialkylated compound 4-methyl-1-isopropyl-2,3-diazabicyclo[2.2.2]-oct-2-ene (2). Compound 3showed a more efficient fluorescence quenching in C–H containing solvents (e.g., in n-hexane: 30 ns for 3versus340 ns for 1and 770 ns for 2), but a less efficient quenching in aqueous solution (e.g., in deaerated H2O: 485 ns for 3versus420 ns for 1and 340 ns for 2), and also by molecular oxygen (kq/109M−1s−1= 0.32 for 3versus2.5 for 1and 1.9 for 2). Towards low-molecular weight antioxidants, compound 3showed a significantly higher reactivity (e.g., for reduced glutathione: kq/109M−1s−1= 1.8 for 3versus0.82 for 1and 0.39 for 2), at the expense of a lower differentiation between the investigated antioxidants (lower selectivity). The increased reactivity of 3and lower, as well as qualitatively different, selectivity is attributed to a combination of factors, most importantly the slightly increased excitation energy of 3and its lower excited-state nucleophilicity. The latter was independently corroborated, besides its longer fluorescence lifetime in aqueous solution, through the trends in quenching rate constants of the azoalkanes 1–3towards electron-deficient versuselectron-rich lactone antioxidants of the benzofuranone type. While common inorganic buffer constituents caused no fluorescence quenching, significant quenching was observed, as a curiosity, for hydrogencarbonate (kq/106M−1s−1= 1.7 for 3versus2.4 for 1and 0.45 for 2), with a fully manifested kinetic deuterium isotope effect (kq(H2O)/kq(D2O) = 12) for 3. [ABSTRACT FROM AUTHOR]

The host–guest interactions of the neutral (AO) and cationic (AOH+) forms of the dye acridine orange with the macrocyclic hosts cucurbit[7]uril (CB7) and β-cyclodextrin (β-CD) were investigated by using ground-state absorption and steady-state as well as time-resolved fluorescence measurements. The cationic form undergoes no significant complexation with β-CD, but binds strongly with CB7 (Keq = 2.0 × 105 M−1), causing a large enhancement in fluorescence intensity and lifetime of the dye in the latter host. The strong and selective binding of AOH+ with CB7 is attributed to ion–dipole interactions involving the ureido carbonyl rims of CB7 and the charged AOH+. In contrast, the neutral AO form of the dye shows quite similar binding with both CB7 and β-CD, but the binding constants are lower by more than two orders of magnitude compared to that of the AOH+–CB7 system. CB7 and β-CD show a contrasting behavior in modifying the acid–base character of the dye, shifting its pKa by about 2.6 units upward and about 0.7 units downward, in the two respective cases. These divergent pKa shifts of the dye arise from the differential affinity of the two host molecules to encapsulate the protonated and neutral form of the dye. [ABSTRACT FROM AUTHOR]