Stern-Volmer Kinetics Research Articles

MECHANISM OF THE PHOTOSENSITIZED REDOX REACTIONS OF ACRIDINE ORANGE IN AQUEOUS SOLUTIONS-A SYSTEM OF INTEREST IN THE PHOTOCHEMICAL STORAGE OF SOLAR ENERGY

-We have carried out a very detailed study, using fluorescence and optical flash photolysis techniques, of the photoreduction of methyl viologen (MV2+) by the electron donor ethylene diamine tetraacetic acid (EDTA) in aqueous solution sensitized by the dye acridine orange (AOH+). A complete mechanism has been proposed which accounts for virtually all of the known observations on this reaction. This reaction is novel in that both the triplet and the singlet state of AOH+ appear to be active photochemically. We have shown that mechanisms previously proposed for this reaction are probably incorrect due to an artifact. At pH 7 the fluorescence quantum yield φs of AOH+ is 0.26 ± 0.02 and the fluorescence lifetime is 1.8 ± 0.2 ns. φs is pH dependent and reaches a maximum of 0.56 at pH 4. The fluorescence of AOH+ is quenched by MV2+ at concentrations above 1 mM and the quenching obeys Stern-Volmer kinetics with a quenching rate constant of (1.0 ± 0.1) × 1010M−1 s−1. The quenching of the AOH+ excited singlet state by MV2+ almost certainly returns the AOH+ to its ground state with no photochemistry occurring. EDTA also quenches the fluorescence of AOH· with Stern-Volmer kinetics but with a smaller rate constant (6.4 ± 0.5) × 108M−1s−1 at pH 7. In this case the quenching is reactive resulting in the formation of semireduced AOH. In the presence of MV2+, flash irradiation of AOH+ does result in the reversible formation of the semireduced MV† which absorbs at 603 nm. We attribute this to a photochemical reaction of the triplet state of AOH+ with MV2+. The initial quantum yield for formation of MV† (φMV:)0 was found to be constant at 0.10 ± 0.05 for [MV2+] from 5 × 10−5 to 1.0 × 10−3 with [AOH+] = 8 × 10−6M. Previous workers had found that (φMV:)0 appears to decrease with decreasing [AOH+]; however, on careful investigation, we found this was most probably due to quenching of the triplet state of AOH+ by trace amounts of oxygen. When EDTA is added to a mixture of AOH + and MV2+ at pH 7, the photochemical formation of MV† becomes irreversible as the [EDTA] is increased. The quantum yield for the irreversible formation of MV† exceeds 0.10 becoming as large as 0.16 for [EDTA] = 0.014M. This fact requires that an alternative photochemical process must be operative and we present evidence that this is a reaction of EDTA with the excited singlet state of AOH+ to produce the semi-reduced AOH- which then reacts with MV2+ to produce MV†. The full kinetic scheme was tested by computer simulation and found to be totally consistent. This also enabled the processing of a full set of rate constants. When colloidal PtO2 was added to the optimal mixture [EDTA] = 3.4 × 10−2M; [MV2+] = 5 × 10−4M; [AOH+] = 4 × 10−5M; pH6 H2 gas was produced at a rate of 0.2μmol H2h−1. Thus, acridine orange should serve as an effective sensitizer in reactions designed to use solar energy to photolyze water.

Saturated amine fluorescence quenching in non-polar solution by various organic molecules

The fluorescence quenching of a prototype saturated amine N,N-diethylmethylamine (DEMA) was examined in n-hexane solution at 25 °C by measuring the quencher concentration dependence of the DEMA fluorescence lifetime. The quenchers studied included N,N-dimethylformamide (DMF), acetic acid, ethyl acetate, acetonitrile, 2-butyne, dimethyl sulfoxide (DMSO), tetramethylethylene, perfluorocyclohexane, n-propyl bromide and oxygen. In all cases, within the respective quencher concentration ranges studied, no evidence of exciplex fluorescence was obtained; moreover, with the exception of oxygen, linear Stern-Volmer kinetics were observed. DEMA fluorescence quenching by these molecules is generally discussed in terms of possible energy transfer and electron (or charge) transfer mechanisms, although hydrogen bonding is considered for the acetic acid system. It was found that, even for those molecules for which energy transfer is highly improbable, efficient quenching is observed if the quencher molecule is polar ( e.g. DMF, acetonitrile, DMSO and n-propyl bromide). In general, non-polar molecules having high-lying S 1 states are inefficient quenchers, perfluorocyclohexane being an exception. Finally, quenching by molecular oxygen and by a secondary amine is mentioned. Complications arising from the ground state association between the amine and oxygen are discussed. The intermolecular quenching by a secondary amine is used to rationalize the very weak emissivity of N-methylpiperazine.

The role of the dipole–dipole interaction in the energy transfer process in aromatic solutions

Fluorescence quantum yield for neat benzene and neat and dilute toluene and energy transfer from toluene and benzene to PBD (2-phenyl-5- (4-biphenylyl) -1,3,4-oxadiazole) have been measured in the 20–160 °C temperature range. The measurements were analyzed assuming Stern–Volmer kinetics. The results can be explained by assuming that the main mechanism of energy transfer in these systems is the diffusion assisted resonance dipole–dipole interaction, which can get saturated in those systems because of the large excitation diffusion rates. At higher temperatures (T?110 °C) short range or collisional interactions begin to play a more important role.

Quenching of toluene fluorescence by oxygen

The oxygen quenching of pure liquid toluene has been investigated to examine departures from simple Stern–Volmer kinetics resulting from transient effects (diffusion control) and static quenching due to the presence of a contact ground state complex. Static quenching is easily observed when excitation is on the red edge of the absorption spectrum. In addition transient effects are seen in the steady state quenching at wavelengths where there is no static quenching. The differential quenching of monomer as opposed to excimer is evident in the lifetime data.

Electronic Processes in Ultraviolet and Pulse Irradiated Halide Ion Systems. The Effect of Heavy Atoms

Air-free, concentrated, aqueous solutions of alkali halides at ∼ 20°C containing small concentrations of heavy metal ions or I2 have been uv irradiated in the induced absorption bands. There was no evidence of chemical change unless an electron acceptor, e.g., N2O was also present when halide oxidation occurred. Luminescence from 1M KCl with 10−4M Tl+ was quenched by N2O and by I−, both systems obeying Stern-Volmer kinetics. In 8M LiCl glass saturated with TlCl, there were three absorption bands at 235, 217, and 187 nm at 77°K. They resemble, both in position and relative intensity, the A, B, and C bands of KCl:Tl+. Analogous results were observed for Tl+-doped aqueous glasses of LiBr and LiI, and all may involve the known halocomplexes of Tl+. The absorption bands in the systems examined are attributed to charge transfer from halide ion to heavy metal cation or to iodine. The relevance of these results to Tl+-doped alkali halide crystals is also considered. The absorption spectra of the diatomic halides MX (GaX, InX, and TlX) with 1Σ+ ground states have in common Π30+, 3π1, and 1π1 excited states. These may correlate with A1, A2, and C bands in ionic lattices. If MX is nearly substitutional in the alkali halide lattice, there are three distinguishable metal-halogen internuclear separations which may account for structure in the C band through Franck-Condon effects. γ-Irradiated LiCl glass at 77°K shows absorption by Cl2− and by trapped electrons and recombination luminescence, induced by optical bleaching of trapped electrons, has both 1 and 9 sec lifetimes. The luminescence and chemical reactivity are attributed to excited halide ions. Pulse radiolysis of neutral chloride ion aqueous solutions is known to yield Cl2−. It has been found that aeration affects G(Cl2−) and, in solutions saturated with N2O or CO2 instead of N2, yields of Cl2− are considerably enhanced.

Reaction between molecular oxygen and photo-excited protoporphyrin IX.

THE absorption spectrum of protoporphyrin IX in tetrahydrofuran solution has maxima at 404 (Soret band), 503, 536, 576 and 633 nm. Excitation at any of these wavelengths produced an intense red emission with its maximum at 635 nm1. The same emission spectrum was observed when added biacetyl was irradiated at its 436 nm absorption. In these conditions the biacetyl phosphorescence was completely quenched. Additionally, molecular oxygen quenched the protoporphyrin IX emission spectrum following Stern-Volmer kinetics. Typical experimental results, obtained at 25° C, are plotted in Fig. 1.

Phosphorescence quenching by diffusion in an isopentane glass

The quenching of toluene phosphorescence by cis-piperylene at 77 K in an isopentane glass obeys Stern–Volmer kinetics; this is in contrast to the “static quenching” previously reported for rigid (i.e. predominantly ethanol) glasses.

Yield of triplet state benzene in the pulse radiolysis of solutions of some aromatics

The yields of triplet states of naphthalene, biphenyl and anthracene formed in the pulse radiolysis of 0.1 mM-50 mM benzene solutions have been determined. When allowance is made for the amount of triplet arising from the singlet states, the remaining triplet yields can be accounted for quantitatively over the whole concentration range by Stern-Volmer kinetics of transfer from the triplet state of benzene. The yields give G(BT)0= 4.2 for the primary yield of triplet benzene, which agrees with the values obtained using the cis-trans isomerization technique. The half-life of the benzene triplet in liquid benzene at 20°C is estimated to be 2.3 ns.

Yields of excited singlet and triplet states of some aromatics in the pulse radiolysis of cyclohexane solutions

The yields of excited singlet G(S) and total triplet, G(T), states of diphenyl, naphthalene and anthracene produced by the pulse radiolysis of their solutions in cyclohexane have been measured at concentrations of 0.5, 2.0, 5.0 and 10.0 mM. Triplet states were measured by absorption, and singlets by comparison of emissions with those of the same solute in benzene. For all solutes in benzene the emissions follow Stern-Volmer kinetics and kinetic parameters have been obtained for energy transfer from the excited singlet of benzene to the solutes. For each solute the total yield of excited states G(S)+G(T)0 is about the same as the yield of anions as calculated from establishing scavenging equations. The ratio G(T)0/G(S) is about 2.0 for diphenyl, 1.0 for naphthalene and 2.0 for anthracene.

Energy transfer in organic systems VII. Effect of diffusion on fluorescence decay

The effect of non-radiative energy transfer and diffusion on the fluorescence decay of phenanthrene in the presence of acridine was observed in six solvents of viscosity η = 0.4 cp to 64.6 cp. The system obeys Stern-Volmer kinetics at low η, Förster kinetics at high η and intermediate kinetics at medium η. At η = 64.6 cp the decay obeys the Förster relation. At lower η it agrees satisfactorily with the relation of Yokota and Tanimoto, but it is less rapid than that predicted by Voltz et al.

Energy transfer in organic systems V. Lifetime studies of anthracene-perylene transfer in benzene solutions

The fluorescence response functions of oxygen-free benzene solutions containing 3 × 10-3M anthracene and concentrations of up to 1·2 × 10-3M perylene were observed using the phase and modulation fluorometer. The response functions of the directly excited anthracene and perylene fluorescence are simple exponential decay functions, while that of the perylene fluorescence, indirectly excited by energy transfer from anthracene, is the difference of the two exponential decay functions. Stern-Volmer kinetics are applicable because of the statistical `mixing' of the molecules by diffusion, which increases the effective critical dipole-dipole transfer radius from 31 Angstrom to 43 Angstrom.