The fate of oxygen in the quenching of excited singlet states

Measurement of the efficiency of rubrene-sensitizied photooxidation of 1,3-diphenylisobenzofuran imply that direct formation of singlet oxygen via oxygen quenching of excited rubrene singlets is inefficient. This contrasts with recent publications based upon studies of self-sensitized rubrene photooxidation. The observed inefficiency can be rationalized in terms of the spin-allowed decay of an initially formed ³(T₁ + ¹Δ) complex state to a lower energy ³(T₁ + ³Σ) state prior to complex dissociation.     

Photo-acoustic detection on electronic quenching rate constants of NO excited states

The electronic quenching rate constants of NO A(2)Σ (υ'=0, 1), E(2)Σ (υ'=2, 3, 4) and F(2)Δ (υ'=1, 2, 3) states by gas air are reported. The experiments were carried out by measuring the total fluorescence intensity of A(2)Σ (υ'=0, 1)→X(2)Π (υ″) transition at various air pressures. It gives the Stern-Volmer plots. The quenching rate constants of A(2)Σ (υ'=0, 1) states are obtained from the slope of Stern-Volmer plots and the known radiative lifetime. Based on the primary results of the work, we have measured the quenching rate constants of high excited E(2)Σ (υ'=2, 3, 4), F(2)Δ (υ'=1, 2, 3) states for the first time with the technique of photo-acoustic (PA) spectroscopy. It is shown that the electronic quenching rate constants of NO E (υ') and F (υ') states are in the order of 10(-10)cm(3)/molecules. They are much larger than those of A(2)Σ (υ') state, whose rate constants are in the order of 10(-13)cm(3)/molecules. For E (υ') and F (υ') states, it is also found that the quenching rate constants increase with the vibrational energy levels. Similar result has been reported also for A(2)Σ (υ'≥2) states in existing literatures. The agreement indicates the potential use of PA spectroscopy for measuring the electronic quenching rate.     

Evidence for static quenching of MLCT excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(deeb)(bpy)(2)(PF(6))(2) [where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine] in acetonitrile or dichloromethane were found to be quenched by iodide at room temperature. The ionic strength dependence of the optical spectra gave evidence for ion pairing. Iodide is found to quench the photoluminescence (PL) intensity and influence the spectral distribution of the emitted light. A static component to the time-resolved PL quenching provided further evidence for ground-state adduct. Stern-Volmer analysis of the static component provided an estimate of the iodide-Ru(deeb)(bpy)(2)(2+) adduct equilibrium constant in dichloromethane, K(sv) = 40,000 M(-)(1). Transient absorption studies clearly demonstrate that an electron-transfer quenching mechanism is operative and that I(2)(-)(*) can be photoproduced in high yield, phi = 0.25. For Ru(bpy)(3)(PF(6))(2) in acetonitrile, similar behavior could be observed at iodide concentrations >100 times that required for dichloromethane.     

Preliminary calculations on excited states of the oxygen molecule

It is well known that the energy interval separating ³ _u ^– and ³ _u ⁺ states of O₂, as given by the conventional ASMO method, is too large. In order to resolve this difficulty, removal of the equivalence restrictions usually employed in the orbital theory is proposed. Thus the orbital exponent of one antibonding _g MO is allowed to take a different value from the other _g's. Variational calculations show that the resulting outermost orbital is much more diffuse than the others. This model of a single diffuse orbital brings about a considerable energy lowering for the ³ _u ^– state and thus the agreement of the ³ _u ^– - ³ _u ⁺ interval with experiment is improved.

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Zusammenfassung Die konventionelle ASMO-Theorie liefert bekanntlich eine viel zu große Differenz der Terme ³ _u ^– und ³ _u ⁺ von O₂, weswegen der Vorschlag gemacht wird, die üblicherweise vorgenommene Äquivalenz-Einschränkung fallen zu lassen. Der Orbital-Exponent eines lockernden MO's kann von dem der übrigen _g's abweichen. Rechnungen zeigen, daß das äußerste MO viel diffuser als die anderen ist und daß die Energie des ³ _u ^– -Zustandes beträchtlich erniedrigt wird.

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Résumé La séparation entre les états ³ _u ^{– 3} _u ⁺ de O₂ donnée par la méthode ASMO conventionnelle est connue pour être trop grande. Afin de résoudre cette difficulté la levée des restrictions d'équivalence ordinairement utilisées est proposée. Ainsi l'exposant orbital d'une des orbitales moléculaires antiliantes _g peut prendre une valeur différente de celui de l'autre orbitale antiliante _g. Des calculs variationnels montrent que l'orbitale la plus haute ainsi obtenue est beaucoup plus diffuse que les autres. Ceci a pour effet de diminuer considérablement l'énergie de l'état ³ _u ^– , améliorant la séparation entre les états ³ _u ^– et ³ _u ⁺ .

     

Photo-oxidation poly(butadiene); quenching of excited states by nitroxides

The phosphorescence from three model bicyclic enones and those formed in thermally oxidized poly(butadiene) has been shown to be quenched efficiently by a nitroxide [3,3,4,4-tetramethyldiazetine N,N′-dioxide] in CH₂Cl₂:tetrahydrofuran glass at 77K. The Perrin model for static quenching was applied, and the interchromophoric distance for half-quenching found to be ～ 12 A. At room temperature the nitroxide additive inhibited the formation of volatile products from thermally oxidized poly(butadiene) films: results also obeyed the Perrin relationship, pointing to a quenching mechanism of photostabilization. By contrast, a commercial hindered amine stabilizer was found not to act as a quencher of phosphorescence at 77K, and to have a concentration dependence for the reduction in volatile product formation at 298K in PBD films different from the nitroxide, indicating different mechanisms of action.     

Rigid medium stabilization of metal-to-ligand charge transfer excited states

The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and [Re(bpy)(CO)3(4-Etpy)](PF6) (bpy=2,2'-bipyridine, py=pyridine, 4-Etpy=4-ethylpyridine, and tpy=2,2':6',2-terpyridine) have been incorporated into poly(methyl methacrylate) (PMMA) films and their photophysical properties examined by both steady-state and time-resolved absorption and emission measurements. Excited-state lifetimes for the metal salts incorporated in PMMA are longer and emission energies enhanced due to a rigid medium effect when compared to fluid CH3CN solution. In PMMA part of the fluid medium reorganization energy, lambdaoo, contributes to the energy gap with lambdaoo approximately 700 cm-1 for [Ru(bpy)3](PF6)2 from emission measurements. Enhanced lifetimes can be explained by the energy gap law and the influence of the excited-to-ground state energy gap, Eo, on nonradiative decay. From the results of emission spectral fitting on [Ru(bpy)3](PF6)2* in PMMA, Eo is temperature dependent above 200 K with partial differentialEo/ partial differentialT=2.8 cm-1/deg. cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 are nonemissive in CH3CN and undergo photochemical ligand loss. Both emit in PMMA and are stable toward ligand loss even for extended photolysis periods. The lifetime of cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is temperature dependent, consistent with a contribution to excited-state decay from thermal population and decay through a low-lying dd state or states. At temperatures above 190 K, coinciding with the onset of the temperature dependence of Eo for [Ru(bpy)3](PF6)2*, lifetimes become significantly nonexponential. The nonexponential behavior is attributed to dynamic coupling between MLCT and dd states, with the lifetime of the latter greatly enhanced in PMMA with tau approximately 3 ns. On the basis of these data and data in 4:1 (v/v) EtOH/MeOH, the energy gap between the MLCT and dd states is decreased by approximately 700 cm-1 in PMMA with the dd state at higher energy by DeltaH0 approximately 1000 cm-1. The "rigid medium stabilization effect" for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is attributed to inhibition of metal-ligand bond breaking and a photochemical cage effect.     

Pulse radiolysis studies of the quenching processes of Xe* excited atoms

The time resolved fluorescence of Xe ₂ ^* excimers in pure xenon and Xe-M (M=H₂,N₂,N₂O,CO₂) mixtures has been observed. The formation rate constant of Xe ₂ ^* , k₂=(1.1±0.1)·10^–30 cm⁶/s and the lifetime of the excimer precursors, ₀(100 ± 40) ns were evaluated. The quenching rate coefficients of Xe(6p) states by M have been found to be in the range of (0.5–1.3)·10^–9 cm³/s. The basic parameters and operating characteristics of the newly constructed pulse radiolysis set up based on SINUS-5 electron accelerator are also presented.     

Specifics of deactivation processes of excited states of thiacarbocyanine dyes adsorbed on cellulose

Spectral and kinetic study on deactivation processes of electronically excited states of molecules of cationic (Dye1, Dye2 and anionic (Dye3–Dye5) thiacarbocyanine dyes adsorbed on cellulose was performed. Adsorption of the dyes leads to an increase in the concentration of dimers. The dimers of adsorbates of Dye3–Dye5 exhibit only phosphorescence, whereas the monomers of the adsorbates are capable of both phosphorescence and delayed fluorescence. Adsorbates of Dye3–Dye5 prepared from water and methanol exhibit phosphorescence of cis-monomers. Adsorbates of Dye1 from water and ethanol as well as adsorbates of Dye2–Dye5 from chloroform exhibit delayed fluorescence of trans-monomers. Phosphorescence and delayed fluorescence are observed at room temperature in the presence of air oxygen.     

Transient absorption spectra and quenching of coumarin excited states by nucleic acid bases.

Triplet-triplet absorption spectra of coumarin show different profiles and maxima in ethanol from those in water, which are very similar to those reported in benzene. Long-lived transient species other than triplet states were generated as initial photoproducts between coumarins and nucleic acid bases. The excited singlet and triplet states of coumarins were quenched by nucleic acid bases. Adenine base quenched the excited singlet state of coumarins more efficiently than thymine base. However, photocycloadducts of furocoumarins are formed predominantly with thymine, and not with adenine. Moreover, it is reported that the poly[dA-dT].poly[dA-dT] sequence region is the most favourable site for the photocycloaddition reaction of furocoumarins. The results imply that adenine contributes to singlet-state photocycloaddition reaction of furocoumarins with thymine, probably through an adenine-furocoumarin-thymine termolecular interaction.     

Theoretical description of pressure shifts and quenching of excited atomic states in liquid helium

The pressure dependence of the linewidth and shift for the 2 ³S→2 ³P and 3 ³S→2 ³P transitions observed in electron-bombarded liquid helium is calculated by applying static line-broadening theory within the framework of a theoretical model. The calculated pressure shift, width and asymmetry are in good agreement with recent measurements over the pressure range of 1 to 25 atm. A new mechanism is proposed for the strong pressure quenching observed for several atomic and molecular emission lines.     

Static and dynamic quenching of Ru(II) polypyridyl excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(bpy)(2)(deeb)(PF(6))(2), where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, in dichloromethane were found to be efficiently quenched by iodide at room temperature. The ionic strength dependence of the UV-visible absorption spectra gave evidence for ion pairing. Iodide was found to quench the excited states by static and dynamic mechanisms. Stern-Volmer and Benesi-Hildebrand analysis of the spectral data provided a self-consistent estimate of the iodide-Ru(bpy)(2)(deeb)(2+) adduct in dichloromethane, K = 59 700 M(-1). Transient absorption studies clearly demonstrated an electron-transfer quenching mechanism with transient formation of I(2)(*)(-) in high yield, phi = 0.25 for 355 or 532 nm excitation. For Ru(bpy)(2)(deeb)(PF(6))(2) in acetonitrile, similar behavior could be observed at higher iodide concentrations than that required in dichloromethane. The parent Ru(bpy)(3)(2+) compound also ion pairs with iodide in CH(2)Cl(2), and light excitation gave a higher I(2)(*)(-) yield, phi = 0.50. X-ray crystallographic, IR, and Raman data gave evidence for interactions between iodide and the coordinated deeb ligand in the solid state.     

Large reaction radii for the quenching of fluorescent excited states of liquid cis-decalin and cyclohexane

The large rates of quenching by different solutes of the fluorescent solvent excited state in liquid cis-decalin and cyclohexane as observed by pulse radiolysis are explained by large reaction radii. Values of 14, 13, and 15 Å are found for the quenching of the excited state in cis-decalin by CCl₄, in cyclohexane by CCl₄, and by O₂ respectively.     

Picosecond isomerisation kinetics of excited p-dimethylaminobenzonitriles studied by oxygen quenching of fluorescence

Fluorescence quenching with O₂ under high pressure was used for the study of reaction kinetics in the 10 picosecond region. Quenching of the dual fluorescence of 2-methyl-4-dimethylaminobenzonitrile (II) in acetonitrile was analysed. The process B^* → A^* occurs with a lifetime of 11 ps, while the solvent reorientation correlation time is only 1 ps, which supports the isomerisation mechanism which involves twisting the amino group. The lifetime of A^* is 4.5 ns.     

The quenching of excited iodine atoms by oxygen molecules as studied by opto-acoustic spectroscopy

The opto-acoustic spectrum of I₂ in the presence of various quenching gases — NO, O₂, CH₃I, SO₂, C₃H_S, N₂, and He — has been studied. Of these, the I₂/O₂ spectrum is quite different due to the near-resonant energy transfer I(²P$\text{1}\text{2}$) + O₂(³Σ) → I(²P$\text{3}\text{2}$) + O₂(^IΔ), wherein the resistance of the O₂((^IΔ) species to collisional relaxation severely distorts the acoustic signal. The photochemical production of excited ²P$\text{1}\text{2}$ iodine atoms commences at wavelengths considerably longer than the dissociation limit of the I₂B? state.     

Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives

The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both experimentally and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron density distributions, which dominate the ground state and the first excited singlet state of fulvenes, respectively, reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, respectively). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state.     

Studies in mixed solvents: Comparison between solvated electron reactions and quenching of excited states

The rate constants for the reaction of electron pulse produced solvated electrons and a number of solutes in water-isopropanol mixtures have been measured. The quenching of the singlet excited state of naphthalene has also been studied in the same mixtures, using triethylamine and acrylamide as quenchers. The variation of the bimolecular solvated electron reaction rate constants with the composition of the solvent has been compared with the variation in the quenching constants with the composition of the solvent. Both these variations are surprisingly similar, with acrylamide behaving in a reverse manner (to the other solutes) in both the cases. It has been possible to quantitatively correlate both sets of data using dielectric constant (?) as a measure of polarity and the viscosity (η) as an index of the microstructure. The curves obtained provide insights with respect to the nature of charge transfer processes involved. © 1995 John Wiley & Sons, Inc.