Rate Constant Kq Research Articles

4-[Diphenyl(trimethylsilyl)methyl]benzophenone as initiator in the photopolymerization of methyl methacrylate and styrene

The polymerization of methyl methacrylate (MMA) and styrene (St) has been studied using 4-[diphenyl(trimethylsilyl)methyl]benzophenone 1 as photoinitiator. The polymerization follows a free radical mechanism; the polymerization rate increases linearly with the monomer concentration and was found to be proportional to the 0.33 and 1.40 power of the photoinitiator and the monomer (MMA) concentration, respectively. The overall activation energy in the case of MMA photopolymerization was calculated to be 25.0 kJ/mol. From 1H NMR studies it is concluded that the obtained polymers contain two different trimethylsilyl moieties, one at the head and the other at the tail of the polymer chain, showing primary termination reactions even at low initiator concentrations. The p-benzoyltrityl radical 1· is incorporated into the polymer chain to a very small extent, acting as a scavenger. This is also concluded by laser flash photolysis (LFP) and ESR spectroscopy measurements. A “living” character of the polymerization was observed only at very low initiator concentrations. The triplet state (31*) of the initiator was quenched by styrene, reducing its efficiency. The rate constant kq of the quenching process of 31* was measured by LFP (kq = 3.1 · 109 M−1 · S−1). The triplet state and the photodissociation efficiency of the initiator is unaffected by MMA at various concentrations.

Monomeric and Dimeric Copper(II) Complexes Coordinated with N, N-Di(Benzotriazol-1-Ylethylene)Imine as Cu-Zn Superoxide Dismutase Mimics

This paper reports the syntheses and characteristics of several monomeric and dimeric copper complexes coordinated with the new tridentate ligand N, N-di(benzotriazol-1-ylethylene)imine (IDBt). The measurements of the superoxide dismutase (SOD) mimetic activity for the model compounds have been performed by means of a modified nitroblue tetrazoliuin (NBT) photoreduction. The rate constants kQ of the catalytic superoxide dismutation have been obtained.

4-[Diphenyl(trimethylsilyl)methyl]benzophenone as initiator in the photopolymerization of methyl methacrylate and styrene

The polymerization of methyl methacrylate (MMA) and styrene (St) has been studied using 4-[diphenyl(trimethylsilyl)methyl]benzophenone 1 as photoinitiator. The polymerization follows a free radical mechanism; the polymerization rate increases linearly with the monomer concentration and was found to be proportional to the 0.33 and 1.40 power of the photoinitiator and the monomer (MMA) concentration, respectively. The overall activation energy in the case of MMA photopolymerization was calculated to be 25.0 kJ/mol. From 1H NMR studies it is concluded that the obtained polymers contain two different trimethylsilyl moieties, one at the head and the other at the tail of the polymer chain, showing primary termination reactions even at low initiator concentrations. The p-benzoyltrityl radical 1· is incorporated into the polymer chain to a very small extent, acting as a scavenger. This is also concluded by laser flash photolysis (LFP) and ESR spectroscopy measurements. A “living” character of the polymerization was observed only at very low initiator concentrations. The triplet state (31*) of the initiator was quenched by styrene, reducing its efficiency. The rate constant kq of the quenching process of 31* was measured by LFP (kq = 3.1 · 109 M−1 · S−1). The triplet state and the photodissociation efficiency of the initiator is unaffected by MMA at various concentrations.

4-[Diphenyl(trimethylsilyl)methyl]benzophenone as initiator in the photopolymerization of methyl methacrylate and styrene

The polymerization of methyl methacrylate (MMA) and styrene (St) has been studied using 4-[diphenyl(trimethylsilyl)methyl]benzophenone 1 as photoinitiator. The polymerization follows a free radical mechanism; the polymerization rate increases linearly with the monomer concentration and was found to be proportional to the 0.33 and 1.40 power of the photoinitiator and the monomer (MMA) concentration, respectively. The overall activation energy in the case of MMA photopolymerization was calculated to be 25.0 kJ/mol. From 1H NMR studies it is concluded that the obtained polymers contain two different trimethylsilyl moieties, one at the head and the other at the tail of the polymer chain, showing primary termination reactions even at low initiator concentrations. The p-benzoyltrityl radical 1· is incorporated into the polymer chain to a very small extent, acting as a scavenger. This is also concluded by laser flash photolysis (LFP) and ESR spectroscopy measurements. A “living” character of the polymerization was observed only at very low initiator concentrations. The triplet state (31*) of the initiator was quenched by styrene, reducing its efficiency. The rate constant kq of the quenching process of 31* was measured by LFP (kq = 3.1 · 109 M−1 · S−1). The triplet state and the photodissociation efficiency of the initiator is unaffected by MMA at various concentrations.

The eosin-5-maleimide binding site on human erythrocyte band 3: investigation of membrane sidedness and location of charged residues by triplet state quenching.

The triplet probe eosin-5-maleimide (EMA) is a specific inhibitor of anion transport mediated by the erythrocyte membrane protein, band 3. It was previously shown that the eosin moiety is located close to the anion binding site when EMA is covalently bound to band 3 [Pan, R.-j., and Cherry, R. J. (1995) Biochemistry 34, 4880-4888]. In the present study the electrostatic properties and membrane sidedness of the EMA binding site of band 3 were further investigated by triplet state quenching. A series of stable nitroxyl free radicals, which are characterized by different charges, and I- were used as the quenchers. Time-resolved laser spectroscopy was employed to measure the triplet lifetime of EMA. It was found that the quenching reaction between the quenchers and band 3-bound EMA follows a linear Stern-Volmer plot. The quenching rate constants (Kq) of the quenchers are in the order of NH3+-TEMPO (Kq = 6.34 x 10(6) M-1 s-1) > TEMPO-Choline+ (Kq = 2.18 x 10(6) M-1 s-1) > TEMPO (Kq = 1.13 x 10(6) M-1 s-1) > I- (Kq = 2.46 x 10(5) M-1 s-1) > pyrroline-COO- (Kq = 2.18 x 10(4) M-1 s-1). Experiments with resealed ghosts and inside-out vesicles revealed that negatively charged quenchers can only access the EMA binding site from the extracellular side of the membrane while the positively charged quenchers acted from the cytoplasmic side. The ionic strength dependence of the quenching rate constants and the effects of pH on the quenching reaction were also studied. For both TEMPO-Choline+ and I-, the Kq values decreased as the ionic strength increased, but quenching by TEMPO was independent of the ionic strength variation over the same range. It was also found that at lower pH, the I- quenching rate constant increases but the TEMPO-choline+ quenching rate constant decreases. In both cases, the dependence of quenching on pH exhibited an apparent pKa of about 6.5, which suggests the involvement of one or more histidine residues. This notion gained further support from the finding that modification of His residues of band 3 by DEPC reduced I- quenching at pH 6. On the basis of these results, it is proposed that eosin is located in the anion transport channel such that it is accessible from both sides of the membrane. Histidine residues, which have previously been proposed to lie in the anion channel, probably are located on either side of the eosin probe where they contribute to electrostatic interactions which determine the Kq values for the charged quenchers.

Discrimination between hydrogen atom and proton abstraction in the quenching of n, π* singlet‐excited states by protic solvents

AbstractThe fluorescence quenching of the azoalkane 2,3‐diazabicyclo[2.2.2]oct‐2‐ene by protic solvents (methanol, methanol‐OD, water, deuterium oxide, acetic acid) has been examined. The pseudo‐unimolecular quenching rate constants (kq) vary from 0.30‐44×106s−1 and decrease upon deuteration of the solvent OH bonds, e.g., the isotope effect for methanol/methanol‐OD is ca. 8.5. This demonstrates that the hydroxylic hydrogens are predominantly responsible for the fluorescence quenching. The activation parameters were determined for methanol and methanol‐OD. The activation enthalpies are unexpectedly low (ΔH‡ = 1.8 kcal mol−1 for methanol) and increase upon deuteration (ΔH‡ = 3.0 kcal mol−1 for methanol‐OD), while the activation entropies remain the same (ΔS‡ ca. 17.5 cal K−1 mol−1. This provides evidence for a fully classical isotope effect related to differences in zero‐point vibrational energies. Tunneling appears to play no significant role. The quenching rate constants display no trend with the acidity of the solvent (pKa values) but with the homolytic bond dissociation energies of the OH bonds. This suggests the involvement of a hydrogen atom rather than a proton transfer. All important aspects (activation enthalpies, isotope effects, etc.) of this novel quenching mechanism of protic solvents are reproduced by MCSCF quantum‐chemical calculations with a complete active space of CAS (12,10) or CAS (8,7). Most importantly, the computed data indicate the occurrence of a conical intersection, i.e., a real surface crossing, which follows the transition state and provides an efficient trigger for radiationless return to the ground‐state energy surface (fluorescence quenching).

The Synthesis and Spectroscopic Properties of a [2]Catenane Incorporating an Anthracene Chromophoric Unit

A modified bis-p-phenylene-34-crown-10 ring in which one of the 1,4-dioxybenzene units has been replaced by a 9,10-dioxyanthracene unit has been employed as a template for the formation of cyclobis(paraquat-p-phenylene). The [2]catenane which results has been shown by 1H NMR spectroscopy to exist in solution exclusively in the translationally isomeric form in which (a) only the 1,4-dioxybenzene ring occupies the central cavity of the tetracationic cyclophane and (b) the crown ether ring is prevented from circumrotating through the cyclophane by the large 9,10-dioxyanthracene unit. The absorption spectrum and luminescence properties of this new [2]catenane and of its crown ether component in its free state have been investigated and compared with those of 1,4-dimethoxybenzene and a model anthracene derivative, carrying methylated triethylene glycol chains on the 9 and 10 positions of the anthracene ring. While the absorption spectrum of the crown ether is the sum of the spectra of the two component chromophoric moieties, its emission spectrum shows only the fluorescence band of the 9,10-dioxyanthracene-type unit. The excitation spectrum shows that the disappearance of the 1,4-dioxybenzene type emission in the crown ether is due to a very efficient (kq ≥ 4 × 1010 s−1) energy-transfer process from the 1,4-dioxybenzene to the 9,10-dioxyanthracene type unit. The absorption spectrum of the [2]catenane is noticeably different from the sum of the spectra of its two cyclic components, particularly as far as the presence of a very broad charge-transfer (CT) band in the visible spectral region (λmax = 545 nm, ϵmax = 615 M−1 cm−1) is concerned.Comparison with the CT band of a model compound shows that the very broad CT band of the [2]catenane is in fact the result of two component bands originating from the interaction of the two different electron-donor units (1,4-dioxybenzene and 9,10-dioxyanthracene type) present in the crown ether with the electron-acceptor bipyridinium-type units of the cyclobis(paraquat-p-phenylene). The emission spectrum of the [2]catenane does not show any band because of the quenching action (rate constant kq ≥ 5 × 1010 s−1) of the low-energy non-luminescent charge-transfer levels on the higher energy, potentially luminescent levels of the crown ether.

Laser photolysis investigation of induced quenching in photoreduction of benzophenone by alkylbenzenes and anisoles

The quenching processes of triplet benzophenone (3BP*) by alkylbenzenes (AB) and anisole derivatives (AD) in benzene (Bz) and a mixture of acetonitrile (ACN) and water (4:1 v/v) have been studied on the basis of rate constants and efficiencies determined by nanosecond laser flash photolysis at 355 nm at 295 K. It was found that (1) the deactivation of 3BP* by ADs in ACN–H2O (4:1 v/v) was governed by electron transfer (ET) to produce the benzophenone anion (BP-) and corresponding cation (AD+) radicals with efficiencies, αET<1 whereas no chemical species were formed in Bz; and (2) photoreduction of 3BP* by ABs resulted in benzophenone ketyl radical (BPK) formation by benzylic hydrogen abstraction (HA) with efficiencies αHA<1 in Bz and ACN–H2O (4:1 v/v). The residual efficiency (αIQ: 1-αET or 1-αHA) was attributed to a bimolecular process with no photochemical product, which was named ‘induced-quenching (IQ)’. The quenching rate constants (kq) of 3BP* by ADs and ABs were less than the diffusion limits of both Bz and ACN–H2O (4:1 v/v). The net bimolecular rate constants for the ET, HA and IQ processes were estimated from the kq values and efficiencies. The rate constants (kET and kIQ) of ET and IQ with AD versus the oxidation potential (Eox) of AD followed Rehm–Weller behaviour while logarithmic rate constants (kHA and kIQ) of HA and IQ by ABs increased linearly with a decrease in the Eox of AB. It was suggested, for the deactivation mechanism of 3BP* by ABs and ADs (RH), that (1) the IQ process was intersystem crossing (ISC) enhanced by the partial charge–transfer (CT) character of the triplet exciplexes, 3(BPδ-···RHδ+)cage*; (2) radical ion formation by ET might be accomplished in a polar solvent by further CT interaction in the exciplex; (3) the process of BPK formation was inferred to be H-atom transfer in the exciplex, where the more protic H-atom was readily mobile, rather than ET followed by proton transfer and (4) the loss of efficiencies of photochemical-product formation was derived not from back ET but from the IQ process, inherent to photoreactions, via triplet exciplexes. The deactivation processes of 3BP* by RH are illustrated in Scheme 1.

Fluorescence quenching of para‐substituted styrenes by tetramethylethylene. Correlation analysis of the quenching coefficient KSV and the quenching rate constant kq by the dual‐parameter equation with polar and spin‐delocalization substituent constants

AbstractThe fluorescence quenching coefficient (KSV) and the quenching rate constant kq of ten para‐substituted styrenes (1‐Ys) have been measured and correlation‐analyzed by both the dual‐parameter equation (Eq. 1) with (ρxσx + ρ· σ·) and the single‐parameter equation (Eq. 2) with ρxσx. Excellent results have been obtained for the correlation of KSV against (ρxσmb + ρ· σ·JJ) or (ρxσ+ + ρ· σ·JJ ). Our results suggest that, possibly, there might be no need to use excited‐state substituent constant for the fluorescence quenching process of excited states of styrenes.

Singlet Oxygen Quenching Abilities of Carotenoids

AbstractThe bimolecular rate constants kq for quenching of singlet oxygen (1Δg state) by 26 different natural and novel synthetic carotenoids were determined at 37 °C in a mixture of chloroform and ethanol. The steady‐state technique used involves the generation of 1O2 by thermal decomposition of disodium 3,3′‐naphtalene‐1,4‐diyl‐dipropionate endoperoxide (NDPO2) and the detection of its luminescence intensity at 1270 nm. Excitation energies (π,π*, 11Ag → 11Bu) and absorption maxima (430–590 nm) vary in the broadest range. Deeply coloured blue carotenoids are also included in the studies for the first time. An empirical correlation between the π,π* (11Ag → 11Bu) excitation energy and carotenoid structure (effective chain length Neff) was found: E(S) = 12642 cm−1 + 92027 cm−1 × 1/Neff. The quenching ability of the investigated carotenoids depends on the excitation energy of their transition at long wavelengths in a characteristic way showing as limiting factors either the thermal Arrhenius activation or the diffusion‐controlled rate. This dependence and the suspected relationship between singlet E(S) and triplet E(T) energies, respectively, are discussed.

Solvent Reorganization Energy in Excited-State Electron-Transfer Reactions. Quenching and Geminate-Pair Back Electron Transfer

The temperature dependencies of the quenching rate constants (kq) and cage escape yields of the redox products (ηce) from the electron-transfer reaction of *Ru(bpy)32+ (bpy = 2,2‘-bipyridine) with nine aromatic amines in deaerated 1:1 (v/v) CH3CN/H2O solutions have been determined. Values of λ, the solvent reorganization energy for electron-transfer quenching and back electron transfer within the solvent cage, have been extracted from plots of log(kqT1/2) vs 1/T and log((ηce-1 − 1)T1/2) vs 1/T, respectively. For the quenching process, λ is not a constant value for the series of quenchers; in general, higher values of λ are exhibited by primary amines and lower values by tertiary amines. The structure and size of the quenchers and the nature of the ring substituents contribute to the value of λ. For the back-electron-transfer reaction within the geminate redox pair formed in the quenching process, the more sterically hindered two-ring amines exhibit a higher value of λ (1.1 ± 0.08 eV) than do the majority ...

Model Studies of DNA Photorepair: Reduction Potentials of Thymine and Cytosine Cyclobutane Dimers Measured by Fluorescence Quenching

The interactions of various pyrimidines (1,3-dimethylthymine, DMT, 1,3-bis(N4,N4-dimethylcytosin-1-yl)propane, DMC) and their corresponding cis-syn cyclobutane dimers (DMTD and DMCD) with a series of excited-state electron donors were examined with the goal of understanding the energetics and mechanism of UV repair by DNA photolyase. For each substrate there is a good correlation between the excited state oxidation potential (Eox*) and the quenching rate constant (kq). The value for kq increases as Eox* becomes more negative, asymptotically approaching a value that is at or below the solvent diffusion limit. These data all showed good fits to the Rehm−Weller equation. Reduction potentials for each of the substrates could be extracted from this analysis: −2.20 V (vs SCE) for DMTD; −2.14 V for DMT; −2.17 V for DMCD; and −2.16 for DMC. These values show that the initial electron transfer step in the photolyase mechanism is exergonic by ca. 10−15 kcal/mol. Thus these data support the reductive electron transfer mechanism for DNA photolyases proposed by Jorns et al. (J. Biol. Chem. 1987, 262, 486−491).

Excited and ground-state complexes of tetraphenylporphyrin (TPP) with some organic electron acceptors

Bimolecular reactions between singlet excited TPP ( 1 TPP*) and various nitroaromatic acceptors were investigated in solvents with various polarity. The reactions between 1 TPP* as electron donor and both picric acid (PIC) and 3,5-dinitrosalicylic acid (DNS) as electron acceptors were accompanied by the appearance of a new emission band in non-polar and slightly polar solvents. A new absorption charge-transfer (CT) band was also observed between TPP and both PIC and DNS. This effect was not observed for other nitroaromatic acceptors. The new emission of the CT complexes was interpreted as a CT excited complex (exciplex). The rate constant (k q ) of the deactivation process, excited-state dipole moment, fluorescence lifetime and activation energies of the CT exciplex are presented. In addition, the association constant, molar absorption coefficients and thermodyn-amic standard reaction quantities of the ground-state complexes were also investigated in solvents of various polarities.

Photooxidation of Tryptophan: O2(1Δg) versus Electron‐Transfer Pathway

Tris (2,2'-bipyridyl)ruthenium(II)chloride hexahydrate (Ru[bpy]3(2+)) free in solution and adsorbed onto antimony-doped SnO2 colloidal particles was used as a photosensitizer for a comparison of the O2(1 delta g) and electron-transfer-mediated photooxidation of tryptophan (TRP), respectively. Quenching of excited Ru(bpy)3(2+) by O2(3 sigma g-) in an aerated aqueous solution leads only to the formation of O2(1 delta g) (phi delta = 0.18) and this compound was used as a type II photosensitizer. Excitation of Ru(bpy)3(2+) adsorbed onto Sb/SnO2 results in a fast injection of an electron into the conduction band of the semiconductor and accordingly to the formation of Ru(bpy)3(2+) and was used for the sensitization of the electron-transfer-mediated photooxidation. The Ru(bpy)3(3+) is reduced by TRP with a bimolecular rate constant kQ = 5.9 x 10(8) M-1 s-1, while O2(1 delta g) is quenched by TRP with kt = 7.1 x 10(7) M-1 s-1 (chemical + physical quenching). Relative rate constants for the photooxidation of TRP (kc) via both pathways were determined using fluorescence emission spectroscopy. With Np, the rate of photons absorbed, being constant for both pathways we obtained kc = (372/Np) M-1 s-1 for the O2(1 delta g) pathway and kc > or = (25,013/Np) M-1 s-1 for the electron-transfer pathway, respectively. Thus the photooxidation of Trp is more than two orders of magnitude more efficient when it is initiated by electron transfer than when initiated by O2(1 delta g).

Effect of chain length on rates of diffusion-limited small molecule-polymer and polymer-polymer reactions: Phosphorescence quenching studies

Model interactions have been studied by phosphorescence quenching to obtain a better understanding of the chain length dependence of interpolymeric chain end-chain end reactions such as those involved in the termination step of free radical polymerization. For small molecule-polymer interactions in dilute cyclohexane solution, quenching rate constant (kq) data agree with the Smoluchowski equation prediction that kq scales as polymer molecular weight (MW) to the -½ power, confirming self-diffusion control. For polymer-polymer interactions in dilute solution, the chain length dependence is weaker than that predicted by translational diffusion control, as described by the Smoluchowski equation, but is stronger than that predicted by renormalization group theory. For interactions between 70000 MW benzil-end-labeled polystyrene and varying MWs of anthracene-end-labeled polystyrene at 300 g/L polymer, kq decreases by a factor of 10 in going from MWs of 100 to 1000 g/mol; beyond 1000 g/mol, kq is nearly independent of chain length. Such effects indicate that the importance of oligomeric radical self-diffusion and polymer radical chain-end segmental mobility must be carefully considered in understanding the termination process in free radical polymerization. © 1996 John Wiley & Sons, Inc.

Electron transfer quenching of tris(2,2?-bipyridine) ruthenium (II) complex derivatives covalently linked to poly (N-isopropylacrylamide) in aqueous solutions

Tris(2,2′-bipyridine) ruthenium(II) complex, ionic probe, was incorporated into poly (N-isopropylacrylamide) (PNIPA), which is known to be a thermoresponsive polymer, by a copolymerization method. Electron transfer quenching of the complex probe by methyl viologen was investigated as a function of temperature. The electron transfer quenching rate constant (kq) in a globular state (higher temperature than the LCST (31°C)) is 4–5 times as large as that in a coil state (lower temperature) from the Stern-Volmer analysis. The result is quite different from the quenching of pyrene probe incorporated into PNIPA in the previous study. This is because hydrophilic ruthenium probe is located at the interface of polymer globular matrix even in a globular state, whereas pyrene probe was embedded into the hydrophobic matrix. The quenching behavior is discussed by a difference in molecular environment of the probes in phase transition of PNIPA in the aqueous solution.

Photophysical triplet state processes of 4-H-1-benzopyrane-4-thione in a perfluoroalkane. Part 1.—Temperature dependence of unimolecular triplet decay

The rate constant k0T for the unimolecular decay of the lowest triplet state, T1, of 4-H-1-benzopyrane-4-thione (BPT) in perfluoro-1,3-dimethylcyclohexane was determined between 230 and 300 K. By assuming a very fast and virtually complete thermal equilibration T1⇌ S1 of the relative population of T1 and the lowest excited singlet state S1, three parameters can be calculated from the temperature dependence of k0T: the individual rate constants βs=(4.0 ± 0.2)× 106 s–1 for the decay of S1 to S0 and βzT=(8.2 ± 2)× 104 s–1 for the decay of the strongly phosphorescent triplet substate Tz1 to S0, and the energy difference ΔE(S1,Tz1)=hc×(757 ± 13) cm–1 between Tz1 and S1. The value of ΔE(S1, Tz1) is in agreement with the positions of T1,0 and S1,0 in the absorption spectrum. The spectrum of the S0→ S1 transition is estimated.The assignment of the hot-band range of the delayed luminescence above the energy hc[graphic omitted]0,0 of T1,0 is facilitated by an inverse Boltzmann weighting (multiplication with exp[hc([graphic omitted] – [graphic omitted]0,0)/kBT]). At the position of S1,0, a contribution of E-type delayed S1→ S0 fluorescence can be discerned.The contribution of diffusion-controlled concentration-quenching (rate constant kQ) to the triplet decay was eliminated by extrapolation to infinite dilution; kQ is proportional to the ratio T/η of temperature T and viscosity η. The temperature dependence of the viscosity and density of perfluoro-1,3-dimethylcyclohexane were measured.

Fluorescence quenching and complexation behaviour of tetraphenylporphyrin with some divalent metal ions

The fluorescence quenching and complexation behaviour of tetraphenylporphyrin (TPP) have been studied using fluoresence, electronic absorption spectra and the stopped-flow technique. The second-order fluorescence quenching rate constant (kq) increases in the order Ni2+ > Co2+ > Hg2+ > Sn2+ in methanol. Both Ni2+ and Co2+ ions form relatively stable complexes with TPP and the complex formation in this case is slow. In the case of Hg2+ and Sn2+, complex formation is fast and the formed complexes are less stable compared with NiTPP and CoTPP complexes. The activation and thermodynamic parameters of the complexation process have been evaluated.

Accessibility of the fluorescent reporter group in native, silica-adsorbed, and covalently attached acrylodan-labeled serum albumins.

Fluorescence quenching techniques are used to investigate the accessibility of a model biorecognition element-reporter group system when in buffer, surface-adsorbed, and covalently attached to a silica surface. The site-selective fluorescent reporter group, 6-acryloyl(dimethylamino)naphthalene (acrylodan, Ac), is attached covalently (at cysteine-34) to bovine and human serum albumin (BSA and HSA, respectively) and serves as a surrogate recognition element-reporter group system. Molecular oxygen is used to quench the Ac fluorescence and the accessibility, in the form of bimolecular rate constants (kq), in each model system is quantified. Although one might expect these systems to exhibit similar behavior, differences in quenching characteristics are observed, such as wavelength dependency of the Stern-Volmer quenching constant (KSV) for the native proteins in buffer. BSA-Ac exhibits wavelength dependent KSV values as well as a blue-shifted emission spectrum on O2 addition. Physisorption of BSA-Ac onto a fused-silica optical fiber lowers the accessibility of Ac to O2, whereas covalent attachment of BSA-Ac to APTES/glutaraldehyde-modified silica enhances the accessibility of the Ac reporter group to O2.

Solvent Reorganization Energy in Excited-State Electron-Transfer Reactions. Quenching and Geminate-Pair Back Electron Transfer

The temperature dependencies of the quenching rate constants (kq) and cage escape yields of the redox products (ηce) from the electron-transfer reaction of *Ru(bpy)32+ (bpy = 2,2‘-bipyridine) with nine aromatic amines in deaerated 1:1 (v/v) CH3CN/H2O solutions have been determined. Values of λ, the solvent reorganization energy for electron-transfer quenching and back electron transfer within the solvent cage, have been extracted from plots of log(kqT1/2) vs 1/T and log((ηce-1 − 1)T1/2) vs 1/T, respectively. For the quenching process, λ is not a constant value for the series of quenchers; in general, higher values of λ are exhibited by primary amines and lower values by tertiary amines. The structure and size of the quenchers and the nature of the ring substituents contribute to the value of λ. For the back-electron-transfer reaction within the geminate redox pair formed in the quenching process, the more sterically hindered two-ring amines exhibit a higher value of λ (1.1 ± 0.08 eV) than do the majority ...