Intramolecular Rate Constants Research Articles

Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence

We study external quantum efficiency (ηEQE) roll-off in organic light-emitting diodes (OLEDs) using thermally-activated delayed fluorescence (TADF) of 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN). Using 2CzPN intramolecular rate constants from optical analyses, we construct an exciton quenching model incorporating intersystem crossing and reverse intersystem crossing. The model indicates that singlet–triplet annihilation and triplet–triplet annihilation dominate ηEQE roll-off because of the relatively long 2CzPN triplet lifetime of 273μs. This work yields a method to relax the exciton quenching process in TADF based OLEDs.

Influence of the lifetime of a metastable triplet state of photostable organic fluorophores on the kinetics of fluorescence fading and recovery

The kinetics of rapid fluorescence fading and recovery (anti-fading) of photostable organic fluorophores under two-stage rectangular modulated photoexcitation was studied using computer modeling in the framework of a three-level energy diagram. The fading and recovery were due to an abrupt change in the population rate of the metastable T 1 state. The calculations revealed the presence of an anti-fading process. The modeling results were confirmed by experimental measurements of the fluorescence of a zinc complex of a substituted tetrabenzoporphyrin in an organic polymer. Precise analytical expressions establishing the relationship between the intramolecular rate constants of the photophysical processes and the measured fluorescence parameters were derived.

Light-induced electron transfer in porphyrin–calixarene conjugates

The fluorescence from a set of porphyrin-calixarene complexes is quenched upon addition of benzo-1,4-quinone (BQ) in fluid solution. In N,N-dimethylformamide solution, fluorescence quenching involves both static and dynamic interactions but there are no obvious differences between porphyrins with or without the appended calixarene. Under such conditions, the static quenching behaviour is attributed to pi-complexation between the reactants and it is concluded that the calixarene cavity does not bind BQ. An additional static component is apparent in dichloromethane solution. This latter effect involves partial fluorescence quenching, for which the intramolecular rate constant can be obtained by time-resolved fluorescence spectroscopy. The derived rate constants depend on molecular structure in a manner consistent with fluorescence quenching being due to electron transfer. In all cases, however, the dominant quenching step involves diffusional contact between the porphyrin nucleus and a non-bound molecule of BQ.

Anomalous distance dependence of electron transfer across peptide bridges.

The first investigation on the distance dependence of a dissociative electron transfer process across peptide bridges is reported. This study was carried out by using a series of donor-peptide-acceptor systems in which the donor is a phthalimido moiety, the peptide bridges are provided by alpha-aminoisobutyric acid (Aib) homooligomers, and the acceptor is a peroxide functional group. The intramolecular electron transfer from the electrogenerated phthalimido radical anion to the peroxide was studied in comparison with the thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular rate constants were determined in N,N-dimethylformamide by taking into account the corresponding intermolecular values. The experimental results point to an unusual non-exponential dependence of the intramolecular electron transfer rate on the number of bridge units. The same trend could be verified also by taking into account the actual donor-acceptor edge-to-edge distance. The peculiar distance dependence that was observed for the intramolecular electron transfer rate is attributed to the mediating effect of the intramolecular C=O...H-N hydrogen bonds.

Insights into the free-energy dependence of intramolecular dissociative electron transfers.

To study the relationship between rate and driving force of intramolecular dissociative electron transfers, a series of donor-spacer-acceptor (D-Sp-A) systems has been devised and synthesized. cis-1,4-Cyclohexanedyil and a perester functional group were kept constant as the spacer and acceptor, respectively. By changing the aryl substituents of the phthalimide moiety, which served as the donor, the driving force could be varied by 0.74 eV. X-ray diffraction crystallography and ab initio conformational calculations pointed to D-Sp-A molecules having the cis-(cyclohexane) equatorial(phthalimido)-axial(perester) conformation and the same D/A orientation. The intramolecular dissociative electron-transfer process was studied by electrochemical means in N,N-dimethylformamide, in comparison with thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular process consists of the electron transfer from the electrochemically generated phthalimide-moiety radical anion to the peroxide functional group. The electrochemical analysis provided clear evidence of a concerted dissociative electron-transfer mechanism, leading to the cleavage of the O-O bond. Support for this mechanism was obtained by ab initio MO calculations, which provided information about the LUMO of the acceptor and the SOMO of the donor. The intramolecular rate constants were determined and compared with the corresponding intermolecular values, the latter data being obtained by using the model molecules. As long as the effective location of the centroid of the donor SOMO does not vary significantly by changing the aryl substituent(s), the intramolecular dissociative electron transfer obeys the same main rules already highlighted for the corresponding intermolecular process. On the other hand, introduction of a nitro group drags the SOMO away from the acceptor, and consequently, the intramolecular rate drops by as much as 1.6 orders of magnitude from the expected value. Therefore, a larger solvent reorganization than for intermolecular electron transfers and the effective D/A distance and thus electronic coupling must be taken into account for quantitative predictions of intramolecular rates.

Dependence of Intramolecular Dissociative Electron Transfer Rates on Driving Force in Donor−Spacer−Acceptor Systems

The voltammetric reduction of a series of phenyl-substituted 4-benzoyloxy-1-methylcyclohexyl bromides has been investigated in DMF. The reduction leads to the cleavage of the C−Br bond. On a thermodynamic ground, the direct reduction of the tertiary C−Br function is easier than that of the selected benzoates by at least 0.5 V. However, since the direct reduction of bromides is affected by a large activation overpotential, the electron is first located in the benzoate moiety. The rate constant for the following exergonic intramolecular dissociative electron transfer was determined by kinetic analysis of the cyclic voltammetry curves. The intermolecular rate constants for the reaction between the radical anions of methyl benzoates and 4-tert-butyl-1-methylcyclohexyl bromide were also determined and found to correlate very well with related literature data pertaining to tert-butyl bromide. The intramolecular rate constants were found to be more sensitive to variation of driving force than the corresponding intermolecular data. This result can be attributed to a shift of the center of the π* orbital of the radical anion donor away from the acceptor moiety, the shift being larger for the most easily reduced donors. The resulting distance increase is therefore envisaged as responsible for a more rapid rate drop, compared to the intermolecular pattern, when smaller driving forces are considered.

Inactivation of xanthine oxidase by oxidative radical attack.

Steady-state radiolysis, pulse radiolysis and epr studies, combined with enzyme activity measurements, were carried out on the mechanism by which radical attack, through one-electron oxidation, inactivates xanthine oxidase. Electron transfer to both the N3 and Br2- radical species was used to initiate oxidative damage on the enzyme. Inactivation was found to occur to a greater extent at low than at high pH and is associated with the initial formation of a tryptophanyl radical which converts by a known intramolecular pathway to a tyrosyl radical with a rate constant of 5 x 10(3) S-1. The tyrosyl radical in turn slowly loses around half of its absorbance at an intramolecular rate constant of 350S-1 and is consistent with the establishment of a radical equilibrium with cysteine residue(s). The sequence of reactions could be repeated several times on the same irradiated sample implying that restitution of the implied cysteinyl radical occurs leading to other damage in the protein. N3+Trp/N-->Trp/N-->Tyr/O<-->Cys/S-->?. Epr evidence implies that inactivation of the enzyme from the above sequence of reactions arises in part from alternations to Fe/S center I in the enzyme.

Intra- and intermolecular electron transfer reactions in covalently linked donor-acceptor molecules

We synthesized a homologous series of molecules (MV nn′ Q) where a methylviologen (MV 2+) and an amino-chloronaphthoquinone (Q) are linked to each other via a flexible chain. Using the electron pulse radiolysis technique, we have measured time-resolved spectra and determined rate constants for intra- and intermolecular electron transfer (ET) between donor and acceptor site of the MV nn′ Q molecules in water and in sodium dodeeyl sulfate (SDS) micellar solution. For comparison, we also irradiated a solution containing a 1:1 mixture of methylviologen and amino-chloronaphthoquinone and measured spectra and intermolecular ET reactions between the separated electron donor and acceptor molecules. We found a remarkably slow intramolecular electron transfer from the reduced methylviologen moiety to the quinone site of all MV nn′ Q molecules both in water and in aqueous SDS micellar suspensions. The intramolecular rate constants measured in water increase with the number of intervening bonds, leading to the conclusion that electron transfer occurs by a through-space rather than through-bond mechanism. The intramolecular rate constants virtually lose their chain length dependence in SDS suspensions where, because of an extended configuration of the micellized MV nn′ Q molecules, through-space interaction is not favored.

Preparation and kinetic properties of 5‐ethylphenazine‐glucose‐dehydrogenase‐NAD+ conjugate, a semisynthetic glucose oxidase

5-Ethylphenazine-glucose-dehydrogenase-NAD+ conjugate (EP(+)-GlcDH-NAD+) was prepared by linking both poly(ethylene glycol)-bound 5-ethylphenazine and poly(ethylene glycol)-bound NAD+ to glucose dehydrogenase. The average number of the ethylphenazine moieties bound/enzyme subunit was 0.8, and that of the NAD+ moieties was 1.2. This conjugate is a semisynthetic enzyme having glucose oxidase activity using oxygen or 3-(4,5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2H-tetrazolium bromide (MTT) as an electron acceptor. When the concentration of oxygen or MTT is varied, the oxidase activity fits the Michaelis-Menten equation with the following values of the kinetic constants: for the system with oxygen, the turnover number per subunit is 0.40 s-1 and Km for oxygen is 1.57 mM; and for the system with MTT, the turnover number is 0.11 s-1 and Km for MTT is 0.072 mM. The catalytic cycle of the semisynthetic oxidase has two catalytic steps: reduction of the NAD+ moiety by the active site of the glucose dehydrogenase moiety and oxidation of the NADH moiety by another catalytic site of the ethylphenazine moiety. The apparent intramolecular rate constants of these steps were estimated, and the values are as follows: 0.39 s-1 for the reductions of the NAD+ moiety, 2.2 s-1 and 0.12 s-1 for the oxidation of the NADH moiety in the systems with oxygen and with MTT, respectively, and 3.2 s-1 and 0.18 s-1 for the reduction of the ethylphenazine moiety in the systems with oxygen and with MTT, respectively. On the bases of these results, the following three rate-acceleration mechanisms of the semisynthetic glucose oxidase are discussed: high effective concentration, intramolecular coupling of successive catalytic reactions, and multiple connection between the two kinds of the catalytic sites.

Preparation and kinetic properties of 5‐ethylphenazine–poly(ethylene‐glycol)–glutamate‐dehydrogenase conjugate

5-Ethylphenazine-poly(ethylene glycol)-glutamate dehydrogenase conjugate (EP(+)-PEG-GluDH) was prepared by linking poly(ethylene glycol)-bound 5-ethylphenazine to glutamate dehydrogenase. The average number of the ethylphenazine moieties bound/enzyme subunit was 0.7. This conjugate is a semisynthetic enzyme having NADH oxidase activity; the ethylphenazine moiety works as a catalytic group, and the coenzyme-binding site of glutamate dehydrogenase works as a substrate-binding site. The effects of the presence of the substrate-binding site near the catalytic group were studied by using EP(+)-PEG-GluDH. Before the preparation of the conjugate, the reactivity of NADH bound in the coenzyme-binding site toward the ethylphenazine moiety was estimated for glutamate and lactate dehydrogenases. The results show that the NADH molecule bound in the site of glutamate dehydrogenase reacts with EP(+)-PEG at a rate of 43% of that of free NADH, but the NADH molecule bound in lactate dehydrogenase does not react with 1-(3-carboxypropyloxy)-5- ethylphenazine. Therefore, glutamate dehydrogenase was used as the substrate-binding site of the semisynthetic NADH oxidase. The results of the kinetic analysis of the activity of EP(+)-PEG-GluDH show that the apparent turnover number of the active site is 0.38 s-1, which corresponds to the apparent intramolecular rate constant of the oxidation of NADH bound in the active site. The apparent effective concentration of bound NADH for the catalytic group of the ethylphenazine moiety is 0.33 mM. This means that the presence of the substrate-binding site near the catalytic group increases the local NADH concentration by at most 0.33 mM, and this is the rate-accelerating effect of the binding site.

Kinetics of disulfide exchange reactions of monomer and dimer loops of cysteine-valine-cysteine peptides.

Rate constants have been determined in 3 M guanidine hydrochloride for disulfide exchange reactions between glutathione and two synthetic peptides containing a cysteine-valine-cysteine region. Equilibrium experiments demonstrate the absence of noncovalent peptide aggregation in this solvent. Procedures are given for separating seven different components in quenched reactions, including the fully reduced cysteine cluster, the monomeric disulfide loop, parallel and antiparallel dimer loops, and the three monomers containing one or two mixed disulfides with glutathione. Intramolecular rate constants for (1) formation of a sterically strained monomer loop, (2) transfer of glutathione between the two cysteines on the same peptide chain, and (3) formation of unstrained dimer loops correspond to a series of processes forming rings of increasing size. In one sequence, these rate constants are 3, 6, and about 21 s-1, respectively. The larger loops are formed more easily. In the other sequence, rate constants for formation and opening of monomer loops are accelerated 180- and 1300-fold, respectively, relative to analogous reactions in a peptide containing eight residues between the two cysteines. This gives a 7-fold smaller equilibrium constant for ring closure in the cysteine cluster. Dimer formation occurs by a mechanism utilizing the accelerated opening of monomer loops. Results provide information assisting efforts to develop strategies for directing disulfide pairing in novel protein structures. Results also help define factors contributing to formation of undesired oligomers during efforts to refold cysteine-containing proteins obtained by bacterial expression of mammalian genes.

Preparation, characterisation, and intramolecular electron-transfer studies on the penta-ammineruthenium histidine-modified high-potential iron–sulphur protein from Chromatium vinosum

The high-potential iron–sulphur protein (Hipip) from Chromatium vinosum(Mrca. 9 500; 85 amino acids) contains a single histidine residue, His 42, the location of which is known from the X-ray crystal structure. The reaction of Hipipred with excess [Ru(NH3)5(H2O)]2+ followed by oxidation, yields the Ru-modified protein Hipipox·RuIII. The product has been purified by column chromatography, and analysed by inductively coupled plasma atomic emission spectroscopy (Fe : Ru ratio 4 : 1). The His 42 of the Ru-modified protein no longer reacts with diethyl pyrocarbonate (depc), and the sharp 1H n.m.r. C2H resonance of His 42 at 8.3 p.p.m. is no longer present due to paramagnetic line broadening by the attached RuIII. The reduction potential of the [Fe4S4]3+/2+ couple remains at 350 mV in the RuIII-modified protein. On pulse radiolysis using eaq–(CO2˙– or O2–) as reductant to generate Hipipox·RuII, a second stage assigned to intramolecualr formation of Hipipred·RuIII is observed, k= 18 s–1(pH 7.0, 20 °C). An additional step is observed as a result of autoreduction of Hipipox·RuIII to Hipipred·RuIII, which occurs on bubbling Ar gas through solutions in anaerobic procedures. Subsequent pulsing generates some Hipipred·RuII, the decay of which gives rise to third stage here assigned as an intermolecular electron transfer between fully oxidised and fully reduced Ru-modified protein. The His 42 is near to the Fe4S4 active site, with Cys 43 attached to one of the Fe atoms. The direct donor–acceptor linkage is unusually short and the intramolecular rate constant is therefore of interest in the context of through-space (7.9 A) or through-bond (13 A) electron transfer. There is no obvious enhancement of the intramolecular electron-transfer process, and at this time no reason for favouring the through-bond assignment.

Modelling of intramolecular reactions in the step-growth polymerization of multifunctional monomers

A general kinetic model for the polymerization of multifunctional monomers RY f valid up to the gel point has been presented, which accounts for the intramolecular reactions. In this model, various molecular species have been distinguished on the basis of the total number of intramolecular bonds existing on them. Various reactions leading to formation and depletion of a molecular species of given degree of polymerization are identified and the differential mole balance equations for these have been written down. The intramolecular rate constant has been assumed to be that proposed by Platé and Noah (based on their Monte Carlo studies of intramolecular reactions) and the various mole balances have been solved numerically. Numerical computations confirm that, as the gel point is approached, larger polymer chains are formed and these have a higher tendency to react intramolecularly. However, even at high conversions, the average number of rings per polymer chain remains small, which is consistent with experimentally observed results.

Transient kinetics of chemical reactions with bounded diffusion perpendicular to the reaction coordinate: Intramolecular processes with slow conformational changes

Intramolecular reactions inside macromolecules (e.g., binding of ligands to iron inside heme proteins) may often be coupled to slow random fluctuations in the reaction center geometry. This motion is ‘‘perpendicular’’ to the reaction coordinate. It can be described as bounded diffusion in the presence of a binding potential field and an intramolecular rate constant which depends on the perpendicular degree of freedom. The diffusion equation is solved under the appropriate reflective boundary conditions. The transient decay of the total population is multiexponential (power law) for small diffusivity, changing to monoexponential kinetics for large diffusivity. For large times or large diffusivity, direct integration is very tedious, but an eigenvalue expansion converges rapidly. It also allows the calculation of the ‘‘average survival time’’ (an extension of the ‘‘first passage time’’) a natural candidate for replacing the reciprocal rate constant in multiexponential kinetics. An example is given for electron transfer between two loosely bound sites in a macromolecule. The average survival time shows a non-Kramers dependence on diffusivity, of the type found in the binding kinetics in heme proteins.

An exact model calculation of the diffusion controlled intramolecular rate constant

An exact calculation of the diffusion controlled first-order intramolecular rate constant k1 is given for the harmonic spring model and for any type of reaction sink in the frame work of the Wilemski–Fixman theory. For a one parameter spherically symmetric sink the resulting exact expansion for k−11 in terms of the sink parameter γ, k−11 = (√π/2Λγ)+(ln2−1)/Λ+0.5(√πγ/Λ)+⋅⋅⋅, gives the same γ→0 behavior of other known asymptotic calculations obtained for simpler diffusion controlled reaction theories. Also, the mean first passage time for end-to-end contact may be evaluated and results coincident to the third term in the expansion with the exact result given by the first passage time approach theory. These results support the vicinity of the Wilemski–Fixman closure approximation.

Optical Pumping of the Lowest Triplet State and Multiple Resonance Optical Techniques in Zero Field

The principles involved in the optical detection of zero-field transitions, electron spin–electron spin double resonance (EEDOR) transitions, and electron spin–nuclear spin double resonance (ENDOR) transitions in zero-field of the lowest triplet state are briefly summarized. Equations are then derived which relate the changes in the phosphorescence intensity originating from any of the zero-field levels to the radiative and nonradiative intramolecular rate constants of processes involving the triplet state upon: (a) saturation of any of the two zero-field transitions involving the emitting level with microwave radiation; (b) simultaneous saturation of two zero-field transitions, only one of which involves the emitting level optical detection of electron–electron double resonance (EEDOR); (c) simultaneous saturation of a zero-field transition involving the emitting level and a hyperfine transition involving either of the two electron spin levels being saturated with microwave (optical detection of electron–nuclear double resonance, ENDOR). These equations are derived for three different systems, in all of which the spin-lattice relaxation processes are assumed negligible (i.e., systems at very low temperatures). In System I, the triplet state is pumped by singlet–singlet absorption followed by the intersystem-crossing processes in the molecule studied (guest). In System II, pumping of the lowest triplet state is carried out by singlet→triplet absorption of the guest. In System III, pumping of the lowest triplet state of the guest is carried out by exciting either the singlet or the triplet states of the host, which is followed by triplet–triplet energy transfer. In System I, the changes in intensity are related to the rate constants of the intersystem-crossing processes to and from the individual zf levels of the molecule being studied. In System II, the radiative rate constants of the optical transition used for pumping, as well as the decay constants of the zf levels of the lowest triplet state of the guest molecule, determine the changes in the phosphorescence intensity upon microwave or radiowave saturation. In System III, in addition to the decay constants of the emitting triplet state, the intersystem-crossing (S1 ↝ T1) rate constant or the singlet→triplet absorption cross section of the host molecule determines the predicted changes in the phosphorescence intensity in the multiple resonance experiment. The relative orientation of the magnetic principal axes of the host to those of the guest is also important in System III. The conservation of spin direction in triplet–triplet energy transfer is briefly discussed. From the equations derived and the observed intensity changes upon microwave saturation, the ratios of important unimolecular rate constants for radiative and nonradiative singlet–triplet processes involving the individual zero-field levels can be determined. Comparison between theoretical predictions and observed effects is made whenever results are available.