Energy Transfer Reactions Research Articles

Triplet State of O-Acyloximes Studied by Time-Resolved Absorption Spectroscopy

Abstract Triplet states of O-acyloximes were studied for the first time by means of laser absorption spectroscopy. Energy transfer experiments showed that the acyl moiety has no effect on the triplet energy, which depends only on the ketone oxime part. It was observed that the difference between the triplet energy of the parent ketone and that of the oxime varies, depending on the oxime structure. This behaviour is explained on the basis of a non-vertical character of the energy transfer reaction.

A new energy transfer chemical laser at 1.315 μm

CW laser action has been demonstrated on the electronic I ∗ ( 2 P 1/2) → I( 2 P 3/2) transition of atomic iodine at 1.315 μm from the NCl (a 1 Δ )+I( 2 P 3/2) energy transfer reaction. The stimulated emission was generated in a transverse subsonic flow device when hydrogen azide, HN 3, was injected into a flow of iodine and chlorine atoms. The measured laser output power was 180 mW.

A Fluorescence Lifetime and Integral Equation Study of the Quenching of Naphthalene Fluorescence by Bromoethane in Super- and Subcritical Ethane

We report on the solvent effect on a kinetic-controlled energy-transfer reaction in super- and subcritical ethane. The reaction chosen, the quenching of naphthalene fluorescence by C2H5Br, occurs well below the diffusion-controlled limit in liquid solvents and in ethane. The reaction was studied in ethane over a wide range of pressures, extending from 34.8 to 111.6 bar at 40 °C, which corresponds to reduced densities (ρr = ρ/ρc, where ρc is the critical density of ethane) between 0.27 and 1.76. The rate constants in super- and subcritical ethane range from 2.42 × 108 to 1.36 × 109 M-1 s-1, which is several orders of magnitude below the diffusion-controlled limit (which is on the order of 1011 M-1 s-1 at these conditions). As in several previous reports, the apparent rate constants, based on bulk concentrations of the reactants, increase dramatically with decreasing pressure in the supercritical region. More importantly, at reduced densities below about 0.44, the apparent rate constants based on bulk concentrations decrease with decreasing pressure. Because the rate constants are solvent insensitive in a variety of liquid solvents, we attribute the behavior of the apparent rate constants in ethane to local composition enhancement of C2H5Br around naphthalene. In addition, we present compelling theoretical (based on integral equation calculation) results that confirm a maximum in the local composition of C2H5Br around naphthalene near a reduced density of 0.44. This is the first experimental study of a simple, bimolecular reaction over such a wide range of densities. Moreover, it is the first to show a maximum in the reaction rate, which corresponds to the expected maximum in the local composition enhancement.

Photoinduced Energy and Electron Transfer in Fullerene−Oligothiophene−Fullerene Triads

A series of fullerene−oligothiophene−fullerene (C60−nT−C60) triads with n = 3, 6, or 9 thiophene units has been synthesized, and their photophysical properties have been studied using photoinduced absorption and fluorescence spectroscopy in solution and in the solid state as thin films. The results are compared to those of mixtures of oligothiophenes (nT) with N-methylfulleropyrrolidine (MP−C60). Photoexcitation of the triads in the film results in an electron-transfer reaction for n = 6 and 9, but not for n = 3. The lifetime of the charge-separated state in the film is on the order of milliseconds. Photoexcitation of the oligothiophene moiety of the C60−nT−C60 triads, dissolved in an apolar solvent, results in a singlet energy-transfer reaction to the fullerene moiety with rates varying between 1012 and 1013 s-1. In more polar solvents, an intramolecular photoinduced charge separation occurs for n = 6 and 9 and, to some extent, for n = 3. The quenching of the MP−C60(S1) fluorescence provides a lower limi...

Long-Range Resonance Energy Transfer to [Ru(bpy)3]2+

We report the first experimental evidence of long-range nonradiative singlet−singlet resonance energy transfer from excited coumarin 460 molecules to [Ru(bpy)3]2+ acceptors (bpy is 2,2‘-bipyridine). The molecules were randomly dispersed in fluid solutions of both high (glycerol) and low (CH3CN) viscosity at room temperature. The frequency-domain donor (coumarin 460) fluorescence decays were satisfactorily analyzed in terms of Förster theory incorporating a term for translational diffusion. Two different [Ru(bpy)3]2+ (quencher) concentrations in each solvent were used, and the coumarin 460 emission was measured and analyzed both independently and globally. In glycerol, diffusion does not play a significant role in the energy transfer process, whereas in CH3CN, diffusion is able to enhance the energy transfer reaction. In both solvents, mutual diffusion coefficients were determined from the experimental data and χ2R error surfaces were constructed to judge their statistical uncertainty. The results demonstrate that metal-to-ligand charge transfer compounds can serve as intermolecular energy transfer acceptors in schemes utilizing long-range Förster-type processes.

On the Ability of Coumarin Derivatives to Interact with Photoinitiators.

The interactions, in the excited states, between a coumarin and a ketocoumarin together with bisimidazole derivative, mercaptobenzoxazole, titanocene have been studied by laser absorption spectroscopy and by photoconductivity. The redox potentials of these different compounds have been measured and led to the calculation of free enthalpy change for a possible electron transfer reaction. The whole results show that the coumarin is able to demonstrate the formation of radicals through an electron transfer reaction with the additives. On the other hand, the ketocoumarin rather leads to an energy transfer reaction with bisimidazole and to a hydrogen abstraction reaction with the benzoxazole derivative. This study presumes that several coumarin or ketocoumarin/additives combinations should be able to initiate a radical polymerization reaction.

Activity of water-soluble fullerenes towards OH-radicals and molecular oxygen

bis-functionalized C60 carrying negatively charged carboxyl groups, C60[C(CO−2)2]2, or positively charged quaternary ammonium groups, C60(C4H10N+)2, have been shown to exhibit high water-solubility without cluster formation. This renders them excellent targets for free radical attack and energy transfer reactions in aqueous environment. Absolute rate constants on the order of 8×109 M−1 s−1 and 1.5×109 M−1 s−1 have been measured for the addition of OH radicals to the fullerene core and for the formation of 1O2 through triplet excited states of the fullerenes, respectively, for a variety of bis-functionalized stereoisomers. Comparatively lower rates have been observed for the corresponding energy transfer reactions with γ-cyclodextrin-encapsulated (C60/γ-CD, C60C(CO−2)2/γ-CD, and C60[C(OCH2CH2)3CH3]2/γ-CD), and water-soluble poly-functionalized fullerenes (e,e,e-C60[C(CO−2)2]3, C60[(CH2)4SO3H]6, and C60(OH)18). The latter three also exhibited a lower reactivity towards OH radicals. In case of the γ-CD complexes, OH reacted exclusively with the macromolecular host.

Rapid detection of the factor V Leiden (1691 G > A) and haemochromatosis (845 G > A) mutation by fluorescence resonance energy transfer (FRET) and real time PCR.

A rapid method based on fluorescence resonance energy transfer (FRET) and real time polymerase chain reaction (PCR) was used to identify the haemochromatosis genotype in 112 individuals and the factor...

Photophysics of 4-methoxy-benzo[b]thiophene in different environments. Its role in non-radiative transitions both as an electron and as an energy donor

The present investigation was made to study the photophysical properties of a newly synthesized benzothiophene compound “4-methoxy-benzo[b] thiophene (4MBT)” in different environments both at the room and at 77 K temperatures. The studies were mainly made by electrochemical and spectroscopic (both by steady-state and time resolved) techniques. The presence of the heteroatom sulfur (S) in 4MBT seems to be responsible for the observed significant changes in the transition energies of this molecule compared to that of analogous compound 4-methylindole (4MI) in spite of the structural similarities of these two compounds. In the presence of the electron acceptors 2-nitrofluorene (2NF) and 9-fluorenone (9FL) the room temperature fluorescence spectra of 4MBT are found to be quenched significantly. In the presence of the latter quencher, a long-wavelength band was found to develop at around 508 nm region with a concomitant reduction in the 4MBT fluorescence band. Fluorescence lifetime data reveal that this long-wavelength band might be due to the formation of acceptor fluorescence by Förster's long-range singlet–singlet energy transfer process. The observed highly negative Δ G 0 values of the present donor-acceptor systems coupled with large overlapping of fluorescence spectra of the donor and electronic absorption spectra of the acceptor are explained by competing energy transfer and photoinduced electron transfer (ET) reactions. In the presence of the acceptors, presently studied, excited donor 4MBT deactivates non-radiatively through both the static and the dynamic quenching processes in which the former process appears to be dominant. Positive deviation from linearity of Stern–Volmer (SV) plot along with the observation of unaffected fluorescence lifetime of 4MBT in the presence of different concentrations of the quenchers (acceptors) demonstrates this fact. In dynamic quenching, Förster's energy transfer process appears to play a major role. However, when acceptors are excited to their first singlet excited states in the presence of unexcited (ground state) donor 4MBT, only dynamic processes (ET associated with some other non-radiative processes) seem to be operative. From the present investigations it was inferred that benzothiophene is a stronger electron donor than the analogous compound 4MI, though the structures of these two donors are similar except for the presence of a heavy atom S in the 1-position of 4MBT. At 77 K as the value of R 0, Förster critical energy transfer distance, is high (∼20 Å), the quenching of the donor fluorescence by the acceptors seems to proceed mainly through the Förster's energy transfer mechanism. The reduction of phosphorescence intensity of this donor in the presence of an acceptor seems to be due to the involvement of a non-radiative pathway through which singlet – singlet ( S D 1→ S A 1) energy transfer occurs efficiently followed by intersystem crossing to populate the triplet level of the acceptor at the expense of the population in donor triplet. At this temperature the exact mechanism of the observed fluorescence quenching effect of the acceptor (2NF and 9FL) in the presence of the donor 4MBT containing S atom is not very clear at the present stage of investigation. Two possible non-radiative mechanisms, one by external heavy-atom effect and the other by photoinduced ET process, seem to have minor role at 77 K as sulfur is known to be weak heavy-atom perturber and ET is thermally activated process.

Triplet Properties and Photoinduced Electron-Transfer Reactions of C120, the [2+2] Dimer of Fullerene C60

Photophysical and photochemical properties of a [2+2] fullerene dimer, C120, have been characterized by laser flash photolysis. By laser irradiation to C120, a triplet−triplet absorption band appeared at 700 nm. From triplet energy-transfer reactions with some energy acceptors, photophysical properties of C120, including triplet energy, extinction coefficient, and quantum yield of the intersystem crossing process, were estimated. Reduction of C120 in the ground state by tetrakis(dimethylamino)ethylene resulted in decomposition into C60•- and C60. Photoinduced electron transfer from N,N,N‘,N‘-tetramethyl-1,4-phenylenediamine to triplet-excited C120 was confirmed by the observation of a transient absorption band due to the radical anion of C120 at 1020 nm. Electron-transfer rates between triplet-excited C120 and various electron-donors were explained by the semiempirical Rehm−Weller relation. Oxidation of triplet-excited C120 was also observed by employing tetracyanoethylene as an electron-acceptor.

Nonequilibrium distribution function theory of diffusion-influenced reversible energy-transfer reactions

Recently, we proposed a new systematic approach to evaluate the many-particle effects on the diffusion-influenced reactions. The method gives an improved result over that obtained by using the superposition approximation. In the present paper, we apply the method to treat the kinetics of reversible energy-transfer reactions of the type A*+B⇄A+B*. Until now, most theories were inapplicable when the lifetime of A* is shorter than that of B*, and a notable exception was the integral encounter theory (IET) of Burshtein et al. The present theory can be applied irrespective of the relative magnitude of the lifetimes of donor and acceptor molecules, and becomes exact for the irreversible target model. In addition, it is applicable to the system with higher reactant densities than IET; the result of IET is recovered as a limiting form in the present theory.

Spectroscopic studies of solvent effects on reactions in supercritical fluids

A number of studies have shown that the local environmental around dissolved solutes in supercritical fluids (SCFs) is significantly different than the bulk. Moreover, in SCF mixtures, there is evidence that the local composition around a dilute solute can be significantly enriched with one of the components. In this paper, we will show how we have used laser flash photolysis and time-resolved fluorescence to study the influence of these molecular scale interactions on the rates of electron transfer and energy transfer reactions in SCFs. In particular, we will present results of the reaction of triplet benzophenone with either triethylamine or 1,4-diazabicyclo[2.2.2]octane in SC CO 2 and ethane. These reactions occur by way of an electron transfer mechanism in normal liquids at rates close to diffusion control. However, in SCFs, they occur over an order of magnitude below the diffusion control limit. We will compare these results with the rates of energy transfer from anthracene to CBr 4 in supercritical CO 2, which occurs at the diffusion control limit in both liquids and SCFs.

A model for the generation of low level chemiluminescence from microbiological growth media and its depletion by bacterial cells

We present a model for the development of chemiluminescence (CL) in autoclaved liquid growth media, as they are used in microbiology for the culturing of microorganisms, or in other related Maillard systems. The model distinguishes between four different stages consisting of sugar fragmentation during heating, autooxidation of highly reducing fragmentation products, radical chain reactions leading to a peroxidation of the media, and finally the formation of excited states, energy transfer reactions and CL emission. The proposed model is also discussed in regard of a recently reported elimination of this CL in growing cultures of microorganisms and possible pathways for this interference are suggested.

Photoinduced Electron Transfer Reactions in a Porphyrin−Viologen Complex: Observation of S2 to S1 Relaxation and Electron Transfer from the S2 State

Photoinduced Energy Transfer Reactions in a Porphyrin-Viologen Complex: Observation of S2 to S1 Relaxation and Electron Transfer from the S2 state

Photoinduced Energy and Electron Transfer Reactions in Lamellar Polyanion/Polycation Thin Films: Toward an Inorganic “Leaf”

Sequential adsorption of polyanions and polycations was used to make a five-component energy/electron-transfer cascade, which mimics some of the functions of natural photosynthetic assemblies. The photon antenna part of the system consists of coumarin- and fluorescein-derivatized poly(allylamine hydrochloride) (Coum-PAH and Fl-PAH), palladium(II)tetrakis(4-N,N,N-trimethylanilinium) porphyrin (PdTAPP4+) or palladium(II)tetrakis(4-sulfonatophenyl) porphyrin (PdTSPP4-) layers, interleaved with anionic Zr(HPO4)2·H2O (α-ZrP) sheets. α-ZrP or HTiNbO5 sheets separate the porphyrin electron donor from a polyviologen electron acceptor layer. Layer-by-layer growth of these thin film assemblies was characterized by atomic force microscopy (AFM) and ellipsometry on planar supports, and by elemental analysis, surface area measurements, and transmission electron microscopy high on surface area silica supports. UV−vis absorption and steady-state emission spectroscopies showed that the overall energy/electron-transfer re...

Energy Transfer, Electron Transfer, and Addition Reactions of Triplet State of 1,3-Dihydroimidazole-2-thiones Investigated by Laser Flash Photolysis

Abstract The transient absorption bands observed at ca. 400 and 550 nm for 1,3-dihydro-2H-imidazole-2-thione (IT), 1,3-dihydro-1-methyl-2H-imidazole-2-thione (MIT), and 1,3-dihydro-2H-benzimidazole-2-thione (BIT) have been assigned to their triplet states (3IT* ’s) by the quenching and sensitizing experiments using laser flash photolysis technique. Short intrinsic triplet lifetimes and relatively high triplet energies have been evaluated as characteristics of 3IT* ’s. For electron acceptors, photoinduced electron-transfer reaction occurs via 3IT* ’s in the diffusion controlled limit. The nucleophilic character of 3IT* ’s was confirmed by changing the substituents of alkenes on the addition reactions. MO calculations indicate that the lowest electronic configurations of both 3IT* and 3MIT* have 3(n,π*) character, while 3BIT* has 3(π,π*) character. The addition reactivity of 3BIT* is slightly higher than those of 3IT* and 3MIT*, which is opposite to the general tendency.

ChemInform Abstract: Energy Transfer, Proton Transfer, and Electron Transfer Reactions Within Zeolites

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Solvation in Supercritical Fluids: Its Effects on Energy Transfer and Chemical Reactions.

Solvation in Supercritical Fluids: Its Effects on Energy Transfer and Chemical Reactions.

Study of the Suppression of Porphyrin Emission upon Addition of Rare Earth Ions

A topic of constant investigation in recent decades has been the study of electron and/or energy transfer reactions in supramolecular systems. The energy transfer between donor-acceptor depends on the influence of the substituent in the intersystem crossing. The competition between the radiative and non-radiative processes also depends on other factors such as temperature, solvent and energy gap. In this work, we have studied the suppression of the emission of monocarboxyphenyl porphyrins after addition of rare earth ions (RE(III)). The porphyrin emission decreases with the addition of RE(III), no matter which ion is present. When Eu(III) was used, the emission of this ion was not observed either. This suppression happens due to the effect of the heavy atom and not due to the energy transfer through electronic levels between the porphyrin and the RE(III). After the addition of RE(III), the lifetime for MCTPPH2 started to decay biexponentially, indicating the formation of a new species (MCTPPH2-RE(III)) of a shorter lifetime. The presence of NO2 groups in the ortho mesoaryl positions of MCTNPPH2 and the presence of Zn(II) in Zn(MCTPP) decreased both the porphyrin lifetime and emission due to an increase in the spin-orbit coupling.

Laser control strategies for energy transfer reactions in atom molecule collisions

We present schemes for the laser pulse control of bimolecular reactions. As a model system we treat the collision complex Na–H2, which is unbound in the electronic ground state X2A1 and weakly bound in the electronic excited state A2B2. This system decays non-adiabatically from the excited state via a conical intersection back to the ground state. The objective of the laser controls presented is the excitation of the collision pair with simultaneous focusing of the wavepacket towards the conical intersection. In the first step an optimal laser pulse is derived by optimal control theory over the expectation values for semi-positive definite operators. In the second step chirped laser pulses are fitted to the optimal one. For the chirped laser pulses the influences of the chirp is discussed. Special emphasis is placed on the excitation of vibrations of the H2 compound, which are considered to be important for the passage through the conical intersection.