Relaxation of internal energy and collisional energy transfer

Using the exponential model for the collisional transition probability, it is shown that relaxation of average internal energy is a measure of bulk-average energy transfer ?ΔE?. This is a macroscopic property which is a complicated function of both time and initial excitation and is only distantly related to average energy transferred per collision ?ΔE?, a microscopic property.     

Reversible energy transfer

Analysis of the transient and steady-state kinetics of reversible energy transfer shows that while the interpretation of lifetime measurements is difficult unless the donor and acceptor lifetimes are appreciably different, quantum yield measurements are relatively easy to interpret.

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Zusammenfassung Die Analyse der Kinetik der Übergangszustände und der stationären Zustände der reversiblen Energieübertragung zeigt, daß im Gegensatz zu einer schwierigen Interpretation der Messungen der Lebensdauer — es sei denn die Lebensdauer von Donor und Acceptor sind wesentlich voneinander verschieden — die Messungen der Quantumausbeute verhältnismäßig einfach zu interpretieren sind.

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Résumé L'analyse de la cinétique de l'état transitoire et de l'état stationnaire du transfert réversible d'énergie montre que, si l'interprétation des mesures de durée de vie est difficile, à moins queles durées de vie du donneur et de l'accepteur soient très différentes, il est par contre relativement facile d'interpréter les mesures de rendement quantique.

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Dedicated to the memory of Professor K. H. Hansen.     

Diffusion and long-range energy transfer

Based on a pair probability method for the statistics of resonance energy transfer the effect of additional diffusion is studied. Simple approximate formulae for luminescence quenching are derived and compared with results previously given in the literature.     

Intramolecular energy transfer in water

The evolution of the vibrational motion of a water molecule which has been excited locally is studied. An exact solution is obtained of a model Hamiltonian representing the non-harmonic behaviour of the molecule. Since this is obtained numerically it does not explain what is observed but it leads to a simplified model of the motion in which different aspects can be isolated and discussed. For many initial modes, especially those of higher energy, there is a large initial drop in the probability of finding the energy in the original local mode. This is due to dephasing. The factors involved in dephasing are discussed. A simple hypothesis is suggested which leads to a formula for this drop and this agrees substantially with the graphical evidence. The relation between the drop and the amplitude of the initial mode is also discussed.     

Long-range energy transfer in DNA

Experimental data on strand breakage in DNA following the direct deposition of ionizing energy seem to require an explanation in terms of long-range energy transfer within the DNA duplex. It is proposed here that the mechanism underlying such energy transfer might involve solitons or solitary waves. These act, at one and the same time, to provide a local environment within the DNA molecule capable of supporting delocalized electronic excitation or charge, and also enable the transportation of such regions along the molecule through the formation of a mobile “open state” in the duplex. Such a mechanism, if established, would have considerable implications for the mechanistic understanding of radiobiology.     

Chemiluminescence and transfer of energy in polymers

Russian Chemical Bulletin -     

Fluorescence and energy transfer in polyvinyl carbazole

A model is proposed to explain the kinetics of fluorescence and energy transfer in thin films of polyvinyl carbazole based on the migration of monomer excitons with activator molecules, dimers, and excimer forming sites competing as traps for the exciton energy.     

Phonon-mediated path-interference in electronic energy transfer

We present a formalism to quantify the contribution of path-interference in phonon-mediated electronic energy transfer. The transfer rate between two molecules is computed by considering the quantum mechanical amplitudes associated with pathways connecting the initial and final sites. This includes contributions from classical pathways, but also terms arising from interference of different pathways. We treat the vibrational modes coupled to the molecules as a non-Markovian harmonic oscillator bath, and investigate the correction to transfer rates due to the lowest-order interference contribution. We show that depending on the structure of the harmonic bath, the correction due to path-interference may have a dominant vibrational or electronic character, and can make a notable contribution to the transfer rate in the steady state.     

Diffusion effects in reversible energy transfer

Back energy transfer reduces the apparent quenching constant, which is an important parameter in the interpretation of energy transfer data. This determination of kinetic results may be erroneous when possible diffusion effects and non-uniform configurational distributions are not taken into account.

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Triplet-triplet energy transfer in Langmuir-Blodgett films

The temperature effect on the efficiency of the triplet energy transfer between different molecules included in molecular layers by the Langmuir-Blodgett (LB) procedure was studied. The efficiency of the triplet energy transfer from the LB film of the donor to the LB film of the acceptor is determined by the homogeneous broadening of the energy donor levels.     

Multiphonon energy transfer in atom-surface scattering

The probability density P(ω,q) of transferring the energy ω and the parallel momentum q to the surface is determined by the maximum entropy subject to constraints procedure. The two important constraints are identified as the recoil energy and the mean parallel energy transfer. Having determined P(ω,q) one can evaluate all observable quantities, e.g., the triple differential reflection probability d³σ/dE_tdΩ or the trapping probability. A three-parameter model for d³σ/dE_tdΩ is derived by assuming a parameterized form for the recoil energy. This model may be regarded as an extension of the hard-cube model, because it reduces to the latter if the third parameter, the speed of sound c, is set to infinity. The comparison of the predicted velocity and angular distributions with recent experiments of Hurst et al. is excellent considering the simplicity of the model.     

Vibration-vibration energy transfer in gaseous HBr

The vibration-vibration energy transfer of HBr gas initially excited to the first vibrational state has been observed. Collisional pumping to the V = 2 level is measured by monitoring the fluorescence of the 2−1 transition. The rate constant for the process: HBr(V=0) + HBr(V=2) → 2HBr(V=1) is found to be 1.4×10⁵ sec⁻¹ torr⁻¹.     

Fluorescence energy transfer methods in bioanalysis

Energy transfer phenomena, in which excited fluorophores transfer energy to neighbouring chromophores, are well characterised in photochemistry and have found a wide range of applications in analytical biochemistry. The transfer of energy from a donor to an acceptor group is only significant over distances of a few nm, so it can be used as a spectroscopic ruler and as a means of detecting molecular interactions and conformational changes. Such methods usually retain the great sensitivity and sample handling flexibility of conventional fluorescence techniques. As a result many assays involving enzymes, antibodies and nucleotides utilise energy transfer measurement principles. This article outlines these principles for the main types of energy transfer, and summarises some of their most important areas of application.     

Quantum effects in intramolecular energy transfer

Anharmonic potentials which are presumed to be classically quasi-ergodic, but which have symmetries leading to degenerate quantum states, fail in the quantum case to transfer energy equivalently among rigorously equivalent phase space locations. This is shown using simple group theory and is illustrated for the case of the Henon-Heiles potential     

Non-radiative energy transfer in N-isopropylacrylamide gel

Non-radiative energy transfer from donor to acceptor probes attached to the network chains of an N-isopropylacrylamide (NIPA) gel was investigated in water at various temperatures through fluorescence spectrum measurements. Naphthalene or biphenyl was used as a donor, and pyrene as an acceptor. Two types of doubly labeled NIPA gels (NIPA/2-vinylnaphthalene/pyrenylacrylamide and NIPA/4-vinylbiphenyl/pyrenylacrylamide) were prepared by copolymerization. The non-radiative transfer was observed for both gels. The ratio of emission intensity from donor to that from acceptor was strongly dependent on temperature, which indicates that the non-radiative energy transfer in doubly labeled NIPA gels could be controlled by means of the volume change with temperature.     

Vibrational energy transfer in methanol and deuterated methanols

The vibrational energy transfer probabilities in methanol and deuterated methanols are calculated by Schwartz, Slawsky and Herzfeld breathing-sphere theory in the temperature range 300–1000 K. The required breathing-sphere parameters are obtained through normal coordinate analysis. All the modes of a manifold, either low-lying or upper vibrational levels, are coupled through rapid V-V exchange processes, whereas the cross-coupling transition probabilities between levels in the different manifolds are smaller by several orders of magnitude.     

Ultrafast energy transfer and structural dynamics in DNA

Ultrafast structural dynamics concomitant to excitation energy transfer in DNA has been studied using a pair of pyrene-labeled DNA bases. The temporal evolution of the femtosecond pump-probe spectra reveals the existence of two electronic coupling pathways, through-base stack and through-space, which lead to excitation energy transfer and excimer formation even when the labeled DNA bases are separated by one AT base pair. The electronic coupling which mediates through-base stack energy transfer is so strong that a new absorption band arises in the excited-state absorption spectrum within 300 fs. From the analysis of time-dependent spectral shifts due to through-space excimer formation, the local structural dynamics and flexibility of DNA are characterized on the picosecond and nanosecond time scale.     

Photoinduced electron and energy transfer in phenylene oligomers

Phenylene oligomers represent a borderline case between very strongly π-conjugated molecular wires such as oligo-p-phenylene vinylenes and saturated molecular bridges. Even subtle chemical modifications of phenylene oligomers can therefore have a strong impact on charge transfer rates and mechanisms. On the basis of recently published selected case studies, this tutorial review discusses the key factors that affect charge transfer kinetics in phenylene oligomers with particular focus on the role of donor-bridge energy matching. Selected examples of triplet-triplet energy transfer reactions across phenylene oligomers are also discussed.     

Sequence, structure and energy transfer in DNA

Excitation energy transfer in DNA has similarities to charge transfer, but the transport is of an excited state, not of mass or charge. Use of the fluorescent, modified adenine base 2‐aminopurine (2AP) as an energy trap in short (3‐ to 20‐base) single‐ and double‐stranded DNA oligomers is reviewed. Variation of 2AP’s neighboring sequence shows (1) relatively efficient transfer from adenine compared to that from cytosine and thymine, (2) efficient transfer from guanine, but only when 2AP is at the 3′ end, (3) approximate equality of efficiencies for 3′ to 5′ and 5′ to 3′ directional transfer in adenine tracks. The overall, average transfer distance at room temperature is about four adenine bases or less before de‐excitation. The transfer fluorescence excitation spectral shape is similar to that of the absorption spectrum of the neighboring normal bases, confirming that initial excitation of the normal bases, followed by emission from 2AP (i.e. energy transfer), is occurring. Transfer apparently may take place both along one strand and cross‐strand, depending on the oligomer sequence. Efficiency increases when the temperature is decreased, rising above 50% (overall efficiency) in decamers of adenine below ?60°C (frozen media). Modeling of the efficiencies of transfer from the nearest several adenine neighbors of 2AP in these oligomers suggests that the nearest two neighbors transfer with near 100% efficiency. As bases in B DNA, as well as in single‐stranded DNA, are separated by less than 5 Å (less than the size of a base), standard Förster transfer theory should not apply. Indeed, while both theory and experiment show efficiency decreasing with donor–acceptor distance, the experimental dependence clearly disagrees with Förster 1/r⁶ dependence. It is not yet clear what the best theoretical approach is, but any calculation must deal accurately with the excited states of bases, including strong base–base interactions and structural fluctuations, and should reflect the increase of efficiency with temperature decrease and the relative insensitivity to strandedness (single, double). Attempts to use DNA as a molecular “fiber optic” face three primary challenges. First, reasonable efficiency over more than a base or two occurs only in adenine stretches at temperatures well below freezing. Second, transfer in these adenine tracks is efficient in both directions. Third, absorption of UV light occurs randomly, making excitation at a specific site on this “fiber optic” a challenge.