Rate Of Electronic Energy Transfer Research Articles

Supersonic Jet Spectroscopy and Intramolecular Electronic Energy Transfer in Naphthalene−(CH2)n−Anthracene Bichromophoric Molecules

The spectroscopy and intramolecular electronic energy transfer (EET) process for a series of molecules, 9-anthryl−(CH2)n−1-naphthyl, n = 1, 3, and 6 (AnN), was investigated in a supersonic jet. For all bichromophoric molecules, the excitation spectra of the anthracene chromophore was slightly red-shifted with respect to bare anthracene and showed low-frequency vibrational progressions associated with the relative motion of the two chromophores. For A1N, when the naphthalene moiety is excited, the emission contains only anthracene-like fluorescence. From the homogeneous broadening of the absorption lines, the EET rate is estimated to be 1.5 ps. For n = 3 (A3N) and n = 6 (A6N), only the fluorescence excitation spectra of the anthracene moiety is observed. This suggests that the EET rate in A3N and A6N is much faster than that in the A1N system. Molecular geometry computations using molecular mechanics methods were applied to the interpretations of experimental results. In the case of A3N and A6N, a face-to-face conformer can be formed in which exchange coupling can be effective in enhancing the EET rate. In A1N, the bridge is sufficiently rigid that a face-to-face conformer cannot be formed.

Time-resolved analysis of intramolecular electronic energy transfer in methylenes-linked naphthalene–anthracene compounds

The mechanism of intramolecular electronic energy transfer (EET) in methylene-linked naphthalene and anthracene (referred to as A nN, n=1, 3 and 6) was examined by time-correlated single-photon counting technique. The chain length dependence of measured EET rate constants is explained in terms of through-bond interaction via the connecting bridge, besides direct Coulombic interaction. In addition, intramolecular energy transfer in A1N and A3N may compete with vibrational relaxation of the corresponding vibrational level of the excited anthracene moiety by comparing the time-resolved spectra of A1N, A3N and A6N.

Electronic Energy Transfer in Multichromophoric Arrays. The Effects of Disorder on Superexchange Coupling and Energy Transfer Rate

The effects of diagonal (site energy) and off-diagonal (intermolecular interaction) disorder arising from the distribution of ionization energies on superexchange coupling and the corresponding electronic energy transfer (EET) rate are considered. The effective donor−acceptor coupling is obtained using the Dyson's equations-based solution of the Green's function for both the orthogonal and nonorthogonal basis sets. In a disordered multichromophoric array, the effective superexchange coupling is shown to be enhanced. Furthermore, the competing roles of the multiple pathways when next-to-nearest-neighbor interactions exist behave differently depending on the type and extent of disorder. It is demonstrated that the exponential falloff of the energy transfer rate with increasing donor−acceptor distance weakens when disorder is present. Moreover, this exponential decay is more apparent when the donor−bridge energy gap is reduced. A method to treat EET at the molecular orbital level using Dyson's equations is a...

Photoinduced energy and electron transfer in bis-porphyrins with quinoxaline Tröger's base and biquinoxalinyl spacers

The photophysical characterisation of bis-porphyrins consisting of two porphyrins bridged by either a quinoxaline Troger's base (1 and 2) or a biquinoxalinyl (3) spacer are reported. Efficient intramolecular electronic energy transfer (EET) between the rigidly linked free-base porphyrins in 1 and from the zinc(II) porphyrin to the free-base porphyrin in 2 has been investigated by steady-state absorption and emission spectroscopy, time-resolved fluorescence spectroscopy and semi-empirical calculations. A resonance dipole–dipole mechanism alone cannot account for the rate of EET in both 1 and 2. It is demonstrated that a superexchange mechanism ia the quinoxaline Troger's base linker is responsible for the enhanced energy transfer rates in these systems. Strong quenching of the fluorescence intensity observed in 3 is interpreted as arising from long-range (>18 A) through-biquinoxalinyl bridge mediated photoinduced electron transfer from the free-base porphyrin to the gold(III) porphyrin. These systems provide useful models for the arrangements of the primary donor–acceptor pair in photosynthetic reaction centres, and for elucidating the role of the connecting bridge in electron and energy transfer processes.

Investigations of Electronic Energy Transfer Dynamics in Multiporphyrin Arrays

A study of the dynamics of electronic energy transfer (EET) in arrays containing three, four, and six tetraphenylporphine units connected with phenylethynyl spacers is reported. For arrays containing the same chromophores, the EET rate constant was determined from the reorientational dynamics of the transition dipole using the crossed grating technique. EET time constants ranging from 150 ps up to 33 ns were measured, depending on the distance between the chromophores and on the metal ion complexed in the porphyrins. For the trimeric planar arrays, the interchromophoric distance varies by a factor of 2, while the ratio of the through space to through bond distances is constant. By comparing the measured EET rate constants with those calculated using Förster theory, the contributions of the Coulombic, through space, mechanism and of the exchange, through bond, mechanism could be estimated. For the arrays with the shortest spacer (through space distance of 23 Å), EET occurs through both exchange and Coulombic interactions with a ratio of about 3:1. This ratio increases up to about 10 as the distance is increased to 34.5 Å. At 46.5 Å, the ratio decreases and it appears that the Coulombic interaction becomes the dominant mechanism at longer distances. In the tetrahedral compound, the presence of a central saturated carbon strongly alters the electronic conducting properties of the spacer and makes the exchange mechanism inoperative.

New electron energy transfer rates for vibrational excitation of N<sub>2</sub>

Abstract. In this paper we present the results of a study of the electron cooling rate, the production rates of vibrationally excited N2(v), and the production frequency of the N2 vibrational quanta arising from the collisions of electrons with unexcited N2(0) and vibrationally excited N2(1) molecules as functions of the electron temperature. The electron energy transfer rates for vibrational excitation of N2 have been calculated and fit to analytical expressions by use of the revised vibrationally excited N2 cross sections. These new analytical expressions are available to the researcher for quick reference and accurate computer modeling with a minimum of calculations.Key words. Atmospheric composition and structure · Thermosphere · Ionosphere · Modeling and forecasting

The interaction of solutes with their environments

The origins of electronic spectral broadening in solutions, glasses and proteins are discussed via three pulse stimulated echo peak–shift measurements. Spectral densities for dye molecules in polar solutions are presented and discussed in terms of molecular models for the dynamics. The breakdown of the harmonic bath picture at low frequencies is presented. Finally, echo measurements on light harvesting complexes of purple bacteria are presented and it is shown that the energy transfer timescale can be obtained from peak–shift measurements.

New electron energy transfer rates for vibrational excitation of N

New electron energy transfer rates for vibrational excitation of N

Damping and higher multipole effects in the quantum electrodynamical model for electronic energy transfer in the condensed phase

The interplay between electronic coupling, spectral linewidth, and rate of electronic energy transfer between chromophores is examined in the context of a quantum electrodynamical (QED) model. The QED framework properly allows us to identify the partitioning between the near and far zone mechanisms for transfer of energy between chromophores dispersed in condensed phase (liquid or solid) host media. The extent to which coupling is modified by the medium is investigated. A general QED treatment of higher multipole contributions to the coupling between transition moments is also derived, whence interactions involving electric dipole, quadrupole and octopole as well as magnetic dipole and quadrupole interactions are examined explicitly. A new formulation is presented wherein expressions for the multipolar coupling tensors are obtained in terms of spherical Bessel functions, providing a clear, compact representation of the retarded coupling interaction and its distance-dependence. The irreducible tensor formulation of the coupling is discussed, highlighting features concerning the exact form of the orientation factors that have often in the past escaped notice. The detailed method of implementing a rotational averaging of the resultant interaction tensors is demonstrated, finally leading to a novel and concise representation for multipolar couplings of arbitrary order. The coupling between bacteriochlorophyll a chromophores is discussed as an example.

Calculated and observed [formula omitted] electronic energy transfer rates in Na 3[Tb 0.01Eu 0.99(oxydiacetate) 3]-2NaClO 4-6H 2O

A complementary experimental and theoretical study of Tb 3+( 5 D 4) → Eu 3 + electronic energy transfer in trigonal Na 3[Tb 0.01Eu 0.99(oxydiacetate) 3]-2NaClO 4-6H 2O (1%Tb:EuODA) is presented. Tb 3+( 5D 4) relaxation rates in 1 % z Tb : GdODA and in 1% Tb : EuODA, and Tb 3+( 5 D 4) → Eu 3 + energy transfer rates in 1% Tb : EuODA are re for sample temperatures between 77 and 343 K. The calculated multipole-multipole transfer rate at room temperature (33 s − 1) is of the same order of magnitude as the measured rate, though smaller by a factor of three. The temperature dependence of the measured energy transfer rate suggests that the transfer is to Eu 3 + ions in thermally populated 7F 1 and 7F 2 states. No transfer is observed to Eu 3+ in the ground, 7F 0 electronic state. The calculation suggests that at room temperature the dominant path is Tb 3+( 5 D 4) + Eu 3+( 7 F 1) → Tb 3+( 7 F 5) + Eu 3+( 5 D 1) with an electric dipole-quadru-pole coupling mechanism.

Mediated Energy Transfer in Covalently Linked Porphyrin Dimers

Donor−acceptor systems where the rate of radiationless electronic energy transfer is influenced by the electronic structure of the intervening medium are studied. In this paper we report the observation of mediated energy transfer in a series of geometrically well-defined porphyrin dimers. The dimers consist of a free base porphyrin (5,15-diphenyl-α,β-octaalkylporphyrin, H2P) and the corresponding zinc porphyrin (ZnP) as acceptor and donor, respectively, connected by aryl chromophores with varying energies of the lowest excited states.

Energy transfer between lanthanide ions in elpasolite lattices

Illustrations are given of the importance of resonant magnetic dipole-magnetic dipole, and resonant electric quadrupole-electric quadrupole interactions in the three-dimensional energy migration and quenching of luminescence of lanthanide ions, Ln 3+, in Cs 2NaLnCl 6. Calculations of resonant electronic energy transfer rates are in agreement with experimental results, but the calculated nonresonant transfer rates are very slow.

Orbital alignment cross sections by stimulated emission probing: The state-to-state Ca Rydberg process Ca(4s17d 1D2)+Xe→Ca(4s18p 1P1)+Xe

The initial state alignment effect vs relative velocity is measured for a state-to-state Ca Rydberg collisional energy transfer process. The stimulated emission detection method is used to determine the alignment effect for the n,l-changing transition: Ca(4s17d 1D2)+Xe→Ca(4s18p 1P1) +Xe+ΔE=−1.7 cm−1. The rate of electronic energy transfer in this state-changing collision is observed to vary with the direction of the Rydberg electron charge cloud relative to the collision axis. Both the expected cos(4β) and cos(2β) dependencies are observed. The alignment data are analyzed to obtain the relative cross sections for the individual Ca(1D2) magnetic sublevels. The values of the m-sublevel cross sections σ0:σ‖1‖:σ‖2‖ are 1.13±0.02:1.11±0.02:0.83±0.02. Qualitative interpretations of the relative cross sections in terms of both molecular (van der Waals) Born–Oppenheimer potentials and the impulse approximation are presented.

Distance dependence of electronic energy transfer between donor and acceptor adlayers: p-terphenyl and 9,10-diphenylanthracene

Investigations of energy transfer between adlayers on single-crystal surfaces provide a unique opportunity to explore electronic energy transfer in restricted geometries. In this study, laser induced fluorescence techniques and donor quantum yield measurements were used to examine the distance dependence of electronic energy transfer between donor and acceptor adlayers on Al2O3(0001). The donor adlayer was p-terphenyl, the acceptor adlayer was 9,10-diphenylanthracene, and n-butane was the variable spacer adlayer. The electronic energy transfer rates vs spacer thickness were determined at both 30 and 85 K in ultra high vacuum. The butane spacer experiments showed that the donor energy transfer rate decreased with a 1/d3 dependence, where d is the thickness of the spacer adlayer. Given a Förster quantum mechanical or a Kuhn classical energy transfer mechanism with randomly oriented dipoles, a 1/d3 distance dependence is consistent with resonance electronic energy transfer from a two-dimensional donor adlayer to a three-dimensional array of acceptors. The spacer measurements yielded a critical transfer distance of d0=44 ±4 Å at 30 K and d0=33 ±6 Å at 85 K. The differences in the critical transfer distance at 30 and 85 K could be explained by the redshift in the p-terphenyl fluorescence spectrum at 85 K that reduces the overlap between the donor fluorescence and acceptor absorption spectra. Values of d0=44 Å at 30 K and d0=35 Å at 85 K were calculated theoretically from a 1/d3 analysis and were in excellent agreement with the experimental measurements. The rate of donor–donor intralayer energy migration was also determined by measuring the electronic energy transfer rate versus donor coverage on the acceptor adlayer. The donor quantum yield measurements versus donor adlayer coverage were consistent with the spacer results and indicated that electronic energy migration does not occur within the p-terphenyl adlayer. These results vs spacer thickness and donor coverage reveal that electronic energy transfer in spatially confined geometries can be described using a modified Kuhn energy transfer mechanism.

Intramolecular electronic energy transfer between rigidly linked naphthalene and anthracene chromophores

The rate of intramolecular singlet-singlet electronic energy transfer (EET) between naphthalene and anthracene chromophores that are held in a well-defined orientation and separation by a rigid bis(norbornyl)bicyclo-[2.2.0]hexane bridge, six σ-bonds in length, is measured to be 2.9 × 1010 s-1 (n-hexane). EET between the lowest-energy naphthalene 1Lb and anthracene 1La states (via a direct Coulombic mechanism) is forbidden in this molecule because of the orthogonality of donor and acceptor transition dipoles. EET involving a Coulombic interaction between the naphthalene 1Lb and the nonfluorescent anthracene 1Lb electronic states is also considered. Although Coulombic EET is vibronically allowed, to account for the high measured rate, it is proposed that relayed mechanisms, involving the bridge, play a dominant role. The significance of these results to EET in aromatic polymers and other multichromophoric systems is discussed. © 1993 American Chemical Society.

Photophysics of metalloazurins

The fluorescence lifetimes of Cu(II), Cu(I), Ag(I), Hg(II), Co(II), and Ni(II) azurin Pae from Pseudomonas aeruginosa and Cu(II), Cu(I), and Hg(II) azurin Afe from Alcaligenes faecalis were measured at 295 K by time-correlated single-photon counting. In addition, fluorescence lifetimes of Cu(II) azurin Pae were measured between 30 and 160 K and showed little change in value. Ultraviolet absorption difference spectra between metalloazurin Pae and apoazurin Pae were measured, as were the fluorescence spectra of metalloazurins. These spectra were used to determine the spectral overlap integral required for dipole-dipole resonance calculations. All metalloazurins exhibit a reduced fluorescence lifetime compared to their respective apoazurins. Forster electronic energy transfer rates were calculated for both metalloazurin Pae and metalloazurin Afe derivatives; both enzymes contain a single tryptophyl residue which is located in a different position in the two azurins. These azurins have markedly different fluorescence spectra, and electronic energy transfers occur from these two tryptophyl sites with different distances and orientations and spectral overlap integral values. Intramolecular distances and orientations were derived from an X-ray crystallographic structure and a molecular dynamic simulation of the homologous azurin Ade from Alcaligenes denitrificans, which contains both tryptophyl sites. Assignments were made of metal-ligand-field electronic transitions and of transition dipole moments and directions for tryptophyl residues, which accounted for the observed fluorescence quenching of Hg(II), Co(II), and Ni(II) azurin Pae and Cu(II) and Hg(II) azurin Afe. The fluorescence of azurin Pae is assigned as a 1Lb electronic transition, while that of azurin Afe is 1La. The marked fluorescence quenching of Cu(II) azurin Pae and Cu(I) azurin Pae and Afe is less well reproduced by our calculations, and long-range oxidative and reductive electron transfer, respectively, are proposed as additional quenching mechanisms. This study illustrates the application of Forster electronic energy transfer calculations to intramolecular transfers in structurally well characterized molecular systems and demonstrates its ability to predict observed fluorescence quenching rates when the necessary extensive structural, electronic transition assignment, and spectroscopic data are available. The agreement between Forster calculations and quenching rates derived from fluorescence lifetime measurements suggests there are limited changes in conformation between crystal structure and solution structures, with the exception of the tryptophyl residue of azurin Afe, where a conformation derived from a molecular simulation in water was necessary rather than that found in the crystal structure.

Direct measurements of energy transfer between identical chromophores in solution

The rate of intramolecular electronic excitation energy transfer in 9,9′ bifluorene has been investigated in a variety of solvents, using both time-correlated single photon counting and femtosecond fluorescence upconversion technique. The kinetics of energy transfer were determined in both cases by time dependent fluorescence anisotropy measurements. The energy transfer dynamics between fluorene moieties has been found to occur on a time scale of approximately 600 fs in different solvents and has been correlated with the T2 value calculated from the absorption linewidth and the β value obtained from jet measurements. The dihedral angle between the fluorene moieties was also calculated from the anisotropy measurement and compared with the values obtained from a solution phase NMR determination.

Temperature dependence of the antistokes luminescence of holmium in yttrium fluoride

This paper analyzes the effect of temperature on the intensity of the green and red luminescence bands of Ho/sup 3 +/ ions in yttrium fluoride excited in the absorption band of ytterbium. The relative intensity of the two luminescence components was measured in the temperature range of 77-400/sup 0/K by integrating the emission spectra in the 0.54- and 0.65-..mu..m regions. The population of the /sup 5/S/sub 2/ and /sup 5/F/sub 3/ levels of Ho/sup 3 +/ were calculated on a computer from the temperature-dependent variation of the rates of many-phonon nonradiative relaxation and electronic excitation energy transfer.

Rate of intramolecular electronic energy transfer in coumarin bichromophoric molecules. An investigation by multifrequency phase-modulation fluorometry

Dynamics of intramolecular electronic energy transfer in bichromophoric molecules, consisting of two coumarins linked by a variable number of methylene groups, is investigated by multifrequency phase-modulation fluorometry. From observation of the acceptor delayed emission, information on the rate of transfer can be obtained.

Electric field effects on quasi-coherent energy transfer

The effect of an external electric field on the rate of electronic excitation energy transfer in a model molecular crystal system is shown to depend on the nature of the site-to-site transfer and on whether the directional coherence is limited by phonon scattering or by defect scattering processes.