Electronic Energy Migration Research Articles

Dispersive transport of triplet excitation of benzaldehyde in solid ethanol solution

Spectral-kinetic studies are made of electronic excitation energy migration over triplet levels of benzaldehyde in a solid solution of ethanol at 77 K. The kinetics of benzaldehyde phosphorescence decay in the conditions of energy migration are nonexponential. The deviation from exponentiality increases with increasing concentration of both the donor and the acceptor. The kinetics of phosphorescence decay are shown to depend on the registration wavelength. The experimental results are analyzed from the point of view of a model with time-dependent rate constants.

Electronic-energy migration and molecular rotation within bichromophoric macromolecules. Part 1.—Test of a model using bis(9-anthrylmethylphoshonate) bisteroid

We report a model for determining the rate of energy migration, within pairs of donor (D) molecules from the fluorescence anisotropy. The D molecules within each pair reside in an anisotropic environment, and undergo rotational motions, similar to what could be the case in a protein molecule. To test the model experimentally, we have synthesized mono- and bis-(9-anthryl-methylphosphonate) bisteroid molecules. A procedure is presented for extracting the rate of energy transfer, as well as the D–D distance from the fluorescence anisotropy. The rate of energy migration obtained from experiments, ω≈ 3.5 × 108 s–1, agrees very well with that predicted. The distance 23.7 ± 2 A between the anthracenes and the mutual angle of 131 ± 3° between their orientational distributions, obtained at different temperatures, are in excellent agreement with independently determined values.

Stochastic theory of molecular radiative transport

The radiative transport (or radiative migration) of electronic excitation energy in a molecular ensemble is analyzed from a stochastic point of view. This approach yields results considerably more general than those of previous treatments, where successive hops are (implicitly) assumed to be uncorrelated, while they constitute in fact a Markov process. The time evolution of the fluorescence intensity emitted by the molecular ensemble, in response to excitation by an external beam of linearly polarized and essentially monochromatic light is obtained. In contrast to the previous treatments, only known parameters are required. The time evolution of the fluorescence anisotropy is also obtained for the first time.

Fluorescence of Ce 3+ in LiREF 4 (RE  Gd, Yb)

Optical absorption, emission and excitation spectra of Ce 3+-doped LiYbF 4 and LiGdF 4 crystals are presented. Vibrational structures were observed and the vibrational modes were described through Raman spectra. Ce 3+ and Gd 3+ energy level schemes were determined and the influences of various processes on the Ce 3+ luminescence intensity are discussed qualitatively. The luminescence efficiency of Ce 3+ is strongly decreased by either electron transfer (LiYbF 4) or energy migration over the Gd 3+ sublattice to impurities (LiGdF 4).

Migration of electronic excitation energy in the charge-transfer crystal of pyromellitic dianhydride-phenanthrene

Singlet and triplet exciton energy migration in the charge-transfer crystal of pyromellitic dianhydride-phenanthrene, undoped and doped with anthracene (10 −4−10 −2M/M), have been studied in the 5–300 K temperature range. At low temperatures singlet and triplet excitons are self-trapped. They become mobile with increasing temperature with activation energies of 380 ± 70 and 400 ± 80 cm −1 for singlet and triplet excitons, respectively. The motion of triplet excitons in the temperature range 50–100 K can be described by one-dimensional hopping within cages, limited by structural defects of the host crystal. Mobile singlet excitons are observed for temperatures above 100 K and their motion is well described by a time-independent excitation transfer rate constant.

Selective steady-state and time-resolved fluorescence spectroscopy of an HLA-A2-peptide complex

The human class I major histocompatibility complex (MHC) encoded molecule HLA-A2 loaded with the high-affinity peptide GILGRVFTL (p790), was studied by means of steady-state and picosecond fluorescence intensity and fluorescence anisotropy methods. The large number of tryptophan residues (W) (10 W/heavy chain, 2 W β 2 m ) as well as their fluorescence sensitivity to the microenvironment, determine the emission of the studied complex. The HLA-A2/peptide complex exhibits a considerable static inhomogeneous broadening (IB) of the W electronic spectra, which results in a dependence of the steady-state fluorescence spectrum on the excitation wavelength. The high concentration of W's chromophores and the spectral IB cause a directed non-radiative migration of electronic excitation energy by Foerster's mechanism from ‘blue’ W residues to ‘red’ ones. This phenomenon manifests itself in a nanosecond fluorescence spectral shift and an accelerated fluorescence depolarization at the red edge of the emission spectrum. Selective excitation at the red edge of the W absorption band (310 nm) provided a space selective reduction in the number of excited chromophores and enabled resolution of the emission of the ‘red’ subset of the protein's tryptophans. This avoided the non-radiative homo-energy transfer and enabled to study the fluorescence anisotropy decay kinetics of these residues without a distortion by the energy transfer (ET) process. Under these experimental conditions the fluorescence anisotropy decays practically from the limiting anisotropy value (0.3) for W in a bi-exponential process. The longer decay constant has a value larger than that expected for a global rotation of the HLA-A2/ peptide complex suggesting that the protein molecules exist in an oligomeric form. No clear assignment for the fast component can currently be made; it may be either a manifestation of a limited internal rotation of the W residues or a result of uncompleted compensation of the homo-ET process.

Electronic energy migration and trapping in quinone-substituted, phenyl-linked dimeric and trimeric porphyrins

The synthesis and photophysical characterization of a series of quinone-substituted, phenyl-linked dimeric and trimeric porphyrin arrays suitable for the study of electron transfer within closely-coupled subunits is presented. The excited singlet state of a porphyrin possessing a quinone substituent at a meso position is shortened to <350 fs due to electron donation to the appended quinone. Energy transfer between adjacent porphyrins occurs with a rate constant of ca. 10[sup 11] s[sup [minus]1], and the photon is immediately trapped by the proximal porphyrin. For the liner porphyrin trimer, the photon can transfer between distal and central porphyrins in an incoherent manner until it is trapped by the proximal porphyrin. These arrays represent interesting models for natural photosynthetic organisms since they combine light harvesting, energy migration and trapping, and photoinduced electron transfer in a single supramolecular entity. 46 refs., 6 figs., 1 tab.

Percolation of triplet excitation of compound molecules in polymeric matrices

The electronic excitation energy migration over the triplet levels of complex molecules in polymeric matrices has been investigated by the method of capture by traps and the annihilation delayed fluorescence method in the 77–320 K temperature range. From experiments with steady-state excitation for benzophenone in PMMA values of the critical exponents β=0.18 and γ=1.8 have been obtained. It is shown that the deviation of the value of the critical exponent β from the theoretical value for the three-dimensional space is due to the influence of the microscopic organization of the amorphous polymer. For comparison, we have investigated the migration of triplet excitations of benzophenone in ethanol. The thus obtained critical exponents β=0.41 and γ=1.8 are in good agreement with theory for the case of three-dimensional space. The application of the notion of connectivity to the description of the energy migration process in the systems under investigation is considered. The dependence of the connectivity radius on the system's properties and experimental conditions for the case of the exchange interaction between molecules is shown.

Electronic energy transfer in anisotropic systems. III. Monte Carlo simulations of energy migration in membranes

Electronic energy migration in membrane systems has been studied by means of Monte Carlo (MC) simulations according to an algorithm described previously [S. Engström, M. Lindberg, and L. B.-Å. Johansson, Chem. Phys. 89, 204 (1988)]. In the systems investigated, the interacting donor molecules are randomly localized in mono-, bi-, and multilayers and are either oriented with their transition dipoles isotropically or parallel to the layers. The mean-square displacement [〈R(t)2〉] of the excitation and experimental observables in terms of different fluorescence depolarizations were determined. All results are relevant for the ‘‘slow case,’’ which means that translational and rotational motions of the donors are much slower as compared to the rate of fluorescence. A two-particle approximation for calculating the excitation probability of the initially excited molecule [denoted by Gs(t) ] and the fluorescence depolarizations in two-dimensional systems has been published previously [J. Baumann and M. D. Fayer, J. Chem. Phys. 85, 4087 (1986)]. By using the MC algorithm, we have in this work tested this model extensively. The excitation probability Gs(t) is found to be in excellent agreement with the MC simulations for all of the systems studied. For isotropically oriented donor molecules in multilamellar systems, the simulations show that Gs(t) is very well approximated by that of a monolayer at distances of d≥3 R0 between the layers. At distances of d≤0.5 R0, the function Gs(t) is equal to that of a three-dimensional solution. For the in-plane oriented dipoles in a multilayer system, Gs(t) is very well approximated by that for a single bilayer at d&amp;gt;2 R0. In general, the depolarizations obtained by the two-particle model and MC simulations differ depending on the particular orientational distribution and the experimental geometry. To obtain a physically correct behavior of the fluorescence anisotropy at long times (i.e., the limiting anisotropy) is not possible within a two-particle approach. We instead propose a phenomenological model for calculating the fluorescence anisotropy which is a function of Gs(t) obtained within the two-particle approximation. This model gives a remarkably good agreement with the MC simulations and has the correct limiting anisotropy. The MC simulations of energy migration show that 〈R(t)2〉 is about twice as large for in-plane oriented donors in mono- and bilayers, as compared to the case of isotropically oriented transition dipoles. For multilayer systems consisting of layers separated by d=R0, we find that 〈R(t)2〉 is only slightly larger for the in-plane oriented transition dipoles as compared to the isotropic orientational distribution.

Rapid migration and trapping of photoexcited electronic energy in amphiphilic polyelectrolytes containing naphthalene chromophores and pyrene traps

Electronic energy migration and transfer from naphthalene (Np) chromophores to pyrene (Py) traps covalently incorporated in terpolymers of sodium 2-acrylamido-2-methylpropanesulfonate, N-(1-naphthylmethyl)methacrylamide, and N-(1-pyrenylmethyl)methacrylamide were investigated. Very efficient energy transfer from Np to Py occurred at room temperature in aqueous solution where the terpolymers form a microphase-separated structure consisting of chromophore clusters

Migration of electronic excitation energy in heterogeneous media under conditions of inhomogeneous broadening of energy levels

The introduction of complex molecules in high concentrations into polymer matrices results in the formation of heterogeneous media with high microscopic nonuniformity. It was shown in [i, 2] that energy migration in such media can be described from the standpoint of percolation theory, which presumes that supermolecular cluster structures form in the system. The experimental results of these works, in particular, show that in many cases as the temperature decreases from room temperature to 77 K the efficiency of migration in the samples studied drops strongly. This effect was explained by the fact that the dispersion of the energy levels of complex molecules, which is associated with fluctuation of the interaction of the molecules with their environment, affects migration. If the molecules forming the system along which energy migration is possible have a different environment and therefore the electronic energy levels are different, there always exists a probability that the migration path will contain an energy well, which at the given temperature the excitation cannot overcome. The probability that an excitation falls into such a well increases as the average distance between traps increases. As the concentration of traps increases this distance increases and the effect of energy factors thereby also decreases. Thus in order for energy migration along a cluster to be efficient at a given temperature the relation

Photophysics of poly(2,3,4,5,6-pentafluorostyrene) film

The temperature-dependent steady-state emission, emission polarization anisotropy, and fluorescence-kinetics of poly(2,3,4,5,6-pentafluoro styrene) film are reported. Two interconverting excited-state conformations of the chromophore have been identified. The fluorescence of the higher energy conformation results from excitation on the red edge of the polymer absorption band at temperatures below 180 K. The energy barrier for conversion of the high energy conformer to the lower energy conformer is estimated to be E/hc = 27 + or - 7/cm. Electronic energy migration is not evident in this polymer.

Generation of nonrandom chromophore distributions by the photo-Fries reaction of 2-naphthyl acetate in poly(methyl methacrylate)

Singlet electronic energy migration and transfer within the nonrandom chromophore distributions are studied by steady-state fluorescence spectroscopy and fluorescence decay measurements. The donor fluorescence decays obtained for nonrandom distributions are clearly different from those predicted by models based on random chromophore distributions. Non-random distributions are converted to random distributions by solvent treatment or annealing for 4 days or more

Irradiation of small molecules isolated in rare gas matrices by high energy electron beams

The matrix isolation technique is used to study radiation chemistry; specifically, rare gas matrices are irradiated with high energy electron beams to (1) spectroscopically probe a high energy electron beam induced reaction, and (2) determine the role of a chemically inert solvent on the electron beam chemistry. In the experiments presented herein, it is shown that the electron beam sensitivity of a component in a mixture may be altered by changing its concentration. A simple two component mixture consisting of small molecular systems like carbon dioxide, and nitrogen dioxide were isolated in chemically inert solvents like solid argon, krypton and xenon, respectively. The net effect of the electron beam exposure, for example, is the conversion of CO 2 to CO. When the concentration of CO 2 in a rare gas matrix was decreased the efficiency for CO 2 decomposition increased. The mechanism for the enhanced decay for CO 2 is related to migration and transfer of electronic energy from the rare gas matrix to CO 2. A kinetic analysis, formulated for a quantitative evaluation of the process, was used to determined that the rate constant for energy transfer from a Wannier exciton to CO 2 is 140 times greater than for transfer back to an argon matrix. The identity of the Wannier exciton as the species transferring energy to the matrix is in part based on energetic considerations.

Picosecond absorption studies of intermolecular electronic energy transfer in micellar systems. II. Polarization-dependent studies of energy migration and energy trapping at low donor excitation densities

A picosecond excite-and-probe beam investigation on intermolecular electronic energy transfer (EET) between dye molecules in one-component systems (electronic energy migration - EM) and two-component systems (electronic energy trapping - ET) is presented and compared with existing theories for EET in restricted volumes. The experiments were carried out on a transient absorption spectrometer based on synchronously pumped dye laser (pulse duration 5 ps). Donor excitation density was less than 1%. In the case of ET good agreement has been obtained with the Förster theory, in which in addition an inhomogeneous spatial distribution of the dye molecules has been considered (part I of this study). In the case of EM direct evidence for the theory of Ediger and Fayer has been obtained by measuring the polarization anisotropy at early times after excitation.

The Pulse-Photoconductivity of Poly(1-vinylpyrene)

Abstract The migration of electronic excitation energy, the steady-state photoconductivity, the hole-drift mobility and the carrier-photogeneration efficiency of holes in poly(1-vinylpyrene)(PVPy) films were investigated and compared with those in poly(N-vinylcarbazole)(PVCz) films. Although the concentration of the excimer-forming sites in PVPy films was about twenty times as large as that in PVCz films, the magnitude of the steady-state photoconductivity of PVPy was comparable to that of PVCz. The hole mobility in PVPy films was about one-half as large as that in PVCz films. The electric field and temperature dependence of the mobility fit Gill’s empirical equation. The carrier-photogeneration efficiency, Φ, of PVPy was larger than that of PVCz. The filed dependence of Φ of PVPy for all the excitation bands was in good agreement with the Onsager theory of geminate recombination. The high photoconductivity of PVPy could be explained on the basis of these results.

Electronic excitation energy migration in chemical dimers of cyclopentaneporphyrin molecule

Electronic excitation energy migration in chemical dimers of cyclopentaneporphyrin molecule

Role of electronic excitation energy migration in tunable lasers utilizing multicomponent dye solutions

The results of studies of electronic excitation energy transfer in dye solutions used as laser active media are summarized. Experimental results are presented, confirming that excitation energy may be transferred during vibrational relaxation. The influence of inhomogeneous broadening of dye spectra on transfer of electronic excitation and the lasing characteristics of dye solutions is discussed. Fluorescence quenching processes that take place as a result of energy migration between dye molecules in solutions and the role of triplet states in these processes are discussed. The results of studies of the influence of energy transfer, association of dye molecules, and their triplet states on the lasing properties of solutions are presented.

Electronic energy transfer and quenching in copolymers of styrene and 2-(2'-hydroxy-5'-vinylphenyl)-2H-benzotriazole: photochemical processes in polymeric systems. 10

Intensity and lifetime quenching of the polystyrene monomer and excimer fluorescence emission by a pendant quencher have been studied in the solid state and in solution for a series of copolymers of styrene and 2-(2-prime-hydroxy-5-prime-vinylphenyl)-2H-benzotriazole. Two mechanisms of quenching have been identified. One involves interception of the migrating monomer excitation by the quencher, and the other involves a single-step Foerster energy-transfer process from the excimer traps to a quencher. The relative efficiencies of these two quenching mechanisms in the solid state and in solution have been compared and related to the efficiency of electronic energy migration. An estimate of the excimer trap concentration in solid polystyrene is also reported.

An experimental method of determining the electronic excitation energy migration coefficient in organic liquids

Abstract A method is described for determining the singlet electronic excitation energy migration coefficient in an organic liquid by using the linear behaviour of the rate parameter measured as a function of temperature for a diffusion-controlled bimolecular reaction involving the excited state of the liquid. A value of (5.5±0.5)×10 −5 cm 2 s −1 is reported for the energy migration coefficient of liquid toluene.