Migration Of Electronic Excitation Research Articles

Photoluminescence Study of Undoped and Eu-Doped Alkali-Niobate Aluminosilicate Glasses and Glass-Ceramics.

In this study, the photoluminescence (PL) behavior of two aluminosilicate glass series containing alkali-niobates ranging from 0.4 to 20 mol% was investigated. The glasses exhibit an intense visible emission centered at ~18,400 cm-1 for the peralkaline series and at higher energies (~19,300 cm-1) for the metaluminous glasses. However, the photoluminescence emission intensity varies significantly with the niobate content and the bulk chemistry. PL and fluorescence lifetime measurements indicate that the broad emission bands result from the overlap of different niobate populations, whose distribution changes with niobate content. The distinct PL behavior in the two glass series was related to the structural evolution of the niobate units upon niobium addition. An enhancement of the visible emission was observed for a higher fraction of distorted [NbO6] units. Eu-doping was carried out as a structural probe of the glass network, and also to determine if these glasses could be used as potential rare earth element (REE) activators. The crystal field strength around Eu ions is strongly dependent on the bulk chemistry and the niobate content. Furthermore, the peralkaline series showed energy transfer from the host [NbO6] to Eu3+, confirming the feasibility of exploring niobate glasses and glass-ceramics as lanthanide ion-activated luminescent materials. In addition, glass-ceramics (GCs) containing alkali-niobate phases with a perovskite-like structure were developed and studied to verify the optical performance of these materials. It was verified that the bulk chemistry influences crystallization behavior, and also the photoluminescence response. The transparent GC from the metaluminous series exhibits a quenching of the Eu3+ emission, whereas an enhanced emission intensity is observed for the peralkaline GC. The latter shows a strong excitation-dependent PL emission, suggesting energy transfer and migration of electronic excitation from one Eu population to another. Additionally, Eu3+ emissions arising from the D15 and D25 excited states were observed, highlighting the low phonon energy achievable in niobo-aluminosilicate hosts.

Energy Transport in SiO2 Crystals: Luminescence Excitation Spectra of Stishovite and α-Quartz

Abstract The migration of elementary electronic excitations was studied in a single crystal of stishovite and compared with migration in a crystal of α-quartz and polycrystalline stishovite powder. The research method is based on comparing the transfer of absorbed energy to luminescence centers, used as detectors of quasiparticles, and the near-surface nonradiative annihilation of electronic excitations. A sign of migration is the appearance of some minima in the photoluminescence (PLE) excitation spectrum in the region of maxima in the intrinsic absorption spectrum. The PLE spectrum of stishovite contains the first minimum at 9.8 eV, indicating the migration of electronic excitations and the existence of an intrinsic absorption band in stishovite at 9.8 eV. In α-quartz, the first minimum in the PLE spectrum is located at 10.5 eV and corresponds well to the intrinsic absorption band of the exciton.

Kinetychna model' prostorovoho rozpodilu elektronnykh zbudzhen' v ridynnykh lyuminoforakh

A model of luminescence in liquid phosphors under X-ray excitation has been proposed. The corresponding spatial distributions for electronic excitations and local heating are obtained. They are described by Gaussian functions, the parameters of which are related to the kinetic energy of an X-ray quantum and the medium characteristics. The explanation to a low quantum yield of luminescence in liquid phosphors under the X-ray excitation in comparison with a high quantum yield at the photoluminescence is given. It is shown that the major losses in this case occur at the stage of electronic excitation migration from the solvent to phosphor molecules.

Temperature dependence of the defect luminescence in La2Be2O5 single crystals

Temperature quenching (TQ) curves in the temperature range of 80–500 K have been studied for both the undoped, and doped with RE3+-ions (RE = Ce, Eu, Er, Pr, Nd) lanthanum beryllate (BLO) single crystals. Photoluminescence spectra and TQ-curves were recorded upon excitation in the absorption bands of the lattice defects. The reaction rate model has been developed to describe the experimental results. The model includes two competing processes with characteristic temperatures: thermal quenching of intracenter PL (T1) and thermally stimulated migration of electronic excitations (T2). The competition between these two processes leads to the observed non-monotonic TQ-curves. The rationalized formulas using three parameters (intensity, activation energy, characteristic temperature), were developed to describe each of these processes. Within the framework of the unified model, all the experimental results were described and the best fit parameters were obtained. Classification of the investigated lanthanum beryllate crystals was carried out in line with the best fit parameters obtained for the TQ-curves.

Excitation transport through Rydberg dressing

We show how to create long-range interactions between alkali atoms in different hyperfine ground states, with the goal of coherent quantum transport. The scheme uses off-resonant dressing with atomic Rydberg states. We demonstrate coherent migration of electronic excitation through dressed dipole–dipole interaction by full solutions of models with four essential states per atom and give the structure of the spectrum of dressed states for a dimer. In addition, we present an effective (perturbative) Hamiltonian for the ground-state manifold and show that it correctly describes the full multi-state dynamics. We discuss excitation transport in detail for a chain of five atoms. In the presented scheme, the actual population in the Rydberg state is kept small. Dressing offers many advantages over the direct use of Rydberg levels: it reduces ionization probabilities and provides an additional tuning parameter for lifetimes and interaction strengths.

Electronic energy transfer on a vibronically coupled quantum aggregate

We examine the transfer of electronic excitation (an exciton) along a chain of electronically coupled monomers possessing internal vibronic structure and which also interact with degrees of freedom of the surrounding environment. Using a combination of analytical and numerical methods, we calculate the time evolution operator or time-dependent Green's function of the system and thereby isolate the physical parameters influencing the electronic excitation transport. Quite generally, we show that coupling to vibrations slows down and inhibits migration of electronic excitation due to dephasing effects on the coherent transfer present without vibrations. In particular, coupling to a continuous spectrum of environment states leads to a complete halting of transfer, i.e., a trapping of the exciton.

Mechanistic principles and applications of resonance energy transfer

Resonance energy transfer is the primary mechanism for the migration of electronic excitation in the condensed phase. Well-known in the particular context of molecular photochemistry, it is a phenomenon whose much wider prevalence in both natural and synthetic materials has only slowly been appreciated, and for which the fundamental theory and understanding have witnessed major advances in recent years. With the growing to maturity of a robust theoretical foundation, the latest developments have led to a more complete and thorough identification of key principles. The present review first describes the context and general features of energy transfer, then focusing on its electrodynamic, optical, and photophysical characteristics. The particular role the mechanism plays in photosynthetic materials and synthetic analogue polymers is then discussed, followed by a summary of its primarily biological structure determination applications. Lastly, several possible methods are described, by the means of which all-optical switching might be effected through the control and application of resonance energy transfer in suitably fabricated nanostructures.Key words: FRET, Förster energy transfer, photophysics, fluorescence, laser.

Atomic-level structural and functional model of a bacterial photosynthetic membrane vesicle

The photosynthetic unit (PSU) of purple photosynthetic bacteria consists of a network of bacteriochlorophyll-protein complexes that absorb solar energy for eventual conversion to ATP. Because of its remarkable simplicity, the PSU can serve as a prototype for studies of cellular organelles. In the purple bacterium Rhodobacter sphaeroides the PSU forms spherical invaginations of the inner membrane, approximately 70 nm in diameter, composed mostly of light-harvesting complexes, LH1 and LH2, and reaction centers (RCs). Atomic force microscopy studies of the intracytoplasmic membrane have revealed the overall spatial organization of the PSU. In the present study these atomic force microscopy data were used to construct three-dimensional models of an entire membrane vesicle at the atomic level by using the known structure of the LH2 complex and a structural model of the dimeric RC-LH1 complex. Two models depict vesicles consisting of 9 or 18 dimeric RC-LH1 complexes and 144 or 101 LH2 complexes, representing a total of 3,879 or 4,464 bacteriochlorophylls, respectively. The in silico reconstructions permit a detailed description of light absorption and electronic excitation migration, including computation of a 50-ps excitation lifetime and a 95% quantum efficiency for one of the model membranes, and demonstration of excitation sharing within the closely packed RC-LH1 dimer arrays.

Effect of energy migration on the emission line shape in InGaN/GaN-based quantum-well structures

Structures with InGaN/GaN-based quantum wells and inhomogenetities in the lateral plane were used to study the effect of the mechanism of electronic excitation migration and carrier trapping on the formation of the emission line of the quantum wells. The stationary and time-resolved photoluminescence spectra were investigated over a wide spectral range under variation of the measurement temperature (4.2, 77, 300 K). The anisotropy (polarization) of radiation in InGaN/GaN structures with different inhomogeneities was studied. Excitation exchange among the inhomogeneities was shown to play an essential part in the formation of the emission line in a quantum-well system.

Excitation Migration along Oligophenylenevinylene-Based Chiral Stacks: Delocalization Effects on Transport Dynamics

Atomistic models based on quantum-chemical calculations are combined with time-resolved spectroscopic investigations to explore the migration of electronic excitations along oligophenylenevinylene-based chiral stacks. It is found that the usual Pauli master equation (PME) approach relying on uncoherent transport between individual chromophores underestimates the excitation diffusion dynamics, monitored here by the time decay of the transient polarization anisotropy. A better agreement to experiment is achieved when accounting for excitation delocalization among acceptor molecules, as implemented in a modified version of the PME model. The same models are applied to study light harvesting and trapping in guest-host systems built from oligomers of different lengths.

Four-center energy transfer and interaction pairs: Molecular quantum electrodynamics

In the photophysics of complex macromolecules, resonance energy transfer is the key mechanism for the migration of electronic excitation. As the ability to engineer specific architectures for such molecules improves, environments for new forms of energy migration are being envisioned. Set in this context, one of our aims in this paper is to expound a new, general way of representing complex energy transfer systems, to obviate semantic and conceptual problems in addressing multicenter interactions. The theory of four-center energy transfer is developed within this framework, through the application of molecular quantum electrodynamics. A variety of mechanisms is identified by which four-center energy transfer may proceed, and a recently developed diagrammatic technique is employed to calculate relevant quantum amplitudes. Symmetry considerations are addressed, and key features of the ensuing rate equations are discussed with regard to their potential exploitation in novel photoactive devices.

Laser assisted resonance energy transfer

At distances beyond wavefunction overlap, and typically running up to 100 Å, the migration of electronic excitation between donor and acceptor species is generally understood to approximate well to the resonance energy transfer theory developed by Förster. Here we demonstrate that the energy transfer rate can be increased by propagation of an auxiliary laser beam through the sample. Results indicate that, under optimum conditions, auxiliary beam intensities of 1010 W cm−2 can produce an increase in the energy migration rate by as much as 15%.

Theoretical study of the migration of electronic excitation along the polymer chains in conformation motion

The approach of the effective medium approximation (EMA) has been used for the description of incoherent migration of electronic excitation. The migration takes place among the chromophores linked into a polymer chain which is in conformational motion. The polynomial equation, corresponding to two relative positions of neighbouring chromophores, yields the hypersurface of the effect rate coefficients W eff( Z) of the migration, where z is the parameter of the Laplace transformation. The inverse Laplace transformation of functions of W eff( Z) gives the mean-square distance and the diffusion coefficient D( t) of the migration of excitation. The dependence of D( t) as well as of its limit value for long time D(∞) on the rate of the trans⇄ gauche (T⇄G) conformational motion has been outlined for the polyvinylnaphthalene (PVN) chain.

Concentration depolarization of fluorescence in the presence of molecular rotation

A theory is presented to describe the observables in fluorescence depolarization experiments when both rotational depolarization and concentration depolarization mechanisms are active. The latter mechanism arises from electronic excitation migration among chromophores and becomes more important as their concentration is increased. We treat the simplest case of randomly distributed chromophores, with fixed centers of mass, undergoing independent rotational Brownian motion. Our formalism reduces to existing theories if either depolarization mechanism is turned off, but makes new predictions in the situation where both are operative. Because electronic excitation transfer and molecular orientation are dynamically correlated, we find that previous ad hoc expressions that neglected such correlations are inappropriate. Approximate expressions for the fluorescence anisotropy are provided that can be tested by time-resolved or photostationary experiments on dye molecules in solution. Extensions of the theory to cases of correlated chromophore positions are straightforward and will allow interpretation of experiments on labeled macromolecular systems.

Electronic excitation migration and trapping among cationic porphyrins adsorbed on anionic vesicle surface. Fractal-like behaviors

Picosecond time-correlated single-photon-counting studies have been made on the electronic excitation migration among positively charged porphyrins, TAPP (5,10,15,20-tetrakis(4-trimethylammonio)phenyl)porphyrin, tosylate) and ZnTMPyP (zinc 5,10,15,20-tetrakis(1-methyl-4-pyridinyl)porphyrin, chloride), adsorbed on a negatively charged surface of DHP (dihexadecyl phosphate) vesicles and its trapping at porphyrin aggregates (dimer or higher associates). The observed fluorescence decay curves cannot be reproduced by decay functions derived assuming single-step transfer from porphyrin monomer to nonfluorescent aggregate uniformly distributed in two dimensions or three dimensions. The decay function for the case of single-step transfer derived on the basis of the fractal theory is also unable to reproduce the results. It has been confirmed that the fluorescence decay function derived for multistep excitation migration on fractal and fast trapping at the first trap encounter can reproduce satisfactorily the observed results, and the fracton dimension relevant to porphyrin monomer distribution on the vesicle has been estimated to be 1.65.

Migration of electronic excitation in alcoholic solutions of rhodamine dyes

Migration of electronic excitation in alcoholic solutions of rhodamine dyes

Förster migration of electronic excitation between randomly distributed molecules

This article deals with the diffusivelike migration of electronic excitation between molecules that are randomly distributed in space. A generalized (non-Markoffian) diffusion equation is obtained, and the generalized diffusion coefficient is expanded to second order in molecular concentration. At early times the mean squared displacement of excitation increases as the 5/6th power of time, instead of the usual first power of time. We conjecture that, at long times, the migration may be described by a conventional (Markoffian) diffusion equation.

The di-π-methane rearrangement of systems with simple vinyl moieties : Mechanistic and exploratory organic photochemistry

3,3-Dimethyl-1,1-diphenyl-1,4-pentadiene and two 5-substituted derivatives were synthesized and studied. The regioselectivity, stereochemistry, quantum efficiency, multiplicity, and excited state reaction rates were studied in each case. The parent hydrocarbon, 5-MeO-derivative, and 5-cyano-diene—all rearranged on direct irradiation to give vinylcyclopropanes. The first compound led to 3,3-dimethyl-2,2-diphenyl-1-vinylcyclopropane. The second afforded 3,3-dimethyl-2,2-diphenyl-1-(2'-methoxyvinyl)cyclopropane. The last gave 1-cyano-3,3-dimethyl-2-(2',2'-diphenylvinyl)cyclopropane. Thus, the vinyl and methoxyvinyl groups survive in the products intact, while the cyanovinyl group is incorporated in the three-ring. In the two substituted dienes, cis-reactant gave cis-product and trans-reactant gave trans-product, both where the substituent was on the vinyl group of the product and where it became a ring substituent. The substituted di-π-methane systems underwent only cis-trans isomerization on sensitization, while the parent, unsubstituted diene led to di-π-methane product on sensitized as well as direct photolysis. While the quantum yields for the hydrocarbon diene were the same at room temperature for the direct and sensitized runs, only the sensitized runs showed a temperature dependence of efficiency with a dramatic, 5-fold increase on a 46° temperature increase. Thus, evidence was obtained for a singlet rearrangement in all cases and a triplet process only in the case of the unsubstituted diene. A sizable activation energy was seen for the triplet but not for the singlet. The room temperature quantum yields in the direct irradiations were: φ(parent diene)=0.011, φ( trans-methoxydiene)=0.051, φ( cis-methoxy-diene)= 0.050, φ( trans-cyanodiene)=0.36, and φ( cis-cyano-diene) = 0.20. A competing side reaction was cis-trans isomerization but these quantum yields were lower. Single photon counting was employed to obtain excited singlet reaction and decay rates at low temperature (i.e. 77°K) and the method of magic multipliers was used to obtain room temperature rates. These were: k r(parent diene) = 4.7 × 10 8 sec −1, k r( trans-cyano-diene)= 1.5 ×10 10 sec −1, k r( cis-cyano-diene)= 8.0 × 10 9sec −1, and k r( trans-methoxy-diene) = 1.9 × 10 9 sec −1. The results are discussed in terms of excited state molecular structure. An SCF-CI molecular orbital treatment of the reaction was developed. This used a cyclopropyldicarbinyl diradical species, with Walsh cyclopropane basis orbitals, as representing the half-reacted species. The energy of formation of this species from vertical excited state reactant was calculated for all three dienes and an excellent correlation with observed excited singlet rates was obtained. Similarly, dissection of the excited diradical energy into bond components led to a correlation between regioselectivity and weakness of the three-ring bond broken in the regioselectivity-determining step. Evidence was adduced for localization of the excitation energy in S 1 of reactant in the diphenylvinyl chromophore with migration of electronic excitation into the cyclopropyldicarbinyl diradical moiety during the vinyl-vinyl bridging process. A general method for quantitatively partitioning excitation energy was developed and applied to the case in hand. Finally, there was predicted a greater probability of di-π-methane three-ring fission in the excited state compared to the diradical ground state where Grob fragmentation proved energetically more favorable.

Theory of Energy Migration in Doped Molecular Crystals

AbstractThe kinetics of diffusion‐controlled sensitized luminescence is analyzed in molecular crystals in which a small concentration of acceptor molecules is embedded. It is shown that if the probability of the excitation energy transfer is determined by the dipole‐dipole interaction between donor and acceptor molecules the host luminescence decay law n(t) changes from Förster's law (n(t) ∼ exp (−α \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt t $\end{document}))to an exponential one (n(t) ∼ exp (−βt)) with the electronic excitation diffusion length increasing from 0 to l ≧ 5R0, where R0, is the effective radius of the energy transfer by resonance. In addition the host luminescence decay law n(t) is found with account for the electronic excitation migration over donor molecules in molecular crystal doped by donor and acceptor impurities of small concentrations. It is shown that for such a system the function n(t) cannot be derived from any appropriate diffusion equation.

Electronic excitation migration in neodymium pentaphosphate

A direct measurement of the diffusion distance of the 4F 3 2 state excitation in NdP 5O 14 along the crystallographic a-axis is reported. At room temperature an upper limit of 32 nm is found which is 20 times shorter than an earlier estimate.