Fluorescence Line Narrowing Research Articles

Laser spectroscopy of Nd 3+ ions in glasses with the 0.8CaSiO 3–0.2Ca 3(PO 4) 2 eutectic composition

In this work we report the study of site-selective laser spectroscopy of Nd 3+ ions in glasses with the 0.8CaSiO 3–0.2Ca 3(PO 4) 2 eutectic composition together with a dynamic study of energy transfer among Nd 3+ ions by using time-resolved fluorescence line narrowing (TRFLN) spectroscopy. The spectral features of the time-resolved fluorescence line narrowed 4 F 3 / 2 → 4 I 9 / 2 emission spectra obtained under resonant excitation reveal the existence of spectral migration of excitation among the Nd 3+ ions. The analysis of the time evolution of the 4 F 3 / 2 → 4 I 9 / 2 narrowed emission shows that the electronic mechanism responsible for the ion–ion interaction can be identified as a dipole–dipole energy transfer process.

Fluorescence line narrowing spectroscopy of Eu3+ in TeO2–TiO2–Nb2O5 glass

In this work we report time-resolved line-narrowed fluorescence spectra of the 5D0→7F0–6 transitions of Eu3+-doped 80TeO2–5TiO2–15Nb2O5 glass obtained at 10K by using different resonant excitation wavelengths along the 7F0→5D0 transition. The wavelength dependence of the 7F1 splitting suggests the existence of a broad distribution of Eu3+ centres possessing a variety of local field environments. The influence of the local coordination ions on the optical properties of Eu3+ has been studied by analysing the dependence of the crystal field (CF) parameters on the excitation energy along the 7F0→5D0 transition.

Light Harvesting in Photosystem II Core Complexes Is Limited by the Transfer to the Trap: Can the Core Complex Turn into a Photoprotective Mode?

A structure-based modeling and analysis of the primary photophysical reactions in photosystem II (PS-II) core complexes is presented. The modeling is based on a description of stationary and time-resolved optical spectra of the CP43, CP47, and D1-D2-cytb559 subunits and whole core complexes. It shows that the decay of excited states in PS-II core complexes with functional (open) reaction centers (RCs) is limited by the excitation energy transfer from the CP43 and CP47 core antennae to the RC occurring with a time constant of 40-50 ps at room temperature. The chlorophylls responsible for the low energy absorbance bands in the CP43 and CP47 subunits are assigned, and their signatures in hole burning, fluorescence line narrowing, and triplet-minus-singlet spectra are explained. The different locations of these trap states in the CP43 and CP47 antennae with respect to the reaction center lead to a dramatic change of the transfer dynamics at low temperatures. The calculations predict that, compared to room temperature, the fluorescence decay at 77 K should reveal a faster transfer from CP43 and a much slower and highly dispersive transfer from CP47 to the RC. A factor of 3 increase in the fastest decay time constant of fluorescence that was reported to occur when the RC is closed (the plastoquinone QA is reduced) is understood in the present model by assuming that the intrinsic rate constant for primary electron transfer decreases from 100 fs-1 for open RCs to 6 ps-1 for closed RCs, leading to a reduction of the primary electron acceptor PheoD1, in 300 fs and 18 ps, respectively. The model suggests that the reduced QA switches the photosystem into a photoprotective mode in which a large part of the excitation energy of the RC returns to the CP43 and CP47 core antennae, where the physiologically dangerous triplet energy of the chlorophylls can be quenched by the carotenoids. Experiments are suggested to test this hypothesis. The ultrafast primary electron transfer inferred for open RCs provides further support for the accessory chlorophyll ChlD1 to be the primary electron donor in photosystem II.

Fluorescence line narrowing and decay dynamics of Er 3+ ions in silicon-rich silicon oxide multilayers

Erbium doped silicon-rich silica layer, known for their potential application for active devices, have been investigated by means of site selective laser spectroscopy and fluorescence dynamics at low temperature. It is pointed out that only one erbium local surrounding is identified within the relaxation limit introduced by the non-resonant fluorescence line narrowing technique. Furthermore, radiative relaxation of erbium ions exhibits a non-exponential behavior tentatively explained by diffusion-limited energy transfer between erbium ions and towards unknown traps.

Perturbation of the ground-state electronic structure of FMN by the conserved cysteine in phototropin LOV2 domains

In LOV2, the blue-light sensitive domain of phototropin, the primary photophysical event involves intersystem crossing (ISC) from the singlet-excited state to the triplet state. The ISC rate is enhanced in LOV2 as compared to flavin mononucleotide (FMN) in solution, which likely results from a heavy-atom effect of a nearby conserved cysteine, C450. Here, we applied fluorescence line narrowing (FLN), resonance Raman (RR) and Fourier-transform infrared (FTIR) spectroscopy to investigate the electronic structure of FMN bound to Avena sativa LOV2 (AsLOV2), its C450A mutant and Adiantum LOV2 (Phy3LOV2). We demonstrate that FLN is the method of choice to obtain accurate vibrational spectra on highly fluorescent flavoproteins. The vibrational spectrum of AsLOV2-C450A showed small but significant shifts with respect to those of wild type AsLOV2 and Phy3LOV2, with a systematic down-shift of Ring I vibrations, upshifts of Ring II and III vibrations and an upshift of the C2=O mode. These trends are similar to those in FMN model systems with an electron-donating group substituted at Ring I, known to induce a quinoid character to the electronic structure of oxidized flavin. Thus, enhancement of the ISC rate in LOV2 is induced through weak electron donation by the cysteine which mixes the FMN pi-electrons with the heavy sulfur orbitals, manifesting itself in a quinoid character of the ground electronic state of oxidized FMN. The proximity of the cysteine to FMN thus not only enables formation of a covalent adduct between FMN and cysteine, but also facilitates the rapid electronic formation of the reactive FMN triplet state.

Optical spectroscopic study of Eu^3+ crystal field sites in Na_3La_9O_3(BO_3)_8 crystal

Time-resolved line-narrowed fluorescence spectroscopy of Eu(3+) ions in a new oxyborate Na(3)La(9)O(3)(BO(3))(8) crystal shows the existence of four independent symmetry crystal field sites for the rare-earth ion. A crystal field analysis and simulation of the experimental results have been performed in order to parametrize the crystal field at the Eu(3+) sites. A plausible argument about the crystallographic nature of these sites is given.

Simulation of Eu 3+ luminescence spectra of borosilicate glasses by molecular dynamics calculations

Simplified inactive rare-earths doped nuclear waste glasses have been obtained by molecular dynamics (MD) simulation in order to investigate the local structure around the rare-earth by luminescence studies. MD calculations were performed with modified Born–Mayer–Huggins potentials and three body angular terms representing Coulomb and covalent interactions. Atomic positions within the glasses are then determined. Simulations of luminescence spectra were then obtained by calculation of the ligand field parameters affecting each luminescent ion. Considering the C 2 v symmetry, it is possible to calculate the radiative transition probabilities between the emitter level, 5D 0, and the splitted receptor levels, 7F J ( J = 0–3) for each Eu 3+ ion. The simulated emission spectra are obtained by convolution of all the Eu 3+ ions contributions. A comparison with the experimental data issue from fluorescence line narrowing and microluminescence spectroscopies allowed us not only to validate the simulation of luminescence spectra from simulated environments, but also to confirm the presence and the identification of two major Eu 3+ sites distribution in the nuclear glasses thanks to spectra-structure correlations.

Resonant fluorescence line narrowing and gain spectral hole burning in erbium-doped fiber amplifier

Gain spectral hole burning and resonant fluorescence line narrowing have been performed at low temperature in standard aluminosilicate erbium-doped fiber amplifier to demonstrate the nature of the line broadening. Comparison of the hole-width measurements with resonant fluorescent line narrowing data shows a good agreement at 77 K, working temperature for which both experiments are feasible and have been performed simultaneously. The FWHM results reported here compare well with the earlier line profile measurements performed on aluminosilicate glass preform of the same chemical composition as the fiber. The pump power dependences are reported and indicate that they influence both the depth and line width, which may induce a limitation for wavelength division multiplexing techniques in the long-haul transmission regime of telecommunications.

Electron–phonon and vibronic couplings in the FMO bacteriochlorophyll a antenna complex studied by difference fluorescence line narrowing

The electron–phonon and vibronic couplings governing the spectral properties have been studied in the Fenna–Matthews–Olson (FMO) bacteriochlorophyll a (BChl a)–protein complex at 4.5 K using a spectrally selective difference fluorescence line-narrowing technique. The complex is a part of the light-harvesting system of green photosynthetic bacteria. Its lowest-energy absorption band, peaking at 826 nm and responsible for the fluorescence, is believed to be due to Q y transitions of largely isolated molecules. One of the main merits of the used method compared with the more common fluorescence line narrowing is that the zero-phonon lines (ZPL) resonant with the excitation laser can be accurately measured, allowing precise determination of the Huang–Rhys (HR) factors, the main characteristics of the linear electron–phonon and vibronic coupling strengths. Over 60 individual vibrational modes of intra- and intermolecular origin have been resolved in the energy range of 45–1600 cm −1. The small HR factors for these modes, ranging between 0.001 and 0.018, add up to a value of S vib=0.38±0.07. The effective HR factor for the phonons, S ph, was found clearly wavelength-dependent, varying from ∼0.7 at short wavelengths to ∼0.3 at the long-wavelength tail of the absorption spectrum. Coupling between the BChl a molecules is likely responsible for this wavelength dependence.

Site selective spectroscopy of Eu3+ in heavy-metal oxide glasses

In this work, we report time-resolved line-narrowed fluorescence spectra of the 5D0→7F0−6 transitions of Eu3+-doped 40Bi2O3–40PbO–10Ga2O3–10GeO2 and 60GeO2–25PbO–15Nb2O5 glasses obtained at 10K by using different resonant excitation wavelengths along the 7F0→5D0 transition. The wavelength dependence of the 7F1 splitting in both glasses suggests the existence of a broad distribution of Eu3+ centres possessing a variety of local field environments. The influence of the local coordination ions on the optical properties of Eu3+ has been studied by analysing the dependence of the crystal field (CF) parameters on the excitation energy along the 7F0→5D0 transition.

Temperature and pressure dependence of the homogeneous width of 7F 0– 5D 0 electronic transition in Sm 2+-doped sodium borate glass

The homogeneous width Γ h of 7F 0↔ 5D 0 transition in Sm 2+ ions doped into sodium borate glass was determined at T=5–293 K under P=1 bar and at T=5, 80 and 120 K under various P up to 8 kbar. The line width Γ h was obtained from spectral hole burning (SHB) and time-resolved resonant fluorescence line narrowing (FLN) experiments. It was found that at P=1 bar the T-dependence of Γ h changes from Γ h( T)∝ T 2 (quadratic) for T<20 K to Γ h( T)∝ T (linear) for T>20 K. Under pressure, Γ h grows slightly at all three T values studied (at 80 K by 1.2%/kbar and at 120 K by 1.6%/kbar in the range 0–8 kbar). Possible mechanisms to explain the observed T- and P-dependences of Γ h are briefly discussed.

Optical and site-selective spectral studies of Eu3+-doped zinc oxyfluorotellurite glass

The Eu3+-doped zinc oxyfluorotellurite (TZLFE) glass of molar composition 59TeO2–20ZnO–20LiF–1Eu2O3 has been prepared and studied by broadband, Raman, and fluorescence-line narrowing (FLN) spectral techniques to investigate the local environments of Eu3+ ions in this glass. From the vibronic spectra, different TeO groups coupled to Eu3+ ions have been identified. From the broadband excitation and emission spectra at RT, energy levels of the Eu3+ ion and the atomic parameters have been calculated. The D05 level of Eu3+ ions in the TZLFE glass has been selectively excited within the inhomogeneously broadened F07→D05 excitation spectrum at 15K. The resulting D05→F17 fluorescence spectra and the lifetimes of the D05 level have been obtained as a function of the excitation wavelength. From the FLN spectra, three Stark levels have been identified and crystal-field (CF) analysis has been carried out to calculate second rank CF parameters, by assuming C2v orthorhombic symmetry, as a function of the excitation wavelength. From the CF parameters, CF strength parameter has been calculated. The observed behavior suggests the existence of a unique kind of site for all the environments of Eu3+ ions in the TZLFE glass. The local structure of Eu3+ ions in the TZLFE glass is almost similar to sodium borosilicate glass but with a shorter distribution of environments.

Band Structure and Local Dynamics of Excitons in Bacterial Light-harvesting Complexes Revealed by Spectrally Selective Spectroscopy

Hole-burned absorption and line-narrowed fluorescence spectra are studied at 5 K in wild type and mutant LH1 and LH2 antenna preparations from the photosynthetic purple bacterium Rhodobacter sphaeroides. Evidence was found in all samples, even in intact membranes, of the presence of a broad distribution of bacteriochlorophyll species that are unable to communicate energy between each other and to the exciton states of functional antenna complexes. The distribution maximum of these localized species determined by zero phonon hole action spectroscopy is at 783.5 nm in purified LH1 complexes and at 786.8 nm in B850-only mutant LH2 complexes. A well-resolved peak at 807 nm in LH1 complexes is assigned to the exciton band structure of functional core antenna complexes. Similar structure in LH2 complexes overlaps with the distribution of localized species. Off-diagonal (structural) disorder may be responsible for this exciton band structure. Our data also imply that pair-wise inter-chlorophyll couplings determine the resonance fluorescence lineshape of excitonic polarons.

The Chemical Interaction between the Estrogen Receptor and Monohydroxybenzo[a]pyrene Derivatives Studied by Fluorescence Line-Narrowing Spectroscopy

A novel approach is presented for studying the chemical interaction between receptor binding sites and ligands. Monohydroxylated polyaromatic compounds were found to be environmentally sensitive ligands when applying a special mode of fluorescence: fluorescence line-narrowing spectroscopy (FLNS). With this technique, solvent dependencies and ligand-receptor interactions can be studied in great detail, due to the high spectral resolution and the fact that at cryogenic temperatures (4 K), no solvent reorientation effects complicate the interpretation. The FLN spectrum of a ligand bound to the receptor is compared to the spectra of the free ligand in solvent mixtures that mimic the functionalities present within the receptor's binding site. It is shown that for the well-known estrogen receptor (ER), the orientations of two xenoestrogenic ligands 3- and 9-hydroxybenzo[a]pyrene (3- and 9-OH-BaP) can be determined. The FLN results clearly indicate that an H-bond accepted by HIS524 plays a major role in the binding of these ligands to the ER. Furthermore, the spectra indicated a pi-pi stacking aromatic interaction for 9-OH-BaP with PHE404. These results are in line with molecular modeling studies published earlier.

Inhomogeneity of Ho3+ activated SrLaGa3O7 and SrLaGaO4 crystals studied by fluorescence line narrowing technique

Spectroscopic properties of Ho3+ in SrLaGa3O7 (SLG) and SrLaGaO4 (SLO) are studied using non-resonant fluorescence line narrowing technique. Spectrally-different ions in inhomogeneous absorption line of holmium are selectively excited. The low temperature emission and fluorescence decays from the 5S2 level of Ho3+ ion after pulsed 5I8 → 5S2 excitation transition are investigated. The observed line-narrowed fluorescence is used to analyze the origin of spectral line broadening phenomenon in SLG and SLO hosts. Based on the experimental results an influence of the statistical distribution of La3+ and Sr2+ ions in crystal lattice on the spectroscopic properties of Ho3+ is discussed.

Assignment of the low-temperature fluorescence in oxygen-evolving Photosystem II

Low-temperature absorption and fluorescence spectra of fully active cores and membrane-bound PS II preparations are compared. Detailed temperature dependence of fluorescence spectra between 5 and 70 K are presented as well as 1.7-K fluorescence line-narrowed (FLN) spectra of cores, confirming that PS II emission is composite. Spectra are compared to those reported for LHCII, CP43, CP47 and D1/D2/cytit b559 subunits of PS II. A combination of subunit spectra cannot account for emission of active PS II. The complex temperature dependence of PS II fluorescence is interpretable by noting that excitation transfer from CP43 and CP47 to the reaction centre is slow, and strongly dependent on the precise energy at which a 'slow-transfer' pigment in CP43 or CP47 is located within its inhomogeneous distribution. PS II fluorescence arises from CP43 and CP47 'slow-transfer' states, convolved by this dependence. At higher temperatures, thermally activated excitation transfer to the PS II charge-separating system bypasses such bottlenecks. As the charge-separating state of active PS II absorbs at >700 nm, PS II emission in the 680-700 nm region is unlikely to arise from reaction centre pigments. PS II emission at physiological temperatures is discussed in terms of these results.

Cross-reactivity and conformational multiplicity of an anti-polycyclic aromatic hydrocarbon mAb

Cross-reactivity and multispecific functionality of antibodies play a central role in the immune system. The Ab's promiscuity is attributed to structural flexibility and conformational multiplicity of its binding sites governed by the rearrangement of hydrogen bonding centers. However, antibodies whose recognition and binding rely on less directional hydrophobic interactions might follow different interaction pathways. We investigated interaction of anti-polycyclic aromatic hydrocarbon mAb with two biologically important cross-reactants, pyrene and benzo(a)pyrene. Complex formation was characterized by means of low-temperature laser-induced fluorescence spectroscopy in both low- and high-resolution fluorescence line-narrowing (FLN) modes. It is shown that the FLN spectroscopy can identify various haptens cross-reacted with an Ab, as well as simultaneously differentiate between free and immunocomplexed haptens. In addition, our results suggest an interesting case of an Ab binding a particular cross-reactant by adopting two distinct conformations of its binding sites. The existence of the multiple conformations for anti-polycyclic aromatic hydrocarbon mAb that are trapped at low temperature can be rationalized through the existing models for Ab binding. Finally, as revealed by FLN spectra of immunocomplexed chromophores, pi-pi interactions, rather than hydrogen bonding, play the central role in complex formation.

Green and Red Fluorescent Proteins: Photo‐ and Thermally Induced Dynamics Probed by Site‐Selective Spectroscopy and Hole Burning

The cloning and expression of autofluorescent proteins in living matter, combined with modern imaging techniques, have thoroughly changed the world of bioscience. In particular, such proteins are widely used as genetically encoded labels to track the movement of proteins as reporters of cellular signals and to study protein-protein interactions by fluorescence resonance energy transfer (FRET). Their optical properties, however, are complex and it is important to understand these for the correct interpretation of imaging data and for the design of new fluorescent mutants. In this Minireview we start with a short survey of the field and then focus on the photo- and thermally induced dynamics of green and red fluorescent proteins. In particular, we show how fluorescence line narrowing and high-resolution spectral hole burning at low temperatures can be used to unravel the photophysics and photochemistry and shed light on the intricate electronic structure of these proteins.

Determination of the environment of lanthanide ions in a simplified non-active nuclear glass and its weathering gel products – Europium as a structural luminescent probe

In order to explore the complex structure of the French nuclear glass (SON68) and its weathering gel products, four simplified Eu 3+ doped matrices relative to the four main glass forming oxides have been studied by fluorescence line narrowing. The precise calculation of their crystal field parameters and the correlation with those of the nuclear glass and its weathering gels allowed us the determination of the nature of the trivalent europium ion environments in a simplified SON68 type glass and gels. The glass presents two sites for europium, the first one is a silicate one and the second is mixed aluminate and borate one. Both sites in the different gels are similar to the glass. The first site is mainly a silicate one with the presence of phosphate groups, and the second is an aluminate one with the possible presence of boron in the neighborhood of the rare-earth. This article led us determine the structure around lanthanide ions in the SON68 glass and its weathering gels in spite of the water leaching on glass surface (lixiviation).

Energy transfer studies in Eu3+-doped lead–niobium–germanate glasses

The spectral migration of energy among Eu 3+ ions in lead-niobium-germanate glass (60GeO 2 -25PbO-15Nb205) with different Eu 3+ concentrations (0.5, 1, 3, and 5 mol%) has been investigated at low temperature by using time-resolved fluorescence line narrowing (TRFLN) spectroscopy. The spectral features of the time-resolved fluorescence line-narrowed 5 D 0 → 7 F 0 emission spectra under resonant excitation at different wavelengths along the 7 F 0 → 5 D 0 transition as a function of time, excitation wavelength, and concentration reveal the existence of energy migration between discrete regions of the inhomogeneous broadened spectral profile. The analysis of the time evolution of the 5 D 0 → 7 F 0 emission shows that the electronic mechanism responsible for the ion-ion interaction can be identified as a dipole-dipole energy transfer process.