Fluorescence Line Narrowing Spectra Research Articles

Fluorescence studies of porphycene in various cryogenic environments

Low temperature fluorescence of porphycene, a structural isomer of porphyrin, has been studied using polymer samples, matrix isolation, and fluorescence line narrowing (FLN) techniques. Contrary to the case of the chromophore embedded in a nitrogen matrix, the emission from polymer samples at temperatures above 10 K exhibits strong dependence on the wavelength of excitation: increasing the excitation energy leads to gradual broadening and, finally, loss of vibrational structure. A rather unusual observation is the similarity of the structured fluorescence spectra obtained for excitations into S1 and S2 states. This finding indicates a correlation between the site distributions in S1 and S2. A similar idea has been put forward earlier for tetraphenylporphyrin [I. Lee, G. J. Small, and J. M. Hayes, J. Phys. Chem. 94, 3376 (1990)]. We propose that the correlation is due to isotropic polarizability in the molecular plane; calculations confirm such hypothesis. For porphycene, an additional factor that can contribute to the effect is a rapid trans-trans tautomerization that leads to the rotation of x and y in-plane axes of the molecule. FLN spectra reveal significant band broadening for excitation into S2. This suggests that the site correlation is not of 1:1 type or that at 4.2 K the site exchange dynamics is frozen in comparison with the situation at higher temperatures.

Exciton Fine Structure and Lattice Dynamics in InP/ZnSe Core/Shell Quantum Dots.

Nanocrystalline InP quantum dots (QDs) hold promise for heavy-metal-free optoelectronic applications due to their bright and size-tunable emission in the visible range. Photochemical stability and high photoluminescence (PL) quantum yield are obtained by a diversity of epitaxial shells around the InP core. To understand and optimize the emission line shapes, the exciton fine structure of InP core/shell QD systems needs be investigated. Here, we study the exciton fine structure of InP/ZnSe core/shell QDs with core diameters ranging from 2.9 to 3.6 nm (PL peak from 2.3 to 1.95 eV at 4 K). PL decay measurements as a function of temperature in the 10 mK to 300 K range show that the lowest exciton fine structure state is a dark state, from which radiative recombination is assisted by coupling to confined acoustic phonons with energies ranging from 4 to 7 meV, depending on the core diameter. Circularly polarized fluorescence line-narrowing (FLN) spectroscopy at 4 K under high magnetic fields (up to 30 T) demonstrates that radiative recombination from the dark F = ±2 state involves acoustic and optical phonons, from both the InP core and the ZnSe shell. Our data indicate that the highest intensity FLN peak is an acoustic phonon replica rather than a zero-phonon line, implying that the energy separation observed between the F = ±1 state and the highest intensity peak in the FLN spectra (6 to 16 meV, depending on the InP core size) is larger than the splitting between the dark and bright fine structure exciton states.

Investigation of the local environment of Eu3+ in a silicophosphate glass using site-selective spectroscopy and Molecular Dynamics simulations

Silicophosphate glasses (SiO2-P2O5) doped with Eu3+ ions were synthesized by the sol-gel process. Optical properties of these glasses were investigated by means of emission spectra and lifetime measurements. The Fluorescence Line Narrowing (FLN) technique was also used to explore the local structure around the Eu3+ ions in this host and to understand the role of phosphate as a codopant. As it is the case for aluminum, the ability of phosphate to avoid the rare earth clustering was investigated, and the role of this codopant in modifying the local order around the rare earth ion was evidenced. The analysis of the FLN spectra and lifetime measurements is consistent with this interpretation. Molecular Dynamics simulations were performed to evaluate and confirm these structural features. Two classes of europium sites were distinguished in agreement with the experimental characterization.

Theoretical Characterization of the Spectral Density of the Water-Soluble Chlorophyll-Binding Protein from Combined Quantum Mechanics/Molecular Mechanics Molecular Dynamics Simulations.

Over the past decade, both experimentalists and theorists have worked to develop methods to describe pigment-protein coupling in photosynthetic light-harvesting complexes in order to understand the molecular basis of quantum coherence effects observed in photosynthesis. Here we present an improved strategy based on the combination of quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations and excited-state calculations to predict the spectral density of electronic-vibrational coupling. We study the water-soluble chlorophyll-binding protein (WSCP) reconstituted with Chl a or Chl b pigments as the system of interest and compare our work with data obtained by Pieper and co-workers from differential fluorescence line-narrowing spectra (Pieper et al. J. Phys. Chem. B 2011, 115 (14), 4042-4052). Our results demonstrate that the use of QM/MM MD simulations where the nuclear positions are still propagated at the classical level leads to a striking improvement of the predicted spectral densities in the middle- and high-frequency regions, where they nearly reach quantitative accuracy. This demonstrates that the so-called "geometry mismatch" problem related to the use of low-quality structures in QM calculations, not the quantum features of pigments high-frequency motions, causes the failure of previous studies relying on similar protocols. Thus, this work paves the way toward quantitative predictions of pigment-protein coupling and the comprehension of quantum coherence effects in photosynthesis.

Fluorescence line narrowing and Δ-FLN spectra in the presence of excitation energy transfer between weakly coupled chromophores.

The results of simulations of fluorescence line narrowed (FLN) and Δ-FLN spectra of weakly coupled chromophore dimers and trimers, connected by slow (hundreds of picoseconds) excitation energy transfer and embedded in amorphous solids such as proteins, are presented. The model includes wavelength-dependent excitation energy transfer (EET) rate distributions converted to nonidentical sub- site distribution function (sub-SDF) (one for each EET rate), as well as spectral hole burning in pigments excited via EET. Emission of the donor molecules and realistic EET probability dependence on the donor-acceptor energy gap are also included. The shapes of the FLN and Δ-FLN spectra are very sensitive to even small variations of interpigment coupling in this weak-coupling regime, and proper modeling is essential for extracting correct parameters from the experimental spectra. Applications of the model to the trimeric FMO complex and the dimeric cytochrome b6f are demonstrated.

Modeling of fluorescence line-narrowed spectra in weakly coupled dimers in the presence of excitation energy transfer.

This work describes simple analytical formulas to describe the fluorescence line-narrowed (FLN) spectra of weakly coupled chromophores in the presence of excitation energy transfer (EET). Modeling studies for dimer systems (assuming low fluence and weak coupling) show that the FLN spectra (including absorption and emission spectra) calculated for various dimers using our model are in good agreement with spectra calculated by: (i) the simple convolution method and (ii) the more rigorous treatment using the Redfield approach [T. Renger and R. A. Marcus, J. Chem. Phys. 116, 9997 (2002)]. The calculated FLN spectra in the presence of EET of all three approaches are very similar. We argue that our approach provides a simplified and computationally more efficient description of FLN spectra in the presence of EET. This method also has been applied to FLN spectra obtained for the CP47 antenna complex of Photosystem II reported by Neupane et al. [J. Am. Chem. Soc. 132, 4214 (2010)], which indicated the presence of uncorrelated EET between pigments contributing to the two lowest energy (overlapping) exciton states, each mostly localized on a single chromophore. Calculated and experimental FLN spectra for CP47 complex show very good qualitative agreement.

Probing the physicochemical interactions of 3-hydroxy-benzo[a]pyrene with different monoclonal and recombinant antibodies by use of fluorescence line-narrowing spectroscopy

Characterization of interactions between antigens and antibodies is of utmost importance both for fundamental understanding of the binding and for development of advanced clinical diagnostics. Here, fluorescence line-narrowing (FLN) spectroscopy was used to study physicochemical interactions between 3-hydroxybenzo[a]pyrene (3OH-BaP, as antigen) and a variety of solvent matrices (as model systems) or anti-polycyclic aromatic hydrocarbon antibodies (anti-PAH). We focused the studies on the specific physicochemical interactions between 3OH-BaP and different, previously obtained, monoclonal and recombinant anti-PAH antibodies. Control experiments performed with non-binding monoclonal antibodies and bovine serum albumin (BSA) indicated that nonspecific interactions did not affect the FLN spectrum of 3OH-BaP. The spectral positions and relative intensities of the bands in the FLN spectra are highly dependent on the molecular environment of the 3OH-BaP. The FLN bands correlate with different vibrational modes of 3OH-BaP which are affected by interactions with the molecular environment (π-π interactions, H-bonding, or van-der-Waals forces). Although the analyte (3OH-BaP) was the same for all the antibodies investigated, different binding interactions could be identified from the FLN spectra on the basis of structural flexibility and conformational multiplicity of the antibodies' paratopes.

Temperature dependent electron–phonon coupling in chlorin-doped impurity glass and in photosynthetic FMO protein containing bacteriochlorophyll a

Difference fluorescence line-narrowing spectra are studied for evaluating the temperature dependence of the linear electron–phonon and vibronic coupling strengths in the two weakly coupled low-temperature glassy systems: chlorin-doped 1-propanol and the native Fenna–Matthews–Olson (FMO) light-harvesting protein complex folded with bacteriochlophyll a chromophores. Roughly three-fold increases of the electron–phonon coupling strength and constant vibronic couplings were observed in both systems in the available temperature range from 4.5 to 70K. However, while the chlorin system is amenable to simple modeling of impurity spectra, the FMO shows significant deviations, consistent with the complex (exciton) nature of its lowest-energy singlet electronic transition.

Pigment–Protein Interactions in Phytochromes Probed by Fluorescence Line Narrowing Spectroscopy

Fluorescence line narrowing (FLN) spectroscopy was used to study bacteriophytochromes and variants from various species in their red-absorbing Pr ground state, including phytochromes Agp1 from Agrobacterium tumefaciens , DrBphP from Deinococcus radiodurans , and RpBphP2 and RpBphP3 from Rhodopseudomonas palustris . A species-dependent narrowing of the fluorescence emission bands is observed. The results suggest varied pigment-protein interactions, possibly connected to chromophore mobility or extended water pyrrole networks inside of the differing binding pockets. Solvent water isotope exchange from H2O-based buffer to D2O-based buffer solutions was used to identify specific vibrational modes of the chromophore. In addition to the expected frequency shifts upon isotope exchange, the line narrowing efficiency is increased in deuterated compared to protonated surroundings. We conclude that proton dynamics inside of the protein binding pocket are a dominant source of spectral diffusion at low temperatures, which possibly relates to the previous observation that the electronic transition is directly coupled to proton transfer. The FLN spectra of Agp1 reconstituted with a synthesized pigment shows strong line narrowing efficiency even in protonated buffer solution. The FLN spectra of a point mutant of RpBphP3 highlight the involvement of aspartate 216 in a hydrogen bond network around the chromophore. On the basis of similar FLN characteristics in RpBphP2 and RpBphP3, we propose a similarly extended hydrogen bond network around their chromophores despite the different photoreactions leading to red- or blue-shifted absorption relative to the respective photoreceptors' ground-state absorption.

Fluorescence line-narrowing difference spectra: Dependence of Huang–Rhys factor on excitation wavelength

It is well known that fluorescence line narrowed (FLN) difference spectra (ΔFLN spectra) reduce to single-site fluorescence line shape function in the low-fluence limit. This finding is confirmed by the observation of different line shapes of FLN and ΔFLN spectra obtained for pyrene (Py) in ethanol glass at various excitation wavelengths using time-resolved fluorescence measurements. A peculiar dependence of the Huang–Rhys factor (S) on excitation wavelength was discovered, and discussed in terms of the solute–solvent, intermolecular Lennard–Jones potential. A V-shaped frequency dependence of the S-factor suggests that the repulsive–dispersive potential is not sufficient to account for a complex ‘color effect’.

Vibrational states of Zn-meso-indolo[3,2-b]carbazolyl-substituted porphyrins: Fluorescence line narrowing study

Highly resolved vibrational spectra for the Zn-complexes of several meso-substituted tetraarylporphyrins were recorded by fluorescence line narrowing (FLN) method at liquid helium temperature. The frequencies of the vibronic lines of the Zn-5,10,15,20-tetraphenylporphyrin (ZnTPP), the Zn-5,10,15,20-tetramesitylporphyrin (ZnTMesP) and four meso-indolocarbazolyl-substituted porphyrin compounds in the FLN spectra have been measured and compared. The frequencies, maximum amplitude changes for natural coordinates and the symmetry of the normal modes for ZnTPP and ZnTMesP were also determined on the basis of DFT quantum-chemical calculations. Differences in the orientation of the meso-aryl rings for ZnTPP and ZnTMesP have been identified and discussed. An explanation for the spectral changes upon substitution with indolo[3,2-b]carbazole units is given providing further elucidation on the structural features arising upon the progressive substitution with ICZ groups.

Accurate modeling of fluorescence line narrowing difference spectra: Direct measurement of the single-site fluorescence spectrum

Accurate lineshape functions for modeling fluorescence line narrowing (FLN) difference spectra (DeltaFLN spectra) in the low-fluence limit are derived and examined in terms of the physical interpretation of various contributions, including photoproduct absorption and emission. While in agreement with the earlier results of Jaaniso [Proc. Est. Acad. Sci., Phys., Math. 34, 277 (1985)] and Funfschilling et al. [J. Lumin. 36, 85 (1986)], the derived formulas differ substantially from functions used recently [e.g., M. Ratsep et al., Chem. Phys. Lett. 479, 140 (2009)] to model DeltaFLN spectra. In contrast to traditional FLN spectra, it is demonstrated that for most physically reasonable parameters, the DeltaFLN spectrum reduces simply to the single-site fluorescence lineshape function. These results imply that direct measurement of a bulk-averaged single-site fluorescence lineshape function can be accomplished with no complicated extraction process or knowledge of any additional parameters such as site distribution function shape and width. We argue that previous analysis of DeltaFLN spectra obtained for many photosynthetic complexes led to strong artificial lowering of apparent electron-phonon coupling strength, especially on the high-energy side of the pigment site distribution function.

Insight into the Electronic Structure of the CP47 Antenna Protein Complex of Photosystem II: Hole Burning and Fluorescence Study

We report low temperature (T) optical spectra of the isolated CP47 antenna complex from Photosystem II (PSII) with a low-T fluorescence emission maximum near 695 nm and not, as previously reported, at 690-693 nm. The latter emission is suggested to result from three distinct bands: a lowest-state emission band near 695 nm (labeled F1) originating from the lowest-energy excitonic state A1 of intact complexes (located near 693 nm and characterized by very weak oscillator strength) as well as emission peaks near 691 nm (FT1) and 685 nm (FT2) originating from subpopulations of partly destabilized complexes. The observation of the F1 emission is in excellent agreement with the 695 nm emission observed in intact PSII cores and thylakoid membranes. We argue that the band near 684 nm previously observed in singlet-minus-triplet spectra originates from a subpopulation of partially destabilized complexes with lowest-energy traps located near 684 nm in absorption (referred to as AT2) giving rise to FT2 emission. It is demonstrated that varying contributions from the F1, FT1, and FT2 emission bands led to different maxima of fluorescence spectra reported in the literature. The fluorescence spectra are consistent with the zero-phonon hole action spectra obtained in absorption mode, the profiles of the nonresonantly burned holes as a function of fluence, as well as the fluorescence line-narrowed spectra obtained for the Q(y) band. The lowest Q(y) state in absorption band (A1) is characterized by an electron-phonon coupling with the Huang-Rhys factor S of approximately 1 and an inhomogeneous width of approximately 180 cm(-1). The mean phonon frequency of the A1 band is 20 cm(-1). In contrast to previous observations, intact isolated CP47 reveals negligible contribution from the triplet-bottleneck hole, i.e., the AT2 trap. It has been shown that Chls in intact CP47 are connected via efficient excitation energy transfer to the A1 trap near 693 nm and that the position of the fluorescence maximum depends on the burn fluence. That is, the 695 nm fluorescence maximum shifts blue with increasing fluence, in agreement with nonresonant hole burned spectra. The above findings provide important constraints and parameters for future excitonic calculations, which in turn should offer new insight into the excitonic structure and composition of low-energy absorption traps.

Effect of compositional variation on optical and structure properties of europium-dopedSiO_2-HfO_2 glasses

Glasses with compositions (100-x)SiO(2)-xHfO(2):0.3Eu(3+) (molar ratio, x=0,10,20,30) for optical applications were prepared using the sol-gel route. The introduction of hafnium into the glass matrix induced the energy splitting of the F27 state of Eu(3+) ions. Furthermore, fluorescence line narrowing (FLN) spectra indicated that Eu(3+) clustering occurred in glasses containing no hafnium. The addition of hafnium promoted better dispersion of Eu(3+) ions in the glass matrix. The role of hafnium on modifying the properties of glasses was discussed with respect to x-ray diffraction and FLN analysis.

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.

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.

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.

Spectral Hole Burning in Sol-Gel-Derived Eu<sup>3+</sup>-Doped Al<sub>2</sub>O<sub>3</sub>-B<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> Glass

Al2O3-B2O3-SiO2 glass containing europium ions was prepared by a sol-gel method. The excitation spectrum of the Eu3+ ions in the glass consists of the charge transfer and f-f transition absorptions of the Eu3+ ions. The emission spectrum indicates the coexistence of the Eu2+ and Eu3+ ions. The formation of some reducing agents in the heat-treatment process should be responsible for the reduction from Eu3+ to Eu2+ ions. The fluorescence line-narrowing spectra reveal that there are two different environments for the Eu3+ ions. Persist spectral hole was burned in the excitation of the 5D0-7F0 transition of the Eu3+ ions. We suggested a possible mechanism on the persistent spectral hole burning of the Eu3+ ion in the glass.

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.

Optical Properties and Valence Change of Europium Ions in a Sol−Gel Al2O3−B2O3−SiO2 Glass by Femtosecond Laser Pulses

The Al2O3-B2O3-SiO2 glass containing europium ions was prepared by a sol-gel method. Fluorescence line-narrowing spectra (FLN) indicate two different environments of the Eu 3+ ions. The calculated second crystal-field parameters exhibit the opposite behaviors of the two different environments. The FLN excitation and emission spectra before and after irradiation show that the change of the emission mainly comes from the Eu 3+ ions at site I, revealing that the concentration ratio of the Eu 3+ ions at site I to site II was decreased. The emission spectra confirmed that some Eu 3+ ions were reduced into Eu 2+ ions. The excitation spectra indicate that the Eu 3+ ions at the sites with higher covalence degree can be easily reduced, implying that the Eu 3+ ions are more easily reduced at site I than at site II. The absorption spectra before and after irradiation exhibit that the absorption of Eu 2+ ions increases and that the positive hole centers appear. These results suggest a mechanism of the formation of the Eu 2+ ions by femtosecond laser irradiation.