Site-selective Fluorescence Spectroscopy Research Articles

A site-selective fluorescence spectroscopy study of the crystal phases of KY3F10: Leveraging the optical response of Eu3+ ions

The present work deals with the investigation of the site-selective symmetries in Eu3+-doped KY3F10 with different crystal phases. Employing time-resolved fluorescence line-narrowing (TRFLN), a study of the luminescent response of the dopant is reported in samples presenting a single δ-phase, a single α-phase, and a mixture of both δ and α. A new, simple, very low time-consuming and high-yield method was developed to obtain nanospheres of the δ-phase. For both crystal phases, the main site symmetry is spectrally corroborated with that expected by the crystallographic substitution position (C2v in δ, and C4v in α). In addition, the accuracy of the measurements also unveils the presence of significantly distorted Eu3+ ions in the surface of the nanoparticles in all the samples studied, a spectral behavior commonly found in glassy systems. All these findings contribute to the proper understanding of the optical response of luminescent dopants in this and related types of complex fluorides. It is worth highlighting the fact that the huge potential of the unexplored δ-phase is demonstrated, since the site-selective emission of the Eu3+-doped δ-phase is up to 20 times more intense than that of the α-phase.

One- and two-exciton states of pair centers of Kramers Nd3+ions in Nd-doped CaF2 and SrF2 crystals, and their possible use as qubits

Using two-color excitation with narrow laser lines of 0.2 cm−1 and time-resolved site-selective fluorescence spectroscopy, we measured the fluorescence excitation spectra of the one-exciton 4G5/2(1) x 4I9/2(1) state monitored in the near IR, and the two-exciton 4G5/2(1) x 4G5/2(1) state monitored in the near UV of the pair M centers in the Nd3+ doped CaF2 and SrF2 crystals at 6.5 K. We observed the splitting of the one-exciton state of the order of 2–3 cm−1 depending on the type of crystal matrix. A significant inhomogeneous splitting ∼1.5 cm−1 of the spectral lines of the one-exciton state of the pair M centers in the Nd3+: SrF2 crystals was found. The measured single spectral line of the two-exciton state of the pair M centers in these crystals has in one-photon scale the linewidth of ∼0.8 cm−1, which is mainly determined by inhomogeneous broadening.

Comparison of Binding Ability and Location of Two Mesoporphyrin Derivatives in Liposomes Explored with Conventional and Site-Selective Fluorescence Spectroscopy

Application of porphyrins as photosensitizers is based on their light-triggered generation of reactive oxygen species (ROS) that may cause oxidative tissue damage and ultimately kill cells. Cellular membranes are the action grounds of many sensitizers due to their hydrophobic or amphiphilic character as well as the location of many of the targets attacked by ROS. Hence, the binding ability and location of porphyrins in liposomes as simple models of cellular membranes are of outstanding interest. Here we compare mesoporphyrin IX dimethyl ester (MPE) and its nonesterified form, mesoporphyrin IX dihydrochloride (MPCl). Monocomponent small unilamellar vesicles formed of various saturated phosphatidylcholines with incorporated mesoporphyrins were investigated. We determined the binding parameters and the inhomogeneous distribution functions (IDFs) by different fluorescence techniques. We found in general that the binding ability of MPE is considerably greater than that of MPCl. In the case of MPCl, the IDFs suggest that only one of the two binding site types identified earlier for MPE ("site II") exists; the other one ("site I") vanishes while a new one appears ("site III"). We can confirm that "site I" is located between the two lipid layers, "site II" is situated between the hydrocarbon chains, while the location of the novel "site III" is along the outer part of the hydrocarbon chains partially inserted between the lipid head groups.

Comparison of Binding Ability and Location of Two Mesoporphyrin Derivatives in Liposomes by Conventional and Site-Selective Fluorescence Spectroscopy

Application of porphyrins as photosensitizers is based on their light-triggered generation of reactive oxygen species (ROS) that may cause oxidative tissue damage and ultimately kill cells. Cellular membranes are the action grounds of many sensitizers due to their hydrophobic or amphiphilic character as well as the location of many of the targets attacked by ROS. Hence, the binding ability and location of porphyrins in liposomes as simple models of cellular membranes are of outstanding interest. Here we compare mesoporphyrin IX dimethyl ester (MPE) and its nonesterified form, mesoporphyrin IX dihydrochloride (MPCl). Monocomponent small unilamellar vesicles formed of various saturated phosphatidylcholines with incorporated mesoporphyrins were investigated. We determined the binding parameters and the inhomogeneous distribution functions (IDFs) by different fluorescence techniques. We found in general that the binding ability of MPE is considerably greater than that of MPCl. In the case of MPCl, the IDFs suggest that only one of the two binding types identified earlier for MPE (site II) exists; the other one (site I) vanishes while a new one appears (site III). We can confirm that site I is located between the two lipid layers, site II is situated between the hydrocarbon chains, while the location of the novel site III is along the outer part of the hydrocarbon chains partially inserted between the lipid head groups.

Location of Mesoporphyrin in Liposomes Determined by Site-Selective Fluorescence Spectroscopy

Binding of photosensitizers to target cells is a crucial step during the photodynamic effect. Sensitizer distribution is a good indication of whether the chemical is a good candidate for perturbing cell membrane integrity. Hence, the photophysical properties of porphyrinoid sensitizers in microheterogeneous systems such as liposomes are of outstanding interest. Here we present a site-selective fluorescence study of liposome systems. Monocomponent, small unilamellar vesicles formed of different phosphatidylcholines with incorporated mesoporphyrin were investigated. The size distribution of liposomes was measured by dynamic light scattering after each step of the experiment. On the basis of fluorescence line narrowing spectra of mesoporphyrin, the inhomogeneous distribution function was determined in order to characterize the photosensitizer location. The dual character of the functions revealed two different locations. Decomposition of the inhomogeneous distribution functions into Gaussians and the analysis of the fit results suggest that one of the locations for mesoporphyrin is between the two lipid layers, and the other one is between the hydrocarbon chains of the lipid molecules.

Fluorescence vibronic analysis in a ladder-type conjugated polymer

The underlying vibronic structure of methyl-substituted ladder-type poly(para-phenylene) absorption and emission was investigated using temperature dependent fluorescence and site-selective fluorescence spectroscopy. Principal contributions in the vibronic manifold were identified from modelling of the data, and were found to be in good agreement with theoretical predictions after a re-evaluation of the position of the zero-phonon transition. A crossover to Raman dominated spectra at low excitation energy and temperature was observed. The influence of Raman effects should form an important point of consideration in the interpretation of fluorescence data.

Single Chromophore Spectroscopy of MEH‐PPV: Homing‐In on the Elementary Emissive Species in Conjugated Polymers

Low-temperature, single-molecule spectroscopy can provide unparalleled access to the primary emissive species of conjugated polymers. We demonstrate this with the example of one of the most commonly studied polymers, poly(2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylenevinylene), MEH-PPV, which is shown to exhibit sharp fluorescence signatures over one hundred times narrower than the ensemble. These unprecedented narrow emission features can be assigned to single chromophores on the polymer chain, which are selectively addressed by the narrow band excitation. As with organic dye systems, the emission from single chromophores is not static with time, but shows a substantial spectral fluctuation. We find that, for single chromophores, this spectral fluctuation always follows a universal Gaussian statistical distribution. High-resolution spectroscopy provides unique insight into low-energy vibrational modes in the polymer emission, which are generally inaccessible with conventional spectroscopic methods such as site-selective fluorescence or Raman spectroscopy. Interchromophoric coupling can also occur owing to the flexible nature of the polymer backbone. This leads to substantial spectral broadening and a loss of resolution in the vibronic progression. We observe reversible switching within one single molecule between narrow and broad emission, which directly correlates with a discrete switching in emission intensity. We conclude that one and the same single molecule can support aggregated and nonaggregated emission, that is, emission from isolated and aggregated chromophores in one single molecule, rather than the tendency for aggregate emission being intrinsic to the molecule.

Spectroscopic characterization of p-phenylene vinylene (PV) oligomers. Part I: A homologous series of 2,5-diheptyloxy substituted PV-oligomers

Abstract A homologous series of metathetically synthesized, monodisperse all- trans oligomers of 2,5-diheptyloxy- p -divinylbenzene 1 (DHepO-OPV n s, with n the number of aromatic rings) is characterized for n =1–7. Experiments were carried out in solution at ambient temperature, under matrix-isolated conditions (dispersed in a 2-methyltetrahydrofuran (2MeTHF) glass) at low temperatures (10 and 80 K) and as spin coated thin layers on quartz substrates at various temperatures. The investigations involve absorption and emission as well as site-selective fluorescence (SSF) spectroscopy. DHepO-OPV n s ( n ⩾2) form nanoaggregates and the tendency of the formation increases with increasing concentration and chain length and decreases with increasing temperature. Absorption spectra of isolated chains as well as aggregated species are simulated and agree nicely with the experimental data. Furthermore, the absorption spectra of the aggregates can be correlated with the spectra of the corresponding spin coated oligomer on a quartz substrate. The singlet (S 1 ←S 0 ) transition energies decrease with increasing conjugation length and converge for the longer oligomers towards a limiting value. The Huang–Rhys parameter S associated with the S 1 →S 0 electronic transition and its evolution with conjugation length has been determined via analysis of the vibronic fine structure. At low temperatures, the values determined can be attributed to aggregate formation. At ambient temperature, S decreases with increasing chain length. Low-temperature SSF spectroscopy is employed to study morphological effects on the conformation of DHepO-OPV3. A vanishing Stokes shift is recorded for the resonantly excited fluorescence spectra.

Spectroscopic characterization of p-phenylene vinylene (PV) oligomers. Part II: Selected 2,5-diheptyl substituted PV-oligomers

Selected, metathetically synthesized, monodisperse all-trans oligomers of 2,5-diheptyl-p-divinylbenzene (DHep-OPVns, with n the number of aromatic rings) are investigated for n=1–6 in solution at (i) ambient temperature, (ii) matrix isolated in a 2MeTHF glass at 10 and 80 K and (iii) as spin coated thin layers on quartz substrates. These materials are compared to their diheptyloxy substituted analogs (DHepO-OPVs), described in the preceding paper, involving absorption and emission as well as site-selective fluorescence (SSF) spectroscopy at various temperatures and as a function of concentration and chain length. Due to both, lower electron density along the conjugated backbone and the presence of methylene groups, which are directly linked to the aromatic rings, the diheptyl substituted oligomers adopt a less planar conformation and, concomitantly, are more disordered as evidenced by the larger inhomogeneous spectral broadening. Consequently, their tendency toward formation of nanoaggregates, though occurring in solution and matrix isolated, is diminished. For both types of oligomers, the tendency of aggregation increases with increasing concentration and chain length and decreases with increasing temperature. Site-selective fluorescence (SSF) at low temperature bear out a finite Stokes shift of 270 and 260 cm−1 for the diheptyl substituted DHep-OPV3 and DHep-OPV6, respectively, while in the case of DHepO-OPV3, the Stokes shift is vanishing small. This shows that rigidity of the oligomer backbone and molecular relaxation upon excitation are correlated.

Site-selective fluorescence spectroscopy of Eu 3+ and Sm 2+ ions in glass

Fluorescence line–narrowing or site-selective fluorescence experiments on Eu 3+ and Sm 2+ ions in glass are reviewed. It is shown that this technique gives detailed information that is difficult to obtain by other means, such as site-dependent homogeneous widths, inhomogeneous broadening mechanisms, distribution of crystal-field components in glass, and so on. A simple theoretical treatment to consider only the second-order terms of the crystal-field potential is discussed and is shown to be very effective. It will be emphasized that J-mixing, i.e., wave function mixing between 4f N states through the static crystal-field potential, plays a significant role in the optical properties of rare-earth ions in glass.

Spectroscopic properties of PSI–IsiA supercomplexes from the cyanobacterium Synechococcus PCC 7942

The cyanobacterium Synechococcus PCC 7942 grown under iron starvation assembles a supercomplex consisting of a trimeric Photosystem I (PSI) complex encircled by a ring of 18 CP43′ or IsiA light-harvesting complexes [Nature 412 (2001) 745]. Here we present a spectroscopic characterization by temperature-dependent absorption and fluorescence spectroscopy, site-selective fluorescence spectroscopy at 5 K, and circular dichroism of isolated PSI–IsiA, PSI and IsiA complexes from this cyanobacterium grown under iron starvation. The results suggest that the IsiA ring increases the absorption cross-section of PSI by about 100%. Each IsiA subunit binds about 16–17 chlorophyll a (Chl a) molecules and serves as an efficient antenna for PSI. Each of the monomers of the trimeric PSI complex contains two red chlorophylls, which presumably give rise to one exciton-coupled dimer and at 5 K absorb and fluoresce at 703 and 713 nm, respectively. The spectral properties of these C-703 chlorophylls are not affected by the presence of the IsiA antenna ring. The spectroscopic properties of the purified IsiA complexes are similar to those of the related CP43 complex from plants, except that the characteristic narrow absorption band of CP43 at 682.5 nm is missing in IsiA.

Nonradiative relaxation and inhomogeneous splitting of aggregated optical centers in the Nd 3+-doped CaF 2 and SrF 2 crystals (FLN and decay study)

Site-selective fluorescence and laser excitation spectroscopy for different types of optical centers in SrF 2 : Nd 3+ (0.3 – 1.0%) and SrF 2 : La 3+ (1.0%) : Nd 3+ (0.2%) were performed at 4.2 K. The crystal-field levels of the 4I 9/2, 4F 3/2 , and 4G 5/2 multiplets for different Nd 3+ optical centers were determined in SrF 2 : Nd 3+ (0.3%). An attempt was made to identify some of the new optical centers on the basis of their fluorescence lifetimes and crystal-field splitting. Some models of these centers have been discussed. Two–six times reduction of the 4F 3/2 multiplet lifetime was observed in the pair M-center in comparison with the single optical centers due to energy cross-relaxation in the pair. A fine splitting of the levels at the 4I 9/2(1)→ 4G 5/2(1) crystal-field transition (1.5 cm −1) similar to that in CaF 2 (2.4 cm −1) was found for the Nd–Nd pair in SrF 2. The variation of the splitting from CaF 2 to SrF 2 nicely demonstrates the R −5 dependence of coherent quadrupole–quadrupole interaction.

Site-Selective Fluorescence Spectroscopy of Conjugated Polymers and Oligomers

Site-Selective Fluorescence Spectroscopy of Conjugated Polymers and Oligomers

Investigation of local structure of glass-forming liquids by fluorescence of doped dye molecules

The weighted density of states of low-frequency vibrational modes has been determined in 1-propanol doped with chlorophyll b by means of site-selective fluorescence spectroscopy. The resonance fluorescence spectra calculated have been found not to reproduce the measured ones even below the glass transition temperature of the solvent. This result is discussed on the basis of the anharmonic character of the vibrational modes.

Polarized site-selective fluorescence spectroscopy of the long-wavelength emitting chlorophylls in isolated Photosystem I particles of Synechococcus elongatus

Isolated trimeric Photosystem I complexes of the cyanobacterium Synechococcus elongatus have been studied with absorption spectroscopy and site-selective polarized fluorescence spectroscopy at cryogenic temperatures. The 4 K absorption spectrum exhibits a clear and distinct peak at 710 nm and shoulders near 720, 698 and 692 nm apart from the strong absorption profile located at 680 nm. Deconvoluting the 4 K absorption spectrum with Gaussian components revealed that Synechococcus elongatus contains two types of long-wavelength pigments peaking at 708 nm and 719 nm, which we denoted C-708 and C-719, respectively. An estimate of the oscillator strengths revealed that Synechococcus elongatus contains about 4-5 C-708 pigments and 5-6 C-719 pigments. At 4 K and for excitation wavelengths shorter than 712 nm, the emission maximum appeared at 731 nm. For excitation wavelengths longer than 712 nm, the emission maximum shifted to the red, and for excitation in the far red edge of the absorption spectrum the emission maximum was observed 10-11 nm to the red with respect to the excitation wavelength, which indicates that the Stokes shift of C-719 is 10-11 nm. The fluorescence anisotropy, as calculated in the emission maximum, reached a maximal anisotropy of r=0.35 for excitation in the far red edge of the absorption spectrum (at and above 730 nm), and showed a complicated behavior for excitation at shorter wavelengths. The results suggest efficient energy transfer routes between C-708 and C-719 pigments and also among the C-719 pigments.

Site-selective fluorescence spectroscopy of metal porphyrins in noncrystalline materials

Site-selective fluorescence spectroscopy has been performed for centro-symmetric metal porphyrins doped in polymer films and for Zn-substituted myoglobin at low temperatures. It has been confirmed that the degeneracy of the excited singlet S 1 state is lifted by the interaction between the guest molecule and its environment.

Determination of weighted density of states of vibrational modes in Zn-substituted myoglobin

Site-selective fluorescence spectroscopy has been performed for Zn-substituted myoglobin in the lowest optical absorption band at 4 K. The single-site fluorescence spectrum as well as the distribution of the zero-phonon transition energy have been obtained. Furthermore, the density of states of vibrational modes of myoglobin weighted by the coupling strength between the chromophore and the polypeptide chain has been determined from the analysis of the obtained spectrum. This is the first case in which the weighted density of states of low-frequency vibrational modes has been determined in biological materials without employing any model shape functions. The results are compared with those for dye-doped polymers.

Site-selective fluorescence spectroscopy in dye-doped polymers. II. Determination of the weighted density of states of the vibrational modes.

The density of states of vibrational modes weighted by the coupling strength between the guest molecule and the host polymer has been obtained in dye-doped polymer films by solving an integral equation numerically using experimentally determined single-site fluorescence spectra at 4 K. The temperature dependence of the laser-induced fluorescence spectrum has been found to be reproduced by a simulation based on the linear electron-phonon coupling theory using the determined weighted density of ``phonon'' states. This result supports the validity of our procedures of experiment and analysis. The spectral profiles of the obtained weighted density of states of the vibrational modes will be discussed.

Site-selective fluorescence spectroscopy in dye-doped polymers. I. Determination of the site-energy distribution and the single-site fluorescence spectrum.

Laser-induced fluorescence spectra have been measured, including narrow resonance fluorescence lines appearing at the same energy as the exciting light, for dye molecules doped in polymer films at low temperatures. This has been made possible by the combination of short light pulses from a cw mode-locked laser for the excitation and a time-correlated single-photon counting method for the detection. The distribution of the number of the molecules as a function of the energy of the zero-phonon line in the sample has been obtained from the excitation profile of the resonance fluorescence line. The fluorescence spectra of dye molecules in a single site have also been determined by making use of the saturation effect of the laser-induced fluorescence. The procedures of the experiment and analysis are reported in detail.

Conformational effects in poly(p-phenylene vinylene)s revealed by low-temperature site-selective fluorescence

Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.