Suchergebnisse

Energy transfer between chromium and neodymium ions in a mixed fluorophosphate glass [21.74Al(PO3)3-57.8BaF2-16.96AlF3] has been investigated in the 4.2-300 K temperature range by using steady-state and time-resolved laser spectroscopy. Radiative and nonradiative energy transfer has been demonstrated from the time-resolved emission spectra and the decrease of the Cr3+ lifetimes. Comparison between time-resolved emission spectra for Cr3+ singly doped and codoped samples with different Nd3+ concentrations shows that the radiative transfer is linearly dependent on Nd3+ concentration. The nonradiative energy transfer is consistent with an electric-dipole-electric-dipole interaction mechanism. Good agreement is found if transfer efficiency corrected for Nd-Nd self-quenching is compared with the measured sensitized Nd3+ luminescence. The transfer efficiency of a Cr3+, Nd3+ system in the investigated fluorophosphate glass has also been compared with previous results in a pure fluoride glass giving a higher efficiency for the fluorophosphate glass if the Nd2O3 concentration is kept below 2 wt %. © 1993 The American Physical Society. ; This work was supported by the Comision Interministerial de Ciencia y Tecnologia (CICYT) of the Spanish Government (Ref. 0434/93), Basque Country Government (Ref. PGV 9012), and Basque Country University (Ref. E126/91). ; Peer Reviewed

The role of carotenoids in chlorosomes of the green sulfur bacterium Chlorobium phaeobacteroides, containing bacteriochlorophyll (BChl) e and the carotenoid (Car) isorenieratene as main pigments, was studied by steady-state fluorescence excitation, picosecond single-photon timing and femtosecond transient absorption (TA) spectroscopy. In order to obtain information about energy transfer from Cars in this photosynthetic light-harvesting antenna with high spectral overlap between Cars and BChls, Car-depleted chlorosomes, obtained by inhibition of Car biosynthesis by 2-hydroxybiphenyl, were employed in a comparative study with control chlorosomes. Excitation spectra measured at room temperature give an efficiency of 60-70% for the excitation energy transfer from Cars to BChls in control chlorosomes. Femtosecond TA measurements enabled an identification of the excited state absorption band of Cars and the lifetime of their S-1 state was determined to be similar to10 ps. Based on this lifetime, we concluded that the involvement of this state in energy transfer is unlikely. Furthermore, evidence was obtained for the presence of an ultrafast (<100 fs) energy transfer process from the S-2 state of Cars to BChls in control chlorosomes. Using two time-resolved techniques, we further found that the absence of Cars leads to overall slower decay kinetics probed within the Qy band of BChl e aggregates, and that two time constants are generally required to describe energy transfer from aggregated BChl e to baseplate BChl a. ; Swedish Natural Science Research Council. European Union (Contract FMRX-CT96-0081). Kempe Foundation ; Peer reviewed

The phytohormone gibberellin (GA) is a small, mobile signaling molecule that plays a key role in seed germination, cellular elongation, and developmental transitions in plants. Gibberellin Perception Sensor 1 (GPS1) is the first Förster resonance energy transfer (FRET)-based biosensor that allows monitoring of cellular GA levels in vivo. By measuring a fluorescence emission ratio of nuclear localized-GPS1 (nlsGPS1), spatiotemporal mapping of endogenously and exogenously supplied GA gradients in different tissue types is feasible at a cellular scale. This protocol will describe how to image nlsGPS1 emission ratios in three example experiments: steady-state, before-and-after exogenous gibberellin A4 (GA4) treatments, and over a treatment time-course. We also provide methods to analyze nlsGPS1 emission ratios using both Fiji and a commercial three-dimensional (3-D) micrograph visualization and analysis software and explain the limitations and likely pitfalls of using nlsGPS1 to quantify gibberellin levels. ; European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement n° 759282)

Fluorescence lifetime microscopy (FLIM) and Förster's resonance energy transfer (FRET) are advanced optical tools that neuroscientists can employ to interrogate the structure and function of complex biological systems in vitro and in vivo using light. In neurobiology they are primarily used to study protein-protein interactions, to study conformational changes in protein complexes, and to monitor genetically encoded FRET-based biosensors. These methods are ideally suited to optically monitor changes in neurons that are triggered optogenetically. Utilization of this technique by neuroscientists has been limited, since a broad understanding of FLIM and FRET requires familiarity with the interactions of light and matter on a quantum mechanical level, and because the ultra-fast instrumentation used to measure fluorescent lifetimes and resonance energy transfer are more at home in a physics lab than in a biology lab. In this overview, we aim to help neuroscientists overcome these obstacles and thus feel more comfortable with the FLIM-FRET method. Our goal is to aid researchers in the neuroscience community to achieve a better understanding of the fundamentals of FLIM-FRET and encourage them to fully leverage its powerful ability as a research tool. Published 2020. U.S. Government.

The ligand HL (6-[3-oxo-3-(2-hydroxyphenyl)propionyl]pyridine-2-carboxylic acid), which exhibits two different coordination pockets, has been exploited to engender and study energy transfer (ET) in two dinuclear [LnLn′] analogues of interest, [EuYb] and [NdYb]. Their structural and physical properties have been compared with newly synthesised analogues featuring no possible ET ([EuLu], [NdLu], and [GdYb]) and with the corresponding homometallic [EuEu] and [NdNd] analogues, which have been previously reported. Photophysical data suggest that ET between Eu and Yb does not occur to a significant extent, whereas emission from Yb originates from sensitisation of the ligand. In contrast, energy migration seems to be occurring between the two Nd centres in [NdNd], as well as in [NdYb], in which Yb luminescence is thus, in part, sensitised by ET from Nd. This study shows the versatility of this molecular platform to further the investigation of lanthanide-to-lanthanide ET phenomena in defined molecular systems. ; This research was supported by the Spanish MINECO through grants CTQ2015-68370-P and PGC2018-098630-B-I00 (G.A., D.A. and V.V.) and MAT2017-86826-R (O.R.), as well as through the Juan de la Cierva program IJCI-2016-29901 (D.A.) and the Aragón government (DGA, consolidated group PLATON E31_17R). G.A. thanks the Generalitat de Catalunya for the ICREA Academia 2018 prize and QUANTERA for project SUMO (through Spanish grant PCI2018-093106). This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231.

Polymethylene-linked bipyrimidine models have been designed with different C5 substitutions and bridge lengths. Selective irradiation of 2′-methoxyacetophenone (2M) with the bipyrimidine models affords cyclobutane pyrimidine dimers, even in the presence of bulky substituents. Substitution at C5 affects both the relative triplet energies (E) of the pyrimidines (Pyr) and the steric hindrance toward intermolecular energy transfer and intramolecular triplet Pyr∗ quenching. Photophysical studies showed that alkyl substitution resulted in a significant decrease in the E value. Quenching of the triplet excited state of 2M by the Pyr derivatives was proven and established their quenching rate constants (k). As a general trend, the thymine-containing compounds showed k values higher than 10 M s, while in the uracil and tert-butyluracil analogues, k was markedly lower. These data are explained considering three different scenarios: (a) triplet energy transfer is the rate controlling step, (b) excited state cyclization is the rate controlling step, and (c) the rate controlling step switches along the reaction. Thus, by introducing variations in the substitution at C5, the length of the linking bridge, or the substrate concentration, it is possible to switch from a process governed by the intrinsic dimerization step to an energy transfer-controlled process. ; Financial support by the Spanish Government (PGC2018-096684 and O.R.A. for FPU14/05294) and GeneralitatValenciana (PROMETEO/2017/075) is gratefully acknowl-edged.

[EN] Polymethylene-linked bipyrimidine models have been designed with different C5 substitutions and bridge lengths. Selective irradiation of 2'-methoxyacetophenone (2M) with the bipyrimidine models affords cyclobutane pyrimidine dimers, even in the presence of bulky substituents. Substitution at CS affects both the relative triplet energies (E-T(rel)) of the pyrimidines (Pyr) and the steric hindrance toward intermolecular energy transfer and intramolecular triplet Pyr* quenching. Photophysical studies showed that alkyl substitution resulted in a significant decrease in the E-T(rel) value. Quenching of the triplet excited state of 2M by the Pyr derivatives was proven and established their quenching rate constants (k(q)). As a general trend, the thymine-containing compounds showed k(q) values higher than 10(9) M(-1)s(-1), while in the uracil and tert-butyluracil analogues, k(q) was markedly lower. These data are explained considering three different scenarios: (a) triplet energy transfer is the rate controlling step, (b) excited state cyclization is the rate controlling step, and (c) the rate controlling step switches along the reaction. Thus, by introducing variations in the substitution at C5, the length of the linking bridge, or the substrate concentration, it is possible to switch from a process governed by the intrinsic dimerization step to an energy transfer-controlled process. ; Financial support by the Spanish Government (PGC2018-096684 and O.R.A. for FPU14/05294) and Generalitat Valenciana (PROMETEO/2017/075) is gratefully acknowledged. ; Rodriguez-Alzueta, O.; Cuquerella Alabort, MC.; Miranda Alonso, MÁ. (2019). Triplet Energy Transfer versus Excited State Cyclization as the Controlling Step in Photosensitized Bipyrimidine Dimerization. The Journal of Organic Chemistry. 84(21):13329-13335. https://doi.org/10.1021/acs.joc.9b01423 ; S ; 13329 ; 13335 ; 84 ; 21

Excitonic coupling, electronic coupling, and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a red-shifted collective emission peak with accelerated dynamics. Here we report that similar spectroscopic features could appear as a result of the nanocrystal reactivity within the self-assembled superlattices. This is demonstrated by studying CsPbBr3 nanocrystal superlattices over time with room-temperature and cryogenic micro-photoluminescence spectroscopy, X-ray diffraction, and electron microscopy. It is shown that a gradual contraction of the superlattices and subsequent coalescence of the nanocrystals occurs over several days of keeping such structures under vacuum. As a result, a narrow, low-energy emission peak is observed at 4 K with a concomitant shortening of the photoluminescence lifetime due to the energy transfer between nanocrystals. When exposed to air, self-assembled CsPbBr3 nanocrystals develop bulk-like CsPbBr3 particles on top of the superlattices. At 4 K, these particles produce a distribution of narrow, low-energy emission peaks with short lifetimes and excitation fluence-dependent, oscillatory decays. Overall, the aging of CsPbBr3 nanocrystal assemblies dramatically alters their emission properties and that should not be overlooked when studying collective optoelectronic phenomena nor confused with superfluorescence effects. ; The work of D.B. was supported by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 794560 (RETAIN). R.X.Y. and L.Z.T. were supported by the Molecular Foundry, a DOE Office of Science User Facility of the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. D.S. acknowledges support from the project PRIN Interacting Photons in Polariton Circuits—INPhoPOL (Ministry of University and Scientific Research, MIUR, 2017P9FJBS_001). We thank P. Cazzato, L. De Marco, D. Ballarini, D. G. Suárez-Forero, V. Ardizzone, L. ...