Journal of Photochemistry and Photobiology A: Chemistry
HIGHLIGHTS The luminescence decays of solid samples are often complex due in part to the presence... more HIGHLIGHTS The luminescence decays of solid samples are often complex due in part to the presence of defects. In the case of these TADF molecular solids, the analysis of the decays after the addition of defects provides the quenching distance of the triplet and singlet states. ABSTRACT Nowadays, Thermally Activated Delayed Fluorescence (TADF) compounds have proven to be attractive for highly efficient optoelectronic devices as they can exploit both singlet and triplet excited states generated by charge recombinations that occur in such devices. However, quenching of excited states limits such benefits at high current densities. We have studied the quenching in the case of cationic copper(I) complexes coordinated by both Nheterolytic carbene (NHC) and 2,2'-dipyridylamine (dpa) ligands. Defects are created in their crystals by a strong laser irradiation. Then we have investigated, by analysing their fluorescence decay as a function of temperature, the quenching mechanism and the exciton mobility. Using a time-resolved version of the Perrin's quenching model, we show that the quenching volume depends on temperature according to the singlet/triplet equilibrium. At room temperature, the singlet states are responsible for 75% of the quenching although they contribute for 2% to the excited states population. From the analysis of the decay curves, we show that the excitons are not mobile in the crystals and are quenched through Förster Resonant Energy Transfer (FRET).
Six polyfluorene derivatives, P1-P6, were synthesized and investigated as responsive materials fo... more Six polyfluorene derivatives, P1-P6, were synthesized and investigated as responsive materials for the optical sensing of metal ions in an aqueous medium. They were designed by combining carbazole with fluorene units within the backbone. Carbazole was N-functionalized with three coordinating groups, 2-pyridyl-benzimidazole (P1 and P4), 2-phenyl-benzimidazole (P2 and P5) and 4-phenyl-terpyridyl (P3 and P6), respectively.
ABSTRACT Four novel phenyl-end-capped p-conjugated polymers comprising alkyl chains of different ... more ABSTRACT Four novel phenyl-end-capped p-conjugated polymers comprising alkyl chains of different lengths were synthesized: the poly(9,9-dihexyl-2,7-fluorene-alt-9,9-dioctylfluorene) (P0), the alternating fluorene– carbazole copolymers P1 and P2 comprising 20% and 50% of carbazole, respectively, and the homopolymer poly(9-hexyl-9H-carbazole) (P3). The non-end-capped alternating fluorene–carbazole copolymer P4 comprising 20% of carbazole was also synthesized for the sake of comparison with P1. P0–P2 and P4 were synthesized by a Pd-catalyzed Suzuki–Miyaura coupling in good yields (80–89%), while P3 was obtained by a Ni-catalyzed Yamamoto coupling reaction from the 3,6-dibromocarbazole monomer in a moderate yield (52%). P0–P4 polymers were characterized by NMR, elemental analysis, and GPC. The molecular weights are 40.30, 23.42, 14.33, 3.92, and 37.49 kDa, with polydispersity indices of 2.5, 1.7, 1.8, 1.3, and 2.6, for P0, P1, P2, P3, and P4, respectively. These polymers were found to show a high thermal stability, with decomposition temperatures in the range of 395–420 C, and the glass transition temperature was found to regularly increase with the amount of carbazole inserted in the conjugated backbone. AFM images obtained on thin films (thickness of about 90 nm) of P0–P2 revealed films with surfaces of good quality, being homogeneous with low roughness (0.2 nm for the smaller ones). These polymers were found to be blue-emitting both in diluted dichloromethane and chlorobenzene solutions as well as in thin films and exhibit relatively high values of the absolute quantum yields in the range of 100–5.5% in dichloromethane and 51.4–7.7% in thin films. Blue-emitting electroluminescent devices were obtained with P0 and P1 as emitting layers, respectively. The device built with P1 showed improved performances (EQE of 1.32%) with respect to the one built with the parent polyfluorene material (EQE of 0.75%).
Calcium green I, a ratiometric probe based on fluorescence lifetime measurements, was used to mon... more Calcium green I, a ratiometric probe based on fluorescence lifetime measurements, was used to monitor intracellular calcium activity ([Ca2+]1) in RINm5F cells using a time‐resolved fluorescence confocal microscope. The probe affinity constant has been recalibrated in single cells using ionomycin as a calcium ionophore and eth‐ylenebis(oxyethylenenitrilo)tetraacetic acid as a calcium buffer; KA was found to equal 150 nmol/L. The kinetics of ionomycin equilibration showed that the calcium release from calcium stores occurs before equilibration with extracellular calcium. The response to the muscarinic agonist carbachol, measured on 17 cells receiving three consecutive applications was characterized both by a [Ca2+]1 peak lasting 50 s without any trailing plateau and by desensitization with a 30% decrease in the response. The dose‐dependent response was obtained for a carbachol concentration from 5 μmol/L to 0.5 mmol/L. The ability of our set‐up to obtain a value every 10 ms enabled us...
Physical parameters, describing the state of chromatinized DNA in living mammalian cells, were re... more Physical parameters, describing the state of chromatinized DNA in living mammalian cells, were revealed by in situ fluorescence dynamic properties of ethidium in its free and intercalated states. The lifetimes and anisotropy decays of this cationic chromophore were measured within the nuclear domain, by using the ultra-sensitive time-correlated single-photon counting technique, confocal microscopy, and ultra-low probe concentrations. We found that, in living cells: 1) free ethidium molecules equilibrate between extracellular milieu and nucleus, demonstrating that the cation is naturally transported into the nucleus; 2) the intercalation of ethidium into chromatinized DNA is strongly inhibited, with relaxation of the inhibition after mild (digitonin) cell treatment; 3) intercalation sites are likely to be located in chromatin DNA; and 4) the fluorescence anisotropy relaxation of intercalated molecules is very slow. The combination of fluorescence kinetic and fluorescence anisotropy dynamics indicates that the torsional dynamics of nuclear DNA is highly restrained in living cells.
HAL (Le Centre pour la Communication Scientifique Directe), Aug 15, 2007
While the use of quantum dot (QD) nanoparticles for bioimaging and sensing has been improved and ... more While the use of quantum dot (QD) nanoparticles for bioimaging and sensing has been improved and exploited during the last several years, most studies have used emission intensity-based techniques. Fluorescence lifetime imaging microscopy (FLIM) can also be employed for sensing purposes, overcoming many of the limitations of the aforementioned systems. Herein, we show that the photoluminescence (PL) lifetime of mercaptopropionic acid-capped QDs (MPA-QDs) collected from FLIM images can be used to determine intracellular pH. The PL average lifetime of MPA-QDs varied from 8.7 ns (pH < 5) to 15.4 ns (pH > 8) in media mimicking the intracellular environment. These long decay times of QD nanoparticles make them easily distinguishable from intrinsic cell autofluorescence, improving selectivity in sensing applications. We demonstrate, for the first time, the successful detection of changes in the intracellular pH of different cell types by examining the PL decay time of QDs. In particular, the combination of FLIM methodologies with QD nanoparticles exhibits greatly improved sensitivity compared with other fluorescent dyes for pH imaging. A detailed description of the advantages of the FLIM technique is presented.
Polyaromatic dyes were used to synthesize molecular nanocrystals in sol-gel thin films for sensor... more Polyaromatic dyes were used to synthesize molecular nanocrystals in sol-gel thin films for sensor applications. Fluorescence Confocal Microscopy (FCM) and Transmission Electron Microscopy (TEM) experiments showed the advantages of our nanocrystallization process compared with microcrystallization in free solutions. Indeed, we obtained well-dispersed and spherical nanocrystals with a narrow size distribution, exhibiting a good crystallinity. Time-resolved fluorescence spectroscopy allowed us to measure fluorescence lifetimes of nanocrystals in presence of molecular probes. Then, chemical sensoring properties of these molecular nanocrystals were demonstrated.
Since Helium-3 shortage announcement, organic scintillators play a major role in neutron detectio... more Since Helium-3 shortage announcement, organic scintillators play a major role in neutron detection. Our laboratory decided to focus on plastic scintillators and their ability to discriminate fast neutrons from gamma rays. In this work, we highlight the influence of the secondary fluorophore in lab-made plastic scintillators. The secondary fluorophore is generally added in the scintillating mixture to shift the emission wavelength towards the transparency domain of the material to improve its attenuation length. Thus, it is considered as a harmless molecule and is barely seen as a key criterion that could enhance the performances of the organic scintillator. In our work, we demonstrate that this molecule, even added at a small concentration (typically in the range 0.02-0.2 wt%), directly impacts the neutron/gamma discrimination ability of plastics. Not only the secondary fluorophore plays a role in self-absorption of the scintillating material, but also the couple it creates with the primary fluorophore has to be carefully chosen, as some specific triplet energy transfers between the two fluorophores can influence the neutron/gamma discrimination abilities of the plastic scintillator. Aside from classical photophysical inconveniences such as self-absorption or diffusion, the possibility that the whole, bulk material was heterogeneous in a photochemical point of view was also checked. Thus, various samples were cut from bulk monoliths and their pulse shape discrimination compared with the parent scintillator in terms of Figure of Merit and light output. Keywords n/γ discrimination; plastic scintillators; secondary fluorophore; self-absorption; triplet-triplet annihilation. Highlights Fast neutrons/gamma discrimination efficiency is usually reduced when the volume of a plastic scintillator is increased. This effect is studied, first in a morphologically way, then chemically and photophysically.
Chemistry (Weinheim an der Bergstrasse, Germany), Jan 13, 2016
The synthesis, photophysical properties, and applications in scintillation counting of N-(2-ethyl... more The synthesis, photophysical properties, and applications in scintillation counting of N-(2-ethylhexyl)carbazole (EHCz) are reported. This molecule displays all of the required characteristics for an efficient liquid scintillator (emission wavelength, scintillation yield), and can be used without any extra fluorophores. Thus, its scintillation properties are discussed, as well as its fast neutron/gamma discrimination. For the latter application, the material is compared with the traditional liquid scintillator BC-501 A, and other liquid fluorescent molecules classically used as scintillation solvents, such as xylene, pseudocumene (PC), linear alkylbenzenes (LAB), diisopropylnaphthalene (DIN), 1-methylnaphthalene (1-MeNapht), and 4-isopropylbiphenyl (iPrBiph). For the first time, an excimeric form of a molecule has been advantageously used in scintillation counting. A moderate discrimination between fast neutrons and gamma rays was observed in bulk EHCz, with an apparent neutron/gamm...
This study presents the influence of various substituents on the photophysical features of hetero... more This study presents the influence of various substituents on the photophysical features of heteroleptic copper(I) complexes bearing both N-heterocyclic carbene (NHC) and dipyridylamine (dpa = dipyridylamine skeleton corresponding to ligand L1) ligands. The luminescent properties have been compared to our recently reported archetypal blue emitting [Cu(IPr)(dpa)][PF6] complex. The choice of the substituents on both ligands has been guided to explore the effect of the electron donor/acceptor and "push-pull" on the emission wavelengths and photoluminescence quantum yields. A selection of the best candidates in terms of their photophysical features were applied for developing the first blue lightemitting electrochemical cells (LECs) based on copper(I) complexes. The device analysis suggests that the main concern is the moderate redox stability of the complexes under high applied driving currents, leading to devices with moderate stabilities pointing to a proof-of-concept for further developments. Nevertheless, under low applied driving currents the blue emission is stable, showing performance levels competitive to those reported for blue LECs based on iridium(III) complexes. Overall, this work provides valuable guidelines to tackle the design of enhanced NHC complexes for lighting applications in the near future.
Four new hindered boron-dipyrromethene (BODIPY) dyes have been engineered and synthesized by modi... more Four new hindered boron-dipyrromethene (BODIPY) dyes have been engineered and synthesized by modification of the pyrrole substituents. The chromophores were designed to shift their absorption and emission wavelengths to the red and to show quantitative emission in solid amorphous deposits. [2.2]Paracyclophane (PcP) was grafted on the BODIPY core as well as alkyl chains of various length to force the molecules away one from each other and prevent π-π stacking. The optical spectroscopic and electrochemical properties in solution are presented. Absorption, emission and FLIM were done on drop-casted films of the fluorophores. The addition of alkyl substituents (-Me,-Et, n-Pr) next to the PcP group clearly affects the emission properties in solid state since the dyes display sharper emission bands than in solution and their relative fluorescence quantum yield increases with the number of carbons in the alkyl chain. Fluorescence lifetime images analysis reveals complex decay profiles. Quite a fair description is given by a biexponential fit: the short component is attributed to aggregates and the long component to monomer-like molecules. This population shows a shorter lifetime (2-3.9 ns) compared to solution (6 ns) due to a quenching by aggregates. This quenching is decreasing as the number of carbons on the alkyl side chain is increasing. The combination of a PcP group next to a n-Pr group on a BODIPY is a good combination to get an excellent fluorophore in the solid state.
Photochemical & Photobiological Sciences, 2006
Latex nanoparticles functionalized with cyclam (1,4,8,11-tetraazacyclotetradecane), a copper chel... more Latex nanoparticles functionalized with cyclam (1,4,8,11-tetraazacyclotetradecane), a copper chelator, have been doped with a fluorescent dye 1 (BODIPY derivative: 4,4-difluoro-8-(2 ,4 ,6-trimethyl)phenyl-2,6-diethyl-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene). The bulky, hydrophobic fluorophore 1 dissolves within the nanoparticles' polymer core up to a concentration of about 88.4 lmol g −1. At this concentration the fluorescence yield is about 0.80. Adding Cu 2+ ions to the solution decreases the fluorescence because of the energy transfer between the dye and the violet copper cyclam complexes. The response is fast: 90% of the quenching occurs within 1 s. The Cu 2+ detection threshold is of 1 nanomolar. No interferences were observed with zinc and nickel ions.
We have analyzed the decays of the fluorescence of colloidal CdSe quantum dots (QDs) suspensions ... more We have analyzed the decays of the fluorescence of colloidal CdSe quantum dots (QDs) suspensions during dilution and titration by the ligands. A ligand shell made of a combination of trioctylphosphine (TOP), oleylamine (OA) and stearic acid (SA) stabilizes the as-synthesized QDs. The composition of the shell was analyzed and quantified using high resolution liquid state 1H nuclear magnetic resonance (NMR) spectroscopy. A quenching of the fluorescence of the QDs is observed upon removal of the ligands by diluting the QDs mother solution. The fluorescence is restored by the addition of TOP. We analyze the results by assuming a binomial distribution of quenchers among the QDs and predict a linear trend in the time resolved fluorescence decays. We have used a non parametric analysis to show that for our QDs, 2.97±0.1 quenching sites per QD on average are revealed by the removal of TOP. We moreover show that the quenching rates of the quenching sites add up. The decay per quenching site can be compared with the decay at saturation of the dilution effect. This provides a value of 2.88±0.02 for the number of quenchers per QD. We extract the quenching dynamics of one site. It appears to be a process with a distribution of rates that does not involve the ligands.
This study provides a rigorous analysis of MOF-5 fluorescence, unifying previously dissimilar res... more This study provides a rigorous analysis of MOF-5 fluorescence, unifying previously dissimilar results. Pure MOF-5 fluorescence was identified as only ligand centered and the rise of other mode of emission were observed when exposed to humidity.
This study provides a rigorous analysis of MOF-5 fluorescence, unifying previously dissimilar res... more This study provides a rigorous analysis of MOF-5 fluorescence, unifying previously dissimilar results. Pure MOF-5 fluorescence was identified as only ligand centered and the rise of other mode of emission were observed when exposed to humidity.
... 155. [31] DE Henn, J. Appl. Cryst., 4 (1971) 256. [32] JB Birks, (ed.) Photophysics of aromat... more ... 155. [31] DE Henn, J. Appl. Cryst., 4 (1971) 256. [32] JB Birks, (ed.) Photophysics of aromatic molecules, 1969. [33] A. Tessier, R. M6allet-Renault, P. Denjean, D. Miller, and RB Pansu, Phys. Chem. Chem. Phys., 1 (1999) 5767. ...
The absolute measurement of calcium activity in cells is hindered by the presence of di †erent fo... more The absolute measurement of calcium activity in cells is hindered by the presence of di †erent forms of the probe (adsorbed, protonated). Time-resolved Ñuorescence spectroscopy can be a sensitive technique to separate contaminating forms of the probe. The Ñuorescent probe Calcium Green 1 was characterised, in vitro, in terms of calcium and also pH dependence, albumin adsorption, extent of hydrolysis, viscosity and medium polarity. A 1 : 1 stoichiometry was obtained for the calcium complexation with a value that agrees with reported K d values. A of the BAPTA moiety was measured as 6.7. The Ñuorescence lifetimes are slightly dependent on pK a polarity but not on viscosity. It was observed that most of the interfering forms of the probe share the common lifetimes of 0.5 and 3.5 ns. An alternative mechanism for Calcium Green 1 sensing is proposed based on a ground state conformational equilibrium a †ecting the photoinduced electron transfer.
A single photon counting system for FLIM and fluorescence anisotropy wide-field imaging is descri... more A single photon counting system for FLIM and fluorescence anisotropy wide-field imaging is described and characterized in this paper. The two polarizations of the fluorescence are divided by a Glan prism and acquired at the same time by the Q A detector. Fluorescence
Journal of Photochemistry and Photobiology A: Chemistry
HIGHLIGHTS The luminescence decays of solid samples are often complex due in part to the presence... more HIGHLIGHTS The luminescence decays of solid samples are often complex due in part to the presence of defects. In the case of these TADF molecular solids, the analysis of the decays after the addition of defects provides the quenching distance of the triplet and singlet states. ABSTRACT Nowadays, Thermally Activated Delayed Fluorescence (TADF) compounds have proven to be attractive for highly efficient optoelectronic devices as they can exploit both singlet and triplet excited states generated by charge recombinations that occur in such devices. However, quenching of excited states limits such benefits at high current densities. We have studied the quenching in the case of cationic copper(I) complexes coordinated by both Nheterolytic carbene (NHC) and 2,2'-dipyridylamine (dpa) ligands. Defects are created in their crystals by a strong laser irradiation. Then we have investigated, by analysing their fluorescence decay as a function of temperature, the quenching mechanism and the exciton mobility. Using a time-resolved version of the Perrin's quenching model, we show that the quenching volume depends on temperature according to the singlet/triplet equilibrium. At room temperature, the singlet states are responsible for 75% of the quenching although they contribute for 2% to the excited states population. From the analysis of the decay curves, we show that the excitons are not mobile in the crystals and are quenched through Förster Resonant Energy Transfer (FRET).
Six polyfluorene derivatives, P1-P6, were synthesized and investigated as responsive materials fo... more Six polyfluorene derivatives, P1-P6, were synthesized and investigated as responsive materials for the optical sensing of metal ions in an aqueous medium. They were designed by combining carbazole with fluorene units within the backbone. Carbazole was N-functionalized with three coordinating groups, 2-pyridyl-benzimidazole (P1 and P4), 2-phenyl-benzimidazole (P2 and P5) and 4-phenyl-terpyridyl (P3 and P6), respectively.
ABSTRACT Four novel phenyl-end-capped p-conjugated polymers comprising alkyl chains of different ... more ABSTRACT Four novel phenyl-end-capped p-conjugated polymers comprising alkyl chains of different lengths were synthesized: the poly(9,9-dihexyl-2,7-fluorene-alt-9,9-dioctylfluorene) (P0), the alternating fluorene– carbazole copolymers P1 and P2 comprising 20% and 50% of carbazole, respectively, and the homopolymer poly(9-hexyl-9H-carbazole) (P3). The non-end-capped alternating fluorene–carbazole copolymer P4 comprising 20% of carbazole was also synthesized for the sake of comparison with P1. P0–P2 and P4 were synthesized by a Pd-catalyzed Suzuki–Miyaura coupling in good yields (80–89%), while P3 was obtained by a Ni-catalyzed Yamamoto coupling reaction from the 3,6-dibromocarbazole monomer in a moderate yield (52%). P0–P4 polymers were characterized by NMR, elemental analysis, and GPC. The molecular weights are 40.30, 23.42, 14.33, 3.92, and 37.49 kDa, with polydispersity indices of 2.5, 1.7, 1.8, 1.3, and 2.6, for P0, P1, P2, P3, and P4, respectively. These polymers were found to show a high thermal stability, with decomposition temperatures in the range of 395–420 C, and the glass transition temperature was found to regularly increase with the amount of carbazole inserted in the conjugated backbone. AFM images obtained on thin films (thickness of about 90 nm) of P0–P2 revealed films with surfaces of good quality, being homogeneous with low roughness (0.2 nm for the smaller ones). These polymers were found to be blue-emitting both in diluted dichloromethane and chlorobenzene solutions as well as in thin films and exhibit relatively high values of the absolute quantum yields in the range of 100–5.5% in dichloromethane and 51.4–7.7% in thin films. Blue-emitting electroluminescent devices were obtained with P0 and P1 as emitting layers, respectively. The device built with P1 showed improved performances (EQE of 1.32%) with respect to the one built with the parent polyfluorene material (EQE of 0.75%).
Calcium green I, a ratiometric probe based on fluorescence lifetime measurements, was used to mon... more Calcium green I, a ratiometric probe based on fluorescence lifetime measurements, was used to monitor intracellular calcium activity ([Ca2+]1) in RINm5F cells using a time‐resolved fluorescence confocal microscope. The probe affinity constant has been recalibrated in single cells using ionomycin as a calcium ionophore and eth‐ylenebis(oxyethylenenitrilo)tetraacetic acid as a calcium buffer; KA was found to equal 150 nmol/L. The kinetics of ionomycin equilibration showed that the calcium release from calcium stores occurs before equilibration with extracellular calcium. The response to the muscarinic agonist carbachol, measured on 17 cells receiving three consecutive applications was characterized both by a [Ca2+]1 peak lasting 50 s without any trailing plateau and by desensitization with a 30% decrease in the response. The dose‐dependent response was obtained for a carbachol concentration from 5 μmol/L to 0.5 mmol/L. The ability of our set‐up to obtain a value every 10 ms enabled us...
Physical parameters, describing the state of chromatinized DNA in living mammalian cells, were re... more Physical parameters, describing the state of chromatinized DNA in living mammalian cells, were revealed by in situ fluorescence dynamic properties of ethidium in its free and intercalated states. The lifetimes and anisotropy decays of this cationic chromophore were measured within the nuclear domain, by using the ultra-sensitive time-correlated single-photon counting technique, confocal microscopy, and ultra-low probe concentrations. We found that, in living cells: 1) free ethidium molecules equilibrate between extracellular milieu and nucleus, demonstrating that the cation is naturally transported into the nucleus; 2) the intercalation of ethidium into chromatinized DNA is strongly inhibited, with relaxation of the inhibition after mild (digitonin) cell treatment; 3) intercalation sites are likely to be located in chromatin DNA; and 4) the fluorescence anisotropy relaxation of intercalated molecules is very slow. The combination of fluorescence kinetic and fluorescence anisotropy dynamics indicates that the torsional dynamics of nuclear DNA is highly restrained in living cells.
HAL (Le Centre pour la Communication Scientifique Directe), Aug 15, 2007
While the use of quantum dot (QD) nanoparticles for bioimaging and sensing has been improved and ... more While the use of quantum dot (QD) nanoparticles for bioimaging and sensing has been improved and exploited during the last several years, most studies have used emission intensity-based techniques. Fluorescence lifetime imaging microscopy (FLIM) can also be employed for sensing purposes, overcoming many of the limitations of the aforementioned systems. Herein, we show that the photoluminescence (PL) lifetime of mercaptopropionic acid-capped QDs (MPA-QDs) collected from FLIM images can be used to determine intracellular pH. The PL average lifetime of MPA-QDs varied from 8.7 ns (pH < 5) to 15.4 ns (pH > 8) in media mimicking the intracellular environment. These long decay times of QD nanoparticles make them easily distinguishable from intrinsic cell autofluorescence, improving selectivity in sensing applications. We demonstrate, for the first time, the successful detection of changes in the intracellular pH of different cell types by examining the PL decay time of QDs. In particular, the combination of FLIM methodologies with QD nanoparticles exhibits greatly improved sensitivity compared with other fluorescent dyes for pH imaging. A detailed description of the advantages of the FLIM technique is presented.
Polyaromatic dyes were used to synthesize molecular nanocrystals in sol-gel thin films for sensor... more Polyaromatic dyes were used to synthesize molecular nanocrystals in sol-gel thin films for sensor applications. Fluorescence Confocal Microscopy (FCM) and Transmission Electron Microscopy (TEM) experiments showed the advantages of our nanocrystallization process compared with microcrystallization in free solutions. Indeed, we obtained well-dispersed and spherical nanocrystals with a narrow size distribution, exhibiting a good crystallinity. Time-resolved fluorescence spectroscopy allowed us to measure fluorescence lifetimes of nanocrystals in presence of molecular probes. Then, chemical sensoring properties of these molecular nanocrystals were demonstrated.
Since Helium-3 shortage announcement, organic scintillators play a major role in neutron detectio... more Since Helium-3 shortage announcement, organic scintillators play a major role in neutron detection. Our laboratory decided to focus on plastic scintillators and their ability to discriminate fast neutrons from gamma rays. In this work, we highlight the influence of the secondary fluorophore in lab-made plastic scintillators. The secondary fluorophore is generally added in the scintillating mixture to shift the emission wavelength towards the transparency domain of the material to improve its attenuation length. Thus, it is considered as a harmless molecule and is barely seen as a key criterion that could enhance the performances of the organic scintillator. In our work, we demonstrate that this molecule, even added at a small concentration (typically in the range 0.02-0.2 wt%), directly impacts the neutron/gamma discrimination ability of plastics. Not only the secondary fluorophore plays a role in self-absorption of the scintillating material, but also the couple it creates with the primary fluorophore has to be carefully chosen, as some specific triplet energy transfers between the two fluorophores can influence the neutron/gamma discrimination abilities of the plastic scintillator. Aside from classical photophysical inconveniences such as self-absorption or diffusion, the possibility that the whole, bulk material was heterogeneous in a photochemical point of view was also checked. Thus, various samples were cut from bulk monoliths and their pulse shape discrimination compared with the parent scintillator in terms of Figure of Merit and light output. Keywords n/γ discrimination; plastic scintillators; secondary fluorophore; self-absorption; triplet-triplet annihilation. Highlights Fast neutrons/gamma discrimination efficiency is usually reduced when the volume of a plastic scintillator is increased. This effect is studied, first in a morphologically way, then chemically and photophysically.
Chemistry (Weinheim an der Bergstrasse, Germany), Jan 13, 2016
The synthesis, photophysical properties, and applications in scintillation counting of N-(2-ethyl... more The synthesis, photophysical properties, and applications in scintillation counting of N-(2-ethylhexyl)carbazole (EHCz) are reported. This molecule displays all of the required characteristics for an efficient liquid scintillator (emission wavelength, scintillation yield), and can be used without any extra fluorophores. Thus, its scintillation properties are discussed, as well as its fast neutron/gamma discrimination. For the latter application, the material is compared with the traditional liquid scintillator BC-501 A, and other liquid fluorescent molecules classically used as scintillation solvents, such as xylene, pseudocumene (PC), linear alkylbenzenes (LAB), diisopropylnaphthalene (DIN), 1-methylnaphthalene (1-MeNapht), and 4-isopropylbiphenyl (iPrBiph). For the first time, an excimeric form of a molecule has been advantageously used in scintillation counting. A moderate discrimination between fast neutrons and gamma rays was observed in bulk EHCz, with an apparent neutron/gamm...
This study presents the influence of various substituents on the photophysical features of hetero... more This study presents the influence of various substituents on the photophysical features of heteroleptic copper(I) complexes bearing both N-heterocyclic carbene (NHC) and dipyridylamine (dpa = dipyridylamine skeleton corresponding to ligand L1) ligands. The luminescent properties have been compared to our recently reported archetypal blue emitting [Cu(IPr)(dpa)][PF6] complex. The choice of the substituents on both ligands has been guided to explore the effect of the electron donor/acceptor and "push-pull" on the emission wavelengths and photoluminescence quantum yields. A selection of the best candidates in terms of their photophysical features were applied for developing the first blue lightemitting electrochemical cells (LECs) based on copper(I) complexes. The device analysis suggests that the main concern is the moderate redox stability of the complexes under high applied driving currents, leading to devices with moderate stabilities pointing to a proof-of-concept for further developments. Nevertheless, under low applied driving currents the blue emission is stable, showing performance levels competitive to those reported for blue LECs based on iridium(III) complexes. Overall, this work provides valuable guidelines to tackle the design of enhanced NHC complexes for lighting applications in the near future.
Four new hindered boron-dipyrromethene (BODIPY) dyes have been engineered and synthesized by modi... more Four new hindered boron-dipyrromethene (BODIPY) dyes have been engineered and synthesized by modification of the pyrrole substituents. The chromophores were designed to shift their absorption and emission wavelengths to the red and to show quantitative emission in solid amorphous deposits. [2.2]Paracyclophane (PcP) was grafted on the BODIPY core as well as alkyl chains of various length to force the molecules away one from each other and prevent π-π stacking. The optical spectroscopic and electrochemical properties in solution are presented. Absorption, emission and FLIM were done on drop-casted films of the fluorophores. The addition of alkyl substituents (-Me,-Et, n-Pr) next to the PcP group clearly affects the emission properties in solid state since the dyes display sharper emission bands than in solution and their relative fluorescence quantum yield increases with the number of carbons in the alkyl chain. Fluorescence lifetime images analysis reveals complex decay profiles. Quite a fair description is given by a biexponential fit: the short component is attributed to aggregates and the long component to monomer-like molecules. This population shows a shorter lifetime (2-3.9 ns) compared to solution (6 ns) due to a quenching by aggregates. This quenching is decreasing as the number of carbons on the alkyl side chain is increasing. The combination of a PcP group next to a n-Pr group on a BODIPY is a good combination to get an excellent fluorophore in the solid state.
Photochemical & Photobiological Sciences, 2006
Latex nanoparticles functionalized with cyclam (1,4,8,11-tetraazacyclotetradecane), a copper chel... more Latex nanoparticles functionalized with cyclam (1,4,8,11-tetraazacyclotetradecane), a copper chelator, have been doped with a fluorescent dye 1 (BODIPY derivative: 4,4-difluoro-8-(2 ,4 ,6-trimethyl)phenyl-2,6-diethyl-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene). The bulky, hydrophobic fluorophore 1 dissolves within the nanoparticles' polymer core up to a concentration of about 88.4 lmol g −1. At this concentration the fluorescence yield is about 0.80. Adding Cu 2+ ions to the solution decreases the fluorescence because of the energy transfer between the dye and the violet copper cyclam complexes. The response is fast: 90% of the quenching occurs within 1 s. The Cu 2+ detection threshold is of 1 nanomolar. No interferences were observed with zinc and nickel ions.
We have analyzed the decays of the fluorescence of colloidal CdSe quantum dots (QDs) suspensions ... more We have analyzed the decays of the fluorescence of colloidal CdSe quantum dots (QDs) suspensions during dilution and titration by the ligands. A ligand shell made of a combination of trioctylphosphine (TOP), oleylamine (OA) and stearic acid (SA) stabilizes the as-synthesized QDs. The composition of the shell was analyzed and quantified using high resolution liquid state 1H nuclear magnetic resonance (NMR) spectroscopy. A quenching of the fluorescence of the QDs is observed upon removal of the ligands by diluting the QDs mother solution. The fluorescence is restored by the addition of TOP. We analyze the results by assuming a binomial distribution of quenchers among the QDs and predict a linear trend in the time resolved fluorescence decays. We have used a non parametric analysis to show that for our QDs, 2.97±0.1 quenching sites per QD on average are revealed by the removal of TOP. We moreover show that the quenching rates of the quenching sites add up. The decay per quenching site can be compared with the decay at saturation of the dilution effect. This provides a value of 2.88±0.02 for the number of quenchers per QD. We extract the quenching dynamics of one site. It appears to be a process with a distribution of rates that does not involve the ligands.
This study provides a rigorous analysis of MOF-5 fluorescence, unifying previously dissimilar res... more This study provides a rigorous analysis of MOF-5 fluorescence, unifying previously dissimilar results. Pure MOF-5 fluorescence was identified as only ligand centered and the rise of other mode of emission were observed when exposed to humidity.
This study provides a rigorous analysis of MOF-5 fluorescence, unifying previously dissimilar res... more This study provides a rigorous analysis of MOF-5 fluorescence, unifying previously dissimilar results. Pure MOF-5 fluorescence was identified as only ligand centered and the rise of other mode of emission were observed when exposed to humidity.
... 155. [31] DE Henn, J. Appl. Cryst., 4 (1971) 256. [32] JB Birks, (ed.) Photophysics of aromat... more ... 155. [31] DE Henn, J. Appl. Cryst., 4 (1971) 256. [32] JB Birks, (ed.) Photophysics of aromatic molecules, 1969. [33] A. Tessier, R. M6allet-Renault, P. Denjean, D. Miller, and RB Pansu, Phys. Chem. Chem. Phys., 1 (1999) 5767. ...
The absolute measurement of calcium activity in cells is hindered by the presence of di †erent fo... more The absolute measurement of calcium activity in cells is hindered by the presence of di †erent forms of the probe (adsorbed, protonated). Time-resolved Ñuorescence spectroscopy can be a sensitive technique to separate contaminating forms of the probe. The Ñuorescent probe Calcium Green 1 was characterised, in vitro, in terms of calcium and also pH dependence, albumin adsorption, extent of hydrolysis, viscosity and medium polarity. A 1 : 1 stoichiometry was obtained for the calcium complexation with a value that agrees with reported K d values. A of the BAPTA moiety was measured as 6.7. The Ñuorescence lifetimes are slightly dependent on pK a polarity but not on viscosity. It was observed that most of the interfering forms of the probe share the common lifetimes of 0.5 and 3.5 ns. An alternative mechanism for Calcium Green 1 sensing is proposed based on a ground state conformational equilibrium a †ecting the photoinduced electron transfer.
A single photon counting system for FLIM and fluorescence anisotropy wide-field imaging is descri... more A single photon counting system for FLIM and fluorescence anisotropy wide-field imaging is described and characterized in this paper. The two polarizations of the fluorescence are divided by a Glan prism and acquired at the same time by the Q A detector. Fluorescence
The contraction process of living Vorticella sp. in polymer solutions with various viscosities ha... more The contraction process of living Vorticella sp. in polymer solutions with various viscosities has been investigated by image processing using a high-speed video camera. The viscosity of the external fluid ranges from 1 to 5 mPa·s for different polymer additives such as hydroxypropyl cellulose, polyethylene oxide, and Ficoll. The temporal change in the contraction length of Vorticella sp. in various macromolecular solutions is fitted well by a stretched exponential function based on the nucleation and growth model. The maximum speed of the contractile process monotonically decreases with an increase in the external viscosity, in accordance with power law behavior. The index values approximate to 0.5 and this suggests that the viscous energy dissipated by the contraction of Vorticella sp. is constant in a macromolecular environment.
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Papers by Robert B Pansu