Low Luminescence Quantum Yield Research Articles

Singlet Oxygen Detection by Chemiluminescence Probes in Living Cells.

Singlet oxygen is a reactive oxygen species that causes oxidative damage to plant cells, but intriguingly it can also act as a signalling molecule to reprogram gene expression required to induce plant physiological/cellular responses. Singlet oxygen photosensitization in plants mainly occurs in chloroplasts after the molecular collision of ground-state molecular oxygen with triplet-excited-state chlorophyll. Singlet oxygen direct detection through phosphorescence emission in chloroplasts is a herculean task due to its extremely low luminescence quantum yield. Because of this, indirect alternative methods have been developed for its detection in biological systems, for example, by measuring the changes in the EPR signal or fluorescence intensity of singlet oxygen reaction-based probes. The singlet oxygen chemiluminescence (SOCL) is a chemiluminescence probe with high sensitivity and selectivity towards singlet oxygen and promising use to detect it in living cells without the inconvenience of low stability of the EPR signal of spin probes in the presence of redox compounds, spurious light scattering coming from the light source required for the excitation of fluorescence probes or the light emission of endogenous fluorescent molecules like chlorophyll in chloroplasts. The protocol presented in this chapter describes the first steps to characterizing singlet oxygen production within the biological system under study; this is accomplished through monitoring molecular oxygen consumption by SOCL using a Clark-type oxygen electrode and measuring the chemiluminescence generated by SOCL 1,2-dioxetane using a spectrofluorometer. For singlet oxygen detection within living cells, a version of SOCL with increased membrane permeability (SOCL-CPP) is described.

Enabling In Vivo Optical Imaging of an Osmium Photosensitizer by Micellar Formulation.

Osmium (Os)-based photosensitizers (PSs) exhibit unique broad, red-shifted absorption, favoring PDT activity at greater tissue depths. We recently reported on a potent Os(II) PS, rac-[Os(phen)2(IP-4T)](Cl)2 (ML18J03) with submicromolar hypoxia activity. ML18J03 exhibits a low luminescence quantum yield of 9.8 × 10-5 in PBS, which limits its capacity for in vivo luminescence imaging. We recently showed that formulating ML18J03 into 10.2 nm DSPE-mPEG2000 micelles (Mic-ML18J03) increases its luminescence quantum yield by two orders of magnitude. Here, we demonstrate that Mic-ML18J03 exhibits 47-fold improved accumulative luminescence signals in orthotopic AT-84 head and neck tumors. We show, for the first time, that micellar formulation provides up to 11.7-fold tumor selectivity for ML18J03. Furthermore, Mic-ML18J03 does not experience the concentration-dependent quenching observed with unformulated ML18J03 in PBS, and formulation reduces spectral shifting of the emission maxima during PDT (variance = 6.5 and 27.3, respectively). The Mic-ML18J03 formulation also increases the production of reactive molecular species 2-3-fold. These findings demonstrate that micellar formulation is a versatile and effective approach to enable in vivo luminescence imaging options for an otherwise quenched, yet promising, PS.

One-pot green synthesis of N,S co-doped biomass carbon dots from natural grapefruit juice for selective sensing of Cr(VI)

The synthesis of biomass carbon dots (BCDs) using natural products has attracted increasing attention in the past few years owing to its advantages in terms of abundant sources and green environmental protection. However, the low luminescence quantum yield (QY) of BCDs is still a bottleneck for practical applications. Herein, nitrogen and sulfur co-doped biomass carbon dots (N,S-BCDs) with high QY (84.93%) were successfully prepared by one-pot green hydrothermal method using natural grapefruit juice and urea as carbon and nitrogen sources, respectively. The as-obtained N,S-BCDs showed an average size of 3.8 nm with strong blue luminescence and excitation-dependence fluorescence behavior. Furthermore, good selectivity and sensitivity were noticed toward hexavalent chromium (Cr(VI)) due to the dynamic quenching of fluorescence. The limit of detection (LOD) was determined as 0.155 μM, and the linearity was extended from 0 to 400 μM. Furthermore, the paper sensors for the visual semi-quantitative rapid detection of Cr(VI) were constructed by using the as-obtained N,S-BCDs, which reveals the great potential applications as fluorescent probes for the detection of heavy metal ions in the environment.

Photoexcitation and radiation regulation mechanism of Ag anchoring WSe2 heterojunction with plasma coupling effect

The noble metal silver (Ag) combined with tungsten selenide (WSe2) was used to solve the problems of the narrow absorption band range, low luminescence quantum yield, and nonlinear transmittance of WSe2 base materials. Ag/WSe2 films and core–shell Ag@WSe2 heterojunction were designed and fabricated by the two-step magnetron sputtering and hydrothermal/magnetron sputtering method, respectively. X-ray diffraction, scanning electron microscopy and X-ray photoelectric spectroscopy results confirmed that the successful preparation of Ag/WSe2 and Ag@WSe2. Raman results demonstrate the presence of interfacial stress between Ag and WSe2. Based on quantum particle coupling in metal–semiconductor nanocomposites, the adsorbed silver particles provide electrons and through charge transfer, which increases the density of electronic states in conduction band of the WSe2 and thus increases the spontaneous emission rate. Electron-hole pairs generated in the semiconductor through resonant coupling between surface plasmas of the Ag and WSe2 interface will provide strongly enhanced radiative recombination. The absorption and emission of WSe2 nanosheets in the visible light range and linear-nonlinear optical properties depend on the regulation the sputtering power and pressure of Ag particles. Our research results demonstrate that Ag&WSe2 have excellent optical and photoelectric properties, indicating that Ag&WSe2 have a wide application potential in photoelectric functional and optoelectronic devices.

Zero-Dimensional Organic-Inorganic Hybrid Copper-Based Halides with Highly Efficient Orange-Red Emission.

Lead-free metal halides are promising luminescent materials with low non-toxicity, tunable emission, and high luminescence quantum yield (PLQY). The development of red light-emitting materials with high PLQY to meet the color coordinate requirements for light-emitting diode (LED) lighting and wide color gamut displays remains a major scientific challenge. Herein, novel lead-free copper-based halide materials are reported. PEA4 Cu4 I4 is an efficient orange-red broad emission with a highest PLQY of 73.6%. All three materials, namely, those of ((R)-(+)-α-PEA)4 Cu4 I4 , ((S)-(-)-α-PEA)4 Cu4 I4 , and (β-PEA)4 Cu4 I4 with zero-dimensional structures have large exciton binding energies and Huang-Rhys factors, thus causing efficient self-trapped exciton emissions. A prototypical orange-red LED device is fabricated successfully that demonstrates the potential of copper-based halide materials in lighting and display applications.

Discriminative Behavior of a Donor-Acceptor-Donor Triad toward Cyanide and Fluoride: Insights into the Mechanism of Naphthalene Diimide Reduction by Cyanide and Fluoride.

A new molecular donor-acceptor-donor (D-A-D) triad, comprised of an electron deficient 1,4,5,8-naphthalene tetracarboxylic diimide (NDI) unit covalently connected to two flanking photosensitizers, i.e., a bis-heteroleptic Ru(II) complex of 1,10-phenanthroline and pyridine triazole hybrid ligand, is described. The single crystal X-ray structure of the perchlorate salt of the triad demonstrates that the electron deficient NDI unit can act as a host for anions via anion-π interaction. Detailed solution-state studies indicate that fluoride selectively interacts with the D-A-D triad to form a dianionic NDI, NDI2-, via a radical anion, NDI•-. On the contrary, cyanide reduces the NDI moiety to NDI•-, as confirmed by UV-vis, NMR, and EPR spectroscopy. Further, femtosecond transient absorption spectroscopic studies reveal a low luminescence quantum yield of the D-A-D triad attributable to the photoinduced electron transfer (PET) process from the photoactive Ru(II) center to the NDI unit. Interestingly, the triad displays "OFF-ON" luminescence behavior in the presence of fluoride by restoring the Ru(II) to phenanthroline/pyridine-triazole-based MLCT emission, whereas cyanide fails to show a similar property due to a different redox process operational in the latter. The reduction of NDI in the presence of fluoride and cyanide in different polar solvents indicates that involvement of such deprotonated solvents in the electron transfer mechanism may not be operative in our present system. Low-temperature kinetic studies support the formation of a charge transfer associative transient species, which likely allows overcoming the thermodynamically uphill barrier for the direct electron transfer mechanism.

Ultrabright bimetallic AuAg complex: From luminescence mechanism to biological application

Metal clusters have attracted wide interests due to their unique electronic and optical properties, but the low luminescence quantum yield (QY) prevents them from potential biomedical applications. In this work, silver-doped Au nanoclusters (NCs) are shown to be able to improve the QY of metal clusters. We succeeded in synthesizing ultrabright glutathione (GSH) protected AuAg clusters with 10.8% QY by a one-pot route. Their florescence is about 7.5 times brighter than pure Au NCs, with super photostability and good biocompatibility in physiological environment. Based on density functional theory (DFT) calculations, we investigated the electronic structures and optical properties of the AuAg NCs. The results show that the increase of the density of states of the lowest unoccupied molecular orbital (LUMO) leads to the fluorescence enhancement. In addition, two-photon excitation fluorescence imaging has been performed to show their great potential for biomedicine.

An Enantiopure Propeller-Like Trityl-Brominated Radical: Bringing Together a High Racemization Barrier and an Efficient Circularly Polarized Luminescent Magnetic Emitter.

A new persistent organic free radical has been synthetized with Br atoms occupying the ortho- and para-positions of a trityl core. After the isolation of its two propeller-like atropisomers, Plus (P) and minus (M), their absolute configurations were assigned by a combination of theoretical and experimental data. Remarkably, no hints of racemization were observed up to 60 °C for more than two hours, due to the higher steric hindrance imposed by the bulky Br atoms. Therefore, when compared to its chlorinated homologue (t1/2 =18 s at 60 °C), an outstanding stability against racemization was achieved. A circularly polarized luminescence (CPL) response of both enantiomers was detected. This free radical shows a satisfactory luminescent dissymmetry factor (|glum (592 nm)|≈0.7×10-3 ) despite its pure organic nature and low luminescence quantum yield (LQY). Improved organic magnetic CPL emitters derived from the reported structure can be envisaged thanks to the wide possibilities that Br atoms at para-positions offer for further functionalization.

Investigations on the interaction of water-soluble semiconductor polymer with thioglycolic acid (TGA) capped CdTe quantum dots

Studies of nanocomposites produced by the association of quantum dots (QDs) and polymers, in general, use the nanocrystals as the energy's donor and the polymer's molecules as acceptors. The production of nanocomposites in which a water-soluble polymer is the energy donor species and the quantum dots the acceptors species is particularly difficult due to low luminescence quantum yield and low fluorescence lifetimes of these molecules when compared to the QDs. In this work, we associated colloidal CdTe QDs stabilized with thioglycolic acid (TGA) with water-soluble semiconductor luminescent polymer Poly{[2,5-bis(2-(N,N-diethylamino)ethoxy)-1,4-phenylene]-alt-1,4-phenylene} (PPE-DBE) to produce nanocomposites that favors the polymers as donors and QDs as acceptors. We characterized the composites by absorption, static fluorescence, and dynamic fluorescence. Our results showed the intensification of the QDs emission and its quantum yield on the nanocomposite and the decrease of the polymer lifetime. Thus, we verified the occurrence of energy transfer from the polymer to the quantum dots, especially for samples which the ratio of PPP-DBE monomers per number of QDs in solution is approximately 465, achieving an increase of 11.34% on the QDs' fluorescence quantum yield.

Brightly and directionally luminescent single-walled carbon nanotubes in a wedge cavity

Single-walled carbon nanotubes suffer severely from their extremely low luminescence quantum yield and spectral purity. In this work, we produced brightly and directionally luminescent, as well as spectrally pure, carbon nanotubes by embedding them into a wedge-shaped planar cavity. By controlling the detuning between the cavity mode and exciton emission, the photoluminescence of carbon nanotubes could be enhanced up to 60 times. Coupling efficiency, i.e., percentage of nanotube luminescence emitted into a cavity mode, was found to be detuning dependent with a maximum efficiency of ∼54%. Moreover, emission from nanotubes inside the cavity becomes highly directional. The emission angle was measured to be less than 1.8°, demonstrating their great potential in device applications of future optoelectronics.

Proton-Coupled Electron-Transfer Processes in Ultrafast Time Domain: Evidence for Effects of Hydrogen-Bond Stabilization on Photoinduced Electron Transfer.

The proton-coupled electron-transfer (PCET) reaction is investigated for a newly synthesized imidazole-anthraquinone biomimetic model with a photoactive RuII -polypyridyl moiety that is covalently coupled to the imidazole fragment. Intramolecular H-bonding interactions between imidazole and anthraquinone moieties favor the PCET process; this can be correlated to an appreciable positive shift in the one-electron reduction potential of the coordinated anthraquinone moiety functionalized with the imidazole fragment. This can also be attributed to the low luminescence quantum yield of the RuII -polypyridyl complex used. The dynamics of the intramolecular electron-transfer (ET) and PCET processes are studied by using femtosecond transient absorption spectroscopy. The steady-state spectroscopic studies and the results of the time-resolved absorption studies confirm that H-bonded water molecules play a major role in both ET and PCET dynamics as a proton relay in the excited state. The electron-transfer process is followed by a change in the H-bonding equilibrium between AQ and imidazole in acetonitrile solvent, and protonation of AQ.- by water leads to PCET in the presence of water. A slower forward and backward electron-transfer rate is observed in the presence of D2 O compared with that in H2 O. These results provide further experimental support for a detailed understanding of the PCET process.

Evidence of Exciton Self-Trapping in Pseudoisocyanine J-Aggregates Formed in Layered Polymer Films

At low temperatures in the luminescence spectrum of pseudoisocyanine (PIC) J-aggregates formed in a layered polymer film an unusual broad red-shifted band appears. The analysis of spectral properties of PIC J-aggregates allowed us to ascribe the additional red band to the exciton self-trapped state. In a layered polymer film, PIC J-aggregates are found to possess a 2D island-like structure, which results in a barrier type of the exciton self-trapping with coexisting free and self-trapped excitons. Both the strong topological disorder and exciton–phonon coupling are suggested to be the reason for the exciton self-trapping in J-aggregates. Nonradiative relaxation of self-trapped excitons at room temperature has been proposed to be responsible for a very low luminescence quantum yield and giant nonradiative rate constant for PIC J-aggregates formed in a layered film.

Luminescence modulations of rhenium tricarbonyl complexes induced by structural variations.

Octahedral d(6) low-spin Re(I) tricarbonyl complexes are of considerable interest as noninvasive imaging probes and have been deeply studied owing to their biological stability, low toxicity, large Stokes shifts, and long luminescence lifetimes. We reported recently the bimodal IR and luminescence imaging of a Re(I) tricarbonyl complex with a Pyta ligand (4-(2-pyridyl)-1,2,3-triazole) in cells and labeled such metal-carbonyl complexes SCoMPIs for single-core multimodal probes for imaging. Re(I) tricarbonyl complexes have unique photophysical properties allowing for their unequivocal detection in cells but also present some weaknesses such as a very low luminescence quantum yield in aqueous medium. Further optimizations would thus be desirable. We therefore developed new Re(I) tricarbonyl complexes prepared from different ancillary ligands. Complexes with benzothiadiazole-triazole ligands show interesting luminescent quantum yields in acetonitrile and may constitute valuable luminescent metal complexes in organic media. A series of complexes with bidentate 1-(2-quinolinyl)-1,2,3-triazole (Taquin) and 1-(2-pyridyl)-1,2,3-triazole (Tapy) ligands bearing various 4-substituted alkyl side chains has been designed and synthesized with efficient procedures. Their photophysical properties have been characterized in acetonitrile and in a H2O/DMSO (98/2) mixture and compared with those of the parent Quinta- and Pyta-based complexes. Tapy complexes bearing long alkyl chains show impressive enhancement of their luminescent properties relative to the parent Pyta complex. Theoretical calculations have been performed to further characterize this new class of rhenium tricarbonyl complexes. Preliminary cellular imaging studies in MDA-MB231 breast cancer cells reveal a strong increase in the luminescence signal in cells incubated with the Tapy complex substituted with a C12 alkyl chain. This study points out the interesting potential of the Tapy ligand in coordination chemistry, which has been so far underexploited.

Photoinduced relaxation processes in self-assembling complexes from CdSe/ZnS water-soluble nanocrystals and cationic porphyrins

Particular features and quenching mechanisms of exciton luminescence of water-soluble nanocomposites that are formed as a result of the interaction of surface charged semiconductor quantum dots (QDs) CdSe/ZnS (dCdSe = 2.8 nm) and cationic porphyrins (H2TMPyrP4+ and ZnTMPyrP4+) have been studied theoretically and experimentally. It has been found that, in CdSe/ZnS⊕Porphyrin conjugates, there occurs long-range inductive resonance electronic excitation energy transfer from surface modified (with thioglycolic or mercaptoundecanoic acid) QDs to porphyrins, which is accompanied by quenching of the exciton luminescence of QDs and an increase in the fluorescence intensity of porphyrin. It has been shown that, when mercaptoundecanoic acid is used as a QD shell, the QD luminescence quenching efficiency by porphyrins follows the Forster-Galanin theory and depends on the overlap integral between the CdSe/ZnS luminescence band and the absorption spectra of free-base porphyrin H2TMPyrP4+ and its metal complex ZnTMPyrP4+. It has been revealed that, as the QDs ↔ Zn-porphyrin intercenter distance decreases from 39.1 (mercaptoundecanoic acid) to 30.1), a considerable QD luminescence quenching is observed; however, the energy transfer efficiency substantially decreases, from 55% in the former case to 23% in the latter one. Based on the spectral-luminescent data and quantum-chemical calculations, it has been found that the chemical change of H2TMPyrP4+ in the structure of the complex with CdSe/ZnS QDs passivated by thioglycolic or mercaptoundecanoic acid is caused by the formation of a metal complex ZnTMPyrP4+. Based on calculations of the redox-potentials, it has been concluded that the low luminescence quantum yield of CdSe/ZnS QDs passivated by residues of mercaptocarboxylic acids S−(CH2)nCOO− and its dependence on the number of CH2 groups are related to the possibility of photoinduced electron transfer from the HOMO of passivating molecules to QDs (QD* ⇐ S−(CH2)nCOO− hole transfer). It has been shown that the quenching of the exciton luminescence of QDs in heterogeneous structures CdSe/ZnS(thioglycolic acid)⊕ZnTMPyrP4+, which is complementary to the energy transfer, can be caused by the photoinduced electron transfer that involves the participation of the LUMO of the ZnTMPyrP4+ molecule (QD* ⇒ ZnTMPyrP4+).

Aryl Bridged 1-Hydroxypyridin-2-one: Effect of the Bridge on the Eu(III) Sensitization Process

The efficiency of Eu(3+) luminescence by energy transfer from an antenna ligand can be strongly dependent on the metal ion coordination geometry. The geometric component of the Eu(III) sensitization has been probed using series of tetradentate 1,2-HOPO derivatives that are connected by bridges of varying length and geometry. The ligands are N,N'-(1,2-phenylene)bis(1-hydroxy-6-oxo-1,6-dihydropyridine-2-carboxamide) for the ligand (L(1)), 1-hydroxy-N-(2-(1-hydroxy-6-oxo-1,6-dihydropyridine-2-carboxamido)benzyl)-6-oxo-1,6-dihydropyridine-2-carboxamide (L(2)) and N,N'-(1,2-phenylenebis(methylene))bis(1-hydroxy-6-oxo-1,6-dihydropyridine-2-carboxamide) (L(3)). Spectroscopic characterization of both the Gd(III) and the Eu(III) metal complexes, time-dependent density functional theory (TD-DFT) analysis of model compounds and evaluation of the kinetic parameters for the europium emission were completed. Some striking differences were observed in the luminescence quantum yield by altering the bridging unit. The [Eu(L(2))(2)](-) derivative shows efficient sensitization coupled with good metal centered emission. For [Eu(L(3))(2)](-), the large quenching of the luminescence quantum yield compared to [Eu(L(2))(2)](-) is primarily a result of one inner sphere water molecule bound to the europium cation while for [Eu(L(1))(2)](-), the low luminescence quantum yield can be attributed to inefficient sensitization of the europium ion.

Rapid Peptide Fragmentation without Electrons, Collisions, Infrared Radiation, or Native Chromophores

Ultraviolet photodissociation of peptides followed by mass analysis has several desirable advantages relative to other methods, yet it has not found widespread use due to several limitations. One shortcoming is the inefficiency with which peptides absorb in the ultraviolet. This issue has a simple solution and can be circumvented by the attachment of noncovalent adducts that contain appropriate chromophores. Subsequent photoactivation of the chromophore leads to vibrational excitation of the complex and eventually to fragmentation of the peptide. Herein, the energetics that control the efficiency of this process are examined as a function of the characteristics of both the peptide and the noncovalently attached chromophore. Fragmentation efficiency decreases with increasing peptide size and is also constrained by the binding energy of the noncovalent adduct. The optimum chromophore should have excellent absorption at the excitation wavelength and a low luminescence quantum yield. It is demonstrated that a naphthyl based 18-crown-6 adduct is ideally suited for attaching to a variety peptides and fragmenting them following absorption of 266 nm light. Potential applications and limitations of this methodology are discussed.

Luminescence of a New Ru(II) Polypyridine Complex Controlled by a Redox-Responsive Protonable Anthra[1,10]phenanthrolinequinone

Redox-controlled luminescence quenching is presented for a new Ru(II)-bipyridine complex [Ru(bpy)2(1)]2+ where ligand 1 is an anthra[1,10]phenanthrolinequinone. The complex emits from a short-lived metal-to-ligand charge transfer, 3MLCT state (τ = 5.5 ns in deaerated acetonitrile) with a low luminescence quantum yield (5 × 10-4). The emission intensity becomes significantly enhanced when the switchable anthraquinone unit is reduced to corresponding hydroquinone. On the contrary, chemical one-electron reduction of the anthraquinone moiety to semiquinone in aprotic tetrahydrofuran results in total quenching of the emission.

Optical properties of highly oriented quaterthiophene thin films grown by organic molecular-beam deposition

The molecular-beam deposition of quaterthiophene (4T) oligomer on potassium acid phtalate (001) single crystals yields thin films exhibiting a peculiar supramolecular organization induced by the interaction with the substrate. The 4T thin films, which exhibit an extremely large optical anisotropy arising from the alignment of the 4T molecular axis along the b crystallographic axis of the substrate, allow one to carry out polarized transmission measurements over the whole \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ absorption band. The observation of a strong polarized band, together with very weak unpolarized absorption peaks originating from isolated molecules, is consistent with the formation of a tilted H aggregate in which the molecular transition moments form a 38\ifmmode^\circ\else\textdegree\fi{} angle with the substrate surface. The low luminescence quantum yield further supports the formation of this very ordered structure. Exciton migration and decay paths in the 4T films have been established by studying, with time-resolved photoluminescence spectroscopy, the emission originating from traps and imperfect aggregates.

Photochemistry at high temperatures - potential of ZnO as a high temperature photocatalyst

Direct conversion of solar radiation into useful, storable and transportable chemical products is the primary goal of solar chemistry. In this paper we discuss some fundamental aspects of photochemistry at elevated temperatures. We show that luminescence can serve as an indicator of the potential use of a system as a photoconverter. As an example we present experimental data on the chemical potential and on the lifetime of the excited states of ZnO. The low luminescence quantum yield together with a lifetime of about 200 ps indicate that an efficient photochemical conversion on ZnO is highly improbable. We believe this to be a general feature of chemical systems based on a semiconductor photocatalyst, in particular of photoreactions at a solid/gas interface.

Singlet oxygen luminescence quantum yields in organic solvents and water

The probability of luminescence f( 1Δ g) for a number of solvents is determined as the ratio of the experimentally measured luminescence quantum yield and the lifetime. The value of f varies over a wide range. The data show specific perturbations of the O 2( 3Σ g −)←O 2( 1Δ g) electronic transition induced by the solvents. The low value of f in water agrees with the extremely low luminescence quantum yield and explains the difficulties of recording the luminescence spectrum of O 2( 1Δ g) in water.