Showing papers on "Stokes shift published in 2017"

Anti-Stokes shift luminescent materials for bio-applications

Abstract: Anti-Stokes shift luminescence is a special optical process, which converts long-wavelength excitation to short-wavelength emission. This unique ability is especially helpful for bio-applications, because the longer-wavelength light source, usually referring to near infrared light, has a larger penetration depth offering a longer working distance for in vivo applications. The anti-Stokes shift luminescence signal can also be distinguished from the auto-fluorescence of biological tissues, thus reducing background interference during bioimaging. Herein, we summarize recent advances in anti-Stokes shift luminescent materials, including lanthanide and triplet–triplet-annihilation-based upconversion nanomaterials, and newly improved hot-band absorption-based luminescent materials. We focus on the synthetic strategies, optical optimization and biological applications as well as present comparative discussions on the luminescence mechanisms and characteristics of these three types of luminescent materials.

Two-photon AIE bio-probe with large Stokes shift for specific imaging of lipid droplets.

Abstract: Lipid droplets are dynamic organelles involved in various physiological processes and their detection is thus of high importance to biomedical research. Recent reports show that AIE probes for lipid droplet imaging have the superior advantages of high brightness, large Stokes shift and excellent photostability compared to commercial dyes but suffer from the problem of having a short excitation wavelength. In this work, an AIE probe, namely TPA-BI, was rationally designed and easily prepared from triphenylamine and imidazolone building blocks for the two-photon imaging of lipid droplets. TPA-BI exhibited TICT+AIE features with a large Stokes shift of up to 202 nm and a large two-photon absorption cross-section of up to 213 GM. TPA-BI was more suitable for two-photon imaging of the lipid droplets with the merits of a higher 3D resolution, lesser photobleaching, a reduced autofluorescence and deeper penetration in tissue slices than a commercial probe based on BODIPY 493/503, providing a promising imaging tool for lipid droplet tracking and analysis in biomedical research and clinical diagnosis.

Origin of the Size-Dependent Stokes Shift in CsPbBr3 Perovskite Nanocrystals

Abstract: The origin of the size-dependent Stokes shift in CsPbBr3 nanocrystals (NCs) is explained for the first time. Stokes shifts range from 82 to 20 meV for NCs with effective edge lengths varying from ∼4 to 13 nm. We show that the Stokes shift is intrinsic to the NC electronic structure and does not arise from extrinsic effects such as residual ensemble size distributions, impurities, or solvent-related effects. The origin of the Stokes shift is elucidated via first-principles calculations. Corresponding theoretical modeling of the CsPbBr3 NC density of states and band structure reveals the existence of an intrinsic confined hole state 260 to 70 meV above the valence band edge state for NCs with edge lengths from ∼2 to 5 nm. A size-dependent Stokes shift is therefore predicted and is in quantitative agreement with the experimental data. Comparison between bulk and NC calculations shows that the confined hole state is exclusive to NCs. At a broader level, the distinction between absorbing and emitting states in...

Tuning of Photoluminescence and Local Structures of Substituted Cations in xSr2Ca(PO4)2–(1 – x)Ca10Li(PO4)7:Eu2+ Phosphors

Abstract: Local structure modification in solid solution is an essential part of photoluminescence tuning of rare earth doped solid state phosphors. Herein we report a new solid solution phosphor of Eu2+-doped xSr2Ca(PO4)2–(1 – x)Ca10Li(PO4)7 (0 ≤ x ≤ 1), which share the same β-Ca3(PO4)2 type structure in the full composition range. Depending on the x parameter variation in xSr2Ca(PO4)2–(1 – x)Ca10Li(PO4)7:Eu2+, the vacancies generated in the M(4) site enable the nonlinear variation of cell parameters and volume, and this increases the magnitude of M(4)O6 polyhedra distortion. The local structure modulation around the Eu2+ ions causes different luminescent behaviors of the two-peak emission and induces the photoluminescence tuning. The shift of the emission peaks in the solid solution phosphors with different compositions has been discussed. It remains invariable at x ≤ 0.5, but the red-shift is observed at x > 0.5 which is attributed to combined effect of the crystal field splitting, Stokes shift, and energy trans...

Band‐Tail Recombination in Hybrid Lead Iodide Perovskite

Abstract: Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH2)2PbI3) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band-structure picture is moderated by a modest degree of energetic disorder.

A fluorescent dye with large Stokes shift and high stability: synthesis and application to live cell imaging

Abstract: A fluorescent dye, 2,5-bis(6-amine-benzoxazol-2-yl)thiophene (BBTA), was synthesized by a two-step reaction from starting material 2,5-bis(benzoxazol-2-yl)thiophene (BBT). BBTA exhibited strong emission and large Stokes shift in solvent, and the largest Stokes shift (Δλ = 186 nm or Δν = 8572 cm−1) was obtained in buffer solution (PBS, pH = 7.2) with 5% (v/v) of polyethylene glycol monomethyl ether (mPEG550, MW = 550) as additive. Application of BBTA to live cell imaging showed that BBTA was clearly expressed in mitochondria with high contrast. Besides, BBTA showed low cytotoxicity and excellent photo-stability.

Cell-Permeant Large Stokes Shift Dyes for Transfection-Free Multicolor Nanoscopy

Abstract: We designed cell-permeant red-emitting fluorescent dye labels with >140 nm Stokes shifts based on 9-iminoanthrone, 9-imino-10-silaxanthone, and 9-imino-10-germaxanthone fluorophores. The corresponding probes selectively targeting mitochondria, lysosomes, and F-actin demonstrate low toxicity and enable stimulated emission depletion (STED) nanoscopy in neurons, human fibroblasts, U2OS, and HeLa cells. In combination with known small Stokes shift dyes, our probes allow live-cell three-color STED nanoscopy of endogenous targets on popular setups with 775 nm STED wavelength.

Light Emission Mechanisms in CuInS2 Quantum Dots Evaluated by Spectral Electrochemistry

Abstract: Luminescent CuInS2 (CIS) quantum dots (QDs) exhibit highly efficient intragap emission and long, hundreds-of-nanoseconds radiative lifetimes. These spectral properties, distinct from structurally similar II–VI QDs, can be explained by the involvement of intragap defect states containing a localized hole capable of coupling with a conduction band electron for a radiative transition. However, the absolute energies of the intragap and band-edge states, the structure of the emissive defect(s), and the role and origin of nonemissive decay channels still remain poorly understood. Here, we address these questions by applying methods of spectral electrochemistry. Cyclic voltammetry measurements reveal a well-defined intragap state whose redox potential is close to that of the Cux defect state (where x = 1+ or 2+). The energy offset of this state from the valence band accounts well for the apparent photoluminescence Stokes shift observed in optical spectra. These results provide direct evidence that Cu-related def...

Broadband Emission with a Massive Stokes Shift from Sulfonium Pb–Br Hybrids

Abstract: Similar to organoammonium cations that template the well-studied layered perovskites, organosulfonium cations template the layered Pb–Br hybrid (tms)4Pb3Br10 (1; tms = (CH3)3S+). Upon ultraviolet excitation, this colorless, organic-inorganic hybrid exhibits broad red/near-infrared photoluminescence (PL) with a massive Stokes shift of ca. 1.7 eV. A suite of PL measurements suggests that the broad emission is an intrinsic property of the material. We ascribe this emission to radiative recombination of self-trapped excitons in the inorganic sublattice, which features both face- and corner-sharing Pb–Br octahedra. We further demonstrate that complex sulfonium-based cations can expand this family of lead-halide hybrids while maintaining the broad, Stokes-shifted PL. The use of more exotic main-group cations affords an exciting opportunity for synthetic chemists to expand the phase-space of semiconducting hybrids with unusual properties for potential applications in optoelectronics.

Molecular Engineering and Design of Semiconducting Polymer Dots with Narrow-Band, Near-Infrared Emission for in Vivo Biological Imaging.

Abstract: This article describes the design and synthesis of donor–bridge–acceptor-based semiconducting polymer dots (Pdots) that exhibit narrow-band emissions, ultrahigh brightness, and large Stokes shifts in the near-infrared (NIR) region. We systematically investigated the effect of π-bridges on the fluorescence quantum yields of the donor–bridge–acceptor-based Pdots. The Pdots could be excited by a 488 or 532 nm laser and have a high fluorescence quantum yield of 33% with a Stokes shift of more than 200 nm. The emission full width at half-maximum of the Pdots can be as narrow as 29 nm, about 2.5 times narrower than that of inorganic quantum dots at the same emission wavelength region. The average per-particle brightness of the Pdots is at least 3 times larger than that of the commercially available quantum dots. The excellent biocompatibility of these Pdots was demonstrated in vivo, and their specific cellular labeling capability was also approved by different cell lines. By taking advantage of the durable brig...

Anomalous Size Dependence of Optical Properties in Black Phosphorus Quantum Dots

Abstract: Understanding electron transitions in black phosphorus nanostructures plays a crucial role in applications in electronics and optoelectronics. In this work, by employing time-dependent density functional theory calculations, we systematically study the size-dependent electronic, optical absorption, and emission properties of black phosphorus quantum dots (BPQDs). Both the electronic gap and the absorption gap follow an inversely proportional law to the diameter of BPQDs in conformity to the quantum confinement effect. In contrast, the emission gap exhibits anomalous size dependence in the range of 0.8–1.8 nm, which is blue-shifted with the increase of size. The anomaly in fact arises from the structure distortion induced by the excited-state relaxation, and it leads to a huge Stokes shift in small BPQDs.

Existence of a Size-Dependent Stokes Shift in CsPbBr3 Perovskite Nanocrystals

Abstract: The existence of a size-dependent Stokes shift is observed in CsPbBr3 perovskite nanocrystals for the first time Stokes shifts range from ∼100 to 30 meV for particles with edge lengths between ∼4 and 12 nm, respectively

Phenyl-doped graphitic carbon nitride: photoluminescence mechanism and latent fingerprint imaging

Abstract: The photoluminescence (PL) emission mechanism of graphitic carbon nitride (g-C3N4) is still ambiguous and the application of PL g-C3N4 powder as a solid sensing platform has not been explored. Herein we highlight a strategy to prepare g-C3N4 powder with strong green PL by doping phenyl groups in a carbon nitride network. Compared with pristine g-C3N4, doping of phenyl groups greatly enhances the PL efficiency and Stokes shift. Theoretical calculations based on density function theory indicate that phenyl groups change the electronic structure of the carbon nitride network and have an obvious contribution to the LUMO of phenyl-doped g-C3N4, which may be the main reason for the enhancement of the PL efficiency and Stokes shift. Taking advantage of the high PL efficiency, large Stokes shift and high photo-stability, phenyl-doped g-C3N4 powder shows promising application for the imaging of latent fingerprints.

Origins of Stokes Shift in PbS Nanocrystals.

Abstract: Stokes shift, an energy difference between the excitonic absorption and emission, is a property of colloidal quantum dots (CQDs) typically ascribed to splitting between dark and bright excitons. In some materials, e.g., PbS, CuInS2, and CdHgTe, a Stokes shift of up to 200 meV is observed, substantially larger than the estimates of dark–bright state splitting or vibronic relaxations. The shift origin remains highly debated because contradictory signatures of both surface and bulk character were reported for the Stokes-shifted electronic state. Here, we show that the energy transfer among CQDs in a polydispersed ensemble in solution suffices to explain the excess Stokes shift. This energy transfer is primarily due to CQD aggregation and can be substantially eliminated by extreme dilution, higher-viscosity solvent, or better-dispersed colloids. Our findings highlight that ensemble polydispersity remains the primary source of the Stokes shift in CQDs in solution, propagating into the Stokes shift in films and...

A fluorescent probe with “AIE+ESIPT” characteristics for Cu2+ and F− ions estimation

Abstract: We report a fluorescent probe 2 possessing hydroxyaryl Schiff’s base onto tetraphenylethylene (TPE) moiety with characteristic of both aggregation induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT). The coupled AIE and ESIPT shows the significant advantages, such as no self quenching at high concentration, large stokes shift, high selectivity towards analyte etc. It has also been used for estimation of F − and Cu 2+ ions amongst various anions and metal ions, respectively. Probe 2 shows the emission at 540 nm with a large stokes shift of 170 nm and presence of Cu 2+ ions induce the restriction of intramolecular motions leading to high emission in the aggregated state (AIE) at longer wavelength (NIR region).

A phenothiazine coumarin-based red emitting fluorescent probe for nanomolar detection of thiophenol with a large Stokes shift

Abstract: By introducing phenothiazine framework as an electron donating component, we have developed a unique deep-red functional fluorescent dye 5b, bearing excellent photophysical properties. Using dye 5b as a platform, we further constructed a novel red emitting fluorescent probe (1) for thiophenol on the basis of a unique thiophenol-mediated substitution-cyclization-hydrolysis reaction cascade. Probe 1 exhibits highly selective and sensitive response to thiophenol over aliphatic thiols, with a detection limit of 2.9 nM (S/N = 3). Moreover, this probe features a remarkable large Stokes shift (141 nm) in PB buffer (pH = 7.4, containing 1 mM CTAB). Importantly, it was successfully applied for detection of thiophenol both in real water samples and in living HeLa cells, indicating that this probe has great potential for practical applications.

Coumarin-Based Boron Complexes with Aggregation-Induced Emission

Abstract: Two coumarin-based boron complexes (HBN and MBN) with aggregation-induced emission were designed and synthesized. The photophysical properties of the complexes were investigated in different solvents and in the solid state. Results showed that the inhibited C═N isomerization by N,O-chelated BF2 caused the significant enhancement of fluorescence in THF. In particular, the complexes displayed red-shifted emissions (>60 nm) in mixed solvents of CH3CN and water because of the aggregation-induced charge-transfer enhancement. In the solid state, the bright red emission appeared at 650 nm (620 nm), with a Stokes shift of 170 nm. Cell-imaging experiments indicated that the complexes have good membrane permeability and can be used as lysosome trackers.

Phenothiazinyl Boranes: A New Class of AIE Luminogens with Mega Stokes Shift, Mechanochromism, and Mechanoluminescence.

Abstract: Phenothiazines with a dimesityl boron moiety, a new class of aminoboranes with B−N linkage, were synthesized. These aminoboranes exhibited interesting photophysical behavior including aggregation-induced emission (AIE), mechanochromism (MC), mechanoluminescence (ML), and a mega Stokes shift (up to 312 nm in hexane). The solid-state emission of the aminoboranes could be switched reversibly by grinding–fuming processes. Furthermore, the phenothiazine derivative with a bromo and an arylborane group at 3- and 7-positions exhibited bright mechanoluminescence.

Luminescence Tuning, Thermal Quenching, and Electronic Structure of Narrow-Band Red-Emitting Nitride Phosphors

Abstract: Exploring high-performance narrow-band red-emitting phosphor is an important challenge for improving white light LEDs. Here, on the basis of three interesting nitride phosphors with similar vierer rings framework structure, two phosphor series, Eu2+-doped Sr(LiAl)1–xMg2xAl2N4 and Sr(LiAl3)1–y(Mg3Si)yN4 (x, y = 0–1), are successfully synthesized by a solid state reaction. They show narrow-band red emission with tunable emission peaks from 614 to 658 nm and 607 to 663 nm. The varying luminescence behaviors with composition and structure are discussed based on centroid shift, crystal field splitting and Stokes shift. On the basis of experimental data, we construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes of divalent/trivalent lanthanide-doped end-member compounds, and further give thermal quenching mechanism of these series phosphors.

A novel ratiometric fluorescent probe for selective and sensitive detection of Cu2+ in complete aqueous solution

Abstract: A novel hemicyanine-based ratiometric fluorescent probe 3 for Cu2+ was described The probe exhibited fluorescence emission at 448 nm Upon addition of Cu2+, the emission at 448 nm quenched and simultaneously generated a significant bathochromic shift of the emission maximum to 503 nm due to the Cu2+-triggered hydrolysis reaction of the picolinate moiety Thus, a single-excitation, dual-emission ratiometric measurement with a large red shift in emission (Δ55 nm) and remarkable changes in the ratio (F503nm/F448nm) of the emission intensity (R/R0 up to 22-fold) could be established Moreover, the probe exhibited a large Stokes shift, high sensitivity (detection limit of 11 nM), and high selectivity for Cu2+ over other metal ions in complete aqueous solution