Low Stokes Shift Research Articles

A Fluorescent Furan-based Probe with Protected Functional Groups for Highly Selective and Non-Toxic Imaging of HT-29 Cancer Cells and 4T1 Tumors.

Most of the previously reported fluorescent organic probes for cancer cell and tumor imaging have significant limitations including chemical toxicity, structural instability, low Stokes shift value, and the inability for selective accumulations in tumors during in vivo imaging. To overcome the mentioned challenges, we synthesized the fluorescent probes with protected polar functional groups to enhance the non-toxicity nature and increase the selectivity toward tumors. In addition, the structural rigidity of the fluorescent probes was increased by embedding aromatic rings in the probe structure. This issue enables us to obtain ultrabright cell images due to enhanced fluorescence quantum yield (ΦFL) values. After synthesis and spectral characterizations, the applicability of two furan-based and imidazole-based fluorescent probes ( abbreviated as DCPEF and DBPPI, respectively) was investigated for ultrabright in vitro and in vivo imaging of cancer cells. The probe DCPEF shows the ΦFL value of 0.946 and the Stocks shift of 86 nm. In addition, probe DBPPI offers the ΦFL value of 0.400 and a Stocks shift of 150 nm. The MTT colorimetric cytotoxicity assay showed that probe DCPEF has minimal effects against HT-29 (cancer) and Vero (normal) cells. The probe DCPEF produced ultrabright fluorescence images from HT-29 cells. In addition, in vivo imaging of cancer cells showed that probe DCPEF selectively accumulates in the 4T1 tumor in mice. The spectral and chemical stability, minimal cytotoxicity, significant Stokes shift, and high degree of selectivity for tumor cells during in vivo imaging make DCPEF an appropriate candidate to be used as a standard probe for cancer cell imaging.

Highly luminescent and stable MAPbBr3 quantum dot fabrication via a hybrid capping ligand

Perovskite quantum dots (PEQDs) are next-generation quantum dot materials that have attracted attention in various areas such as display, radiation scintillator, and photodetector. However, PEQD defects that occur for various reasons hinder the commercialization of PEQDs. Herein, we reported a new strategy for a hybrid ligand comprising 1-octanethiol (OT) and octylammonium bromide that can passivate the defects of PEQD and effectively reduce the Stokes shift. First, the increase in the OT concentration in hybrid ligands leads to an increase in the PEQD size and a decrease in the Stokes shift. Second, OT in hybrid ligands can stabilize the PEQD solution by binding to an MA ion, which is easily degraded by atmospheric oxygen, thereby suppressing the generation of metallic Pb that serves as a defect in PEQDs. Consequently, PEQDs by hybrid ligands reduced the defects, particularly metallic Pb, leading to strong green emission with low Stokes shift with a photoluminescence quantum yield of 99.3 % and high stability under atmospheric conditions.

N-Functionalized Graphene Quantum Dots with Ultrahigh Quantum Yield and Large Stokes Shift: Efficient Downconverters for CIGS Solar Cells

Copper indium gallium selenide (CIGS) is the most promising thin film solar cell technology. However, its high performance is hampered by its poor short-wavelength response. The short-wavelength response can be enhanced via photon downconversion using quantum dots. Unfortunately, most graphene quantum dots (GQDs) are not suitable as downconverters in CIGS cells owing to their low photoluminescence quantum yield (PL QY) and/or low Stokes shift. Herein, an ultrahigh PL QY (99%) and a large Stokes shift (98 nm) are achieved for N-doped GQDs via a novel method. The performance of a CIGS solar cell is enhanced via photon downconversion and the light-trapping effect using the NGQDs. The effectiveness of the NQGDs is manifested in a conversion efficiency (η) of 15.31%. In addition, improvements in the short-circuit current density from 30.69 mA/cm2 to 31.77 mA/cm2 and fill factor from 71.25% to 73.09% are observed. The n and J0 values are decreased by insertion of NGQDs, indicating a reduction in the recombinati...

Excited State and Non-linear Optical Properties of NIR Absorbing β-Thiophene-Fused BF2-Azadipyrromethene Dyes-Computational Investigation.

Density functional theory and time-dependent density functional theory computations were used to understand the electronic and photophysical parameters of NIR β-thiophene-fused BF2-azadipyrromethene dyes. The computed data are in good agreement with those obtained experimentally and they provide insights into the origin of red shifted optical spectra compared to the parent aza-BODIPY, low Stokes shift, non-linear optical responses and quantitative description of the singlet-triplet energy gap. The resultant decrease in the HOMO - LUMO energy gap is responsible for the red shift. The possible use as non-linear optical materials is supported by large enhancement in the non-linear optical properties. On the basis of vertical triplet energies, their possible potential therapeutic use as a photosensitizer in photodynamic therapy is proposed. The singlet-triplet energetic gaps suggest that the β-thiophene-fused BF2-azadipyrromethene dyes can act as a sensitizer to produce an efficient generation of singlet oxygen. Their optimal use as an efficient singlet fission materials has been proposed on the basis of excitation energies in the ground, lowest singlet and triplet excited states.

Molecular evolution of versatile derivatives from a GFP-like protein in the marine copepod Chiridius poppei

Fluorescent proteins are now indispensable tools in molecular research. They have also been adapted for a wide variety of uses in cases involving creative applications, including textiles, aquarium fish, and ornamental plants. Our colleagues have previously cloned a yellow GFP-like protein derived from the marine copepod Chiridius poppei (YGFP), and moreover, succeeded in generating transgenic flowers with clearly visible fluorescence, without the need for high-sensitivity imaging equipment. However, due to the low Stokes shift of YGFP (10 nm), it is difficult to separate emitted light of a labeled object from the light used for excitation; hence, limitations for various applications remain. In this study, which was aimed at developing YGFP mutants with increased Stokes shifts, we conducted stepwise molecular evolution experiments on YGFP by screening random mutations at three key amino acids, based on their fluorescent characteristics and structural stabilities, followed by optimization of their fluorescence output by DNA shuffling of the entire coding sequence. We successfully identified an eYGFPuv that had an excitation maximum in UV wavelengths and a 24-fold increase in fluorescence intensity compared to the previously reported YGFP mutant (H52D). In addition, eYGFPuv exhibited almost 9-fold higher fluorescence intensity compared to the commercially available GFPuv when expressed in human colon carcinoma HCT116 cells and without any differences in cytotoxicity. Thus, this novel mutant with the desirable characteristics of bright fluorescence, long Stokes shift, and low cytotoxity, may be particularly well suited to a variety of molecular and biological applications.

Emissive Molecular Aggregates and Energy Migration in Luminescent Solar Concentrators.

Luminescent solar concentrators (LSCs) are light harvesting devices that are ideally suited to light collection in the urban environment where direct sunlight is often not available. LSCs consist of highly luminescent compounds embedded or coated on a transparent substrate that absorb diffuse or direct solar radiation over a large area. The resulting luminescence is trapped in the waveguide by total internal reflection to the thin edges of the substrate where the concentrated light can be used to improve the performance of photovoltaic devices. The concept of LSCs has been around for several decades, and yet the efficiencies of current devices are still below expectations for commercial viability. There are two primary challenges when designing new chromophores for LSC applications. Reabsorption of dye emission by chromophores within the waveguide is a significant loss mechanism attenuating the light output of LSCs. Concentration quenching, particularly in organic dye systems, restricts the quantity of chromophores that can be incorporated in the waveguide thus limiting the light absorbed by the LSC. Frequently, a compromise between increased light harvesting of the incident light and decreasing emission quantum yield is required for most organic chromophore-based systems due to concentration quenching. The low Stokes shift of common organic dyes used in current LSCs also imposes another optimization problem. Increasing light absorption of LSCs based on organic dyes to achieve efficient light harvesting also enhances reabsorption. Ideally, a design strategy to simultaneously optimize light harvesting, concentration quenching, and reabsorption of LSC chromophores is clearly needed to address the significant losses in LSCs. Over the past few years, research in our group has targeted novel dye structures that address these primary challenges. There is a common perception that dye aggregates are to be avoided in LSCs. It became apparent in our studies that aggregates of chromophores exhibiting aggregation-induced emission (AIE) behavior are attractive candidates for LSC applications. Strategic application of AIE chromophores has led to the development of the first organic-based transparent solar concentrator that harvests UV light as well as the demonstration of reabsorption reduction by taking advantage of energy migration processes between chromophores. Further developments led us to the application of perylene diimides using an energy migration/energy transfer approach. To prevent concentration quenching, a molecularly insulated perylene diimide with bulky substituents attached to the imide positions was designed and synthesized. By combining the insulated perylene diimide with a commercial perylene dye as an energy donor-acceptor emitter pair, detrimental luminescence reabsorption was reduced while achieving a high chromophore concentration for efficient light absorption. This Account reviews and reinspects some of our recent work and the improvements in the field of LSCs.

On the ability of Förster resonance energy transfer to enhance luminescent solar concentrator efficiency

Developing means to reduce the cost of solar energy is vital to curb our carbon footprint over the upcoming decades. A luminescent solar concentrator (LSC) is a potential solution as it provides light concentration without any tracking device and can be readily integrated into the built environment. In this study we report on an advanced LSC design that employs quantum dots as absorption fluorophores and organic dye molecules as emission fluorophores. By linking the two types of fluorophores to each other, energy is transferred efficiently via Förster resonance energy transfer (FRET) from the quantum dot to the dye molecule. This novel method makes use of the quantum dot's spectrally wide absorption profile and the higher quantum yield of the dye. We show that our design can overcome the losses normally incurred due to a low quantum yield emitter by transferring the absorbed energy to a linked fluorophore with a higher quantum yield. Our experimental measurements show FRET can enhance the optical efficiency of a LSC by at least 24.7%. The maximum theoretical efficiency has been investigated by ray-tracing and has been found to be 75.1%; this represents a relative improvement of even 215.5% compared to a LSC doped with quantum dots only (23.8%), showing the great potential of our concept. Our design will initiate interest in fluorophores which have not been considered for LSC applications thus far because of their low quantum yield or small Stokes shift.

Temperature-Dependent Luminescence Characteristic of SrSi2O2N2:Eu2+ Phosphor and Its Thermal Quenching Behavior

The yellow SrSi2O2N2:Eu2+ phosphor has been synthesized by using a simple solid-state reaction method with Sr2SiO4:Eu2+ as the precursor. It shows a broad excitation band extending from 250 to 520 nm and an asymmetric emission band with a main peak at about 550 nm. The emission intensity of the SrSi2O2N2:Eu2+ is about 1.2 times higher than the commercial yellow phosphor YAG:Ce3+ (P46-Y3). The temperature-dependent luminescence characteristic of SrSi2O2N2:Eu2+ has been investigated in this paper. With increasing temperature, the emission band of SrSi2O2N2:Eu2+ shows anomalous blue-shift along with decreasing emission intensity and the broadening full width at half maximum (FWHM). Particularly, compared with YAG:Ce3+ (P46-Y3), the yellow SrSi2O2N2:Eu2+ phosphors exhibit higher thermal stability due to their weaker electron-phonon coupling strength (1.1), lower stokes shift (0.0576 eV) and larger activation energy (0.288 eV). All these results indicate that SrSi2O2N2:Eu2+ yellow phosphors have potential application for white light-emitting diodes (LEDs). What's more, an energy level scheme is constructed to explain the anomalous blue-shift phenomenon.

UV-Photoelectron Spectroscopic Studies on 2-Arylethynyl-1,3,2-benzodiazaboroles

Gas-phase He I photoelectron spectra of a series of 2-arylethynyl-1,3,2-benzodiazaboroles BEP-X [1 (X = H), 2 (X = Me), 3 (X = OMe), 4 (X = SMe), 5 (X = NMe2), 6 (X = Br)] have been recorded and assessed by density functional theory calculations. The first ionization energies of these benzodiazaboroles are in the order 5 (7.0 eV) < 3 (7.15 eV) < 2 (7.2 eV) < 1 = 4 = 6 (7.3 eV). As an important result of this study it was disclosed that in contrast to the remaining diazaboroles the frontier orbitals of the amino derivative 5 lack contributions from the diazaborole part of the molecule. In keeping with this, the dipole moment of the ground state (5.51 D) is only slightly changed, to 6.60 D, upon electronic excitation. This also rationalizes the low Stokes shift of 2600 cm−1 in comparison to the other diazaboroles in this study (5900−7300 cm−1).

The photoluminescence of Ce-doped Lu 4Si 2O 7N 2 green phosphors

Cerium-doped Lu 4Si 2O 7N 2 green phosphors were synthesized by a high-temperature solid-state reaction method under nitrogen atmosphere. Compared with Ce-doped Y 4Si 2O 7N 2 phosphors, Ce-doped Lu 4Si 2O 7N 2 phosphors showed longer wavelengths of both excitation and emission, lower Stokes shift, and much stronger emission intensity. Based on the first-principles calculations of the two phosphors, the strong emission intensity originates from the large density of states. At last, the effects of Ce 3+ concentration on photoluminescence were also examined.

Stability and photodegradation of phthalocyanines and hematoporphyrin doped PMMA as solar concentrators

Luminescent solar concentrator samples using polymethylmethacrylate (PMMA) as the matrix with phthalocyanine, nickel-phthalocyanine and hematoporphyrin IX dichloride laser dyes were prepared by a casting method. Optical absorption measurements were carried out at room temperature across the 200–900 nm wavelength region both before and after the samples were exposed to sunlight for two weeks. The degradation of PMMA–dye samples, measured by absorption, was found to obey a first-order kinetic equation. Values of the optical band gaps ( E g) have been obtained for the direct allowed transitions before and after the samples were exposed to sunlight. The tail width of localized states in the band gap ( E u) was calculated from Urbach’s formula. Photodegradation studies revealed that the phthalocyanine dye systems are the most stable. Emission spectra and quantum efficiency were also investigated, and phthalocyanine dyes were shown to have a lower Stokes shift and a higher fluorescence quantum yield ( Q f) than the hematoporphyrin IX dichloride. Collectively, these results suggest that PMMA doped with phthalocyanines provide the better system for use in fluorescent solar collector systems.

Synthesis, molecular structure and optical spectral studies of syn-dithia benzothiazolophane and anti-bis-lactone benzothiazolophane

Synthesis of syn-dithia-benzothiazolophane 1 and anti-bis-lactone benzothiazolophane 2 has been carried out by well established protocols as outlined in Schemes 1 and 2, respectively. The 1H NMR spectra of heterophanes 1 and 2 were found to be temperature independent in the range of −55 to +120 °C which indicates conformationally mobile nature of these molecules. In comparison to plain CHCl 3, the UV–visible spectra of 1 and 2 measured in CHCl 3 containing trifluoroacetic acid (TFAA) exhibited bathochromic shifts by 25 and 19 nm, respectively. The red shifts are attributed to the increased charge transfer interaction arising from the donor phenyl ring to the acceptor benzothiazolium ring. The emission spectra of thia-bridged 1, both in the neutral as well as the protonated forms are accompanied by large Stokes shifts (Δ ν=127–156 nm) with the emissions most likely originating from the locally excited states. The bis-lactone 2 in neutral CHCl 3 solvent also exhibited a single locally excited emission, but with a relatively lower Stokes shift (Δ ν=69 nm). Interestingly, unlike 1, bis-lactone 2 in the protonated form displayed both local as well as what appears to be an intramolecular charge transfer emission. The emission behavior of 1 and 2 has been tentatively rationalized on the basis of HOMO–LUMO interaction. Molecular modelling of 1 and 2 generated five reasonable conformations within 5 kcal/mol depending upon the orientation of the benzothiazole rings and the heteroatoms in the connecting bridges. The transannular distances in 1 and 2 were found to be ca. 2.8 and 3.6 Å, respectively. While the rings in 2 are nearly planar, however, in the case of 1 the stacks are slightly curved signifying molecular strain in the letter system. Based on dynamic 1H NMR spectral analysis and molecular modelling, we propose that these molecules exist as a rapidly equilibrating mixtures of conformers.

Response to “Comment on ‘Low Stokes shift in thick and homogeneous InGaN epilayers’ ” [Appl. Phys. Lett 81, 1353 (2002)

Response to “Comment on ‘Low Stokes shift in thick and homogeneous InGaN epilayers’ ” [Appl. Phys. Lett <b>81</b>, 1353 (2002)

Wavelength- and Time-Resolved Luminescence Spectroscopy for Investigation of the Matrix-Assisted Laser Desorption Process

This work deals with investigations of fluorescence phenomena during interaction of ultraviolet laser radiation with four substances (2,5-dihydroxybenzoic acid, 3-hydroxypicolinic acid, dithranol and ferulic acid) commonly used as matrices in matrix-assisted laser desorption/ionization (MALDI). Wavelength-resolved fluorescence measurements allowed classification of UV-MALDI matrices on the basis of their Stokes shift. Matrices showing a high Stokes shift dissipated high amounts of laser energy by means of intra- and intermolecular energy transfer processes, hence contributing less to the ionization of analyte compounds. Conversely, matrix substances with a low Stokes shift retained more of the original photonic energy which may then be available for ionization within the matrix or after desorption of molecular species. Therefore matrix compounds can be designated as “hard” (high ionization energy) in the case of small Stokes shifts or “soft” (low ionization energy) in the case of high Stokes shifts. Time-resolved measurements showed fluorescence lifetimes below 5 ns for all four substances.

Self-absorption effects in a LEC with low Stokes shift

The electroluminescence spectra both in forward and reverse directions of a light-emitting electrochemical cell prepared from a methyl-substituted ladder-type poly(paraphenylene) (mLPPP), blended with a crown ether, Dicyclohexano18crown6 (DCH18C6), and a Li salt, lithium trifluoromethanesulfonate (litriflate, LiTf for short) are compared. Different from the forward direction, in reverse direction the spectrum is strongly influenced by self-absorption effects. We discuss this behavior by different degrees of doping of the p- and n-type doped zones. This effect can be applied for a new method to realize bias-dependent color emission from single-layer devices at low voltages that can be prepared by only one simple and easy spin-coating process.

Low Stokes shift in thick and homogeneous InGaN epilayers

The optical properties of thick InxGa1−xN layers have been studied using optical absorption and cathodoluminescence techniques. The indium composition x of the layers ranged from 0.03 to 0.17 as determined by Rutherford backscattering measurements. The difference between the band gap and the peak emission energy (Stokes shift) was found to be considerably smaller than reported in the past for these alloys. Monochromatic images show that light emission from most of the film is homogeneous and is associated with a low Stokes shift. A second emission band at longer wavelengths is observed for x⩾0.08. This band originates from indium-rich regions in the vicinity of extended defects, and exhibits a larger Stokes shift. Our observations indicate that it is possible to grow InGaN epilayers with high indium composition, high homogeneity, and lower Stokes shift.