Blue-shifted Emission Spectra Research Articles

Theoretical insights into long-range coupling of electron-hole pairs in TCTA–PO-T2T exciplex

This work reports theoretical investigations concerning long-range coupling of electron-hole pairs in spatially separated exciplex (SSE) systems via Density Functional Theory and Time-Dependent Density Functional Theory. Based on TCTA−PO-T2T parent exciplex, where 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TCTA) serves as donor (D) and 2,4,6-Tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) as acceptor (A), SSEs are constructed by intentionally tuning the D−A distance. The calculation results demonstrate that all SSEs, even if the D−A distance is over 14 Å, exhibit the clear and strong charge transfer character in terms of DCT, qCT, and t indexes. As the D−A distance increases, the energy gap of SSEs is increased, resulting in the blueshift of emission spectra. Calculation and experiment results show a good consistency, indicating that our model could well describe SSEs. Meanwhile, the reduced energy gap between CT and 3LE and more degenerate states could boost reverse intersystem crossing via vibronic coupling and hyperfine coupling, eventually improving the electroluminescent performance of SSEs. Our study suggests that manipulation of relative energy alignment via controlling D−A distance not only promotes the properties of exciplexes, but also offers guidance for designing thermally activated delayed fluorescence emitters through-space charge transfer.

A novel ethylene linkage-based covalent organic framework for turn-on fluorescence sensing for Al3+ with excellent selectivity and sensitivity

Covalent organic Framework (COFs) has become a new platform for functional research and material design. A novel covalent organic framework (CN-COF) was first synthesized with p-xylylene dicyanide and 2-hydroxy-1,3,5-benzenetrialdehyde through the Knoevenagel condensation reaction. CN-COF is a porous crystal material with strong thermal and chemical stability. CN-COF exhibits a selective “turn-on” fluorescence response to Al3+ in ethanol with blue-shifted emission spectra over the other tested metal ions. The color changes from pink to earth yellow, and the fluorescence effect is clearly visible. The fluorescence intensity of CN-COF was linearly related to the concentration of Al3+, and the detection limit was 1.815 μM. Importantly, CN-COF exhibits a satisfactory recovery for detecting Al3+ in drinking water and fish samples. CN-COF also showed the intuitive semi-quantitative detection ability for Al3+ via the color change with the naked eyes. The special pore structure is conducive to allow Al3+ enter to coordinate with O and N atoms on the wall of CN-COF scaffold. The revisable fluorescence change upon the selective addition of Al3+ and XRD, EDTA, XPS and DFT results demonstrated the complex process. The inhibition of the photoinduced electron transition from O atoms to Al3+ induced the fluorescence enhancement. This study not only presents a synthesis idea for a new structural organic framework, but also offers a highly selective and sensitive fluorescence chemical sensor for the identification and detection of Al3+.

Solid-State [2+2] Photoreaction of Isostructural Cd(II) Metal Complexes and Solid-State Fluorescence.

A green method to synthesize cyclobutane derivatives has been developed over the past three decades in the form of solid-state [2+2] photochemical reactions. These solid-state reactions also play a major role in the structural transformation of hybrid materials. In this regard, crystal engineering has played a major role in designing photoreactive molecular systems. Here, we report three novel binuclear Cd(II) complexes with the molecular formula [Cd2(4spy)4L4], where 4spy = 4-styryl pyridine and L = p-toluate (1); 4-fluorobenzoate (2); and 3-fluorobenzoate (3). Although three different benzoates are used, all three complexes are isostructural, as corroborated through SCXRD experiments. Structural analysis also helped in identifying two potential photoreactions. These are both intra- and intermolecular in nature and are driven by the head-to-head (HH) and head-to-tail (HT) alignment of 4spy linkers within these metal complexes. 1H NMR spectroscopy studies showed evidence of a quantitative head-to-head photoreaction in all these three complexes, and SCXRD analysis of the recrystallization of the photoproducts also provided confirmation. TGA studies of these photoreactive complexes showed an increase in the thermal stability of the complexes due to the solid-state photoreaction. Photoluminescence studies of these complexes have been conducted, showing a blue shift in emission spectra across all three cases after the photoreaction.

Evaluation of a trinuclear heteroleptic bis-cyclometalated iridium(III) complex as a photoredox catalyst for visible light-mediated hydrothiolation reactions

A trinuclear bis-cyclometalated iridium(III) polypyridyl complex was synthesised and evaluated as a photoredox catalyst for visible light- mediated thiol-ene coupling reactions. Electronic absorption and emission studies revealed that the complex shows a blue-shifted emission spectrum, with a wavelength at 578 nm, relative to the canonical iridium complex, [Ir(ppy)3] (598 nm). The emissions are attributed to the transition from the triplet MLCT excited state (3MLCT) to the ground state. Cyclic voltammetry studies demonstrated that this complex is a promising candidate for the photoredox catalysed reactions, with the trinuclear iridium complex increasing the oxidation potential without sacrificing the reduction ability. The iridium(III) complex was evaluated as a photoredox catalyst in the model hydrothiolation coupling reaction and benchmarked against a trinuclear ruthenium congener. Reactions conducted with the trinuclear iridium polypyridyl complex resulted in the highest NMR yields.

Synthesis and Unexpected Optical Properties of Ionic Phosphorus Heterocycles with P-Regulated Noncovalent Interactions.

Optoelectronic properties of organic chromophores (OCPs) are to a large extent dictated by the chemical structures. Herein, we synthesized a new series of ionic phosphorus(P)-heteropines via the methylation of the P(III) center. Our studies revealed that methylation is highly dependent on the P(III) environments (NPN, NPC, and CPC), in which adjacent nitrogen atoms greatly withdraw electron density of the P(III) center. The observation of noncovalent interactions between solvent molecules and the molecular backbones of the related P-heterocycle in the single crystal structure implied tunable molecular conformations. Different from the red-shifted absorption and emission spectra of ionic P-OCPs induced by either decreased lowest unoccupied molecular orbital (LUMO) or intramolecular charge transfer (ICT) state in previous studies, current ionic P-heterocycles exhibit blue-shifted absorption and emission spectra compared to the nonionic counterparts. Our experimental and theoretical studies suggest that the unexpected photophysical characters are probably due to the counter-anion induced structure twisting via intermolecular noncovalent interactions between NH-indole and O(OTf), and/or strong intermolecular O···F bonding between O(MI) and F(OTf). Our studies also revealed that the P-environments (NPN, NPC, and CPC) conjunctly impact the photophysical properties of the ionic P-heteropines. Overall, the fact that the P-environment-regulated noncovalent interactions induce the rich structure dynamics and photophysics offers us with a new and effective strategy to fine-tune the optical properties of OCPs.

Characterization of unexpected blue-shifted single molecule fluorescence of JF646 and JF669 for SMLM.

JF-Halotag dyes have become increasingly popular for single-molecule localization microscopy (SMLM) because of their favorable photo-physics, ease of use, and compatibility with live-cell imaging. However under a variety of conditions, the excitation and emission wavelengths from individual emitters can deviate significantly from the commonly accepted characterization, leading to artifacts in two color SMLM data. Here I show results from multiple in vivo and in vitro experiments demonstrating 640nm mediated photo switching of JF669 from a conventional far-red signal to a 561nm excitable red single molecule signal. This result is exciting because it enables UV-free PALM experiments. UV radiation causes cell death, increases auto fluorescence, and induces cell stress, which may affect the behavior of labeled proteins or cellular processes of interest. Additionally, I present evidence of a subpopulation of JF646 molecules that can be excited by 561nm light, but emit red instead of far-red light. This unexpected fluorescence causes single molecule cross talk in the red channel, despite the commonly accepted characterization. This phenomenon is more apparent at low dye concentrations as well as high temperatures, suggesting that a self-interaction between JF646 molecules may bias the well characterized fluorescence, while in isolation JF646 may have significantly blue-shifted excitation and emission spectra. Together, these results show potentially useful photo-physical behavior of JF dyes, but also highlight the importance of careful controls when using synthetic dyes for single molecule localization microscopy.

One-Pot Synthesis and Characterization of CuCrS2/ZnS Core/Shell Quantum Dots as New Blue-Emitting Sources.

In this paper, we introduce a new blue-emitting material, CuCrS2/ZnS QDs (CCS QDs). To obtain bright and stable photoluminescent probes, we prepared a core/shell structure; the synthesis was conducted in a one-pot system, using 1-dodecanethiol as a sulfur source and co-ligand. The CCS QDs exhibited a semi-spherical colloidal nanocrystalline shape with an average diameter of 9.0 nm and ZnS shell thickness of 1.6 nm. A maximum photoluminescence emission peak (PL max) was observed at 465 nm with an excitation wavelength of 400 nm and PLQY was 5% at an initial [Cr3+]/[Cu+] molar ratio of one in the core synthesis. With an off-stoichiometric modification for band gap engineering, the CCS QDs exhibited slightly blue-shifted PL emission spectra and PLQY was 10% with an increase in initial molar ratio of 2.0 (462 nm PL max). However, when the initial molar ratio exceeded two, the CCS QDs exhibited a lower photoluminescence quantum yield of 4.5% with 461 nm of PL max at the initial molar ratio of four due to the formation of non-emissive Cr2S3 nanoflakes.

A comparative study on photophysics of meso-substituted mono- and bis-BODIPY carbazoles

A comparative study of the photophysics of mono- and bis-BODIPY (BDP) substituted N-butylcarbazoles (CBZ): CBZ-BDP and CBZ-bisBDP has been carried out. The longest wavelength electronic absorption band at ∼495 nm corresponds to BODIPY moiety and does not exhibit solvatochromism. The absorbance value of CBZ-bisBDP is twice that of CBZ-BDP due to the presence of two BODIPY chromophores in the former. For both the molecules, fluorescence spectra show locally excited (LE) emission in non-polar solvents and red-shifted broad Intramolecular Charge Transfer (ICT) emission in polar solvents. CBZ-BDP shows a larger red-shift and more significant intensity reduction of the ICT band than the bis-conjugated molecule. Theoretical calculations reveal that the Highest Occupied Molecular Orbitals (HOMO) of the two molecules are localized primarily on the carbazole unit. In contrast, the Lowest Unoccupied Molecular Orbitals (LUMO) are mainly concentrated on BODIPY unit. This suggests that excited state charge transfer in both the fluorophores is from the carbazole unit to the BODIPY unit. The S1 states of both CBZ-BDP and CBZ-bisBDP are Twisted Intramolecular Charge Transfer (TICT) states. The temperature-dependent study in ethyl acetate has shown an increase in emission intensity with temperature accompanied by a blue shift of emission spectrum in CBZ-BDP. In contrast, CBZ-bisBDP displayed a decrease in intensity along with blue shift. Besides, CBZ-bisBDP showed an additional LE emission whose emission intensity gets enhanced with increasing temperature. The Critical Micelle Concentration (CMC) of sodium deoxycholate (NaDC) bile salt in aqueous medium was determined by using CBZ-BDP as a fluorescent probe. Three independent parameters: absorbance, fluorescence intensity and anisotropy of the molecule were employed to determine the CMC value. Cell imaging studies using confocal fluorescence microscopy showed that, unlike the mono-BODIPY, CBZ-bisBDP could penetrate the cells and reveal the cytoplasmic distribution in Hela cells. The IC50 values obtained via MTT assay indicate that the cytotoxicity against cancer cells is significant in CBZ-bisBDP compared to the mono-BODIPY.

Molecular Mechanism of Interaction of Curcumin with BSA, Surfactants and Live E. Coli Cell Membrane Revealed by Fluorescence Spectroscopy and Confocal Microscopy.

Clinical trials on the therapeutic effect of curcumin have proven to be highly effective against many diseases including cancer and Alzheimer's. However, the molecular mechanism of interaction of curcumin with protein and live cell membrane is poorly understood. Here, we report the mechanism of interaction of curcumin with bovine serum albumin (BSA) and live E. coli cell membrane in the presence of organized assemblies of sodium dodecyl sulfate (SDS) and cetrimonium bromide (CTAB) by fluorescence spectroscopy, laser-scanning confocal microscopy, and computation. Enhanced binding constant, blue-shifted emission spectra, and imaging of heterogeneous FRET on live bacteria cell membrane strongly indicate the complex formation of curcumin with strong hydrophobic interaction, which is further validated by computation. Finally, our results may shed light on the efficient strategy of applications of curcumin as a natural therapeutic lead in clinical trials against many life-threatening diseases.

Theoretical Design of Blue-Color Phosphorescent Complexes for Organic Light-Emitting Diodes: Emission Intensities and Nonradiative Transition Rate Constants in Ir(ppy)2(acac) Derivatives.

Theoretical calculations of phosphorescent spectra and nonradiative transition (NRT) rate constants for S1 ⇝ T1, T1 ⇝ S0, and S1 ⇝ S0 were carried out to determine the best candidate for a blue-color phosphorescent complex among several derivatives of bis(2-phenylpyridine)(acetylacetonate)iridium(III). The geometries of the ground state (S0), the lowest triplet state (T1), and the lowest excited singlet state (S1) were optimized at the levels of density functional theory, in which B3LYP functionals and SBKJC+p basis sets were used. The NRT rate constants were derived by using a generating function method within the displaced harmonic oscillator model. The results of the calculation for phosphorescence showed that the introduction of F and/or CN substituents at the 4'/6'-th and 5'-th sites in 2-phenylpyridinate (ppy) ligands, respectively, causes a blue shift of the emission spectra. They also suggest that Ir(5-CN,6-F-ppy)2(acac), denoted 3(56) in the text, is a good candidate for a blue-color phosphorescent complex because a blue shift of emission spectra and a moderate intensity are obtained for phosphorescence and, furthermore, this complex is calculated to have a large rate constant for S1 ⇝ T1 and relatively smaller rate constants for T1 ⇝ S0 and S1 ⇝ S0 based on the calculations of spin-orbit coupling and nonadiabatic coupling constants.

Guest-Regulated Luminescence and Force-Stimuli Response of a Hydrogen-Bonded Organic Framework.

Guest molecules may endow porous materials with new or enhanced properties as well as functions. Here, a porous hydrogen-bonded organic framework (HOF) constructed from a three-armed triphenylamine derivative is used to investigate how guests regulate photoluminescence and trigger force-stimuli response. It was found that guest solvents in pores might regulate HOF's luminescence. Interestingly, acetic acid as a guest endowed HOF materials with longer emission wavelengths and triggered the responses to mechanical force stimuli. Under shear force, an obvious blueshift in emission spectra was observed because of the loss of free guests and the conversion of π-stacking model. Further blue-shifted emission appeared while the bound guests were completely removed by heating. Mechanofluorochromic HOF materials could be regenerated through recrystallization and adsorbing guest. Conversely, HOFs with other guests and activated HOFs only resulted in a slight change in their fluorescence behaviors after force stimuli.

Effect of annealing on metal nanoclusters tuned luminescence shift in Ag and Cu embedded nanocomposite glasses

Nanoscale silver and copper clusters are prepared in the soda-lime glass by sequential ion-exchange technique and thermal annealing process. The observed peak values of optical absorption spectroscopy result in the confirmation of embedded Ag and Cu nanoclusters in soda-lime glass. The effect of temperature on the plasmonic response has been investigated for various temperatures. The temperature dependence of surface plasmon resonance (SPR) shifted towards the blue wavelength region, due to growth of silver and copper nanoclusters. Similar to the absorption peak shift, the blue shift in emission spectra has been observed in the photoluminescence spectra of embedded Ag and Cu nanoclusters. The optical bandgap values of Ag and Cu nanoclusters are determined from Tauc's plot. The bandgap variations with luminescence spectra and their emission peak shift with the temperature make them as a promising material for temperature sensors. The FESEM images confirm the cluster distribution of these embedded metal nanoparticles.

State filling effects on photoluminescence and photovoltaic characteristic of aluminium-doped CdTe colloidal quantum dots stabilized in aqueous medium

CdTe and aluminium (Al)-doped CdTe (Al:CdTe QDs) colloidal quantum dots (QDs) were synthesized through an aqueous route. CdTe and Al:CdTe QDs colloidal quantum dots of different size were obtained by collecting the samples at different refluxing time. The size of the synthesized QDs collected after 6 h of refluxing time was measured to 3.3 and 4.8 nm for Al:CdTe QDs and CdTe QDs, respectively. The size shrinkages of Al:CdTe QDs were due to chlorine ions from AlCl3 precursor, which controls the growth kinetics of QDs and stabilizes the QDs in the aqueous phase. The size shrinkages of Al:CdTe QDs result in an increase of bandgap from 2.13 to 2.3 eV and congruently blue-shifted absorbance and emission spectra were discerned. The state filling effect, gradual filling of energy state, observed clearly in photoluminescence spectra of Al:CdTe QDs for samples collected after 30 min of refluxing time. We also fabricated Gratzel-type QDs-sensitized solar cells by loading CdTe and Al:CdTe QDs on mesoporous TiO2 photoanode. Photovoltaic efficiency is marginally increased (13%) from 0.45 to 0.51% for Al-doped CdTe QDs as compared to CdTe QDs.

Chlorine distribution management for spectrally stable and efficient perovskite blue light-emitting diodes

Abstract Blue emitting perovskite electroluminescent (EL) devices are mainly realized based on chlorine mixing in bromide perovskites including CsPbBr3, CH3NH3PbBr3 (MAPbBr3), and CH(NH2)2PbBr3 (FAPbBr3). However, the inhomogeneous distribution and varied mobility of Cl− and Br− ions in alloyed perovskite films always result in EL spectra gradually shifting from blue to green region during device operation. Herein, alloyed FAPbBr3−xClx nanocrystal (NC) films with optimized Cl distribution are fabricated through employment of Cl terminated amine ligands (Cl-ligands). The as fabricated FAPbBr3−xClx NC films exhibit a Cl rich surface and 9 nm blue shifted emission spectrum compared to FAPbBr3−xClx films fabricated by conventional Br terminated ligands (Br-ligands). Furthermore, EL devices based on Cl-ligands achieve blue EL emissions with maximum luminance of 2810 cd/m2 and EQE of 3.1%, which is almost three times higher than that of EL devices based on Br-ligands. Importantly, Cl-ligands derived EL devices show greatly enhanced spectra stability with only slightly red-shifted EL spectra during device operation in a wide bias range, providing an alternative strategy for spectrally stable perovskite blue light-emitting diodes.

A new colorimetric and ratiometric probe for highly selective recognition and bioimaging of ClO- and Al3.

In this study, a new fluorescence probe HMAQ based on quinazoline and diaminomaleonitrile was constructed for sensing ClO- and Al3+. A fluorescence blue-shift with 102nm together with a color change from golden-yellow to colorless was found by hypochlorite-induced hydrolysis of -CH=N- group to release the initial fluorophore. Besides, Al3+ could cause a 72-nm blue-shifted emission spectra and a color change from golden-yellow to brown. As expected, HMAQ exhibited a satisfactory selectivity and sensitivity to ClO-/Al3+ with a quick response. Most notably, the reversibility of the [HMAQ+Al3+] complex could be used to detect ClO- and Al3+ simultaneously without mutual interferences. The detection limits of HMAQ for ClO- and Al3+ were turned out to be 10.2nM and 1.56nM, respectively. The high-performance results of real-time detections demonstrated the enormous potential of HMAQ in real-water samples and living cells.

Strong Solid-State Fluorescence Induced by Restriction of the Coordinate Bond Bending in Two-Coordinate Copper(I)-Carbene Complexes.

The photophysical properties of two-coordinate copper(I) complexes have become a new research hotspot due to their nearly perfect luminescent properties and low price and promising applications in organic light-emitting diodes (OLEDs). In this work, we employ the hybrid quantum mechanics and molecular mechanics (QM/MM) approach, coupled with our early developed thermal vibration correlation function (TVCF) rate formalism, to study the aggregation effect on the luminescent properties of the cyclic (alkyl)(amino)carbene-copper(I)-Cl complex. Our calculations reveal that the transition properties changes from metal-ligand-charge-transfer (MLCT) in solution to hybrid halogen ligand charge-transfer (XLCT) and MLCT in solid state, which induces the blue-shifted emission spectra from solution to solid phase. Upon aggregation, the restriction of the bending vibrations of the C-Cu-Cl and Cu-C-N bonds largely slow down the nonradiative decay, which induces strong fluorescence. This study provides a clear rationalization for the highly efficient fluorescence character of two-coordinate Cu(I) complexes.

Promising color controllable phosphors CBSOCNy:Eu2+ with excellent thermal stability

Novel Ca1.96(B1-ySiy)(O3-yNy)Cl:0.04Eu2+ phosphors were designed and synthesized by introducing [Si4++N3−]/[B3++O2−] co-substitutions into Ca2BO3Cl:Eu2+ lattice. Fourier Transform Infrared Spectroscopy (FTIR) show that typical planar [BO3]3− units gradually transformed into [BO4]5− units with the increase of y, which was firstly realized in luminescence materials. Uneven expansion of cell parameters a,b, and c were confirmed by the combination of X-ray diffraction spectra (XRD) and results of Rietveld refinement. Structure variation caused smaller centroid shift (εc) and 5d energy level splitting (εcfs), inducing the narrowing of excitation spectra (y = 0 → y = 0.2, 168 → 144 nm) and blue-shift of emission spectra (y = 0 → y = 0.2, 582 → 525 nm). Excellent thermal stability (6% loss at 150 °C) was obtained, which resulted from more covalent bonding. Using the combination of representative sample CBSOCN0.2: 0.04Eu2+ and red K2SiF6:Mn4+ in a blue LED driven lamp, white light with high CRI ~92.8 and color temperature closed to the summer sun (5770 K) was obtained.

A coumarin based fluorescent probe for rapidly distinguishing of hypochlorite and copper (II) ion in organisms.

A dual-functional fluorescent probe based on coumarin fluorophore for monitoring ClO- and Cu2+ was synthesized and characterized. The identification mechanisms for ClO- and Cu2+ induced different colorimetric and ratiometric changes. ClO- caused a colorimetric change from yellow to colorless and 120 nm blue-shifted emission spectra from red to blue. Besides, Cu2+ induced a remarkable fluorescence quenching behavior and 36 nm blue-shifted absorption spectra accompanied by the color change from yellow to luminous yellow. As expected, Probe 1 displayed excellent selectivity and sensitivity for detecting ClO- and Cu2+ over other competing ions in their respective systems. The limits of detection for ClO- and Cu2+ were 24.62 nM and 0.39 nM, respectively. The recognition mechanisms were proved by 1H NMR, mass spectrum and theoretical calculations. More importantly, the reversibility of Cu2+ could be applicable to rapid quantification for ClO-/Cu2+. Thus, the strategy for sensing ClO-/Cu2+ was carried out, which revealed broad applications of Probe 1 in multiple actual water and biology.

Remarkable enhancement of La(1-x)SmxCrO3 nanoperovskite properties: An influence of its doping concentrations

Rare earth orthochromites of type La(1-x)SmxCrO3 ((LSCO) x=0, 0.1, 0.3, 0.5) have been synthesized through solid state reaction. These polycrystalline samples exhibit orthorhombic crystal structure and its corresponding distortion depends on the dopant concentrations. With increasing dopant stoichiometry, the A-O bond length changes resulting in the augmentation of perovskite’s physical properties. The occupancy of relatively less ionic radii dopant influences the decrease in crystallite size, blue shift in emission spectrum and intensification of the phonon characteristic vibrations. The finer particles aggregate to form dendritic morphology and become distinct on Sm/La ratio increments as identified from electron micrographs and atomic force microscopy. Ambient temperature magnetization analysis reveals the enhancement of antiferromagnetic nature upon dopant increments.

What Is the Optoelectronic Effect of the Capsule on the Guest Molecule in Aqueous Host/Guest Complexes? A Combined Computational and Spectroscopic Perspective

Encapsulation of dye molecules is used as a means to achieve charge separation across different dielectric environments. We analyze the absorption and emission spectra of several coumarin molecules that are encapsulated within an octa-acid dimer forming a molecular capsule. The water-solvated capsule effect on the coumarin’s electronic structure and absorption spectra can be understood as due to an effective dielectric constant where the capsule partially shields electrostatically the dielectric solvent environment. Blue-shifted emission spectra are explained as resulting from a partial intermolecular charge transfer where the capsule is the acceptor, and which reduces the coumarin relaxation in the excited state.