Small Stokes Shift Research Articles

Building Molecular Complexity from Quinizarin: Conjoined Coumarins and Coronene Analogs.

The double Knoevenagel condensation of 1,4-dibenzoyloxyanthraquinone with methyl esters of arylacetic acids affords a series of compounds based upon a previously unknown 1,8-dioxa-benzo[e]pyrene-2,7-dione heterocyclic core. The aryl groups incorporated in the 3- and 6-positions can be oxidatively coupled to the π-expanded backbone to produce a further new heterocyclic core: 1,10-dioxa-dibenzo[dj]coronene-2,9-dione. The intriguing optical properties of these π-expanded coumarin derivatives are discussed and rationalized through quantum chemical calculations. The broad absorption bands of 1,8-dioxa-benzo[e]pyrene-2,7-dione-based dyes are attributed to both HOMO-1→LUMO and HOMO→LUMO transitions, which have a similar energy. Weakly coupled electron-donating aryl substituents result in a moderate bathochromic shift of both the absorption and emission by 30-60 nm in toluene. The emissive properties of these compounds are in part determined by the oscillator strength of the main transition, lifetimes of the excited state, and by the energy match of the excited state with a triplet state of a similar energy. The 1,10-dioxa-dibenzo[dj]coronene-2,9-dione displays a much smaller Stokes shift, yet a markedly increased fluorescence quantum yield of 90 % owing to the increased rigidity compared with the 1,8-dioxa-benzo[e]pyrene-2,7-dione core.

Excitation- and Emission-Wavelength-Based Multiplex Spectroscopy Using Red-Absorbing Near-Infrared-Emitting Lanthanide Complexes

Multiplex imaging in the red and near-infrared (NIR) should be an enabling tool for the real-time investigation of biological systems. Currently available emitters have short luminescent lifetimes, broad absorption and emission bands, and small Stokes shifts, which limits multiplexing in this region to two colors. NIR-emitting luminescent lanthanide (Ln) complexes carrying hydroporphyrin (chlorin) sensitizing antennae are excitable in the red through the narrow, intense and tunable chlorin absorptions. Both emission- and excitation-based multiplexing are possible, the former by exciting the same antenna appended to different Lns, the latter by attaching different chlorins with nonoverlapping absorptions to the same Ln. The combination of excitation and emission spectroscopies allows for the straightforward differentiation of up to four different complexes.

Learning from a Mineral Structure toward an Ultra‐Narrow‐Band Blue‐Emitting Silicate Phosphor RbNa3(Li3SiO4)4:Eu2+

AbstractLearning from natural mineral structures is an efficient way to develop potential host lattices for applications in phosphor converted (pc)LEDs. A narrow‐band blue‐emitting silicate phosphor, RbNa3(Li3SiO4)4:Eu2+ (RNLSO:Eu2+), was derived from the UCr4C4‐type mineral model. The broad excitation spectrum (320–440 nm) indicates this phosphor can be well matched with the near ultraviolet (n‐UV) LED chip. Owing to the UCr4C4‐type highly condensed and rigid framework, RNLSO:Eu2+ exhibits an extremely small Stokes shift and an unprecedented ultra‐narrow (full‐width at half‐maximum, FWHM=22.4 nm) blue emission band (λem=471 nm) as well as excellent thermal stability (96 %@150 °C of the initial integrated intensity at 25 °C). The color gamut of the as‐fabricated (pc)LEDs is 75 % NTSC for the application in liquid crystal displays from the prototype design of an n‐UV LED chip and the narrow‐band RNLSO:Eu2+ (blue), β‐SiAlON:Eu2+ (green), and K2SiF6:Mn4+ (red) components as RGB emitters.

Learning from a Mineral Structure toward an Ultra-Narrow-Band Blue-Emitting Silicate Phosphor RbNa3 (Li3 SiO4 )4 :Eu2.

Learning from natural mineral structures is an efficient way to develop potential host lattices for applications in phosphor converted (pc)LEDs. A narrow-band blue-emitting silicate phosphor, RbNa3 (Li3 SiO4 )4 :Eu2+ (RNLSO:Eu2+ ), was derived from the UCr4 C4 -type mineral model. The broad excitation spectrum (320-440 nm) indicates this phosphor can be well matched with the near ultraviolet (n-UV) LED chip. Owing to the UCr4 C4 -type highly condensed and rigid framework, RNLSO:Eu2+ exhibits an extremely small Stokes shift and an unprecedented ultra-narrow (full-width at half-maximum, FWHM=22.4 nm) blue emission band (λem =471 nm) as well as excellent thermal stability (96 %@150 °C of the initial integrated intensity at 25 °C). The color gamut of the as-fabricated (pc)LEDs is 75 % NTSC for the application in liquid crystal displays from the prototype design of an n-UV LED chip and the narrow-band RNLSO:Eu2+ (blue), β-SiAlON:Eu2+ (green), and K2 SiF6 :Mn4+ (red) components as RGB emitters.

Self-Activated Low-Dimensional Metal Halide Phosphors and Scintillators

We report theoretical studies of exciton excitation and emission in low-dimensional metal halides, (C4N2H14X)4SnX6 (X = Br, I), Cs4PbBr6, (Ph4P)2SbCl5 (Ph4P = tetraphenylphosphonium), (C9NH20)2SnBr4, and (C4N2H14)PbCl4. These are bulk crystalline materials, in which isolated metal halide clusters/wires are separated from each other by large inorganic or organic countercations, forming 0D/1D structures. For example, in (C9NH20)2SnBr4, the anionic (SnBr4)2- clusters are separated from each other by (C9NH20)+ molecular countercations. The metal halides shown above all have 0D structures except (C4N2H14)PbCl4, which has a 1D wire structure. High photoluminescence quantum efficiencies (PLQE) (close to 100% in some cases) have been reported for the 0D metal halides. We calculated exciton excitation and emission energies in these low-dimensional metal halides; the results are in excellent agreement with the experimental values. This enables us to study the luminescence mechanisms and suggest new compounds for synthesis. We show that exciton self-trapping in the metal-halide cluster/wire leads to localized excitons, which is important for efficient radiative recombination. The relatively low PLQE (6% - 18%) in 1D (C4N2H14)PbCl4 is likely due to the exciton diffusion along the wire and the subsequent nonradiative recombination at defects. The large organic countercations in hybrid organic-inorganic 0D halides plays the important role of decoupling metal halide clusters (which are luminescent centers), leading to highly immobile excitons and high PLQE. On the other hand, in all inorganic 0D Cs4PbBr6, the significant electronic coupling between PbBr6 clusters and the small Stokes shift should enable resonant transfer of excitation energy and the subsequent energy loss at defects, which explains the strong thermal quenching of luminescence in Cs4PbBr6. We further suggest that Cs4EuX6 (X = Br,I), which are analogs to Cs4PbBr6, should exhibit improved luminescent properties compared to Cs4PbBr6 due to stronger exciton localization at Eu2+ than at Pb2+. Indeed, recent synthesis and characterization of these compounds show high light yields under gamma-ray irradiation at room temperature. In particular, the light yield of Cs4EuBr6 (78,000 photons/MeV) is the highest among the self-activated scintillators.

Accounting for Secondary Inner Filter Effect in Fluorescence Spectra from Solid Samples

Fluorescence experiments at high fluorophore concentrations or in solid state have biological relevance and application potential in devices. Inner filter effect, arising from reabsorption of emitted photons, is the simplest of several factors that complicate the emission in these systems. This effect is prominent, especially for systems with small Stokes shift between absorption and emission. Solid state ‘dilution’ of a fluorophore with an optically transparent, non-interfering substance can significantly minimize this effect. This has been shown in the present study, using salophen as a fluorophore and NaCl as the dilutant. The general applicability of this method is tested with three more fluorophores.

Ultra‐Narrow‐Band Blue‐Emitting Oxoberyllates AELi2[Be4O6]:Eu2+ (AE=Sr,Ba) Paving the Way to Efficient RGB pc‐LEDs

AbstractHighly efficient phosphor‐converted light‐emitting diodes (pc‐LEDs) are popular in lighting and high‐tech electronics applications. The main goals of present LED research are increasing light quality, preserving color point stability and reducing energy consumption. For those purposes excellent phosphors in all spectral regions are required. Here, we report on ultra‐narrow band blue emitting oxoberyllates AELi2[Be4O6]:Eu2+ (AE=Sr,Ba) exhibiting a rigid covalent network isotypic to the nitridoalumosilicate BaLi2[(Al2Si2)N6]:Eu2+. The oxoberyllates’ extremely small Stokes shift and unprecedented ultra‐narrow band blue emission with fwhm ≈25 nm (≈1200 cm−1) at λem=454–456 nm result from its rigid, highly condensed tetrahedra network. AELi2[Be4O6]:Eu2+ allows for using short‐wavelength blue LEDs (λem<440 nm) for efficient excitation of the ultra‐narrow band blue phosphor, for application in violet pumped white RGB phosphor LEDs with improved color point stability, excellent color rendering, and high energy efficiency.

Ultra-Narrow-Band Blue-Emitting Oxoberyllates AELi2 [Be4 O6 ]:Eu2+ (AE=Sr,Ba) Paving the Way to Efficient RGB pc-LEDs.

Highly efficient phosphor-converted light-emitting diodes (pc-LEDs) are popular in lighting and high-tech electronics applications. The main goals of present LED research are increasing light quality, preserving color point stability and reducing energy consumption. For those purposes excellent phosphors in all spectral regions are required. Here, we report on ultra-narrow band blue emitting oxoberyllates AELi2 [Be4 O6 ]:Eu2+ (AE=Sr,Ba) exhibiting a rigid covalent network isotypic to the nitridoalumosilicate BaLi2 [(Al2 Si2 )N6 ]:Eu2+ . The oxoberyllates' extremely small Stokes shift and unprecedented ultra-narrow band blue emission with fwhm ≈25 nm (≈1200 cm-1 ) at λem =454-456 nm result from its rigid, highly condensed tetrahedra network. AELi2 [Be4 O6 ]:Eu2+ allows for using short-wavelength blue LEDs (λem <440 nm) for efficient excitation of the ultra-narrow band blue phosphor, for application in violet pumped white RGB phosphor LEDs with improved color point stability, excellent color rendering, and high energy efficiency.

Temperature dependent Cr3+ photoluminescence in garnets of the type X3Sc2Ga3O12 (X = Lu, Y, Gd, La)

Broad band near-infrared (NIR) light sources have great potential, e.g., in optical communication and non-invasive medical imaging. Here we report the optical properties of X3Sc2Ga3O12 (X = Lu, Y, Gd, La) garnets doped with Cr3+ showing efficient 4T2 → 4A2 broad band NIR emission between 600 and 1000 nm. The chromium-doped garnets were investigated in view of their capability for blue to NIR conversion in high power phosphor converted LEDs (pcLEDs). Typical high power (In,Ga)N LEDs reach junction temperatures up to 450 K. Hence, the photoluminescence (PL) quenching temperature is a crucial parameter for on-chip phosphors. The Cr3+-doped garnets reveal very high quenching temperatures for the broad band NIR emission, ~ 700 K for Lu3Sc2Ga3O12, Y3Sc2Ga3O12, Gd3Sc2Ga3O12, and 450 K for La3Sc2Ga3O12. The high quenching temperatures are in line with a small Stokes shift (~ 2400 cm−1) and weak electron-phonon coupling. To gain further insight in the luminescence properties, low temperature photoluminescence measurements were done. The optical properties of Cr3+ ions in garnets were analyzed in terms of the crystal field strength, Racah parameters, and phonon coupling parameters. The suitability for application in pcLEDs for high power broad band NIR sources was confirmed by emission measurements of ceramic pellets upon blue LED excitation.

A General Method To Increase Stokes Shift by Introducing Alternating Vibronic Structures.

Fluorescent dyes have enabled much progress in the broad range of biomedical fields. However, many commercially available dyes suffer from small Stokes shifts, resulting in poor signal-to-noise ratio and self-quenching on current microscope configurations. In this work, we have developed a general method to significantly increase the Stokes shifts of common fluorophores. By simply appending a 1,4-diethyl-decahydro-quinoxaline (DQ) moiety onto the conjugated structure, we introduced a vibronic backbone that could facilely expand the Stokes shifts, emission wavelength, and photostability of 11 different fluorophores by more than 3-fold. This generalizable method could significantly improve the imaging efficiency of commercial fluorophores. As a demonstration, we showed that the DQ derivative of hemicyanine generated 5-fold signal in mouse models over indocyanine green. Furthermore, the DQ-modified fluorophores could pair with their parent molecules to conduct one-excitation, multiple emission imaging, allowing us to study the cell behavior more robustly. This approach shows promise in generating dyes suitable for super-resolution microscopy and second window near-infrared imaging.

Endoplasmic Reticulum-Targeted Fluorescent Nanodot with Large Stokes Shift for Vesicular Transport Monitoring and Long-Term Bioimaging.

Herein, a highly stable aggregation-induced emission (AIE) fluorescent nanodot assembled by an amphiphilic quinoxalinone derivative-peptide conjugate, namely Quino-1-Fmoc-RACR (also termed as Q1-PEP), which exhibits large Stokes shift and an endoplasmic reticulum (ER)-targeting capacity for bioimaging is reported. It is found that the resulting nanodot can effectively enter the ER with high fluorescent emission. As the ER is mainly involved in the transport of synthesized proteins in vesicles to the Golgi or lysosomes, the Q1-PEP nanodot with ER-targeting capacity can be used to monitor vesicular transport inside the cells. Compared to conventional fluorescent dyes with small Stokes shifts, the self-assembled fluorescent nanodot shows superior resistance to photobleaching and aggregation-induced fluorescence quenching, and elimination of the spectra overlap with autofluorescence of biosubstrate owning to their AIE-active and red fluorescence emission characteristics. All these optical properties make the fluorescent nanodot suitable for noninvasive and long-term imaging both in vitro and in vivo.

A two-photon ESIPT based fluorescence probe for specific detection of hypochlorite

Hypochlorous acid (HOCl) is one important member of reactive oxygen species (ROS) familyresponsible for various human diseases. Although many contributions have been made to develop fluorescent probes for sensing HOCl, many of them suffer small Stokes shifts, sophisticated synthetic procedure, and slow reaction rate. In the current study, we designed and synthesized an ESIPT based fluorescence probe with diaminomeleonitrile (DMN) as reactive site. The probe can detect HOCl with high selectivity and sensitivity. It also has large Stokes shift and rapid reaction rate. Moreover, it can be utilized to monitor exogenous and endogenous HOCl in living cells and tissues via two-photon microscopy.

Understanding and Controlling the Emission Brightness and Color of Molecular Cerium Luminophores.

Molecular cerium complexes are a new class of tunable and energy-efficient visible- and UV-luminophores. Understanding and controlling the emission brightness and color are important for tailoring them for new and specialized applications. Herein, we describe the experimental and computational analyses for series of tris(guanidinate) (1-8, Ce{(R2N)C(N iPr)2}3, R = alkyl, silyl, or phenyl groups), guanidinate-amide [GA, A = N(SiMe3)2, G = (Me3Si)2NC(N iPr)2], and guanidinate-aryloxide (GOAr, OAr = 2,6-di- tert-butylphenoxide) cerium(III) complexes to understand and develop predictive capabilities for their optical properties. Structural studies performed on complexes 1-8 revealed marked differences in the steric encumbrance around the cerium center induced by various guanidinate ligand backbone substituents, a property that was correlated to photoluminescent quantum yield. Computational studies revealed that consecutive replacements of the amide and aryloxide ligands by guanidinate ligand led to less nonradiative relaxation of bright excited states and smaller Stokes shifts. The results establish a comprehensive structure-luminescence model for molecular cerium(III) luminophores in terms of both quantum yields and colors. The results provide a clear basis for the design of tunable, molecular, cerium-based, luminescent materials.

How to study picosecond solvation dynamics using fluorescent probes with small Stokes shifts

Xanthene dyes have wide ranging applications as fluorescent probes in analytical, biochemical and medical contexts. Being cationic/anionic in nature, the solvation dynamics of xanthene dyes confined within a negatively/positively charged interface are very interesting. Unfortunately, the floppy structure and small Stokes shift render any xanthene dye unsuitable for use as a solvation probe. Using di-sodium fluorescein, we present our work on the picosecond solvation dynamics of bulk and confined water (at pH = 9.2). We also propose a new methodology for studying picosecond solvation dynamics using any fluorescent dye with a small Stokes shift. We discuss how scattering contributions can be effectively removed, and propose an alternative way of defining zero time of solvation. Finally, we demonstrate the tuning location of the probe within confinement.

Electrogenerated Chemiluminescence of a BODIPY Derivative with Extended Conjugation

AbstractThe electrochemical properties and electrogenerated chemiluminescence (ECL) of a borondipyrromethane (BODIPY) derivative, 2,8‐bis(4‐isopropylphenyl)‐difuro[2, 3‐b][3, 2‐g]‐5,5‐difluoro‐5‐bora‐3a,4a‐diaza‐s‐indacene (Derivative 1, Figure 1) are described. Derivative 1 emitted ECL in the range of 650∼800 nm by using tripropylamine (TPA) as a coreactant. In the cyclic voltammogram, Derivative 1 showed a reversible oxidation wave, whereas the reduction wave was irreversible. Energetically, it is likely that triplet‐triplet annihilation is the main pathway to emit ECL through the ion annihilation of the radical anions and cations of Derivative 1 whereas the lowest excited singlet state of Derivative 1 is directly produced with TPA. The maximum ECL wavelengths (670 nm at 0.01 mM) were red‐shifted with increasing the concentration of Derivative 1 because of the self‐absorption resulted from a small Stokes shift (∼10 nm). Relative ECL efficiency of Derivative 1 was 0.13 compared to that of a Ru(bpy)2+/TPA system.

Reaction of 3,5-di-(tert-butyl)-o-benzoquinone with arylamines developing to the formation of a pentaheterocyclic 12Н-quinoxaline[2,3-b]phenoxazine system. A deeper insight into the reaction mechanism

Reaction between sterically crowded 3,5-di-(tert-butyl)-o-benzoquinone and aromatic amines occurring at the oxidative conditions and developing to the formation of compounds 8, derivatives of the pentaheterocyclic 12Н-quinoxaline[2,3-b]phenoxazine system 2 was studied. The products obtained at the intermediate stages, including di-(tert-butyl) derivatives of the tricyclic 10H-phenoxazine system: (E)-N-3Н-phenoxazin-3-ylidene-4-methoxyaniline 3, N-(aryl)-3-arylimine-3Н-phenoxazine-2-amine 4, 1-phenyl-10H-phenoxazine 5 and 1-phenyl-3H-phenoxazin-3-one 6 were preparatively isolated and their molecular structures determined by X-ray crystallography. A stable 1-phenyl-6,8-di-(tert-butyl)-10H-phenoxazine radical 7, a primary product of oxidation of 5 was prepared and characterized by the ESR spectroscopy. Compounds 8 exhibit strong red luminescence distinguished by small Stokes shifts, short fluorescent lifetimes and high quantum yields.

A simple method of measuring the effective SRS coefficient in single-mode optical fibres and the range of its applicability

This paper describes techniques for measuring the SRS coefficient in a wide spectral range, including the region of small Stokes shifts. A simple, approximate method is proposed for evaluating the SRS coefficient near a gain peak. Spectral dependences of the SRS coefficient are presented for various telecom fibres.

Photophysical Characterization and BSA Interaction of Direct Ring Carboxy Functionalized Symmetrical squaraine Dyes

A series of far-red sensitive symmetrical squaraine dyes bearing direct –COOH functionalized indole ring were synthesized, characterized and subjected to photophysical investigations. These symmetrical squaraine dyes were then subjected to investigate their interaction with bovine serum albumin (BSA) in Phosphate buffer solutions. All the squaraine dyes under investigation exhibit intense and sharp optical absorption mainly in the far-red wavelength region from 550 nm -700 nm having very high molar extinction coefficients from 1.3 × 105 dm3.mol−1.cm−1. A very small Stokes shift of 10-17 nm indicates the rigid conformational structure of squaraine chromophore. Interaction of these dyes with BSA leads to not only enhanced emission intensity but also bathochromically shifted absorption maximum due to formation of dye-BSA conjugate. These dyes bind strongly with BSA having about an order of magnitude higher binding constant as compared to the reported squaraine dyes. Amongst the symmetrical squaraine dyes investigated in this work one bearing substituents like trifluorobutyl as alkyl chain at N-position of indole ring and carboxylic acid on benzene ring at the terminal (SQ-26) exhibited highest association with the BSA having very high binding constant 8.01 × 106 M−1.

Through-Bond Energy Transfer Cassette with Dual-Stokes Shifts for "Double Checked" Cell Imaging.

Organic dyes generally suffer from small Stokes shift that usually leads to self‐quenching and ‐gaining errors during the fluorescent imaging process. Here, a through‐bond energy transfer (TBET) cassette is developed with large Stokes shift to pursue precise cell imaging. The TBET is constructed by covalently conjugated tetraphenylethene (acts as donor) and rhodamine (acceptor) through an acetylene bond. The constructed TBET cassette distinctly behaves as dual‐Stokes shifts, including a large pseudo‐Stokes shift caused by energy transfer, from donor's absorption to acceptor's emission (up to 260 nm) and a smaller Stokes shift of acceptor molecules itself. Due to the intrinsic dual‐Stokes shifts, TBET cassette exhibits specific “dual distinct absorbances, single shared emission” properties, which can be excitated under two different laser channels. By colocalization of the imaging readouts of these two channels, the precisely “double checked” fluorescent imaging is achieved in living cells.

Aza-BOIMPYs: A Tetrazole Auxochrome for Highly Red-Emissive Dipyrromethene-Based Fluorophores.

A structural derivative of the recently designed BOIMPY motif is introduced; this derivative engages a meso tetrazole moiety and a dipyrromethene scaffold in a twofold BF2 -coordination scheme. A one-pot synthesis allows the isolation of unusual dipyrro-perazafulvene precursors and subsequent access to nonfluorescent mono-BF2 and fluorescent bis-BF2 species. The novel fluorophore, denoted Aza-BOIMPY, is highly emissive (ΦF up to 0.82), structurally compact, and provides useful inherent polarity. Absorption and emission events focus at roughly 600 nm with exceptionally small Stokes shifts (209 cm-1 ) and high attenuation coefficients (up to 120 000 m-1 cm-1 ). X-ray crystallographic and computational investigations provide insights into geometrical and electronic properties of the novel dye type.