Nanocrystals for large Stokes shift-based optosensing

The influence of Stokes shift in optosensing was discussed. Then, the current status of large Stokes shift-based optosensing was reviewed here.     

Nanocrystals for large Stokes shift-based optosensing

Stokes shift is an important feature of fluorescence, which reveals the energy loss between the excitation and the emission. For most fluorescent materials(e.g., organic dyes and proteins), the large overlap between the absorption and emission spectra endow them a small Stokes shift that induced reabsorption by fluorophore itself. Although the self-absorption can be effectively reduced due to the emergence of fluorescent nanomaterials, fluorescence attenuation is still observed in aggregated or concentrated nanocrystals, causing reduced sensitivity of biosensors. Therefore, increasing the Stokes shift can effectively improve the performance of nano-agents based biosensing. In this critical review, through understanding the Stokes shift from the viewpoint of self-absorption, the influence of Stokes shift on fluorescence properties are discussed. Based on the principle of changing the Stokes shift of fluorescent nanomaterials, we described the methods for constructing various optically large Stokes shift-based nanomaterials, and the application of these nanocrystals in biosensing is especially concerned in this review.     

A novel fluorescent probe with a large Stokes shift for real-time imaging mitochondria in different living cell lines

Fluorescent dyes with large Stokes shift play a key role in avoiding self-quenching and scattered light of dyes in the process of biological imaging. In this work, a novel mitochondria-targetable fluorescent dye (PI-C2) with large Stokes shift (e. g. Maximum value is 219 nm in DMSO) have been developed. Compared to the commercial mitochondria probes MTR and MTG (Less than 30 nm in various solution), the newly constructed PI-C2 has a much larger Stokes shift in various solutions (169–219 nm in various solutions). Furthermore, the probe can successfully be applied for sensing mitochondria, and exhibited excellent photostability in different living cell lines. The novel fluorescent platform with the large Stokes may be extended to construct powerful fluorescent probes with large Stokes shift for detecting a wide variety of biomolecules in mitochondria.     

Experimental and theoretical study of the 2-alkoxyethylidene rearrangement

The rearrangement of 2-ethoxyethylidene, generated photochemically from a nonnitrogenous precursor, leads to ethyl vinyl ether. Although this product could result, in principle, from a 1,2-hydrogen shift and/or a 1,2-ethoxy shift in the carbene, a deuterium labeling study indicates an essentially exclusive preference for hydrogen migration. The experimental results are in agreement with CCSD and W1BD calculations for the simpler 2-methoxyethylidene system that show a prohibitively large barrier for the methoxy shift and a negligible barrier for the hydride shift.     

Design and synthesis of an enzyme-cleavable sensor molecule for phosphodiesterase activity based on fluorescence resonance energy transfer

Ratiometric measurement is a technique that can provide precise data and even quantitative detection. To carry out ratiometric measurements, it is necessary that the sensor molecule exhibits a large shift in its emission or excitation spectrum after reaction with the target molecule. Fluorescence resonance energy transfer (FRET) is one mechanism used to obtain a large spectral shift. In this study, our aim was to develop a ratiometric fluorescent sensor molecule for phosphodiesterase activity based on FRET. We designed and synthesized CPF4 with a coumarin donor, a fluorescein acceptor, and two phenyl linkers having the phosphodiester moiety interposed between them. In the emission spectrum of CPF4 in aqueous buffer excited at 370 nm, the emission of the coumarin donor was strongly quenched and the emission of the fluorescein acceptor was observed. This emission spectrum demonstrates that energy transfer from the coumarin donor to the fluorescein acceptor proceeds efficiently. Addition of a phosphodiesterase to an aqueous solution of CPF4 resulted in an increase in the donor fluorescence and a decrease in the acceptor fluorescence. CPF4 exhibited a large shift in its emission spectrum after the hydrolysis of the phosphodiester group by the enzyme. This large shift of the emission spectrum indicates that ratiometric measurements can be made by using CPF4. The method described in this paper for designing enzyme-cleavable sensor molecules based on FRET should be readily applicable to other hydrolytic enzymes.     

Naphthalene derivatives of a conformationally locked GFP chromophore with large stokes shifts

The active development of fluorescence microscopy requires an increase in the variety of the dyes and their characteristics. Compounds with a large Stokes shift, i.e., a large difference between the positions of the absorption and emission maxima, have attracted the interest of researchers as a tool that can be used for multicolor labeling. One of the known approaches to increase the Stokes shift is the introduction of additional polycyclic fragments. Herein, we report novel derivatives of a conformationally locked GFP chromophore containing the naphthalene ring. The proposed modification leads not only to the enhancement of the Stokes shift up to 100 nm but also leads to the noticeable red-shift of the emission and absorption spectra in contrast with the corresponding derivatives with one benzene ring.     

Conformational studies on hexahelicenes—I: The NMR spectra of the benzohexahelicenes

The NMR spectra of the benzohexahelicenes have been analyzed. The conformation of the helical structure is not changed to a large extent by the benzo-groups. Some flattening of a part of the helix results in a small downfield shift of the protons on the terminal rings and in a larger upfield shift of proton A in II ascribed to a change in the ring current in the pitch of the helix. A gradual upfield shift is found for protons of the terminal rings by an increase in overcrowding of the benzo-groups.     

Origin of the blue shift of the CH stretching band for 2-butoxyethanol in water

The blue shift of the isolated CD stretching band of 2-butoxyethanol (C4E1), which is observed for the aqueous solution during the dilution process, has been investigated by infrared (IR) spectroscopy and quantum chemical calculations. Mono-deuterium-labeled C4E1's were employed to remove the severe overlapping among the CH stretching bands. The isolated CD stretching mode of the alpha-methylene in the butoxy group shows a large blue shift, while those of the beta-methylene and methyl groups are not largely shifted. The spectral simulation results for the C4E1/H2O complexes indicate that the large blue shift of the CD stretching band of the butoxy group arises mainly from the hydration of the ether oxygen atom.     

Importance of structural tightening, as opposed to partially bound States, in the determination of chemical shift changes at noncovalently bonded interfaces

Two models (A and B) have been proposed to account for decreased downfield chemical shifts of a proton bound by noncovalent interactions at a ligand/antibiotic interface as the number of ligand/antibiotic interactions is decreased. In model A, the proton involved in the noncovalent bond suffers a smaller downfield shift because the bond is, with a relatively large probability, broken, and not because it is longer. In model B, the proton involved in the noncovalent bond suffers a smaller downfield shift because the bond is longer, and not because it is, with a relatively large probability, broken. We show that model A cannot account for the chemical shift changes. Model B accounts for the process of positively cooperative binding, in which noncovalent bonds are reduced in length and thereby increase the stability of the organized state.     

Two-photon Lee-Goldburg nuclear magnetic resonance: Simultaneous homonuclear decoupling and signal acquisition

We present NMR signals from a strongly coupled homonuclear spin system, (1)H nuclei in adamantane, acquired with simultaneous two-photon excitation under conditions of the Lee-Goldburg experiment. Small coils, having inside diameters of 0.36 mm, are used to achieve two-photon nutation frequencies of approximately 20 kHz. The very large rf field strengths required give rise to large Bloch-Siegert shifts that cannot be neglected. These experiments are found to be extremely sensitive to inhomogeneity of the applied rf field, and due to the Bloch-Siegert shift, exhibit a large asymmetry in response between the upper and lower Lee-Goldburg offsets. Two-photon excitation has the potential to enhance both the sensitivity and performance of homonuclear dipolar decoupling, but is made challenging by the high rf power required and the difficulties introduced by the inhomogeneous Bloch-Siegert shift. We briefly discuss a variation of the frequency-switched Lee-Goldburg technique, called four-quadrant Lee-Goldburg (4QLG) that produces net precession in the x-y plane, with a reduced chemical shift scaling factor of 13.     

NMR solvent shifts of acetonitrile from frozen density embedding calculations

We present a density functional theory (DFT) study of solvent effects on nuclear magnetic shielding parameters. As a test example we have focused on the sensitive nitrogen shift of acetonitrile immersed in a selected set of solvents, namely water, chloroform, and cyclohexane. To include the effect of the solvent environment in an accurate and efficient manner, we employed the frozen-density embedding (FDE) scheme. We have included up to 500 solvent molecules in the NMR computations and obtained the cluster geometries from a large set of conformations generated with molecular dynamics. For small solute-solvent clusters comparison of the FDE results with conventional supermolecular DFT calculations shows close agreement. For the large solute-solvent clusters the solvent shift values are compared with experimental data and with values obtained using continuum solvent models. For the water --> cyclohexane shift the obtained value is in very good agreement with experiments. For the water --> chloroform NMR solvent shift the classical force field used in the molecular dynamics simulations is found to introduce an error. This error can be largely avoided by using geometries taken from Car-Parrinello molecular dynamics simulations.     

[IrBr6]2-的反对称极化率与圆偏振激光诱导的ESR谱频移

在理论上研究了顺磁性粒子体系的圆偏振激光诱导的ＥＳＲ谱频移效应。给出磁场中分子反对称极化率的公式，并以［ＩｒＢｒ６］＾２－离子为例讨论了吸收带半高宽的影响。计算了［ＩｒＢｒ６］＾２－的ＥＳＲ谱在圆偏振激光作用下的频移。结果发现：在光学共振区附近，ＥＳＲ频移可达１ＭＨｚ的量级，与钠原子体系结果类似。     

Unpaired and sigma bond electrons as H, Cl, and Li Bond Acceptors: an anomalous one-electron blue-shifting chlorine bond

Ab initio, DFT, and AIM theoretical studies on H-, Cl-, and Li-bonded complexes have been carried out with typical lone pair (H2O), pi (C2H4) and sigma (H2) bonded pairs, and unpaired (CH3) electrons as acceptors and HF, ClF, and LiF as donors. Optimization and frequency calculations have been carried out at reasonably high levels (MP2, DFT(B3LYP), and QCISD) with large basis sets up to aug-cc-pVTZ. Not surprisingly, all HF complexes show red shift in stretching frequency and the shift is correlated to the binding energy. However, the FCl...CH3 complex shows a large blue shift (about 200 cm-1), which appears to be the largest blue shift predicted for any weakly bound complex yet. Analysis of the normal modes of the complex indicates that the shift is due to the mixing of modes between donor and acceptor and it is qualitatively different from the blue shifts reported thus far in hydrogen-bonded complexes. For Cl- and Li-bonded complexes, a correlation between frequency shift and binding energy is not found. However, AIM theoretical analysis shows the similarity in all these interactions. The electron density at the bond critical point shows a strong correlation with the binding energy for H-, Cl-, and Li-bonded complexes. This appears to be the first report on a one-electron chlorine bond.     

Finite-element calculations on the behaviour of a DSC in temperature-modulated mode

The influence of changes of sample properties on the amplitude and phase shift of the differential-temperature signal as well as the influence of frequency changes has been calculated for a one-dimensional model of a temperature-modulated DSC (TMDSC) using a computer program for finite-element-method (FEM) calculations. Amplitude and phase shift of the measured signal ΔT (which is proportional to the differential heat-flow rate) is strongly influenced by the heat capacity of the sample. The connection is only linear for rather small heat capacities. The influence of the heat-transfer coefficient between sample and sample pan on amplitude and phase shift of the signal is not so large and linear (within the framework of our calculations). The influence of the heat-transfer coefficient between sample and sample pan on amplitude and phase shift of the signal is not so large and linear (within the framework of our calculations). For precise measurements, a very careful “calibration” is needed, which must take all the aforementioned influences into account.     

Synthesis,Characterization, and Computational Investigation of Bright Orange‐Emitting Benzothiadiazole [10]Cycloparaphenylene

Conjugated aromatic macrocycles are attractive due to their unique photophysical and optoelectronic properties. In particular, the cyclic radially oriented π‐system of cycloparaphenylenes (CPPs) gives rise to photophysical properties unlike any other small molecule or carbon nanomaterial. CPPs have tunable emission, possess large extinction coefficients, wide effective Stokes shifts, and high quantum yields. However, accessing bright CPPs with emissions beyond 500 nm remains difficult. Herein, we present a novel and bright orange‐emitting CPP‐based fluorophore showing a dramatic 105 nm red‐shift in emission and striking 237 nm effective Stokes shift while retaining a large quantum yield of 0.59. We postulate, and experimentally and theoretically support, that the quantum yield remains large due to the lack of intramolecular charge transfer.     

Coupled Hartree-Fock calculation of the H-bond chemical shift in FHF

A coupled Hartree-Fock calculation of the proton shielding constant in FHF⁻ with a reasonably good basis set of contracted gaussian orbitals leads to an extremely large upfield H-bond chemical shift with respect to the HF molecule. An approximate scheme, based on the requirement of the gauge origin independence of the finite basis set coupled Hartree-Fock calculations of the magnetic susceptibility, provides a considerable improvement of the computed proton shielding constant and results in the required downfield H-bond chemical shift of the proton resonance. The computed H-bond shift for FHF⁻ agrees with the experimental data.     

Theoretical Insight into the Origin of Large Stokes Shift and Photophysical Properties of Anilido‐Pyridine Boron Difluoride Dyes

The geometric and electronic structures and photophysical properties of anilido‐pyridine boron difluoride dyes 1 – 4 , a series of scarce 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) derivatives with large Stokes shift, are investigated by employing density functional theory (DFT) and time‐dependent DFT (TD‐DFT) calculations to shed light on the origin of their large Stokes shifts. To this end, a suitable functional is first determined based on functional tests and a recently proposed index—the charge‐transfer distance. It is found that PBE0 provides satisfactory overall results. An in‐depth insight into Huang–Rhys (HR) factors, Wiberg bond indices, and transition density matrices is provided to scrutinize the geometric distortions and the character of excited states pertaining to absorption and emission. The results show that the pronounced geometric distortion due to the rotation of unlocked phenyl groups and intramolecular charge transfer are responsible for the large Stokes shift of 1 and 2 , while 3 shows a relatively blue‐shifted emission wavelength due to its mild geometric distortion upon photoemission, although it has a comparable energy gap to 1 . Finally, compound 4 , which is designed to realize the rare red emission in BODIPY derivatives, shows desirable and expected properties, such as high Stokes shift (4847 cm^?1), red emission at 660 nm, and reasonable fluorescence efficiency. These properties give it great potential as an ideal emitter in organic light‐emitting diodes. The theoretical results could complement and assist in the development of BODIPY‐based dyes with both large Stokes shift and high quantum efficiency.     

Stokes Shift in Inorganic Lead Halide Perovskites: Current Status and Perspective

Inorganic metal halide perovskite system is considered as a promising candidate for applications from display to biomedical industry. Intrinsic inorganic lead halides possess small Stokes shift or self-absorption, providing negative impact for both photo voltaic and biomedical applications. Therefore, the development of an inorganic halide perovskite system with large Stokes shift is a significant venture. This review aims to provide an updated survey of the Stokes shift phenomena in the inorganic lead halide perovskites. The first section focuses about the mechanism, the second section gives different approaches in preparing inorganic perovskites with distinct Stokes shift, while the third section highlights the potential applications in both photovoltaic and biomedical areas. This review provides deep insight about the importance and usefulness of such phenomena in inorganic lead halides, essential for various applications.     

Deuterium isotope effects on 13C chemical shifts of nitromalonamide

The OH chemical shift of the enol form of nitromalonamide is found at 18.9 ppm both in DMSO-d(6) and in DMF-d(7) indicating a very strong hydrogen bond. The OH chemical shift is insensitive to temperature changes. Contrary to the large OH chemical shift, a small two-bond deuterium isotope effect of 0.135 ppm due to deuteration at the OH position is found at the enolic carbon. This is confirmed by density functional theory calculations. The observed effects are interpreted as due to an equilibrium between identical enolic forms. These show a strong OH...O hydrogen bond as well as a NH...O-N=O hydrogen bond.