Long-wavelength Fluorescence Research Articles

Unravelling the role of charge transfer state during ultrafast intersystem crossing in compact organic chromophores.

When organic electron donor (D) and acceptor (A) chromophores are linked together, an electron transfer (ET) state can take place. When a short bridge such as one Sigma bond is used to link the donor and the acceptor, complete charge separation is difficult to access and one usually observes an intramolecular charge transfer (CT) state instead. Due to the inevitable coupling between the donor and the acceptor in compact organic chromophores, the most common decay pathway for the CT state is charge recombination, which may lead to a distinct longer wavelength fluorescence emission or non-radiative dissipation of the excited state energy. However, recent studies have shown that unique excited state dynamics can be observed when the CT state is involved during both forward and backward intersystem crossing (ISC) from singlet excited states to triplet excited states in organic chromophores. Analysis of the mechanism for ISC involving the CT state has received much attention over the last decade. In this perspective, we present a collection of molecular design rationales, spectroscopy and theoretical investigations that provide insights into the mechanism of the ISC involving the CT state in compact organic chromophores. We hope that this perspective will prove beneficial for researchers to design novel compact organic chromophores with a predictable ISC property for future biochemical and optoelectronic applications.

The direct evidence for ESPT route and ICT emission of N6-Methyladenine in aqueous solution

N6-Methyladenine (6MeAde) was regarded as an important epigenetic biomarker in gene regulation. Recently, Zhou et al. observed a dual peak fluorescence emission of 6MeAde in aqueous solution (Phys. Chem. Chem. Phys., 2019, 21, 6878), in which the long-wavelength fluorescence was speculated to the isomerized intramolecular charge transfer (ICT). Nevertheless, this speculation still lacked a direct evidence and the essential mechanism of how 6MeAde was regulated by water molecules remained ambiguous. In present work, we theoretically studied the effect of water molecules on excited-state proton transfer (ESPT) process of 6MeAde based on the time-dependent DFT (TDDFT) methods. 6MeAde possessed both hydrogen acceptor and donor that formed intermolecular hydrogen bonds with water molecules. Thus, the excited-state hydrogen bonding dynamics may occur in three scenarios. The constructed potential energy surfaces provided direct evidence that proton transfer cannot occur for 6MeAde monomer and 6MeAde + H2O, but could occur spontaneously for 6MeAde+2H2O due to the coplanar closed-loop hydrogen bond structure. This result pointed out that determining the energy barrier of a potential energy surface was the key to explore the real ESPT channel. Besides, our analyses of electron spectra and electronic structures confirmed that the long-wavelength fluorescence originated from the ICT emission.

Optical characterization of LiF:Mg,Cu,P – Towards 3D optically stimulated luminescence dosimetry

The introduction of radiotherapy beam delivery with high spatial modulation, including spot-scanning proton therapy, calls for development of 3D dosimeters for patient-specific treatment verifications. Inherently reusable methods like optically stimulated luminescence (OSL) pose a potential way to introduce a clinically viable 3D-dosimetry solution. Here we present a characterization of the OSL properties of the well-known thermoluminescence material LiF:Mg,Cu,P (MCP). The characterization was performed by monitoring the wavelength-dispersed emission of continuous-wave stimulated MCP pellets. We show that OSL emission is observed under blue (460 nm) and green (532 nm) laser stimulation, while no OSL signal is seen in MCP under red (664 nm) laser stimulation. Long-wavelength fluorescence including accumulating radiophotoluminescence is very intense under blue stimulation and must be suppressed for continuous use of a future 3D-dosimetry system. An evaluation of the number of emitted photons is used to estimate a measurable dose range of 0.1–10 Gy, which prompts further research in the use of MCP for 3D OSL-based dosimetry.

First fluorescence sensor for simultaneously detecting three kinds of IIB elements (Zn2+, Cd2+ and Hg2+) based on aggregation-induced emission

This work report the first AIE-based fluorescence sensor that can simultaneously detect three kinds of IIB elements (Zn2+, Cd2+ and Hg2+) in aqueous media. The thiourea-bridging bis-tetraphenylethylene (Bis-TPE) was synthesized by the condensation reaction of formyltetraphenylethylene and bis-thiosemicarbazide derivative in 82 % yield. Due to the AIE effect in the aqueous medium, the long-wavelength fluorescence was observed at 500–526 nm. It displayed high selective Zn2+-sensing ability with red-to-green fluorescence change, selective Cd2+-sensing ability with red-to-yellow fluorescence change and selective Hg2+-sensing ability with fluorescence enhancement, respectively. Moreover, it could detect simultaneously Zn2+, Cd2+ and Hg2+ in the mixed solution of these three kinds of cations by the fluorescence change with the assistance of Cl− and ATP. The sensing mechanism was confirmed as 1:1 stoichiometric ratio between host and guest. The detection limits were as low as 8.22×10-8 M, 2.36×10-7 M and 3.43×10-8 M for Zn2+, Cd2+ and Hg2+, respectively. These sensing abilities were applied as test strips for detecting Zn2+, Cd2+ and Hg2+, and were used for living cell imaging, implying a high efficient and simple method for analyzing trace IIB elements (Zn2+, Cd2+ and Hg2+) simultaneously in complex environments and living bodies.

LC-BLYP Calculations of the Structures and Photophysical Properties of [1,3]Thiazolo[4,5-b]pyrazine Derivatives in Cyclohexane and Methanol.

Applications of the B3LYP and LC-BLYP functionals with the 6-311+G(d,p) basis set for predicting absorption and fluorescence spectra of benzothiazole and 11 [1,3]thiazolo[4,5-b]pyrazine (TPy) derivatives in implicit solvents (cyclohexane and methanol) were tested and compared with experimental results. The damping parameter of LC-BLYP was tuned for simulating absorption and fluorescence spectra of each compound. For the 11 TPy derivatives, four and seven compounds are small- and medium-sized TPy derivatives, respectively. From this calculation, TD-B3LYP can be used to simulate the absorption spectra of the small-sized compounds in cyclohexane and methanol, but it gives poor results for the fluorescence spectra in cyclohexane and methanol. Contrarily, the LC-BLYP functional with the optimal damping parameter can reproduce the absorption and fluorescence spectra of all compounds in cyclohexane and methanol. The values of the damping parameter of LC-BLYP depend on the types of substituents and solvents. Moreover, the conformations of the compounds do not affect the spectra, and the molecular size and molecular dipole moment do not affect the damping parameter. In addition, types and the number of substitutions attached to the phenyl rings of TPy as well as solvent polarity play the key roles in the long-wavelength fluorescence emission and fluorescence brightness of the compounds. The intramolecular charge transfer of the compounds containing the -NMe2 and -CN groups is predominant in methanol, resulting in a large Stokes shift and oscillator strength.

Retrosynthesis of Tunable Fluorescent Carbon Dots for Precise Long-Term Mitochondrial Tracking.

Mitochondria play a significant role in many cellular processes. Precise long-term tracking of mitochondrial status and behavior is very important for regulating cell fate and treating mitochondrial diseases. However, developing probes with photostability, long-term tracking capability, and tunable long-wavelength fluorescence has been a challenge in mitochondrial targeting. Carbon dots (CDs) as new fluorescent nanomaterials with low toxicity and high stability show increasing advantages in bioimaging. Herein, the mitochondria tracking CDs (MitoTCD) with intrinsic mitochondrial imaging capability and tunable long-wavelength fluorescence from green to red are synthesized where the lipophilic cation of rhodamine is served as the luminescent center of CDs. Due to the excellent photostability, superior fluorescence properties and favorable biocompatibility, these MitoTCD are successfully used for mitochondrial targeting imaging of HeLa cells in vitro and can be tracked as long as six passages, which is suitable for long-term cell imaging. Moreover, these MitoTCD can also be used for zebrafish imaging in vivo.

Experimental and theoretical investigation of long-wavelength fluorescence emission in push-pull benzazoles: intramolecular proton transfer or charge transfer in the excited state?

This study presents the synthesis, characterisation and theoretical calculations of compounds that contain electron donor and withdrawing groups connected through a π-conjugated benzazolic structure. The compounds in solution show an absorption maximum in the UV-visible spectrum (380-390 nm) due to spin and symmetry allowed electronic 1ππ* transitions with no clear evidence for charge transfer in either compound in the ground state. A fluorescence emission located in the violet-blue-green region, tailored by solvent polarity, with a large Stokes shift was observed. Taking the long-wavelength emission into account, the Lippert-Mataga plot indicates a positive solvatochromism in the solvent polarity function (Δf) range 0.02-0.20, related to the occurrence of an ICT mechanism in the excited state. At Δf greater than 0.20, the polarity of the medium seems no longer to increase the stabilization of the compounds, reaching a plateau. Time-dependent density functional theory (TD-DFT) and resolution-of-identity second-order approximate coupled-cluster (RI-CC2) calculations were also used to better understand the excited state of these compounds. The results indicated that ESIPT was disfavoured in the compounds, mainly in polar solvents, and the emission wavelengths were primarily associated with ICT. In summary, in these push-pull compounds, the electron donating and withdrawing groups do not favour the ESIPT process.

Suppression of Kasha’s rule in higher excited states of 4-pyrilocyanines

Spectral properties of the series of cationic polymethine dyes containing conjugated chain of different length and 4-pyrylium terminal groups were studied in details. Besides, the steady-state long-wavelength fluorescence from the lowest excited state and the fluorescence from higher excited states were experimentally revealed. The steady-state fluorescence band from the lowest excited state is shifted bathochromically with lengthening of the polymethine chain, which is also reflected in the absorption shift. According to quantum-chemical modeling, the appearance of unconventional fluorescence, which suppresses Kasha’s rule, is associated with the different nature of the lowest excited states: the first and forth transitions are typical polymethine electron transitions involved only totally delocalized molecular orbitals (MOs), whereas the second and third transitions correspond to the electron jumping from the degenerated MOs localized only within the terminal groups (so called quasi-local MOs and hence quasi-local transitions). The forbidden transitions between the excited states of different types stimulate the fluorescence from the higher excited states.

Excimer Emission of Acridine Orange Adsorbed on Gadolinium-Yttrium Orthovanadate Nanoparticles.

Studying the complexes of inorganic nanoparticles - organic dye molecules is of great importance for their theranostics application. In this paper, we report gadolinium-yttrium orthovanadate nanoparticles (VNPs) - Acridine Orange (AO) complex formation in water solutions. To study the interactions between VNPs and AO, the methods of steady-state and time-resolved spectroscopy were used. It was shown that in aqueous solutions containing VNPs, AO aggregation takes place with a sandwich-like stacking of AO molecules in the near-surface layer of VNPs. The VNPs-AO complex formation causes significant changes in the AO fluorescence spectrum, namely, the appearance of a new broad, structureless band in the long-wavelength spectral edge, which was not observed in AO spectrum in a pure water solution. By analyzing of the absorption, fluorescence excitation spectra and fluorescence decay, the static excimer origin of the long-wavelength fluorescence band has been established.

Unusually Long-Wavelength Emissions of Cyclopropanes: New Insight into C-C Bond Homolysis.

Bond homolysis (BHo) is a fundamental concept in chemical-bonding phenomena. To date, research studies on the BHo concept have provided crucial information for understanding the nature of chemical bonding and reactions. Two potential-energy minima, a σ-bonding isomer and a singlet-diradical isomer, have been known to exist in carbon-carbon BHo. Herein, a third isomer, that is, a puckered singlet diradical exhibiting unstructured long-wavelength fluorescence beyond 460 nm, was first observed in the excited states of 1,4-diarylbicyclo[2.1.0]pentane derivatives. The careful selection of appropriate substituents in the bicyclic structures enabled direct spectral detection. State-of-the-art ab initio quantum chemical calculations quantitatively reproduced the experimental observations. This new finding provides new insight into carbon-carbon bond-breaking and -forming processes.

Dynamic feedback of the photosystem II reaction centre on photoprotection in plants.

Photosystem II of higher plants is protected against light damage by thermal dissipation of excess excitation energy, a process that can be monitored through non-photochemical quenching of chlorophyll fluorescence. When the light intensity is lowered, non-photochemical quenching largely disappears on a time scale ranging from tens of seconds to many minutes. With the use of picosecond fluorescence spectroscopy, we demonstrate that one of the underlying mechanisms is only functional when the reaction centre of photosystem II is closed, that is when electron transfer is blocked and the risk of photodamage is high. This is accompanied by the appearance of a long-wavelength fluorescence band. As soon as the reaction centre reopens, this quenching, together with the long-wavelength fluorescence, disappears instantaneously. This allows plants to maintain a high level of photosynthetic efficiency even in dangerous high-light conditions.

Scalable synthesis of organic-soluble carbon quantum dots: superior optical properties in solvents, solids, and LEDs.

Carbon quantum dots (CQDs) have attracted much attention owing to their unique optical properties and a wide range of applications. The fabrication and control of CQDs with organic solubility and long-wavelength emission are still urgent issues to be addressed for their practical use in LEDs. Here, organic-soluble CQDs were produced at a high yield of ∼90% by a facile solvent engineering treatment of 1,3,6-trinitropyrene, which were simultaneously used as the nitrogen and carbon sources. The optical properties of the organic-soluble CQDs (o-CQDs) were investigated in nonpolar and polar solvents, films, and LED devices. The CQDs have a narrow size distribution around 2.66 nm, and can be dispersed in different organic solvents. Significantly, the as-prepared CQDs present an excitation-independent emission at 607 nm with fluorescence quantum yields (QYs) up to 65.93% in toluene solution. A pronounced solvent effect was observed and their strong absorption bands can be tuned in the whole visible region (400-750 nm) by changing the solvent. The CQDs in various solvents can emit bright, excitation-independent, long-wavelength fluorescence (orange to red). Furthermore, benefiting from the unique oil-solution properties, the as-prepared CQDs can be processed in thin film and device forms to meet the requirements of various applications, such as phosphor-based white-light LEDs. The color coordinate for these CQD modified LEDs is realized at (0.32, 0.31), which is close to pure white light (0.33, 0.33).

Nonplanar Monocyanines: Meso-Substituted Thiazole Orange with High Photostability and Their Synthetic Strategy as well as a Cell Association Study.

A convenient approach for the direct synthesis of meso-substituted thiazole orange (meso-TO) analogues has been unprecedentedly developed through the AlCl3-catalyzed reaction of parent TO with benzyl alcohol derivatives. Single-crystal X-ray structures show that the prepared new meso-TO analogues are nonplanar, forming a sharp contrast to planar TO. The spectral properties show that nonplanar meso-TO analogues do not aggregate, existing in monomer form (M) in PBS buffer, and have little effect of solvatochromism in different solvents. In comparison with the parents, meso-TO analogues exhibit a large Stokes shift, excellent light fastness, and inertness to singlet oxygen. A cellular association study demonstrates that incorporation of a benzyl group at the meso position methine of parent TO can decrease the cytotoxicity, change staining area in cells, and emit long-wavelength fluorescence for an extended time, which are useful for the development of smarter TOs for imaging in biological science.

A Sensitive Ratiometric Long-Wavelength Fluorescent Probe for Selective Determination of Cysteine/Homocysteine.

The development of sensitive fluorescence probes to detect biothiols such as cysteine and homocysteine has attracted great attention in recent times. Herein, we described the design and synthesis of coumarin based long-wavelength fluorescence probe, Bromo-2-benzothiazolyl-3-cyano-7-hydroxy coumarin (BBCH, 2) for selective detections of cysteine and homocysteine. The probe is rationally designed in such a way that both sulfhydryl and adjacent amino groups of thiols are involved in sensing process. Only cysteine/homocysteine able to react with BBCH to release fluorescence reporter (BCH, 1); while, glutathione and other amino acids unable to react with BBCH due to the absence of adjacent amino groups. In presence of cysteine, the color of BBCH is turns from colorless to red and thus BBCH is a naked eye fluorescence indicator for cysteine. Besides, BBCH can discriminate cysteine and homocysteine based on color changes and different reaction rates. The described sensing platform showed good sensing performances to detect cysteine and homocysteine with detection limits of 0.87 and 0.19μM, respectively. Practical applicability was verified in biological and pharmaceutical samples.

Stable fluorescence conjugation of ZnO nanoparticles and their size dependent cellular uptake

We evaluated size dependent cellular uptake of ZnO nanoparticles utilizing stably introduced Cy5.5, which emits long-wavelength fluorescence. Through (3-aminopropyl)triethoxysilane modification, ZnO nanoparticles of different sizes (20 and 70nm) were functionalized with amine moiety, which was further reacted with Cy5.5-N-hydroxylsuccinimide ester to make covalently conjugated Cy5.5 dye on ZnO nanoparticles. Field emission-scanning electron microscopic images revealed that average particle size as well as particle morphology of ZnO nanoparticles were not altered by Cy5.5 conjugation. Zeta potential measurement confirmed that the positive surface charge of ZnO nanoparticles was well preserved after successive conjugation reactions. Based on infrared, ultraviolet-visible light and photoluminescence spectroscopies, we verify that the Cy5.5 was stably introduced to ZnO nanoparticles without serious aggregation. Surface conjugated Cy5.5 showed high stability in deionized water, phosphate buffered saline and cell culture medium, showing less than 2% of release during 85h. Confocal microscopy and fluorescence-activated cell sorting analysis demonstrated that smaller ZnO nanoparticles were more taken up in greater quantities by HaCaT cells. Moreover, systematic study on cellular uptake pathway showed that smaller ZnO nanoparticles were internalized into cells mainly by clathrin-mediated endocytosis, while larger ZnO nanoparticles entered cells via several pathways.

Near-Infrared Phosphorus-Substituted Rhodamine with Emission Wavelength above 700 nm for Bioimaging.

Phosphorus has been successfully fused into a classic rhodamine framework, in which it replaces the bridging oxygen atom to give a series of phosphorus-substituted rhodamines (PRs). Because of the electron-accepting properties of the phosphorus moiety, which is due to effective σ*-π* interactions and strengthened by the inductivity of phosphine oxide, PR exhibits extraordinary long-wavelength fluorescence emission, elongating to the region above 700 nm, with bathochromic shifts of 140 and 40 nm relative to rhodamine and silicon-substituted rhodamine, respectively. Other advantageous properties of the rhodamine family, including high molar extinction coefficient, considerable quantum efficiency, high water solubility, pH-independent emission, great tolerance to photobleaching, and low cytotoxicity, stay intact in PR. Given these excellent properties, PR is desirable for NIR-fluorescence imaging in vivo.

The diadinoxanthin diatoxanthin cycle induces structural rearrangements of the isolated FCP antenna complexes of the pennate diatom Phaeodactylum tricornutum

The study investigated the influence of the xanthophyll cycle pigments diadinoxanthin (DD) and diatoxanthin (Dt) on the spectroscopic characteristics, structure and protein composition of isolated fucoxanthin chlorophyll protein (FCP) complexes of the pennate diatom Phaeodactylum tricornutum. 77 K fluorescence emission spectra revealed that Dt-containing FCP complexes showed a characteristic long wavelength fluorescence emission at 700 nm at a pH-value of 5 whereas DD-enriched FCPs retained the typical 680 nm fluorescence emission maximum of isolated FCPs. The 700 nm emission in Dt-containing FCPs indicates an aggregation of antenna complexes and is a typical feature of the quenching site Q1 in recent models for non-photochemical fluorescence quenching (NPQ). A comparable long-wavelength fluorescence emission was found in FCP complexes prepared with either triton X-100 or n-dodecyl β-D-maltoside as detergent. A treatment of the FCP complexes at low pH-values in the presence of a high concentration of Mg2+ ions showed that the extent of FCP aggregation which leads to the 700 nm fluorescence emission is different from the macro-aggregation of antenna complexes in higher plants. Protein analyses by mass spectrometry showed that the protein composition of the DD- and Dt-enriched FCP complexes was comparable. However, the Lhcf6 and Lhcr1 polypeptides were only found in Dt-enriched FCPs isolated with dodecyl maltoside whereas the Lhcf17 protein was only detected in DD-enriched FCPs prepared with triton. With respect to low pH-induced antenna aggregation it is important that the Lhcx1 protein was found in both DD- and Dt-enriched FCPs, albeit with only two peptides with confident scores.

A Photostable Silicon Rhodamine Platform for Optical Voltage Sensing.

This paper describes the design and synthesis of a photostable, far-red to near-infrared (NIR) platform for optical voltage sensing. We developed a new, sulfonated silicon rhodamine fluorophore and integrated it with a phenylenevinylene molecular wire to create a Berkeley Red Sensor of Transmembrane potential, or BeRST 1 ("burst"). BeRST 1 is the first member of a class of far-red to NIR voltage sensitive dyes that make use of a photoinduced electron transfer (PeT) trigger for optical interrogation of membrane voltage. We show that BeRST 1 displays bright, membrane-localized fluorescence in living cells, high photostability, and excellent voltage sensitivity in neurons. Depolarization of the plasma membrane results in rapid fluorescence increases (24% ΔF/F per 100 mV). BeRST 1 can be used in conjunction with fluorescent stains for organelles, Ca(2+) indicators, and voltage-sensitive fluorescent proteins. In addition, the red-shifted spectral profile of BeRST 1, relative to commonly employed optogenetic actuators like ChannelRhodopsin2 (ChR2), which require blue light, enables optical electrophysiology in neurons. The high speed, sensitivity, photostability and long-wavelength fluorescence profiles of BeRST 1 make it a useful platform for the noninvasive, optical dissection of neuronal activity.

Single-Step Assembly of Multimodal Imaging Nanocarriers: MRI and Long-Wavelength Fluorescence Imaging.

Magnetic resonance imaging (MRI)- and near-infrared (NIR)-active, multimodal composite nanocarriers (CNCs) are prepared using a simple one-step process, flash nanoprecipitation (FNP). The FNP process allows for the independent control of the hydrodynamic diameter, co-core excipient and NIR dye loading, and iron oxide-based nanocrystal (IONC) content of the CNCs. In the controlled precipitation process, 10 nm IONCs are encapsulated into poly(ethylene glycol) (PEG) stabilized CNCs to make biocompatible T2 contrast agents. By adjusting the formulation, CNC size is tuned between 80 and 360 nm. Holding the CNC size constant at an intensity weighted average diameter of 99 ± 3 nm (PDI width 28 nm), the particle relaxivity varies linearly with encapsulated IONC content ranging from 66 to 533 × 10(-3) m(-1) s(-1) for CNCs formulated with 4-16 wt% IONC. To demonstrate the use of CNCs as in vivo MRI contrast agents, CNCs are surface functionalized with liver-targeting hydroxyl groups. The CNCs enable the detection of 0.8 mm(3) non-small cell lung cancer metastases in mice livers via MRI. Incorporating the hydrophobic, NIR dye tris-(porphyrinato)zinc(II) into CNCs enables complementary visualization with long-wavelength fluorescence at 800 nm. In vivo imaging demonstrates the ability of CNCs to act both as MRI and fluorescent imaging agents.

Rational design of long-wavelength absorbing and emitting carbostyrils aided by time-dependent density functional calculations

We have used computational chemistry methods to aid the rational design of long-wavelength absorbing and emitting organic materials. For this purpose, the vertical electronic transition energies of 3,4-dicyano carbostyrils (quinolone-2(1H)-ones) substituted by electron-donating substituents (methoxy, methylamino, dimethylamino) at positions 6 and 7, are calculated by time-dependent density functional theory (B3LYP) within the Tamm–Dancoff approximation. Bulk solvent effects (DMSO, CH3CN, H2O) were taken into account by the CPCM solvation model. Particular long-wavelength absorptions (∼540nm) are predicted for derivatives containing an amino group in position 6 irrespective of whether a 7-methoxy or a 7-amino group is present. In contrast, considerably shorter wavelength absorption (∼440nm) can be expected for 6-methoxy-7-amino substituted 3,4-dicyano carbostyrils. Optimization of the first excited singlet state of 6-dimethylamino substituted carbostyrils leads to a perpendicular arrangement of the (CH3)2N-group with respect to the heterocyclic ring system accompanied by an extremely low electronic transition energy (1000–1500nm) with vanishingly small intensity (oscillator strength f<0.000). 6-methoxy-7-amino substituted carbostyrils are predicted to emit at 490–520nm. An especially long-wavelength fluorescence (∼660nm) is calculated for 6-amino-7-methoxy- as well as 6,7-bis(methylamino)-3,4-dicyanocarbostyril.