Structure and Two-photon Excited Blue Fluorescence of an Asymmetrical Substituted Stilbene

In common fluorescence process, molecular excitation is caused by the absorption of at least a single photon with shorter wavelength. When certain laser is used as the pump source, some compounds can be excited by simultaneous absorption of at least two photons and then the emission of up-converted fluorescence may occur. This process is called two-photon excited fluorescence (TPEF)1,2. Because of several advantages of TPEF, including intrinsic high three-dimensional resolution and the p…     

Red carbon dots as label-free two-photon fluorescent nanoprobes for imaging of formaldehyde in living cells and zebrafishes

Direct,in situ selective detection of intracellular formaldehyde(FA)is of great significance for understanding its function in FA-related diseases.Herein,red carbon dots(RCD)are reported as label-free two-photon fluorescent nanoprobes for detecting and imaging of FA.Upon addition of FA,the-NH2 groups of RCD could quickly and specially react with aldehydes to form Schiff base and then the strong fluorescence of RCD with blue-shift emission is recovery due to the destruction of the hydrogen bond interaction between RCD and water.In addition,the nanoprobes exhibit outsta nding photo stability,rapid response(<1 min),high sensitivity(~9.9μmol/L)and excellent selectivity toward FA over other aldehyde group compounds.Notably,owing to the good cell-membrane permeability and biocompatibility,as well as the large two-photon absorption cross-section,the as-prepared RCD can be used as label-free nanoprobes for selectively detecting and imaging FA in living cells and zebrafishes through one-photon and two-photon excitation.Moreover,RCD could stain the tissue of zebrafishes at depths interval of up to 240μm under two-photon excitation.This research implied that RCD are promising tools for directly and in situ imaging FA in vivo,thus providing critical insights into FA-related pathophysiological processes.     

Biocompatible organic dots with aggregation-induced emission for in vitro and in vivo fluorescence imaging

Fluorescent probes play a key role in modern biomedical research. As compared to inorganic quantum dots (QDs) composed with heavy metal elements, organic dye-based fluorescent nanoparticles have higher biocompatibility and are richer in variety. However, traditional organic fluorophores tend to quench fluorescence upon aggregation, which is known as aggregation-caused quenching (ACQ) effect that hinders the fabrication of highly emissive fluorescent nanoparticles. In this work, we demonstrate the synthesis of organic fluorescent dots with aggregation-induced emission (AIE) in far-red/near-infrared (FA/NIR) region. A conventional ACQ-characteristic fluorescent dye, 3,4:9,10-tetracarboxylic perylene bisimide (PBI), is converted into an AIE fluorogen through attaching two tetraphenylethylene (TPE) moieties. The fluorescent dots with surface folic acid groups are fabricated from PBI derivative (DTPEPBI), showing specific targeting effect to folate receptor-overexpressed cancer cells. In vivo studies also suggest that the folic acid-functionalized AIE dots preferentially accumulate in the tumor site through enhanced permeability and retention (EPR) effect and folate receptor-mediated active targeting effect. The low cyto-toxicity, good FR/NIR contrast and excellent targeting ability in in vitro/in vivo imaging indicate that the AIE dots have great potentials in advanced bioimaging applications.     

Photophysical properties of octupolar chromophore based on triazine core and fluorene divinylene conjugated bridge

A novel octupolar chromophore with 1,3,5-triazine as core,2,7-divinylene-9,9-dimethylfluorene as extendedπ-conjugated bridge,triarylamine as the electron-donating end-groups was successfully synthesized and characterized.Their linear photophysical and two-photon absorption(TPA) properties were investigated by UV absorption,excited fluorescence(SPEF) spectra and nonlinear transmission method,respectively.The absorption cut-off of the chromophore is below 520 nm and it has stronger fluorescence emission in a nonpolar solvent.In addition,the chromophore exhibits larger TPA cross-section(226.0 GM) in the femtosecond regime at 800 nm.     

Two-photon spectroscopic behaviors and photodynamic effect on the BEL-7402 cancer cells of the new chlorophyll photosensitizer

The spectroscopic properties of a new chlorophyll derivate photosensitizer(CDP) are studied under the excitation wavelengths at 800 and 400 nm using femtosecond pulses from a Ti:sapphire laser.The damaging effect of CDP on the BEL-7402 cancer cells is also investigated upon two-photon illumination at 800 nm.The normalized fluorescence spectra of CDP in tetrahydrofuran(THF) show that two-photon and one-photon spectra have the same distributions and the same emission bands(675 nm).The life-times of two-and one-photon induced fluorescence of this molecule are of the order of 5.0 ns.By comparing the data it is shown that there is some difference between the two lifetimes,but the differ-ence is less than one nanosecond.The two-photon absorption cross section of the molecule is also measured at 800 nm and estimated as about σ′2 ≈ 31.5×10-50 cm4·s·photon-1.The results of two-photon photodynamic therapy(TPPDT) tests show that CDP can kill all of the tested cancer cells according to the usual Eosine assessment.Our results indicate that the two-photon-induced photophysical,photo-chemical and photosensitizing processes of CDP may be basically similar to those of one-photon ex-citation.These behaviors of the sample suggest that one may find other possible methods to estimate some photosensitizers' effects in details such as their distribution in cells and the reactive targets of the sub-cellular parts of some tumor cells via two-photon excitation techniques.     

一个新的具有大的吸收截面的双光子引发剂的合成和非线性光学性质

E,E-1,4-Bis（4＇-N,N-diphenylaminostyryl）-2,5-dimethoxybenzene （DPAMOB） has been synthesized by a simple and effective solid phase Wittig reaction and characterized by 1^H NMR spectra and elemental analysis, Linear absorption, single-photon induced fluorescence and two-photon induced fluorescence spectra were experimentally studied. The new dye has a large two-photon absorption （TPA） cross-section of σr= 1007,2 GM [1 GM= 1 × 10^-50 cm^4·s/（photon molecule）] at 800 nm measured by the two-photon induced fluorescence method. The experimental results confirm that DPAMOB is a good TPA chromophore and can successfully initiate two-photon photopolymerization of ethoxylated trimethylolpropane triacrylate esters （SR454）. Finally, a microstructure has been fabricated by use of DPAMOB as initiator.     

Synthesis and properties of tribranched chromophores with triazine and fluorene units

Two tribranched chromophores,1,3,5-triazine as electron deficient core,9,9-dimethylfluorene asπ-conjugated bridge, diphenylamino(4a) and naphthylamino(4b) as electron-donating end-groups,were successfully prepared via Buchwald-Hartwig coupling reaction.Their linear photophysical and two-photon absorption(TPA) properties were investigated by absorption, fluorescence and nonlinear transmission method,respectively.The absorption cut-offs of the chromophores are below 550 nm and both chromophores have strong fluorescence emission.The compound 4a(206.3 GM) exhibits larger TPA cross-section than 4b (57.8 GM) in the femtosecond regime at 800 nm.     

两种吩噻嗪衍生物的合成，光学性质及其电化学性质

The synthesis, luminescence and electrochemical properties of two novel phenothiazine derivatives were presented, which exhibit good one-photon fluorescence emission. The quantum yields and solvent effects of them were studied in detail. Compound 3,7-bis（2-4＇-imidazolylbenzylidenehydrazonoethyl）-（0-ethylphenothiazine） （5） was used as an effective initiator, of which two-photon fluorescence spectra were investigated under 800 nm fs laser pulse and the measured two-photon absorption cross-section was 18 × 10^-50 cm^4·s per photon. Two-photon initiated polymerization microfabrication experiments were carried out. X-ray diffraction analyses revealed that the shape of the compound 3,7-bis（2-pyrid-4＇-ylmethylidenehydrazonoethyl）-（0-ethylphenothiazine） （4） looks like a butterfly with nearly planar wings. The dihedral angle of the two benzene rings is 37.6° and there is an obvious π-π stacking interaction between the molecules in the crystal.     

Synthesis,Characterization, and Linear and Nonlinear Optical Properties of Phenylene-vinylene Based Chromophores for Singlet Oxygen Generation

A series of symmetrical and unsymmetrical phenylene-vinylene （PV） based chro- mophores with the molecular configuration of donor-π-donor （D-g-D） were prepared and characterized. Iodine was first introduced into the Jr-conjugation backbone of the PV based chromophores in order to study the heavy atom effect on their linear absorption, two-photon absorption （TPA） properties, as well as singlet oxygen generation properties. TPA cross-sections of these chromophores were investigated by using the two-photon excited fluorescence method. The unsymmetrical chromophores were found to have larger TPA cross-section values compared to their symmetrical counterparts. For one of the unsymmetrical chromophores with the iodine incorporation, a large TPA cross section value with quenched emission was found. The decreased fluorescence quantum yield of a molecule can be ascribed to the increased intersystem crossing, which is favorable for enhancing the singlet oxygen generation. Therefore, the unsymmetrical PV based chromophores with heavy atom incorporation are promising singlet oxygen sensitizers for the photodynamic therapy application.     

Syntheses of Novel Asymmetric Cyclopentanone Dyes and Measurement of Two－photon Absorption Cross－section

A simple synthesis route with a high yield of novel asymmetric cyclopentanone dyes 3a—3e and their highly two-photon up-converted fluorescences are reported. The dyes have good solubilities in most of ordi-nary solvents, a wide UV absorption wavelength range from 380—540 nm, and high fluorescence quantum yields. The two-photon absorption cross-sections of dyes 3a—M were measured in chloroform by a two-pho-ton induced fluorescence method. All of these properties of the new dyes make them suitable for being used as two-photon fluorescent probes.     

Tuning molecular aggregation to achieve highly bright AIE dots for NIR-II fluorescence imaging and NIR-I photoacoustic imaging

Currently, bright aggregation-induced emission luminogens (AIEgens) with high photoluminescence quantum yields (PLQYs) in the NIR-II region are still limited, and thus an efficient strategy to enhance NIR-II fluorescence performance through tuning molecular aggregation is proposed here. The synthesized donor–acceptor tailored AIEgen (DTPA-TBZ) not only exhibits an excellent absorptivity in the NIR-I region, but also good fluorescence signals in the NIR-II region with an emission extending to 1200 nm. Benefiting from such improved intramolecular restriction and aggregation, a significant absolute PLQY value of 8.98% was obtained in solid DTPA-TBZ. Encouragingly, the resulting AIE dots also exhibit a high relative PLQY of up to 11.1% with IR 26 as the reference (PLQY = 0.5%). Finally, the AIE dots were applied in high performance NIR-II fluorescence imaging and NIR-I photoacoustic (PA) imaging: visualization of abdominal vessels, hind limb vasculature, and cerebral vessels with high signal to background ratios was performed via NIR-II imaging; Moreover, PA imaging has also been performed to clearly observe tumors in vivo. These results demonstrate that by finely tuning molecular aggregation in DTPA-TBZ, a good NIR-I absorptivity and a highly emissive fluorescence in the NIR-II region can be achieved simultaneously, finally resulting in a promising dual-modal imaging platform for real-world applications to achieve precise cancer diagnostics.

A highly efficient dual-modal imaging platform by using bright AIE dots was constructed to achieve precise cancer diagnostics.     

Sub-10 nm Aggregation-Induced Emission Quantum Dots Assembled by Microfluidics for Enhanced Tumor Targeting and Reduced Retention in the Liver

A robust platform is developed to assemble sub-10 nm organic aggregation-induced emission (AIE) particles using four different AIE luminogens (AIEgens) with emissions from green to the second near-infrared window (NIR-II). They are called AIE quantum dots (QDs) to distinguish from typical AIE dots which are larger than 25 nm. Compared with AIE dots that are larger than 25 nm, AIE QDs allow more efficient cellular uptake and imaging without surface modification of any membrane-penetrating peptides or other targeting molecules. NIR-II AIEgens, which have nearly no background fluorescence from organisms, are used to demonstrate that AIE QDs can achieve high contrast at the tumor as small as 80 mm³ and evade the liver more efficiently than AIE dots. AIE QDs hold a good promise for sensitive and precise diagnosis of the latent solid tumor in clinical medicine with much lower off-targeting to the liver than AIE dots.     

具有聚集诱导发光性质的近红外荧光染料

聚集诱导发光(AIE)现象的发现为解决传统有机荧光分子在高浓度和聚集形态下存在的荧光猝灭问题提供了最佳方案,并实现了在光电器件、化学传感、生物成像和靶向治疗等众多领域的广泛应用.随着对AIE 发光机理研究的不断深入,AIE 分子体系得到了极大的扩展.其中,一类具有给体-受体结构的AIE分子能够显著降低分子能隙,使发光分...     

Facile Suzuki Coupling Strategy toward New Nile Red Derivatives with Improved Two-Photon Brightness

Chemical fine-tuning of fluorophores is a pivotal step towards development of next generation fluorescent dyes for microscopy. With the advent of high-resolution two-photon excitation fluorescence imaging, there is a growing demand for very sensitive laser dyes that can be efficiently excited using commercial Ti:sapphire laser sources in the first near-infrared window (NIR-I, 780–1020 nm). Using the fluorescent dye Nile Red as the lead structure, we report a robust and concise Suzuki coupling approach for the synthesis of 14 new Nile Red analogues that feature extended π ring systems and diverse functionalities. For this set, we gauged their two-photon excitation efficiency in NIR-I as well as evaluated their general fluorescent properties (emission wavelength, Stokes shift, quantum yield and solvatochromism). Several of the new fluorophores were found to display very favorable characteristics. In particular, the derivative featuring a 4-aminophenyl group in the 2-position of Nile Red exhibited extreme solvent sensitivity, and the thien-2-yl substituted Nile Red derivative showed significantly redshifted emission, large Stokes shift and high two-photon brightness.     

NIR-II Fluorescent Brightness Promoted by “Ring Fusion” for the Detection of Intestinal Inflammation

Fluorophores with emission in the second near-infrared window (NIR-II) have displayed salient advantages for biomedical applications. However, the common strategy of reducing the energy bandgap of fluorophores so as to achieve red-shifted wavelengths always leads to compromised fluorescent brightness. Herein, we propose a molecular design concept of “ring-fusion” to modify the acceptor of AIEgen that can extend the luminous wavelength from NIR-I to NIR-II. The fused-acceptor-containing fluorophore yielded, TTQP, has an enhanced absorption coefficient with a higher brightness in nanoparticle formation compared to its NIR-I emissive counterpart (TTQ-DP) with a non-fused acceptor. Theoretical calculation further confirms that the ring fusion can efficiently promote the rigidity and planarity of the electron-deficient core, leading to a lower reorganization energy and nonradiative decay. The TTQP NPs yielded thus allow sensitive NIR-II fluorescence imaging of vasculature and intestinal inflammation in mice models. Therefore, we anticipate that our work will provide a promising molecular-engineering strategy to enrich the library and broaden the application scope of NIR-II fluorophores.     

Structural and process controls of AIEgens for NIR-II theranostics

Aggregation-induced emission (AIE) is a cutting-edge fluorescence technology, giving highly-efficient solid-state photoluminescence. Particularly, AIE luminogens (AIEgens) with emission in the range of second near-infrared window (NIR-II, 1000–1700 nm) have displayed salient advantages for biomedical imaging and therapy. However, the molecular design strategy and underlying mechanism for regulating the balance between fluorescence (radiative pathway) and photothermal effect (non-radiative pathway) in these narrow bandgap materials remain obscure. In this review, we outline the latest achievements in the molecular guidelines and photophysical process control for developing highly efficient NIR-II emitters or photothermal agents with aggregation-induced emission (AIE) attributes. We provide insights to optimize fluorescence efficiency by regulating multi-hierarchical structures from single molecules (flexibilization) to molecular aggregates (rigidification). We also discuss the crucial role of intramolecular motions in molecular aggregates for balancing the functions of fluorescence imaging and photothermal therapy. The superiority of the NIR-II region is demonstrated by fluorescence/photoacoustic imaging of blood vessels and the brain as well as photothermal ablation of the tumor. Finally, a summary of the challenges and perspectives of NIR-II AIEgens for in vivo theranostics is given.

Structural and process controls of NIR-II AIEgens realize manipulating of radiative (R) and nonradiative (NR) decay for precise theranostics.     

Nanoaggregate Probe for Breast Cancer Metastasis through Multispectral Optoacoustic Tomography and Aggregation-Induced NIR-I/II Fluorescence Imaging

An activatable nanoprobe for imaging breast cancer metastases through near infrared-I (NIR-I)/NIR-II fluorescence imaging and multispectral optoacoustic tomography (MSOT) imaging was designed. With a dihydroxanthene moiety serving as the electron donor, quinolinium as the electron acceptor and nitrobenzyloxydiphenylamino as the recognition element, the probe can specifically respond to nitroreductase and transform into an activated D-π-A structure with a NIR emission band extending beyond 900 nm. The activated nanoprobe exhibits NIR emission enhanced by aggregation-induced emission (AIE) and produces strong optoacoustic signal. The nanoprobe was used to detect and image metastases from the orthotopic breast tumors to lymph nodes and then to lung in two breast cancer mouse models. Moreover, the nanoprobe can monitor the treatment efficacy during chemotherapeutic course through fluorescence and MSOT imaging.     

Two-photon excitation of a phytofluor protein

Phytofluors are highly fluorescent proteins in which the chromophore in a phytochrome is replaced with phycoerythrobilin (PEB), the pigment precursor of the cyanobacterial light harvesting protein phycoerythrin. We examined the fluorescence spectra of the N-terminal region of the cyanobacterial phytochrome 1 from cyanobacterium Synechocystis sp. Pcc 6803 bound to PEB. This protein, Cph1(N514)-PEB, displayed a good two-photon cross-section of 20–30 GM for excitation at 792 nm. This phytofluor also exhibits a high fundamental anisotropy at most practical two-photon excitation (2PE) wavelengths from 700 to 900 nm. Identical lifetimes and correlation times with one and 2PE indicates that the phytofluor is not adversely affected by the intensities needed for 2PE. The one-photon absorption extends well beyond the absorption spectrum and even beyond the emission spectrum to 700 nm. The phytofluor thus appears to be a suitable probe for 2PE and/or cellular imaging.     

Enhanced emission efficiency and excited state lifetime due to restricted intramolecular motion in silole aggregates

The aggregation-induced emission (AIE) properties of 1,1,2,3,4,5-hexaphenylsilole (HPS) and poly{11-[(1,2,3,4,5-pentaphenylsilolyl)oxy]-1-phenyl-1-undecyne} (PS9PA) were studied by time-resolved fluorescence technique. The enhanced fluorescence and long fluorescent lifetime were obtained for the sample in an aggregate state as compared to the sample in solution. The time-decay of fluorescence of HPS and PS9PA in high viscosity solvents and low-temperature glasses has also been measured in detail to further investigate the possible mechanism for AIE. Enhanced light emission and long fluorescence lifetime were detected for both HPS and PS9PA in the solution-thickening and -cooling experiments. These results provided direct evidence that the enhanced photoluminescence (PL) efficiency is due to restricted intramolecular motion, which ascribes AIE to the deactivation of nonradiative decay caused by restricted torsional motions of the molecules in the solid state or aggregate form.     

Calixarene‐Based Supramolecular AIE Dots with Highly Inhibited Nonradiative Decay and Intersystem Crossing for Ultrasensitive Fluorescence Image‐Guided Cancer Surgery

Host–guest complexation between calix[5]arene and aggregation‐induced emission luminogen (AIEgen) can significantly turn off both the energy dissipation pathways of intersystem crossing and thermal deactivation, enabling the absorbed excitation energy to mostly focus on fluorescence emission. The co‐assembly of calix[5]arene amphiphiles and AIEgens affords highly emissive supramolecular AIE nanodots thanks to their interaction severely restricting the intramolecular motion of AIEgens, which also show negligible generation of cytotoxic reactive oxygen species. In vivo studies with a peritoneal carcinomatosis‐bearing mouse model indicate that such supramolecular AIE dots have rather low in vivo side toxicity and can serve as a superior fluorescent bioprobe for ultrasensitive fluorescence image‐guided cancer surgery.