Aggregation‐Enhanced Fluorescence and Two‐Photon Absorption in Nanoaggregates of a 9,10‐Bis[4′‐(4″‐aminostyryl)styryl]anthracene Derivative

This paper reports the design, synthesis, and theoretical modeling of two‐photon properties of a new class of chromophore that exhibits enhanced two‐photon absorption (TPA) and subsequently generated strong up‐converted emission in nanoaggregate forms. This chromophore utilizes the basic structural unit of 9,10‐bis[4′‐(4″‐aminostyryl)styryl]anthracene that exhibits large internal rotation in the monomer form in organic solvents, whereby the fluorescence is greatly reduced. In nanoaggregates formed in water, the internal rotation is considerably hindered, leading to significant increases of TPA and fluorescence quantum yield. Theoretical modeling of the conformational structure and dynamics has utilized a semiempirical pm3 formalism. The TPA cross sections of the monomer and the aggregate states have been calculated on the basis of the quadratic response theory applied to a single‐determinant self‐consistent field reference state making use of a split‐valence 6‐31G* basis set.     

Cyano‐Substituted Oligo(p‐phenylene vinylene) Single Crystals: A Promising Laser Material

Organic crystals that combine high charge‐carrier mobility and excellent light‐emission characteristics are expected to be of interest for light‐emitting transistors and diodes, and may offer renewed hope for electrically pumped laser action. High‐luminescence‐efficiency cyano‐substituted oligo(p‐ phenylene vinylene) (CN‐DPDSB) crystals (η ≈ 95%) grown by the physical vapor transport method is reported here, with high mobilities (at ≈10^?2 cm² V^?1 s^?1 order of magnitude) as measured by time‐of‐flight. The CN‐DPDSB crystals have well‐balanced bipolar carrier‐transport characteristics (μ_hole≈ 2.5–5.5 × 10^?2 cm² V^?1 s^?1; μ_electron ≈ 0.9–1.3 × 10^?2 cm² V^?1 s^?1) and excellent optically pumped laser properties. The threshold for amplified spontaneous emission (ASE) is about 4.6 μJ per pulse (23 KW cm^?2), while the gain coefficient at the peak wavelength of ASE and the loss coefficient caused by scattering are ≈35 and ≈1.7 cm^?1, respectively. This indicates that CN‐DPDSB crystals are promising candidates for organic laser diodes.     

A New Series of Quadrupolar Type Two‐Photon Absorption Chromophores Bearing 11, 12‐Dibutoxydibenzo[a,c]‐phenazine Bridged Amines; Their Applications in Two‐Photon Fluorescence Imaging and Two‐Photon Photodynamic Therapy

A new series of quadrupolar type two‐photon absorption (2PA) chromophores 3 – 9 bearing a core arylamine‐[a,c]phenazine‐arylamine motif are synthesized in high yields. Palladium‐catalyzed Stille coupling and C? N coupling reactions are utilized to prepare target chromophores. Detailed characterization and systematic studies of these molecules, including absorption and fluorescence emission, are conducted. These compounds are found to exhibit very large 2PA cross section values, for example, ～7000 GM at 800 nm for 8 in toluene. Two‐photon‐induced fluorescence imaging is successfully demonstrated in vitro using compound‐ 8 ‐encapsulated silica nanoparticles with excellent bio‐compatibility. In combination with the capability of both one‐ and two‐photon singlet‐oxygen sensitizations, this nanocomposite demonstrates its promising potential in dual functionality toward two‐photon fluorescence imaging and two‐photon photodynamic therapy.     

Acrylate‐Based Photopolymer for Two‐Photon Microfabrication and Photonic Applications

This paper provides a photopolymerizing material suitable for stereolithography of complex submicrometer‐sized three‐dimensional (3D) structural elements to a broad scientific public. Here, we present the formulation of a polymer (LN1 resin) that allows further research in the field of nanofabrication and ‐technology as it surpasses current material limitations. The polymer consists of multifunctional acrylate oligomers as binder, polyfunctional monomers, and a photoinitiator (PI). The chemistry to form 3D structures is based on photopolymerization of the acrylate system initiated by free‐radical species that are triggered by two‐photon absorption of a PI. Important parameters of photocuring, such as the effects of PI concentration, temperature, and light intensity, were studied using photocalorimetry. The thermal stability of the material was tested using thermal gravimetric analysis, providing key information for electronic and photonic applications. Photonic‐crystal structures generated from this resin exhibiting photonic stop gaps in near‐infrared‐ and telecommunication‐wavelength regions are presented.     

Multibranched Octupolar Module Embedded Covalent Organic Frameworks Enable Efficient Two‐Photon Fluorescence

Covalent organic frameworks (COFs) have emerged as potential light emitting polymers for optoelectronic and optical devices, but their nonlinear optical properties, particularly two‐photon absorption and fluorescence (TPA/TPF), have seldom been explored. Herein, to construct octupolar three‐branched modules (e.g., acceptor 3‐(donor‐core), triphenylbenzene core) within a 2D cyano‐sp²c‐conjugated framework is proposed that results in two‐photon luminescent COFs, combining a large TPA cross section and high quantum yield (QY). Such octupolar module‐embedded sp²c‐conjugated COFs emit not only intense one‐photon fluorescence with QY of 27.2% in the solid state and 38.1% in tetrahydrofuran—superior to almost all reported COFs, but also efficient two‐photon fluorescence with large TPA cross section of 1225 GM—remarkably surpassing the corresponding cyano‐sp²c‐linked model compounds (104 GM). The finding highlights the synergy between sp²c‐conjugated framework and octupolar modules that leads to markedly improved TPA response owing to extended conjugated length, enhanced planarity and multidimensional intramolecular interaction. In view of the versatility of the branched chromophore, the proposed design idea is expected to be used to exploit more two‐photon active COF materials for a range of applications. Multiple uses of the COF in information encryption and warm white light‐emitting diodes are also exemplified.     

Synthesis,Characterization, Two‐Photon Absorption,and Optical Limiting Properties of Ladder‐Type Oligo‐p‐phenylene‐Cored Chromophores

This paper reports on the two‐photon absorption (TPA) and related up‐converted emission properties of a novel series of chromophores containing ladder‐type oligo‐p‐phenylenes with various π‐conjugation lengths. The design and synthesis of these ladder‐type two‐photon chromophores are first discussed. An increase in the π‐conjugated length of the ladder‐type oligo‐p‐phenylene for these chromophores leads to an increase in TPA cross‐section together with an increased fluorescence quantum yield. These chromophores exhibit high fluorescence quantum yields because of the rigid planar structure of the ladder‐type oligomers. The chromophore with an enhanced TPA cross‐section together with an increased fluorescence quantum yield would provide significant benefits for two‐photon excited fluorescence based applications. An improved optical limiting behavior was also demonstrated using the ladder‐type pentaphenylene cored chromophore.     

Hydrophobic Chromophores in Aqueous Micellar Solution Showing Large Two‐Photon Absorption Cross Sections

A series of new hydrophobic two‐photon absorbing (2PA) chromophores with varied electron‐donating groups in quasi‐linear and multibranched structures are synthesized to correlate their structure/photophysical property relationships. The feasibility of using these large two‐photon absorption cross‐sectional (δ, expressed in GM = 1 × 10^–50 cm⁴ s photon^–1 molecule^–1) materials in aqueous solution is also explored. All four hydrophobic 2PA materials can be encapsulated into micelles generated by dispersing an amphiphilic block copolymer, poly(methacrylic acid)‐block‐polystyrene (PMAA‐b‐PS), into water. The micellar nanostructures are characterized using dynamic light scattering, atomic force microscopy, and transmission electron microscopy. After these dyes are incorporated into micelles, they exhibit strong fluorescence in water. It is found that the quantum yield and δ values of these chromophores are strongly dependent on the diameters of the micelles, concentrations of the PMAA‐b‐PS, and molecular structures of the 2PA chromophores. One of the compounds that has a strong triarylamino donor and a multibranched structure exhibits a large δ value of 2790 GM and high quantum yield (0.56) in micelle‐containing water. Although this value is smaller than the original value of 5300 GM in toluene, it is still substantially larger than the values of most water‐soluble 2PA materials, which have δ values of less than 100 GM.     

Fabrication and Characterization of Two‐Photon Polymerized Features in Colloidal Crystals

The fabrication and characterization of two‐photon polymerized features written within and outside of colloidal crystals is presented. Two‐photon polymerization (TPP) response diagrams are introduced and developed to map the polymerization and damage thresholds for features written via modulated beam rastering. The use of tris[4‐(7‐benzothiazol‐2‐yl‐9,9‐diethylfluoren‐2‐yl)phenyl]amine (AF‐350) as an initiator for TPP is demonstrated for the first time and TPP response diagrams illustrate the polymerization window. These diagrams also demonstrate that the polymerization behavior within and outside of colloidal crystals is similar and electron microscopy reveals nearly identical resolution. Fluorescence confocal microscopy further enables visualization of non‐self‐supporting, three‐dimensional TPP features within self‐assembled photonic crystals. Finally, microspot spectroscopy is collected from a two‐photon feature written within a colloidal crystal and this is compared with simulation.     

Hyperbranched Polyglycerol‐Doped Mesoporous Silica Nanoparticles for One‐ and Two‐Photon Activated Photodynamic Therapy

Two‐photon activated photodynamic therapy (TPA‐PDT) is a recently developed technique that shows a potential for medical application. In contrast to traditional one‐photon activated PDT, TPA‐PDT can increase the treatment depth and decrease the damage to healthy tissue by using a near‐infrared two‐photon laser. However, this technique also suffers from the fact that approved photosensitive drugs have a low two‐photon absorption cross section. In this study, it is demonstrate that doped polyglycerol mesoporous silica nanoparticles can carry a photosensitizer, Rose bengal, and can be applied in one‐ and two‐photon PDT. TPA dye‐doped mesoporous silica nanoparticles have been synthesized using a surfactant‐free route, which can be considered a TPA‐PDT platform after loading normal photosensitive drugs. The doped TPA dyes in the silica nanoparticles can transfer energy to the loading drugs via an intraparticle fluorescence resonance energy transfer (FRET) mechanism. The fluorescence lifetime and confocal laser scanning microscopy (CLSM) images obtained under different conditions demonstrated a FRET effect through both one‐ and two‐photon activated modes. The results of cytotoxicity experiments proved that this TPA‐PDT system could induce cellular apoptosis under one‐ or two‐photon irradiation. This system in principle extends the application range of TPA‐PDT.     

Frequency Upconversion of 800 nm Ultrashort Pulses by Two‐Photon Absorption in a Stilbenoid Compound‐Doped Polymer Optical Fiber

We present the results of a study of frequency upconversion of femtosecond optical pulses in a step‐index polymer optical fiber that uses a stilbenoid compound as an active dopant. Intense blue emission is observed in the doped poly(methyl methacrylate) (PMMA) fiber when it is longitudinally pumped at 800 nm by 175 fs optical pulses. By means of the intensity‐dependent transmission method, the two‐photon absorption cross‐section is deduced. Our study illustrates that the combination of a well‐designed organic chromophore incorporated into a fiber geometry is appealing for the development of an upconversion blue polymer laser.     

Biocompatible Nanoparticles Based on Diketo‐Pyrrolo‐Pyrrole (DPP) with Aggregation‐Induced Red/NIR Emission for In Vivo Two‐Photon Fluorescence Imaging

Compared with traditional one‐photon fluorescence imaging, two‐photon fluorescence imaging techniques have shown advantages such as increased penetration depth, lower tissue autofluorescence, and reduced photo­damage, and therefore are particularly useful for imaging tissues and animals. In this work, the design and synthesis of two novel DPP ‐based compounds with large two‐photon absorption (2PA) cross‐sections (σ ≥ 8100 GM) and aggregation‐induced emission (AIE) properties are reported. The new compounds are red/NIR emissive and show large Stokes shifts (Δλ ≥ 3571 cm^?1). 1,2‐Distearoyl‐sn‐glycero‐3‐phosphoethanol amine‐N‐[maleimide(polyethylene glycol)‐2000 (DSPE‐PEG‐Mal) is used as the encapsulation matrix to encapsulate DPP‐2 , followed by surface functionalization with cell penetrating peptide (CPP) to yield DPP‐2‐CPP nanoparticles with high brightness, good water dispersibility, and excellent biocompatibility. DPP‐2 nanoparticles have been used for cell imaging and two‐photon imaging with clear visualization of blood vasculature inside mouse ear skin with a depth up to 80 μm.     

Role of Dimensionality on the Two‐Photon Absorption Response of Conjugated Molecules: The Case of Octupolar Compounds

A comparative study of the two‐photon absorption (TPA) properties of octupolar compounds and their dipolar one‐dimensional counterparts is presented on the basis of correlated quantum‐chemical calculations. The roles of dimensionality and symmetry are first discussed on the basis of a simple exciton picture where the ground‐state and excited‐state wavefunctions of three‐arm octupolar systems are built from a linear combination of the corresponding single‐arm wavefunctions. This model predicts a factor of 3 increase in the TPA cross section in the limiting case of three independent charge‐transfer pathways. When taking into account the full chemical structures of representative octupolar molecules, the results of the calculations indicate that a much larger enhancement associated with an increase in dimensionality and delocalization can be achieved when the core of the chromophore allows significant electronic coupling among the individual arms. These theoretical predictions are in agreement with the experimental determination of the TPA cross sections for crystal violet and the related compound, brilliant green, and suggest new strategies for the design of conjugated materials with large TPA cross sections.     

Spiro‐Bridged Ladder‐Type Oligo(para‐phenylene)s: Fine Tuning Solid State Structure and Optical Properties

A set of ladder‐type quaterphenyls with an incremental number of spiro‐bifluorene units in the bridge positions as well as an in‐plane bent quaterphenyl carrying all bridges on one and the same side of the ribbon are synthesized and characterized. While spiro‐bifluorene substituents lead to bathochromically shifted maxima in the UV–vis absorption spectra, this effect can be compensated by in‐plane bending. The influence of different deposition techniques on the solid state structure is analyzed by X‐ray diffraction of single crystals obtained by crystallization from solution as well as sublimation. An increasing number of spiro‐bifluorene substituents are found to aid thin‐film formation.     

Lattice‐Registered Two‐Photon Polymerized Features within Colloidal Photonic Crystals and Their Optical Properties

In this work we demonstrate a significant advance in the introduction of embedded defects in 3D photonic crystals by means of two‐photon polymerization. We have developed the ability to precisely position embedded defects with respect to the lattice of 3D photonic crystals by imaging the structure concurrently with two‐photon writing. Defects are written with near‐perfect lattice registration and at specifically defined depths within the crystal. The effect of precise defect position on the optical response is investigated for embedded planar cavities written in a photonic crystal. The experimental data are compared to spectra calculated using the Scalar Wave Approximation (SWA).     

One‐ and Two‐Dimensionally Structured Polymer Networks in Liquid Crystals for Switchable Diffractive Optical Applications

We have created one‐ and two‐dimensionally structured polymer networks dispersed in a liquid‐crystal solvent using a holographic exposure technique. These structures have potential for electrically switchable, reverse‐mode, polarization selective and non‐selective diffractive optical elements. Using a simple phenomenological model to describe our diffraction measurements in conjunction with microscopic studies, we are able to estimate the structured polymer wall thickness as a function of monomer concentration.     

Biocompatible PEG‐Chitosan@Carbon Dots Hybrid Nanogels for Two‐Photon Fluorescence Imaging,Near‐Infrared Light/pH Dual‐Responsive Drug Carrier,and Synergistic Therapy

This work designs a class of biocompatible PEG‐chitosan@CDs hybrid nanogels by integrating nonlinear poly(ethylene glycol) (PEG), chitosan, and graphitic carbon dots (CDs) into a single nanoparticle for two‐photon fluorescence (TPF) bioimaging, pH and near‐infrared (NIR) light dual‐responsive drug release, and synergistic therapy. Such hybrid nanogels can be simply prepared from a one‐pot surfactant‐free precipitation polymerization of the PEG macromonomers complexed with chitosan and CDs in water, resulting in a semi‐interpenetration of chitosan chains and an immobilization of CDs in the nonlinear PEG networks. The embedded CDs in hybrid nanogels not only serve as an excellent confocal and TPF imaging contrast agent and fluorescent pH‐sensing probe, but also enhance the loading capacity of the hybrid nanogels for hydrophobic anticancer drug. The chitosan can induce a pH‐sensitive swelling/deswelling of the hybrid nanogels for pH‐regulated drug release over the physiologically important range of 5.0–7.4 and surface modulation of embedded CDs to realize fluorescent pH sensing. The thermosensitive nonlinear PEG network can promote the drug release through the local heat produced by the embedded CDs under NIR irradiation. The in vitro results indicate that the hybrid nanogels demonstrated high therapeutic efficacy through the synergistic effect of combined chemo–photothermal treatments.     

Porous Porphyrin‐Based Organosilica Nanoparticles for NIR Two‐Photon Photodynamic Therapy and Gene Delivery in Zebrafish

Periodic mesoporous organosilica nanoparticles emerge as promising vectors for nanomedicine applications. Their properties are very different from those of well‐known mesoporous silica nanoparticles as there is no silica source for their synthesis. So far, they have only been synthesized from small bis‐silylated organic precursors. However, no studies employing large stimuli‐responsive precursors have been reported on such hybrid systems yet. Here, the synthesis of porphyrin‐based organosilica nanoparticles from a large octasilylated metalated porphyrin precursor is described for applications in near‐infrared two‐photon‐triggered spatiotemporal theranostics. The nanoparticles display unique interconnected large cavities of 10–80 nm. The framework of the nanoparticles is constituted with J‐aggregates of porphyrins, which endows them with two‐photon sensitivity. The nanoparticle efficiency for intracellular tracking is first demonstrated by the in vitro near‐infrared imaging of breast cancer cells. After functionalization of the nanoparticles with aminopropyltriethoxysilane, two‐photon‐excited photodynamic therapy in zebrafish is successfully achieved. Two‐photon photochemical internalization in cancer cells of the nanoparticles loaded with siRNA is also performed for the first time. Furthermore, siRNA targeting green fluorescent protein complexed with the nanoparticles is delivered in vivo in zebrafish embryos, which demonstrates the versatility of the nanovectors for biomedical applications.     

Efficient Red/Near‐Infrared Fluorophores Based on Benzo[1,2‐b:4,5‐b′]dithiophene 1,1,5,5‐Tetraoxide for Targeted Photodynamic Therapy and In Vivo Two‐Photon Fluorescence Bioimaging

Red/near‐infrared dyes are highly demanded for biological applications but most of them are far from satisfactory. In this work, a series of red/near‐infrared fluorophores based on electron‐withdrawing benzo[1,2‐b:4,5‐b′]dithiophene 1,1,5,5‐tetraoxide (BDTO) are synthesized and characterized. They possess both aggregation‐induced emission, and hybridized local and charge‐transfer characteristics. Crystallographic, spectroscopic, electrochemical and computational results reveal that the oxidation of benzo[1,2‐b:4,5‐b′]dithiophene to BDTO can endow the fluorophores with greatly red‐shifted emission, enhanced emission efficiency, reduced energy levels, enlarged two‐photon absorption cross section, and increased reactive oxygen species generation efficiency. The nanoparticles fabricated with a near‐infrared fluorophore TPA‐BDTO show high photostability and biocompatibility with good performance in targeted photodynamic ablation of cancer cells and two‐photon fluorescence imaging of intravital mouse brain vasculature.