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.     

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.     

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.     

Two‐Photon Absorption Cross Sections of Dithienothiophene‐Based Molecules

We performed nonlinear transmission measurements and quantum‐chemical calculations on dithienothiophene (DTT)‐based molecules to gain insight into the effect of acceptor and donor groups on two‐photon absorption (TPA) properties. The TPA intensity showed dispersion characteristics of the single‐photon absorption spectrum. When the molecules included an asymmetric donor‐acceptor pair, the single‐ and two‐photon absorption maximum wavelengths were red‐shifted more than when the molecules had a symmetric donor‐donor structure. We interpreted this result as indicating that the S₂ state plays the dominating role in the absorption process of molecules with a symmetric structure. The experimental TPA δ values at the absorption peak wavelength showed a dependence on the structural variations. We found the self‐consistent force‐field theory and Hartree‐Fock Hamiltonian with single configuration interaction formalism to be valid for evaluating TPA δ. Although the quantum‐chemical calculations slightly underestimated the experimental δ values obtained from nonlinear transmission measurements, they reasonably predicted the dependence of the δ value on the structural variations. We confirmed the role of molecular symmetry by observing that donor‐donor substituted structure gave the highest experimental and theoretical TPA δ values and that the donor‐acceptor substituted structure showed a greater red‐shift in the TPA absorption maximum wavelength. Overall, the theoretical δ values of DTT‐based molecules were in the order of 10^??46 cm⁴ · s · photon^?1 and are higher than that of AF‐50 by nearly two orders of magnitude.     

Self‐Assembly of a Micelle Structure from Graft and Diblock Copolymers: An Example of Overcoming the Limitations of Polyions in Drug Delivery

A novel mixed micelle with a multifunctional core and shell is successfully prepared from a graft copolymer, poly(N‐isopropyl acrylamide‐co‐methacrylic acid)‐g‐poly(d,l ‐lactide) (P(NIPAAm‐co‐MAAc)‐g‐PLA) and two diblock copolymers, poly(ethylene glycol)‐b‐poly(d,l ‐lactide) and poly (2‐ethyl‐2‐oxazoline)‐b‐poly(d,l ‐lactide). This nanostructure completely screens the highly negative charges of the graft copolymer and exhibits multifunctionality because it has a specialized core/shell structure. An example of this micelle structure used in intracellular drug delivery demonstrates a strong relationship between drug release and the functionality of the mixed micelle. Additionally, the efficiency of the screening feature is also displayed in the cytotoxicities; mixed micelles exhibit higher drug activity and lower material cytotoxicity than micelles from P(NIPAAm‐co‐MAAc)‐g‐PLA ([NIPAAm]/[MAAc]/[PLA] = 84:5.9:2.5 mol/mol) copolymer. This study not only presents a new micelle structure generated using a graft–diblock copolymer system, but also elucidates concepts upon which the preparation of a multifunctional micelle from a graft copolymer with a single (or many) diblock copolymer(s) can be based for applications in drug delivery.     

Diphenylamine‐Substituted Cruciform Oligo(phenylene vinylene): Enhanced One‐ and Two‐Photon Excited Fluorescence in the Solid State

1,4‐di(4′‐N,N‐diphenylaminostyryl)benzene (DPA‐DSB) is a well known compound with a large two‐photon absorption (TPA) section and strong fluorescence in solution. However, the ease with which it crystallizes results in the formation of discontinuous crystalline phases during vacuum deposition processes, thereby greatly limiting its applicability in solid‐state devices. A cruciform dimer of DPA‐DSB, 2,5,2′,5′‐tetra(4′‐N,N‐diphenylaminostyryl)biphenyl (DPA‐TSB) is reported, wherein two DPA‐DSB molecules are linked through a central biphenyl bond. The DPA‐TSB molecules take on a cruciform configuration because of the steric crowding around the central biphenyl core, which has the effect of efficiently suppressing crystalline and intermolecular interactions. The neat DPA‐TSB solid shows strong green–blue fluorescence because of both steady‐state absorption as well as TPA. The DPA‐TSB solid exhibits a photoluminescence (PL) efficiency (η_solid) of 29 % and a solid‐state two‐photon action cross section (δη_solid) of 954 GM (1 GM = 1 × 10^–50 cm⁴ s photon^–1 molecule^–1), which is much greater than for the model compound DPA‐DSB (η_solid = 16 % and δη_solid = 150 GM, where δ is the TPA cross section and η is the fluorescence quantum yield). Based on its high PL efficiency, good film‐forming ability, and strong TPA, DPA‐TSB seems to be a good candidate for applications in solid‐state optical devices.     

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.     

High‐Order Shift Current Induced Terahertz Emission from Inorganic Cesium Bromine Lead Perovskite Engendered by Two‐Photon Absorption

High‐order nonlinear optical phenomena are interesting from a fundamental point of view as they reveal intrinsic symmetries of the materials. Potentially they can also be used for practical optoelectronic applications. High‐order shift current is one of these phenomena, and yet it has never been detected in experiments, primarily due to the difficulty in conventional contact detection. In this work, the shift current due to the two‐photon absorption (TPA) from all‐inorganic perovskite CsPbBr₃ is first observed by a contactless and nondestructive terahertz (THz) emission method. The results reveal that the THz emission is dominated by a high‐order shift current (fourth‐order nonlinear optical effect) with the below‐bandgap photon energy of femtosecond laser excitation. The high‐order shift current origins from the broken inversion symmetry induced by the dynamic stereochemical activity of the Pb²⁺ lone pair. A microscopic TPA‐assisted nonlinear optical model is presented to describe the photophysical process of the THz emission. The model matches well with the quadratic pump fluence and angular dependence of the THz emission. This work can not only open a new venue for the all‐inorganic perovskite‐based nonlinear optoelectronics and THz devices, but also afford a THz technology for the high‐order nonlinear effect analysis.     

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.     

A Europium Complex With Excellent Two‐Photon‐Sensitized Luminescence Properties

We report the synthesis and excellent two‐photon‐sensitized luminescence properties of a new complex [Eu(tta)₃dmbpt] (tta = henoyltrifluoroacetonate; dmbpt = 2‐(N,N‐diethyl‐2,6‐dimethylanilin‐4‐yl)‐4,6‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1,3,5‐triazine) that exhibits the highest efficiency of lanthanide luminescence when excited by near‐infrared (NIR) laser pulses (action cross section of two‐photon‐excited fluorescence δ × Φ_F: 85 GM at 812 nm and 56 GM at 842 nm; 1 GM = 10^–50 cm⁴ s photon^–1 molecule^–1). Compared to a previously reported [Eu(tta)₃dpbt] complex, (dpbt = 2‐(N,N‐diethylanilin‐4‐yl)‐4,6‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1,3,5‐triazine), [Eu(tta)₃dmbpt] has two excess methyl groups at the 2,6‐positions of the phenyl ring. Crystallographic data of dmbpt show that the 2,6‐dimethyl substitutes bring about a significant twist in the conformation of the diethylamino group compared to that in dpbt, which severely influences the conjugation in the ground state between the electron lone pair of N in the –N(CH₂–)₂ moiety and the aromatic electron system in dmbpt. The large two‐photon absorption (TPA) cross section of dmbpt is mainly derived from its large static dipole moment difference between the S₀ and the S₁ states, which is partly responsible for the high capability of two‐photon‐sensitized luminescence of [Eu(tta)₃dmbpt]. The broader two‐ and single‐photon excitation windows and the superior two‐photon‐sensitized luminescent properties in the long‐wavelength NIR region of [Eu(tta)₃dmbpt] compared to [Eu(tta)₃dpbt] are also explained according to the calculated results and twisted structure.     

Photosensitizers for Two‐Photon Excited Photodynamic Therapy

Photodynamic therapy (PDT) is a noninvasive protocol for the treatment of various cancers and nonmalignant diseases. Light, oxygen, and photosensitizer (PS) are the essential three elements in a typical PDT process. Currently, there are two major barriers limiting the further development of PDT. One issue is limited tissue penetration, and the other is the lack of high‐performance PSs. Therefore, the newly emerging two‐photon excited PDT (2PE‐PDT) has attracted considerable attention in recent years due to its advantages such as a higher spatial resolution and a greater penetration depth. In this review, focus is on (i) the principle of 2PE‐PDT, (ii) the progression of PSs for 2PE‐PDT, and (iii) the potential indications and future directions in this field.     

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.     

Direct Writing of Three‐Dimensional Polymer Scaffolds Using Colloidal Gels

Polymer scaffolds intended to provide a substrate for cell attachment and proliferation benefit if the geometric architecture, mechanical properties, and surface chemistry are controllable within the range applicable for the target tissue. Such scaffolds may be made bioinductive through the inclusion of surface proteins and release of growth factors. Furthermore, the polymer support may be formed of biodegradable polymers for use as tissue‐engineering scaffolds. In this study, a new scaffold‐fabrication technique based on the direct writing of polymer colloidal‐gel‐based inks is described. The colloidal approach allows for the modular design of inks where the structure and composition of the colloidal particles, surface adsorbed molecules, and dissolved species may be easily controlled. Polyacrylate latex particles are formulated into colloidal gels by using a thermoreversible gel‐forming poly(ethylene oxide)–poly(propylene oxide) block‐copolymer adsorbed layer. The resulting colloidal gels are laced with the model protein bovine serum albumin (BSA) either dissolved in the solvent phase of the ink or dispersed in chitosan nanoparticles as a second colloid. Ink development and rheological characterization are presented along with demonstration of assembly of mesoporous scaffolds. After assembly and drying of the scaffold structure, the drug‐release kinetics are measured upon re‐exposure to an aqueous environment. Protein activity appears to be unaffected by the processing route of these scaffolds. Finally, the assembly of heterogeneous scaffolds is demonstrated to illustrate the possibilities for staged or heterogeneous drug release. This approach to scaffold fabrication offers a new route for scaffold assembly from water‐insoluble polymers while allowing the inclusion of sensitive biomolecules without risk of denaturation.     

Sub‐20 nm Magnetic Dots with Perpendicular Magnetic Anisotropy

A new and simple method for the preparation of magnetic dot arrays is introduced. Diblock copolymer micelles with a silica core are used as template for the generation of nanostructure arrays. The silica cores are utilized as mask for ion milling preparation. The morphology and size of the silica and magnetic dot arrays are discussed. The magnetic dots are made from Co/Pt multilayer films. Ferromagnetic dots with a diameter well below 20 nm and perpendicular easy axis of magnetization are created. The switching behavior changes from domain wall motion, dominant in the film, to single domain particle switching in the dots. The magneto‐optic saturation signals and the evolution of magnetic anisotropy are discussed.     

Micellar Nanoreactors—Preparation and Characterization of Hexagonally Ordered Arrays of Metallic Nanodots

The preparation of hexagonally ordered metallic nanodots was studied in detail with emphasis on the chemical state of the resulting particles. To obtain these dots, in a first step micellar structures were formed from diblock copolymers in solution. The reverse micelles themselves are capable of ligating defined amounts of a metal salt within their cores, acting as nanoreactors. After transfer of the metal‐loaded reverse micelles onto a substrate, the polymer was removed by means of different plasmas (oxygen and/or hydrogen), which also allow the metal salt to be reduced to the metallic state. In this way, ordered arrays of metallic nanodots can be prepared on various substrates. By adjusting the appropriate parameters, the separation and the size of the dots can be varied and controlled. To determine their purity, chemical state, and surface cleanliness—all of which are crucial for subsequent experiments since nanoscale structures are intrinsically surface dominated—in‐situ X‐ray photoelectron spectroscopy (XPS) and ex‐situ transmission electron microscopy (TEM) were applied, also giving information on the formation of the nanodots.     

Near‐Field Lithography by Two‐Photon Induced Photocleavage of Organic Monolayers

We prove that the enhanced electromagnetic near‐field around metallic nanostructures can be used for localized two‐photon induced activation of surfaces, obtaining a defined chemical pattern with nanometric resolution. Gold nanoparticles (Au‐NP) are deposited on glass slides that were modified with a polysiloxane layer containing a nitroveratrylcarbonyl (NVoc) photoremovable group. Upon illumination with a femtosecond laser, the NVoc entity is removed. Due to the electromagnetic field enhancement of the nanoparticles, the threshold of this process is lowered in the nm‐scale vicinity of the metal structures. Upon cleavage, an amine functional group is released, which can be used to site‐selectively bind species with complementary chemical functionality on the surface. This method can be utilized for sub‐wavelength chemical structuring.     

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.     

Dual Near‐Infrared Two‐Photon Microscopy for Deep‐Tissue Dopamine Nanosensor Imaging

A key limitation for achieving deep imaging in biological structures lies in photon absorption and scattering leading to attenuation of fluorescence. In particular, neurotransmitter imaging is challenging in the biologically relevant context of the intact brain for which photons must traverse the cranium, skin, and bone. Thus, fluorescence imaging is limited to the surface cortical layers of the brain, only achievable with craniotomy. Herein, this study describes optimal excitation and emission wavelengths for through‐cranium imaging, and demonstrates that near‐infrared emissive nanosensors can be photoexcited using a two‐photon 1560 nm excitation source. Dopamine‐sensitive nanosensors can undergo two‐photon excitation, and provide chirality‐dependent responses selective for dopamine with fluorescent turn‐on responses varying between 20% and 350%. The two‐photon absorption cross‐section and quantum yield of dopamine nanosensors are further calculated, and a two‐photon power law relationship for the nanosensor excitation process is confirmed. Finally, the improved image quality of the nanosensors embedded 2‐mm‐deep into a brain‐mimetic tissue phantom is shown, whereby one‐photon excitation yields 42% scattering, in contrast to 4% scattering when the same object is imaged under two‐photon excitation. The approach overcomes traditional limitations in deep‐tissue fluorescence microscopy, and can enable neurotransmitter imaging in the biologically relevant milieu of the intact and living brain.     

Two‐Photon Nanolithography Enhances the Performance of an Ionic Liquid–Polymer Composite Sensor

Continuous development of fabrication technologies, such as two‐photon polymerization (2PP), allows the exact reconstruction of specific volume shapes at micro‐ and nanometer precision. Advancements in the engineering of new materials, such as ionic liquids (ILs), are bringing superior advantages in terms of material characteristics, facilitating a combination of optical and electrical properties, as well as lithographic capabilities. In this paper, 2PP is utilized for structuring of a novel IL–polymer composite in a single‐step manufacturing process with high resolution, down to 200 nm, and high aspect ratio, up to 1:20. The composition, based on a photosensitive photoresist (e.g., IP‐L 780 or SU‐8) and the IL 1‐butyl‐3‐methylimidazolium dicyanamide, possesses a good ionic conductivity (in the range of 1–10 mS cm^?1) over a wide frequency bandwidth (1 kHz–1 MHz), an electrochemical window of 2.7 V, and a good optical transparency (transmission value of 90% for a 170 μm thick film). The fabricated structures are characterized and the phenomenon of enhanced conductivity (up to 4 S cm^?1) is explained. Two potential applications, including temperature and relative humidity sensing, are demonstrated as examples. The results suggest a new advanced approach for material structuring that can be regarded as highly most promising for a wide range of applications.