Cooperative Veratryle and Nitroindoline Cages for Two‐Photon Uncaging in the NIR

Tandem uncaging systems in which a two‐photon absorbing module and a cage moiety, linked via a phosphorous clip, that act together by Förster resonance energy transfer (FRET) have been developed. A library of these compounds, using different linkers and cages (7‐nitroindolinyl or nitroveratryl) has been synthesized. The investigation of their uncaging and two‐photon absorption properties demonstrates the scope and versatility of the engineering strategy towards efficient two‐photon cages and reveals surprising cooperative and topological effects. The interactions between the 2PA module and the caging moiety are found to promote cooperative effects on the 2PA response while additional processes that enhance the uncaging efficiency are operative in well‐oriented nitroindoline‐derived dyads. These synergic effects combine to lead to record two‐photon uncaging cross‐section values (i.e., up to 20 GM) for uncaging of carboxylic acids.     

Three‐Dimensional Protein Networks Assembled by Two‐Photon Activation

Spatial and temporal control over chemical and biological processes plays a key role in life and material sciences. Here we synthesized a two‐photon‐activatable glutathione (GSH) to trigger the interaction with glutathione S‐transferase (GST) by light at superior spatiotemporal resolution. The compound shows fast and well‐confined photoconversion into the bioactive GSH, which is free to interact with GST‐tagged proteins. The GSH/GST interaction can be phototriggered, changing its affinity over several orders of magnitude into the nanomolar range. Multiplexed three‐dimensional (3D) protein networks are simultaneously generated in situ through two‐photon fs‐pulsed laser‐scanning excitation. The two‐photon activation facilitates the three‐dimensional assembly of protein structures in real time at hitherto unseen resolution in time and space, thus opening up new applications far beyond the presented examples.     

Cloaked Caged Compounds: Chemical Probes for Two‐Photon Optoneurobiology

Caged neurotransmitters, in combination with focused light beams, enable precise interrogation of neuronal function, even at the level of single synapses. However, most caged transmitters are, surprisingly, severe antagonists of ionotropic gamma ‐aminobutyric acid (GABA) receptors. By conjugation of a large, neutral dendrimer to a caged GABA probe we introduce a “cloaking” technology that effectively reduces such antagonism to very low levels. Such cloaked caged compounds will enable the study of the signaling of the inhibitory neurotransmitter GABA in its natural state using two‐photon uncaging microscopy for the first time.     

Highly Efficient,Conjugated‐Polymer‐Based Nano‐Photosensitizers for Selectively Targeted Two‐Photon Photodynamic Therapy and Imaging of Cancer Cells

Two‐photon photodynamic therapy (2P‐PDT) is a promising noninvasive treatment of cancers and other diseases with three‐dimensional selectivity and deep penetration. However, clinical applications of 2P‐PDT are limited by small two‐photon absorption (TPA) cross sections of traditional photosensitizers. The development of folate receptor targeted nano‐photosensitizers based on conjugated polymers is described. In these nano‐photosensitizers, poly{9,9‐bis[6′′‐(bromohexyl)fluorene‐2,7‐ylenevinylene]‐co‐alt‐1,4‐(2,5‐dicyanophenylene)}, which is a conjugated polymer with a large TPA cross section, acts as a two‐photon light‐harvesting material to significantly enhance the two‐photon properties of the doped photosensitizer tetraphenylporphyrin (TPP) through energy transfer. These nanoparticles displayed up to 1020‐fold enhancement in two‐photon excitation emission and about 870‐fold enhancement in the two‐photon‐induced singlet oxygen generation capability of TPP. Surface‐functionalized folic acid groups make these nanoparticles highly selective in targeting and killing KB cancer cells over NIH/3T3 normal cells. The 2P‐PDT activity of these nanoparticles was significantly improved, potentially up to about 1000 times, as implied by the enhancement factors of two‐photon excitation emission and singlet oxygen generation. These nanoparticles could act as novel two‐photon nano‐photosensitizers with combined advantages of low dark cytotoxicity, targeted 2P‐PDT with high selectivity, and simultaneous two‐photon fluorescence imaging capability; these are all required for ideal two‐photon photosensitizers.     

Two‐Photon STED Spectral Determination for a New V‐Shaped Organic Fluorescent Probe with Efficient Two‐Photon Absorption

Two‐photon stimulated emission depletion (STED) cross sections were determined over a broad spectral range for a novel two‐photon absorbing organic molecule, representing the first such report. The synthesis, comprehensive linear photophysical, two‐photon absorption (2PA), and stimulated emission properties of a new fluorene‐based compound, (E)‐2‐{3‐[2‐(7‐(diphenylamino)‐9,9‐diethyl‐9H‐fluoren‐2‐yl)vinyl]‐5‐methyl‐4‐oxocyclohexa‐2,5‐dienylidene} malononitrile ( 1 ), are presented. Linear spectral parameters, including excitation anisotropy and fluorescence lifetimes, were obtained over a broad range of organic solvents at room temperature. The degenerate two‐photon absorption (2PA) spectrum of 1 was determined with a combination of the direct open‐aperture Z‐scan and relative two‐photon‐induced fluorescence methods using 1 kHz femtosecond excitation. The maximum value of the 2PA cross section ～1700 GM was observed in the main, long wavelength, one‐photon absorption band. One‐ and two‐photon stimulated emission spectra of 1 were obtained over a broad spectral range using a femtosecond pump–probe technique, resulting in relatively high two‐photon stimulated emission depletion cross sections (～1200 GM). A potential application of 1 in bioimaging was demonstrated through one‐ and two‐photon fluorescence microscopy images of HCT 116 cells incubated with micelle‐encapsulated dye.     

Photon Counting Histogram Analysis for Two‐Dimensional Systems

Photon counting statistics in 3D photon counting histogram analysis for one‐photon excitation is a function of the number of molecules of particular brightness in the excitation‐detection volume of a confocal microscope. In mathematical form that volume is approximated by a three‐dimensional Gaussian function which is embedded in the PCH theoretical equations. PCH theory assumes that a molecule can be found anywhere inside the excitation‐detection volume with equal probability. However, one can easily imagine systems in which this assumption is violated because molecules are constrained by the geometry of the sample. For example, molecules on a surface or in a membrane would be constrained to two dimensions. To enable the analysis of such systems by PCH, the theoretical framework requires modification. Herein, we present an extension of the PCH analysis to systems where molecules exist in thin structures that are effectively two‐dimensional. The method, aptly called two‐dimensional photon counting histogram (2D PCH), recovers the number of fluorescent particles per unit area and their molecular brightness. Both theoretical background and experimental results are presented. The theory was tested using computer‐simulated and experimental 2D PCHs obtained from confocal experiments. We demonstrate that this modification of the theoretical framework provides a tool to extract data that reveal states of aggregation, surface photophysics, and reactivity.     

Two‐Photon Absorption Cross‐Sections of Reference Dyes: A Critical Examination

The electronic structure and one‐ and two‐photon absorption spectra of four fluorophores, p‐bis(o‐methoxystyryl)benzene (Bis‐MSB), coumarin 307, fluorescein and rhodamine B, commonly used as reference compounds for two‐photon absorption spectra, have been theoretically calculated and compared with available experimental data. The possible reasons for the wide discrepancies in two‐photon absorption cross‐sections reported in the literature are discussed on the basis of the theoretical findings. The role of a solvent environment on the electronic one‐ and two‐photon absorption spectra is also studied. We highlight some necessary precautions that one needs to take when comparing literature results of two‐photon absorption cross‐sections.     

Emission Turn‐On and Solubility Turn‐Off in Conjugated Polymers: One‐ and Two‐Photon‐Induced Removal of Fluorescence‐Quenching Solubilizing Groups

The synthesis of highly efficient two‐photon uncaging groups and their potential use in functional conjugated polymers for post‐polymerization modification are reported. Careful structural design of the employed nitrophenethyl caging groups allows to efficiently induce bond scission by a two‐photon process through a combination of exceptionally high two‐photon absorption cross‐sections and high reaction quantum yields. Furthermore, π‐conjugated polyfluorenes are functionalized with these photocleavable side groups and it is possible to alter their emission properties and solubility behavior by simple light irradiation. Cleavage of side groups leads to a turn‐on of the fluorescence while solubility of the π‐conjugated materials is drastically reduced.

     

Sensitized Two‐NIR‐Photon Z→E Isomerization of a Visible‐Light‐Addressable Bistable Azobenzene Derivative

Two‐NIR‐photon‐triggered Z→E isomerization of an azobenzene was accomplished by covalently linking a two‐photon‐harvesting triarylamine antenna to a thermally stable ortho‐fluorinated azobenzene derivative. The obtained photoswitch is fully addressable with visible and NIR light by using one‐photon and two‐photon excitation, respectively, with the latter offering enhanced penetration depth and improved spatial resolution.     

A Highly K+‐Selective Two‐Photon Fluorescent Probe

A highly K⁺‐selective two‐photon fluorescent probe for the in vitro monitoring of physiological K⁺ levels in the range of 1–100 mM is reported. The two‐photon excited fluorescence (TPEF) probe shows a fluorescence enhancement (FE) by a factor of about three in the presence of 160 mM K⁺, independently of one‐photon (OP, 430 nm) or two‐photon (TP, 860 nm) excitation and comparable K⁺‐induced FEs in the presence of competitive Na⁺ ions. The estimated dissociation constant (K_d) values in Na⁺‐free solutions (K_d^OP=(28±5) mM and K_d^TP=(36±6) mM ) and in combined K⁺/Na⁺ solutions (K_d^OP=(38±8) mM and K_d^TP=(46±25) mM ) reflecting the high K⁺/Na⁺ selectivity of the fluorescent probe. The TP absorption cross‐section (σ_2PA) of the TPEF probe+160 mM K⁺ is 26 GM at 860 nm. Therefore, the TPEF probe is a suitable tool for the in vitro determination of K⁺.     

Efficient Two‐Photon Excited Amplified Spontaneous Emission from Organic Single Crystals

E, E‐1, 4‐bis[4′‐(N,N‐dibutylamino)styryl]‐2,5‐dimethoxy‐benzene (DBASDMB) organic crystals with high crystalline quality, large size and excellent optical properties are prepared. The linear and nonlinear properties in the crystal are comparatively studied. The relaxation dynamics pumped by two‐photon are very similar with that pumped by one‐photon. The crystal exhibits very strong two‐photon excited fluorescence and amplified spontaneous emission. Efficient two‐photon absorption, reasonably high fluorescent quantum efficiency, and high crystal quality together with stimulated emission make organic crystals ideal for the application in frequency upconversion and other optoelectronic fields.     

Glycosylated Star‐Shaped Conjugated Oligomers for Targeted Two‐Photon Fluorescence Imaging

A glucopyranose functionalized star‐shaped oligomer, N‐tris{4,4′,4′′‐[(1E)‐2‐(2‐{(E)‐2‐[4‐(benzo[d]thiazol‐2‐yl)phenyl]vinyl}‐9,9‐bis(6‐2‐amido‐2‐deoxy‐1‐thio‐β‐D ‐glucopyranose‐hexyl)‐9H‐fluoren‐7‐yl)vinyl]phenyl}phenylamine (TVFVBN‐S‐NH₂), is synthesized for two‐photon fluorescence imaging. In water, TVFVBN‐S‐NH₂ self‐assembles into nanoparticles with an average diameter of ～49 nm and shows a fluorescence quantum yield of 0.21. Two‐photon fluorescence measurements reveal that TVFVBN‐S‐NH₂ has a two‐photon absorption cross‐section of ～1100 GM at 780 nm in water. The active amine group on the glucopyranose moiety allows further functionalization of TVFVBN‐S‐NH₂ with folic acid to yield TVFVBN‐S‐NH₂FA with similar optical and physical properties as those for TVFVBN‐S‐NH₂. Cellular imaging studies reveal that TVFVBN‐S‐NH₂FA has increased uptake by MCF‐7 cells relative to that for TVFVBN‐S‐NH₂, due to specific interactions between folic acid and folate receptors on the MCF‐7 cell membrane. This study demonstrates the effectiveness of glycosylation as a molecular engineering strategy to yield water‐soluble materials with a large two‐photon absorption (TPA) cross‐section for targeted cancer‐cell imaging.     

Two‐Photon‐Induced Fluorescence in New π‐Expanded Diketopyrrolopyrroles

Structurally unique π‐expanded diketopyrrolopyrroles (EDPP) were designed and synthesized. Strategic placement of a fluorene scaffold at the periphery of a diketopyrrolopyrrole through tandem Friedel–Crafts‐dehydration reactions resulted in dyes with supreme solubility. The structure of the dyes was confirmed by X‐ray crystallography verifying a nearly flattened arrangement of the ten fused rings. Despite the extended ring system, the dye still preserved good solubility and was further functionalized by using Pd‐catalyzed coupling reactions, such as the Buchwald–Hartwig amination. Photophysical studies of these new functional dyes revealed that they possess enhanced properties when compared with expanded DPPs in terms of two‐photon absorption cross‐section. It is further demonstrated that in addition to the initial diacetals, the final electrophilic cyclization step can also be applied to diketones. By placing two amine groups at peripheral positions of the resulting dyes, values of two‐photon absorption cross‐section on the level of 2000 GM around 1000 nm were achieved, which in combination with high fluorescence quantum yield (Φ_fl), generated a two‐photon brightness of approximately 1600 GM. These characteristics in combination with strong red emission (665 nm) make these new π‐expanded diketopyrrolopyrroles of major promise as two‐photon dyes for bioimaging applications. Finally, the corresponding N‐alkylated DPPs displayed a solid‐state fluorescence.     

Two‐Photon Absorption and Time‐Resolved Stimulated Emission Depletion Spectroscopy of a New Fluorenyl Derivative

The synthesis, comprehensive linear photophysical characterization, two‐photon absorption (2PA), steady‐state and time‐resolved stimulated emission depletion properties of a new fluorene derivative, (E)‐1‐(2‐(di‐p‐tolylamino)‐9,9‐diethyl‐9H‐fluoren‐7‐yl)‐3‐(thiophen‐2‐yl)prop‐2‐en‐1‐one ( 1 ), are reported. The primary linear spectral properties, including excitation anisotropy, fluorescence lifetimes, and photostability, were investigated in a number of aprotic solvents at room temperature. The degenerate 2PA spectra of 1 were obtained with open‐aperture Z‐scan and two‐photon induced fluorescence methods, using a 1 kHz femtosecond laser system, and maximum 2PA cross‐sections of ～400–600 GM were obtained. The nature of the electronic absorption processes in 1 was investigated by DFT‐based quantum chemical methods implemented in the Gaussian 09 program. The one‐ and two‐photon stimulated emission spectra of 1 were measured over a broad spectral range using a femtosecond pump–probe‐based fluorescence quenching technique, while a new methodology for time‐resolved fluorescence emission spectroscopy is proposed. An effective application of 1 in fluorescence bioimaging was demonstrated by means of one‐ and two‐photon fluorescence microscopy images of HCT 116 cells containing dye encapsulated micelles.     

Two‐dimensional ZnII and one‐dimensional CoII coordination polymers based on benzene‐1,4‐dicarboxylate and pyridine ligands

Coordination polymers constructed from metal ions and organic ligands have attracted considerable attention owing to their diverse structural topologies and potential applications. Ligands containing carboxylate groups are among the most extensively studied because of their versatile coordination modes. Reactions of benzene‐1,4‐dicarboxylic acid (H₂BDC) and pyridine (py) with Zn^II or Co^II yielded two new coordination polymers, namely, poly[(μ₄‐benzene‐1,4‐dicarboxylato‐κ⁴O:O′:O′′:O′′′)(pyridine‐κN)zinc(II)], [Zn(C₈H₄O₂)(C₅H₅N)]_n, (I), and catena‐poly[aqua(μ₃‐benzene‐1,4‐dicarboxylato‐κ³O:O′:O′′)bis(pyridine‐κN)cobalt(II)], [Co(C₈H₄O₂)(C₅H₅N)₂(H₂O)]_n, (II). In compound (I), the Zn^II cation is five‐coordinated by four carboxylate O atoms from four BDC²⁻ ligands and one pyridine N atom in a distorted square‐pyramidal coordination geometry. Four carboxylate groups bridge two Zn^II ions to form centrosymmetric paddle‐wheel‐like Zn₂(μ₂‐COO)₄ units, which are linked by the benzene rings of the BDC²⁻ ligands to generate a two‐dimensional layered structure. The two‐dimensional layer is extended into a three‐dimensional supramolecular structure with the help of π–π stacking interactions between the aromatic rings. Compound (II) has a one‐dimensional double‐chain structure based on Co₂(μ₂‐COO)₂ units. The Co^II cations are bridged by BDC²⁻ ligands and are octahedrally coordinated by three carboxylate O atoms from three BDC²⁻ ligands, one water O atom and two pyridine N atoms. Interchain O—H…O hydrogen‐bonding interactions link these chains to form a three‐dimensional supramolecular architecture.     

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.     

Rational Design of Fluorescent Phthalazinone Derivatives for One‐ and Two‐Photon Imaging

Phthalazinone derivatives were designed as optical probes for one‐ and two‐photon fluorescence microscopy imaging. The design strategy involves stepwise extension and modification of pyridazinone by 1) expansion of pyridazinone to phthalazinone, a larger conjugated system, as the electron acceptor, 2) coupling of electron‐donating aromatic groups such as N,N‐diethylaminophenyl, thienyl, naphthyl, and quinolyl to the phthalazinone, and 3) anchoring of an alkyl chain to the phthalazinone with various terminal substituents such as triphenylphosphonio, morpholino, triethylammonio, N‐methylimidazolio, pyrrolidino, and piperidino. Theoretical calculations were utilized to verify the initial design. The desired fluorescent probes were synthesized by two different routes in considerable yields. Twenty‐two phthalazinone derivatives were synthesized and their photophysical properties were measured. Selected compounds were applied in cell imaging, and valuable information was obtained. Furthermore, the designed compounds showed excellent performance in two‐photon microscopic imaging of mouse brain slices.     

噻吩基卟啉衍生物的单光子和双光子吸收性质的理论研究

本文理论上研究了两个系列的噻吩基卟啉衍生物,这种衍生物在可见光区具有大的双光子吸收截面。用密度泛函理论和ZINDO-SOS方法,计算了分子的几何构型、电子结构,单光子和双光子吸收性质。结果显示噻吩单元的数目影响分子的单光子和双光子吸收性质。具有两个或三个噻吩基团的噻吩基卟啉衍生物在较大范围内具有可用于实际应用中的双光子吸收响应,这一性质有利于这类分子在光限幅中的应用。插入乙炔基有利于扩大共轭范围,增加分子的双光子吸收截面。同时,乙炔基团的加入导致了单光子和双光子波长的红移。从高透明性和相对大的非线性光学响应考虑,噻吩基卟啉衍生物是一类有应用前景的双光子吸收材料。     

Two‐photon excitation and optical limiting in polyfluorene derivatives

Fluorene‐based polymers are widely known materials due to a combination of features such as photoluminescence and electroluminescence, oxidative stability, and film‐forming ability. However, studies reporting nonlinear optical properties in this class of conjugated polymer are scarce. Here, we report a new class of polyfluorene derivatives poly(9,9′‐n‐dihexyl‐2,7‐fluorenedilvinylene‐alt‐1,4‐phenylenevinylene), poly(9,9′‐n‐dihexyl‐2,7‐fluorenedilvinylene‐alt‐2,5‐thiophene), and poly[(9,9‐di‐hexylfluorenediylvinylene‐alt‐1,4‐phenylenevinylene)‐co‐((9,9′‐(3‐t‐butylpropanoate) fluorene‐1,4‐phenylene)] displaying high two‐photon absorption (2PA) in the spectral range from a 490 to 1100 nm. The 2PA cross‐section peak values for these materials are as high as 3000 Göppert Mayer (1 GM = 1 × 10^?50 cm⁴ s/photon), which is related to the high degree of conjugation along the polymer backbone. The polymers that were used in this study presented a strong two‐photon luminescence and also displayed optical limiting behavior, which, in combination with their well‐established properties, make them highly suitable for nonlinear optical devices. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 148–153, 2012