Action cross sections of two-photon excited luminescence of some Eu(iii) and Tb(iii) complexes

Giant Star-shaped meso-substituted Fluorescent Porphyrins with Fluorenyl-containing Arms Designed for Two-photon Oxygen Photosensitization

In the continuation of previous studies on carbon-rich meso-tetraarylporphyrins featuring 2,7-fluorene units at their periphery, the effect of changing the peripheral dendritic arms for linear arms on their oxygen-photosensitizing ability, their fluorescence and their two-photon absorption (2PA) properties is now analyzed. Thus, starburst porphyrins possessing up to twenty conjugated fluorenyl units were isolated and studied. More precisely, a series of five new free-base porphyrins featuring fully conjugated arms incorporating an increasing number of fluorenyl groups connected via 1,2-alkenyl spacers were synthesized, along with their Zn(II) complexes. Upon excitation in the arm-centred π-π* absorption band, an efficient energy transfer takes place from the peripheral fluorenyl units to the central porphyrin core, leading to intense red-light emission and oxygen photosensitization by the latter. More interestingly, while the linear optical properties of these porphyrins were only slightly improved compared to those of their dendrimer analogues for photodynamic therapy (PDT) or fluorescence imaging, their 2PA cross-sections were much more significantly boosted, evidencing the key role played by different structures on nonlinear optical properties. Finally, by comparison with other porphyrin-based two-photon photosensitizers reported in the literature, we show that these new "semi-disconnected" starburst systems exhibit a remarkable trade-off between intrinsic 2PA, fluorescence and oxygen photosensitization.

Synthesis, characterization and optical properties of new tetrafluorenyl-porphyrins peripherally functionalized with conjugated 2-fluorenone groups

Star shaped porphyrins decorated with external fluorenonyl arms have been isolated. Their luminescence and selected photophysical properties are discussed in the frame of two-photon-induced theranostics.

Two-Photon Antenna Sensitization of Curium: Evidencing Metal-Driven Effects on Absorption Cross Section in f-Element Complexes

Two-photon-excited fluorescence spectroscopy is a powerful tool to study the structural and electronic properties of optically active complexes and molecules. Although numerous lanthanide complexes have been characterized by two-photon-excited fluorescence in solution, this report is the first to apply such a technique to actinide compounds. Contrasting with previous observations in lanthanides, we demonstrate that the two-photon absorption properties of the complexes significantly depend on the metal (4f vs 5f), with Cm(III) complexes showing significantly higher two-photon absorption cross sections than lanthanide analogues and up to 200-fold stronger emission intensities. These results are consistent with electronic and structural differences between the lanthanide and actinide systems studied. Hence, the described methodology can provide valuable insights into the interactions between f-elements and ligands, along with promising prospects on the characterization of scarce compounds.

Multimodal optical contrast agents as new tools for monitoring and tuning nanoemulsion internalisation into cancer cells. From live cell imaging to in vivo imaging of tumours

Tailor-made NIR emitting dyes were designed as multimodal optical probes. These asymmetric amphiphilic compounds show combined intense absorption in the visible region, NIR fluorescence emission, high two-photon absorption in the NIR (with the maximum located around 1000 nm) as well as large Stokes' shift values and second-harmonic generation ability. Thanks to their structure, high loading into nanoemulsions (NEs) could be achieved leading to very high one- and two-photon brightness. These dyes were demonstrated to act as multimodal contrast agents able to generate different optical modalities of interest for bioimaging. Indeed, the uptake and carrier behaviour of the dye-loaded NEs into cancer cells could be monitored by simultaneous two-photon fluorescence and second-harmonic generation optical imaging. Multimodal imaging provided deep insight into the mechanism and kinetics of dye internalisation. Quite interestingly, the nature of the dyes was also found to influence both the kinetics of endocytosis and the internalisation pathways in glioblastoma cancer cells. By modulating the charge distribution within the dyes, the NEs can be tuned to escape lysosomes and enter the mitochondria. Moreover, surface functionalization with PEG macromolecules was realized to yield stealth NIRF-NEs which could be used for in vivo NIRF imaging of subcutaneous tumours in mice.

Phthalocyanine-Cored Fluorophores with Fluorene-Containing Peripheral Two-Photon Antennae as Photosensitizers for Singlet Oxygen Generation

A series of free base and Zn(II) phthalocyanines featuring fluorenyl antennae linked by methoxy or oxo bridges to the phthalocyanine core (Pc) were synthesized and characterized. Selected linear and nonlinear (two-photon absorption) optical properties of these new compounds were subsequently studied. As previously observed for related porphyrin dendrimers bearing 2-fluorenyl peripheral dendrons, an efficient energy transfer occurs from the peripheral antennae to the central phthalocyanine core following excitation in the fluorenyl-based π–π* absorption band of these chromophores. Once excited, these compounds relax to the ground state, mostly by emitting intense red light or by undergoing intersystem crossing. As a result, the tetrafunctionalized Zn(II) phthalocyanines are fluorescent, but can also efficiently photosensitize molecular oxygen in tetrahydrofurane (THF), forming singlet oxygen with nearly comparable yields to bare Zn(II) phthalocyanine (ZnPc). In comparison with the latter complex, the positive role of the fluorenyl-containing antennae on one- and two-photon brightness (2PA) is presently demonstrated when appended in peripheral (β) position to the phthalocyanine core. Furthermore, when compared to known porphyrin analogues, the interest in replacing the porphyrin by a phthalocyanine as the central core to obtain more fluorescent two-photon oxygen photosensitizers is clearly established. As such, this contribution paves the way for the future development of innovative biphotonic photosensitizers usable in theranostics.

New porphyrin dendrimers with fluorenyl-based connectors: a simple way to improving the optical properties over dendrimers featuring 1,3,5-phenylene connectors

Dendrimers with new fluorenyl-based connectors have been synthesized and characterized. Their detailed luminescence properties and selected photophysical properties are discussed in the frame of two-photon-induced theranostics.

Hydrophilic Fluorescent Nanoprodrug of Paclitaxel for Glioblastoma Chemotherapy

Highly water-soluble, nontoxic organic nanoparticles on which paclitaxel (PTX), a hydrophobic anticancer drug, has been covalently bound via an ester linkage (4.5% of total weight) have been prepared for the treatment of glioblastoma. These soft fluorescent organic nanoparticles (FONPs), obtained from citric acid and diethylenetriamine by microwave-assisted condensation, show suitable size (Ø = 17-30 nm), remarkable solubility in water, softness as well as strong blue fluorescence in an aqueous environment that are fully retained in cell culture medium. Moreover, these FONPs were demonstrated to show in vitro safety and preferential internalization in glioblastoma cells through caveolin/lipid raft-mediated endocytosis. The PTX-conjugated FONPs retain excellent solubility in water and remain stable in water (no leaching), while they showed anticancer activity against glioblastoma cells in two-dimensional and three-dimensional culture. PTX-specific effects on microtubules reveal that PTX is intracellularly released from the nanocarriers in its active form, in relation with an intracellular-promoted lysis of the ester linkage. As such, these hydrophilic prodrug formulations hold major promise as biocompatible nanotools for drug delivery.

Luminescent Mesoporous Silica Nanohybrid Based on Drug Derivative Terbium Complex

Mesoporous silica nanoparticles prepared by organic template-driven synthesis have been successfully explored as carriers of the drug-derivate green luminescent complex of terbium (III) with the nonsteroidal anti-inflammatory drug ketoprofen. The terbium (III) complex was synthesized by reacting ketoprofen sodium salt with terbium (III) chloride, which was further adsorbed onto the surface of mesoporous nanoparticles with a mean particle size of 47 ± 4 nm and pore size of 11 nm. The incorporation of the complex into mesoporous silica nanoparticles was tracked by the decrease in the surface area and pore size of the nanoparticles, and successfully demonstrated by substantial changes in the adsorption isotherms and thermal and vibrational spectroscopy results. The cytotoxicity assay and confocal microscopy have shown that the novel luminescent nanohybrid presents high cell viability and the characteristic terbium (III) emission can be assessed through two-photon excitation, which paves the way for bioimaging applications in nanomedicine.

Dendrimeric Nanoparticles for Two-Photon Photodynamic Therapy and Imaging: Synthesis, Photophysical Properties, Innocuousness in Daylight and Cytotoxicity under Two-Photon Irradiation in the NIR

The synthesis and the photophysical properties of a new class of fully organic monodisperse nanoparticles for combined two-photon imaging and photodynamic therapy are described. The design of such nanoparticles is based on the covalent immobilization of a dedicated quadrupolar dye that combines excellent two-photon absorbing (2PA) properties, fluorescence and singlet oxygen generation ability, in a phosphorous-based dendrimeric architecture. First, a bifunctional quadrupolar dye bearing two different grafting moieties, a phenol function and an aldehyde function, was synthesized. It was then covalently grafted through its phenol function to a phosphorus-based dendrimer scaffold of generation 1. The remaining aldehyde functions were then used to continue the dendrimer synthesis up to generation 2, introducing finally 24 water-solubilizing triethyleneglycol chains at its periphery. A dendrimer confining 12 photoactive quadrupolar units in its inner scaffold and showing water solubility was thus obtained. Interestingly, the G1 and G2 dendrimers retain some fluorescence as well as significant singlet oxygen production efficiencies while they were found to show very high 2PA cross-sections in a broad range of the NIR biological spectral window. Hydrophilic dendrimer G2 was tested in vitro on breast cancer cells, first in one- and two-photon microscopy, which allowed for visualization of their cell internalization, then in two-photon photodynamic therapy. While being nontoxic in the dark and, more importantly, under exposure to daylight, dendrimer G2 proved to be a very efficient cell-death inducer only under two-photon irradiation in the NIR.

New porphyrin-based dendrimers with alkene linked fluorenyl antennae for optics

The biphotonic (σ₂) singlet oxygen generation capability (Φ_Δ) of new TFP-based photosensitizers peripherally functionalized with various dendrons containing 2-ethynylflorenyl chromophores is reported along with remarkable fluorescence yields (Φ_F) of up to 17%.

Diphenylamino-substituted tristyryl vs. triphenyl isocyanurates: improved conjugation has minimal impact on two-photon absorption

Planarization and extension of the central π-system does not increase two-photon absorption cross-section.

Cooperative porphyrin-quadrupolar based triad for combined two-photon induced fluorescence and singlet oxygen generation

The design and synthesis of a cooperative multichromophoric triad system which combines the large two-photon absorption properties of fluorene-cored bis-donor quadropolar dyes and the remarkable sensitizing properties of the porphyrin subunit (i.e. high intersystem crossing and ability to produce singlet oxygen by energy transfer to oxygen from its triplet excited state) is described. After irradiation the energy can be transferred from the quadrupolar chromophores to the porphyrin with an estimated 80% efficiency via a FRET process. Moreover both the two-photon absorption properties of the quadrupolar subunits and the sensitizing and fluorescence properties of the porphyrin are retained indicating that deleterious competing processes (such as photo-induced electron transfer) are prevented in such molecular architectures thanks to the implemented design. As a result, the two-photon absorption induced singlet oxygen generation efficiency of the triad in the NIR region is found to be enhanced by an order of magnitude as compared to the porphyin subunit. Potential applications of these porphyrin-based multichromophoric systems for photodynamic therapy based upon two-photon excitation in the NIR region might be possible since it overcomes the low two-photon absorption response of porphyrin while fully retaining their remarkable photosensitizing properties.

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.

Weak two-photon absorption applied to the rapid prototyping of cell scaffolds

Two-photon polymerization (TPP) has been employed to generate deep structures using the biocompatible and optically transparent monomer ethoxylated bisphenol A dimethacrylate (EO=6) (EBPADMA) and 4, 4'-Bis(diethylamino)benzophenon as the photoinitiator. The two-photon absorption cross section of the initiator was measured to be 1 GM (1  GM=1×10(-50)  cm(4) s photon(-1)) in EBPADMA. Here we have explored a weak absorption regime whereby deep structures (∼300  μm) can be generated in a single pass. This allows rapid fabrication of structures suitable for cell scaffolds where the length scales are small, ∼10  μm, but are required over long ranges, ∼cm. The dependence of the TPP properties on the writing power, speed, exposure time and NA, of the focusing lens were studied in detail. Diffraction calculations for the focusing optics employed show that spherical aberration plays a significant role in determining the feature sizes achieved.

Solvatofluorochromic, non-centrosymmetric π-expanded diketopyrrolopyrrole

A donor–acceptor type π-expanded diketopyrrolopyrrole behaves as non-centrosymmetric as far as linear optical properties are concerned but as ‘pseudo-symmetric’ for two-photon absorption.

Optical and photophysical properties of anisole- and cyanobenzene-substituted perylene diimides

Substituent and solvent effects on the one- and two-photon absorption and the photophysical properties of eight bay-substituted perylene-diimides are reported and analyzed.

An internal charge transfer-dependent solvent effect in V-shaped azacyanines

New V-shaped non-centrosymmetric dyes, possessing a strongly electron-deficient azacyanine core, have been synthesized based on a straightforward two-step approach. The key step in this synthesis involves palladium-catalysed cross-coupling of dibromo-N,N'-methylene-2,2'-azapyridinocyanines with arylacetylenes. The resulting strongly polarized π-expanded heterocycles exhibit green to orange fluorescence and they strongly respond to changes in solvent polarity. We demonstrate that differently electron-donating peripheral groups have a significant influence on the internal charge transfer, hence on the solvent effect and fluorescence quantum yield. TD-DFT calculations confirm that, in contrast to the previously studied bis(styryl)azacyanines, the proximity of S1 and T2 states calculated for compounds bearing two 4-N,N-dimethylaminophenylethynyl moieties establishes good conditions for efficient intersystem crossing and is responsible for its low fluorescence quantum yield. Non-linear properties have also been determined for new azacyanines and the results show that depending on peripheral groups, the synthesized dyes exhibit small to large two-photon absorption cross sections reaching 4000 GM.

Study of the two-photon excitation of photoinitiator in various solvents, and the two-photon polymerization process

In this study, the two-photon absorption excited fluorescence of the photosensitizer 4,4^'-Bis(diethylamino)benzophenonin in different solvents is investigated by using mode-locked Ti:sapphire excitation having a wavelength of 800 nm with pulse duration of 150 fs at the rate of 1 kHz. The fluorescence signals excited by wavelengths of 800 and 400 nm have been compared. Contribution of these cross-section measurements for the two-photon polymerization processes is also reported.

Determination of two-photon absorption cross sections of photosensitizers and its implications for two-photon polymerization

We report two-photon absorption cross sections and two-photon excited fluorescence of the photosensitizer 4,4'-Bis(diethylamino)benzophenon and thioxanthone using a femtosecond laser delivering pulses of wavelength 800 nm and 150 fs duration at rates of 1 kHz. Values for the two-photon absorption cross section are derived by comparing the fluorescence signals excited by wavelengths of 800 and 400 nm using a second-harmonic generator. Fluorophore fluorescence intensities dependence with quadratic illumination laser power showed no significant deviation from the two-photon absorption process. Implications of these cross-section measurements for two-photon polymerization are discussed.

Strong solvent dependence of linear and non-linear optical properties of donor–acceptor type pyrrolo[3,2-b]pyrroles

The pyrrolo[3,2-b]pyrrole core was determined to be an efficient linker allowing the conjugation of peripheral benzene rings. The resulting dipolar compounds displayed strong solvatochromism of fluorescence.

Two-photon multiplexing bio-imaging using a combination of Eu- and Tb-bioprobes

Two europium and terbium luminescent bio-probes whose luminescence can be sensitized by a two-photon sensitisation process have been designed and the proof-of-concept of biphotonic multiplexing experiment is described.

A cyclometallated fluorenyl Ir(iii) complex as a potential sensitiser for two-photon excited photodynamic therapy (2PE-PDT)

A cyclometallated fluorenyl Ir(iii) complex that exhibits enhanced triplet oxygen sensing properties for two photon photodynamic therapy is reported.

Supramolecular self-assembly enhanced europium(III) luminescence under visible light

In this paper, we report on the luminescence of europium by directly exciting europium ions with visible light in aqueous medium. Upon replacing all the water molecules that coordinate around a central europium ion with a ditopic ligand 1,11-bis(2,6-dicarboxypyridin-4-yloxy)-3,6,9-trioxaundecane (L2EO4), the quenching from water molecules is efficiently eliminated, offering considerable europium emission. By stoichiometrically mixing with a positively charged block polyelectrolyte, the negatively charged L2EO4-Eu coordinating complex can be transformed into a coordination 'polymer', which simultaneously forms electrostatic micelles with further enhanced europium fluorescence emission, owing to the increased fraction of L2EO4-coordinated Eu(III) as revealed by the fluorescence lifetime measurements. This approach avoids the use of the antenna effect that often utilizes UV light as the irradiation source. We further use those micelles for bio-imaging, and for the first time demonstrate the use of directly excited Eu-containing nano-probes for in vivo fluorescence imaging in small animals under visible excitation. Although literature results have shown that the direct excitation of europium ions in water may lead to emissions in the presence of coordinating ligands, those emissions were too weak to be applied due to the remaining water molecules in the coordination sphere. Our work points out that the direct excitation of europium can generate considerable europium emission given that all the water molecules in the coordination sphere are excluded, which does not only greatly reduce tedious lab work in synthesizing antenna molecules, but also facilitates the application of europium in aqueous medium under visible light.

Millisecond lifetime imaging with a europium complex using a commercial confocal microscope under one or two-photon excitation

We demonstrate that a commercial confocal microscope can perform time-gated and long lifetime imaging in the μs to ms range under classical one photon or nonlinear two photon excitation.

Two-photon polarity probes built from octupolar fluorophores: synthesis, structure-properties relationships, and use in cellular imaging

A series of octupolar fluorophores built from a triphenylamine (TPA) core connected to electron-withdrawing (EW) peripheral groups through conjugated spacers has been synthesized. Their photoluminescence, solvatochromism, and two-photon absorption (2PA) properties were systematically investigated to derive structure-property relationships. All derivatives exhibit two 2PA bands in the 700-1000 nm region: a first band at low energy correlated with a core-to-periphery intramolecular charge transfer that leads to an intense 1PA in the blue-visible range, and a second more intense band at higher energy due to an efficient coupling of the branches through the TPA core. Increasing the strength of the EW end groups or the length of the conjugated spacers and replacing triple-bond linkers with double bonds induces both enhancement and broadening of the 2PA responses, thereby leading to cross-sections up to 2100 GM at peak and higher than 1000 GM over the whole 700-900 nm range. All derivatives exhibit intense photoluminescence (PL) in low- to medium-polarity environments (with quantum yields in the 0.5-0.9 range) and display a strong positive solvatochromic behavior (with Lippert-Mataga specific shifts ranging from 15,000 to 27,500 cm(-1)), triple bonds, and phenyl moieties in the conjugated spacers, thereby leading to larger sensitivities than those of double bonds and thienyl moieties. More hydrophilic derivatives were also shown to be biocompatible, to retain their 2PA and PL properties in biological conditions, and finally to be suitable as polarity sensors for multiphoton cell imaging.