Non-Linear Optical Activity of Chiral Bipyrimidine-Based Thin Films.

An original series of bipyrimidine-based chromophores featuring alkoxystyryl donor groups bearing short chiral (S)-2-methylbutyl chains in positions 4, 3,4 and 3,5, connected to electron-accepting 2,2-bipyrimidine rings, has been developed. Their linear and non-linear optical properties were studied using a variety of techniques, including one- and two-photon absorption spectroscopy, fluorescence measurements, as well as Hyper-Rayleigh scattering to determine the first hyperpolarizabilities. Their electronic and geometrical properties were rationalized by TD-DFT calculations. The thermal properties of the compounds were also investigated by a combination of polarized light optical microscopy, differential scanning calorimetry measurements and small-angle X-ray scattering experiments. The derivatives were found not to have mesomorphic properties, but to exhibit melting temperatures or cold crystallization behavior that enabled the isolation of well-organized thin films. The nonlinear optical properties of amorphous or crystalline thin films were studied by wide-field second harmonic generation and multiphoton fluorescence imaging, confirming that non-centrosymmetric crystal organization enables strong second and third harmonic generation. This new series confirms that our strategy of functionalizing 3D organic octupoles with short chiral chains to generate non-centrosymmetric organized thin films enables the development of highly second order nonlinear optical active materials without the use of corona-poling or tedious deposition techniques.

Thioxanthone-Based Siloxane Photosensitizer for Cationic/Radical Photopolymerization and Photoinduced Sol-Gel Reactions.

In this investigation, a multifunctional visible-light TX-based photosensitizer containing a siloxane moiety (TXS) was designed with a good overall yield of 54%. The addition of a siloxane moiety enabled the incorporation of a TX photosensitizer into a siloxane network by photoinduced sol-gel chemistry, thus avoiding its release. Both liquid 1H and solid-state 29Si NMR measurements undeniably confirmed the formation of photoacids resulting from the photolysis of the TXS/electron acceptor molecule (Iodonium salt), which promoted the photoinduced hydrolysis/condensation of the trimethoxysilane groups of TXS, with a high degree of condensation of its inorganic network. Notably, the laser flash photolysis, fluorescence, and electron paramagnetic resonance spin-trapping (EPR ST) experiments demonstrated that TXS could react with Iod through an electron transfer reaction through its excited states, leading to the formation of radical initiating species. Interestingly, the TXS/Iod was demonstrated to be an efficient photoinitiating system for free-radical (FRP) and cationic (CP) polymerization under LEDs@385, 405, and 455 nm. In particular, whatever the epoxy monomer mixtures used, remarkable final epoxy conversions were achieved up to 100% under air. In this latter case, we demonstrated that both the photoinduced sol-gel process (hydrolysis of trimethoxysilane groups) and the cationic photopolymerization occurred simultaneously.

New Multifunctional Bipyrimidine-Based Chromophores for NLO-Active Thin-Film Preparation.

New synthesized bipyrimidine-based chromophores presenting alkoxystyryl donor groups carrying aliphatic achiral and chiral chains in the 4 position, connected to electron-accepting 2,2-bipyrimidine cores have been synthesized. Their linear and nonlinear optical (NLO) properties were investigated as well as their mesomorphic properties by various techniques (light-transmission measurements, polarized-light optical microscopy, differential scanning calorimetry measurements and two-photon excited fluorescence). The derivatives with achiral linear carbon chains were found to exhibit liquid-crystal properties with the formation smectic phases over large temperature ranges, which were confirmed by small-angle X-ray scattering analysis via stacking models. The nonlinear optical properties in the solid state for derivatives with C14 and the citronellol chains have been studied by wide-field second-harmonic generation and multi-photon fluorescence imaging, confirming centrosymmetry for these achiral mesogens and their excellent third-order nonlinearity whereas the chiral compound exhibits non-centrosymmetric organization resulting in a strong Second Harmonic Generation at the crystal state.

Acetylacetonate Ruthenium Nitrosyls: A Gateway to Nitric Oxide Release in Water under Near-Infrared Excitation by Two-Photon Absorption.

A fundamental challenge for phototriggered therapies is to obtain robust molecular frameworks that can withstand biological media. Photoactivatable nitric oxide (NO) releasing molecules (photoNORMs) based on ruthenium nitrosyl (RuNO) complexes are among the most studied systems due to several appealing features that make them attractive for therapeutic applications. Nevertheless, the propensity of the NO ligand to be attacked by nucleophiles frequently manifests as significant instability in water for this class of photoNORMs. Our approach to overcome this limitation involved enhancing the Ru-NO π-backbonding to lower the electrophilicity at the NO by replacing the commonly employed 2,2'-bipyridine (bpy) ligand by an anionic, electron-rich, acetylacetonate (acac). A versatile and convenient synthetic route is developed and applied for the preparation of a large library of RuNO photoNORMs with the general formula [RuNO(tpy)(acac)]2+ (tpy = 2,2':6',2″-terpyridine). A combined theoretical and experimental analysis of the Ru-NO bonding in these complexes is presented, supported by extensive single-crystal X-ray diffraction experiments and by topological analyses of the electron charge density by DFT. The enhanced π-back-bonding, systematically evidenced by several techniques, resulted in a remarkable stability in water for these complexes, where significant NO release efficiencies were recorded. We finally demonstrate the possibility of obtaining sophisticated water-stable multipolar NO-delivery platforms that can be activated in the near-IR region by two-photon absorption (TPA), as demonstrated for an octupolar complex with a TPA cross section of 1530 GM at λ = 800 nm and for which NO photorelease was demonstrated under TPA irradiation in aqueous media.

Tris-benzo[cd]indole Cyanine Enables the NIR-photosensitized Radical and Thiol-ene Polymerizations at 940†nm.

A near-infrared-absorbing heptamethine (HM+ ) incorporating three bulky benzo[cd]indole heterocycles was designed to efficiently prevent self-aggregation of the dye, which results in a strong enhancement of its photoinitiating reactivity as compared to a parent bis-benzo[cd]indole heptamethine (HMCl+ ) used as a reference system. In this context, we highlight an efficient free-radical NIR-polymerization up to a 100 % acrylates C=C bonds conversion even under air conditions. Such an important initiating performance was obtained by incorporating our NIR-sensitizer into a three-component system leading to its self-regeneration. This original photoredox cycle was thoroughly investigated through the identification of each intermediary species using EPR spectroscopy.

Micropatterning of electrochemiluminescent polymers based on multipolar Ru-complex two-photon initiators.

In this work, we present an original stereolithography strategy based on multibranched Ru-complexes with a high two-photon initiating ability allowing the 'one-pot' direct laser writing of ECL-active materials deposited onto electro-active surfaces at the µm scale.

Bimetallic Ruthenium Nitrosyl Complexes with Enhanced Two-Photon Absorption Properties for Nitric Oxide Delivery.

One monometallic and three bimetallic ruthenium nitrosyl (RuNO) complexes are presented and fully characterized in reference to a parent monometallic complex of formula [FTRu(bpy)(NO)]3+ , where FT is a fluorenyl-substituted terpyridine ligand, and bpy the 2,2'-bipyridine. These new complexes are built with the new ligands FFT, TFT, TFFT, and TF-CC-TF (where an alkyne C≡C group is inserted between two fluorenes). The crystal structures of the bis-RuNO2 and bis-RuNO complexes built from the TFT ligand are presented. The evolution of the spectroscopic features (intensities and energies) along the series, at one-photon absorption (OPA) correlates well with the TD-DFT computations. A spectacular effect is observed at two-photon absorption (TPA) with a large enhancement of the molecular cross-section (σTPA ), in the bimetallic species. In the best case, σTPA is equal to 1523±98 GM at 700 nm, in the therapeutic window of transparency of biological tissues. All compounds are capable of releasing NO⋅ under irradiation, which leads to promising applications in TPA-based drug delivery.

Two-Photon Absorption Cooperative Effects within Multi-Dipolar Ruthenium Complexes: The Decisive Influence of Charge Transfers.

One- and two-photon characterizations of a series of hetero- and homoleptic [RuL3-n(bpy)n]2+ (n = 0, 1, 2) complexes carrying bipyridine π-extended ligands (L), have been carried out. These π-extended D-π-A-A-π-D-type ligands (L), where the electron donor units (D) are based on diphenylamine, carbazolyl, or fluorenyl units, have been designed to modulate the conjugation extension and the donating effect. Density functional theory calculations were performed in order to rationalize the observed spectra. Calculations show that the electronic structure of the π-extended ligands has a pronounced effect on the composition of HOMO and LUMO and on the metallic contribution to frontier MOs, resulting in strikingly different nonlinear properties. This work demonstrates that ILCT transitions are the keystone of one- and two-photon absorption bands in the studied systems and reveals how much MLCT and LLCT charge transfers play a decisive role on the two-photon properties of both hetero- and homoleptic ruthenium complexes through cooperative or suppressive effects.

o-Carborane-based fluorophores as efficient luminescent systems both as solids and as water-dispersible nanoparticles.

A set of o-carborane-appended π-conjugated fluorophores and their light-emitting properties in the solid state are reported. The aggregation-induced emission enhancement (AIEE) exhibited for one of the fluorenyl derivatives paved the way to successfully preparing o-carborane-containing organic nanoparticles (NPs) homogeneously dispersed in aqueous media that maintain their luminescence properties. Notably, NPs processed as thin films also show high fluorescence efficiency, suggesting potential optical and optoelectronic applications.

Thionolactone as a Resin Additive to Prepare (Bio)degradable 3D Objects via VAT Photopolymerization.

Three-dimensional (3D) printing and especially VAT photopolymerization leads to cross-linked materials with high thermal, chemical, and mechanical stability. Nevertheless, these properties are incompatible with requirements of degradability and re/upcyclability. We show here that thionolactone and in particular dibenzo[c,e]-oxepane-5-thione (DOT) can be used as an additive (2 wt %) to acrylate-based resins to introduce weak bonds into the network via a radical ring-opening polymerization process. The low amount of additive makes it possible to modify the printability of the resin only slightly, keep its resolution intact, and maintain the mechanical properties of the 3D object. The resin with additive was used in UV microfabrication and two-photon stereolithography setups and commercial 3D printers. The fabricated objects were shown to degrade in basic solvent as well in a homemade compost. The rate of degradation is nonetheless dependent on the size of the object. This feature was used to prepare 3D objects with support structures that could be easily solubilized.

Kinetically stable sub-50 nm fluorescent block copolymer nanoparticles via photomediated RAFT dispersion polymerization for cellular imaging.

Self-assembled block copolymer nanoparticles (NPs) have emerged as major potential nanoscale vehicles for fluorescence bioimaging. The preparation of NPs with high yields possessing high kinetic stability to prevent the leakage of fluorophore molecules is crucial to their practical implementation. Here, we report a photomediated RAFT polymerization-induced self-assembly (PISA) yielding uniform and nanosized poly((oligo(ethylene glycol) acrylate)-block-poly(benzyl acrylate) particles (POEGA-b-PBzA) with a concentration of 22 wt%, over 20 times more than with micellization and nanoprecipitation. The spherical diblock copolymer nanoparticles have an average size of 10-50 nm controllable through the degree of polymerization of the stabilizing POEGA block. Subsequent dialysis against water and swelling with Nile red solution led to highly stable fluorescent NPs able to withstand the changes in concentration, ionic strength, pH or temperature. A PBzA/water interfacial tension of 48.6 mN m-1 hinders the exchange between copolymer chains, resulting in the trapping of NPs in a "kinetically frozen" state responsible for high stability. A spectroscopic study combining fluorescence and UV-vis absorption agrees with a preferential distribution of fluorophores in the outer POEGEA shell despite its hydrophobic nature. Nile red-doped POEGA-b-PBzA micelles without initiator residues and unimers but with high structural stability turn out to be noncytotoxic, and can be used for the optical imaging of cells. Real-time confocal fluorescence microscopy shows a fast cellular uptake using C2C12 cell lines in minutes, and a preferential localization in the perinuclear region, in particular in the vesicles.

Azacalixphyrins as an innovative alternative for the free-radical photopolymerization under visible and NIR irradiation without the need of co-initiators.

Azacalixphyrins are unique aromatic macrocycles featuring strong absorption from the visible to the near-infrared (NIR) spectral ranges. This work demonstrates through EPR spin-trapping experiments that the N-alkyl tetrasubstituted azacalixphyrin (ACP) can lead to the formation of carbon-centered radicals initiating for the free-radical photopolymerization (FRP) of bio-based acrylate monomer upon the irradiation of several light emitting diodes, which emissions range from 455 to 660 nm. Compared to other previously reported systems, the tremendous advantage of the ACP photoinitiating system is its ability to promote photopolymerization on its own, avoiding the introduction of co-initiators. A new potential application of this promising photoinitiator is highlighted through the fabrication of well-defined microstructures under NIR laser diode irradiation at λ = 800 nm.

Optical, third order non-linear optical and electrochemical properties of dipolar, centrosymmetric and C2v organoimido polyoxometalate derivatives.

A family comprising seven arylimido-polyoxometalate (POM) hybrid chromophores (three of which are new), with linear dipolar, C2v and linear centrosymmetric geometries have been synthesised and studied by electronic absorption spectroscopy, electrochemistry, Z-scans (two photon absorption, TPA) and computation (DFT/TD-DFT). These reveal that POM acceptor units are an effective basis for TPA materials: the centrosymmetric bis-POM chromophores produce significant cross sections (δ up to 82 GM) from a single aryl bridge, a similar performance to larger dipolar π-systems combining carbazole or diphenylamino donors with the imido-POM acceptor. DFT/TD-DFT calculations indicate strong communication between POM and organic components is responsible for the linear and non-linear optical behaviour of these compounds, while electrochemical measurements reveal class II mixed valence behaviour resulting from an interplay of through-bond and through-space effects.

Unexpected disruption of the dimensionality-driven two-photon absorption enhancement within a multipolar polypyridyl ruthenium complex series.

The dimensionality-driven two-photon absorption (2PA) enhancement effect is investigated in a series of functionalized bipyridyl Ru-complexes. Our design strategy leads to very high 2PA responses up to ∼1500 GM. However, we highlight that the 2PA performance vs. dimensionality correlation reaches an unexpected limit stemming from 'anti-cooperative' interchromophoric couplings.

Two-Photon Initiating Efficiency of a Ditopic Alkoxynitrostilbene Reacting through a Self-Regenerative Mechanism.

The photophysical properties and the photoinitiating reactivity of a ditopic alkoxynitrostilbene were compared to those of its single branch chromophore used as a reference. Whereas a trivial additive effect is observed when considering the one- and two-photon absorption properties, a clear and very significant amplification has been highlighted for the photoreactivity of this free radical photoinitiator which was used as a hydrogen abstractor in presence of an aliphatic amine co-reactant. We indeed demonstrate that the proximity of two nitroaromatics moieties within the same molecular architecture gives rise to an original cycling mechanism based on a stepwise photo triggering of each photoredox center followed by a subsequent regenerative process. The combination of a high two-photon absorption cross-section (δ780nm ≈330 GM) with a strong enhancement in photoreactivity makes this nitrostilbene bichromophore a very suitable candidate for two-photon polymerization applications.

Two-Photon-Triggered NO Release via a Ruthenium-Nitrosyl Complex with a Star-Shaped Architecture.

We report herein a molecular engineering strategy based on the design of a multipolar ruthenium-nitrosyl (Ru-NO) complex with a three-branched architecture. The three Ru-NO units are introduced at the periphery of a highly π-delocalized truxene core bearing three terpyridine ligands. The two-photon absorption capabilities of the complex were investigated by the Z-scan technique. The strong electronic coupling among the individual arms gives rise to a very strong two-photon absorption response (δ800 nm ∼ 1600 GM), which corresponds to a 16-fold enhancement of the capability of a single-arm reference, thereby promoting an efficient light-driven NO release process in aqueous media.

Laser Direct Writing of Arbitrary Complex Polymer Microstructures by Nitroxide-Mediated Photopolymerization.

In this paper, we demonstrate the possibility of generating arbitrary polymer microstructures covalently linked to a first polymer layer by laser direct writing. At the molecular scale, the process relies on nitroxide-mediated photopolymerization triggered by a light-sensitive alkoxyamine. In addition to the proof of concept and examples of achievable structures, including multichemistry patterns and 3D structures, this paper aims at investigating the physicochemical phenomena involved under such conditions. In particular, the parameters influencing the repolymerization process are considered, and special attention is paid to the study of the impact of oxygen on the spatial control of the polymerization. Such a work opens many possibilities toward the fabrication of on-demand high-resolution (multi)functional polymer micro and nanostructures.

Branching effect on the linear and nonlinear optical properties of styrylpyrimidines.

This contribution aims at investigating the branching effect on the steady state, time resolved fluorescence and two-photon absorption (2PA) properties of dimethylamino and diphenylamino substituted styrylpyrimidine derivatives, by means of a combined experimental and theoretical study. In contrast to classical branched molecules with a triphenylamine central core and electron accepting groups at the periphery, here, branched molecules with reverse topology and different symmetries are examined, namely a styrylpyrimidine group is used as the electron withdrawing core and dimethylamino or diphenylamino donors are incorporated at the periphery. Besides, compared to the great majority of existing branched systems, the herein studied molecules do not have C3 symmetry. For this reason, the region of the linear and non-linear optical spectra of the two and three branched chromophores is actually similar. Interestingly, while the one-photon absorption spectra of one-branched systems versus two- or three-branched ones are spectrally shifted, there is almost no spectral shift in the main 2PA spectral region. Meanwhile, there is still an enhancement of both linear and nonlinear optical responses. Overall, here we developed a strategy that enhances the 2PA response while maintaining the spectral position. Specifically, 2PA cross section values as high as 500 GM have been obtained for the diphenylamino A-(π-D)3 molecule in dichloromethane.

Highly Virulent Bactericidal Effects of Curcumin-Based µ-Cages Fabricated by Two-Photon Polymerization.

A new antibacterial strategy is reported based on two-photon fabrication of three-dimensional curcumin-embedded µ-cages. Such devices were designed to entrap and kill Staphylococcus aureus bacteria upon visible light irradiation. The proposed concept mainly relies on the pivotal role of curcumin, which is sequentially used as a two-photon active free radical initiator and as a photogenerator of reactive oxygen species within the cage µ-volumes. We show that these µ-cages exhibit extremely high antimicrobial properties, leading to 95% bacteria mortality after only 10 min visible irradiation. A preconcentration mechanism of photogenerated oxygen species is proposed to account for this highly performing bactericidal effect whose virulence can be strikingly switched on by increasing the light exposure time from 5 to 10 min.

Porosity-driven large amplitude dynamics for nitroaromatic sensing with fluorescent films of alternating D-π-A molecules.

We report herein the structural properties and nitroaromatic sensing performances of fluorescent thin films formed by alternating donor-acceptor π-conjugated chromophores. The incorporation of a flexible one-dimensional alkyl chain in the chromophore backbone drastically accelerates by more than one order of magnitude the sensing dynamics for the detection of 2,4-dinitrotoluene (DNT) vapors.