This contribution reports on the synthesis and the photochemical behavior of two new sulfonium-based photoacid generators (PAGs). We demonstrate that a para-to-meta substitution of a methyl (p-cyanobenzyl) sulfonium group in a 4-alkoxystilbene core strongly influences the photodissociation efficiency of the PAGs and leads to an increase of the quantum yield for acid generation by a factor 2.4. This substantial effect, which was also corroborated by a reactivity enhancement in cationic photopolymerization, is assigned to the modulation of the electronic interaction between two low lying excited states whose energy gap is strongly influenced by this substitution effect. Moreover, it was found that the position of the sulfonium moiety hardly affects the two-photon absorption properties of these push−pull chromophores. By the two-photon fabrication of microstructures, we finally show the potential use of the meta derivative as cationic two-photon initiator.

Noncentrosymmetric molecules with a π-conjugated system and, among them, push-pull molecules such as pyridinium phenoxide, are a promising new class of materials for applications in optoelectronics due to their nonlinear optical (NLO) properties. Modelling studies have indicated that an increase in the twist angle between the two aromatic rings leads to an enhancement of the NLO properties. In order to

A series of pyridinium phenoxides that differ by the dihedral angle between the pyridinium and the phenoxide rings because of substituents with increasing steric encumbrance has been investigated by ultrafast spectroscopy. Like the related betaine-30, these molecules are characterised by a zwitterionic electronic ground state and a weakly polar S(1) state. Their fluorescence lifetime was found to lie between 200 to 750 fs, decreasing with increasing dihedral angle, and increasing with solvent viscosity. This was assigned to a non-radiative deactivation of the emissive state coupled to a large amplitude motion involving the dihedral angle. The transient absorption spectra suggested that emission occurs from the Franck-Condon S(1) state, which decays to a dark excited state, that itself most probably corresponds to the relaxed S(1) state. Finally, this relaxed state decays to the vibrationally hot ground state through an intramolecular charge separation process with a time constant ranging between 0.4 and 3 ps, increasing with the dihedral angle and with the solvent relaxation time. These variations were discussed in terms of the Jortner-Bixon model of electron transfer, where the charge separation dynamics depends on both electronic coupling and solvent relaxation. The results suggested that charge separation slows down with increasing dihedral angle.

A one step method for the fabrication of silver nanoparticle-embedded polymer is detailed from the mechanistic aspects to the structural analysis of the material. A 2,7-diaminofluorene derivative is employed both as a photoreductant agent of silver precursors and as a free radical photoinitiator to produce silver nanoparticles in a poly(ethylene glycol) matrix at the same time. Each photophysical mechanism is separately investigated and quantified by steady state and time-resolved spectroscopy. The analysis confirms the key role the dye radical cation in the photopolymerization. Moreover, the addition of an amine co-initiator induces a clear acceleration of the photopolymerization rate and leads to a new distinctive mechanism. As a consequence, the morphology of the resultant silver nanocomposite exhibits different size distributions characterized by transmission electron microscopy. This structural observation was also correlated by an unexpected metal-enhancement fluorescence process arising from residual dyes entrapped in the metal-polymer.

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The photophysical behavior of the trans-4-propoxy 4′-nitrostilbene (PNS) as well as its one-and two-photon polymerization properties are reported. A detailed solvatochromic analysis of the steady-state and time-resolved fluorescence indicates that the locally excited state of PNS (LE) undergoes multiple deactivation pathways which are sequentially triggered by the solvent polarity. In low polar solvent, an efficient intersystem crossing (ISC) identified as a S 1 fT n process leads to the population of the lowest triple state from which the trans-cis photoisomerization is mainly proceeding. From ns-laser flash photolysis measurements, it also appears that the T 1 state which exhibits a nπ* character can efficiently abstract a hydrogen from aliphatic amines and subsequently produces R-aminoalkyl radicals, highly reactive species for the photoinitiation of free-radical polymerization. As a consequence, PNS constitutes a good photoinitiating system. However, the further increase of the solvent polarity opens a new relaxation pathway which populates a highly polar TICT level to the detriment of ISC and trans-cis photoisomerization. A strong increase of the fluorescence quantum yield with a concomitant decrease of the trans-cis photoisomerization one is then observed. At the high end of the solvent-polarity scale, an adding non-radiative deactivation process associated with the twisting of the nitro group leads to a noticeable decrease of the fluorescence process. The photoinduced charge transfer process which largely impacts the photophysical feature of such a D-π-A system also promotes the two-photon absorption properties of PNS. The chromophore exhibits a sizable two-photon absorption spectrum in the 700-900 nm region with δ of about 180 ( 15 GM at 750 nm. In conjunction with its photoinitiating properties, the two-photon polymerization ability of PNS was finally demonstrated. A systematic comparison with a 2,7 diaminofluorene derivative used as reference for two-photon polymerization confirms the substantial improvement of the two-photon polymerization efficiency.

In this current contribution, we provide a detailed investigation into the photochemistry and the free radical photoinitiating reactivity of LED light-sensitive photoinitiators (PIs). This series was designed on the basis of a judicious association of a carbazole-coumarin fused subunit and an O-acyl-α-oxooxime branch, which integrates an N−O photocleavable bond. Within this molecular framework, several substitution changes affecting specifically two distinctive sites of the oxime group have been proposed to rationalize some relevant structure−reactivity relationships. We show that the photobleaching rates of the oxime esters (OXEs) are clearly influenced by an ethyl-to-isopropyl substitution effect on the oxime methine carbon whereas the photoinitiating efficiency is mainly driven by a O-benzyl-to-O-acetyl substitution change. Of particular interest, we show that the photoinitiating efficiencies of these OXEs largely depart from their respective absorption spectra in such manner that their photopolymerization performance can be amplified by more than 2 orders of magnitude between 365 and 425 nm LED irradiation. This effect clearly outperforms the photoinitiating efficiency of the commercially available Irgacure OXE-02 oxime ester used as a reference. In the proposed mechanism that accounts for this original wavelength-dependent photopolymerization property, we highlighted the role of an imine-based transient species whose reactivity toward the acrylate monomer can be phototriggered promoting thereby an alternative competing reaction sequence.

MFI-type materials with a lamellar morphology were successfully synthesized by using mononitrogen surfactants specifically designed by molecular modelling. The mononitrogen surfactants directed the recrystallization of a crystalline layered polysilicate formed in situ, the magadiite, into a zeolite ZSM-5.Moreover, the surfactants allow the preservation of the lamellar shape of the magadiite and inhibit a further growth into one dimension, leading to the formation of zeolite ZSM-5 nanosheets with a thickness comprised between 2 and 3 nm and a Si/Al ratio of 24. This simple approach paves a new way for obtaining zeolite materials of controlled size and shape for specific catalytic applications.