Six novel oxime carboxylate photoinitiators were designed on the basis of a π-conjugated (E)-1-(4-styrylphenyl)-1-one subunit. Different substituent groups were introduced into three distinctive sites of the chromophore to rationalize their related electronic and structural effects on free-radical photoinitiating reactivities. The electronic properties, photodissociation mechanism, and structure–reactivity relationships were investigated by UV–visible spectroscopy, DFT calculations, real-time ¹H NMR, electron spin resonance spectroscopy, and photo-DSC. We demonstrate that the structural change affecting the photoinitiating properties corresponds to a para-substitution effect on the stilbene subunit using an electron-acceptor group. Such a structural change leads to a 10-fold enhancement in initiation efficiency (Φ_i) upon visible irradiation of light-emitting diodes. The photoinitiating performances of these new derivatives are strikingly sensitive to the excitation wavelengths, leading to an impressive enhancement of Φ_i by more than three orders of magnitude from 365 nm to 405 nm irradiation. This wavelength-dependent reactivity previously observed for commercially available oxime esters can be generalized in our series and has high potential photocuring applications in low-power visible LEDs.

We present herein the properties of a highly reactive type I photoinitiator with significant 2PA cross-sections (δ_720nm ∼ 90 GM). We demonstrate that this new type of photocleavable system exhibits very efficient two-photon polymerization abilities with performances amplified by more than two orders of magnitude with regards to those of a commercially available type I photoinitiator (Lucirin TPO-L) which is extensively employed for multiphoton 3D stereolithography.

The cationic photoinitiating abilities of a series of ‘push–pull’ sulfonium-based photoacid generators (PAGs) have been investigated. In this linear π-conjugated series, a 4-N,N-diphenylaminostilbene subunit is associated with different types of sulfonium substituents, which are connected to the stilbene moiety either in the 4′ position or in the 3′ position. This para-to-meta substitution effect leads to a strong increase of the quantum yield for acid generation with a maximum value of ca. 0.5. Such a positioning effect has a strong influence on the efficiency of the S–C bond cleavage. A detailed photolysis mechanism has been proposed. In contrast to commercially available sulfonium salts, these highly reactive π-conjugated PAGs all exhibit large absorption in the visible range as well as large two-photon absorption cross-sections (δ_max > 600 GM) in the near-infrared region. As a consequence, efficient one and two-photon polymerization reactions are observed at 405 nm and 800 nm, respectively, using typical monomers such as cyclohexene oxide, n-butyl vinyl ether or SU-8 photoresists. By the fabrication of well resolved two-dimensional microstructures, we finally demonstrate the potential use of these new generation PAGs in the fields of one and two-photon lithography.

This study describes the role of two novel visible-light sensitive mono-allyl (TXA) and di-allyl sulfone (TXo2A) thioxanthone derivatives as photosensitizers and photoinitiators (PIs) for free-radical and cationic polymerizations (FRP and CP) and thiol–ene processes upon visible-light emitting diode exposure (LEDs@405 nm, 455 and 470 nm). The photoactivable groups incorporated into TXA and TXo2A have extended their absorption to the visible region up to 510 nm. TXo2A is the first example of a monomeric photoinitiator having two allyl side-chain groups. These new photoinitiating systems have first demonstrated unambiguously that they can be used as PIs and H donor molecules upon visible LED irradiation for the FRP of acrylates without additives. TXA and TXo2A also show highly efficient initiating properties when associated with judicious co-initiators i.e. an iodonium salt (Iod) as an electron acceptor, a tertiary amine (methyldiethanol amine, MDEA) as an electron donor, or a thiol derivative (trimethylolpropane tris (3-mercaptopropionate, TT) as a H-donor, under air and in laminate, for FRP, CP and thiol–ene reactions. These unprecedented initiating combinations promote high acrylate and epoxy conversions even under air, with a higher efficiency than the well-known CQ (and TX)-based photoinitiating systems used as references. As evidenced by nanosecond laser flash photolysis (LFP), electron paramagnetic resonance spin-trapping (EPR ST) and fluorescence/phosphorescence experiments, TXA (or TXo2A) can play a triple role, first as a proton/proton-coupled electron transfer initiator using MDEA or TT, as an electron donor when associated with Iod, and free-radical PIs following an intermolecular electron/proton transfer reaction. Interestingly, the photoinduced 3D interpenetrated polymer network (IPN) synthesized via the concomitant FRP and CP of an acrylate/epoxide monomer mixture upon LED@405 or 455 nm exposure has shown the capability to support viable cell proliferation without any cytotoxicity.

A comparative study of the linear and nonlinear optical properties of a novel triphenylamine–pyrimidine alternated oligomer and its corresponding V-shaped quadrupolar monomer is presented. Both chromophores strikingly exhibit the same spectral shape when considering their respective one- and two-photon absorption spectra. This effect was attributed to a weak interchromophore coupling within the oligomer which exhibits a highly distorted geometry resulting in a strong reduction of the effective conjugation length. The recursive implementation of nine monomers into a three-dimensional architecture leads however to a cooperative enhancement of the two-photon absorption (2PA) cross-section with a δ_MAX of 5093 GM at 800 nm. This very high 2PA ability has been oriented to improve the two-photon induced polymerization efficiency of a bicomponent photoinitiator system implying a hexaarylbiimidazole used as a H-abstractor and an aliphatic amine used as a H-donor. The photosensitizing mechanism is investigated and we clearly show that the intrinsic photoinitiation efficiency of the oligomer is increased by a factor 3 as compared to its corresponding monomer. We therefore demonstrate that such a two-photon sensitizing strategy leads to a synergy effect combining a higher photoinitiation reactivity and a very large two-photon absorption cross-section.