The design of highly emissive and stable blue emitters for organic light emitting diodes (OLEDs) is still a challenge, justifying the intense research activity of the scientific community in this field. Recently, a great deal of interest has been devoted to the elaboration of emitters exhibiting a thermally activated delayed fluorescence (TADF). By a specific molecular design consisting into a minimal overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) due to a spatial separation of the electron-donating and the electron-releasing parts, luminescent materials exhibiting small S₁–T₁ energy splitting could be obtained, enabling to thermally upconvert the electrons from the triplet to the singlet excited states by reverse intersystem crossing (RISC). By harvesting both singlet and triplet excitons for light emission, OLEDs competing and sometimes overcoming the performance of phosphorescence-based OLEDs could be fabricated, justifying the interest for this new family of materials massively popularized by Chihaya Adachi since 2012. In this review, we proposed to focus on the recent advances in the molecular design of blue TADF emitters for OLEDs during the last few years.

This study highlights the use of four new phenothiazine derivatives (denoted as B compounds: B106, B107, B108, and B111), which are synthesized and proposed as high performance visible light photoinitiators, in combination with an iodonium salt, in both the free radical polymerization (FRP) of acrylates and the cationic polymerization (CP) of epoxides upon visible light exposure using a cheap and safe Light Emitting Diode (LED) at 405 nm. Remarkably, two of them (B108 and B111) are also evaluated as photoredox catalysts (PCs). These phenothiazine compounds can also be used in three-component photoinitiating systems with an iodonium salt and an amine (e.g. N-phenylglycine (NPG)) for the FRP of acrylates upon irradiation with a LED@405 nm. They showed an outstanding polymerization photoinitiation ability; that is, excellent polymerization rates and high final reactive function conversions were obtained. The photoinitiation mechanisms were investigated through different techniques including RT-FTIR, UV–visible spectroscopy, fluorescence spectroscopy, cyclic voltammetry and electron paramagnetic resonance. A full picture of the involved photochemical mechanisms is provided. As high performance photoinitiators, phenothiazine-based systems are especially used in new radical photosensitive 3D printing resins to improve the depth of cure, ensuring the formation of very thick 3D printed polymers. Finally, the use of these new phenothiazine derivatives for the access to thick glass fiber composites using a near-UV conveyor (LED@395 nm) is outlined i.e. thick and filled samples again requiring improved depth of cure.

Radical chemistry is a very convenient way to produce polymer materials. Here, an application of a particular photoinduced radical chemistry is illustrated. Seven new carbazole derivatives Cd1–Cd7 are incorporated and proposed as high performance near-UV photoinitiators for both the free radical polymerization (FRP) of (meth)acrylates and the cationic polymerization (CP) of epoxides utilizing Light Emitting Diodes LEDs @405 nm. Excellent polymerization-initiating abilities are found and high final reactive function conversions are obtained. Interestingly, these new derivatives display much better near-UV polymerization-initiating abilities compared to a reference UV absorbing carbazole (CARET 9H-carbazole-9-ethanol) demonstrating that the new substituents have good ability to red shift the absorption of the proposed photoinitiators. All the more strikingly, in combination with iodonium salt, Cd1–Cd7 are likewise preferred as cationic photoinitiators over the notable photoinitiator bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO) for mild irradiation conditions featuring their remarkable reactivity. In particular their utilization in the preparation of new cationic resins for LED projector 3D printing is envisioned. A full picture of the included photochemical mechanisms is given.