NIR-Sensitized Photoinitiated Radical Polymerization and Proton Generation with Cyanines and LED Arrays

Abstract

Abstract Photoinitiated radical polymerization using as photoinitiator system a combination of NIR-sensitizer (Sens) and a radical initiator (RI) was investigated in different multifunctional monomers derived from the multifunctional acrylic esters HDDA, TPGDA, PEGDA, and TMPTA. NIR-LEDs were applied for exposure exhibiting their emission maximum either at 750 nm, 790 nm, 830 nm or 940 nm. Solubility of Sens in the respective monomers, their absorption data (λabsmax, emax), fluorescence data (Φf, λfmax), electrochemical data (Eox, Ered) and the free energy of electron transfer (ΔGel) between the excited state of Sens and RI were compiled. It showed for many systems an exothermic behavior, which does not always correlate with reactivity. Iodonium salts with low coordinating anions ([B(Ph)4]−, [B(Ph-F5)4]−, [(CF3SO2)2N]-, PF6−) and Triazine A (2,4-Bis(trichloromethyl)-6-(4-methoxy)phenyl-1,3,5-triazine) were applied as RI. Sens was selected from the group of cyanines in which neutral compounds exhibited the highest reactivity. Cationic sensitizers showed almost no initiating reactivity. Furthermore, the sensitizer (5-(6-(2-(3-Ethyl-1,1-dimethyl-1H-benzo[e]indol-2(3H)-ylidene)ethylidene)-2-(2-(3-ethyl-1,1-dimethyl-1H-benzo[e]indol-3-ium-2-yl)vinyl)cyclohex-1-en-1-yl)-1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate) (S9) was taken to study the effect of wavelength dependent LED excitation. The highest reactivity was found for the LED emitting at 850 nm where the system possesses the highest excitation volume and the lowest absorption of photoproducts. Furthermore, thermal investigation approved that the combination S9 and iodonium salt can be considered as a thermal initiation system either. Thus, these systems can be seen as hybrid initiator systems. Finally, we show for the first time the generation of solvated protons using different iodonium salts and sensitizers in acetone as surrounding. The activity of solvated protons was evaluated showing a certain reactivity tendency. The number of solvated protons with respected to the number of sensitizer molecules was lower than unity showing their consumption by nucleophilic centers.