Photochemical Properties Of 4-Benzoylbenzylammonium Borates

Abstract

Several new 4-benzoylbenzylammonium borates were prepared in order to study their photochemical properties. It was found that a quantitative yield of biphenyl and a small amount of benzene are formed from photolysis of the title compounds as tetraphenylborates in the absence of oxygen. In addition, amines and methylbenzophenone were detected after the steady-state photolysis. The quantum yields of the photolysis of the title compounds and the formation of biphenyl, amines, and methylbenzophenone were determined. Radical trapping experiments showed that the photogenerated amine is converted to an R-aminoalkyl radical, which is an important initiating species for the polymerization of acrylates when tetraphenylborate is chosen as a counteranion. It was found that even 1-(4-benzoylbenzyl)pyridinium tetraphenylborate is an active photoinitiator, however, indicating that R-aminoalkyl radical is not the only initiating species formed from 4-benzoylbenzylammonium tetraphenylborates. The effect of structure on initiating activity was determined and discussed.

High-Efficiency Radical Photopolymerization Enhanced by Autonomous Dark Cure

“…The electrostatically induced proximity of the photo–redox pair in the nonpolar monomer environment motivated our hypothesis that ET 1 can be operational in both the excited singlet or triplet states ( 1 or 3 [BP–DMPT + ]*), as the excited singlet state is not limited by its short lifetime and the first triplet excited state ( = 67.2 kcal/mol) is still sufficiently oxidizing to induce spontaneous ET 1 ( = 56.0). , ET 1 produces a reduced chromophore ( 2 BP–DMPT • ) and an oxidized borate (BPh 4 • ), both of which are transient intermediates destined to undergo homolysis via separate pathways. BPh 4 • degrades by homolysis (HM 1 ) into triphenyl borane (BPh 3 ) and a phenyl radical that efficiently initiates a monomer with a large exergonicity of −51.9 kcal/mol. − Similar photoradical-generation mechanisms with redox pairs of photosensitizers and borates have been exploited in various photopolymer applications. ,− As previously shown, photopolymerization rates can also be controlled by utilizing different borate anions with various reduction potentials. ,,− …”

“…We hypothesized that within an ideal DCPI framework, a single photon reliably generates four separate initiating radicals. When compared to Norrish type I and II photoinitiators that generate one or two initiating radicals per photon absorption event, the DCPI significantly outperforms other photoinitiators, possibly making it the most photon-efficient photoinitiator for radical polymerization to date, especially considering the auxiliary initiating abilities of the products of the DCPI chemistry. , In particular, the chain-end groups from 4-methylbenzophenone and alpha-aminoalkyl radicals can be recycled to function as a tethered photoinitiator and an amine initiator to provide additional radicals through additional photo- and redox cycles . Such a recycling ability of tethered amines enhances their dark-cure potential within the APRP framework, especially in regions of limited light access where the free amine, relative to the formulated BPO concentration, is the limiting redox reagent (see Figure S15 for comparison of amine and tethered amine undergoing redox reactions).…”

Relative Activity of Possible Initiating Species Produced from Photolysis of Tetraphenyl and Triphenylbutyl Borates As Measured by Fluorescence Probe Techniques

“…Since 3a releases pyridine, the radical cation of which cannot easily lose a proton, therefore, the only radicals capable of initiation in the case of 3a are the phenyl radical and p-benzoylbenzyl radical. 14 The formation of these radicals can be avoided if one photolyzes an equimolar mixture (7.1 × 10 -3 M) of pyridine and benzophenone in TEGDA, and this mixture was shown not to polymerize even after 2000 s irradiation at 350 nm. When pyridine was replaced in this system with either tributylamine or dimethyl-ptoluidine, a slow polymerization was detected (Table 1).…”

“…Compound 3b and compounds containing the imidazolium moiety (as in 5) in combination with the benzophenone chromophore could not be synthesized. Synthetic procedure for compounds 1a,b,c, 8 2a,b,c, 17 3a,c, 14 and 4a,b 18 have been described previously. The synthesis of 5a,b,c is described below.…”

Visible-Light Photoinitiation of (Meth)acrylate Polymerization with Autonomous Post-conversion

“…After ET 1 , two HM reactions ensue (Figures and ). The HM of tetraphenylborate (HM 1 ) is well-known and discussed in our previous publication and many others. − On the other hand, the HM of the chromophore–DMPT radical (HM 2 ) is a novel aspect of the new visible DCPI design that enables the photo-release of an amine, where HM 2 of the zwitterionic radical leads to an amine radical cation and chromophore anion that form an ionic complex (SI8, ionic complexation). We computed that the HM 2 of the BP-DMPT radical was exergonic with a reaction free energy of −16.8 kcal/mol, while the HM 2 of the ARC-DMPT radical, ARC-5,7-diOMe-DMPT radical, and ARC-5,7-diOMe-6-Br-DMPT radical was calculated to be exergonic with free energy changes of −7.7, −11.5, and −6.5 kcal/mol, respectively.…”