Mechanistic Studies of Photobase Generation from Ammonium Tetraorganyl Borate Salts¹

Abstract

The photochemistry and photophysics of N,N,N-tributyl-N-acetonaphthone ammonium borate (1) and N,N,N-tributyl-N-acetobenzo[b]furan ammonium borate (2) have been investigated by steady-state spectroscopy, laser flash photolysis, and product analysis. Reaction of excited precursors leads to formation of tertiary amines by carbon-nitrogen bond cleavage. Laser flash photolysis and product analysis confirm homolytic carbonnitrogen bond scission and the formation of respective acetyl radicals which, in acetonitrile, dimerize to form coupled products. In all cases, photodissociation occurs with such a short lifetime that it could not be measured by nanosecond flash photolysis. The quantum yield of disappearance was found to be higher for salts of the triphenylbutyl borate anion than for those of the tetraphenyl borate anion. A proposed mechanism involves electron transfer from borate to the excited acceptors as the primary photochemical step. The rate constants for excited-state quenching correlate with the free energy for electron transfer, as estimated from the excited-state energies and the reduction potentials of the acceptors.

Photochemical reactions of quaternary ammonium dithiocarbamates as photobase generators and their use in the photoinitiated thermal crosslinking of poly(glycidyl methacrylate)

“…Still, resulting bases that are generated from PBGs do not have such unfavorable properties and may be used as crosslinkers of epoxy compounds and as catalysts for the imidization of poly(amic acid)4 and the surface modification of polymers 5, 6. Thus far, Co‐amine complexes,7 carbamates,8–12 O ‐acyloximes,5, 6, 13–16 O ‐carbamoyloximes,17–19 formamide,20 and onium salts21–23 are known as PBGs, and several reviews have been published 3, 24–27. Most articles on them, however, describe photoproduction of primary or secondary amines from them, and PBGs that generate tertiary amines are scarcely reported.…”

Branched Base‐Amplifying Oligomers Enhancing UV‐Curing of Epoxy Resins

“…Representative PBG generating primary or secondary amines includes o ‐nitrobenzyl derivatives,3 cobalt ammine complexes,4 acyl oximes,5 and urethane oximes 6. Aliphatic tertiary amines are generated by the photolysis of quarternary ammonium salts,7–9 whereas PBGs enhancing the basicity of photogenerated tertiary amines have been reported and applied to various types of UV‐curing materials 1. Recent work disclosed that a tetraphenylborate salt gives rise to the photogeneration of a bicyclic guanidine as a superbase, which initiates the anionic polymerization of ε ‐caprolactone 10…”

“…Usually, much more than one order larger magnitude crosslinked density is required for UV‐curable materials when compared with photocrosslinkable polymers exhibiting solubility changes. Since primary or secondary amines generated from PBGs are consumed by the addition to electrophilic residues like epoxy, isocyanate, or electron‐poor CC double bond, predominant studies have been focused on achieving photocrosslinkable polymers with epoxy side chains,4, 7–9, 12 and scarce reports have been made on UV‐curing based on photobase generation. This situation stems not only from low quantum efficiencies for the generation of amines, but also from the stepwise nature of the addition of amine to epoxy groups.…”

Synthesis and photoinitiation properties of a novel quaternary ammonium tetraphenylborate salt

“…Cobalt-amine complexes, 2,3 acyl oximes, 1,[4][5][6][7][8] carbamates, 1,9,10 formamides, 11 and quaternary ammonium salts have been used as PBGs. [12][13][14][15][16] Among these PBGs, quaternary ammonium salts have been develop recently. As we know, arylalkylborate salts are also free-radical chain initiators, so quaternary ammonium arylalkylborate salts can produce not only amines but also free radicals.…”