Photoinduced Inter- and Intra-Ion-Pair Electron Transfer Reactions in N-(p-Benzoylbenzyl)-N,N,N-tri-n-butylammonium Triphenyl-n-butylborate and -Gallate Salts

Abstract

The photoreduction of N-(p-benzoylbenzyl)-N,N,N-tri-n-butylammonium triphenyl-n-butylborate (1) and gallate (2) was studied using nano- and picosecond laser flash photolysis. An electron transfer reaction from the borate or the gallate counteranion to the excited triplet state of the benzophenone moiety (BP*3) was demonstrated. This reaction leads to the formation of benzophenone radical anion (6) and the boranyl radical, the latter of which dissociates rapidly to form butyl radical. The electron transfer rate was found to depend on the polarity of the solvent. In neat benzene, the short lifetime of the triplet obtained (300 ± 150 ps) indicates an intramolecular process and that the compounds exist as tight ion pairs. The addition of 1% MeCN increases the triplet lifetime up to 1.2 ns. The formation of a solvent-separated ion pair was suggested in this solvent mixture. However, in the polar solvent MeCN, 1 and 2 exist partially as free ions as determined by their dissociation constants. A diffusion-controlled intermolecular electron transfer process was shown in this solvent. The p-benzoylbenzyl radical (BPCH2 •), most likely formed by the C−N cleavage bond in the radical anion (6), was identified as an intermedate in the reaction pathway. After the initial electron transfer reaction, this produced tributylamine. The quantum yield obtained for the reaction of 1, 10-2 M in MeCN, is 0.5 ± 0.05. Triplet quenching as well as coupling of the radical anion (6) with the butyl radical deriving from decomposition of the borate was suggested to compete with the C−N cleavage reaction.

Photoreduction of triplet thioxanthone derivative by azolium tetraphenylborate: a way to photogenerate N-heterocyclic carbenes

“…In addition, these values correspond reasonably well to previously reported values for these spin adduct in literature. 16,19 This result supports that the transition state of oxidized BPh 4 could dissociate into Ph  and Ph 3 B.…”

“…were also used as electron donor in inter-and intra-PET reactions (route 2) with cationic or neutral electron acceptor (EA) sensitizers such as carbocyanine, 15 fullerene, 16 coumarine, 17 fluorone, 18 benzophenone derivatives. 19 A boranyl radical is formed, which in turn undergoes rapid cleavage yielding a triphenylborane and a phenyl (or alkyl) radical. Only alkyl radicals have been applied for initiation of radical polymerizations, 17 or alkylation of the EA.…”

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

“…[2][3][4][5][6][7][8] Triphenylbutylborates have been studied most extensively, and the butyl radical formed when they are oxidized has been found to be the dominant initiating species for radical polymerization. 3,[7][8][9][10] Interestingly, tetraphenylborates paired with a similar cationic partner have been observed to have similar initiating efficiencies. [11][12][13] Tetraphenylborates, when oxidized, are often prone to back electron transfer.…”

Left recurrent laryngeal nerve palsy secondary to left pulmonary artery stent in a child

“…It is apparent that borate anions are especially beneficial electron transfer donors, and they have many important applications in free‐radical polymerizations initiated by different electron accepting sensitizers. The one electron photooxidation of triarylalkylborates forms a boranyl radical which lives for a very short time and cleaves rapidly yielding triarylborane and an alkyl radical, the latter initiating polymerization 10 Electron transfer photooxidation of borates may occur with uncharged and charged acceptors 11–13.…”