The photoreduction of 5,7-diiodo-3-butoxy-6-fluorone (DIBF) in the presence of the electron donors, tetrabutylammonium triphenylbutylborate (BORATE), and AVV-dimethyl-2,6-diisopropylaniline (DIDMA) was studied in acetonitrile solution. The transient phenomena examined using nanosecond laser flash photolysis confirm that the reduction is a one-electron transfer process from the electron donor to the excited triplet state of the dye. Triplet quenching rate constants were measured for BORATE and DIDMA as well as oxygen. The relative quenching rates for oxygen and DIDMA are «20:1. The electron transfer rates calculated are 6.7 x 107 and 1.2 x 108 s-1 in the BORATE and DIDMA systems, respectively. The dye radical anion (DIBP, Amax « 420 nm) formed by the electron transfer reaction is the only product obtained in the BORATE case. However, with DIDMA the dye "neutral" radical (DIBP, Amax « 360 nm) was detected as the only product. DIBP formed after protonation of DIBF'by proton transfer from the amino cation radical. This implies that the proton transfer is too fast to measure on the nanosecond time scale. This is the first such observation in these systems. Singlet oxygen formed by oxygen quenching of the dye triplet was found to be quenched by the electron donors via an electron transfer process also. No quenching of DIBF triplet occurred in the presence of the electron acceptor (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate (OPPI). The principal photobleached products detected by HPLC were reduced quinomethines in the DIDMA case and the ammonium salts of the reduced quinomethines in the BORATE case. Reoxygenation of these products regenerates the original dye in a reaction the rate of which depends on the polarity of the solution. However, the reoxygenation of the quinomethines is faster than that of their ammonium salts under the same conditions.
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.
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