In
this work, 2,4′-dichlorobiphenyl (1) yielded
4′-chloro-2-hydroxybiphenyl (2) after photolysis
in neutral acetonitrile aqueous (ACN–H2O) solutions.
Ultrafast spectroscopic measurements and density functional theory
(DFT) computations were performed for 2 in ACN and ACN–H2O (v/v, 1:1). These results were compared with previously
published results for 2-hydroxybiphenyl (3). The counterparts 2 and 3 went through a singlet excited state
intramolecular proton transfer (ESIPT) in ACN but behaved differently
in ACN–H2O with a dehydrochlorination process occurring
for 2 and an ESIPT taking place for 3. Computational
results indicate that the phenol O–H bond elongates after photoexcitation
to induce a concerted asynchronous process with the C–Cl bond
increasing first followed by HCl elimination. A biradical intermediate
(IM1) is then formed with some spin located at the phenyl 4′-C
radical that appears to favor a hydrogen atom transfer (HAT) process
and some spin located on phenoxyl that appears to prefer a subsequent
•CH2CN radical rebound. The hydrogen bond promotes
HCl elimination, while this is disfavored for ESIPT, making 4′-Cl
extrusion the predominant process in ACN–H2O solutions.
The mechanistic investigations have fundamental and significant implications
for the understanding of polychlorinated biphenyl photolysis in an
aqueous environment and hence the photodegradation of these kinds
of pollutants in the natural environment.
Time-resolved spectroscopies and DFT calculations were
utilized
to investigate the photoredox mechanisms of naphthoquinone compounds.
5-Methoxy-8-tetrahydropyrane-1,4-naphthoquinone (NQ) and 2-methyl-3-(3-methylbut-2-en-1-yl)
1,4-naphthoquinone (MNQ) were excited to singlet excited species (labeled
NQ(S1) and MNQ(S1), respectively). NQ(S1) underwent intersystem crossing to produce a triplet NQ,
which further underwent hydrogen atom transfer to form a biradical
intermediate. The biradical underwent electron transfer to form a
zwitterion, followed by cyclization and proton transfer to generate
a photoproduct. MNQ(S1) underwent a 1,4-proton transfer
process to produce a quinone methide intermediate (1,3-QM) with zwitterionic
character, which tautomerized to 1,2-QM. Then, 1,2-QM underwent electrocyclization.
The substituent on the parent naphthoquinone is the key factor leading
to the different reaction processes for NQ and MNQ.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.