While heteroatom-centered radicals are understood to
be highly
electrophilic, their ability to serve as transient electron-withdrawing
groups and facilitate polar reactions at distal sites has not been
extensively developed. Here, we report a new strategy for the electronic
activation of halophenols, wherein generation of a phenoxyl radical
via formal homolysis of the aryl O–H bond enables direct nucleophilic
aromatic substitution of the halide with carboxylate nucleophiles
under mild conditions. Pulse radiolysis and transient absorption studies
reveal that the neutral oxygen radical (O•) is indeed
an extraordinarily strong electron-withdrawing group [σp
–(O•) = 2.79 vs σp
–(NO2) = 1.27]. Additional mechanistic
and computational studies indicate that the key phenoxyl intermediate
serves as an open-shell electron-withdrawing group in these reactions,
lowering the barrier for nucleophilic substitution by more than 20
kcal/mol relative to the closed-shell phenol form of the substrate.
By using radicals as transient activating groups, this homolysis-enabled
electronic activation strategy provides a powerful platform to expand
the scope of nucleophile–electrophile couplings and enable
previously challenging transformations.
One-electron reduced photosensitizers have been invoked as crucial intermediates in photoredox catalysis, including multiphoton excitation and electrophotocatalytic processes. However, such reduced chromophores have been less investigated, limiting mechanistic studies of their associated electron transfer processes. Here, we report a total of 11 different examples of isolable singly reduced iridium chromophores. Chemical reduction of a cyclometalated iridium complex with potassium graphite affords a 19-electron species. Structural and spectroscopic characterizations reveal a ligand-centered reduction product. The reduced chromophore absorbs a wide range of light from ultraviolet to near-infrared and exhibits photoinduced bimolecular electron transfer reactivity. These studies shed light on elusive reduced iridium chromophores in both ground and excited states, providing opportunities to investigate a commonly invoked intermediate in photoredox catalysis.
The morphology of organic semiconductors
is critical to their function
in optoelectronic devices and is particularly crucial in the donor–acceptor
mixture that comprises the bulk heterojunction of organic solar cells.
Here, energy landscapes can play integral roles in charge photogeneration,
and recently have been shown to drive the accumulation of charge carriers
away from the interface, resulting in the buildup of large nanoscale
electric fields, much like a capacitor. In this work we combine morphological
and spectroscopic data to outline the requirements for this interdomain
charge accumulation, finding that this effect is driven by a three-phase
morphology that creates an energetic cascade for charge carriers.
By adjusting annealing conditions, we show that domain purity, but
not size, is critical for an electro-absorption feature to grow-in.
This demonstrates that the energy landscape around the interface shapes
the movement of charges and that pure domains are required for charge
carrier buildup that results in reduced recombination and large interdomain
nanoscale electric fields.
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