Substituted triphenylamine (TPA) radical cations show
great potential
as oxidants and as spin-containing units in polymer magnets. Their
properties can be further tuned by supramolecular assembly. Here,
we examine how the properties of photogenerated radical cations, intrinsic
to TPA macrocycles, are altered upon their self-assembly into one-dimensional
columns. These macrocycles consist of two TPAs and two methylene ureas,
which drive the assembly into porous organic materials. Advantageously,
upon activation the crystals can undergo guest exchange in a single-crystal-to-single-crystal
transformation generating a series of isoskeletal host–guest
complexes whose properties can be directly compared. Photoinduced
electron transfer, initiated using 365 nm light-emitting diodes, affords
radicals at room temperature as observed by electron paramagnetic
resonance (EPR) spectroscopy. The line shape of the EPR spectra and
the quantity of radicals can be modulated by both polarity and heavy
atom inclusion of the encapsulated guest. These photogenerated radicals
are persistent, with half-lives between 1 and 7 d and display no degradation
upon radical decay. Re-irradiation of the samples can restore the
radical concentration back to a similar maximum concentration, a feature
that is reproducible over several cycles. EPR simulations of a representative
spectrum indicate two species, one containing two N hyperfine interactions
and an additional broad signal with no resolvable hyperfine interaction.
Intriguingly, TPA analogues without bromine substitution also exhibit
similar quantities of photogenerated radicals, suggesting that supramolecular
strategies can enable more flexibility in stable TPA radical structures.
These studies will help guide the development of new photoactive materials.
In order to design a series of photoredox compounds with a broad range of reactivity, eosin Y, a xanthene derivative, was chosen as a precursor to synthesize a new series of organic photocatalysts. The synthesis and characterization of these four new organic photocatalysts was undertaken. Redox potentials of this series of photocatalysts varied by 110 mV, which shows that these catalysts can be tuned for specific reactions. The measured fluorescence quantum yields ranged from 0.33 to 0.65 which outperform most transition metal photocatalysts. The excited state lifetimes (ns) of the new photocatalysts are comparable to those of the parent complex, but the λ max value for absorption was red-shifted into the green light region of the solar spectrum. Despite the absorbance shift to lower energy wavelengths, the new photocatalysts were more potent reductants compared to the parent complex and were able to undergo oxidative quenching and promote the photocatalytic enol arylation reaction.
Absorption of electronic acceptors in the accessible channels of an assembled triphenylamine (TPA) bis-urea macrocycle 1 enabled the study of electron transfer from the walls of the TPA framework to...
Natural photosynthesis uses an array of molecular structures
in
a multiphoton Z-scheme for the conversion of light energy into chemical
bonds (i.e., solar fuels). Here, we show that upon excitation of both
a molecular photocatalyst (PC) and a substituted naphthol (ROH) in
the presence of a sacrificial electron donor and proton source, we
achieve photocatalytic synthesis of H2. Data support a
multiphoton mechanism that is catalytic with respect to both PC and
ROH. The use of a naphthol molecule as both a light absorber and H2 producing catalyst is a unique motif for Z-scheme systems.
This molecular Z-scheme can drive a reaction that is uphill by 511
kJ mol–1 and circumvents the high-energy constraints
associated with the reduction of weak acids in their ground state,
thus offering a new paradigm for the production of solar fuels.
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