The
development of homogeneous photoredox catalysts in radical coupling reactions
has been truly phenomenal, and the generation of new carbon–carbon bonds in an
atom economical manner is a continuous pursuit in synthetic organic chemistry.
This enables the production of many pharmaceutical compounds and bio-active
molecules. The use of photoredox catalysts has quickly realized the huge
potential in organic synthesis to afford facile coupling reactions enabled by
photo-energy. Many excellent examples have been seen in the literature,
including couplings of carbonyl-a-radicals [1]
and ketyl radicals (e.g. the Pinacol Coupling Reaction, PCR) [2]. However, despite many successful applications
of photoredox catalysts in radical coupling reactions, with some exceptions the
scarce and expensive ruthenium (Ru) and iridium (Ir)-based homogeneous
catalysts are predominantly used, which are difficult to isolate and recycle at
the end of the reactions.
Over the past few years, engineering of group
III-nitride semiconductor nano-structures and their applications as
photocatalysts are well explored. Compared to common semiconductor catalysts [3] such as CdS, TiO2 and polymeric
carbon nitride (PCN), the gallium nitride (GaN) semiconductor shows
extraordinary stability [4, 5].
Figure 1. Scheme for radical coupling of
carbonyl reactions carried out under blacklight irradiation using GaN nanowire
(NW) as a reusable photoredox catalyst [5].
By
carefully repositioning of the conduction band (CB) energy level and the valence
band (VB) energy level of GaN NWs with doping, the desired direction of the
electron flow can be established upon light irradiation on the GaN NWs. The
photo-excited electron (e-) can efficiently be donated from the
semiconductor to the reactant (H2O, N2, or CO2,
respectively) to form the desired product (H2, NH3, or
CO/hydrocarbon, respectively). At the same time, the photo-excited hole (h+)
can be readily neutralized by many common sacrificial reagents. Inspired by
these studies, we contemplate the possibility of using those unique characters
of GaN NWs to design a more suitable and highly recyclable photoredox catalyst
for radical coupling reactions under a light.
The surface energy band bending significantly influences
the electronic properties of semiconductors [6].
Upon light irradiation, the excited e- in the CB of the
semiconductor tends to migrate to the potential energy well. In contrast, the h+
on the VB tends to migrate to high potential energy (high ground). These
surface band properties inhibit carrier recombination and give rise to unique
reactivities. By doping the NW with a tetravalent element (e.g. silicon,
germanium, etc.) as the n-type dopant, the surface energy band of GaN bends
upward to give n-GaN NWs. Upon light irradiation [6],
the excited e-in the CB tends to migrate to the internal region of
the NW instead of the surface, while the h+ in the VB tends to
migrate to the surface instead of the internal region of the NW. Such doping
prohibits e- donation to the reactant and enhances the neutralization
of h+ by the sacrificial reagent (Figure 2A).
On the other hand, by using a divalent element (e.g., magnesium) as the p-type
dopant, the surface energy band of the corresponding NW bends downwards (p-GaN
NW).
Figure 2. (A)
Surface energy band bending of the GaN NW. (B) EPR identification
of band bending [5].
The
photoexcited e- for the p-GaN NWs will therefore tend to aggregate on
the NW surface, making the donation of e- to the reactant easier,
while the h+ tends to migrate to the internal region of the NWs and
inhibits the consumption of the sacrificial reagent (Figure 2A right). To characterize these electronic properties, electron
paramagnetic resonance (EPR) spectra were recorded for the as-synthesized
intrinsic- (i-), n-, and p-GaN NWs; they were irradiated under 365 nm light (Figure 2B) for 1 hour [7].
Since both Ga3+ and N3- do not possess unpaired electron, the
ground state of GaN is diamagnetic. Therefore, the observed EPR signal should
exclusively come from photoexcited electrons (EPR spectra of i-, n-, and p-GaN
NWs in the dark were silent). It can be observed from the EPR spectra that both
n- and p-GaN NWs show stronger EPR signals than i-GaN NWs. This is because the
photoexcited e- and h+ are more prone towards
recombination in i-GaN NWs are due to less influential band bending.
In
concluding, they report a photo-pinacol coupling reaction catalyzed by GaN
nanowires under ambient light at room temperature with methanol as a solvent
and sacrificial reagent. By simply tuning the dopant, the GaN nanowire shows
significantly enhanced electronic properties. The catalyst showed excellent
stability, reusability and functional tolerance. All reactions could be
accomplished with a single piece of nanowire on Si-wafer.
References
D. A.
Nicewicz and D. W. C. MacMillan, Science, 322, 77 (2008).
L. J. Rono et al., J. Am. Chem. Soc.,
135, 17735 (2013).
A. Caron et al., ACS Catal., 9, 9458 (2019).
M. G. Kibria et al., Adv. Mater. 28, 8388 (2016).
M. Liu et al., Chem. Sci. 11,
7864 (2020).
M. G. Kibria et al., Nat. Commun. 5, 3825 (2014).
M. Foussekis,
Band Bending in GaN, Master dissertation, Virginia Commonwealth University,
Richmond, Virginia (2009).
Blog Written By
Dr. Y. Sasikumar
School of Materials Science & Engineering
Tianjin University of Technology
Tianjin, China
Editors
Dr. A. S. Ganeshraja
Dr. K. Rajkumar
Dr. K. Chandrasekar
Reviewers
Dr. K. Vaithinathan
Dr. S. Thirumurugan
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