Journal of the American Chemical Society
Article
a redox relay is the hydrogen-bonded tyrosine-histidine (Tyrz-
H190) pair located in Photosystem II (PSII).11 Sunlight
generates the excited state of the reaction-center (P680*) in
PSII, which then reduces a nearby chlorophyll (Chl), forming
an intermolecular charge-separated state, P680•+-Chl•−. This
high energy state is responsible for the light-driven electron
transfer reactions needed for water splitting, but it is
simultaneously susceptible to charge recombination.12,13 To
avoid recombination and to move the oxidizing equivalent
closer to the site of water oxidation, Tyrz, with its hydrogen-
•+
bonded partner H190, rapidly fills the hole on P680 using a
proton-coupled electron transfer (PCET) process.
Figure 2. Chemical structures of cyclometalated Ir(III) photocatalysts
in this study.
Inspired specifically by the function of the Tyrz-H190 pair in
PSII, model systems have been developed for charge transport,
utilizing charge-separated states.14,15 These systems include a
benzimidazole-phenol (BIP, see Figure 1B) platform which
features PCET mimicking the Tyrz-H190 pair.16−18 By
incorporating the BIP as an electron transfer mediator, Zhao
et al. have shown an improvement in the quantum yield for
water splitting using dye-based systems, which is generally low
because the charge recombination reaction is faster than the
catalytic four-electron oxidation (Figure 1B).19
recombination in the intermolecular charge separated state of
oxidized photocatalyst 2+ and MV•+, when compared to an
Ir(III) photocatalyst without an appended BIP (photocatalyst
1). These results are then extended into preparative-scale
photoredox catalysis, where quantum yield enhancement is
observed with photocatalyst 2 in the reduction of redox-active
N-hydroxyphthalimide esters (NHPI esters).
RESULTS AND DISCUSSION
■
In the design of PSII mimics, the incorporation of a
hydrogen-bonded, phenol-imidazole pair into the ligand
framework of a heteroleptic Ru(II) photosensitizer was
shown to similarly impede charge recombination (Figure
Synthesis. The Ir(III) photocatalysts 1 and 2 were
synthesized in accordance with previous reports.23,24 We
hypothesized that the electron-withdrawing ppy ligands were
necessary for efficient oxidation of the BIP. The complete
synthetic route, conditions, and structural characterization of
the BIP-bipyridine ligand (BIP-bpy L) and photocatalysts 1
and 2 are described in the Supporting Information (SI 1.2).
Steady-State Absorption and Emission. The photo-
physics of photocatalyst 2 are intriguingly distinct from those
of the reference photocatalyst 1. The electronic absorption and
emission properties of photocatalysts 1 and 2 are summarized
in Table 1. The absorption spectra of 1 and 2 feature ππ*
1C).20 Emission from the MLCT state of Ru(II) complexes
3
with coordinated imidazole-phenol pairs occurs at identical
energies and with the same quantum yields as control
complexes lacking the PCET ligand, indicating PCET does
not occur in the excited state.20 However, upon addition of
methyl viologen dication (MV2+), electron transfer from the
3MLCT excited state of a similar Ru(II) complex to MV2+
generates an oxidized Ru(III) species.21 Rapid PCET from the
coordinated imidazole-phenol pair fills the hole on Ru,
effectively slowing charge recombination between Ru(III)
and MV•+.
Table 1. Electronic Absorption and Emission of 1 and 2
λmaxabs/nm
However, these concepts and their attendant advantages are
less explored in general photocatalytic organic synthesis, and
we questioned whether BIP appended to an Ir(III) photo-
sensitizer could play a similar role in h+ transfer, effectively
delaying charge recombination and extending the lifetime of
the substrate radical (Figure 1D). When compared to
ruthenium photocatalysts, iridium photocatalysts offer greater
stability and tunability of photophysical properties, due to
larger ligand field splitting and the ease with which they
support cyclometallating C−N ligands (such as 2-phenyl-
pyridine, ppy).22 There are two possible mechanisms by which
the appended BIP can slow charge recombination. If BIP
oxidation is thermodynamically capable of quenching the Ir-
a
a,
b
a
,
b
,
c
d
photocatalyst
(ε/M−1 cm−1
)
λ
em/nm
Φem
0.90
0.0060
λ
em/nm
1
2
380 (5100)
370 (18000)
498
515
460
570
a
b
c
In CH3CN at room temperature. In deoxygenated solvent. Relative
d
to Ir(ppy)3. In 4:1 MeOH:EtOH glassy matrix at 77 K.
transitions in the UV and a lowest energy transition in the
near-UV assigned as a mixed ligand centered (LC) (ppy π →
π*) and Ir(d) → ppy(π*) transition (Figure S9).25 The
electronic absorption and emission spectra of 1 are consistent
with literature reports.23,26 Emission quantum yields were
determined relative to Ir(ppy)3 (Φ = 0.97 in MeTHF).27 The
emission quantum yield of 2 is approximately 2 orders of
magnitude lower than that of 1, indicating quenching of the
emissive excited state (Figure S10). Addition of 10 mM
phenylphosphoric acid to 2 increases the emission quantum
yield to 0.70 (Figure S11). Protonation of the imidazole
proton acceptor in acidic solution prevents intramolecular
PCET. The emission recovery upon acid addition suggests
PCET quenches the emission of 2.
3
complex MLCT state, intramolecular PCET will result in a
charge-separated state (3CS), leaving a positive charge
localized on BIP. On the other hand, BIP can be oxidized by
Ir(IV), formed by electron transfer to the substrate from the
3MLCT, as is the case in the aforementioned Ru(II)
complexes.
We report herein an Ir(III) complex (Figure 2, photocatalyst
2) inspired by the Tyrz-H190 redox relay of PSII. Electro-
chemical, spectroscopic, and computational characterization
demonstrates that excitation of 2 with blue light rapidly
produces an intramolecular, triplet charge separated state
(Figure 1D, middle). Characterization of charge recombination
kinetics with MV2+ quencher illustrates that 2 slows charge
Electrochemistry. Cyclic voltammetry experiments were
performed in degassed acetonitrile solution to gain insight into
the electrochemical parameters of 2 compared with 1. Figure 3
displays cyclic voltammograms of 1 and 2 and Table 2
summarizes the most relevant electrochemical parameters. For
13036
J. Am. Chem. Soc. 2021, 143, 13034−13043