Developing broadband and strong visible-light-absorbing photosensitizer is highly desired for dramatically improving the utilization of solar energy and boosting artificial photosynthesis. Herein, we develop a facile strategy to co-sensitize Ir-complex with Coumarins and boron dipyrromethene to explore photosensitizer with a broadband covering ca. 50% visible light region (Ir-4). This type of photosensitizer is firstly introduced into water splitting system, exhibiting significantly enhanced performance with over 21 times higher than that of typical Ir(ppy)
2
(bpy)
+
, and the turnover number towards Ir-4 reaches to 115840, representing the most active sensitizer among reported molecular photocatalytic systems. Experimental and theoretical investigations reveal that the Ir-mediation not only achieves a long-lived boron dipyrromethene-localized triplet state, but also makes an efficient excitation energy transfer from Coumarin to boron dipyrromethene to trigger the electron transfer. These findings provide an insight for developing broadband and strong visible-light-absorbing multicomponent arrays on molecular level for efficient artificial photosynthesis.
The
prototypical [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine, Ru-1) with 3MLCT state (metal-to-ligand charge-transfer,
πM → πL*) is one of the most
widely used photosensitizers (PSs) for photocatalytic hydrogen production.
However, its photostability and excited state lifetime (<1 μs)
are eagerly to be improved to further enhance the performance of hydrogen
production. Herein, [Ru(bpy)2(3-pyrenyl-1,10-phenanthroline)]2+ (Ru-3) with 3IL/3MLCT
equilibrated state and [Ru(bpy)2(3-pyrenyl ethynylene-1,10-phenanthroline)]2+(Ru-4) with 3IL state (intraligand
charge transfer, πL → πL*)
as lowest excited state were first introduced into the photocatalytic
hydrogen evolution system. Photophysical and photocatalytic characteristics
manifest that the 3IL state complex (Ru-4)
shows a long-lived excited state (up to 120 μs) and much enhanced
photostability with no photobleaching over 13 h in stark contrast
to Ru-1 and [Ru(bpy)2(1,10-phenanthroline)]2+ (Ru-2). Photocatalytic reactions with these
Ru(II) complexes as PSs, Co(dmgH)2pyCl (C-1) as a catalyst, and N, N-dimethyl-p-toluidine (DMT) as an electron donor indicate that
the catalytic performance of Ru-4 and Ru-3 is dramatically enhanced compared to that of Ru-2 and Ru-1, and the TON and TOF toward Ru-4 can reach
up to 9140 and 6.3 min–1 under the optimized condition.
Photoluminescence studies reveal that the Stern–Volmer quenching
constant of excited state Ru-4 by DMT is
determined as 2.8 × 104 M–1, which
is 4.5-, 42-, and 44-fold higher than those of Ru-3 (6.2
× 103 M–1), Ru-2 (6.7
× 102 M–1), and Ru-1 (6.3 × 102 M–1), respectively.
Transient absorption spectra confirmed that the reductive quenching
mechanism is the dominated process, and the quenching constant of
electron transfer from reduced PSs of Ru-1–Ru-4 to C-1 catalyst has the same order of magnitude
(∼105 M–1). The increased photocatalytic
activity of Ru-3 and Ru-4 is due to their
prominent photostability and efficient electron transfer from DMT to PSs. This work not only contributes to a deep understanding
in the photocatalytic process with the PSs of three different excited
state types but also opens up an avenue to explore robust and long-lived
PSs with 3IL state for efficient hydrogen production.
Developing strong visible‐light‐absorbing (SVLA) earth‐abundant photosensitizers (PSs) for significantly improving the utilization of solar energy is highly desirable, yet it remains a great challenge. Herein, we adopt a through‐bond energy transfer (TBET) strategy by bridging boron dipyrromethene (Bodipy) and a CuI complex with an electronically conjugated bridge, resulting in the first SVLA CuI PSs (Cu‐2 and Cu‐3). Cu‐3 has an extremely high molar extinction coefficient of 162 260 m−1 cm−1 at 518 nm, over 62 times higher than that of traditional CuI PS (Cu‐1). The photooxidation activity of Cu‐3 is much greater than that of Cu‐1 and noble‐metal PSs (Ru(bpy)32+ and Ir(ppy)3+) for both energy‐ and electron‐transfer reactions. Femto‐ and nanosecond transient absorption and theoretical investigations demonstrate that a “ping‐pong” energy‐transfer process in Cu‐3 involving a forward singlet TBET from Bodipy to the CuI complex and a backward triplet‐triplet energy transfer greatly contribute to the long‐lived and Bodipy‐localized triplet excited state.
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