A mechanism study of efficient photoreduction of CO2 to CO using BODIPY-derived heavy-atom-free electron donor-acceptor dyads as photosensitizers showing strong absorption of green light.
Abstract
With a model CO2 photoreduction molecular system, we investigated the detailed photophysics of the intermolecular electron transfer (ET) between the photosensitizer (PS), sacrificial electron donor, and the Fe(II) catalyst. To achieve strong visible-light absorption and long-lived triplet excited states, boron dipyrromethene (BDP) chromophores were used to fabricate heavy-atom-free electron donor-acceptor dyads as PSs. The PSs show strong absorption of green light (ε = 7.51 × 104 M-1 cm-1 at 502 nm) and long-lived triplet states (52-118 µs). Under CO2 photoreduction conditions, efficient ET was observed between the sacrificial electron donor 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) and the PSs at triplet excited states, generating stable long-lived radical anions (PSs˙-) that further transfer electrons to the catalyst [Fe(qpy)(OH2)]2+ (C-1, qpy = 2,2':6',2'':6'',2'''-quaterpyridine). The Stern-Volmer constant (KSV) of the quenching of PSs˙- by C-1 ranged from 1.67 × 104 M-1 to 2.30 × 104 M-1, determined by nanosecond transient absorption spectroscopy. The quenching of the singlet excited state of the PSs by BIH is less efficient (3.54 × 102 M-1). The solvent polarity dependence of the ISC process of the PSs was tuned by using different electron donors in the molecular structure. Moreover, upon oxidation of the phenothiazine (PTZ) moiety, the resulting PTZ-O-BDP dyad exhibits excellent ISC efficiency (ΦΔ = 43%) in polar solvents (τT = 52.5 µs), whereas the unoxidized analogue only shows ISC in nonpolar solvents. Interestingly, the oxidized PTZ-O-BDP dyad exhibited photocatalytic CO2 reduction activity (TONPTZ-O-BDP = 3024), much higher than the TON of PTZ-BDP (TONPTZ-BDP = 4). Importantly, quenching of the T1 state of the PSs by the catalyst C-1 was observed (KSV = 5.63 × 103 M-1), which is detrimental to the CO2 photoreduction. We postulate that suppressing this undesired quenching pathway may improve the CO2 photoreduction.
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