Efficient Bimolecular Mechanism of Photochemical Hydrogen Production Using Halogenated Boron-Dipyrromethene (Bodipy) Dyes and a Bis(dimethylglyoxime) Cobalt(III) Complex.

Abstract

A series of Boron-dipyrromethene (Bodipy) dyes were used as photosensitizers for photochemical hydrogen production in conjunction with [Co(III)(dmgH)2pyCl] (where dmgH = dimethylglyoximate, py = pyridine) as the catalyst and triethanolamine (TEOA) as the sacrificial electron donor. The Bodipy dyes are fully characterized by electrochemistry, X-ray crystallography, quantum chemistry calculations, femtosecond transient absorption, and time-resolved fluorescence, as well as in long-term hydrogen production assays. Consistent with other recent reports, only systems containing halogenated chromophores were active for hydrogen production, as the long-lived triplet state is necessary for efficient bimolecular electron transfer. Here, it is shown that the photostability of the system improves with Bodipy dyes containing a mesityl group versus a phenyl group, which is attributed to increased electron donating character of the mesityl substituent. Unlike previous reports, the optimal ratio of chromophore to catalyst is established and shown to be 20:1, at which point this bimolecular dye/catalyst system performs 3-4 times better than similar chemically linked systems. We also show that the hydrogen production drops dramatically with excess catalyst concentration. The maximum turnover number of ∼ 700 (with respect to chromophore) is obtained under the following conditions: 1.0 × 10(-4) M [Co(dmgH)2pyCl], 5.0 × 10(-6) M Bodipy dye with iodine and mesityl substituents, 1:1 v:v (10% aqueous TEOA):MeCN (adjusted to pH 7), and irradiation by light with λ > 410 nm for 30 h. This system, containing discrete chromophore and catalyst, is more active than similar linked Bodipy-Co(dmg)2 dyads recently published, which, in conjunction with our other measurements, suggests that the nominal dyads actually function bimolecularly.