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

A series of Boron-dipyrromethene (Bodipy) dyes were used as photosensitizers for photochemical hydrogen production in-Conjunction with [Co-III(dmgH)(2)pyCl] (where dmgH = dimethylglyoximate, py = pyridine). as the catalyst and triethanolamine (TBOA) 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 chromo-phores 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 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 similar to 700 (with respect to chromophore) is obtained under the following conditions: 1.0 x 10(-4) M [Co(dmgH)(2)pyCl], 5.0 x 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 lambda > 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 biomolecularly.