Intersystem Crossing in Halogenated Bodipy Chromophores Used for Solar Hydrogen Production

A series of halogenated boron-dipyrromethene (Bodipy) chromophores with potential applications in solar energy conversion were synthesized and characterized by steady state and ultrafast laser spectroscopy. The ultrafast dynamics of the chromophores were compared between a series containing H, Br, or I at the 2,6 positions of the Bodipy dye. The parent Bodipy has a fluorescent lifetime (τ_fl) of 3−5 ns, a fluorescence quantum yield (Φ_fl) of 0.56, and negligible triplet state yield. Bromination enhances the intersystem crossing (ISC) such that τ_fl and Φ_fl decrease to ∼1.2 ns and 0.11, respectively, while iodination further accelerates ISC so that τ_fl is only ∼130 ps and Φ_fl is 0.011. Transient absorption experiments lead to the observation of excited state absorption bands from the singlet (S₁) and triplet (T₁) states at ∼345 and 447 nm, respectively, and characterization of ISC via the dynamics of these bands and the decay of S₁ stimulated emission

Photoinduced Charge Transfer in Porphyrin–Cobaloxime and Corrole–Cobaloxime Hybrids

We report on the synthesis of hybrid molecules consisting of a porphyrin or corrole chromophore axially coordinated to a [Co^III(dmgH)₂(Cl)]^±0 (dmg = dimethylglyoxime) unit via a pyridine group as potential hydrogen forming entities in H₂O/THF medium. Photophysical, electrochemical, and pulse radiolysis studies on the hybrids and/or their separate components show that selective excitation of the porphyrin or corrole chromophore in its first singlet excited state leads to fast charge separation due to chromophore to cobalt electron transfer. However, this charge separation is followed by even faster charge recombination thereby preventing the accumulation of a reduced cobalt species which would lead to hydrogen production. It is important, nevertheless, that addition of a sacrificial electron donor slows the charge recombination down. In light of the latter it comes as hardly surprising that the photocatalysis experiments in the presence of a sacrificial electron donor (i.e., triethylamine) show modest rates of hydrogen production