Intersystem Crossing in Halogenated Bodipy Chromophores Used for Solar Hydrogen Production

A series of halogenate 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 (tau(f1)) of 3-5 ns, a fluorescence quantum yield (P-q) of 0.56, and negligible triplet state yield. Bromination enhances the intersystem crossing (ISC) such that tau(n) and Phi(f1) decrease to similar to 1.2 ns and 0.11, respectively, while iodination further accelerates ISC so that tau(f1) is only similar to 130 ps and Phi(f1) is 0.011. Transient absorption experiments lead to the observation of excited state absorption bands from the singlet (S-1) and triplet (T-1) states at similar to 345 and 447 nm respectively and characterization of ISC via the dynamics of these bands and the decay of S-1 stimulated emission.