Intersystem Crossing in Halogenated Bodipy Chromophores Used for Solar Hydrogen Production

Abstract

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