Quantitative Profiling of the Heavy‐Atom Effect in BODIPY Dyes: Correlating Initial Rates, Atomic Numbers, and 1O2 Quantum Yields
Direct oxidation using molecular oxygen is both attractive and atom-efficient. However, this process first requires the catalyst-based activation or electronic reconfiguration of inert O2. The most expedient strategy relies on the generation of singlet oxygen (1O2; a1Δg) from the triplet state (3O2; X3Σg–) by a photosensitizer. In the current arsenal of photosensitizers, boron-dipyrromethene (BODIPY) cores are considered privileged on account of their unique photophysical characteristics and the ability to tune their behavior through facile structural modifications such as halogen (X) incorporation. Thus, the scaffold has become synonymous with the renowned heavy-atom effect (HAE), a phenomenon that correlates the increasing atomic number (ZX) of pendant halogen atoms with an enhanced probability of intersystem crossing (S1→T1). Herein, a facile GC-based method to assess catalyst performance has been developed and validated with a focused set of halogenated BODIPY scaffolds. An initial-rate approximation was applied to a model transformation and follows the HAE trend (v0,H < v0,Cl < v0,Br < v0,I). This operationally simple approach was corroborated by complementary determinations of absolute singlet oxygen and photoluminescence quantum yields and time-resolved luminescence decays to evaluate lifetimes. For double logarithmic plots, linear correlations between relative intersystem-crossing rates kXisc/kYisc and relative atomic numbers ZX/ZY for the respective substituents with corresponding slopes of approximately 4 were obtained, that is, kisc ~ Z4, which also was shown to hold for the fluorescence-lifetime-corrected singlet-oxygen quantum yields as independent measurements. This substantiates theoretical predictions pertaining to the heavy-atom effect.