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Quantitative Profiling of the Heavy‐Atom Effect in BODIPY Dyes: Correlating Initial Rates, Atomic Numbers, and 1 O 2 Quantum Yields
Author(s) -
Rey Yannick P.,
Abradelo Dario G.,
Santschi Nico,
Strassert Cristian A.,
Gilmour Ryan
Publication year - 2017
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201601372
Subject(s) - intersystem crossing , bodipy , chemistry , singlet oxygen , catalysis , photochemistry , atom (system on chip) , halogen , hypsochromic shift , quantum yield , singlet state , excited state , fluorescence , oxygen , atomic physics , physics , organic chemistry , computer science , embedded system , alkyl , quantum mechanics
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 O 2 . The most expedient strategy relies on the generation of singlet oxygen ( 1 O 2 ; a 1 Δ g ) from the triplet state ( 3 O 2 ; X 3 Σ g – ) 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 ( Z X ) of pendant halogen atoms with an enhanced probability of intersystem crossing (S 1 →T 1 ). 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 ( v 0,H < v 0,Cl < v 0,Br < v 0,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 k X isc / k Y isc and relative atomic numbers Z X / Z Y for the respective substituents with corresponding slopes of approximately 4 were obtained, that is, k isc ~ Z 4 , 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.