Luminescence of meso-pyrimidinylcorroles: relationship with substitution pattern and heavy atom effects

The luminescence properties of a series of corroles containing multiple meso-pyrimidinyl groups have been studied. In particular, nine corroles containing two pyrimidinyl moieties, four corroles containing three pyrimidinyl groups and one corrole carrying a single pyrimidinyl substituent have been investigated, and their properties have been compared with some model species. The results indicate that the energy of the emissive π-π* corrole-core-based state is not significantly perturbed by the various substituents, whereas the emission lifetimes and quantum yields depend on the number of appended meso-dichloropyrimidinyl substituents. In particular, both emission lifetime and quantum yield decrease with increasing the number of meso-dichloropyrimidinyl substituents, whereas pyrimidinyl substituents which do not carry further electron withdrawing groups such as the chlorine atoms do not affect the corrole emission properties. Two hypotheses are taken into consideration to rationalize the results: (i) the presence of meso-dichloropyrimidinyl substituents could introduce low-lying CT states which mix with the corrole π-π* emissive level, so reducing emission efficiency; (ii) the ortho,ortho'-chlorine groups of the meso-pyrimidinyl substituents, which lie in the proximity of the corrole macrocycle, can increase the intersystem crossing rate constant of the corrole-based fluorescent state via the heavy-atom effect. A comparison of the results of the studied corrole compounds with formerly investigated species and the linear dependence of the radiationless decay rate constants and emission quantum yields on the squared spin-orbit coupling constants, calculated considering the number of chlorine atoms in ortho-position of the corrole meso-substituents, suggest that hypothesis (ii) is most likely the valid one. These results are of particular interest for the design of (a) corrole compounds featuring highly efficient triplet formation and (b) multicomponent systems containing photo-active corrole subunits and pyrimidinyl spacers.