Trans-A2B-corroles bearing a coumarin moiety--from synthesis to photophysics

Four dyads comprised of corrole and coumarin units have been synthesised. Three coumarincarboxaldehydes were synthesized and transformed into the corresponding trans-A(2)B-corroles by reaction with 5-(pentafluorophenyl)dipyrromethane. It has been proven that this type of direct condensation can lead to the corresponding corroles in moderate yields. The reaction of hydroxybenzaldehydes with vinylphosphonium salts has been identified as the most general method for the preparation of formyl-coumarins with various patterns of substituents. The dyad consisting of ketobiscoumarin and corrole was synthesized by Sonogashira coupling. Spectroscopic and photophysical investigations revealed that there is an efficient energy transfer from the coumarin moiety to corrole in all four dyads. Energy transfer can be clearly ascribed to a dipole-dipole mechanism (Förster) for all dyads that contain luminescent coumarins and to an electron exchange mechanism (Dexter) for the dyad with the non-luminescent one. In the case of the dyad that bears coumarin with a hydroxy group at position 5, an electron-transfer was detected from corrole to coumarin. The latter process is possible because of the suitably low reduction potential of coumarins of this type.