Rhenium-Selenido Corroles: Reflections on 5d Metalloporphyrins and Metallocorroles as Triplet Emitters and Photosensitizers

Following the successful synthesis of rhenium-oxido, -imido, and -sulfido corroles, a series of para-X-substituted rhenium-selenido triarylcorroles (X = OCH3, CH3, H, F, CF3) have been prepared in 70-76% yields. A one-pot, two-step procedure was used, analogous to that used for ReS corroles, involving rhenium insertion via high-temperature reaction of Re2(CO)10 and free-base corroles in refluxing decalin, followed by exposure to PCl3 (which is thought to deoxygenate ReO corroles formed under the reaction) and elemental selenium. An analogous procedure, however, failed to yield rhenium-tellurido corroles, presumably reflecting, according to DFT calculations, the limited stability of the species. Unlike ReO corroles, ReS and ReSe corroles were found not to exhibit phosphorescence in the NIR-I regime (600-1000 nm); nor did they sensitize singlet oxygen formation. In the hope of obtaining a broader perspective of this negative result, we also examined ReN porphyrins and found them not to phosphoresce or to sensitize singlet oxygen formation. These results were explained by DFT and TDDFT calculations in terms of low-energy triplet states, which are not energetic enough to excite molecular oxygen to its lowest singlet state. Whether some of the new Re corroles exhibit phosphorescence in the NIR-II regime remains an interesting question for the future.