Pyrenyl-Substituted Imidazo[4,5- f ][1,10]phenanthroline Rhenium(I) Complexes with Record-High Triplet Excited-State Lifetimes at Room Temperature: Steric Control of Photoinduced Processes in Bichromophoric Systems.

Photochemical applications based on intermolecular photoinduced energy triplet state transfer require photosensitizers with strong visible absorptivity and extended triplet excited-state lifetimes. Using a bichromophore approach, two Re(I) tricarbonyl complexes with 2-(1-pyrenyl)-1 H -imidazo[4,5- f ][1,10]phenanthroline (pyr-imphen) and 1-(4-(methyl)phenyl)-2-(1-pyrenyl)-imidazo[4,5- f ][1,10]phenanthroline (pyr-tol-imphen) showing extraordinary long triplet excited states at room temperature (>1000 μs) were obtained, and their ground- and excited-state properties were thoroughly investigated by a wide range of spectroscopic methods, including femtosecond transient absorption (fs-TA). It is worth noting that the designed [ReCl(CO) 3 (pyr-imphen)] ( 1 ) and [ReCl(CO) 3 (pyr-tol-imphen)] ( 2 ) complexes form a unique pair differing in the mutual chromophore arrangement due to introduction of a 4-(methyl)phenyl substituent into the imidazole ring at the H1-position, imposing an increase in the dihedral angle between the pyrene and {ReCl(CO) 3 (imphen)} chromophores. The magnitude of the electronic coupling between the pyrene and {ReCl(CO) 3 (imphen)} chromophores was found to be an efficient tool to tune the photophysical properties of 1 and 2 . The usefulness of designed Re(I) compounds as triplet photosensitizers was successfully verified by examination of their abilities for 1 O 2 generation and triplet-triplet annihilation upconversion. The phosphorescence lifetimes, ∼1800 μs for 1 and ∼1500 μs for 2 , are the longest lifetimes reported for Re(I) diimine carbonyl complexes in solution at room temperature.