Pyrenyl-Substituted Imidazo[4,5-<i>f</i>][1,10]phenanthroline Rhenium(I) Complexes with Record-High Triplet Excited-State Lifetimes at Room Temperature: Steric Control of Photoinduced Processes in Bichromophoric Systems
Katarzyna Choroba, Mateusz Penkala, Joanna Palion‐Gazda, E. Malicka, B. Machura
Abstract
High Resolution Image Download MS PowerPoint Slide 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.