Rhenium(I) Complexes with Neutral Monodentate Coligands and Monoanionic 2-(1,2,4-Triazol-5-yl)pyridine-Based...

In this work, we describe the synthesis as well as structural, photophysical, and theoretical investigation of a new coordination chemical concept involving rhenium(I) complexes bearing monoanionic 1,2,4-triazolylpyridine-based bidentate chromophores. The X-ray diffractometric analysis of single crystals revealed particular packing features: the trifluoromethylated exemplar displayed two kinds of arrangements of the coordination centers, where the bidentate ligands at the edges of the unit cell are staggered parallel to each other, whereas those inside show antiparallel stacking with respect to the external ligands. On the other hand, the complexes bearing an adamantyl substituent yield a linear arrangement, where the bulky moiety of one luminophore points to the pyridine center of the adjacent ligand of the neighboring complex while including methanol molecules hydrogen-bonded to the triazolato unit. We observed that the photophysical properties of the complexes (photoexcited-state lifetimes, photoluminescence maxima and quantum yields) can be adjusted by tuning of the substitution pattern at the bidentate luminophore as well as by variation of the monodentate coligand. The photoluminescence spectra and photoexcited-state lifetimes of the crystalline phases were measured by phosphorescence lifetime micro(spectro)scopy. Interestingly, the vibrationally resolved emission spectra of the crystals closely resemble those of diluted frozen glassy matrixes at 77 K, in contrast with the broad bands observed in amorphous solids and in fluid solutions, where the charge-transfer character is enhanced. While the photoluminescence quantum yields (Φ_L) reach up to 15%, the complexes are able to attain up to 55% efficiency regarding the photosensitization of ¹O₂ (Φ_Δ), depending on the combination of luminophore and coligand. Theoretical calculations showed that the photoexcited triplet (T₁) state has a metal-ligand-to-ligand charge-transfer character, where promotion to the excited electronic configuration shortens the Re(I)-N bond involving the bidentate triazolylpyridine while stretching the three fac-CO-Re(I) bonds as well as the linkage to the axial monodentate coligand. The calculated vertical (E^vl) and 0-0 (E^(0-0)) radiative transition energies are in very good agreement with the experimental values (E_exp^lum).