Computational and Spectroscopic Studies of New Rhenium(I) Complexes Containing Pyridylimidazo[1,5-<i>a</i>]pyridine Ligands: Charge Transfer and Dual Emission by Fine-Tuning of Excited States

Three new Re(CO)3Cl complexes (ReL1−ReL3) containing the N,N-bidentate ligands 1-(2-pyridyl)-3-phenylimidazo[1,5-a]pyridine (L1), 1-(2-pyridyl)-3-(4-tert-butylphenyl)imidazo[1,5-a]pyridine (L2), and 1-(2-pyridyl)-3-(4-dimethylaminophenyl)imidazo[1,5-a]pyridine (L3) were synthetized and fully characterized. Photophysical properties of L1−L3 and ReL1−ReL3 were studied with absorption and emission spectroscopy. The X-ray structure of ReL3 was determined. Time-dependent DFT (TDDFT) calculations were performed in order to elucidate the electronic structures and the excited states of ligands and complexes. Ligands L1 and L2 show 1π−π* emission with limited charge-transfer character (CT), while L3 emits from an excited state with higher CT character due to the presence of a dimethylamino group. No emissive metal-to-ligand charge-transfer (MLCT) states are found for the rhenium complexes. ReL1 and ReL2, although similar to their ligands, display a ligand-centered 1π−π*/3π−π* dual emission; singlet emissions fall at 21.8 × 103 (458 nm) and 22.6 × 103 cm−1 (443 nm), respectively, and the structured triplet emissions have two peaks at 17.9 × 103 (558 nm) and 16.3 × 103 cm−1 (613 nm) in both complexes. ReL3 emits from a ligand-centered CT state at 18.9 × 103 cm−1 (530 nm). Finally, the complex [Re(L1)(CO)3py]PF6 (ReL1py) (where py = pyridine) was prepared and studied by spectroscopy and computational methods. The complex has high-energy emission centered at 22.9 × 103 cm−1 (437 nm). DFT calculations show that dual fluorescence almost disappears due to the reduced spin–orbit coupling. Finally, electrochemical properties of ligands and rhenium complexes have been investigated.