Role of pH in the Ground and Excited-State Properties of Rhenium(I) Imidazole-Phenanthroline Amine Complexes: A Spectroscopic, Electrochemical, and Theoretical Study

This work reports the synthesis, characterization, and systematic study of fac-[Re(CO)₃(Ip)Br] with imidazo-phenanthroline (Ip) ligand derivatives (2a-2d), whose optical properties can be tuned by changes in pH. UV-Vis and photoluminescence studies revealed pronounced pH-dependent spectral shifts and intensity changes. Ground-state acid-base titrations showed Ip deprotonation across the series, yielding pK_a2 in the range of 9.77-10.89, while the complex 2b displays an additional pK_a1 of 3.75 (-NH₂ → NH₃⁺). Complexes 2a/2c possess photoluminescence quantum yields of less than 1%. The photoluminescence response for amine complex 2b/2d is weaker than that of 2a/2c and depends on the excitation wavelength and pH, indicating mixed MLCT/ILCT contributions. TD-DFT, including spin-orbit coupling (SOC-TDDFT), shows that acid-base perturbations change the frontier orbital composition and SOC, where the protonation of -NH₂ increases the SOC and stabilizes the ³MLCT emission for [2b]⁺ and [2d]⁺. In contrast, Ip deprotonation results in the triplet being localized on the ligand (³ILCT) (low ZFS), promoting emission deactivation by a PET process in basic media ([2a]^-, [2b]^-, [2c]^-, and [2d]^-). These results provide experimental and computational support for the photochemical tuning of MLCT/IL/ILCT transitions with pH, contributing to the design of new molecular tools for colorimetric and photoluminescent pH sensing based on Re(I)-Ip complexes.