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 <i>fac</i>-[Re(CO)₃(Ip)Br] with imidazo-phenanthroline (Ip) ligand derivatives (<b>2a-2d</b>), 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 p<i>K</i>_a2 in the range of 9.77-10.89, while the complex <b>2b</b> displays an additional p<i>K</i>_a1 of 3.75 (-NH₂ → NH₃⁺). Complexes <b>2a/2c</b> possess photoluminescence quantum yields of less than 1%. The photoluminescence response for amine complex <b>2b/2d</b> is weaker than that of <b>2a/2c</b> 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 [<b>2b</b>]⁺ and [<b>2d</b>]⁺. 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 ([<b>2a</b>]^-, [<b>2b</b>]^-, [<b>2c</b>]^-, and [<b>2d</b>]^-). 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.