Unveiling the Mechanistic Shift in Singlet Oxygen Generation by Re(I) Complexes: A Conceptual DFT and Kinetic Decomposition Approach

Abstract Predicting and rationalizing the singlet oxygen (1O2) photosensitization efficiency of Re(I) tricarbonyl complexes remains challenging due to the complex interplay of electronic and environmental factors. In this work, we employ Density Functional Theory (DFT) and Conceptual DFT (C-DFT) analysis to systematically rationalize the substituent- and solvent-dependent sensitization trends of a family of pyrazolyl–pyridazine fac-Re(I)(CO)3Br complexes. While the global quantum yields (ΦΔ) initially exhibited an unintelligible vertical dispersion in high-polarity media when plotted against the electronic descriptors, a rigorous mathematical deconstruction into triplet quenching efficiency (PT) and the fraction of productive energy transfer (fT) successfully unraveled the underlying photophysics. In non-protic environments (benzene, dichloromethane, and acetonitrile), the systems adhere to a conventional diffusion-controlled dynamic quenching regime successfully mirrored by continuous-dielectric C-DFT indices─specifically the global electrophilicity (ω) and the rhenium contribution to the HOMO (% Re). In contrast, analyzing these relationships in dimethylformamide and ethanol revealed an “unphysical” regime in which fT approaches 2 or dynamic quenching fails (PT = 0) yet significant 1O2 generation is detected. This mathematical divergence provides compelling diagnostic proof of a solvent-induced mechanistic switch toward ground-state static aggregates or upper-precursor-mediated pathways. These results highlight that the boundaries where continuous computational models collapse can serve as a diagnostic tool to map specific solute-solvent interactions, providing new principles for designing high-efficiency transition-metal photosensitizers.