Rhenium(I) Complexes with 2-(1,2,4-Triazol-5-yl)-β-Carboline-Based Bidentate Luminophores and Neutral Co-Ligands: Towards Tunable Phosphorescence and Efficient Singlet Dioxygen Photoproduction

A bidentate ligand concept based on <i>β</i>-carbolines functionalized with a 1,2,4-triazolyl-moiety was designed and realized, enabling the development of a series of neutral rhenium(I) complexes. This new class of anionic ligands, incorporating either an unsubstituted 9<i>H</i>-pyrido[3,4-<i>b</i>]indole core (<b>L_nHo</b>) or a 9-methyl-substitued variant (<b>L_Me-nHo</b>), was developed towards tailored photofunctionality. Structural modification via methyl substitution at the indole moiety was found to enhance overall phosphorescence efficiency. Comparative studies of two monodentate auxiliary units revealed that 1,3,5-triaza-7-phosphaadamantane (<b>PTA</b>) significantly reduces the photoluminescence efficiency compared to pyridine (<b>Py</b>). Solvent-dependent photoluminescence studies indicated that a lowered polarity leads to an increase in photoluminescence quantum yields (<i>Φ</i>_L). The complex <b>Re(L_Me_-nHo)Py</b> emerged as the most efficient emitter, displaying a <i>Φ</i>_L of 44% in dichloromethane (DCM). Notably, all complexes exhibited efficient quenching of excited triplet states by diffusional collision with triplet dioxygen (³O₂), yielding good singlet dioxygen (¹O₂) photoproduction efficiencies (<i>Φ_Δ</i>) with a maximum of 45% observed for <b>Re(L_nHo)Py</b>. These results highlight the suitability of these complexes for applications requiring efficient phosphorescence and oxygen photosensitization, such as bioimaging, and photodynamic therapy or photooxidation catalysis, while underscoring the central role of the tailored <i>β</i>-carboline-based chromoluminophores in enabling precise tuneability of photophysical properties.