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

Abstract

A bidentate ligand concept based on β-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 9H-pyrido[3,4-b]indole core (L_nHo) or a 9-methyl-substitued variant (L_Me-nHo), 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 (PTA) significantly reduces the photoluminescence efficiency compared to pyridine (Py). Solvent-dependent photoluminescence studies indicated that a lowered polarity leads to an increase in photoluminescence quantum yields (Φ_L). The complex Re(L_Me-nHo)Py emerged as the most efficient emitter, displaying a Φ_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 (Φ_Δ) with a maximum of 45% observed for Re(L_nHo)Py. 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 β-carboline-based chromoluminophores in enabling precise tuneability of photophysical properties.

Keywords: bidentate ligands; phosphorescence; photoluminescence; photoproduction of singlet dioxygen; photosensitization; photosensitizer; rhenium(I) complexes; β-carboline alkaloids.

Scheme 1

Bidentate ligand precursors incorporating a β-carboline developed by Mao et al. [32] for Ir(III), Re(I) [33], and Ru(II) [34] complexes (left); by Lu et al. [35] for Pt(II), Ni(II), Cu(II), and Co(II) complexes (center); by Chen et al. [36] for Ru(II) complexes (right).

Scheme 2

Synthetic route towards the β-carboline-derived chelators. The additional reaction step for the introduction of the methyl group in L_Me-nHo is shown in a dashed box, as it is not required for the synthesis of L_nHo.

Scheme 3

Synthetic route towards the rhenium(I) complexes.

Figure 1

Absorption spectra (molar absorption coefficients as a function of wavelength) of Re(L_nHo)Py (black), Re(L_nHo)PTA (blue), Re(L_Me-nHo)Py (red) and Re(L_Me-nHo)PTA (green). Validity range: c = 1 × 10⁻⁵–1 × 10⁻⁶ M in ACN at 298 K.

Figure 2

Photoluminescence emission spectra of Re(L_nHo)Py (black), Re(L_nHo)PTA (blue), Re(L_Me-nHo)Py (red) and Re(L_Me-nHo)PTA (green), recorded in different solvents and conditions (c = 10⁻⁵ M, λ_exc = 314–360 nm). Left: Emission spectra of Re(L_nHo)Py and Re(L_nHo)PTA in ACN (solid lines) and DCM (dotted lines) at 298 K. Right: Emission spectra of Re(L_Me-nHo)Py and Re(L_Me-nHo)PTA in ACN (solid lines) and DCM (dotted lines) at 298 K.

Figure 3

Photoluminescence emission spectra of Re(L_nHo)Py (black), Re(L_nHo)PTA (blue), Re(L_Me-nHo)Py (red), and Re(L_Me-nHo)PTA (green) recorded in a frozen glassy matrix of DCM/MeOH (V:V = 1:1) at 77 K (λ_exc = 320 nm, c = 10⁻⁷ M).

Figure 4

Singlet dioxygen phosphorescence exemplarily shown for Re(L_nHo)Py in ACN. Left: ¹O₂ phosphorescence spectra at different concentrations of the complex. Right: ¹O₂ phosphorescence intensity versus the fraction of light absorbed (i.e., 1–10^−A) for Re(L_nHo)Py (cyan) and the reference (black). l_ex = 350 nm. m_s and m_r are the slopes of the sample and reference, respectively. For the other complexes, see Figures S88−S91.