Pyrimidoquinazolinophenanthroline Opens Next Chapter in Design of Bridging Ligands for Artificial Photosynthesis

Abstract

The synthesis and detailed characterization of a new Ru polypyridine complex containing a heteroditopic bridging ligand with previously unexplored metal-metal distances is presented. Due to the twisted geometry of the novel ligand, the resultant division of the ligand in two distinct subunits leads to steady state as well as excited state properties of the corresponding mononuclear Ru(II) polypyridine complex resembling those of prototype [Ru(bpy)₃ ]²⁺ (bpy=2,2'-bipyridine). The localization of the initially optically excited and the nature of the long-lived excited states on the Ru-facing ligand spheres is evaluated by resonance Raman and fs-TA spectroscopy, respectively, and supported by DFT and TDDFT calculations. Coordination of a second metal (Zn or Rh) to the available bis-pyrimidyl-like coordination sphere strongly influences the frontier orbitals, apparent by, for example, luminescence quenching. Thus, the new bridging ligand motif offers electronic properties, which can be adjusted by the nature of the second metal center. Using the heterodinuclear Ru-Rh complex, visible light-driven reduction of NAD⁺ to NADH was achieved, highlighting the potential of this system for photocatalytic applications.

Keywords: artificial photosynthesis; bridging ligand; light-induced redox catalysis; quantum chemistry; ruthenium photosensitizer.

Figure 1

Top: Existent bridging ligand structures and selected metal‐to‐metal distances from solid state structures of [(bpy)₂Ru(bpym)Ir(Cp*)]²⁺, [(L)Ru(tape)Ru(L)]⁴⁺ (with L=N,N’‐dimethyl‐2,11‐diaza‐[3.3](2,6)‐pyridinophane) and [(tbbpy)₂RutpphzRu(tbbpy)₂]⁴⁺ (tbbpy=4,4'‐di‐tert‐butyl‐2,2'‐bipyridine). Bottom: Synthetic protocol and positions of the respective protons (in black) and carbon atoms (coloured; with ligand (sphere) abbreviations); inset shows a part of the all‐carbon analog dibenzo[g,p]chrysene (DBC) or pqp framework and the discussed bond angles/lengths and the twisting of Rupqp; color code: Rudpymp (black), Rupqp (red), DBC (light blue).

Figure 2

¹H NMR spectra of Rudpymp (top), Rupqp (middle) and RupqpRh (bottom) in acetonitrile‐d₃ (due to absence of concentration dependence as shown in Figures S23 and S24 no specific concentrations were used).

Figure 3

Solid‐state structure of Rudpymp (left) and Rupqp (right) (thermal ellipsoids are drawn at a probability level of 50 %). Hydrogen atoms, PF₆ ⁻ counter anions and solvent molecules are omitted for clarity; additionally, intermolecular Rupqp‐toluene interactions are depicted.

Figure 4

Cyclic voltammograms (top) and differential pulse voltammograms (bottom) of Rudpymp (black), Rupqp (red) and RupqpRh (green) in acetonitrile (1 mM) referenced against the Fc/Fc⁺ couple. Conditions: scan rate 100 mVs⁻¹ (black, green), 200 mVs⁻¹ (red), [Bu₄N][PF₆] at 0.1 M as supporting electrolyte.

Figure 5

Top left: Extinction coefficients of Rudpymp (black) and Rupqp (red) with relative emissions (dashed) intensities (λ(ex)=451 nm), simulated transitions underlying the absorption bands are indicated in the inlet; top right: rR spectra of Rudpymp (black) and Rupqp (red) upon 405 nm (bottom) and 473 nm (top) excitation in acetonitrile. The tbbpy‐, and dpymp‐ or pqp‐type modes are indicated by violet and grey bars, respectively. The integral ratios of selected modes (tbbpy in blue relative to dpymp/pqp in orange) for the respective excitation wavelength are included as insets for Rupqp and Rudpymp; integral values are normalized to the integral of the modes of Rudpymp upon 405 nm excitation, respectively; bottom: transitions of Rudpymp and Rupqp visualized by charge density differences; charge transfer occurs from red to blue.

Figure 6

Ultrafast transient absorption (top) and decay‐associated spectra (bottom) of Rupqp. The red, dashed line in the upper panel shows the scaled and inverted ground state absorption spectrum. For comparison the transient absorption spectrum of Rudpymp collected at a delay time of 1.5 ns is shown in yellow. The spectra are collected upon 400 nm excitation (excitation densities of circa 5 %) in a delay time range between 0.3 and 2 ns in acetonitrile. The lifetime of the long‐lived state is obtained from nanosecond time‐resolved emission spectroscopy (see Figures S70 and S71).

Figure 7

Top: excitation energies, wavelengths and oscillator strengths of low‐lying MLCT_pqp excitations and involved π_pqp* for Rupqp (S₄), RupqpZn (S₂₉) and RupqpRh (S₄); bottom: absorption and emission spectra of Rupqp, RupqpRh and Rupqp+[Zn(BF₄)₂]; absorption spectra were normalized at 451 nm; emission spectra were recorded upon excitation at 451 nm (OD 0.1).

Figure 8

Light‐induced redox catalysis of RupqpRh at room temperature (black), 40 °C (blue) and 50 °C (red) in water/acetonitrile mixtures (4 : 1, v:v); concentrations used RupqpRh (5 μM), NAD⁺ (200 μM), TEA (0.12 M) and NaH₂PO₄ (0.1 M), excitation with 470 nm (50 mWcm⁻²).