Photocatalytic Reduction of Nicotinamide Co-factor by Perylene Sensitized RhIII Complexes

Abstract

The ambitious goal of artificial photosynthesis is to develop active systems that mimic nature and use light to split water into hydrogen and oxygen. Intramolecular design concepts are particularly promising. Herein, we firstly present an intramolecular photocatalyst integrating a perylene-based light-harvesting moiety and a catalytic rhodium center (Rh^III phenPer). The excited-state dynamics were investigated by means of steady-state and time-resolved absorption and emission spectroscopy. The studies reveal that photoexcitation of Rh^III phenPer yields the formation of a charge-separated intermediate, namely Rh^II phenPer⋅⁺ , that results in a catalytically active species in the presence of protons.

Keywords: charge separation; perylene; photocatalysis; rhodium catalyst; time-resolved spectroscopy.

Figure 1

Extinction coefficients of Rh^IIIphenPer (green) and phenPer (gold) and emission spectrum of phenPer (scaled relative to the absorption maximum) in acetonitrile. Structural formula of Rh^IIIphenPer photocatalyst, highlighting the phenPer ligand in gold. Charge density differences of two selected optical transitions of Rh^IIIphenPer (excitation occurs from yellow to blue). The respective TD‐DFT simulated vertical transition energies and oscillator strengths are represented as green bars in the absorption spectrum (see also Table S2).

Figure 2

a) Transient absorption and b) decay associated spectra of Rh^IIIphenPer in acetonitrile upon 400 nm excitation. For reasons of comparability the scaled long‐lived TA spectrum of phenPer (lifetime of 2.1 ns) collected at 400 nm excitation (excitation density of circa 5 %) in acetonitrile is depicted in gold (a). The scaled inverted ground state absorption spectrum is shown in panel b (filled, green curve).

Figure 3

The Jablonski diagram shows the excited state dynamics of Rh^IIIphenPer upon photoexcitation at 400 nm in acetonitrile. The colours in the ground‐state structure indicate the origin of the of the optically accessible ππ* and ILCT states (yellow). The excess electron density in the excited states is highlighted in green.

Figure 4

a) Normalized absorption spectrum of Rh^IIIphenPer (green, 5 μM) and upon LED‐illumination at 470 nm (at room temperature for 70 minutes in steps of 10 minutes) in water:acetonitrile mixture (4 : 1, v:v) in the presence of phosphate‐buffered (NaH₂PO₄, 0.1 M) triethylamine (TEA, 0.12 M) and NAD⁺ (0.2 mM). The increase in absorbance at around 340 nm reflects the catalytic conversion of NAD⁺ to NADH, which is further reflected in b) the turnover number (TON, ▵) and turnover frequency (TOF, ○) as shown in panel b.