Lighting the Flavin Decorated Ruthenium(II) Polyimine Complexes: A Theoretical Investigation.

The emission properties of a series of flavin (FL) decorated Ru (II) polyimine complexes were investigated by extensive time-dependent (TD) density functional theory (DFT) and DFT based calculations. We attributed the moderate emission properties of FL decorated Ru(II) polyimine complex (Ru-1), such as triplet lifetime and luminescence quantum yield, to the dominant fast nonradiative decay due to the small adiabatic energy gap between the ground state and the lowest lying triplet state (Δ Ead) and the slow radiative decay owing to the ligand localized triplet (3IL) nature of the emissive state. Electron withdrawing groups such as F and Cl were attached to the FL moiety of Ru-1 to alter Δ Ead. Both the radiative and nonradiative decay rates were found to be sensitive to Δ Ead and may result in a drastic change of the photophysical properties of the Ru(II) complexes. Specifically, substitution with F leads to an increase in the Δ Ead from 1.85 to 1.93 eV, resulting in a nearly doubled phosphorescent quantum yield and triplet lifetime with respect to Ru-1. These findings are vital for the rational design of phosphorescent transition metal complexes.

Synthesis and photophysical properties of ruthenium(ii) polyimine complexes decorated with flavin.

A bipyridine ruthenium(ii) complex (Ru-1) with a flavin moiety connected to one of the bipyridine ligands via an acetylene bond was designed and synthesized, and its photophysical properties were investigated. Compared with the tris(bipyridine) Ru(ii) complex (Ru-0), which has an extinction coefficient ε = 1.36 × 104 M-1 cm-1 at 453 nm, the introduction of the flavin moiety endows Ru-1 with strong absorption in the visible range (ε = 2.34 × 104 M-1 cm-1 at 456 nm). Furthermore, Ru-1 exhibits phosphorescence (λem = 643 nm, ΦP = 1%, τP = 1.32 µs at 293 K and 4.53 µs at 77 K). We propose that the emission of Ru-1 originates from the low lying triplet excited state of 3IL according to the time-resolved transient difference absorption spectra, the calculated T1 spin density and the T1 thermo-vibration modes localized on the flavin-decorated bipyridine ligand. This is the first time that the phosphorescence of flavin was observed within Ru(ii) complexes. Consequently, Ru-1 was used for triplet-triplet annihilation upconversion, showing a reasonable quantum yield of 0.7% with respect to the phosphorescence quantum yield of 1%. These findings pave the way for the rational design of phosphorescence transition metal complexes. Also, further approaches that may improve the performance of flavin-decorated Ru(ii) bipyridine complexes are proposed.

Efficient Photooxidation of Sulfides with Amidated Alloxazines as Heavy-atom-free Photosensitizers.

Photooxidation utilizing visible light, especially with naturally abundant O2 as the oxygen source, has been well-accepted as a sustainable and efficient procedure in organic synthesis. To ensure the intersystem crossing and triplet quantum yield for efficient photosensitization, we prepared amidated alloxazines (AAs) and investigated their photophysical properties and performance as heavy-atom-free triplet photosensitizers and compared with those of flavin (FL) and riboflavin tetraacetate (RFTA). Because of the difference in the framework structure of AAs and FL and the introduction of carbonyl moiety, the absorption of FL at ∼450 nm is blue-shifted to ∼380 nm and weakened (ε = 8.7 × 103 for FL to ∼6.8 × 103 M-1 cm-1), but the absorption at ∼340 nm is red-shifted to ∼350 nm and enhanced by ∼50% (from ε = 6.4 × 103 for FL to ∼9.9 × 103 M-1 cm-1) in AAs. The intersystem crossing rates from the S1 to T1 are also enhanced in these AAs derivatives, while the fluorescence quantum yield decreases from ∼30 to ∼7% for FL and AAs, respectively, making the triplet excited state lifetime and the singlet oxygen quantum yield of AAs at least comparable to those of FL and RFTA. We examined the performance of these heave-atom-free chromophores in the photooxidation of sulfides to afford sulfoxides. In accordance with the prolonged triplet excited state lifetime and enhanced triplet quantum yield, 2-5-fold performance enhancements were observed for AAs in the photooxidation of sulfides with respect to FL. We proposed that the key reactive oxygen species of AA-sensitized photooxidation are singlet oxygen and superoxide radical anion based on mechanistic investigations. The research highlights the superior performance of AAs in photocatalysis and would be helpful to rationalize the design of efficient heavy-atom-free organic photocatalysts.