Photoinduced Energy and Electron-Transfer Reactions by Polypyridine Ruthenium(II) Complexes Containing a Derivatized Perylene Diimide

The [Ru­(II) (phen)₂(pPDIp)]²⁺ complex, where pPDIp is the symmetric bridging ligand phenanthroline–perylene–phenanthroline, shows strong electronic absorption bands attributed to the pPDIp and {Ru­(phen)₂}²⁺ moieties in acetonitrile. The charge-separated intermediate {Ru­(III) (phen)₂(pPDIp^–•)} was detected by transient absorption spectroscopy upon electronic excitation in either the pPDIp or the complex moieties. The charge-separated intermediate species decays to generate the triplet state ³*pPDIp-Ru­(II) (τ_P = 1.8 μs) that sensitizes the formation of singlet molecular oxygen with quantum yield ϕ_Δ = 0.57. The dyad in deaerated acetonitrile solutions is reduced by triethylamine (NEt₃) to the [Ru­(II) (phen)₂(pPDIp^•–)] radical anion in the dark. The electron-transfer reaction is accelerated by light absorption. By photolysis of the radical anion, a second electron transfer reaction occurs to generate the [Ru­(II) (phen)₂(pPDIp^2–)] dianion. The changes of the color of solution indicate the redox states of complexes and offer a sensitive reporter of each stage of redox reaction from start to finish. The reduced complexes can be converted to the initial complex, using methyl viologen or molecular oxygen as an electron acceptor. The accumulation of electrons in two well-separated steps opens promising opportunities such as in catalysis

Excited-State Proton Transfer of Fluorescein Anion as an Ionic Liquid Component

Fluorescent ionic liquids (FILs) incorporating the fluorescein anion have been prepared by anion exchange of the parent quaternary ammonium chloride (Quat⁺Cl^–) ionic liquid. By controlling the molar ratio of fluorescein to Quat⁺Cl^–, ionic liquids incorporating different prototropic forms of fluorescein were prepared. The 1:1 molar ratio ionic liquid (FIL1) is essentially composed of monoanionic fluorescein, while dianionic fluorecein is predominant in the FIL with a 1:2 molar ratio (FIL2). The fluorescence excitation spectrum of FIL2 is markedly different from its absorption spectrum. Absorption features the fluorescein dianion, while the excitation spectrum is exclusively due to the monoanion. In FIL1, the absorption and excitation spectra are both characteristic of the monoanion. In both FILs, emission of the dianion is observed upon excitation of the monoanion. This unusual behavior is interpreted in the context of a fast deprotonation of the monoanion in the excited state. The presence of residual water in the ionic liquid is important for the proton transfer process. By lowering the pH of FIL1, the transient proton transfer is inhibited, and the emission of the monoanion could be observed. The FILs have completely different spectroscopic properties from solvated fluorescein in Quat⁺Cl^–, where the prototropic equilibrium is shifted toward the neutral forms