Fig. 3
Formation of radical ion pairs. a Photoluminescence spectra of 100 μM H with added Ir3 (0–60 μM). Photoexcitation wavelength used was 300 nm. The inset figure is a Stern–Volmer analysis of the fluorescence of H in the absence (I0) and presence (I) of Ir3. The value of the intensity of the host fluorescence was corrected by considering the absorbance of the dopant, following the relationship I = Iobs × (Abs/Abs0) × 1/(1 − 10−Abs) where Iobs, Abs, and Abs0 are the observed fluorescence intensity, the absorbance at 300 nm in the presence of Ir3, and the absorbance at 300 nm in the absence of Ir3, respectively. The I0/I values were fit to the Stern–Volmer equation I0/I = (1 + Ka·[Ir3]) × (1 + kq·τ0·[Ir3]). In this equation, Ka, kq, τ0, and [Ir3] are the association constant, the quenching constant, the fluorescence lifetime in the absence of Ir3 (3.8 ns), and the molar concentration of Ir3, respectively. The Stern–Volmer analysis yielded kq and Ka to be >1012 M−1 s−1 and 6.8 × 103 M−1, respectively. The non-negligible contribution of the static quenching (i.e., the (1 + Ka·[Ir3]) term in the Stern–Volmer equation) may indicate the existence of excited-state interactions between H and Ir3. Stern–Volmer analyses for other Ir dopants are shown in Supplementary Fig. 2. b Phosphorescence intensities of 10, 20, 30, 40, 50, and 60 μM Ir3 (deaerated THF) upon photoexcitation at 300 nm in the absence (gray bars) and presence (black bars) of equimolar concentrations of H. The decrease in the difference between the gray and black bars corresponded to the formation of radical ion pairs. c Photoinduced EPR spectra of Ar-saturated THF solutions of 1.0 mM H, 1.0 mM Ir3, and a mixture of 1.0 mM H and 1.0 mM Ir3 in the absence and presence of photoirradiation. Peaks marked with asterisks may correspond to the rhombic signals due to an Ir(IV) species of the radical cation of Ir3
