Fig. 4

Annihilation of radical ion pairs. a Transient absorption spectra of a THF solution containing 150 μM Ir3 and 3.0 mM H after nanosecond pulsed photoexcitation. The negative and positive absorbance were due to the emission of Ir3 and the generation of Ir3^•+, respectively. b Comparisons of the simulated (TD−B3LYP) electronic transition spectra of the radical anion of H (magenta curve) and the radical cation of Ir3 (blue curve) with the transient absorption spectra obtained at delay times of 0.2 μs (black triangles) and 16.4 μs (empty circles), and the absorption spectrum of Ir3 obtained during oxidative electrolysis at 1.30 V vs. SCE (black curve). c Decay traces (top panel) and a second-order plot (bottom panel) of the transient absorption signals observed at a wavelength of 1100 nm. d Plot of the rate constant for back electron transfer for charge recombination (k_BeT, black squares) as a function of the driving force and the Marcus plots for outer-sphere electron transfer calculated for the reorganization energies (λ) at 1.7 eV (magenta), 1.9 eV (orange), 2.2 eV (turquoise), and 2.3 eV (blue). The Marcus plots were constructed from the equation, k_BeT = Z exp[−(ΔG_BeT + λ)²/4λk_BT]. In this equation, Z, k_B, and T are the collisional frequency taken as 1.0 × 10¹² M⁻¹ s⁻¹, the Boltzmann constant, and absolute temperature (298 K), respectively