Light Excitation of a Bismuth Iodide Complex Initiates I–I Bond Formation Reactions of Relevance to Solar Energy Conversion

Abstract

The titration of iodide into acetonitrile solutions of BiI resulted in the formation of [BiI]. Ligand-to-metal charge transfer (LMCT) excitation of [BiI] yielded a transient species assigned as the diiodide anion I directly ligated to Bi, [Bi(I)I]. With 20 ns time resolution, transient absorption measurements revealed the appearance of two species assigned on the analysis of the iodine molecular orbitals as an η ligated I, [(η-I)BiI] (λ = 640 nm), and an η species [(η-I)BiI] (λ = 750 nm). The rapid appearance of this intermediate was attributed to intramolecular I-I bond formation. The [(η-I)BiI] subsequently reacted with 1 equiv of iodide to yield [(η-I)BiI]. Interestingly, [(η-I)BiI] decayed to ground state products with a first-order rate constant of k = 2 × 10 s. Under the same experimental conditions, I in CHCN rapidly disproportionates with a tremendous loss of free energy, ΔG = -2.6 eV. The finding that metal ligation inhibits this energy wasting reaction is of direct relevance to solar energy conversion. The photochemistry itself provides a rare example of one electron oxidized halide species coordinated to a metal ion of possible relevance to reductive elimination/oxidation addition reaction chemistry of transition metal catalysts.

Dimethylammonium iodide stabilized bismuth halide perovskite photocatalyst for hydrogen evolution

“…Upon further increase of the DAI concentration (up to 0.15 M), the absorption peaks became stronger, suggesting the increasing amount of iodobismuthate(III) ions. Meanwhile, the peaks gradually shifted to 347 and 483 nm, that are reasonably ascribed to the characteristic peaks of BiI6 3− ions [48]. The difference (347 nm vs. 350 nm and 483 nm vs. 486 nm) for the peak positions may result from the presence of DAI in the aqueous solution, which is known to have a great effect on the absorption spectra [49].…”

“…Besides, two peaks appeared to UV-vis spectra located at 350 and 461 nm, which can be assigned to [BiI4]ions and BiI3 (Figs. S3 and S4 in the ESM), respectively [48]. Upon further increase of the DAI concentration (up to 0.15 M), the absorption peaks became stronger, suggesting the increasing amount of iodobismuthate(III) ions.…”

Halide Photoredox Chemistry

“…The photochemistry of a bismuth iodide complex was reported by Maurer et al to probe fundamental LMCT processes. Several intriguing results emerged . For instance, upon halide-to-metal charge-transfer excitation of the lowest energy transition, λ ex = 532 nm, halide oxidation was observed.…”

“…A proposed mechanism is shown in Figure and is described below . Excitation with a 532 nm laser pulse resulted in an iodide to Bi III charge transfer with a formal oxidation state of Bi II in the Franck–Condon excited state.…”

Crystal Structure and Computational Analysis of a Two-Dimensional Coordination Polymer, BiI3(DppeO2)3/2

“…Our group and others have reported on the various electronic transitions bismuth(III) iodide complexes undergo upon excitation, which include ligand-to-metal charge transfer (LMCT), mixed halide/metal-to-ligand charge transfer (X/MLCT), and cluster-centered metal halide rearrangement (CC) [23], [37][38][39][40][41][42]. The high degree of conjugation in the DppeO2 ligand further offers the possibility of ligand ➝ * transitions in 1 [37], [38].…”