Ultrafast Solution-Phase Photophysical and Photochemical Dynamics of Hexaiodobismuthate(III), the Heart of Bismuth Halide Perovskite Solar Cells.

The ultrafast relaxation pathways in a hexaiodide bismuth(III) complex, BiI 6 3- , excited at 530 nm in acetonitrile solution are studied by means of femtosecond transient absorption spectroscopy supported by steady-state absorption/emission measurements and DFT computations. Radiationless relaxation out of the Franck-Condon, largely metal-centered (MC) triply degenerate 3 T 1u state (46 ± 19 fs), is driven by vibronic coupling due to the Jahn-Teller effect in the excited state. The relaxation populates two lower-energy states: a ligand-to-metal charge transfer (LMCT) excited state of 3 π I(5p π ) → Bi(6p) nature and a luminescent "trap" 3 A 1u ( 3 P 0 ) MC state. Coherent population transfer from the initial 3 T 1u into the 3 π LMCT state occurs in an oscillatory, stepwise manner at ∼190 and ∼550 fs with a population ratio of ∼4:1. The 3 π LMCT state decays with a 2.9 ps lifetime, yielding two short-lived reaction intermediates of which the first one reforms the parent ground state with a 15 ps time constant, and the second one decays on a ∼5 ps timescale generating the triplet product species, which persists to the longest 2 ns delay times investigated. This product is identified as the η 2 metal-ligated diiodide-bismuth adduct with the intramolecularly formed I-I bond, [(η 2 -I 2 )Bi( II )I 4 ] 3- , which is the species of interest for solar energy conversion and storage applications. The lifetime of the "trap" 3 A 1u state is estimated to be 13 ns from the photoluminescence quenching of BiI 6 3- . The findings give insight into the excited-state relaxation dynamics and the photochemical reaction mechanisms in halide complexes of heavy n s 2 metal ions.