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, BiI₆^3-, 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 ³T_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 ³π I(5p_π) → Bi(6p) nature and a luminescent "trap" ³A_1u(³P₀) MC state. Coherent population transfer from the initial ³T_1u into the ³π LMCT state occurs in an oscillatory, stepwise manner at ∼190 and ∼550 fs with a population ratio of ∼4:1. The ³π 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 η² metal-ligated diiodide-bismuth adduct with the intramolecularly formed I-I bond, [(η²-I₂)Bi(II)I₄]^3-, which is the species of interest for solar energy conversion and storage applications. The lifetime of the "trap" ³A_1u state is estimated to be 13 ns from the photoluminescence quenching of BiI₆^3-. The findings give insight into the excited-state relaxation dynamics and the photochemical reaction mechanisms in halide complexes of heavy ns² metal ions.

Structural, Hirshfeld Surface Analysis, Morphological Approach, and Spectroscopic Study of New Hybrid Iodobismuthate Containing Tetranuclear 0D Cluster Bi4I16·4(C6H9N2) 2(H2O)

The Bi4I16·4(C6H9N2) 2(H2O) compound was synthesized by slow evaporation at room temperature. It exhibits a zero-dimensional (0D) tetrameric structure, comprising [Bi4I16]4− distorted octahedra, with strong I⋯I interactions among adjacent anionic clusters. We used Hirshfeld surface analysis to discuss the strength of hydrogen bonds and to quantify the inter-contacts (two-dimensional (2D) fingerprint plots). It revealed that the hydrogen bonding interactions H⋯I (56.3%), π–π stacking (11.7%), and I⋯I interactions (5.9%) play the major role in the stability of the crystal structure. The crystal morphology was simulated using Bravais–Friedel, Donnay–Harker (BFDH) and growth morphology (GM) methods. The experimental habit of the title compound was adequately reproduced by the two models. The calculated results show that the crystal morphology of the title compound in a vacuum is dominated by five facets: (020), (011), (110), (10−1), and (11−1). The (020) facet is the largest among all the facets calculated. Projection of the facet showed that there are a few polar groups on the (020) facet. In the 50–400 and 400–4000 cm−1 frequency regions, we measured the Raman and infrared spectra, respectively, of the title compound, and we assigned the observed vibration modes.