# Light-Driven Intraoctahedral Halide Isomerization in Two-Dimensional Mixed Halide Perovskites

## TL;DR

This paper shows that light can trigger reversible halide isomerization in 2D perovskites, changing their electronic properties without structural changes.

## Contribution

The discovery of light-driven intraoctahedral halide isomerization in 2D perovskites, enabling tunable optoelectronic properties.

## Key findings

- Halide ions switch between local configurations within PbX6 octahedra without long-range migration.
- Optical bandgap modulation of ∼0.1 eV and reversible electronic bandgap shift of up to ∼0.5 eV are achieved.
- Valence band redistribution creates optoelectronic isomers activated by light.

## Abstract

Two-dimensional metal halide perovskites are emerging
materials
for quantum light emission and neuromorphic computing owing to their
quantum-confined structures and tunable optoelectronic properties.
Beyond structural dimensionality, the presence of multiple crystallographically
distinct halide sites within a single metal halide octahedron presents
a unique opportunity to engineer functionality at the subunit-cell
level. Here, we report a light-driven, reversible halide-ion isomerization
in single-crystalline BA2PbBr
x
I4–x
(BA = butylammonium, x = 1–3), where ions switch between distinct local
configurations within individual PbX6
4– octahedra, without long-range migration or macroscopic phase segregation.
Through a combination of hyperspectral imaging, in situ X-ray diffraction,
and first-principles calculations, we demonstrate that this intraoctahedral
halide site switching modulates the optical bandgap by ∼0.1
eV and enables an estimated reversible electronic bandgap shift of
up to ∼0.5 eV. Density functional theory reveals that these
changes stem from a redistribution of valence band character, effectively
creating chemically distinct optoelectronic isomers that can be activated
by light. These results uncover a mechanism of structurally encoded,
site-selective photoisomerization in 2D perovskites, offering a new
strategy for reconfigurable optoelectronic devices, nonvolatile optical
memory, and quantum photonics.