Kinetic Control of Cycloaddition Reaction in Hybrid Metal Halide Perovskitoids for Tunable Photomechanical and Photochromic Response

In this work, we tune the photomechanical and photochromic response in zero-dimensional halide perovskitoids (BM) 2 Sn 1– y Te y X 6 ·2H 2 O (BM = Brooker’s merocyanine; X = Cl, Br) by controlling cycloaddition reaction kinetics. By alloying the B-site with different ratios of Sn 4+ /Te 4+, the kinetics of the solid-state [2 + 2] cycloaddition reaction between BM cations are precisely modulated. Using the Johnson–Mehl–Avrami–Kolmogorov kinetic model, the rate constant of BM was determined with values increasing from 0.059 min –1 for (BM) 2 TeCl 6 ·2H 2 O ( BMTeCl ) to 0.129 min –1 for (BM) 2 SnCl 6 ·2H 2 O ( BMSnCl ) under 405 nm irradiation. The faster reaction kinetics results in a stronger photomechanical and photochromic response with shorter responsive time for BMSnCl . Spectroscopic studies and time-dependent density functional theory calculations reveal that the incorporation of Te 4+ with 5s 2 lone pair electrons allows the electronic transition 1 S 0 → 3 P x of Te 4+ and through-space charge transfer from [TeX 6 ] to BM. These transitions overlap well with the intramolecular charge transfer of BM → BM*, leading to competitive light absorption between BM and the [TeX 6 ] sublattice. Therefore, varying the Sn 4+ /Te 4+ ratio modulates the excited state population of BM and results in a Te-dependent photomechanical and photochromic response. This study highlights the potential of B-site alloying in MHPs as a powerful strategy for designing stimuli-responsive smart materials.