Solid state molecular motion in Cd-based halides monitored by photoluminescence switching†

The research on low-dimensional organic–inorganic hybrid metal halide materials is attractive due to their flexible structures and outstanding optoelectronic properties. This paper reports on a series of novel (PIP)CdX₄ compounds (PIP = protonated piperazine), including (PIP)CdI₄ (Cd-I), (PIP)CdI₂Cl₂ (Cd-ICl) and (PIP)CdI_2.4Br_1.6 (Cd-IBr). It is notable that PIP·2Cl can diffuse into their structures resulting in a luminescent phase, (PIP)CdI₄·PIP·2Cl (CdI-P). As a result, the molecular motion of PIP can be monitored by photoluminescence switching leading to a bulk luminescent heterostructure. The photophysical properties of CdI-P were studied using temperature-dependent photoluminescence, photoluminescence excitation, time-resolved photoluminescence spectra, and transient femtosecond absorption spectroscopy. We propose that the green emission of CdI-P should arise from self-trapped excitons (STEs) that undergo radiative recombination, which might be related to the isolated Cl⁻ in the structure determined by the comparison study with (PIP)CdI₄·PIP·2I and density functional theory calculations. The superior water solubility should enhance the processability of the title compounds. Finally, an advanced time-resolved information encryption application was developed.