Near-Infrared Phosphorescent Hybrid Organic–Inorganic Perovskite with High-Contrast Dielectric and Third-Order Nonlinear Optical Switching Functionalities

Hybrid organic–inorganic
perovskites providing integrated
functionalities for multimodal switching applications are widely sought-after
materials for optoelectronics. Here, we embark on a study of a novel
pyrrolidinium-based cyanide perovskite of formula (C4H10N)2KCr­(CN)6, which displays thermally
driven bimodal switching characteristics associated with an order–disorder
phase transition. Dielectric switching combines two features important
from an application standpoint: high permittivity contrast (Δε′
= 38.5) and very low dielectric losses. Third-order nonlinear optical
switching takes advantage of third-harmonic generation (THG) bistability,
thus far unprecedented for perovskites and coordination polymers.
Structurally, (C4H10N)2KCr­(CN)6 stands out as the first example of a three-dimensional stable
perovskite among formate-, azide-, and cyanide-based metal–organic
frameworks comprising large pyrrolidinium cations. Its stability,
reflected also in robust switching characteristics, has been tracked
down to the Cr3+ component, the ionic radius of which provides
a large enough metal–cyanide cage for the pyrrolidinium cargo.
While the presence of polar pyrrolidinium cations leads to excellent
switchable dielectric properties, the presence of Cr3+ is
also responsible for efficient phosphorescence, which is remarkably
shifted to the near-infrared region (770 to 880 nm). The presence
of Cr3+ was also found indispensable to the THG switching
functionality. It is also found that a closely related cobalt-based
analogue doped with Cr3+ ions displays distinct near-infrared
phosphorescence as well. Thus, doping with Cr3+ ions is
an effective strategy to introduce phosphorescence as an additional
functional property into the family of cobalt-cyanide thermally switchable
dielectrics.