Pyridinium Cations Act as Charge Reservoirs in Expanded Halide Perovskite Analogs at High Pressure

Introducing charge reservoirs within semiconductors is a promising strategy for controlled electronic doping. We previously showed that dmpz²⁺ (N,N'-dimethylpyrazinium) molecules placed within the cavities of an expanded analog of halide perovskites, (dmpz)[Sn₂I₆], could serve as electron acceptors and hole-dope the material upon compression. However, the number of such small redox-active cations is limited, and they tend to engage in unwanted redox reactions that compromise material stability. Herein, we show that redox-innocent pyridinium cations (H₂apy²⁺ = 3-amidinopyridinium) contained within the cavities of an expanded perovskite, (H₂apy)[Sn₂I₆], can also act as charge reservoirs upon compression. At ambient pressure, the energy of the H₂apy²⁺ π* acceptor orbital is ≥1.84 eV above the valence-band maximum of (H₂apy)[Sn₂I₆]. Upon compression, this energy difference is decreased to <0.2 eV (32 GPa), enabling electron transfer from the Sn-I framework into the H₂apy²⁺ π* orbital. This electron transfer increases the valence-band hole concentration, affording substantial electronic conductivity (7 S•cm^-1, 36 GPa). Notably, this behavior is comparable to that of (dmpz)[Sn₂I₆], albeit at higher pressure, and is not matched in (H₂bda)[Sn₂I₆] (H₂bda²⁺ = butane-1,4-diammonium), where the organic aliphatic cations do not have π* acceptor orbitals. This work expands charge reservoir candidates in perovskite analogs to include simple, stable, and abundant aromatic molecules.