Organic Layer Reorganization across a Phase Transition in Benzotriazole-Based 2D Hybrid Perovskites with Humidity-Assisted Reversibility

Abstract

Two-dimensional (2D) hybrid organic–inorganic perovskites (HOIPs) offer structural versatility and tunable optoelectronic behavior arising from the interplay between their organic spacer layers and inorganic frameworks. In this work, we investigate how modifying the alkyl spacer length in benzotriazole (BTa)-based perovskites influences both the distortion of the lead-iodide layers and the resulting optical properties. Comparison between (BTaC2)2PbI4 and its three-carbon analogue (BTaC3)2PbI4 shows that the longer spacer unexpectedly decreases the penetration of the ammonium group into the inorganic layer, while accommodation of the longer alkyl chain increases in-plane Pb–I bond lengths and intraoctahedral distortion. Experimentally, (BTaC3)2PbI4 exhibits negligible octahedral tilting, unusually long average Pb–I bond lengths, and a pronounced alternation between long and short Pb–I bonds of 3.68 and 3.04 Å, respectively. First-principles calculations demonstrate that this atypical inorganic framework significantly influences its electronic structure. This dependence also manifests with a blue-shifted band gap and reduced photoluminescence intensity. Temperature-dependent X-ray diffraction and differential scanning calorimetry reveal that (BTaC3)2PbI4 undergoes a phase transition near 145 °C, in which the interdigitated organic bilayer reorganizes into a more conventional noninterdigitated arrangement, which preserves the 2D perovskite structure but removes intraoctahedral distortion, while reintroducing the interoctahedral tilting, as confirmed by three-dimensional electron diffraction (3DED). This phase transformation results in a substantial increase in photoluminescence intensity, accompanied by a blue shift in the emission band. The high-temperature phase is metastable at room temperature and converts back to the low-temperature interdigitated phase when exposed to elevated humidity. These findings clarify the connection between organic layer ordering, inorganic distortions, and the optical response in layered perovskites and indicate that humidity is an important trigger able to assist transitions between polymorphs.