Unique Hydrogen Bonding Correlating with a Reduced Band Gap and Phase Transition in the Hybrid Perovskites (HO(CH₂)₂NH₃)₂PbX₄ (X = I, Br)

The first hybrid perovskites incorporating alcohol-based bifunctional ammonium cations, (HO(CH2)2NH3)2PbX4 (X = I, Br), have been prepared and characterized. (HO(CH2)2NH3)2PbI4 adopts a monoclinic cell, a = 8.935(1) Å, b = 9.056(2) Å, c = 10.214(3) Å, β = 100.26(1)°, V = 813.3(3) Å3, P21/a, and Z = 2, and (HO(CH2)2NH3)2PbBr4 is orthorhombic, a = 8.4625(6) Å, b = 8.647(1) Å, c = 19.918(2) Å, V = 1457.5(2) Å3, Pbcn, and Z = 4. In the layered structures, a unique hydrogen-bond network connects adjacent perovskite layers, owing to OH····X, NH3+····X, and intermolecular NH3+···OH interactions. Its impact on the bonding features of the inorganic framework and on the quite short interlayer distance, in the case of (HO(CH2)2NH3)2PbI4, is shown. As a result, a significant red shift of the exciton peaks (λ = 536 nm (X = I), λ = 417 nm (X = Br)), compared to other PbX42--based perovskite hybrids, is observed, revealing a reduced band gap. A reversible structural transition occurs at T = 96 °C (X = I) and T = 125 °C (X = Br). An orthorhombic cell of the high-temperature phase of (HO(CH2)2NH3)2PbI4 with aHT = 18.567(6) Å, bHT = 13.833(6) Å, cHT = 6.437(2) Å, and V = 1653 Å3 is proposed from powder X-ray diffraction. A change in the hydrogen bonding occurs, with molecules standing up in the interlayer space and OH parts probably interacting together, leading to a more conventional situation for ammonium groups and a more distorted perovskite layer. This is in accordance with the blue shift of the exciton peak to λ = 505 nm (X = I) or to λ = 374 nm (X = Br) during the phase transition.