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)(2)NH3)(2)PbX4 (X = I, Br), have been prepared and characterized. (HO(CH2)(2)NH3)(2)Pbl(4) adopts a monoclinic cell, a = 8.935(l) Angstrom, b = 9.056(2) Angstrom, c = 10.214(3) Angstrom, p = 100.26(l)degrees, V = 813.3(3) Angstrom(3), P2(1)/a(1) and Z = 2, and (HO(CH2)(2)NH3)(2)PbBr4 is orthorhombic, a = 8.4625(6) Angstrom, b = 8.647(l) Angstrom, c = 19,918(2) Angstrom, V = 1457.5(2) Angstrom(3), 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)2Pbl4, is shown. As a result, a significant red shift of the exciton peaks (lambda = 536 nm (X = 1), lambda = 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 degreesC (X = 1) and T= 125 degreesC (X = Br). An orthorhombic cell of the high-temperature phase of (HO(CH2)(2)NH3)(2)Pbl(4) with a(HT) = 18.567(6) Angstrom, b(HT) = 13.833(6) Angstrom, C-HT = 6.437(2) (,)Angstrom and V = 1653 Angstrom(3) 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 lambda = 505 nim (X = I) or to Angstrom = 374 nm (X = Br) during the phase transition.