Design, Structure, and Optical Properties of Organic−Inorganic Perovskites Containing an Oligothiophene Chromophore

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Abstract
A quaterthiophene derivative, 5,5‘ ‘‘-bis(aminoethyl)-2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene (AEQT), has been selected for incorporation within the layered organic−inorganic perovskite structure. In addition to having an appropriate molecular shape and two tethering aminoethyl groups to bond to the inorganic framework, AEQT is also a dye and can influence the optical properties of lead(II) halide-based perovskites. Crystals of C20H22S4N2PbBr4 were grown from a slowly cooled aqueous solution containing lead(II) bromide and quaterthiophene derivative (AEQT·2HBr) salts. The new layered perovskite adopts a monoclinic (C2/c) subcell with the lattice parameters a = 39.741(2) Å, b = 5.8420(3) Å, c = 11.5734(6) Å, β = 92.360(1)°, and Z = 4. Broad superstructure peaks are observed in the X-ray diffraction data, indicative of a poorly ordered, doubled supercell along both the a and b axes. The quaterthiophene segment of AEQT2+ is nearly planar, with a syn−anti−syn relationship between adjacent thiophene rings. Each quaterthiophene chromophore is ordered between nearest-neighbor lead(II) bromide sheets in a herringbone arrangement with respect to neighboring quaterthiophenes. Room temperature optical absorption spectra for thermally ablated films of the perovskites (AEQT)PbX4 (X = Cl, Br, I) exhibit an exciton peak arising from the lead(II) halide sheets, along with absorption from the quaterthiophene moiety. No evidence of the inorganic sheet excitonic transition is observed in the photoluminescence spectra for any of the chromophore-containing perovskites. However, strong quaterthiophene photoluminescence is observed for X = Cl, with an emission peak at approximately λmax = 532 nm. Similar photoluminescence is observed for the X = Br and I materials, but with substantial quenching, as the inorganic layer band gap decreases relative to the chromophore HOMO−LUMO gap.