Design, structure, and optical properties of organic-inorganic perovskitescontaining an oligothiophene chromophore

A quaterthiophene derivative, 5,5'''-bis(aminoethyl)-2,2':5',2 ":5 ",2'''-q uaterthiophene (AEQT), has been selected for incorporation within the layer ed organic-inorganic perovskite structure. In addition to having an appropr iate molecular shape and two tethering aminoethyl groups to bond to the ino rganic framework, AEQT is also a dye and can influence the optical properti es of lead(II) halide-based perovskites. Crystals of C20H22S4N2PbBr4 were g rown 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 ) Angstrom, b = 5.8420(3) Angstrom, c = 11.5734(6) Angstrom, beta = 92.360( 1)degrees, and Z = 4. Broad superstructure peaks are observed in the X-ray diffraction data, indicative of a poorly ordered, doubled supercell along b oth the a and b axes. The quaterthiophene segment of AEQT(2+) is nearly pla nar, with a syn-anti-syn relationship between adjacent thiophene rings. Eac h quaterthiophene chromophore is ordered between nearest-neighbor lead(II) bromide sheets in a herringbone arrangement with respect to neighboring qua terthiophenes. Room temperature optical absorption spectra for thermally ab lated films of the perovskites (AEQT)PbX4 (X = CI, Br, I) exhibit an excito n peak arising from the lead(II) halide sheets, along with absorption from the quaterthiophene moiety. No evidence of the inorganic sheet excitonic tr ansition is observed in the photoluminescence spectra for any of the chromo phore-containing perovskites. However, strong quaterthiophene photoluminesc ence is observed for X = Cl, with an emission peak at approximately lambda( max) = 532 nm. Similar photoluminescence is observed For the X = Br and I m aterials, but with substantial quenching, as the inorganic layer band gap d ecreases relative to the chromophore HOMO-LUMO gap.