Design, Structure, and Optical Properties of Organic-Inorganic Perovskites Containing an Oligothiophene Chromophore David B. Mitzi,* Konstantinos Chondroudis, and Cherie R. Kagan T. J. Watson Research Center, IBM P.O. Box 218, Yorktown Heights, New York 10598 ReceiVed August 31, 1999 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 C 20 H 22 S 4 N 2 PbBr 4 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,  ) 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 AEQT 2+ 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)PbX 4 (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 λ max ) 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. Introduction Substantial recent attention has been focused on organic- inorganic perovskites as a result of interesting and potentially useful electrical, optical, and magnetic properties that arise in these compounds. 1 The hybrid perovskites naturally form layered structures, consisting of sheets of corner-sharing metal halide octahedra separated by bilayers or monolayers of organic cations. One example, (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) n-1 Sn n I 3n+1 , con- sists of “n”-layer-thick perovskite sheets interleaved with bilayers of butylammonium cations. 2 In contrast to most metal halides, which are electrically insulating, this family undergoes a semiconductor-metal transition as a function of increasing perovskite sheet thickness. The group IVA metal-based per- ovskites also exhibit sharp, tunable resonances in their room temperature optical absorption and emission spectra. 3 These features arise from excitonic transitions associated with the band gap of the metal halide framework and are therefore charac- teristic of the choice of metal and halogen making up the inorganic sheets. The strong room temperature exciton peaks in the optical spectra attest to the large exciton binding energy (>200 meV) in these self-assembling quantum well structures. In most of the organic-inorganic systems studied to date, the relatively simple organic layer of the hybrid plays a secondary role in distinguishing the interesting physical proper- ties associated with each compound. For example, in the known highly conducting tin(II)-based perovskites, 2 while the organic layers serve to define the dimensionality of the compound, the large conductivity arises from the small band gap and substantial carrier mobility associated with the metal halide sheets. Similarly, in the luminescent systems, the organic layers define the dimensionality of the structure and enhance the exciton binding energy through a “dielectric confinement” effect. 4 However, the strong photoluminescence arises from the radiative decay of excitons within the metal halide sheets. In each of these cases, the organic component of the structure is relatively simple, consisting of alkylammonium or single ring aromatic ammonium cations. The HOMO-LUMO energy gap for the organic molecule is therefore large compared to the band gap of the inorganic framework, and the molecules are optically and electrically inert. One motivation of this study is to design organic dye molecules that fit within the perovskite structure and that also have the appropriate molecular energy levels so that the HOMO-LUMO gap of the organic component can be engi- neered relative to the band gap of the inorganic layer. The oligothiophenes are particularly useful organic moieties in this respect because the energy at which the molecules absorb (and subsequently emit) light can be controlled by choosing the length of the thiophene chain. The absorption maximum for “n” * To whom correspondence should be addressed. (1) For a recent review, see: Mitzi, D. B. Prog. Inorg. Chem. 1999, 48, 1. (2) Mitzi, D. B.; Feild, C. A.; Harrison, W. T. A.; Guloy, A. M. Nature 1994, 369, 467. (3) Ishihara, T. In Optical Properties of Low-Dimensional Materials; Ogawa, T., Kanemitsu, Y., Eds.; World Scientific: Singapore, 1995, pp 288-339. (4) Hong, X.; Ishihara, T.; and Nurmikko, A. V. Phys. ReV.B 1992, 45, 6961. 6246 Inorg. Chem. 1999, 38, 6246-6256 10.1021/ic991048k CCC: $18.00 © 1999 American Chemical Society Published on Web 12/08/1999