Targeted synthesis of trimeric organic-bromoplumbate hybrids that display intrinsic, highly Stokes-shifted, broadband emission

Zero-dimensional (0D) hybrid organic-inorganic lead halides have been shown to display efficient broadband photoluminescence and are, therefore, of significant interest for artificial lighting applications. However, work that investigates the formability of the materials as a function of templating organic cation structure is rare. This severely limits our ability to rationally design new materials displaying specific structural and photophysical properties. With the goal of accessing rare 0D trimeric bromoplumbates, we have systematically varied templating Nalkylpyridinium cations and examined their impact upon inorganic lattice structure. Whereas comparatively short and flexible N-alkyl substituents (ethyl, 2-hydroxyethyl, and pentyl) yield one-dimensional (1D) inorganic chains, more rigid substituents (benzyl, acetamidyl, and cyanomethyl) afford hybrids composed of lead bromide face-sharing trimers (i.e., [Pb3Br12](6-)). Of the rigid substituents studied, benzyl groups were found to enforce the highest level of distortion of the [PbBr6](4-) octahedra that comprise their trimeric structures. Upon exposure to ultraviolet (UV) light, N-benzylpyridinium lead bromide (1)(6) [Pb3Br12] exhibits a broadband emission, centered at 571 nm, which spans from 400 to 800 nm. More specifically, it displays a large Stokes shift of ca. 1.39 eV and a full width at half-maximum of ca. 146 nm. This broadband emission decays with a comparatively long lifetime of 426 ns at room temperature, which increases to 5.8 mu s at 77 K. The reduced size and dimensionality of its inorganic lattice also result in a photoluminescence quantum yield (of at least 10%) that is approximately one order of magnitude higher than that of its 1D congeners. Mechanistically, broadband emission in (1)(6) [Pb3Br12] is believed to originate from triplet excited state(s) obtained from excited-state structural reorganization of the [Pb3Br12](6) - moiety.