Luminescent Zero-Dimensional Hybrid Lead Thiohalide Nanostructures for High Quantum Yield and Broadband Excitation

Luminescent trans-[Pb­(DMTU-S)4Cl2] (DMTU: N,N′-dimethylthiourea)
was designed and prepared via either mechanochemical or solvothermal
methods, and the structures of DMTU and trans-[Pb­(DMTU-S)4Cl2] have been resolved using X-ray single-crystal
diffraction. Upon excitation over broadband covering the range from
450 to 250 nm, trans-[Pb­(DMTU-S)4Cl2] shows yellow-green emission peaking at 549 nm with a spectral
width of 110 nm, which is assigned to the triplet–singlet transition
of Pb2+ ions within distorted heterogeneous S4Cl2 octahedra. The broadband excitation comprised singlet–singlet
transitions of Pb2+ ions and energy transfer from orbitals
involving those of organic ligands. Simultaneous analysis of the luminescent
bandwidth and Stokes shift gives for Pb2+ ions in S4Cl2 octahedra the value of the Huang–Rhys
parameter S = 4.25 and the energy of phonon involved
in the formation of the luminescence spectrum of the order of 90 meV.
Quantum yield as high as 91% is detected for excitation at 365 nm.
This high quantum yield indicates the absence of noticeable concentration
quenching at an average distance of 9.4 Å between the Pb2+ ions within the structure of trans-[Pb­(DMTU)4Cl2]. The weak spin–orbit intersystem crossing
is deduced from a high photoluminescence quantum yield (PLQY) value.
Time dependent-density functional theory (TD-DFT) calculations of
the nanocluster indicate the red shift of absorption bands in Pb­(DMTU)4Cl2 with respect to parent DMTU. The high-performance
photoluminescence and stability demonstrated promising applications
in photonics.