Ligand-Induced Ground- and Excited-State Chirality in Silicon Nanoparticles: Surface Interactions Matter

• Published in: Journal of the American Chemical Society 2022-03, Vol.144 (11), p.5074-5086
• Main Authors: Sujith, Meleppatt, Vishnu, E. Krishnan, Sappati, Subrahmanyam, Oliyantakath Hassan, Muhammed Shafeek, Vijayan, Vinesh, Thomas, K. George
• Format: Article
• Language: English
• Subjects: Indoles; Ligands; Nanoparticles - chemistry; Nitrogen; Silicon - chemistry; Tryptophan
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.1c13698

Silicon-based light-emitting materials have emerged as a favorable substitute to various organic and inorganic systems due to silicon’s high natural abundance, low toxicity, and excellent biocompatibility. However, efforts on the design of free-standing silicon nanoparticles with chiral non-racemic absorption and emission attributes are rather scare. Herein, we unravel the structural requirements for ligand-induced chirality in silicon-based nanomaterials by functionalizing with D- and L-isomers of a bifunctional ligand, namely, tryptophan. The structural aspects of these systems are established using high-resolution high-angle annular dark-field imaging in the scanning transmission electron microscopy mode, solid-state nuclear magnetic resonance, Fourier transform infrared, and X-ray photoelectron spectroscopy. Silicon nanoparticles capped with L- and D-isomers of tryptophan displayed positive and negative monosignated circular dichroic signals and circularly polarized luminescence indicating their ground- and excited-state chirality. Various studies supported by density functional theory calculations signify that the functionalization of indole ring nitrogen on the silicon surface plays a decisive role in modifying the chiroptical characteristics by generating emissive charge-transfer states. The chiroptical responses originate from the multipoint interactions of tryptophan with the nanoparticle surface through the indole nitrogen and −CO2 – groups that can transmit an enantiomeric structural imprint on the silicon surface. However, chiroptical properties are not observed in phenylalanine- and alanine-capped silicon nanoparticles, which are devoid of Si–N bonds and chiral footprints. Thus, the ground- and excited-state chiroptics in tryptophan-capped silicon nanoparticles originates from the collective effect of ligand-bound emissive charge-transfer states and chiral footprints. Being the first report on the circularly polarized luminescence in silicon nanoparticles, this work will open newer possibilities in the field of chirality.