Fullerene superlattices containing charge transfer complexes for an improved nonlinear optical performance

Abstract

To improve the nonlinear optical (NLO) properties of fullerene C₆₀, chemical modifications are normally needed to construct a donor–π-acceptor (D–π-A) system, which requires tedious and time-consuming synthesis procedures. In addition, the conjugated structure of C₆₀ will inevitably be destroyed, which is disadvantageous for other applications. Here, we use solvent-based nanoarchitectonics to obtain highly ordered, three-dimensional (3D) C₆₀ supramolecular structures. For this purpose, a liquid–liquid interfacial precipitation (LLIP) method was employed using quinoline as the good solvent. Hollow polyhedra (HPH) and multilayer flowers (MFs) were obtained when methanol and ethanol were selected as the poor solvents, respectively. While quinoline failed to enter the HPH, it was found to be successfully intercalated with the MFs, which induced a transition of the C₆₀ organization from a pristine face-centered-cubic (fcc) phase to a hexagonal close packed (hcp) lattice. When embedded in a poly(methyl methacrylate) (PMMA) matrix, the HPH and MFs both show reverse saturable absorption (RSA) and optical limiting (OL) properties. The MFs-based film showed a third-order nonlinear absorption coefficient (β) of 1.25 × 10⁵ cm·GW⁻¹ and an optical limiting threshold (F_ol) of 0.00625 J·cm⁻². Comparatively, the HPH-based film exhibited a lower β value of 9.80 × 10⁴ cm GW⁻¹ and a higher F_ol value of 0.00834 J cm⁻². The better NLO performance of the MFs was mainly ascribed to the formation of the charge transfer complexes between quinoline and C₆₀, proven by UV-vis and electrochemical measurements. The fine tuning of the NLO properties of C₆₀ without chemical modification provides new opportunities for C₆₀ to be applied in nonlinear optics.