Single vacancy-defect endohedral metallofullerene-superhalogens: molecular topology and nonlinear optical responses of Na@C59[9-4]([8-5])-AlX4 (X = Cl, Br) systems

Abstract

By combining density-functional theory and wavefunction analysis, the photophysical properties and optical nonlinearity responses of the single vacancy-defect endohedral metallofullerene-superhalogen compounds (Na@C59[9-4]([8-5])-AlX₄ (X = Cl, Br)) are comprehensively investigated in static and dynamic regimes. This work unveils unusual optical properties by changing the incident wavelengths and increasing the involved halogen atoms. Charge-transfer spectra are calculated by the Sum-Over-States method to characterize the electron excitation nature from the interfragment charge-transfer. Strong electron transfer characteristics are observed within and between Na@C59[9-4] and AlBr₄. Meanwhile, the electron redistribution inside C59[8-5] reveals a major proportion of optically active electron excitations. Regarding the optical nonlinearity responses, it is found that, from λ = 1908 nm to λ = 589 nm, the frequency-dependent anisotropic polarizability for Na@C59[9-4]-AlBr₄ at λ = 1064 nm (793.17 a.u.) is 4 times larger than the static regime (α_anisotropy (λ = ∞ nm) = 199.79 a.u.), showing a notable polarization anisotropy character. The dispersion of the optical nonlinearity of Na@C59[9-4]-AlBr₄ achieves the maximum at λ = 1064 nm (β_xxx = 6 448 870 a.u.), confirming that the resonance effect is inversely dependent on the incident wavelength. The π-conjugated electrons and charge states bring a considerable change to the electronic properties, so that the first hyperpolarizability of Na@C59[9-4]-AlBr₄ is about 981-fold larger than that of Na@C60[6]-AlBr₄. By calculating the two-dimensional second order nonlinear optical spectra, Na@C59[9-4]-AlBr₄ shows remarkable optical rectification effect, in contrast to the larger Electro-Optics Pockels value of Na@C59[8-5]-AlBr₄ at (0.0,0.10).