Design of donor–π–acceptor type cyclo[18]carbon derivatives for infrared nonlinear optical materials: a theoretical perspective

Abstract

The geometries, electronic structures, photophysical properties, and optical nonlinearities of four cyclo[18]carbon (C₁₈) derivatives containing hydrogen (–H), amino (–NH₂) and/or nitro (–NO₂) groups were theoretically explored. The carbon-atom skeletons of molecules with different functional groups do not differ obviously, but their electronic properties are noticeably different. Electronic excitation analysis shows that with the introduction of –NH₂ and/or –NO₂ groups, the maximum wavelength absorption of derivatives red-shifts slightly, the absorption intensity increases gradually, and the difference in dipole moment between the ground state and the crucial excited state increases sharply, indicating that their first hyperpolarizability increases continuously. The four molecules all have an excellent infrared (IR) transparency in the wavelength range of 800 to 4000 nm. The essence of electronic transition in derivatives mainly concentrated on the C₁₈ unit and greater charge separation in NH₂–C₁₈–NO₂ was elucidated through the charge-transfer spectrum (CTS) analysis, hole–electron analysis, and electrostatic potential (ESP) analysis. Derivative molecules with different combinations of functional groups exhibit markedly different response properties, and the first hyperpolarizability reaches the maximum when –NH₂ and –NO₂ are introduced simultaneously to form NH₂–C₁₈–NO₂. The anisotropy and origin of the first hyperpolarizability of four C₁₈ derivatives are revealed by analyzing the hyperpolarizability tensor, hyperpolarizability density, and hyperpolarizability decomposition. The comprehensive analysis indicated that donor–π–acceptor (D–π–A) type NH₂–C₁₈–NO₂ can be considered as a potential candidate for novel IR nonlinear optical (NLO) materials.