Design of D-π-A type cyclo[18]carbon derivatives with excellent optical nonlinearity: A theoretical perspective

The geometric and electronic structures, photophysical property, and optical nonlinearity of four cyclo[18]carbon (C18) derivatives containing hydrogen (-H), amino (-NH2) and/or nitro (-NO2) groups were theoretically explored. The carbon-atom skeleton 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 and the absorption intensity decreases gradually, and 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 was illustrated through charge-transfer spectrum (CTS) analysis and hole-electron 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 differences in first hyperpolarizability between four C18 derivatives are revealed by analyzing hyperpolarizability tensor, hyperpolarizability density, and hyperpolarizability decomposition. The comprehensive analysis indicated that donor-π-bridge-acceptor (D-π-A) type NH2-C18-NO2 can be considered as potential candidates for novel IR nonlinear optical (NLO) materials.