Excellent NLO materials should be transparent in the frequency band of their application, which is a requirement that cannot be ignored in practice. With that in mind, the TD-DFT calculations were performed on studied C18 and (BN)nC(18-2n) (n = 1-9) to simulate their electronic absorption spectra. As illustrated in Figure (a), the absorption peak of C18 obtained in this work is located at λmax = 219.6 nm with oscillator strength f0 = 3.06, which is perfectly consistent with our previous report at the ωB97XD/def2-TZVP level (λmax = 219.4 nm and f0 = 3.02). The absorption bands of all molecules lie in the ultraviolet range of 125-400 nm, indicating that they are almost colorless like pristine C18, that is, they all have optical transparency in the visible light region. The bands of (BN)nC(18-2n) (n = 1-9) corresponding to the maximum absorption of C18 at 219.6 nm are centered at a slightly shorter wavelength, and their absorption intensities are lower than that of C18. As more C-C units are replaced by B-N units in the ring, these absorptions gradually blue-shifts accompanied by a continuous decrease in intensity. Besides, (BN)nC(18-2n) (n = 1-9) appear an additional absorption bond in the long wavelength region. The successive replacement of C-C units by B-N groups causes a sustained blue-shift in the absorption peak, while their f0 show a trend of first increasing and then decreasing. After all C-C units were replaced, the additional absorption of the formed (BN)9 ring merged with the absorption corresponding to C18 at 193.3 nm. Taking (BN)4C10 as an example, we performed charge-transfer spectrum (CTS) analysis by dividing it into two fragments, C-C chain and B-N chain, and plotted the spectra in Figure . As can be seen, for electronic absorption at all wavelengths, the order of contribution of the CTS to the total absorption spectrum is electron redistribution in C-C units > electron redistribution in B-N units > charge transfer of C-C units to B-N units > charge transfer of B-N units to C-C units. The crucial excited states (Sn) involved in maximum wavelength absorption and selected characteristic parameters of C18 and (BN)nC(18-2n) (n = 1-9) are arranged in Table . Once again, we take (BN)4C10 as an example to examine the excitation properties and transition nature related to the maximum wavelength absorption. The S0→S7 transition of the electrons was identified as the crucial excitation for the electronic absorption of (BN)4C10. Configurational coefficients corresponding to different MO transitions calculated by TD-DFT indicate that S7 state displays mixed characteristics with partial in-plane MO transitions (π in →π in* ) and partial out-of-plane MO transitions (π out →π out* ), as shown in Figure (b). For a detailed description of inplane and out-of-plane π systems, please refer to reference 5 and 11 . a "H" and "L" represent HOMO and LUMO, respectively.