, where ΔE, f0, and Δμ are the transition energy of Sn, the oscillator strength of Sn, and the difference of the dipole moment between S0 and Sn, respectively. The two-level model shows that the βtot(∞) is inversely proportional to the cube of the ΔE, and directly proportional to the f0 and Δμ. We can see from Table that the β*(∞) of (BN)nC(18-2n) (n = 1-9) calculated according to the two-level model also show a rough trend of first increasing and then decreasing with the increase of n, just like the order shown by the βtot(∞). The difference, however, is that the species with the largest β*(∞) is (BN)5C8, most likely because the excited states that contribute substantially to the βtot(∞) of the studied molecules are not only the Sn considered. For (BN)5C8, the reason for its remarkable β*(∞) in the twolevel model framework is due to the smaller ΔE, the larger f0, and the largest Δμ.