Size Dependence of [n]Cycloparaphenylenes (n = 9–20): Relationship between Aromaticity and Third-Order Nonlinear Optical Properties

As influence factors in generating third-order nonlinear optical (NLO) responses, several parameters, including the length of π-conjugated linkers, donor/acceptor substituents, and shape of the π electron networks, have been investigated previously to explore the structure–property relationships. This work is aimed at presenting the novel interplay between the size of cycloparaphenylenes (CPPs) and the third-order NLO responses, as well as the relationship between the aromaticity and NLO properties. Herein, the electronic structures, aromaticities, spectra, and second hyperpolarizabilities (γ) of [9]CPP – [20]CPP are investigated using the density functional theory computations. It is found that the odd-numbered node CPP has smaller E gap, larger λ max, and more obvious charge transfer characteristic with respect to that of the foregoing even-numbered ones. Neutral CPPs present antiaromaticities because of the 4 n π electrons. The results of the γ values show that larger NLO responses are proportional to the size of the CPPs, which is due to the fact that larger CPPs possess larger density amplitude and longer distance, resulting in the larger γ values with respect to the smaller CPPs. The divalent cations and anions of CPPs have 4 n + 2 π electrons, so their aromaticities are very obvious and the aromaticities of anions are generally larger with respect to that of cations. On the other hand, the γ values of CPPs 2– are generally larger with respect to that of CPPs 2+ . The results of this investigation will not only provide a new structure–property relationship but also yield novel insights into the relationship between aromaticity and third-order nonlinear optical properties.