Nonlinear optical (NLO) materials are the smartest materials of the era, and have the ability to generate new electromagnetic fields with changed frequencies, phases, and other physical properties. Recently, many cutting edge research reports have been focused on NLO materials especially on those which are composed of sp² hybridized carbon nanostructures. As the carbon nanostructures are composed of abundant π-electrons and have significant delocalization, these are potential candidates for modern NLO materials. Generally, sp² hybridized carbon nanostructures can be divided into zero-dimensional fullerenes, one-dimensional nanotubes and two-dimensional graphene nanoribbons and quantum dots etc. These dimensionally different carbon nanomaterials are promising candidates for a wide range of applications in next-generation nanotechnologies. In present feature article, we first briefly explain a theoretical structure–NLO property relationship based on perturbation theory and then elucidate the crucial factors to control the NLO responses. We put together the different random investigations of sp² hybridized carbon nanostructures for NLO application by highlighting the importance of their several structural designs to tune NLO amplitudes. Furthermore, we make a comparative and updated analysis of the NLO properties of dimensionally different sp² hybridized carbon nanomaterials i.e. fullerenes, carbon nanotubes, and graphene nanoribbons and quantum dots. Finally, we make a brief discussion about different aspects and opportunities to use the sp² hybridized carbon nanomaterials as high performance NLO materials of the future. This review is a focused perspective based on different updated quantum chemical investigations about fullerenes, nanotubes and graphene nanoribbons and quantum dots for their possible use in nonlinear optical applications.

Excess electron compounds have been proposed to be novel candidates of high-performance nonlinear optical (NLO) materials because of their large static first hyperpolarizabilities (β₀). To enhance the stability of an unstable excess electron compound (LiCN⋯Li) with an extremely large β₀ value (310196 a.u.), we designed a boron nitride nanotube (BNNT) as a protective shield molecule to encapsulate it (in theory). The stability of LiCN⋯Li was enhanced: the vertical ionization potentials (VIP) of LiCN⋯Li increased after encapsulating. Therefore, by comparison with LiCN⋯Li, the encapsulated complexes are more difficult to oxidize. Significantly, the BNNT encapsulated LiCN⋯Li complex exhibits a considerable β₀ value (10645 a.u.), which is significantly (almost 380 times) larger than 28 a.u. of BNNT. Our further investigations into the intrinsic hyperpolarizabilites (β_int) of these compounds show that there are clearly dependencies of the NLO response on the transition energy. Furthermore, it is easy to encapsulate LiCN⋯Li from the B-rich edge rather than N-rich edge of BNNT due to the lower energy barrier, which makes our calculations more useful to experimentalists who may try to synthesize these compounds. Knowledge of the encapsulation process of LiCN⋯Li within BNNT provides a new strategy for the design and synthesis of stable high-performance NLO materials.