Contrary to the enormous number of previous studies on carbon nanotubes (CNTs), herein, we realized the origin of the intrinsic open-shell diradical character and second hyperpolarizability γ using a broken symmetry approach. This study was inspired by our recent findings (S. Muhammad, et al ., Nanoscale , 2016, 8 , 17998 and Nakano, et al. , J. Phys. Chem. C , 2016, 120 , 1193). We performed structural modifications through a unique asymmetric donor-nanotube framework, which led to a novel paradigm of modified CNTs with tunable open-shell diradical character and remarkably superior NLO response properties. Interestingly, asymmetry and diradical character were found to be the crucial factors to modulate the second hyperpolarizability γ . We initially performed a comparative analysis of the diradical characters and γ amplitudes of boron nitride nanotubes (BNNTs) and CNTs possessing significant ionic characters and covalent characters, respectively. The basic findings for these simple configurations were further extended to the donor-acceptor CNT paradigm, which finally led to excellent asymmetric donor-CNT configurations with remarkably larger γ amplitudes. Furthermore, among the CNTs, finite length zigzag CNT(6,0) 3 were modified with different donor-acceptor configurations. Interestingly, for the first time, unique donor-nanotube configurations 1,4-(NH 2 ) 2 CNT-(6,0) 3 and 1,4-(NH 2 ) 2 CNT-(6,0) 5 were found; they showed significantly robust γ amplitudes as large as 2519 × 10 3 and 4090 × 10 3 a.u. at the LC-UBLYP( μ = 0.33)/6-31G* level of theory. Additionally, several molecular level insights have been obtained for these novel donor-nanotube configurations using their odd electron densities, molecular electrostatic maps, densities of states and γ density analyses to highlight the realization of these novel materials for highly efficient optical and NLO applications. Open-shell diradical effects on carbon nanotubes. Fascinating nonlinear optical response properties are observed for modified asymmetric open-shell carbon nanotubes.