A Series of Novel Derivatives with Giant Second Hyperpolarizabilities, Based on Radiaannulenes, Tetrathiafulvalene, Nickel Dithiolene, and Their Lithiated Analogues

The main goal of this study was the design/recognition of a series of derivatives with exceptionally large hyperpolarizabilities, the understanding of the mechanism through which these properties are derived, and the generalization of the resulting findings. Thus, we have studied the structure, a series of properties involving the excitation energies and the diradical character, but mainly the (hyper)polarizabilities of some oligomers of radiaannulene-tetrathiafulvalene (RA-TTF), radiaannulene-bis(ethylene-1,2-dithiolato)nickel (RA-NiBDT), and their lithiated analogues. The employed methodology involves MP2 and DFT techniques, approaches for the computation of static and frequency-dependent properties, and techniques for the calculation of electronic and vibrational contributions. The polarizability has been partitioned be employing the fractional occupational Hirshfeld iterative method. Indices for the calculation of the diradical character of the NiBDT oligomers have been employed. We have found that (i) oligomerization increases γ zzzz (second hyperpolarizability), (ii) substitution of TTF with NiBDT leads to a very large increase of γ zzzz, and (iii) lithiation leads to extremely large nonlinearities. The importance of NiBDT and lithium as building blocks for the production of NLO materials is highlighted.