Theoretical Design of Substituted Tetrathia‐[7]‐Helicenes with Large Second‐Order Nonlinear Optical Responses

Abstract

Second-order nonlinear optical (NLO) materials have attracted a lot of interest because of foreseeable applications in optoelectronic technology. In particular, organic compounds are attractive for this purpose because of their large and fast responses combined with the flexibility of organic synthesis which hastens the design of new chromophores. The first generation of NLO organic chromophores consists of one-dimensional (1D) p-conjugated systems end-capped with donor (D) and acceptor (A) moieties. Experimental and theoretical work has shown during the last decades how to choose the D/A strengths and p segments for optimizing the second-order NLO responses. Later, two-dimensional (2D) octupolar molecules such as 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) were shown to exhibit second-order NLO responses that can be as large as those of their dipolar analogues. In addition, with these 2D structures, some of the limitations of the 1D dipolar structures were displaced, if not overcome. Indeed, 1D compounds suffer from the transparency–efficiency tradeoff, which is difficult to achieve because the desirable increase in b(the first hyperpolarizability) is accompanied by a bathochromatic shift of the electronic transitions. Moreover, their dipolar nature and the predominance of dipole–dipole intermolecular interactions favor the formation of centrosymmetric crystal arrangements which exhibit no second harmonic generation (SHG) responses. In addition, phase-matching conditions cause only a fraction of the microscopic response to be preserved at the macroscopic scale. On the other hand, octupolar compounds present an improved nonlinearity/transparency tradeoff, enable non-centrosymmetric crystal structures owing to their lack of permanent dipole moment, and present large offdiagonal b tensor components that can give rise to large macroscopic SHG responses.