Hyperpolarizability of novel carbo-meric push-pull chromophores

A systematic carbo-meric comparison of geometrical and NLO properties of push-pull aromatic NLO chromophores is undertaken at the B3PW91/6-31G* level and the INDO-SOS level respectively. In a first approach, merocyanine-like chromophores all containing a p-oxyphenyl donor and a N-methyl-4-pyridyl acceptor with various conjugated hydrocarbon bridges are considered. Carbo-merization of ethylene, 2,5-thiophenylene or 1-oxo-2,5-cyclopentadienylene bridges results in a ca ten times exaltation of static hyperpolarizability (β$_{0}$). Surprisingly, carbo-merization of the p-phenylene unit of $^{-}$O-C$_{6}$H$_{4}$-C$_{6}$H$_{4}$-C$_{5}$H$_{4}$N$^{+}$-Me results in a three-fold decrease of β$_{0}$. The negative carbo-meric effect is even more pronounced for the $^{-}$O-C$_{6}$H$_{4}$-C$_{6}$H$_{4}$-C≡C-C$_{5}$H$_{4}$N$^{+}$-Me ethynylogue. This effect is however relative to the exceptional value of the latter parent molecule (β$_{0}$ = 825 × 10$^{-30}$ cm$^{5}$esu$^{-1})$. The $^{-}$O-C$_{6}$H$_{4}$-C≡C-C$_{6}$H$_{4}$-C$_{5}$H$_{4}$N$^{+}$-Me isomer exhibits a similar hyperpolarizability (β$_{0}$ = 774 × 10$^{-30}$ cm$^{5}$ esu$^{-1})$. Inserting an additional $_{2}$ unit results in the double ethynylogue structure $^{-}$O-C$_{6}$H$_{4}$-C≡C-C$_{6}$H$_{4}$-C≡C-C$_{5}$H$_{4}$N$^{+}$-Me, for which β$_{0}$ = 1982 × 10$^{-30}$ cm$^{5}$esu$^{-1}$. The "phenylethynylene effect" becomes dramatic in the $^{-}$O-C$_{6}$H$_{4}$-C≡C-(C$_{6}$H$_{4}$-C≡C)$_{2}$-C$_{5}$H$_{4}$N$^{+}$-Me homologue which exhibits an unprecedented calculated value among small organic chromophores, namely: β$_{0}$ = 33856 × 10$^{-30}$ cm$^{5}$ esu$^{-1}$. A generalized absolute bond length alternation parameter (ABLA) is propounded and calculated for a homogeneous family of merocyanine-like chromophores. An astonishing regular variation of β$_{0}$ vs ABLA is empirically observed: drawing nearer to a critical value ABLA° a 0.040 A results in a steep increase of β$_{0}$. A Lorentzian-based fit is empirically found (R > 0.999) and heuristically derived from simple approximations in a two-level model. The curve interpolates points corresponding to all kinds of chromophores bearing p-C$_{6}$H$_{4}$O and 4-C$_{5}$H$_{4}$N termini, including the non-zwitterionic isomer MeO-C$_{6}$H$_{4}$-C$_{18}$H$_{4}$-C$_{5}$H$_{4}$N, the experimental synthesis of which is addressed. As in the case of p-nitroaniline, a positive carbo-meric effect on β$_{0}$ is restored upon carbo-merization of the p-phenylene bridge of the non-zwitterionic Me$_{2}$N-C$_{6}$H$_{4}$-C$_{6}$H$_{4}$-C$_{6}$H$_{4}$-NO$_{2}$ congener of $^{-}$O-C$_{6}$H$_{4}$-C$_{6}$H$_{4}$-C$_{5}$H$_{4}$N$^{+}$-Me. It is also shown that β$_{0}$ exaltation is larger when carbo-merization is applied to the terminal benzene ring of 4-nitro-biphenyl derivatives, thus confirming the excellent intrinsic donor properties of the carbo-benzene ring.