Hyperpolarizability of novel carbo-meric push-pull chromophores

Abstract

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 (β₀). Surprisingly, carbo-merization of the p-phenylene unit of ^-O-C₆H₄–C₆H₄–C₅H₄N⁺–Me results in a three-fold decrease of β₀. The negative carbo-meric effect is even more pronounced for the ^-O-C₆H₄–C₆H₄–C≡C–C₅H₄N⁺–Me ethynylogue. This effect is however relative to the exceptional value of the latter parent molecule (β₀ = 825 × 10^-30 cm⁵ esu^-1). The ^-O-C₆H₄–C≡C-C₆H₄–C₅H₄N⁺–Me isomer exhibits a similar hyperpolarizability (β₀ = 774 × 10^-30 cm⁵ esu^-1). Inserting an additional ₂ unit results in the double ethynylogue structure ^-O-C₆H₄–C≡C–C₆H₄–C≡C–C₅H₄N⁺–Me, for which β₀ = 1982 × 10^-30 cm⁵ esu^-1. The “phenylethynylene effect” becomes dramatic in the ^-O-C₆H₄–C≡C–(C₆H₄–C≡C)₂–C₅H₄N⁺–Me homologue which exhibits an unprecedented calculated value among small organic chromophores, namely: β₀ = 33856 × 10^-30 cm⁵ 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 β₀ vs ABLA is empirically observed: drawing nearer to a critical value ABLA° ≈ 0.040 Å results in a steep increase of β₀. 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-C₆H₄O and 4-C₅H₄N termini, including the non-zwitterionic isomer MeO-C₆H₄–C₁₈H₄–C₅H₄N, the experimental synthesis of which is addressed. As in the case of p-nitroaniline, a positive carbo-meric effect on β₀ is restored upon carbo-merization of the p-phenylene bridge of the non-zwitterionic Me₂N-C₆H₄–C₆H₄–C₆H₄–NO₂ congener of ^-O-C₆H₄–C₆H₄–C₅H₄N⁺–Me. It is also shown that β₀ 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.