Hyperpolarizability of novel carbo-meric push-pull chromophores

Issue title: Computational aspects of electric polarizability calculations: Atoms, Molecules and clusters Part II

Article type: Research Article

Authors: Lepetit, Christine | Lacroix, Pascal G. | Peyrou, Viviane | Saccavini, Catherine | Chauvin, Remi^{; *}

Affiliations: Laboratoire de Chimie de Coordination, UPR 8241 CNRS, 205 Route de Narbonne, F-31077 Toulouse cedex 04, France

Correspondence: [*] Corresponding author. Tel.: +33 05 61 33 31 13; Fax: +33 05 61 55 30 03; E-mail: [email protected].

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.

Keywords: non-linear Optics, carbo-mers, bond-length alternation, chromophores, aromaticity, hyperpolarizability, ZINDO/DFT calculations

DOI: 10.3233/JCM-2004-4404

Journal: Journal of Computational Methods in Sciences and Engineering, vol. 4, no. 4, pp. 569-588, 2004