Very Large Second Order Non-Linear Optical Activity Shown by Heterocycle-Based Dicyanomethanide Zwitterions
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ABSTRACT The new series of recently synthesized chromophores 2 - 6 in which a positively charged Nalkylpyridinium acceptor and a negatively charged dicyanomethanide donor are spaced by thiophene-based and/or ethylenic bridging units show dramatically enhanced first molecular hyperpolarizabilities (pt3 as high as 27000 x 10-48 esu), in comparison with some of the best recently reported neutral systems 7 and 8. INTRODUCTION A wide range of applications are responsible for the increasingly greater interest being shown in non linear optic (NLO) materials [1,2], including telecommunications, optical data processing, and data storage. Some of these materials have already been marketed [3], but manufacturing companies feel that substantial improvements are needed in order to consolidate and possibly extend their production. 4-Dimethylamino-4'-nitrostilbene DANS (1) is the prototype of a large family of second order NLO active systems in which the acceptor and donor units represent primary organic functionalities. Instead, we have exploited Rt-excessive and 7C-deficient heterocycles as donors and acceptors, respectively. Two approaches in the molecular design of second order NLO materials have been reported in the literature. In the first, and conventional approach, where DANS is often referred as a standard, the NLO activity originates from an efficient long-range charge transfer between the donor and the acceptor groups. In these molecules the ground state is described by an efficient resonance between two limit formula ("the cyanine limit" or "bond length alternation" approach), one aromatic, which slightly predominates, and one quinoid. The aromatic limit formula is always neutral and, by contrast, the quinoid structure is dipolar. The alternative approach draws the attention to the fact that, according to the two-state model, 13increases as the difference between the dipole moments of the ground- and excited-state increases. One way to achieve this is to design molecules with a high dipolar character in the ground state, which necessarily strongly decreases in the excited state. However so far this could be obtained in heterocyclic pyridinium betaines with no conjugation between the positively- and negatively-charged fragment, resulting in low values of hyperpolarizability [4]. EXPERIMENT AND RESULTS Our approach presented here, described by compounds 2 - 6, implies the design of an heterocyclic analogue of DANS, with the ground state presenting a highly charge-separated zwitterionic structure. In this set the azinium ion is the acceptor and the donor a dicyanomethanide unit: spacer groups are either ethylenic or 2,5-thienylene fragments. The 819 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
aromatic nature of the dipolar structure, that is exactly the reverse of the more popular class of DANS-like push-pull systems where the aromatic form is neutral, provides stability to the high charge-separation structure. The presence of a variable it-conjugation pathway between the donor and the acceptor
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