A distinctive example of the cooperative interplay of structure and environment in tuning of intramolecular charge transfer in second-order nonlinear optical chromophores.

The strongly enhanced cooperative influence of medium polarity and org. structural design on the first hyperpolarizability b of a novel family of highly polarizable azinium-(CH=CH-thienyl)-dicyanomethanido chromophores (e.g. I; R=H, n-C10H21, Me) is described. The dyes can be efficiently synthesized by regioselective protonation/alkylation of the corresponding bidentate anion precursors. Consecutive annelation of the pyridyl ring (pyridine -> quinoline -> acridine) and medium polarity effects are responsible for an extraordinarily variable range of intramol. charge transfer (ICT), leading to a large set of p-electron distribution patterns. Accordingly, systems with remarkably different zwitterionic/quinoid character in the ground and excited states present b values in a broad range, eventually switching from neg. to pos. Our investigation is based on a combination of exptl. (UV/Vis spectroscopy, multinuclear NMR spectroscopy, and electrooptical absorption measurements) and computational (ab initio) approaches. It is shown that. b And mb are dramatically influenced, even by orders of magnitude, by a complex, non-monotonic interplay of structure and medium action, which in turn affects mol. ICT and bond length alternation (BLA),. The computations, validated by different exptl. data, are to be recommended as an extremely useful tool in the search for a greatly improved set of mol. nonlinear optical (NLO) responses (the compds. examd. show that such conditions may be attained only in a narrow and limited range of dielec. consts. in which the annelation effect operates most efficiently), and. The search for the most favorable mol. NLO response of a highly polarizable chromophore both in soln. and in solid matrixes should simultaneously take into account not only the mol. design supplemented by annelation effects but also the polarity of the medium.