A Distinctive Example of the Cooperative Interplay of Structure and Environment in Tuning of Intramolecular Charge Transfer in Second‐Order Nonlinear Optical Chromophores

Abstract

The strongly enhanced cooperative influence of medium polarity and organic structural design on the first hyperpolarizability beta of a novel family of highly polarizable azinium-(CH=CH-thienyl)-dicyanomethanido chromophores 1-3 is described. The dyes can be efficiently synthesized by regioselective protonation/alkylation of the corresponding bidentate anion precursors. Consecutive annelation of the pyridyl ring of 1 (pyridine-->quinoline-->acridine) and medium polarity effects are responsible for an extraordinarily variable range of intramolecular charge transfer (ICT), leading to a large set of pi-electron distribution patterns. Accordingly, systems with remarkably different zwitterionic/quinoid character in the ground and excited states present beta values in a broad range, eventually switching from negative to positive. Our investigation is based on a combination of experimental (UV/Vis spectroscopy, multinuclear NMR spectroscopy, and electrooptical absorption measurements) and computational (ab initio) approaches. It is shown that: 1) beta and mubeta are dramatically influenced, even by orders of magnitude, by a complex, non-monotonic interplay of structure and medium action, which in turn affects molecular ICT and bond length alternation (BLA), 2) the computations, validated by different experimental data, are to be recommended as an extremely useful tool in the search for a greatly improved set of molecular nonlinear optical (NLO) responses (in the case of 1-3 they show that such conditions may be attained only in a narrow and limited range of dielectric constants in which the annelation effect operates most efficiently), and 3) the search for the most favorable molecular NLO response of a highly polarizable chromophore both in solution and in solid matrices should simultaneously take into account not only the molecular design supplemented by annelation effects but also the polarity of the medium.

Relationship between the Molecular Structure of Merocyanine Dyes and the Vibrational Fine Structure of Their Electronic Absorption Spectra

“…Equation (1) describes two possible limiting forms of acceptor/ donor-substituted polyenes and, therefore, cannot be used for merocyanines. Equation (2) (4) and (5) do not represent polyene-like forms as often claimed, [4,6,[9][10][11][12] but are equivalent to structure b in Equation (3) for cyanines and are polymethines.…”

“…The concepts of the average bond-length alternation (BLA), [7] and the closely related average p-bond order alternation (BOA), [8] between adjacent bonds in the hydrocarbon chain are most popular. [4,6,[9][10][11][12]] Surprisingly, within the same concept merocyanines have been described by two quite different models, namely Equations (1), [7a,c, 8b,c] and (2), [7b, 8a] in which A (electron acceptor) and D (electron donor) are capable of charge exchange. Polymethines are characterized by a chain of conjugated double bonds with an odd number n of p centers and (n + 1) p electrons, whereas polyenes are characterized by an even number of p centers and the same number of p electrons.…”

Konjugierte Oligomere mit terminaler Donor‐Acceptor‐Substitution

“…[138,139] Neue Ansätze in präparativer Hinsicht bieten Sternverbindungen (Abschnitt 3.3) [140] und dipolare Strukturen wie 101, bei denen -im Unterschied zu der bei 2 im Schema 3 beschriebenen Mesomerie -aromatische und dipolare Resonanzstrukturen vereinbar sind (Schema 30). [141] Das vielversprechende Ziel sind schlussendlich maßgeschneiderte D-p-A-Strukturen (molecular engineering) für bestimmte elektrooptische Anwendungen wie die optische Datenspeicherung, Datenverarbeitung und Datenübertragung. [142] Für die Förderung der im Artikel zitierten eigenen Arbeiten bedanke ich mich bei der Deutschen Forschungsgemeinschaft, bei der Volkswagen-Stiftung, beim Fonds der Chemischen Industrie und beim Materialwissenschaftlichen Forschungszentrum der Universität Mainz.…”

Cyanines Bearing Quaternary Azaaromatic Moieties

“…76 In the search for push-pull conjugated compounds promising in the design of NLO materials, dyes 41-43 were investigated. 51,77,78 It was observed that the quinoid character of 41-43 increases with higher annelation, i.e., in the order 41 < 42 < 43.…”

“…Dyes 41-43 give access to large second-order molecular NLO activities, provided the solvent polarity is suitable (Scheme 21). 77 Organic transition metal complexes may combine redox or magnetic behavior with NLO properties, a fact interesting for construction of multifunctional materials. It was established that some ruthenium pyridyl ammines containing pyridine groups often have very high b o values, associated with intense, low energy metal-to-ligand CT (MLCT) excitations.…”