Design of Coupled Porphyrin Chromophores with Unusually Large Hyperpolarizabilities

Abstract

A new series of push−pull porphyrin-based chromophores with unusually large static first hyperpolarizabilities are designed on the basis of coupled-perturbed Hartree−Fock and density functional calculations. The proper combination of critical building blocks, including a ruthenium(II) bisterpyridine complex, proquinoidal thiadiazoloquinoxaline, and (porphinato)zinc(II) units, gives rise to considerable predicted enhancements of the static nonlinear optical (NLO) response, computed to be as large as 11 300 × 10 −30 esu, 2 orders of magnitude larger than the benchmark [5-((4′-(dimethylamino)phenyl)ethynyl)-15-((4″-nitrophenyl)ethynyl)porphinato]zinc(II) chromophore. A two-state model was found to be useful for the qualitative description of the first hyperpolarizabilities in this class of NLO chromophores, which are predicted to have hyperpolarizabilities approaching the fundamental limit predicted to be attainable by empirical theoretical models. ■ INTRODUCTIONNonlinear optical (NLO) materials have attracted considerable attention due to potential applications in optics and optoelectronics that include information storage, image processing, frequency conversion, optical signal processing, optical computing, and dynamic imaging. 1−3 Inorganic crystals such as LiNbO 3 and KTiOPO 4 have been widely used in commercial NLO devices. 2 Since the 1990s, however, a large number of organic compounds with extended conjugation have emerged as candidates for electrooptically functional elements in NLO materials. 1,4−40 The advantages of organic materials over traditional inorganic crystals stem from their lower dielectric constants, potentially faster and larger NLO responses, and ease of processing; furthermore, the considerable topological and electronic structural diversity made available through chemical synthesis offers the opportunity for application-specific chromophore optimization. With respect to organic NLO materials, the donor-bridge (π-electron system)-acceptor or push−pull structure, has served as a classic design motif. 2,4−7,9−36,38−40 Although many theoretical 41−52 and experimental groups have been working for decades on the design of new structures with large second order NLO properties, relatively few examples of chromophores have been delineated that possess β λ values (dynamic hyperpolarizabilities) that exceed 1000 × 10 − 3 0 esu at telecommunications-relevant wavelengths. 9,13,14,17,24,25,[28][29][30]33,37,53,54 Due in part to the fact that a large fraction of these highly hyperpolarizable chromophores exploits a porphyrinic component, there has been increased i n t e r e s t i n p o r p h y r i n -b a s e d N L O m a t e r ials. 12−14,17,18,25,26,29−36,55−62 Porphyrins and their corresponding ethynylated and ethyne-linked derivatives are attractive building blocks for such materials because of their extended electronic delocalization, large oscillator strengths, and substantial polarizabilities; 12−14,38,61,63−76 in addition to providing for substantial β λ values, such porphyrin-based donorbridge-...

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